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Effects of ultrasonic pretreatment on the solubilization and kinetic study of biogas production from anaerobic digestion of waste activated sludge
Abstract and Figures
Waste activated sludge (WAS) is a polluting waste with severe management problems that must be treated to prevent pollution and human health risks. Anaerobic digestion (AD) is the most used process to stabilize sludge; however, it must be improved because the biomethanation of sludge entails low biodegradability, pathogen inactivation, and biogas production. This study investigated the effects of ultrasonic pretreatment (USp) of WAS as a strategy to improve AD. Macromolecule solubilization, heavy metal behavior, pathogen inactivation and biogas production were evaluated. USp was applied at a range of 5000–35000 kJ/kg TS (total solids). The maximum solubilization degree of soluble chemical oxygen demand was 26%, and 22.9% of proteins at 35000 kJ/kg TS. The highest USp reduced only 2 log units of pathogens; nevertheless, a high inactivation was obtained when TS were reduced to 2% and continuous stirring was applied. AD of raw and sonicated WAS were compared with biochemical methane potential tests, and a biogas overproduction of 31.43% (219.5 mL/g VS) was achieved at highest USp. A modified Gompertz model was used for kinetic study of biogas production. The kinetic parameters at highest energy were: biogas production potential, G0 = 634.2 mL; maximum biogas production rate, Rmax = 57.23 mL/day.
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... To evaluate the solubilization degree (SD) of APT, it was necessary to know the solubilization factor typified by the relation of the soluble chemical oxygen demand (sCOD) before and after APT. For this purpose, the soluble fractions of the raw and pretreatment samples were obtained by centrifugation at 12,000 rpm for 15 min and filtration of the supernatant with a 0.2-μm membrane [19]. ...
... During this period, the operating parameters monitored were biogas production (mL of biogas), biogas yield (mL of biogas per gVS removed ), and concentration of organic matter expressed as grams of VS. The biogas produced was collected, and its volume was measured based on water displacement [19]. Each experiment was performed two times for all suggested pretreatments and then analyzed. ...
... The soluble fraction was obtained after centrifugation at 13,000 rpm for 15 min and subsequent filtration with a 0.45mm nylon membrane filter [19]. The total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (TCOD) were analyzed according to the closed reflux colorimetric method (5220D) of the standard methods [22] using a spectrophotometer (Thermo Scientific™, Genesys 20). ...
In Mexico, productive activities related to poultry farming and the sugarcane industry generate large amounts of three byproducts, sewage sludge, chicken litter, and sugarcane wastes, which have high concentrations of organic matter and lignocellulosic material. Therefore, the objective of the present study was to evaluate the effect of acid pretreatment with different doses of acetic acid (2%, 3%, and 4% v/v) and exposure times of 30, 60, and 90 min on the solubilization of organic matter and biogas production in the anaerobic codigestion of a mixture of agroindustrial wastes. The biodegradability and biogas production were evaluated by anaerobic digestion for 30 days. Finally, for 90 days of semi-continuous operation, the increase in applied organic load of 1 KgVS/(m 3 .d) was considered with Δ1 KgVS/(m 3 .d) every 30 days until reaching 3 KgVS/(m 3 .d). Acid pretreatment with a dose of 4% acetic acid and an exposure time of 90 min improved the solubility and hydrolysis, considering the limiting stage of the anaerobic process, reducing the hydraulic retention time from 19 to 11 d to reach-Δ 38% VS, and increasing biogas yields (Y bio = 609 ± 11.7 to 1857.2 ± 7.5 L bio /gVS rem) and methane yields (YCH 4 = 426.9 ± 8.19 to 1392.9 ± 5.65 LCH 4 /gVS rem).
... Heat, acid, ozone and ultrasound pretreatments may improve this process and yield more digestible organics for the steps succeeding hydrolysis [23,24]. Especially ultrasound pretreatment, at levels up to 35 kJ/g total solids (TS) specific energy, was shown to increase the soluble COD (sCOD) concentration by 26%, resulting in 31% higher biogas production [25]. In a different study, comparing ozone and ultrasound pretreatments, sonication at 2.5 kJ/g TS input was shown to increase both volatile solids (VS) removal and biogas yield while ozone was largely ineffective at improving digestion [26]. ...
... Specific energy input was calculated by multiplying sonication time and power, which was then divided by the DS mass of sonicated sludge, resulting in 12.93 kJ/g TS. This value is compatible with the previous ultrasound pretreatment studies of [25]. Effectiveness of sonication was assessed by measuring the sCOD concentrations prior to and after sonication. ...
... Given in Fig. 2 are the percent sCOD concentrations with respect to specific energy inputs that were reported in other lab-scale ultrasound pretreatment studies. In comparison, we were able to achieve a significant sCOD increase for our specific ultrasound energy input, within the boundaries set by the previous studies [22,25,43,44]. The impact of this sCOD increase was primarily reflected as higher total biogas and methane productions during digestion. ...
With stricter regulations and concerns over sustainability, combustion can be a beneficial handling method as dried sludge can be used as an energy source. Anaerobic digestion of sludge also provides energy in the form of biogas without the need for sludge drying. Yet, anaerobic digestion reduces the calorific value (CV) of sludge. In this study, the effect of anaerobic digestion (with or without ultrasound pretreatment) on the fuel properties of sludge were examined through laboratory experiments. Additionally, a number of sludge management strategies were evaluated to maximize energy gain. Temporal changes in fuel properties of sludge were tracked during digestion. Results showed that sonication provided a 3.4-fold increase in soluble COD, 15% increase in biogas production and 32% increase in methane production, while the CV of sludge reduced from 14.7 kJ/g to 10 kJ/g. Energy balances for a number of sludge management process trains showed that the optimal method was the combination of thickening, dewatering, thermal drying and combustion. Addition of digestion and ultrasound pretreatment to this scenario reduced the energy gained from sludge. Yet, high ambient temperatures impacted results such that the net energy gain by the scenario including digestion and combustion was close to that of combustion only.
... Pretreatment methods, like ultrasound and thermal, may also impact sludge hygienization and could be used as both pretreatment and post-treatment, depending on the requirements of the WWTP (Ruiz-Hernando et al. 2014). According to studies mesophilic anaerobic digestion is inactive around 2 log 10 of pathogens and sludge containing up to 7 log 10 (Lizama et al. 2017). ...
... log units, respectively, at 35,000 kJ/kg. The authors tested TS concentration without continuous stirring and did not achieve the same inactivation, so these two conditions interfere on ultrasound pathogen inactivation capability (Lizama et al. 2017). According to Kumar (2011), the pathogen concentration decreased as sonication time and frequency increased, and reduction is mainly caused by the effects of cavitation and decreased the bacterial cells showing ruptured shapes. ...
High-strength wastewater (HSWW) has become a major issue that has been continuously affecting the environment, soil, and other freshwater resources. However, HSWW contains large amounts of organic matter which can be recovered in the form of energy as biogas by using anaerobic digestion. High-rate anaerobic reactors are being employed for treating the wastewater and converting the organics into biogas. In this study, several high-rate anaerobic reactors and the parameters affecting their performance have been discussed. The challenges and future perspectives have been detailed for exploring a better solution to recover bioenergy from the wastewater. Among the different types of reactors, the anaerobic membrane reactor (AMR) with a dynamic module (DM) is the best and most efficient option because it can be employed for treating high-strength wastewater (HSWW) with minimum losses. These reactors also reduce the methane emissions generated from wastewater sludge in an open area.KeywordsBiogasEnergy recoveryHigh-rate anaerobic digestionWastewater
... A study by Rasapoor, et al., (2016) carried out sonification (1200-18000kJ/TS) on the OFMSW at different solid contents (6 -10 %) and concluded that biogas yields were much improved at lower TS content (6 & 8 %) than at TS of 10 %. Lizama et al., (2017) also mentions that the total solid content limits effectiveness of ultrasonification as does the type of ultrasonic equipment used. Ultrasonic pretreatment of substrate reportedly enhances biogas and methane yield by improving solubility of organic matter. ...
... Ultrasonic pretreatment of substrate reportedly enhances biogas and methane yield by improving solubility of organic matter. The results of this study also confirms that in general, as the levels of SE treatment increase the methane yield also increases, a trend reported Accepted Article also by Lizama, et al., (2017), who observed increased cumulative methane production from 544 to 616 ml in the range 5000 -35000 kJ/kg TS. Likewise, Boni et al., (2016), reported higher cumulative methane yield with increasing US energy in the range SE 6000 to SE 50000. ...
The goal of this paper was to assess the possibilities for one‐stage reactor anaerobic digestion processing of fruit and vegetable biomass within a biorefinery concept for optimum products. Co‐digestion was the simplest strategy to implement to increase product yield among the variables tested (pH, ultrasonification, and co‐digestion). Methane yields from co‐digestion of fruit and vegetable waste (FVW) and pig manure (PM) ranged from 318 to 434 NmL /g VS, with the highest yield (434 NmL /g VS) achieved at a co‐digestion ratio of FVW75:PM25, representing a 20% increase over the methane production from FVW alone. The pH value was determined to be the most effective variable for enhancing methane production, with pH = 8 producing the maximum cumulative methane yield of 497 NmL/g VS. There was no significant change in the composition of volatile fatty acids (VFA) with pH in terms of dominant product type; valeric acid was the predominant VFA synthesized at all starting pH values except pH = 12, where formic acid was the predominant acid formed. As a digestate post‐treatment procedure, hydrothermal carbonization was combined with anaerobic digestion. At 200 °C, the maximum hydrochar yield production was 58.2%, with a higher heating value of 10.75 MJ/kg. © 2022 The Authors. Biofuels, Bioproducts and Biorefining published by Society of Industrial Chemistry and John Wiley & Sons Ltd.
... Numerous studies have demonstrated that the application of US pretreatment leads to a significant increase in methane yield [189,[235][236][237][238][239][240][241]284]. This suggests that utilizing ultrasound before AD can enhance the production of methane gas from biomass. ...
The increasing global population has led to a surge in waste production across various fields including agriculture, industry, marine, and household, posing significant waste management challenges. Concurrently, the world is facing an energy crisis, emphasizing the crucial need for sustainable and renewable energy sources. This comprehensive review examines the potential of biomethane production from diverse waste biomass. Feedstock characteristics; anaerobic digestion (AD); biochemical pathways; factors influencing AD; various pretreatment methods such as physical, chemical, biological, and combined; existing policies supporting biomethane production; and potential new policy implications are discussed in this review along with the significance of waste‐to‐energy integration. Our findings indicate that lignocellulosic wastes, primarily agricultural waste, stand out as the most efficient biomass source for biomethane production due to their characteristics such as high carbon/nitrogen ratio, low ash content, and their abundant availability. Among pretreatment methods, combined pretreatment emerges as the most promising option, offering flexibility and effectiveness in enhancing biomethane production. Additionally, the two‐stage digester configuration proves advantageous in overcoming limitations associated with single‐stage digesters such as pH inhibition. Altogether, the review highlights that biomethane production from waste biomass through AD offers a sustainable solution.
... The study shows that the lower specific energy input enhances biogas production by 40%, whereas the moderate specific energy increases it by only 15% (Appels et al., 2008). In a study by Lizama et al. (2017), the specific energy input of 35000 kJ/kg TS shows an increase in biogas production by 31.4%. Moreover, the VFA concentration also enhances to 233.96 mg/L. ...
The growing necessity for energy worldwide has led to the hunt for an interminable solution in the form of sustainable energy generation. To accomplish sustainability, these problems can be resolved using renewable waste biomass, which is readily accessible and low priced. Moreover, the ecological issue due to the disposal of this waste biomass into the environment is also counteracted by the use of this biomass for energy generation along with the substantial solid reduction for disposal. The presence of complex biopolymers in biomass, which hasten the hydrolysis step during energy generation, was enhanced by the application of a pretreatment method. The efficiency of the pretreatment methods was enhanced by maintaining the cost and energy usage since the commercialization of this method is largely limited. The major economic drivers are based on solid concentration and, thus, lead to higher capital costs. This study reveals the wide assortment of current progression in pretreatment techniques for treating waste biomass with special focus on combined and phase-separated pretreatment. Additionally, it converses the advantages and limitations of pretreatment methods. This pivotal investigation brings about the cost- and energy-effective conversion solution that paves the way for a sustainable energy system.
... A volatile free acid mix (SUPELCO, CRM46975) was used as the standard. The analysis conditions were the same as those reported by Lizama et al. [45]. The ORP was measured using a multiparameter (WTW Multi 3430). ...
Recently, different metallic additives have been studied to improve biohydrogen production, such as salts or oxides with iron and nickel. The results have been positive in simple systems, but there are very few studies of the use of nanoparticles of iron and cobalt in systems that use complex substrates such as sludge. In the present study, the effects on hydrogen production from anaerobic digestion of waste activated sludge by zero valent iron nanoparticles (NZVI) and cobalt (CoNP) dosage were investigated. The maximum hydrogen yields were reached with 7 mg/gVS for both additives, 5.74 and 5.40 mLH2/gVSadded, for NZVI and CoNP, respectively. In contrast, a low yield was observed in the control reactor (1.79 mLH2/gVSadded), representing increases of over 200%. The dosage of CoNP and NZVI decreased the redox potential and increased the volatile fatty acid concentration, mainly acetic acid. The results indicate that NZVI and CoNP stimulate the early stages of anaerobic digestion of sludge.
... 108,263,301−305 Mostly, low frequencies, ranging from 20 to 40 kHz, are used to obtain the physical effects required for the sludge treatment. The ultrasonic disintegration of sludge can offer several advantages, including effective sludge dewatering, 306,307 reduced foaming, 308 detoxification and disinfection, 309,310 enhanced biogas production, 311,312 and activation of ammonia-oxidizing bacteria (AOB). 313−315 The application of ultrasonic technology to sludge disintegration has already been successfully transferred to industrial applications, proving an energy-efficient and cost-effective alternative to conventional technologies. ...
... Hu et al. (2020) investigated the possibility of competence of biosolids using microbial fuel cells (MFC) by applying a voltage of 0.5 V, biogas manufacture enhanced 3.68-fold. Lizama et al. (2017) reported that after ultrasonic pretreatment, two different microorganisms, such as fecal coliforms and Salmonella spp. become deactivated. ...
Waste activated sludge (biosolids) treatment is intensely a major problem around the globe. Anaerobic treatment is indeed a fundamental and most popular approach to convert organic wastes into bioenergy, which could be used as a carbon-neutral renewable and clean energy thus eradicating pathogens and eliminating odor. Due to the sheer intricate biosolid matrix (such as exopolymeric substances) and rigid cell structure, hydrolysis becomes a rate-limiting phase. Numerous different pretreatment strategies were proposed to hasten this rate-limiting hydrolysis and enhance the productivity of anaerobic digestion. This study discusses an overview of previous scientific advances in pretreatment options for enhancing biogas production. In addition, the limitations addressed along with the effects of inhibitors in biosolids towards biogas production and strategies to overcome discussed. This review elaborated the cost analysis of various pretreatment methods towards the scale-up process. This review abridges the existing research on augmenting AD efficacy by recognizing the associated knowledge gaps and suggesting future research.
... In literature, low temperature thermal pretreatment (50-95 • C) was shown to enhance hydrolysis rate and, when application times of 60 min (or longer) were selected, methane yield was also commonly augmented (Gonzalez et al., 2018). Comparable results to the present ones related to ultrasonication were claimed by (Lizama et al., 2017), with CH 4 yield increase up to 31.4%, by applying ultrasonic energies of 5,000-35,000 kJ/kg TS (similar to the present values). Another relevant study (Yan et al., 2020) highlighted that biochar addition to sludge AD reduces VFA accumulation, leading to significant CH 4 yield increases and enhanced sCOD abatements. ...
In this work, different pretreatment technologies (low temperature thermal treatment, ultrasonication, alkali treatment, combined alkali-thermal treatment, icing-thawing) were investigated together with biochar addition to increase biogas yield from sewage sludge. The aim of the work was to develop a standardized protocol, including physicochemical characterization, biochemical methane potential (BMP) tests, digestate characterization, life cycle assessment (LCA) and economic analysis, to verify the techno-economic suitability and the related environmental impacts of upgrading conventional full-scale anaerobic digesters. Low-temperature (65–85 °C) thermal pretreatment showed the best performances in enhancing sludge solubilization and methane yield (up to 110%), followed by ultrasonication (up to 53%) and biochar addition (about 16%). The solubilization rate obtained after each pretreatment was similar for municipal and industrial sludge; however, it did not correspond to a proportional increase in methane yield. LCA showed that ultrasonication led to the worst environmental impacts at laboratory scale, due to the huge electricity request; however, at full-scale conditions, green-house gases (GHG) emissions could be reduced by 22%, fossil depletion by 25% and stratospheric ozone depletion by 18%, when compared to the baseline scenario (raw sludge anaerobic digestion). The economic analysis proved that the high ultrasonication capital costs could not be recovered in the considered timeframe (15 yr), while biochar price should be significantly lowered (down to 6.5 €/ton) to give an overall positive outcome. Finally, the thermal scenario would be convenient only when considering waste heat recovery. The proposed protocol, including physicochemical sludge characterization, BMP tests, digestate characterization, LCA and economic analysis, could be extended to other existing digesters to compare alternative pretreatment strategies, enhancing renewable energy generation in biogas form.
... The contributing factors of ultrasonic sludge disintegration include sonochemical effects, high mechanical forces, and thermal effects (Pilli et al., 2011). Sonication leads to the formation of cavitation bubbles in the liquid phase; these bubbles grow and explode violently, causing high shearing forces in the surrounding liquid phase of the waste and radicals' formation (Lizama et al., 2017;Luo et al., 2014;Pilli et al., 2011). Therefore, it could be inferred that the thermal combined ultrasonic pre-treatment was simultaneously affected and strengthened by the above two mechanisms, which had also been verified in our work. ...
Thermal/ultrasonic pre-treatment of sludge has been proven to break the hydrolysis barriers of sewage sludge (SS) and improve the performance of anaerobic digestion (AD). In this study, the objective was to investigate whether the combination of two pre-treatment methods can achieve better results on the AD of SS. The results indicated that, compared with the control group and separate pre-treatment groups, the thermal combined ultrasonic pre-treatment presented more obvious solubilization of soluble proteins, polysaccharides, and other organic matters in SS. The combined method promoted the dissolution of protein-like substances more effectively, with biogas production increased by 19% and the volatile solid (VS) removal rate improved to above 50% compared with the control group. The capillary suction time is reduced by about 85%, which greatly improved the dewatering performance of SS. In addition, the combined method has advantages in degrading sulfonamide antibiotics, roxithromycin and tetracycline. Particularly, by analyzing the interaction between the degradation of different antibiotics and the composition of dissolved organic matters (DOMs), it was found that the composition of DOMs could affect the degradability of different antibiotics. Among them, the high content of tyrosine-like and tryptophan-like was conducive to the degradation of sulfamethoxazole, and the high content of fulvic acid-like and humic acid-like was conducive to the degradation of roxithromycin and tetracycline. This work evaluated the comprehensive effect of thermal combined ultrasonic pre-treatment on SS disposal and provided useful information for its engineering.
... During this period, the biogas production (mL biogas), biogas yield (mL biogas per gVS removed ) and organic matter concentration expressed as gram volatile solids (VS) were monitored. The biogas produced during AD was collected, and its volume was measured based on the water displacement [36,41]; all the assays were performed in triplicate. ...
The coffee pulp is the solid waste most generated during the coffee humid pulping. It contains an elevated concentration of organic matter and lignocellulosic materials. In addition, the low rate of biodegradability of the coffee pulp waste may cause severe environmental damage. Therefore, the aim of this study was to evaluate the effect of acid and thermal pre-treatments in the biodegradability rate, followed by an anaerobic digestion of the coffee pulp and collect biogas production. Thermal and acid pre-treatments (50, 70, and 90 °C, and concentration of 2.5, 5, and 10% V·V⁻¹ of acetic acid, respectively) were applied, both during 1 h, evaluating the organic matter solubilization. Posteriorly, the biodegradability and biogas production using anaerobic digestion for 35 days were evaluated. Finally, during 90 days in semicontinuous, the increase of applied organic load of 1 kg VS m⁻³ day⁻¹ with Δ 1 kg VS m⁻³ day⁻¹ every 30 days until reaching 3 kg m⁻³ day⁻¹ was evaluated. The thermal pretreatment to 90 °C and 1 h improved the solubility and hydrolysis considered limiting of the anaerobic process, reducing the hydraulic retention time from 21 to 15 days, and increasing the biogas yields (0.92 L g VSrem⁻¹ year 79.8% CH4).
... Several studies have been conducted to explain this process. Lizama et al. (2017) reported that when the ultrasonic pretreatment range was 5000−35000 kJ/kg TS, the SCOD, soluble protein, and biogas production are improved by 26 %, 22.9 %, and 31.4 % respectively. Also, with the increase of the intensity of ultrasound pretreatment, the VFAs concentration increased from 138.57 ...
Owing to the development of urbanization, the amount of sewage sludge generated through biological activated sludge process has increased dramatically. Anaerobic fermentation of sludge is recognized as an expedient and efficient treatment process, widely applied for biogas generation, resource recovery, and volatile fatty acid production. Volatile fatty acids are one of the most widely used carbon sources and have great use in biological nutrient removals. Volatile fatty acids production in anaerobic sludge fermentation is affected by sludge properties, metabolic pathways, and operating parameters. This paper aims to present an overview of the recent advancement in volatile fatty acids production from waste activated sludge. Standalone and hybrid pretreatment methods prior to the sludge fermentation were introduced and assessed based on VFAs accumulation rate and system performance. In addition, different metabolic steps involved in anaerobic fermentation (i.e. hydrolysis, acidification, and methanogenesis) were deeply evaluated. More importantly, the effects of humic substances were evaluated, among which, the electron transfer, the enzyme activity of microbial species, and the interaction between exogenous electron transporters and humic substances were illustrated. Furthermore, the influence of nitrogen and phosphorus ingredients in sludge fermentation and volatile fatty acids production was introduced. It was found that the additives and pretreatment of waste activated sludge are energetically preferred for the hydrolysis improvement and accelerating the volatile fatty acids accumulation. It was concluded that different structures of humic substances may have different effect on the fermentation process and volatile fatty acids production. The synergistic addition of hydrolytic enzymes assisted to reverse the negative effect of humic acids in some cases, and mitigated the adverse effect of humic substances on the inhibition of bacterial growth. The composition and properties of waste activated sludge may limit its biodegradability and hamper the volatile fatty acids production.
... Especially, hot air oven and autoclave pretreatment provide more biodegradability with increased biogas production. The presented kinetic data offer the scientific basis for further improvement, basic design criteria, simulation, and upscaling of anaerobic bioprocesses, which obey the stringent environmental regulation [2,27]. The application of thermal pretreatment especially hot air oven and autoclave pretreatment prior to anaerobic digestion offers a sustainable pathway. ...
Pretreatment plays an important role in enhanced methane production from lignocellulose material. In this study four different types of heating processes such as hot air oven, autoclave, liquid hot water bath, and microwave oven were used for comparison. The study was carried out in a biochemical methane potential assay to evaluate the different thermal heating process for enhanced methane production. Methane yield was increased in varied proportions with reduced digestion time for different heating processes of thermal pretreatment with low heating temperature at high residence time. Amongst, hot air oven pretreatment (17.92 %) showed a high yield of methane production followed by autoclave (13.44 %), microwave oven (9.54 %) and hot water bath (7.54 %) than control samples. In addition, the effect of pretreatment on biodegradability was assessed with three different kinetic models. Modified Gompertz model presents similar experimental data with a high degree of best fit when compared to the other two models.
... However, in the case of S2 and S3, BY values obtained in AD1 (TES subject to the HC process) were as much as 37% and 53% higher, respectively, in contrast to AD2. For example, Lizama et al. [26] subjecting waste activated sludge to ultrasounds obtained a 31% increase in BY. It should be emphasized, however, that the increase was obtained for ES = 35,000 kJ/kgTS, which constituted a value 9.6 times higher than the value of the parameter for series 2 in this study. ...
The main purpose of this study was the assessment of the possibility of increasing the production of biogas through the pre-treatment of thickened excess sludge (TES) by means of the hydrodynamic cavitation (HC) conducted at different levels of energy density (EL) i.e., 70, 140 and 210 kJ/L. The experiments were performed on a pilot scale, and a mixture of thickened primary sludge (TPS) and TES was used as digester feed. The results documented that an important parameter determining the possibility of obtaining an enhanced methane production is the value of energy input in the HC process. This parameter determines the changes occurring in sludge as a result of disintegration (i.e., sludge floc deagglomeration, lysis of cells, re-flocculation process and the related release of compounds susceptible to biodegradation from sludge flocs). The maximum increase in methane yield (MY) of 152% was obtained for EL = 140 kJ/L. In this case, HC mainly caused sludge floc deagglomeration. An increase in MY was also recorded when TES was subject to the disintegration process at EL = 210 kJ/L. However, it was 4.3 times lower than that observed for EL = 140 kJ/L. Pre-treatment of TES at EL = 70 kJ/L did not contribute to an increase in methane production.
... Nonetheless, under the investigated conditions, SCOD concentrations increased linearly with an increase in ultrasound specific energy inputs. Previous studies also reported that ultrasound treatment <10,000 kJ/kg TS could linearly increase COD solubilization for different feedstocks, including sewage sludge, lignocellulosic waste, manure, etc. [20,[27][28][29]. Additionally, SCOD solubilization efficiencies achieved in this study (0.9-1.2 mg SCOD/(kJ/kg TS)) were within the range of values (0.12-1.35 mg SCOD/(kJ/kg TS)) previously reported in the literature [21,30]. ...
High-solids anaerobic digestion of organic fraction of municipal solid waste often shows inefficient biomethane recovery due to mass transfer limitations. Consequently, this study presents a two-stage anaerobic digestion process combining high-solids anaerobic digestion followed by ultrasonication of digestate and wet-type anaerobic digestion for effective biomethane recovery from the organic fraction of municipal solid waste. The high-solids anaerobic digestion yielded methane production of 210 L CH4/kg volatile solids (VS). The digestate from the high-solids anaerobic digestion process was ultrasonicated at three different specific energy inputs (1000, 2500, and 5000 kJ/kg total solids (TS)). The increases in the soluble chemical oxygen demand (SCOD) concentrations (8%–32%) and volatile solids (VS) removal efficiencies (3.5%–10%) at different specific energy inputs were linearly correlated (R2 = 0.9356). Thus, ultrasonication led to the solubilization of particulate organics and released soluble organic matters. All ultrasonicated digestate samples showed significantly higher biomethane yields than that observed for the untreated digestate samples. The highest methane yield of 132 L CH4/kg VS was observed for a specific energy input of 5000 kJ/kg TS, which was 1.94 times higher than the control (68 L CH4/kg VS). Although specific energy inputs of 1000 kJ/kg TS and 2500 kJ/kg TS showed comparable methane yields (113–114 L CH4/kg VS), they were ~1.67 times higher than the control. Overall, our results suggest that an integrated system of high-solids and wet-type anaerobic digestion with pre-ultrasonication of digestate has the potential to provide a technically viable solution to enhance biomethane recovery from the organic fraction of municipal solid waste.
... The mechanisms of ultrasonic disintegration of organic matter included mechanical, sonochemical and thermal effects. Three hypotheses can be proposed for the ultrasonication mechanisms (Al-Juboori et al., 2016;Pilli et al., 2016;Lizama et al., 2017). First, ultrasonication treatment reduced the particle size (~70%, Wong et al., 2018a) and increased the surface area of the organic matter in POME for microbial degradation, and thus increased the anaerobic digestion rate. ...
The conventional practice in enhancing the larvae growths is by co-digesting the low-cost organic wastes with palatable feeds for black soldier fly larvae (BSFL). In circumventing the co-digestion practice, this study focused the employment of exo-microbes in a form of bacterial consortium powder to modify coconut endosperm waste (CEW) via fermentation process in enhancing the palatability of BSFL to accumulate more larval lipid and protein. Accordingly, the optimum fermentation condition was attained by inoculating 0.5 wt% of bacterial consortium powder into CEW for 14–21 days. The peaks of BSFL biomass gained and growth rate were initially attained whilst feeding the BSFL with optimum fermented CEW. These were primarily attributed by the lowest energy loss via metabolic cost, i.e., as high as 22% of ingested optimum fermented CEW was effectively bioconverted into BSFL biomass. The harvested BSFL biomass was then found containing about 40 wt% of lipid, yielding 98% of fatty acid methyl esters of biodiesel upon transesterification. Subsequently, the protein content was also analyzed to be 0.32 mg, measured from 20 harvested BSFL with a corrected-chitin of approximately 8%. Moreover, the waste reduction index which represents the BSFL valorization potentiality was recorded at 0.31 g/day 20 BSFL. The benefit of fermenting CEW was lastly unveiled, accentuating the presence of surplus acid-producing bacteria. Thus, it was propounded the carbohydrates in CEW were rapidly hydrolysed during fermentation, releasing substantial organic acids and other nutrients to incite the BSFL assimilation into lipid for biodiesel and protein productions simultaneously.
... Additionally, their results revealed that the net energy gain increased by 395% for 500 g VS/ m 3 -day and 237% for 1500 g VS/ m 3day with the ultrasonic pretreatment in comparison to the control [106]. Moreover, Lizama et al. [107] reported that pathogens such as fecal coliforms and Salmonella spp. became inactivated after ultrasonic pretreatment. ...
Biogas production from different waste resources still has limitations due to its complex structure and slowly biodegradable nature. To improve methane yield and anaerobic digestion performance, various substrate pre-treatment methods have been suggested. This paper reviews the latest trends, progress, and research achievements about pretreatment technologies to improve anaerobic digestion efficiency. The pretreatment techniques T are divided into four main groups which are physical, chemical, biological, and combined. The effect of inhibitor formation during the pretreatment process is discussed. The energy performance, economics, and environmental impact of these pretreatment technologies are revealed. This study concludes with future trends and emphasizes the necessity of pretreatment methods.
The waste biological sludge disintegration by using microwave irradiation was investigated at a ramping rate of 2°C/min and 5 min holding time at various target temperatures. Significant disintegration of biosludge was observed and the highest disintegration degree was determined about 82% at the temperature of 110°C. Increase of target temperature elevated
the energy needs to 98, 123 and 148 kWh/kg TS at the temperatures of 75°C, 90°C and 110°C, respectively. The gradual increase of sugar and protein in the sludge slurry with increasing temperatures indicates successful degradation. The microwave pretreatment increased the specific surface area of the sludge by particle size reduction. The specific surface area of raw sludge was 70 m2 /kg and rose to approximately 253.7 m2 /kg at 110°C with an increment ratio of 260%. Although a significant NH4–N release was not observed, PO4–P concentrations increased from 11.0 mg/L to 16.3, 20.7 and 29.2 mg/L at the temperatures of 75°C, 90°C, 110°C, respectively. While the specific filter resistance of waste biological sludge was about 1.0 × 1013, increasing the microwave target temperature, the ability of dewatering decreased and the highest SFR value of 5.1 × 1014 was observed at the temperature of 110°C.
This study investigates the effect of microwave treatment on the disintegration of municipal activated sludge. Sludge samples underwent heating at a targeted temperature of 90 °C for 35 min, with a 5-min retention time. Soluble chemical oxygen demand, sugars, proteins, nitrogen, and phosphorus exhibited notable increases compared to untreated samples. Results indicate a substantial (42–45%) rise in CH4 production during the anaerobic digestion process of the disintegrated sludge compared to the untreated counterpart CH4 production was estimated using a transference model, which showed the best fit compared to other models. Further experimentation involved testing digested sludge with excess soluble NH4–N and PO4–P for the recovery of struvite at a 1.50/1/1 (Mg/N/P) ratio. The findings reveal that up to 90.1% and 90.4% of PO4–P and NH4–N, respectively, can be efficiently removed from the solution. Despite the increased CH4 output, the energy recovered was insufficient to offset the electrical energy used by the microwave. There was a significant deterioration in sludge filter resistance due to the increase in fine particles and bound water after anaerobic digestion of the pretreated sludge.
Sewage sludge, generated in significant quantities globally, poses a formidable challenge for environmentalists due to its volume and management. Various treatment methods have been explored to address this issue, among which anaerobic digestion stands out as an environmentally friendly approach. However, the hydrolysis step within anaerobic digestion presents a bottleneck, hindering its efficiency. To overcome this limitation, diverse pretreatment methods—physical, chemical, biological, and their hybrids—have been investigated to enhance sewage sludge solubilization and accelerate hydrolysis, thereby improving overall anaerobic digestion efficiency. This chapter systematically reviews different pretreatment technologies preceding anaerobic digestion. Furthermore, it explores methods to enhance methane production through pretreatment strategies. Economic feasibility analysis of these pretreatment methods is also conducted, providing valuable insights for practical implementation in biogas production plants utilizing sewage sludge as a substrate. Overall, this chapter serves as a valuable resource and reference for integrating pretreatment technologies into biogas production plants, contributing to more efficient sewage sludge management and sustainable energy production.
Ultrasound and combined alkaline–ultrasound pretreatment (AUP) strategies were examined for primary sewage sludge (SS) disintegration and were utilized to evaluate the degree of solubilization (DS). Further, the pretreated primary SS was operated in microbial electrolysis cells (MECs) to maximize methane production and thereby improve the reactor performance. The highest DS of 67.2% of primary SS was recorded with the AUP. MEC reactors operated with the AUP showed the highest methane production (240 ± 6.4 mL g VSin−1). VS (61.1%) and COD (72.2%) removal in the MEC ALK-US showed the best organic matter removal efficiency. In the modified Gompertz analysis, the substrate with the highest degree of solubilization (AUP) had the shortest lag phase (0.2 ± 0.1 d). This implies that forced hydrolysis via pretreatment could enhance biodegradability, thereby making it easy for microorganisms to consume and leading to improved MEC performances. Microbial analysis implicitly demonstrated that pretreatment expedited the growth of hydrolytic bacteria (Bacteroidetes and Firmicutes), and a syntrophic interaction with electroactive microorganisms (Smithella) and hydrogenotrophic methanogens (Methanoculleus) was enriched in the MECs with AUP sludge. This suggests that the AUP strategy could be useful to enhance anaerobic digestion performance and provide a new perspective on treating primary SS in an economical way.
Treating municipal wastewater is a complex and costly process. With rising energy costs and sustainability targets, wastewater treatment plants (WWTPs) are looking for alternatives to reduce operating costs and carbon dependence. Anaerobic digestion is the most common and established technology used in WWTPs to treat sludge since it can potentially improve energy recovery and reduce sewage treatment costs, mainly due to the generation of biogas. Biogas is a renewable energy resource and can be used in several applications, including heating and producing electricity. By exploring the biogas potential, WWTPs can reduce their operating costs and energy demands. The objective of this paper is to conduct a scoping literature review in order to provide the key concepts underpinning alternatives to improve biogas production and utilisation in WWTPs. In addition, this study aims to provide an overview of the current state-of-the-art that may serve as a quick reference for the research community, WWTP operators, and engineers, including definitions and a general overview of the current state of biogas technologies around the world. Methods to increase biogas production, including co-digestion, pre-treatment, and biological hydrogen methanation, are reviewed, and the alternatives to using biogas are also summarised. This review has identified that co-digestion was the most efficient technique to improve biogas production and methane yield, while pre-treatment of sludge improved sludge biodegradability and reduced sludge treatment costs but also enhanced biogas production. Although many studies have explored different methods to improve biogas production in WWTPs, there is still a need for further investigation, especially regarding the techno-economic feasibility of these methods in full-scale facilities. The current challenges are mainly related to the need for extra investment and increased operating costs to integrate the new techniques into the current system. There is a great interest in alternatives to improve energy efficiency and self-sufficiency in WWTPs. This work provides an important review of the increasing number of recently published research papers that focus on improving biogas generation from sewage sludge in WWTPs.
Urine wastewater derived from source separation technology was employed to pretreat waste activated sludge (WAS) for anaerobic digestion in this study. In comparison to the control group, cumulative methane production was identified to be improved by 23% after pretreated by urine wastewater with its volumetric proportion of 1:8. Urea and hydrolyzed free ammonia (FA) in urine wastewater were observed to pose a synergistic effect on sludge disintegration, benefiting the reduction of sludge volume and enrichment of available substrates for bio-processes. Analysis of microbial community further revealed that acid-producing microbes were enriched in the experimental digesters. Although FA was found to be the major inhibitor to several microbial stages, metal ions supplemented by urine pretreatment might have alleviated such suppression. The proposed strategy would pave a more sustainable way for sludge pretreatment with waste materials.
The main goal of this study was to investigate the novel combined Ultrasonication and Free Nitrous Acid (FNA) pretreatment on biodegradability and kinetics of thickened waste-activated sludge (TWAS). Partial factorial design with four levels of (0, 600, 1500, and 3000 kJ/kg) for ultrasonication and 0, 0.7, 1.4, and 2.8 mg HNO2-N/L for FNA dose were examined creating 16 different combinations. Results revealed that combined pretreatment could significantly improve solubilization and solid destruction compared to solo pretreatments. The highest organic matter solubilization of 25.6% and volatile suspended solids destruction of 21.7% were observed when 2.8 mg HNO2-N/L and 1500 KJ/Kg were combined. Moreover, combining the pretreatments further enhanced biodegradability up to the highest percentage of 50.3% when pretreatment of 3000 KJ/Kg and 2.8 mg HNO2-N/L was applied. Also, the experimental data from a biochemical methane potential test was fitted well into First Order Kinetic and Modified Gompertz models, given that the coefficients of determination, R2, for models at all treatment levels were above 99%.
The treatment of municipal wastewater is considered a cornerstone for the protection of public health and environment. However, a major issue derived from this process is the large quantities of produced sewage sludge. Although anaerobic digestion is a widely applied method in Wastewater Treatment Plants (WWTPs) aiming to stabilize the sludge and to recover energy in the form of methane, it is usually limited due to the reduced decomposition efficiency and slow biodegradation rate of this recalcitrant substrate. For this reason, various pretreatment methods have been proposed aiming to modify the sludge structure, solubilize the organic matter, and decrease the crystallinity of sludge so as to accelerate hydrolysis and consequently enhance methane production. The current research is a comprehensive collection of recent advances in pretreatment technologies that can be potentially applied in wastewater treatment facilities. The critical review analysis presented herein reveals the several advantages and drawbacks, as well as the technical opportunities of the pretreatment methods and provides an assessment of their feasibility/applicability from an energetic, environmental, and economic point of view.
Anaerobic digestion (AD) is the most widely used technology for the treatment of sewage sludge. It allows the production of biogas, prevents environmental and health negative impacts, and generates biosolids. However, despite its potential, the sludge AD has certain disadvantages such as low biodegradability of sludge, high hydraulic retention time, instability at high organic loading rates and long periods, and low pathogen inactivation. These disadvantages make it necessary to improve the process, especially using pretreatments. Ultrasonic pretreatment is the most effective sludge disintegration technology. Therefore, the main objectives of this work were reviewing the effects of sonication on the sludge AD, highlighting effects on solubilization of sludge, dewaterability, pathogen inactivation, and biosolid quality. As well as the current developments, challenges, and future perspectives, such as coupling with other pretreatments or the nanomaterials incorporation, highlighting the novel process of nanoferrosonication.
Sludge, a by-product or residue of wastewater treatment facilities, has considerably increased the generation over the years. Due to its large amount and content, organic matter, metals, and pathogens, sludge poses an environmental and health risk if not properly managed. Furthermore, stabilization and management of this residue maintain affordable costs on wastewater treatment plants (WWTPs). Anaerobic digestion (AD) is a promising technology to sludge valorization; however, it needs to be made more effective because this waste leads to low degradability and consequently low energy production. Pretreatments can be used to hydrolyze sludge and consequently improve biogas production, solid removal, and sludge quality after digestion, increasing the applicability of AD. Different technologies are being studied by physical-chemical and biological methods. This chapter addresses an overview of different technologies for pretreatment, focusing on thermal, ultrasonic, and enzymatic processes, discussing their effects on sludge properties and anaerobic digestion. Concerns related to pretreatment implementation, pathogen distribution, and directives around the world are also addressed.KeywordsAnaerobic digestionThermal pretreatmentEnzymesUltrasonicPathogen
Organic wastes are regarded worldwide as a large prospective source of renewable energy, which may be used in a sustainable manner, provided that it is processed in controlled conditions in order to reduce its environmental impact and energy recovery. An acknowledged method of the recovery and treatment of the mentioned waste, supported by more than 30 years of experience, is anaerobic digestion (AD). Its advantages include low energy consumption, waste stabilization, waste volume reduction and the fact that the area that is needed for process facilities does not have to be large. What is equally important is the production of an energy carrier – biogas, whose share in the year 2020 was to constitute 5% of the overall energy balance within the meaning of the EU policy. This chapter reviews the current methane‐producing potential of various organic wastes, the basis of the process, including the classification of AD systems, their pros, and cons, as well as examples of commercial AD technologies. Optimization techniques associated with anaerobic digestion such as co‐digestion and pre‐treatment of feedstock are also discussed. It should be emphasized at this point that it is difficult to indicate the best option because each of them has its strong and weak points. For this reason, the selection of a technology should depend on the properties of the organic waste, and it should be preceded with the calculation of energy consumption, outlays, etc. Nonetheless, the more comprehensive industrial use of AD requires development offering the system of biodegradable organic waste collection.
In Mexico, productive activities related to poultry farming and the sugarcane industry generate large amounts of three byproducts: physicochemical residual sludge, poultry manure, and sugarcane wastes, which have high concentrations of organic matter and lignocellulosic material. Therefore, the objective of the present study was to evaluate the effect of acid pre-treatment, with different doses of acetic acid (2%, 3%, and 4% v.v − 1 ) and exposure times of 30, 60, and 90 min on the solubilization of organic matter and biogas production in the anaerobic co-digestion of a mixture of agroindustrial wastes (residual sludge, chicken manure, and sugarcane wastes). The biodegradability and biogas production were evaluated by anaerobic digestion for 30 days. Finally, for 90 days semi-continuously, the increase in applied organic load of 1 kg VS m − 3 d − 1 was considered with Δ1 kg VS m − 3 d − 1 every 30 days until reaching 3 kg m − 3 d − 1 . Acid pre-treatment with a dose of 4% acetic acid and an exposure time of 90 min improved the solubility and hydrolysis, considered the limiting stage of the anaerobic process, reducing the hydraulic retention time from 19 to 11 d, and increasing biogas yields (Y bio =609 ± 11.7 to 1857.2 ± 7.5 L bio gVS rem ⁻¹ ) and methane (Y CH4 = 426.9 ± 8.19 to 1392.9 ± 5.65 L CH4 gVS rem ⁻¹ ).
Objectives:This study examined the effect of ultrasonic pretreatment on primary sewage sludge (raw sludge) solubilization and its subsequent microbial electrolysis cells (MECs) operation performance.
Methods:To compare the effect of ultrasound on raw sludge solubilization, ultrasonic pretreatment was conducted at 1~4 W/mL energy density for 5~30 min. In MECs operation, raw sludge was used as a control group, and ultrasound pretreated sludge was used as an experimental group. For comparing MECs performance, biogas production, and organic matter removal were analyzed.
Results and Discussion:The optimal experimental condition for ultrasonic pretreatment were 30 min of sonication time at 3 W/mL. In methane production, MEC with ultrasound pretreatment (MEC 3W) produced 243 mL/L more methane than that of unpretreated MEC (MEC) by 4,970 mL/L at 1, 3 cycles. In the modified Gompertz model analysis, the lag phase of MEC 3W was 0.46 days, which was 0.12 days longer than MEC. The maximum methane production rate of MEC 3W by 938.5 mL/L/day was also higher than MEC. MEC 3W showed a 1.8% higher TS removal rate, 2.4% VS removal rate than MEC. COD removal rate also improved by 2.0% when ultrasound pretreatment was applied. The methane yield of MEC with ultrasound pretreatment (377.4 mL/g VSin.) was 0.4% higher than that of MEC without pretreatment.
Conclusion:Ultrasonic pretreatment of sewage sludge improved the methane production and organic removal in microbial electrolysis cells. It is necessary to find the optimal operating conditions to obtain the maximize the performance.
In this study, palm oil mill effluent (POME) treated by ultrasonication at optimum conditions (sonication power: 0.88 W/mL, sonication duration: 16.2 min and total solids: 6% w/v) obtained from a previous study was anaerobically digested at different hydraulic retention times (HRTs). The reactor biomass was subjected to metagenomic study to investigate the impact on the anaerobic community dynamics. Experiments were conducted in two 5 L continuously stirred fill-and-draw reactors R1 and R2 operated at 30 ± 2 °C. Reactor R1 serving as control reactor was fed with unsonicated POME with HRT of 15 and 20 days (R1-15 and R1-20), whereas reactor R2 was fed with sonicated POME with the same HRTs (R2-15 and R2-20). The most distinct archaea community shift was observed among Methanosaeta (R1-15: 26.6%, R2-15: 34.4%) and Methanobacterium (R1-15: 7.4%, R2-15: 3.2%). The genus Methanosaeta was identified from all reactors with the highest abundance from the reactors R2. Mean daily biogas production was 6.79 L from R2-15 and 4.5 L from R1-15, with relative methane gas abundance of 85% and 73%, respectively. Knowledge of anaerobic community dynamics allows process optimization for maximum biogas production.
The accelerated chemical-industry has caused a rapid increase of calcium-containing alkali wastes, containing a large amount of calcium, magnesium, aluminum, and iron ions and caused a great risk to environment. Anaerobic digestion or dark fermentation is one of the most promising technologies to recover biogas, such as methane and hydrogen. Nevertheless, the hydrolysis processes of lignocellulosic biomass and waste activated sludge were the rate-limiting step of the biochemical reactions, which focused on pretreatment to improve the biodegradability of substrate. In addition, when some easily acidified wastes, such as kitchen residue, fruit and vegetable waste, and high concentration organic wastewater, are used as substrate to produce hydrogen and methane, volatile fatty acid accumulation often occurs, causing the process instability. Thus, this paper reviewed the main roles of calcium-based alkali materials such as calcium oxide, calcium peroxide and calcium hydroxide on the substrate pretreatment for obtaining high biodegradability, while others (e.g. calcium carbonate, lime and red muds) used as additives for maintaining process stability, thereby increasing biogas yield from anaerobic digestion and dark fermentation.
Recovering phosphorus (P) from excess sludge of wastewater treatment plants (WWTPs) has attracted considerable attention. An efficient P release method is undoubtedly critical for a satisfactory recovery performance. In this study, the effectiveness of three sludge pretreatment methods, i.e. anaerobic digestion (AD), EDTA-anaerobic digestion (EA) and ultrasound combined with EA (U-EA), on P release and struvite recovery from excess sludge was investigated. The results showed that different pretreatment methods resulted in the different characteristics of P release and recovery. For P release, the highest P release rate (57.14% of sludge total phosphorus, TP) was achieved by U-EA pretreatment, followed by EA and AD. Furthermore, U-EA was beneficial for sludge disintegration and reduction, by which the mixed liquor suspended solids (MLVSS) reduction rate reached 42.00% at a specific energy of 110,000 kJ/kg TS. For the P recovery (in the form of struvite), there was only a little difference in the optimal conditions and P recovery rate (89.29–94.49% of TP in the supernatant). AD pretreatment was beneficial for the purity of products and achieved the highest struvite purity (85.14%), followed by EA (80.95%) and U-EA (77.56%). In summary, the highest recovery rate of TP from excess sludge (53.50% of sludge TP) and struvite yield (26.10 mg/gSS) was obtained by U-EA.
Corn straw (CS) was pretreated by ultrasonic combined aerobic with biogas slurry as medium for anaerobic digestion (AD), that strengthened the degradation efficiency CS, varied in the composition of digestion slurry, thereby the methane production was increased. Central combinatorial design (CCD) test was used to treat CS at ultrasonic power (200, 400, and 600 W), time (10, 20, and 30 min) and AD for 25 days, at 37±1℃. According to data showed that the pH and volatile fatty acids (VFAs) affected methane production directly. With an ultrasonic power 309W, time 26 min, it reached the maximum content of VFAs with 16.24 g/L, the cumulative methane production achieved the highest with 198.56 mL/g VS, which was 46.73% higher than unpretreated raw material as CK. Ultrasonic-aerobic hydrolysis pretreatment can obtain higher VFAs and methane production content in a short period of time that is great significance to biogas engineering.
Excess sludge is rich in proteins that can be recovered to prepare protein foaming agents and other products with high added value. To extract sludge proteins efficiently, more sludge proteins can be dissolved into the liquid phase by destroying sludge flocs and microbial cell structures. The commonly used alkali-thermal hydrolysis method has the disadvantages of high energy consumption and the additional Maillard reaction. Ultrasonic-alkali hydrolysis (UA) has been widely used as pretreatment for sludge anaerobic digestion, and the effect of UA on cell wall cracking gives it potential application prospects in sludge protein extraction. This study evaluated the potential of UA for sludge protein extraction, and the experimental conditions were optimized to explore the foaming performance of sludge proteins based on a single-factor experiment and orthogonal experiment. The results showed that under optimum hydrolysis conditions (pH of 12, sludge moisture content of 94%, ultrasonic power density of 1.7 W/mL, and ultrasonication duration of 24 min), the protein yield and protein extraction efficiency (RP) were 366.3 mg/g total solids (TS) and 81.4%, respectively. UA converted more than half of the proteins into polypeptides and thus promoted the foaming performance of the sludge proteins to meet the relevant standards for foaming agents. The dewatering performance of the hydrolysed sludge was improved by 93.4%, which was convenient for subsequent treatment. Further research illustrated that UA could shorten the treatment time of alkali hydrolysis (AH) by 36 times when achieving a similar protein yield, and the protein yield produced by UA was 11.4 times greater than that produced solely by ultrasonic hydrolysis (US), indicating that a synergistic effect occurred when AH was combined with US.
A strategy of directly recovering organics for the production of volatile fatty acids (VFAs) was introduced to make wastewater treatment profitable. A sludge adsorption section was proposed to add before the conventional wastewater process for adsorbing the organics in wastewater by the excess activated sludge which was produced during biological wastewater treatment and returned from the secondary settling tank. Then, the excess activated sludge combining wastewater organics, called organics-enriched sludge, was anaerobically fermented for the production of high value-added VFAs. Results indicated about 70 % of wastewater organics could be source-recovered within 30 min. Most of the organics in wastewater could avoid being lost via carbon dioxide or synthesized into bacterial organisms, therefore, the total amount and overall bioavailability of the organics recovered from wastewater were greatly improved. Pilot-scale tests indicated VFAs production from wastewater organics was enhanced by 62.2 %. Mass balance analysis indicated 46.8 % of wastewater organics could be finally transferred into VFAs, only 12 % in conventional processes. Economic evaluations indicated source-recovering wastewater organics for VFAs production was a promising alternative to make wastewater treatment plants (WWTPs) profitable. Finally, an idealized organics transformation route for wastewater treatment was proposed based on this concept to make wastewater treatment profitable.
The technical feasibility of TiO2-photocatalysis towards palm oil mill effluent (POME) treatment is well-proven in previous studies. As a continuity, current study evaluated the strengths, weaknesses, opportunities and threats (SWOT) in a concise way, subsequently revealed its practicality in palm oil industry of Malaysia. Indeed, TiO2-photocatalysis displays a promising technical feasibility in treating POME, but its wide application is economically-suppressed. It is positing that biological-based treatments (including the existing open-ponding system) are more likely to be employed as the major treating approach for POME over TiO2-photocatalysis. This is particularly true as biological-based treatments offer better performance index for concentrated POME with comparatively lower treatment cost and technicality needed. Furthermore, it is also prevailed with high biogas generability, therefore being irreplaceably benchmarked for POME treatment in Malaysia. Instead of replacing biological treatment entirely, the adoption of TiO2-photocatalysis as complementing tertiary treatment for biological-treated-POME is more practical. Operationally, it can support the inadequacy of biological-based treatment towards treating low concentration POME, bestowed to its robust organic-mineralizing feature. Such integrated system is expected to augment the POME degradation efficiency, hence effectively preserve the environment from POME pollution.
A new sludge pretreatment approach was developed to disintegrate waste activated sludge by mechanical cutting in this study, and the effect of mechanical cutting pretreatment on sludge fermentation and anaerobic digestion was analysed. The study results showed that a sludge disintegration degree of 6.7 % was achieved when the sludge was mechanically cut for 8 min. Moreover, the sludge fermentation was promoted by mechanical pretreatment. After the pretreatment, the soluble COD (SCOD) production in the sludge fermentation system was as high as 3052 mg/L, which was 2.28 times higher than that of the control group. More short-chain fatty acids (SCFAs), especially acetic acid, were observed in the pretreatment group. The mechanical pretreatment accelerated the sludge digestion process and increased biogas production, resulted in a maximum yield of 539.30 mL and 3.06 times higher than that of the control group. High-throughput sequencing results indicated that the microbial community composition in the sludge fermentation system was affected by the mechanical pretreatment, and these changes were conducive to sludge fermentation. The composition of methanogens in the anaerobic digestion reactors was changed by mechanical pretreatment, the relative abundance of Methanosaeta in the control group was 52.70 %, while that in the pretreatment group was as high as 94.05 %. The methanogens in the sludge anaerobic digestion system involving and without involving pretreatment was 5.47 × 10⁷ and 5.12 × 10⁷ copies/g, respectively. Overall, the mechanical pretreatment was beneficial for the enrichment of methanogens, especially acetotrophic Methanosaeta, and it increased methane production.
The aim of the present work is to investigate the impact of acetic acid assisted ultrasound pre-treatment of food waste (FW) and cardboard (CB) to overcome the barriers of substrate characteristics. Anaerobic co-digestion (ACoD) experiments were conducted with FW and CB in the mixing ratios of 100:0, 80:20, 60:40 and 50:50 at a food to inoculum (F/I) ratio of 1:1 and 1:2 to investigate its impact on biogas yield, volatile solids (VS) removal, solubilization of organic matter and the specific energy (SE) required.
Results revealed that the biogas yield obtained from ultrasound pre-treated samples regardless of mixing ratio was higher at a F/I ratio of 1:2 compared to 1:1. The formation of bubbles, cavities and floc matrix significantly increased and the particle size decreased from 100 μm to 10 μm for FW:CB of 60:40 and 50:50 after pre-treatment. Ultrasound pre-treatment for 45 min was adequate for FW:CB of 100:0 and 80:20 while it was 60 min for 60:40 and 50:50 for increased sCOD between 37 % - 53 %. The VS removal efficiencies increased between 3–15 % from pre-treated samples compared to untreated. The CB mixing ratio is inversely related to SE while directly related to sCOD. Overall, during ACoD of FW and CB, the FW:CB of 80:20 gave a higher biogas yield of 471 mL/g VSadded followed by 60:40 and 50:50.
Proper pretreatment of waste activated sludge is commonly needed before biohydrogen fermentation due to its complex floc structure. Considering that the outer-layer extracellular polymeric substances (EPS) could restrict the disintegration of inner-layer microbial cells during the pretreatment of sludge, this study firstly applied sodium citrate pretreatment (0.3 g/g-TSS) for removing EPS from sludge flocs, followed by ultrasonic pretreatment (2 W/mL, 15 min), which aimed to enhance the disintegration efficiency. The subsequent hydrogen fermentation performance was tested in batch mode at 37 °C and initial pH 7.0. Results showed that the combined sodium citrate-ultrasonic pretreatment significantly disrupted the sludge floc structure and promoted soluble COD concentration by 157.5 times, which synergistically enhanced the biohydrogen fermentation performance. The maximum hydrogen yield of 38.8 mL/g-VSadded was achieved with the combined pretreatment, which was 604.2%, 155.1%, and 92.6% higher compared to the control, individual ultrasonic pretreatment, and individual sodium citrate pretreatment, respectively. Correspondingly, the energy conversion efficiency increased from 7.4% to 32.8% through the combined pretreatment. The combined pretreatment also enhanced the organics utilization, induced a more efficient fermentative pathway, and shortened the hydrogen-generating lag phase. More enrichment of hydrogen-producing bacteria, especially the genera Clostridium sensu stricto and Paraclostridium, was responsible for the synergistic enhancement in hydrogen yield and energy conversion efficiency. The present work puts forward an effective and eco-friendly pretreatment approach for enhancing bioenergy recovery from waste activated sludge.
The extensive use of fossil fuels and the environmental effect of their combustion products have attracted researchers to look into renewable energy sources. In addition, global mass production of waste has motivated communities to recycle and reuse the waste in a sustainable way to lower landfill waste and associated problems. The development of waste to energy (WtE) technology including the production of bioenergy, e.g. biogas produced from various waste through Anaerobic Digestion (AD), is considered one of the potential measures to achieve the sustainable development goals of the United Nations (UN). Therefore, this study reviews the most recent studies from relevant academic literature on WtE technology (particularly AD technology) for biogas production and the application of a solar-assisted biodigester (SAB) system aimed at improving performance. In addition, socio-economic factors, challenges, and perspectives have been reported. From the analysis of different technologies, further work on effective low-cost technologies is recommended, especially using SAB system upgrading and leveraging the opportunities of this system. The study found that the performance of the AD system is affected by a variety of factors and that different approaches can be applied to improve performance. It has also been found that solar energy systems efficiently raise the biogas digester temperature and through this, they maximize the biogas yield under optimum conditions. The study revealed that the solar-assisted AD system produces less pollution and improves performance compared to the conventional AD system.
Microwave-induced plasma optical spectrometry (MIP OES) and induced coupled plasma optical spectrometry (ICP OES) were implemented in parallel for the determination of Cd, Cu, Cr, Ni, Pb, and Zn in sediments, as well as in unexplored matrices such as sewage sludge and fired brick. Initially, a microwave-assisted digestion procedure was applied to solubilize the samples. A selective approach has recommended for the detection of elements with low and high concentration ranges in different environments to improve sensitivity. Limit of detection (LOD) and limit of quantification (LOQ) were found between 0.026–0.039 mg L−1 and 0.08–0.11 mg L−1, respectively. Precision and trueness were controlled using certified reference materials with the same matrices for all analysts. MIP OES showed results in accordance with ICP OES and LOD in the same range (or better, i.e., Pb) than ICP OES. MIP OES found a potential alternative with comparable performance and reduced costs for the analysis of trace metals.
Persulfate-based oxidation has attracted much attention in recent years for the relatively wider adaptability range of pH compared with classical Fenton reaction, and compared to traditional remediation mathods, persulfate salts (PS) catalyzed through heat, light and interim metal activation process have a higher reduction potential (2.6 V), which have been proved could almost completely decompose refractory hazardous organic species existed in reaction system. In this study, persulfate salts activated with various methods adopted as replacement of the classical Fenton-like reagent for the release of nutrients from excess waste activated sludge(WAS). The results indicated that this tecnology can efficiently disintegrate WAS zoogloea, improve dewatering performance and filterability, and the mechanism of WAS decomposition and nutruits recovery by PS process was elucidated. Finally, the degradation proccess of trichlorfon(one kind of organophosphorus pesticides widely used to control a variety of arthropod pests in the agricultural production process) in the aquatic environment through thermally activated persulfate has also been investigated.
In order to reveal the mechanism of phosphate release by chemically enhanced process and recovery from waste activated sludge respectively, potassium persulfate (K2S2O8) was adopted as a replacement of the classical Fenton-like reagent for the release of nutrients from excess sludge. The results indicated that the increases in the temperature and PS dosage could significantly improve the release of phosphate from sludge by PS process.
When the PS process the aid of ultraviolet radiation was performed at 65oC and PS: WAS mass ratio 1.5:1 for 6 h, the phosphate and ammonia nitrogen concentrations in the supernatant reached 204 mg/L and 216 mg/L, respectively, accompanying with sludge reduction by 64%. In the phosphate recovery process, 98.9% of the phosphate and 34.8% of the ammonia nitrogen can be recovered from the supernatant by struvite precipitation at pH 11.5 and Mg/P molar ratio 2. Hybrid alkali-persulfate disintegration of waste activated sludge in low dosage could not efficaciously destroy the structure of cell membrane of zoogloea, while EPS mainly composed of macromolecular microbial secretions and products of cellular lysis or macromolecule hydrolysis will be drastically destroyed, the proteins and polysaccharides therein were further decomposed by SO4• – and •OH radicals. And the organic phosphorus and organic nitrogen contained in these substances are converted into inorganic states, resulting in an increase of PO4-P, NH4-N and COD concentrations in the supernatant. During the dynamic characteristics research of organophosphorus pesticide degeneration process, the experiment data obviously indicated that the release of phosphate in the degradation process of trichlorfon using persulfate oxidant could be enhanced in higher temperature, which removal efficiency of trichlorfon in synergistic degeneration system with ultraviolet irradiation (UV，80oC , PS/ Trichlorfon =7) could reach 95.63%. And 93.4% of the phosphate could be recovered from the supernatant by struvite precipitation at pH 9.5 and Mg/P molar ratio 1.3 in the phosphate recovery process. We have found that the addition of ascorbic acid could improve the effectiveness of waste activated sludge decomposition and dewatering process when the WAS was treated with different disposal methods. And the experiment results shows that the high dosage ascorbic acid addition at the early stage of persulfate WAS oxidation process will substantially improve the dewaterability and filterability of sludge with shorter response time, conversely, which almost has no influence on variation tendency of capillary suction time (CST) of waste activated sludge in the alkali pre-treatments process. Finally, it is noteworthy that the improvement of WAS dewaterability and filterability in alkali-persulfate disintegration may attribute to decomposation of EPS which rich in proteinaceous organic matters such as protein, humic acid and DNA, because there is a coincidence phenomenon of the TN concentration change in supernatant and the variation characteristic of capillary suction time (CST) of WAS.
Mexico, wastewater sludge is characterized by high concentrations of organic compounds and pathogenic microorganisms. This study proposes the development and comparison of the anaerobic digestion enhanced with thermal pre-treatment denominated Pre-anaerobic digestion (Pre-AD) and with thermal post-treatment denominated Post-anaerobic digestion (Post-AD) processes for treating wastewater sludge. Additionally, the results from a kinetic model that was developed for thermal inactivation were used to describe the inactivation of faecal coliforms, Salmonella spp. and helminth ova during the thermal treatment stages. In the Post-AD operational batch test, it was determined that a period of 27 d was necessary to reduce the volatile solids (VS) by 38% at an organic loading rate of 0.65 kg VS m(-3)d(-1). When the Pre-AD process was evaluated, 13 d of treatment was sufficient for meeting the stabilisation criteria at an organic loading rate (ORL) of 1.25 kg VS m(-3)d(-1) in the batch test. In a semicontinuous operation tests, the Pre-AD process could be operated at an OLR of 3 kg VS m(-3)d(-1), while the Post-AD process could only be operated at an OLR of 1.5 kg VS m(-3)d(-1). The bacteria and helminth ova were inactivated at 70 degrees C for 1 h and 80 degrees C for 2 h, respectively.
Ultrasonication (US), which creates hydro-mechanical shear forces in cavitation, is an advanced technology in sludge pretreatment. However, there are many factors affecting the efficacy of cavitation and ultrasonication disintegration of sludge as a consequence. The objective of this work is to present an extensive review of evaluation approaches of sludge US pretreatment efficiency. Besides, optimization methodologies of related parameters, the differences of optimum values and the similarities of affecting trends on cavitation and sludge pretreatment efficiency were specifically pointed out, including ambient conditions, ultrasonic properties, and sludge characteristics. The research is a prerequisite for optimization of sludge US pretreatment efficiency in lab-scale and practical application. There is not-yet a comprehensive method to evaluate the efficiency of sludge US pretreatment, but some main parameters commonly used for this purpose are degree of sludge disintegration, proteins, particle size reduction, etc. Regarding US parameters, power input PUS, intensity IUS, and frequency FS seem to have significant effects. However, the magnitude of the effect of PUS and probe size in terms of IUS has not been clearly detailed. Investigating very low FS seems interesting but has not yet been taken into consideration. In addition, static pressure effect has been marginally studied only and investigation on the effect of pH prior to US process has been restricted. Their effects therefore should be varied separately and simultaneously with other related parameters, i.e. process conditions, ultrasonic properties, and sludge characteristics, to optimize sludge US pretreatment process. © 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.
There is a lack of literature reporting the methane potential of several livestock manures under the same anaerobic digestion conditions (same inoculum, temperature, time, and size of the digester). To the best of our knowledge, no previous study has reported biochemical methane potential (BMP) predicting models developed and evaluated by solely using at least five different livestock manure tests results. The goal of this study was to evaluate the BMP of five different livestock manures (dairy manure (DM), horse manure (HM), goat manure (GM), chicken manure (CM) and swine manure (SM)) and to predict the BMP using different statistical models. Nutrients of the digested different manures were also monitored. The BMP tests were conducted under mesophilic temperatures with a manure loading factor of 3.5 g volatile solids (VS)/L and a feed to inoculum ratio (F/I) of 0.5. Single variable and multiple variable regression models were developed using manure total carbohydrate (TC), crude protein (CP), total fat (TF), lignin (LIG) and acid detergent fiber (ADF), and measured BMP data. Three different kinetic models (first order kinetic model, modified Gompertz model and Chen and Hashimoto model) were evaluated for BMP predictions. The BMPs of DM, HM, GM, CM and SM were measured to be 204, 155, 159, 259, and 323 mL/g VS, respectively and the VS removals were calculated to be 58.6%, 52.9%, 46.4%, 81.4%, 81.4%, respectively. The technical digestion time (T80–90, time required to produce 80–90% of total biogas production) for DM, HM, GM, CM and SM was calculated to be in the ranges of 19–28, 27–37, 31–44, 13–18, 12–17 days, respectively. The effluents from the HM showed the lowest nitrogen, phosphorus and potassium concentrations. The effluents from the CM digesters showed highest nitrogen and phosphorus concentrations and digested SM showed highest potassium concentration. Based on the results of the regression analysis, the model using the variable of LIG showed the best (R2 = 0.851, p = 0.026) for BMP prediction among the single variable models, and the model including variables of TC and TF showed the best prediction for BMPs (R2 = 0.913, p = 0.068–0.075) comparing with other two-variable models, while the model including variables of CP, LIG and ADF performed the best in BMP prediction (R2 = 0.999, p = 0.009–0.017) if three-variable models were compared. Among the three kinetic models used, the first order kinetic model fitted the measured BMPs data best (R2 = 0.996–0.998, rRMSE = 0.171–0.381) and deviations between measured and the first order kinetic model predicted BMPs were less than 3.0%.
Physical methods like ultrasound, ultraviolet and nanoparticles are very useful in wastewater purification and recycling. The ultrasound irradiation in a liquid leads to the acoustic cavitation phenomenon, which can affect a number of mechanical, acoustic, chemical and biological changes in waste analysis. In present study, ultrasonic irradiation treatment technique was used to treat the sewage sludge effluent, which was collected from Delhi and another sample of pure E-coli strain was processed. Samples were treated in ultrasonic bath at 35 and 130 kHz of irradiation for different time periods of 5 min, 10 min, 20 min and 30 min including control (untreated). Treated samples were tested for different parameters, viz. bacterial cell count, chemical oxygen demand (COD), degree of disintegration COD (DDCOD), scanning electron microscope (SEM), optimum density (OD) of cells and reactive oxygen species (ROS). Result shows significant disintegration in ultrasonic treated sewage sludge and E-coli samples as compared with untreated (control). We observed an increased level of ROS and decreased bacterial population in treated samples with 35 kHz and 130 kHz of frequency. Result suggests that the ultrasonic treatment was more effective by increasing time and frequency. Study concludes that low-frequency ultrasonic bath at 130 kHz is more effective as compared to 35 kHz.
A study was conducted to determine whether differences in the levels of volatile fatty acids (VFAs) in anaerobic digester plants could result in variations in the indigenous methanogenic communities. Two digesters (one operated under mesophilic conditions, the other under thermophilic conditions) were monitored, and sampled at points where VFA levels were high, as well as when VFA levels were low. Physical and chemical parameters were measured, and the methanogenic diversity was screened using the phylogenetic microarray ANAEROCHIP. In addition, real-time PCR was used to quantify the presence of the different methanogenic genera in the sludge samples. Array results indicated that the archaeal communities in the different reactors were stable, and that changes in the VFA levels of the anaerobic digesters did not greatly alter the dominating methanogenic organisms. In contrast, the two digesters were found to harbour different dominating methanogenic communities, which appeared to remain stable over time. Real-time PCR results were inline with those of microarray analysis indicating only minimal changes in methanogen numbers during periods of high VFAs, however, revealed a greater diversity in methanogens than found with the array.
Sonication and thermalization can be applied successfully to disrupt the complex waste activated sludge (WAS) floc structure and to release extra and intra cellular polymeric substances into soluble phase along with solubilization of particulate organic matters, before sludge digestion. In this study, sonication has been combined with thermalization to improve its disintegration efficiency. It was aimed that rise in temperature occurring during the sonication of sludge was used to be as an advantage for the following thermalization in the combined pre-treatment. Thus, the effects of sonication, thermalization and sono-thermalization on physical and chemical properties of sludge were investigated separately under different pre-treatment conditions. The disintegration efficiencies of these methods were in the following descending order: sono-thermalization>sonication>thermalization. The optimum operating conditions for sono-thermalization were determined as the combination of 1-min sonication at 1.0W/mL and thermalization at 80°C for 1h. The influences of sludge pre-treatment on biodegradability of WAS were experienced with biochemical methane potential assay in batch anaerobic reactors. Relative to the control reactor, total methane production in the sono-thermalized reactor increased by 13.6% and it was more than the sum of relative increases achieved in the sonicated and thermalized reactors. Besides, the volatile solids and total chemical oxygen demand reductions in the sono-thermalized reactor were enhanced as well. However, it was determined that sludge pre-treatment techniques applied in this study was not feasible due to their high energy requirements.
Treatment of poultry industry effluents produces wastewater sludge with high levels of organic compounds and pathogenic microorganisms. In this research, the thermal pre-treatment of poultry slaughterhouse sludge (PSS) was evaluated for low temperatures in combination with different exposure times as a pre-hydrolysis strategy to improve the anaerobic digestion process. Organic compounds solubilization and inactivation of pathogenic microorganisms were evaluated after treatment at 70, 80 or 90°C for 30, 60 or 90 min. The results showed that 90°C and 90 min were the most efficient conditions for solubilization of the organic compounds (10%). In addition, the bacteria populations and the more resistant structures, such as helminth eggs (HE), were completely inactivated. Finally, the thermal pre-treatment applied to the sludge increased methane yield by 52% and reduced hydraulic retention time (HRT) by 52%.
This paper presents a review of the main sludge treatment techniques used as a pretreatment to anaerobic digestion. These processes include biological (largely thermal phased anaerobic), thermal hydrolysis, mechanical (such as ultrasound, high pressure and lysis), chemical with oxidation (mainly ozonation), and alkali treatments. The first three are the most widespread. Emphasis is put on their impact on the resulting sludge properties, on the potential biogas (renewable energy) production and on their application at industrial scale. Thermal biological provides a moderate performance increase over mesophilic digestion, with moderate energetic input. Mechanical treatment methods are comparable, and provide moderate performance improvements with moderate electrical input. Thermal hydrolysis provides substantial performance increases, with a substantial consumption of thermal energy. It is likely that low impact pretreatment methods such as mechanical and thermal phased improve speed of degradation, while high impact methods such as thermal hydrolysis or oxidation improve both speed and extent of degradation. While increased nutrient release can be a substantial cost in enhanced sludge destruction, it also offers opportunities to recover nutrients from a concentrated water stream as mineral fertiliser.
Acid phase digestion pretreatment resulted in an increase in biogas production and volatile solids (VS) removal at the West Camden plant which was fed with only waste activated sludge. Without the acid phase digesters, the specific methane yield of waste activated sludge (WAS) was 190 L/kgVSadded, whereas a specific methane yield of WAS of 231 L/kg VSadded was observed from sludge sampled from the acid phase digester. The specific methane yield obtained from BioWin® simulation was 331 L/kgVSadded and was slightly higher than that from BMP assessment (231 L/kgVSadded). In addition, the overall VS removal values obtained from BioWin® simulation (44%) and biomethane potential (BMP) evaluation (49%) were close to the actual VS removal value (45%) achieved by the plant. The consistency between full scale evaluation data, BioWin® simulation, and BMP assessment suggests that BioWin® simulation and BMP study can be used to guide future design and optimisation of acid phase digestion pretreatment to intensify anaerobic digestion of sewage sludge.
Sewage sludge management is now becoming a serious issue all over the world. Anaerobic digestion is a simple and well-studied process capable of biologically converting the chemical energy of sewage sludge into methane-rich biogas, as a carbon-neutral alternative to fossil fuels whilst destroying pathogens and removing odors. Hydrolysis is the rate-limiting step because of the sewage sludge complex floc structure (such as extracellular polymeric substances) and hard cell wall. To accelerate the rate-limiting hydrolysis and improve the efficiency of anaerobic digestion, various pretreatment technologies have been developed. This paper presents an up-to-date review of recent research achievements in the pretreatment technologies used for improving biogas production including mechanical (ultrasonic, microwave, electrokinetic and high-pressure homogenization), thermal, chemical (acidic, alkali, ozonation, Fenton and Fe(II)-activated persulfate oxidation), and biological options (temperature-phased anaerobic digestion and microbial electrolysis cell). The effectiveness and relative worth of each of the studied technologies are summarized and compared in terms of the resulting sludge properties, the digester performance, the environmental benefits and the current state of real-world application. The challenge and technical issues encountered during sludge cotreatment are discussed, and the future research needs in promoting full-scale implementations of those approaches are proposed.
Anaerobic digestion is a widely accepted approach to the treatment of organic waste and uses a complex consortium of microorganisms. In this study, a metaproteomic approach was used to analyze the proteins expressed in an anaerobic digester that ran on swine manure at 55 °C. The extracted proteins were separated by sodium dodecylsulfate polyacrylamide gel electrophoresis, and identified by LC-ESI-MS/MS. The study identified proteins with functions that are related to hydrolysis, acidogenesis and methanogenesis. The majority of the identified proteins (42%) were involved in the translation, ribosomal structure and biogenesis, which were followed by proteins associated with energy production and conversion (28%). The majority of the above proteins were eubacterial, with about 87% being Proteobacteria. Among the methanogenic archaeal proteins present, acetyl-CoA decarbonylase from Methanosarcina spp., which are able to produce methane from both acetate and CO2, were identified. The present study is the first to investigate the metaproteome of a thermophilic anaerobic digester and to demonstrate the presence in the system of a high bacterial diversity where Proteobacteria are dominant. The study also provides evidence of types of microbial activity taking place during thermophilic anaerobic digestion when swine manure is used as the sole feedstock to produce biogas.
The effect of pretreatment and anaerobic co-digestion of food waste (FW) and waste activated sludge (WAS) was assessed by the reductions of total suspended solids (TSS), volatile suspended solids (VSS), COD removal and methane production. Thermal treatment and anaerobic digestion reduced VSS up to 43.4% (FW) and 43.1% (FW + WAS) by increasing the solubilization of the feedstock. Methane yields of ultrasonic treatment reached 206.4 (FW) and 326.3 (FW + WAS) mL CH4 g−1 VSS removed which were 50.5 and 56.2% higher than that of the control. Results showed that each pretreatment gave distinctive effect on different feedstocks due to dissimilar composition. Co-digestion conferred superior result than mono digestion with FW or WAS. Estimated parameters (methane production potential and the rate) with Gompertz equation also inferred that co-digestion increased methane production significantly.
Hydrolysis is the most rate limiting step in almost all anaerobic digestion systems. To tackle long time duration, ultrasonic pretreatment proved that it can effectively improve biogas yield efficiency by effecting on soluble particles. In this study, the effect of three different ultrasonic power densities (0.2 W/mL, 0.4 W/mL and 0.6 W/mL) at three different times (10 min, 20 min and 30 min) on biogas yield of organic fraction of municipal solid waste (OFMSW) at three different total solid content (6%, 8% and 10%) were analyzed. Results showed significant (p < 0.01) effect of both sonication density and time of sonication on biogas final yield and biogas yield after 72 h digestion with the 6% TS content. Parameters like specific energy input and total volatile fatty acid (TVFA) content were also evaluated to find the best sonication treatments for OFMSW. For lower TS contents (6% and 8%), sonication treatment significantly (p < 0.01) increased TVFA concentration before digesting. It is also proved that specific energy input between 5000 kJ/kg TS and 10,000 kJ/kg TS can effectively increase the biogas yields, especially for 6% TS content, and caused maximum biogas yield produced after 72 h of digestion.
Sonication is an effective way for sludge disintegration, which can significantly improve the efficiency of anaerobic digestion to reduce and recycle use of sludge. But high energy consumption limits the wide application of sonication. In order to improve ultrasonic sludge disintegration efficiency and reduce energy consumption, aeration was introduced. Results showed that sludge disintegration efficiency was improved significantly by combining aeration with ultrasound. The aeration flow rate, gas bubble size, ultrasonic density and aeration timing had impacts on sludge disintegration efficiency. Aeration that used in later stage of ultrasonic irradiation with low aeration flow rate, small gas bubbles significantly improved ultrasonic disintegration sludge efficiency. At the optimal conditions of 0.4 W/mL ultrasonic irradiation density, 30 mL/min of aeration flow rate, 5 min of aeration in later stage and small gas bubbles, ultrasonic sludge disintegration efficiency was increased by 45% and one third of ultrasonic energy was saved. This approach will greatly benefit the application of ultrasonic sludge disintegration and strongly promote the treatment and recycle of wastewater sludge.
An increasing amount of sewage sludge produced as a result of water treatment processes still poses a key problem in many communes and the cities in Poland. Therefore new methods of their utilization are indispensable. One of such methods is their natural and agricultural use. However, a drained secondary sludge that has not been subjected to hygenization poses a great environment hazard in terms of sanitary qualities. A sewage sludge contains considerable amount of pathogenic bacteria, parasitic worms and pathogenic fungi, making up for human health potential threats. Therefore sewage sludge used in agriculture ought to be analysed not only for pathogenic bacteria (belonging to Salmonella and coliforms) and the helminth eggs, but also for presence of pathogenic fungi. We present the results of investigations on the use of ultrasounds as a method for hygienization of sewage sludge with special regard to the pathogenic fungi survival rate.
Waste activated sludge is a valuable resource containing multiple nutrients, but is currently treated and disposed of as an important source of pollution. In this work, waste activated sludge after ultrasound pretreatment was reused as multiple nutrients for biofuel production. The nutrients trapped in sludge floc were transferred into liquid medium by ultrasonic disintegration during first 30 min, while further increase of pretreatment time only resulted in slight increase of nutrients release. Hydrogen production by Ethanoligenens harbinense B49 from glucose significantly increased with the concentration of ultrasonic sludge, and reached maximum yield of 1.97 mol H2/mol glucose at sludge concentration of 7.75 g volatile suspended solids/l. Without addition of any other chemicals, waste molasses rich in carbohydrate was efficiently turned into hydrogen with yield of 189.34 ml H2/g total sugar by E. harbinense B49 using ultrasonic sludge as nutrients. The results also showed that hydrogen production using pretreated sludge as multiple nutrients was higher than those using standard nutrients. Acetic acid produced by E. harbinense B49 together with the residual nutrients in the liquid medium were further converted into hydrogen (271.36 ml H2/g total sugar) by Rhodopseudomonas faecalis RLD-53 through photo fermentation, while ethanol was the sole end product with yield of 220.26 mg/g total sugar. Thus, pretreated sludge was an efficient nutrients source for biofuel production, which could replace the standard nutrients. This research provided a novel strategy to achieve environmental friendly sludge disposal and simultaneous efficient biofuel recovery from organic waste.
This study investigated the effects on methane production of combining diverse organic wastes as a method to improve the anaerobic digestion (AD) of sewage sludge in a wastewater treatment plant (WWTP). Evaluation of methane production from the AD of selected organic wastes showed the AD of food waste (FW) attained the highest methane production (522.9 mL CH4/g VS), while the digestion of septage produced the lowest– (164.9 mL CH4/g VS). Type of substrate determined the microbial community, possibly relating to differences in chemical properties of intermediates produced from the AD of wastes. Compared to the AD of primary sludge alone, co-digestion of primary sludge with FW achieved 72% higher methane production, while co-digestion with septage exhibited 20% lower production. Thus, the anaerobic co-digestion of sewage sludge with FW can provide a viable alternative to circumvent problems of separate digestion in WWTPs.
In this study, the anaerobic fermentation was carried out for volatile fatty acids (VFAs) production at different pH (between 7.0 and 10.0) conditions with untreated sludge and heat-alkaline pretreated waste activated sludge. In the fermentation with untreated sludge, the extent of hydrolysis of organic matters and extent of acidification at alkaline pH are 54.37% and 30.37%, respectively, resulting in the highest VFAs yield at 235.46mg COD/gVS of three pH conditions. In the fermentation with heat-alkaline pretreated sludge, the acidification rate and VFAs yield at neutral pH are 30.98% and 240.14mg COD/gVS, respectively, which are higher than that at other pH conditions. With the glucose or bovine serum albumin as substrate for VFAs production, the neutral pH showed a higher VFAs concentration than the alkaline pH condition. The results of terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that the alkaline pH caused low microbial richness. Based on the results in this study, we demonstrated that the alkaline pH is favor of hydrolysis of organic matter in sludge while neutral pH improved the acidogenesis for the VFAs production from sludge. Our finding is obvious different to the previous research and helpful for the understanding of how heat-alkaline pretreatment and alkaline fermentation influence the VFAs production, and beneficial to the development of VFAs production process.
Nanoparticles (NPs) of trace metals such as Co, Ni, Fe and Fe3O4 were implemented in this study to compare their effects on biogas and methane production from anaerobic digestion of livestock manure. The most effective concentrations of NPs additives were determined based on our previous studies, and were 1 mg/L Co NPs, 2 mg/L Ni NPs, 20 mg/L Fe NPs and 20 mg/L Fe3O4 NPs. These concentrations of NPs additives were further investigated and compared to each other in this study and were found to significantly (p < 0.05) increase the biogas yield by 1.7, 1.8, 1.5 and 1.7 times in comparison with control, respectively. The methane yield significantly (p < 0.05) increased by 2, 2.17, 1.67 and 2.16 times the methane volume of the control, respectively. The results of this study showed that the Ni NPs yielded the highest biogas and methane production compared to Co, Fe and Fe3O4 NPs.
Ultrasonic disintegration of excess sludge is used as a process preceding the stabilization of sludge in wastewater treatment plants. It has a task of intensifying the anaerobic digestion of the organic fraction of sludge due to the fragmentation of its particles and destruction of microorganisms. Amount of energy put into process is the strongest factor determining type and intensity of disintegration. Physicochemical properties of sludge, operational variables and construction of installation have to be considered as well. The research have shown the effects of disintegration of various sludge conducted in disintegrators of varying heads and emitter structures, but at the same energy density EV kW h m−3.
Biogas production via anaerobic digestion (AD) of waste is a very attractive, yet a challenging task. This mini-review focuses on the recent efforts done to explore the effect of various nanomaterials additives on the biogas production rate. The materials are classified into three categories: (1) metal oxides, (2) zero-valent metals, and (3) nano-ash and carbon-based materials. The positive and negative effects of such nanomaterials on the AD process and consequently on the biogas production are discussed. Also, future perspectives to increase the biogas production are listed.
In the present study, biogas production from food waste through anaerobic digestion was carried out in a 2 l laboratory-scale batch reactor operating at different temperatures with a hydraulic retention time of 30 days. The reactors were operated with a solid concentration of 7.5% of total solids and pH 7. The food wastes used in this experiment were subjected to characterization studies before and after digestion. Modified Gompertz model and Logistic model were used for kinetic study of biogas production. The kinetic parameters, biogas yield potential of the substrate (B), the maximum biogas production rate (Rb) and the duration of lag phase (λ), coefficient of determination (R2) and root mean square error (RMSE) were estimated in each case. The effect of temperature on biogas production was evaluated experimentally and compared with the results of kinetic study. The results demonstrated that the reactor with operating temperature of 50 °C achieved maximum cumulative biogas production of 7556 ml with better biodegradation efficiency.
This study is an assessment of hydrothermal pre-treatment of waste activated sludge for enhancement of biogas production by anaerobic digestion. This assessment was carried out in order to ascertain the optimal hydrothermal pre-treatment temperature. Anaerobic digestion efficacy was investigated by biochemical methane potential (BMP) tests and subsequent statistical analyses. Hydrothermal pre-treatment was found to improve solubilization of waste activated sludge and increase the response surface, resulting in increased biogas production. BMP tests revealed that hydrothermal pre-treatment of waste activated sludge increased COD solubilization (30–37%) and soluble COD (SCOD) levels; in addition, it reduced volatile solids (VS), and improved final methane yields. The optimum temperature for hydrothermal pre-treatment using subcritical water was found to be ∼180–210 °C. Methane yields of 130.2 (180 °C pre-treatment) and 126.6 (210 °C pre-treatment) ml CH4/g-VS were measured following hydrothermal pre-treatment. In pre-treated waste activated sludge samples, methane composition was found to be 63.4 and 58.8% (180 and 210 °C pre-treatment, respectively) and VS removal was measured to be ∼60 and 58% (180 and 210 °C pre-treatment, respectively). These findings show that in order to achieve high conversion efficiency, an accurately designed pre-treatment step must be included in the overall anaerobic digestion process for wastewater treatment.
The use of photosynthetic microalgae for nutrient removal and biofuel production has been widely discussed. Anaerobic digestion of waste microalgal biomass to produce biogas is a promising technology for bioenergy production. However, the methane yield from this anaerobic process was limited because of the hard cell wall of Chlorella vulgaris. The use of ultrasound has proven to be successful at improving the disintegration and anaerobic biodegradability of Chlorella vulgaris. Ultrasonic pretreatment in the range of 5-200 J ml−1 was applied to waste microalgal biomass, which was then used for batch digestion. Ultrasound techniques were successful and showed higher soluble COD at higher applied energy. During batch digestion, cell disintegration due to ultrasound increased in terms of specific biogas production and the degradation rate. Compared to the untreated sample, the specific biogas production was increased in the ultrasound-treated sample by 90% at an energy dose of 200 J ml−1. For the disintegrated samples, volatile solids reduction was also increased according to the energy input and degradation. These results indicate that the hydrolysis of microalgal cells is the rate-limiting step in the anaerobic digestion of microalgal biomass.
The effect of thermal pre-treatment on sludge anaerobic digestion (AD) efficiency was studied at different total solids (TS) concentrations (20.0, 30.0 and 40.0 g TS/L) and digestion times (0, 5, 10, 15, 20 and 30 days) for primary, secondary and mixed wastewater sludge. Moreover, sludge pre-treatment, AD and disposal processes were evaluated based on a mass-energy balance and corresponding greenhouse gas (GHG) emissions. Mass balance revealed that the least quantity of digestate was generated by thermal pre-treated secondary sludge at 30.0 g TS/L. The net energy (energy output-energy input) and energy ratio (energy output/energy input) for thermal pre-treated sludge was greater than control in all cases. The reduced GHG emissions of 73.8 × 10(-3) g CO2/g of total dry solids were observed for the thermal pre-treated secondary sludge at 30.0 g TS/L. Thermal pre-treatment of sludge is energetically beneficial and required less retention time compared to control.
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Several surfactant macromonomers (SMMxyz) were prepared by using triblock polymers of ethylene oxide (EO) and propylene oxide (PO) (POx-EOy-POz triblock polymer, x: y: z represent the weight ratio of the different blocks). Then graft copolymers of acrylamide and SMMxyz (PAM-g-SMMxyz) were synthesized. The effects of EO/PO mole ratio, length of grafted SMMxyz and the three relative block sizes on the surface activity and thermo-associativity of PAM-g-SMMxyz dilute solutions were studied. The results showed that the surface tension decreased with increasing PO in the graft at 30°C and decreased with the length of the graft when the temperature was higher than the cloud points of the grafted POx-EOy-POz chains, regardless of EO/PO ratio. When the EO/PO ratio was the same in the SMMxyz, the PAM-g-SMMxyz with higher PO content at the graft end had higher surface activity. With increasing temperature, the hydrodynamic radius (Rh) of PAM-g-SMMxyz in dilute aqueous solution increased. The polymer with shorter grafts may have much more intermolecular interaction points and there were more polymer chains in the polymeric aggregates, which led to Rh increasing with decreasing length of graft. In addition, when the EO/PO was not more than 1:1.74, the three relative block sizes had no great influence on the temperature-associativity of PAM-g-SMMxyz. However, for PAM-g-SMM217 and PAM-g-SMM712, the Rh of PAM-g-SMM217 was smaller than PAM-g- SMM712.
In this study, batch tests were conducted to investigate the effect of extracellular polymeric substances (EPS) disintegration on anaerobic fermentation of waste activated sludge (WAS). Ultrasonic pretreatment was used to disintegrate EPS and accelerate WAS hydrolysis. The experimental results showed that more organic substances were released from sludge with increase of ultrasonic density, and the EPS were completely disintegrated at ultrasonic parameters beyond 2.0 W/mL and 15 min. The optimal ultrasonic density for short chain fatty acids (SCFAs) accumulation was 2.0 W/mL, and the SCFAs concentration reached 3166 mg/L after 5 days of anaerobic fermentation. The SCFAs mainly consisted of acetic and propionic acids, accounting for 88% of total SCFAs. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that ultrasonic density significantly altered the bacterial communities by affecting the EPS disintegration degree. Firmicutes, Proteobacteria and Bacteroidetes were main species, contributing to proteins (PN) and polysaccharides (PS) degradation and SCFAs production.
Anaerobic digestion is increasingly being used to treat wastes from many sources because of its manifold advantages over aerobic treatment, e.g. low sludge production and low energy requirements. However, anaerobic digestion is sensitive to toxicants, and a wide range of compounds can inhibit the process and cause upset or failure. Substantial research has been carried out over the years to identify specific inhibitors/toxicants, and their mechanism of toxicity in anaerobic digestion. In this review we present a detailed and critical summary of research on the inhibition of anaerobic processes by specific organic toxicants (e.g., chlorophenols, halogenated aliphatics and long chain fatty acids), inorganic toxicants (e.g., ammonia, sulfide and heavy metals) and in particular, nanomaterials, focusing on the mechanism of their inhibition/toxicity. A better understanding of the fundamental mechanisms behind inhibition/toxicity will enhance the wider application of anaerobic digestion.
Anaerobic digestion is a well-known technique for the recovery of energy from waste sludge. Pre-treatment methods are useful tools to improve the biodegradability of the sludge and to enhance the digestion efficiency. In this study, an ultrasound (US) and a microwave (MW) pre-treatment were compared in a long-term digestion experiment, using 3 small pilot scale semi-continuous digesters (SRT = 20 days). A specific energy of 96 kJ/kg sludge was applied, hence enabling to compare the effectiveness of both pre-treatment methods towards sludge solubilisation and biogas production enhancement. Total and volatile solids (TS and VS), COD, carbohydrates and proteins were monitored throughout the digestion experiment. It was seen that US was most effective in COD solubilisation. The average biogas increment was 20% for the microwave pre-treatment and 27% for the ultrasonic pre-treatment. However, this additional biogas production did not outweigh the energy consumed by the pre-treatment, leading to a negative energy balance.
Short chain fatty acids (SCFAs) are value-added products from waste activated sludge (WAS) anaerobic fermentation. The effects of alkaline adjustment, in the range of pH 7–10, on SCFAs accumulation and microbial community of ultrasonic-pretreated WAS at 20 ± 2 °C were investigated in this study. Experimental results indicated that optimum conditions for SCFAs accumulation were pH 9 and fermentation time of 7 days. The changes of protease activity showed that on the initial phase, ultrasonic pretreatment and alkaline adjustment rather than biotic effect was the leading cause of SCFAs improvement at high pHs. DGGE analysis revealed that alkaline adjustment had a strong impact on microbial community. Some bacteria belonged to Firmicutes, Proteobacteria and Bacteroidetes could adapt to high alkaline environments and contribute to proteins and carbohydrates degradation and SCFAs production in the later phase of anaerobic fermentation.
During the past decades, ultrasonic technology has gained wide interest as an effective mechanical pretreatment method for sludge, due to its excellent performance, good technical and operational stability, compactness and environmental friendly processing. This review is intended to provide an overview on the applications of ultrasonics in sludge solubilisation and subsequent digestion, sludge sanitization, enhancement of enzymatic activity, resource recovery, extraction of chemicals, ultrasound assisted lysis-cryptic growth and degradation of hazardous pollutants. Moreover, the working mechanisms behind the treatment and the influence of various process conditions, including power density/intensity, specific energy input, ultrasonic frequency, duration of ultrasonication, i.e., partial and complete ultrasonication, solids concentration and type of sludge on the ultrasonic processing of sludge is specifically emphasized. Also, a discussion is included on the future research directions, an economical assessment of ultrasonic technique and feasibility of full-scale implementation.
Improving anaerobic digestion of sewage sludge through pretreatment techniques is a suitable solution for better sludge management. In this sense, modeling may present itself as an important tool to assess and predict process performance and pretreatment effects. In this study, the feasibility of using biochemical methane potential (BMP) tests data for calibrating the Anaerobic Digestion Model No. 1 (ADM1) was evaluated, in order to simulate the operation of continuous digesters fed, at different HRTs, with raw and autohydrolysis-pretreated waste activated sludge. This was achieved using a simplified COD fractioning methodology proposed to define ADM1 inputs. Hydrolysis constant rates were determined as the most sensitive parameters, and estimated using BMP tests. The calibrated model was then cross-validated with continuous digesters data sets. Good model performance was attained employing these techniques. The ADM1 was able to successfully represent the consumption of slowly biodegradable organic matter in BMP tests, the changes in hydrolytic limiting steps due to the autohydrolysis pretreatment and the behavior of the continuous digesters in overall. The COD fractioning methodology and the Xc variable manipulation proposed seemed to be crucial for proper model predictions. Results indicate that BMP tests are a suitable data source for ADM1 calibration, and that the model can be a powerful tool to assess the effect of the autohydrolysis pretreatment on the anaerobic digestion of sewage waste activated sludge.
In this work, the influence of an ultrasonic pre-treatment on anaerobic digestion of waste activated sludge is studied. Attention is paid to the solubilisation of the main organic (proteins, carbohydrates) and inorganic (heavy metals) sludge components during ultrasonic treatment and the influence of the dry solids content (DS) on the degree of solubilisation. The second part of the paper focuses on the relationship between the applied specific energy of the ultrasonic treatment and methane production. In general, a higher specific energy and a higher DS content are beneficial for the release of organic matter, resulting in an increased methane production. The efficiency of the subsequent anaerobic digestion is similar for both sludge types (2.1 and 3.2% DS). However, at lower DS contents (2.1%), the methane production increase was more significant.
Aerobic and anaerobic digestions were compared in reactors fed with sonicated activated sludge. Sonication treatment of activated sludge led to solubilisation of matter and especially of organic compounds. An important improvement of aerobic and anaerobic biodegradability was observed for a sonication treatment of 108,000 kJ kg TS−1 due to the increase of the instantaneous specific soluble COD uptake rate. Sonication led to an increase of biogas production due to the increase of available soluble COD. In this study, sludge sonication prior to aerobic digestion in the aim of enhancing sludge reduction was inconclusive. Under anaerobic conditions, the enhancement of sludge reduction due to sonication depended on the disintegration degree of the sludge. The combination of high disintegration degree of sonicated sludge prior to an anaerobic digestion led to very good results in term of sludge reduction (80%). Energy balance was also studied.
The effect of thermal and sonication pre-treatment on the anaerobic degradation of sewage sludge was evaluated through the calculation of performance parameters by using three simplified mathematical models and one kinetic model. The Modified Gompertz equation, the Logistic function, Reaction Curve and First-Order models were all used with experimental data from the anaerobic biodegradability tests fed with primary and secondary thermal pre-treated sludge, and secondary sonicated sludge. All the models fit well with the experimental data, but the Reaction Curve model presented the best agreement in the fitting process. From the first-order equation no significant changes were observed in the hydrolysis constant under all conditions. Thermal pre-treatment (175 °C and 30 min) showed an important effect on the secondary sludge reaching an improvement of around 90% and 80% in the maximum production rate and the total biogas produced respectively. With regards to the sonication experiment, the best result was obtained when 12,400 kJ/kgTS were used, reaching an improvement of 40% in the total biogas production.
When treating municipal wastewater, the disposal of sludge is a problem of growing importance, representing up to 50% of the current operating costs of a wastewater treatment plant. Although different disposal routes are possible, anaerobic digestion plays an important role for its abilities to further transform organic matter into biogas (60–70 vol% of methane, CH4), as thereby it also reduces the amount of final sludge solids for disposal whilst destroying most of the pathogens present in the sludge and limiting odour problems associated with residual putrescible matter. Anaerobic digestion thus optimises WWTP costs, its environmental footprint and is considered a major and essential part of a modern WWTP. The potential of using the biogas as energy source has long been widely recognised and current techniques are being developed to upgrade quality and to enhance energy use. The present paper extensively reviews the principles of anaerobic digestion, the process parameters and their interaction, the design methods, the biogas utilisation, the possible problems and potential pro-active cures, and the recent developments to reduce the impact of the problems. After having reviewed the basic principles and techniques of the anaerobic digestion process, modelling concepts will be assessed to delineate the dominant parameters. Hydrolysis is recognised as rate-limiting step in the complex digestion process. The microbiology of anaerobic digestion is complex and delicate, involving several bacterial groups, each of them having their own optimum working conditions. As will be shown, these groups are sensitive to and possibly inhibited by several process parameters such as pH, alkalinity, concentration of free ammonia, hydrogen, sodium, potassium, heavy metals, volatile fatty acids and others. To accelerate the digestion and enhance the production of biogas, various pre-treatments can be used to improve the rate-limiting hydrolysis. These treatments include mechanical, thermal, chemical and biological interventions to the feedstock. All pre-treatments result in a lysis or disintegration of sludge cells, thus releasing and solubilising intracellular material into the water phase and transforming refractory organic material into biodegradable species. Possible techniques to upgrade the biogas formed by removing CO2, H2S and excess moisture will be summarised. Special attention will be paid to the problems associated with siloxanes (SX) possibly present in the sludge and biogas, together with the techniques to either reduce their concentration in sludge by preventive actions such as peroxidation, or eliminate the SX from the biogas by adsorption or other techniques. The reader will finally be guided to extensive publications concerning the operation, control, maintenance and troubleshooting of anaerobic digestion plants.
Ultrasonication is an emerging and very effective mechanical pretreatment method to enhance the biodegradability of the sludge, and it would be very useful to all wastewater treatment plants in treating and disposing sewage sludge. Ultrasonication enhances the sludge digestibility by disrupting the physical, chemical and biological properties of the sludge. The degree of disintegration depends on the sonication parameters and also on sludge characteristics, therefore the evaluation of the optimum parameters varies with the type of sonicater and sludge to be treated. The full-scale installations of ultrasonication have demonstrated that there is 50% increase in the biogas generation, and in addition evaluation of energy balance showed that the average ratio of the net energy gain to electric consumed by the ultrasound device is 2.5. This review article summarizes the benefits of ultrasonication of sludge, the effect of sonication parameters, impact of sludge characteristics on sludge disintegration, and thereby the increase in biogas production in anaerobic digester. Due to uncertainty in the unit representation by many researchers and nonavailability of the data, comparison of these results is complicated. Comparison of ultrasonication with other pretreatment options is necessary to evaluate the best economical and environmental pretreatment technology for sludge treatment and disposal. The optimum parameters for the ultrasonication vary with sludge characteristics.
In this work, the influence of a low temperature (70-90 degrees C) thermal treatment on anaerobic digestion is studied. Not only the increase in biogas production is investigated, but attention is also paid to the solubilisation of the main organic (proteins, carbohydrates and volatile fatty acids) and inorganic (heavy metals, S and P) sludge constituents during thermal treatment and the breakdown of the organic components during the subsequent anaerobic digestion. Taking into account the effects of the treatment on the sludge composition is of prime importance to evaluate its influence on the subsequent anaerobic digestion and biogas production using predictive models. It was seen that organic and inorganic compounds are efficiently solubilised during thermal treatment. In general, a higher temperature and a longer treatment time are beneficial for the release. The efficiency of the subsequent anaerobic digestion slightly decreased for sludge pre-treated at 70 degrees C. At higher pre-treatment temperatures, the biogas production increased significantly, up to a factor 11 for the 60 min treatment at 90 degrees C.
The individual effects of alkaline (pH 8-13) and ultrasonic (3750-45,000kJ/kg TS) pretreatments on the disintegration of sewage sludge were separately tested, and then the effect of combining these two methods at different intensity levels was investigated using response surface methodology (RSM). In the combined pretreatment, ultrasonic treatment was applied to the alkali-pretreated sludge. While the solubilization (SCOD/TCOD) increase was limited to 50% in individual pretreatments, it reached 70% in combined pretreatment, and the results clearly showed that preconditioning of sludge at high pH levels played a crucial role in enhancing the disintegration efficiency of the subsequent ultrasonic pretreatment. By applying regression analysis, the disintegration degree (DD) was fitted based on the actual value to a second order polynomial equation: Y=-172.44+29.82X(1)+5.30x10(-3)X(2)-7.53x10(-5)X(1)X(2)-1.10X(1)(2)-1.043x10(-7)X(2)(2), where X(1), X(2), and Y are pH, specific energy input (kJ/kg TS), and DD, respectively. In a 2D contour plot describing the tendency of DD with respect to pH and specific energy input, it was clear that DD increased as pH increased, but it seemed that DD decreased when the specific energy input exceeded about 20,000kJ/kg TS. This phenomenon tells us that there exists a certain point where additional energy input is ineffective in achieving further disintegration. A synergetic disintegration effect was also found in the combined pretreatment, with lower specific energy input in ultrasonic pretreatment yielding higher synergetic effect. Finally, in order to see the combined pretreatment effect in continuous operation, the sludge pretreated with low intensity alkaline (pH 9)/ultrasonic (7500kJ/kg TS) treatment was fed to a 3 L of anaerobic sequencing batch reactor after 70 days of control operation. CH(4) production yield significantly increased from 81.9+/-4.5mL CH(4)/g COD(added) to 127.3+/-5.0mL CH(4)/g COD(added) by pretreatment, and this enhanced performance was closely related to the solubilization increase of the sludge by pretreatment. However, enhanced anaerobic digestion resulted in 20% higher soluble N concentration in the reactor, which would be an additional burden in the subsequent nitrogen removal system.
The production of short-chain fatty acids (SCFAs) from excess sludge was conducted in batch fermentation tests at different pH values ranging from 4.0 to 11.0. Experimental results of the impacts of different pHs on SCFAs production showed that during the first 8-day fermentation time the total SCFAs production at either pH 9.0 or pH 10.0 was much greater than that at acidic or neutral pH, and the maximal yield of 256.2 mg SCFAs-COD per gram of volatile suspended solids (VSS) was at pH 10.0, which was, respectively, over3 and 4times that at pH 5.0 and uncontrolled pH. Clearly, SCFAs production from excess sludge could be significantly improved and maintained stable by controlling the fermentation pH at 10.0. The composition of SCFAs and the percent distribution of individual SCFAs accounting for total SCFAs at pH 10.0 were analyzed. The SCFAs consisted of acetic, propionic, iso-butyric, n-butyric, iso-valeric, and n-valeric acids, and acetic acid was the most prevalent product with a fraction of 40-55%. Because the results of this study were differentfrom those of previous studies of SCFAs production, the mechanism of increased SCFAs production under alkaline conditions was investigated. Results showed that as soluble COD increased, more soluble protein was provided as the substrate for producing SCFAs. In addition, less or even no SCFAs were consumed by methanogens at alkaline pH, so the SCFAs production was therefore remarkably improved. Further investigation revealed thatthe formation of SCFA at pH 10.0 was dominated by biological effects rather than by chemical hydrolysis.
The application of sonication to wastewater or sludge contributes to the dispersion of aggregates, the solubilisation of particulate matter with an increase in its biodegradability, the damage of microorganisms due to the loss of cellular membrane integrity. This research is aimed at investigating the effects of sonication at 20kHz frequency on viability of microorganisms present in raw wastewater and activated sludge taken from a municipal wastewater treatment plant, as well as pure strains of Escherichia coli and E. faecalis. Flow cytometry was applied for the identification and quantification of viable and dead bacteria free in the bulk liquid, after the fluorescent staining of cellular nucleic acids. The main results showed that: (i) cells of E. coli were highly sensitive to sonication, even at low specific ultrasonic energy (E(s)), and disintegration of a large amount of cells was observed; (ii) on the contrary E. faecalis were more resistant than E. coli, even if high levels of E(s) were applied; (iii) bacteria in raw wastewater exhibited a dynamic of viable and dead bacteria similar to E. coli; (iv) in activated sludge samples, low levels of E(s) produced a prevalent disaggregation of flocs releasing single cells in the bulk liquid, while disruption of bacteria was induced only by very high levels of E(s).
The effects of sonication of activated sludge on heavy metal uptake were in a first time investigated in respect with potential modifications of floc surface properties. The treatment led to the simultaneous increase of specific surface area and of the availability of negative and/or hydrophilic sites. In parallel, organic matter was released in the soluble fraction. Sorption isotherms of cadmium and copper showed that uptake characteristics and mechanisms were highly dependent on both heavy metal species and specific energy supplied. The increase of both specific surface area and fixation sites availability led to the increase of Cd(II) uptake. For Cu(II), organic matter released in soluble phase during the treatment seemed to act as a ligand and to limit adsorption on flocs surface. Three different heavy metals uptake mechanisms have been identified: proton exchange, ion exchange and (co)precipitation.
EU Directive 86/278/EEC of 12 June 1986 on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture
CEC, 1986. EU Directive 86/278/EEC of 12 June 1986 on the protection of the
environment, and in particular of the soil, when sewage sludge is used in
agriculture. Off. J. Eur. Communities L. 181/6 1e7.