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Effects of microwave irradiation at various temperatures on biosludge disintegration
Abstract
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.
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Fourier Transform Infrared (FTIR) has been developed as a tool for the simultaneous determination of organic components, including chemical bond, as well as organic content (e.g., protein, carbohydrate, and lipid). However, until now, there is no further documents for describing the detailed information in the FTIR peaks. The objective of this study was to demonstrate how to read and assess chemical bond and structure of organic material in the FTIR, in which the analysis results were then compared with the literatures. The step-by-step method on how to read the FTIR data was also presented, including reviewing simple to the complex organic materials. This study is potential to be used as a standard information on how to read FTIR peaks in the biochemical and organic materials. .
In order to improve the removal efficiency of refractory organic matters in micro-polluted source water, biological manganese oxides (BMOs) were generated in situ in the biological aerated filter (BAF) (BAF 2#), which could oxidize the refractory organic matters into biodegradable organic matters. CODMn and NH4+-N in the effluent of BAF 2# both stabilized on the 39th day, while CODMn and NH4+-N in the effluent of the control BAF (BAF 1#) stabilized on the 38th and 42nd days, respectively. In the steady phase, the removal rates of CODMn and NH4+-N in BAF 1# were 41.51% and 94.79%, respectively, while in BAF 2#, they were 54.52% and 95.55%, respectively. BMOs generated in BAF 2# evidently improved the efficiency of CODMn removal. With the increase in the influent Mn2+ in BAF 2#, the rate of CODMn removal was gradually improved to 63.60%, while the efficiency of NH4+-N removal was slightly improved, CODMn was evidently removed in each section of the filter layer, and ammonia was mainly removed in the 0~0.8 m layer of the filter. CODMn was evidently removed in each section of the filter layer, and NH4+-N was mainly removed in the 0~0.8 m layers of the filter. Biological CODMn, Mn2+, and NH4+-N removal all followed the first-order kinetic reaction. As the influent Mn2+ gradually increased from 0 to about 0.5, 1, and 2 mg/L, the efficiency of CODMn removal along the filter layer was significantly improved, but the efficiency of NH4+-N removal was slightly improved. The kinetic constant k of biological CODMn removal significantly increased, while the kinetic constant k of biological Mn2+ and NH4+-N removal gradually increased.
A large amount of sludge is produced in the process of municipal sewage treatment. The recovery and utilization of large amounts of sugar, protein, lipids and other organic matter from sewage sludge (SS) is of great significance for reducing environmental pressure and producing clean energy. In this study, microwave combined with thermal-alkaline pretreatment was used to accelerate the dissolution of primary sedimentation sludge and the release of intracellular substances, and to promote the extraction of sugar from SS. The results showed that the yield of crude sugar and the extraction efficiency of pure sugar increased with the increase in NaOH dosage. The extraction of crude sugar reached the equilibrium at about 30 min. During the response surface analysis, the optimal pretreatment conditions were determined as follows: the dosage of NaOH was 9.93 mL, and the leaching time and the microwave time were 27.65 min and 33.2 s, respectively. The crude sugar yield and extraction efficiency obtained under this condition were 39.80 ± 3.57% and 89.74 ± 3.61%, respectively. The pretreated sludge and crude sugar were characterized with scanning electron microscopy and Fourier transform infrared spectroscopy. The results showed that the combined use of thermal-alkaline and microwave effectively destroyed the structure of the sludge and increased the yield of crude sugar.
This paper aims to assess purification performances and explore wastewater byproducts valorization strategies from a full scale urban wastewater treatment plant (WWTP). Process performance indicators and effluent discharge parameters were assessed during a review period of one year and compared to WWTP design criteria and applied discharge standards. Treated wastewater, sewage sludge and energy production lines were investigated. Results highlight that Marrakesh WWTP performed adequately depending its design characteristics and generally fulfilled applied discharge standards depending established requirements. Sludge line constitute an important component of Marrakesh WWTP, it allows clean energy production, which contributes up to 50% of its electrical self-sufficiency.
Enhanced biological phosphorus removal at wastewater treatment plants that use anaerobic digesters for sludge treatment have historically encountered phosphate precipitation problems in the form of struvite. Literature on struvite is thin which is surprising given it can foul/block the sludge return lines and associated pumps and valves, causing significant operational problems. This study has evaluated if a typical large wastewater treatment plant can overcome this problem by adopting circular economy thinking. The struvite profile based on the supersaturation ratio of (Mg:NH4:PO4²⁻), pH and temperature demonstrates the potential operational hotspots that can present uncontrolled struvite formation. Based on current struvite monitoring technologies and a cost-benefit analysis, the controlled struvite recovery via an Ostara crystallization reactor has been demonstrated to be economically viable with a pay-back period of less than a decade. An integrated evaluation illustrates the positive environmental impact arising from the utilisation of the recovered product. Economic viability and payback periods will vary according to circumstances, but we recommend that WWTP operators globally consider fitting a crystallisation reactor to appropriate plants, The outcomes and recommendation from this study are particularly timely given the global fertiliser shortage (2022) that is driving up food prices and reducing crop sizes.
The disintegration of biosludge provides the release of organic and inorganic substances into the solution. An increase of organic substances concentration in the inlet of anaerobic reactor improves the digestion efficiency. In the disintegration process, a slow and partial degradable organic fractions of biosludge are converted into the readily biologically available compounds in the anaerobic digestion process. In order to improve the biogas production in the anaerobic sludge digestion, several disintegration methods such as thermal, chemical, mechanical, and advance oxidation processes or their combinations are applied. The main goal of the present review paper is to introduce the biological sludge disintegration by the microwave (MW) radiation considering the biogas production in the anaerobic digestion process. Under various operating conditions, the effectiveness of MW disintegration method was investigated in terms of heating principles, sludge disintegration, and biogas production. Additionally, the efficiency of hybrid systems such as MW/H2O2, MW/UV, etc. were compared with the singular MW radiation process
As an environmentally friendly energy recovery technology, microwave pyrolysis has huge development potential in sludge resource treatment. This paper comprehensively reviews the progress of microwave pyrolysis of sludge, focusing on the mechanisms and development status of microwave pyrolysis equipment. The effects of pyrolysis temperature, heating rate, microwave absorbers, sludge properties and catalysts on microwave pyrolysis efficiency and its products are also discussed. Finally, the differentiation compared with conventional pyrolysis is summarized. It is suggested that target products can be controlled directionally by changing the pyrolysis conditions and exploring the harmful products produced in the microwave pyrolysis process. Future research directions are proposed to help the subsequent extensive application of microwave pyrolysis technology in sludge treatment.
Municipal sludge is a by-product of urban sewage treatment; it has high water content and contains many hydrophilic extracellular polymeric substances (EPS). It is difficult to achieve deep dewatering of sludge. Therefore, it is necessary to find an efficient method to treat sludge. In this study, an acid-microwave treatment method was proposed to improve the dewatering ability of sludge. A vacuum filtration test and particle size analysis test were used to explore the improvement effect of dehydration performance. After centrifugation, the concentrations of soluble proteins and polysaccharides in the supernatant were measured to explore the disintegration of acid-microwave treatment on EPS. Finally, the microstructure of sludge treated by acid-microwave was studied via the SEM test. The results showed that short-term microwave radiation was conducive to sludge dewatering. However, the sludge dewatering performance deteriorated significantly when the treatment time was too long. The optimal microwave treatment condition was 700 W for 180 s, and the SRF of sludge was the lowest, which was 10.11×1012 m/kg. After acid-microwave treatment, sludge particle size increased significantly. The concentration of protein and polysaccharide in supernatant increased with the increase of microwave treatment time. Acid-microwave treatment could significantly improve the release efficiency of the polysaccharide. Through the SEM test, it can be verified that the bulk floc structure of sludge is broken after microwave treatment, and acid-microwave treatment is beneficial to the aggregation of floc fragments to form large particles, which can provide larger pores for drainage.
The migration, transformation and ecological risk of heavy metals (Cr, As, Ni, Cu, Zn, Cd and Pb) in sewage sludge during the microwave-assisted thermal hydrolysis process were investigated under different temperatures (80 °C, 100 °C, 120 °C, 140 °C, and 160 °C). The potential relationship between the bio-availability of heavy metals and the variables of microwave treatment, including pH, ammonium-nitrogen, soluble chemical oxygen demand, pH, soluble protein, soluble polysaccharide and volatile solids, was also explored. The results showed that the migration of heavy metals between solid–liquid phase mainly depended on the temperature. The percentage of all heavy metals (except Cu) in mobile (acid-soluble/exchangeable and reducible) forms decreased after microwave-assisted thermal hydrolysis treatment. The solubilisation of compounds with C = O and O-H accompanied with the generation of organic and inorganic metal halides were also observed in the treated sludge through Fourier transform infrared spectroscopy analysis. NH4+-N showed the highest negative correlation to the bio-availability of most heavy metals (except Cu and Cr) with coefficients (absolute value) over 0.87 (P < 0.05). VS showed a positive correlation to the bio-availability of most heavy metals (except Cu). The total potential ecological risk index (RI) decreased by 46.65% after microwave treatment at 160 °C. HIGHLIGHTS
Effect of microwave treatment on heavy metals speciation in sludge was reported.;
The metal halides related to organic and inorganic compounds were observed.;
NH4+-N showed the greatest correlation to the bio-availability of most heavy metals.;
Environmental risk of treated sludge after microwave treatment was alleviated.;
Every year, the human impact on the world’s water sources becomes more pronounced. One of the triggers to this increase is the use of ineffective wastewater and sludge treatment systems. Recently, the number of studies of microwave processing in handling liquid municipal and industrial waste has increased. This paper discusses heat treatment, change in properties, decomposition of substances, removal of metals, demulsification, pyrolysis, biogas processing, disinfection, and other topics. The findings of European, Chinese, Russian, and other authors are summarised and presented in this review. In addition, the most notable Russian patents for microwave installations/devices and reactors suitable for a wide variety of applications are discussed. In this article, the authors look at microwave wastewater and sludge treatment from the perspective of practical application in various fields of human economic activity.
The abatement of nutrient compounds from aqueous waste and wastewater is currently a priority issue. Indeed, the uncontrolled discharge of high levels of nutrients into water bodies causes serious deteriorations of environmental quality. On the other hand, the increasing request of nutrient compounds for agronomic utilizations makes it strictly necessary to identify technologies able to recover the nutrients from wastewater streams so as to avoid the consumption of natural resources. In this regard, the removal and recovery of nitrogen and phosphorus from aqueous waste and wastewater as struvite (MgNH4PO4·6H2O) represents an attractive approach. Indeed, through the struvite precipitation it is possible to effectively remove the ammonium and phosphate content of many types of wastewater and to produce a solid compound, with only a trace of impurities. This precipitate, due to its chemical characteristics, represents a valuable multi-nutrients slow release fertilizer for vegetables and plants growth. For these reasons, the struvite precipitation technology constantly progresses on several aspects of the process. This manuscript provides a comprehensive review on the recent developments in this technology for the removal and recovery of nutrients from aqueous waste and wastewater. The theoretical background, the parameters, and the operating conditions affecting the process evolution are initially presented. After that, the paper focuses on the reagents exploitable to promote the process performance, with particular regard to unconventional low-cost compounds. In addition, the development of reactors configurations, the main technologies implemented on field scale, as well as the recent works on the use of struvite in agronomic practices are presented.
In this study, ultrasound disruption was employed to enhance the efficiency of microwave disintegration of dairy sludge. Results revealed that ultrasound specific energy input of 1,500 kJ/kg TS was found to be optimum with limited cell lysis at the end of the disruption phase. Biodegradability study suggested an enhancement in suspended solids reduction (16%) and biogas production (180 ml/gVS) in floc disrupted (deflocculated) samples when compared to sole microwave pretreatment (8.3% and 140 ml/gVS respectively). Energy assessment to attain the 15% optimum solubilization revealed a positive net production of 26 kWh per kg sludge in deflocculated samples compared to 18 kWh in flocculated (sole microwave) samples. Thus, ultrasound disruption prior to microwave disintegration of dairy sludge was considered to be a feasible pretreatment technique.
The present study highlights microbial assimilation of carbon (glucose) and nitrogen (NH3–N) from wastewater using heterotrophic bioconversion process. Experiments were conducted to study the role of heterotrophic microbes towards removal of carbon and nitrogen at varying initial concentrations of carbon (COD, 533 to 1600 mg/l) and nitrogen (NH3–N, 73 to 249 mg/l) keeping the initial biomass of microorganisms constant. Removal of COD and ammonia from wastewater represented a first-order rate reaction, upon analysis of kinetics, indicating that the rate of reaction is dependent on the initial concentration of nutrients available. Rate equations were developed using the Monod model, and the maximum specific consumption rate (k4) and half saturation constant (Ks) values for NH3–N and COD were found to be 2.59 mg/l/h and 64.13 mg/l/h and 38.46 mg/l and 2162.69 mg/l, respectively. Assimilation of NH3–N followed the Freundlich isotherm model. The mass transfer coefficient for COD and NH3–N were found to be 0.13 h⁻¹ and 0.81 h⁻¹ respectively. The NH3–N is converted to N2O during nitrification, and observed values of N2O coincided with the empirically predicted values indicating the activity of heterotrophic nitrifiers. The regeneration/doubling time of heterotrophic microbial biomass varied from 26 to 121 h. Statistical techniques, viz. analysis of variance, multi-linear regression analysis and principal component analysis, validated the results.
This study focused on the separate and combined applications of ozonation and microwave treatment to enhance the phosphorus and ammonia release from waste activated sludge. Twenty-six batch experiments were run with or without acidic (pH 2) and alkaline (pH 10) pretreatments and different ozone dosages. Also, microwave post-treatments were applied to enhance phosphorus release efficiency. Results showed that ozonation is an effective technology for solubilization and release without any pre or post-treatment, reactive phosphorus content increased from 1.9 to 3.6 mg PO 4 -P/g MLSS in (89.5% increase) with 19.4% COD release. Alkaline pretreatment enhanced sludge solubilization and phosphorus release at most (23.9% COD release and 152.6% PO 4 -P increase); however, decreases in ammonia, calcium and magnesium concentrations pointed out a loss of a part of released phosphorus, due to struvite or apatite precipitation. Acidic pretreatment reduced the sludge solubilization during ozonation (10% COD release) but prevented the uncontrolled precipitation and enhanced the phosphorus release (115.8% PO 4 -P increase). For microwave treatment, acid pretreated sludge showed higher release than alkaline pretreated or neutral sludge. Among different process combinations, acid pretreatment/ozonation/microwave experiments have shown the highest sludge solubilization and nutrient release (48% COD release and 579% PO 4 -P increase); however, the difference between acid pretreatment/microwave and acid pretreatment/ozonation/microwave was not significant in terms of phosphorus release (479% PO 4 -P increase, p = 0.082). Thus, pH 2/microwave may be a cost-effective and feasible alternative for nutrient recovery from waste sludge. For struvite precipitation, pH 8.5 were determined as optimum level. Also using fine struvite particles as seed increased struvite precipitation efficiency.
This study investigated the disintegration of sewage sludge through the Fenton process. The study was conducted by both the conventional Fenton-type process (CFP) (Fe²⁺/H2O2) and the Fenton-type process (FTP) (nZVI/H2O2). Experiments were performed using different pH, catalyst iron (Fe²⁺ and nZVI), and H2O2 dosages. Different parameters such as the degree of disintegration (DD), soluble chemical oxygen demand (SCODS), and particle size distribution were studied to investigate the effects of CFP and FTP processes on the disintegration of sludge. In addition to these parameters, scanning electron microscope, Fourier-transform infrared spectroscopy, and X-ray diffraction analyses were done to determine the changes in sludge characterization before and after disintegration. In the study, the optimal catalyst iron was determined to be (Fe²⁺ and nZVI) 4 g/kg total solids (TS), and the H2O2 dosage was determined to be 10 g/kg TS. The experiments were performed with consideration of the 1-h oxidation time. While DD was found to be 31.8% and the SCODS was found to be 364 mg/L for FTP, DD and SCODS were found to be 14.1% and 256 mg/L for CFP, respectively.
This investigation deals with ultrasonic disintegration of sludge taken from a meat processing wastewater treatment plant. Different specific energy inputs ranging between 0 and 100 MJ/kg Total Solids (TS) were used in the study. The effect of ultrasonic pre-treatment on anaerobic biodegradability and filterability characteristics of sludge were evaluated using lab-scale experiments. 30 MJ/kg TS of supplied energy is efficient for cell lyses. Ultrasonic pre-treatment significantly enhanced the biodegradability of sludge. Biochemical methane potential test results also supported this result. For 30 MJ/kg TS, 2.4 times higher methane production was achieved compared to raw sludge. Sludge's supernatant characteristics were also affected by the ultrasonic pre-treatment. For 30 MJ/kg TS, oil content, total nitrogen and total phosphorus in the sludge supernatant increased by 3.8 times, 58 times, and 12 times, respectively. The other result is that ultrasonic treatment adversely affected filterability characteristics of sludge even for very low specific energy levels.
This study presents mid and near-infrared (7500-375 cm ⁻¹ ) total reflection mode spectra of several natural organic materials used in artworks as binding media, consolidants, adhesives, or protective coatings. A novel approach to describe and interpret reflectance bands as well as calculated absorbance after Kramers-Kronig transformation (KKT) is proposed. Transflection mode spectra have represented a valuable support both to study the distorted reflectance bands and to validate the applicability and usefulness of the KK correction. The aim of this paper is to make available to scientists and conservators a comprehensive infrared reflection spectral database, together with its detailed interpretation, as a tool for the noninvasive identification of proteins, lipids, polysaccharides, and resins by means of portable noncontact FTIR spectrometers.
Sewage sludges generation and their disposal have become one of the greatest challenges of the 21st century. They have great microbial diversity that may impact wastewater treatment plant (WWTP) efficiency and soil quality whether used as fertilizers. Therefore, this research aimed to characterize microbial community diversity and structure of 19 sewage sludges from São Paulo, Brazil, as well as to draw their relations to sludge sources [domestic and mixed (domestic+industrial)], biological treatments (redox conditions and liming), and chemical attributes, using molecular biology as a tool. All sludges revealed high bacterial diversity, but their sources and redox operating conditions as well as liming did not consistently affect bacterial community structures. Proteobacteria was the dominant phylum followed by Bacteroidetes and Firmicutes; whereas Clostridium was the dominant genus followed by Treponema, Propionibacterium, Syntrophus, and Desulfobulbus. The sludge samples could be clustered into six groups (C1 to C6) according their microbial structure similarities. Very high pH (≥11.9) was the main sludge attribute segregating C6, that presented very distinct microbial structure from the others. Its most dominant genera were Propionibacterium > > Comamonas > Brevundimonas > Methylobacterium ∼Stenotrophomonas ∼Cloacibacterium. The other clusters’ dominant genera were Clostridium > > Treponema > Desulfobulbus ∼Syntrophus. Moreover, high Fe and S were important modulators of microbial structure in certain sludges undertaking anaerobic treatment and having relatively low N-Kj, B, and P contents (C5). However, high N-Kj, B, P, and low Fe and Al contents were typical of domestic, unlimed, and aerobically treated sludges (C1). In general, heavy metals had little impact on microbial community structure of the sludges. However, our sludges shared a common core of 77 bacteria, mostly Clostridium, Treponema, Syntrophus, and Comamonas. They should dictate microbial functioning within WWTPs, except by SS12 and SS13.
The need to apply innovative technologies for maximizing the efficiency and minimizing the carbon footprint of sewage treatment plants has upgraded sewage sludge management to a highly sophisticated research and development sector. Sewage sludge cannot be regarded solely as 'waste'; it is a renewable resource for energy and material recovery. From this perspective, legislation on sewage sludge management tends to incorporate issues related to environmental protection, public health, climate change impacts and socio-economic benefits. This paper reviews the existing legislative frameworks and policies on sewage sludge management in various countries, highlighting the common ground as well as the different priorities in all cases studied. More specifically, the key features of legislation regarding sludge management in developed countries such as the USA, Japan, Australia, New Zealand and the European Union (EU27) are discussed.
The management of secondary sludge from aerobic treatment of effluents from the cellulose industry is a current problem. The usual ways of disposal do not provide added value to the waste as they assume an economy based on "take-make-waste" (linear economy). In this work, thermal hydrolysis (TH) and anaerobic digestion (AD) are proposed to valorize this biosludge. Based on a Doehlert experimental plan, a response surface methodology (RSM) defined by seven different TH conditions is proposed. After TH, biomethanation potential (BMP) tests were performed to evaluate the AD possibilities. The TH conditions cover a temperature range between 125 °C and 205 °C and a reaction time from 15 min to 45 min. The TH process was successful in enhancing the bioavailability of the waste, increasing the concentration of soluble organic matter quantified by chemical oxygen demand of the soluble fraction (CODs), and decreasing the concentration of volatile suspended solids (VSS). However, response surfaces performed for CODs and VSS revealed the existence of optimums, which demonstrated the adverse effects of the more severe TH conditions. Organic matter solubilization was confirmed by microscopic observations. The amount of suspended organic matter after TH is reduced by two to three times compared to the untreated value. The subsequent BMP of the hydrolyzed waste increases between 100% and 220% compared to the untreated condition, wich had a BMP value of 84 NmL CH4 gVS-1. The response surface determined for the BMP reveals the presence of a maximum point of methane production at 202 °C for 31 min, which differs from the maximum CODs value observed at 196 °C for 40 min.
While sludge bulking often occurring in activated sludge processes generally leads to serious membrane fouling in membrane bioreactors (MBR), the underlying causes are still unclear. In this study, fouling behaviors of a MBR operated at stages of normal and sludge bulking were compared, and the fouling mechanisms of the different behaviors were explored. It was found that, the MBR could be stably operated in normal stage without membrane cleaning for about 60 days, whereas, daily membrane cleaning had to be carried out when operated in sludge bulking stage. The bulking sludge possessed a rather high specific filtration resistance (SFR) of about 1.36×10¹⁴ m·kg⁻¹, which is over 5.33 times than that of the normal sludge. A series of characterizations demonstrated that the bulking sludge had rather lower dewaterability, smaller particle size, higher fractal dimension, higher viscosity, abundant filamentous bacteria and different functional groups of extracellular polymer sustains (EPS). It was suggested that microbial community transition was responsible for the occurrence of sludge bulking, further affecting membrane fouling. Based on these characterizations, it was reported that adhesion propensity (indicated by the thermodynamic interaction) of the bulking sludge to the membrane surface is about 3.6 times than that of the normal sludge. It was proposed that, extra force should be provided to offset a chemical potential gap caused by foulant layer structure transition in order to sustain filtration of the bulking sludge, resulting in extremely high SFR. This study offered deep thermodynamic mechanisms of MBR fouling during occurrence of sludge bulking.
To avoid the generally deteriorated dewaterability of sludge in waste activated sludge (WAS) anaerobic acidogenesis, the combination of varied calcium oxide (CaO) dosage (i.e., 0.01-0.07 g/g TS) and freezing/thawing pretreatment (5 F/T cycles) was investigated for concurrently improving the production of volatile fatty acids (VFAs) and dewatering performance of sludge. The highest release of soluble chemical oxygen demand (SCOD) (1836±96 mg/L) and accumulation of VFAs (448.0 mg COD/g VS) were reached through the co-pretreatment of CaO (0.07 g/g TS) and F/T (50 h at -24 °C) (i.e., 0.07 CaO−F/T). Meanwhile, optimal dewaterability of sludge was also achieved in 0.07 CaO−F/T co-pretreated WAS fermentation, which was reflected by large particle size (98.32 μm), low capillary suction time (41.6 s), decreased specific resistance to filtration (SRF, reduced 47.5% against blank) and high zeta potential (−9.59 mV). Co-pretreatment of CaO and F/T reduced the species number of total microbial population, but improved the abundance of acid-producing bacteria. Increased abundance of Bacteroides, Macellibacteroides, Petrimonas, Prevotella, Clostridium, and Megasphaera was positively relevant to the high yields of VFAs. The economic analysis indicated that CaO−F/T was economically acceptable with considerable estimated net profits, which provided a feasible and efficient alternative for further WAS treatment at large scale.
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.
To alleviate difficult degradation of lipids and its inhibition on anaerobic digestion of food waste, ultrasound and microwave heating were used to pretreat lipid and food wastes for promoting methane production. The soluble COD (10,130 mg/L) of lipid waste pretreated with ultrasound of 50,000 kJ/kg-total volatile solids (TVS) was higher than that (1910 mg/L) with microwave heating of the same energy input. This was because the thermal and mechanical effects of the ultrasound pretreatment were stronger than the thermal and solid dissolution effects of the microwave heating pretreatment. The methane yield of lipid waste pretreated with ultrasound during anaerobic digestion increased by 43.3% to 927.97 mL/g-TVS, higher than that of 738.63 mL/g-TVS with microwave heating. Fourier transform infrared spectrometer (FTIR) and Scanning electronic microscopy (SEM) analysis demonstrated that the residual lipids were reduced and microorganism coating was relieved after anaerobic digestion of lipid waste pretreated with ultrasound. The energy conversion efficiency of lipid waste pretreated with ultrasound to methane product was 69.89%, which was higher than that of 58.98% with microwave heating. Ultrasound efficiently degraded lipid waste and released more energy to be converted into methane.
In this study, the dewaterability characteristics of electroplating sludge have been investigated after treatment with microwave irradiation. While specific resistance to filtration (SRF) was used to evaluate sludge dewaterability, water content (WC) in sludge and soluble chemical oxygen demand (SCOD) were determined to describe the observed changes in sludge dewaterability. SRF of sludge decreased from 3.54E + 09 m/kg to 4.33E + 08 m/kg, and WC decreased from 98.56 wt.% to 94.37 wt.% after microwave treatment at 800 W and 120 s. Sludge solubilization increased with microwave power, but deteriorated at higher contact times due to adverse effects of sustained microwave irradiation. A full factorial design along with response surface analysis was applied to determine an optimum combination of microwave power and contact time for effective dewaterability enhancement. Based on statistical observations and numerical optimization using desirability function analysis, 800 W and 141 s was the optimal conditioning requirement which yielded minimum SRF and WC, while also ensuring maximum sludge disintegration.
Hydrothermal pretreatment (HTP) conditions were optimized for continuous mesophilic (MAD) and thermophilic (TAD) anaerobic digestion of high-solid sludge (10–11% total solids). COD solubilization increased with prolonged HTP durations, and became not significant after 210 min. According to the methane production rate and energy consumption, the optimal HTP temperature was determined at 160 °C. Regarding continuous operation without HTP, TAD achieved higher methane yield and volatile solids (VS) reduction, at 0.12 L/g VSadded and 23.9%, respectively. After HTP, methane yield and VS reduction in MAD and TAD were increased by 400% and 191% (MAD), 67% and 72% (TAD), respectively. TAD was limited due to the inhibition from about 2800 mg/L of NH4⁺-N concentration. The methanogenic activity of MAD was enhanced, whereas TAD displayed a reduced value owing to ammonia inhibition. Ultimately, MAD with HTP and TAD without HTP achieved the higher energy balance, 5.25 and 3.27 kJ/g VS, respectively.
Future sustainable technological solutions need to balance the development of sewage sludge as resource against the paramount necessity to protect human health and environment. Nutrients, high value metals, embedded energy, and the avoidance of sludge handling costs are important examples of recoverable assets. Aim of this study was to assess the feasibility of sludge pre-treatments as ultrasound and thermal hydrolysis to promote the release of metals due to organics disintegration, and to investigate the efficiency of the anaerobic process to leach out metals from municipal sludge through an enhanced hydrolysis/acidification step. A preliminary deep analysis of elements concentration (Al, As, Ca, Cd, Co, Cr, Cu, Fe, Ga, Mg, Mn, Na, Ni, Pb, Se, V and Zn) in solid residue and water phase of waste activated sludge and anaerobic digestate sampled at full scale wastewater treatment plant (WWTP) allowed to calculate partitioning coefficients and equilibrium concentrations, in order to predict metal mobility and bioavailability. The gradual sludge disintegration and organics hydrolysis significantly contribute to the leaching of toxic elements as As, Ga, Se and Zn, increasing their removal potential. A new anaerobic bioleaching approach will be presented, aiming to develop a sustainable platform to recover metals, together with methane and a clean recyclable digestate.
This work studied the influence of low-thermal pretreatment (60-120 °C) on anaerobic digestion of saline waste activated sludge. The findings showed higher temperature and longer pretreatment time considerably improve organics hydrolysis (soluble chemical oxygen demand increased by 4.2-11.9 times) and volatile solid reduction (maximum 24.6%). Carbohydrate and proteins solubilization accelerated by 5.6-43.8 times and 8.9-35.9 times, respectively by temperature rose from 60 to 120 °C. Low temperature (60 °C) promotes faster release of ammonia and phosphate. Thermal treatment had positive effect on biogas production because methane yield was enhanced by 13.7, 27.0, 29.0 and 29.6% when pretreated at 60, 80, 100 and 120 °C, respectively. Significant positive relationships observed between pretreatment temperature/duration and sludge properties. Energy and economic assessment displayed anaerobic digestion of 80 °C pretreated sludge is more economically feasible. Thus, low-thermal pretreatment technology could be useful for improvement of methane yield in anaerobic digestion.
Protein recovery from sludge is a simple and well-studied waste-to-resource approach to convert the “wasted” sludge to useful “product”. This article reviews different pretreatment methods to accelerate the release/solubilization of protein from sludge, including physical, thermal, chemical, biological options, and their combinations. Based upon the papers reviewed, protein release/solubilization from sludge was found to be the determining step prior to recovery. The alkaline pretreatment at pH 12 was the most effective method for protein solubilization when compared with ultrasonic (1 W mL−1) and thermal (80 oC) pretreatments, if only considering the released protein concentration. Moreover, pretreatments can change protein properties (e.g., molecular weight, conformation, type, etc.), and the details were summarized. The recovered protein can be alternatives for poultry feedstuff, wood adhesive and fire-extinguishing foam, and its separation from sludge liquor can considerably reduce the nitrogen load to the wastewater treatment plant and the environment. Finally, the integration of circular economy into protein recovery from sludge was proposed, and the related system boundaries for life cycle assessment of this technology were defined. Future research should focus on promoting its full-scale implementation by tackling with the technical, economic, social, and environmental challenges.
Sewage sludge generation is steadily rising because of urbanization and population increase. Handling the excess sludge is a tedious and costly task. Although anaerobic digestion (AD) is a widely accepted practice, a notable portion of the substrate leaves digesters undigested. Microwave pretreatment offers a distinct advantage to increase digester performances by solubilizing intracellular organic matter and disturbing the floc assemblies in waste-activated sludge. This article summarizes 70 + microwave pretreatment articles testing irradiation of waste-activated sludge prior to AD published in the past 15 years. Our systematic analysis reveals critical sludge properties, microwave operational conditions, and gaps in today's literature while providing a mechanistic insight for microwave enabled solubilization and subsequent AD.
Urbanization and population growth have resulted in the accumulation of heavy metals in biosolids, and these metals act as potent environmental contaminants. In this study, a novel microwave-mediated method of extracting heavy metals from dewatered biosolids was developed. With an electromagnetic power of 140 W for a contact time of 10 s, microwave irradiation effectively induced the immobilization of heavy metals. The treated biosolids were subsequently mixed with acetic acid, sulfuric acid, or ethylenediaminetetraacetic acid (EDTA) for heavy metal extraction. The biosolids in this study were contaminated by different heavy metals, including Cd, Cu, Fe, Pb, Ni, and Zn. Among them, the concentrations of Cd (94.3 ± 14.2 mg/kg) and Pb (888.7 ± 79.8 mg/kg) were considerably above the limits allowed for land application. Conventional extraction methods were found to be insufficient in lowering heavy metal contents below regulation limits, while the microwave-mediated method efficiently increased heavy metal removal by as much as ∼3x. After the biosolids were treated, Cd and Pb concentrations decreased to 80.2 ± 2.7 and 159.8 ± 22.1 mg/kg, respectively. The treated biosolids and their products were eligible for land application as an alternative treatment. The microwave-mediated method also lowered the demand for extractants. Using a reduced concentration of sulfuric acid, acetic acid, or EDTA, at least 90% of Cu, 70% of Zn and Pb, 45% of Fe, and 20% of Ni were simultaneously removed from the contaminated biosolids. Cost analyses revealed that the microwave-mediated method could decrease the net total cost of biosolid handling by as much as 62.7%. Considering its simplicity, cost-effectiveness, and minimal environmental impacts, the proposed method offers a promising solution to the problem of heavy metal accumulation in biosolids.
To develop aerobic granules based sustainable wastewater treatment, it is necessary to view wastewater treatment process and excess sludge treatment as a whole to evaluate resource recovery and sustainability. We thus investigated in this study how mineral characteristics of aerobic granules with/without calcium phosphate precipitates for phosphorus removal in treatment process affect the excess sludge digestion for energy recovery. Steam explosion at 170 C as an effective thermal sludge treatment approach was studied in parallel with normal thermal treatment in an autoclave at 70, 100 and 125 C, respectively. A liner relationship was found between the thermal treatment temperature in the autoclave and biogas production of aerobic granules. The untreated granules with only 10% mineral content (G1) generated 30% more biogas than the untreated granules with 39% mineral content (G2), but steam explosion
is more effective to G2 with high mineral content and relatively poor methane yield potential. In
addition, steam explosion improved methane production from G2 more compared with activated sludge although both untreated activated sludge and G2 had comparable methane production, i.e. around 0.235 L CH4/g VS. Therefore, steam explosion is potential to be used to increase methane production especially when the untreated granular sludge has low methane yield due to high mineral content. This work provides a good basis for a holistic evaluation of resource recovery based on aerobic granular sludge, i.e. combined energy recovery and phosphorus removal and recovery via CaP precipitates, and trade-off between different factors with steam explosion.
In this study, microwave irradiation has been employed to disintegrate the sludge biomass profitably by deagglomerating the sludge using a mechanical device, ultrasonicator. The outcomes of the study revealed that a specific energy input of 3.5 kJ/kg TS was found to be optimum for deagglomeration with limited cell lysis. A higher suspended solids (SS) reduction and biomass lysis efficiency of about 22.5% and 33.2% was achieved through ultrasonic assisted microwave disintegration (UMWD) when compared to microwave disintegration –MWD (15% and 20.9%). The results of biochemical methane potential (BMP) test were used to estimate biodegradability of samples. Among the samples subjected to BMP, UMWD showed better amenability towards anaerobic digestion with higher methane production potential of 0.3 L/g COD representing enhanced liquefaction potential of disaggregated sludge biomass. Economic analysis of the proposed method of sludge biomass pretreatment showed a net profit of 2.67 USD /Ton respectively.
For pretreating waste-activated sludge, the combined alkali and microwave pretreatment is a promising hybrid method. To make this pretreatment more economical, moderate conditions were applied for central composite design of the experiments. A quadratic model was established to describe the influence of NaOH dosage and microwave processing time on methane production. The optimal condition was 0.12 g NaOH/g TS treatment for 24 h and 240 W microwave treatment for 10 min. Either single or hybrid pretreatment can significantly accelerate the hydrolysis, but the degree of degradation increases positively as correlated with treatment intensity. Comparing to untreated sludge, the methane yields of microwave-, alkali-, and combined-treated increased to 1.9, 3.4, and 4.6 times, respectively. The SCOD removal rates of the three kinds of treated sludge were 20.6%, 23.0%, and 36.5%, respectively. The single microwave pretreatment efficiently broke the sludge flocs and promoted the release of biodegradable organics as well as nutrients, but there was no advantage in methane production. The existence of NaOH not only eliminated the inhibition caused by microwave, but also improved the degree of degradation.
The present study investigates the effects of microwave irradiation on the performance of anaerobic digestion processes. A first set of experiments is performed to distinguish the upper limit of the applied energy levels. Secondly, the effects of these treatments on the performance of the digestion process are evaluated in 3 experimental setups: (i) monitoring the acetic acid degradation, (ii) performing a biological methane potential (BMP) assay and (iii) conducting a specific methanogenic activity (SMA) test. The solubilisation experiment reveals a limited degree of disintegration of anaerobic biomass up to a microwave treatment of 10000 kJ/kg TS. Above this threshold value the soluble COD level started to rise, with up to 350% at 30000 kJ/kg TS regardless of the microwave output power. Because solubilisation of the biomass increases the easily degradable content, this would lead to false observations regarding increased activity. Therefore, solubilisation is minimized by limiting the microwave treatment to a maximum of 6000 kJ/kg TS during the second part of the experiments. Monitoring the degradation of acetic acid after a low intensity microwave treatment, reveals that microwave irradiation shortens the lag phase, e.g., from 21 to 3 h after a microwave treatment of 1000 kJ/kg TS at 100 W. However most treatments also result in a decrease of the maximum degradation and of the degradation rate of acetic acid. BMP assays are performed to evaluate the activity and performance of the entire anaerobic community. Every treatment results in a decreased biogas production potential and decreased biogas production rate. Moreover, each treatment induced an increase of the lag phase. The SMA experiments show no influence of the microwave irradiation in terms of biogas or methane production.
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.
Anaerobic digestion (AD) is the most commonly applied end-treatment for the excess of waste activated sludge (WAS) generated in biological wastewater treatment processes. The efficacy of different typologies of pre-treatments in liberating intra-cellular organic substances and make them more usable for AD was demonstrated in several studies. However, the production of new extracellular polymeric substances (EPSs) that occur during an AD process, due to microbial metabolism, self-protective reactions and cell lysis, partially neutralizes the benefit of pre-treatments. The efficacy of post- and inter-stage treatments is currently under consideration to overcome the problems due to this unavoidable byproduct.
The present study examines the relationship between the degree of solubilization and biodegradability of wastewater sludge as a result of low-temperature thermal pre-treatment. The main effect of thermal pre-treatment is the disintegration of cell membranes and thus solubilization of organic compounds. There is an established correlation between chemical oxygen demand (COD) solubilization and temperature of thermal pre-treatment, but results of thermal pre-treatment in terms of biodegradability are not well understood. Aiming to determine the impact of low temperature treatments on biogas production, the thermal pre-treatment process was first optimized based on an experimental design study on waste activated sludge in batch mode. The optimum temperature, reaction time and pH of the process were determined to be 80 °C, 5 h and pH 10, respectively. All three factors had a strong individual effect (p < 0.001), with a significant interaction effect for temp. pH² (p = 0.002). Thermal pre-treatments, carried out on seven different municipal wastewater sludges at the above optimum operating conditions, produced increased COD solubilization of 18.3 ± 7.5% and VSS reduction of 27.7 ± 12.3% compared to the untreated sludges. The solubilization of proteins was significantly higher than carbohydrates. Methane produced in biochemical methane potential (BMP) tests, indicated initial higher rates (p = 0.0013) for the thermally treated samples (khyd up to 5 times higher), although the ultimate methane yields were not significantly affected by the treatment.
This research evaluates whether there is any advantage of selecting one of the thermal methods of sludge pretreatment, conventional heating (CH) and microwave hydrolysis (MW), over another to enhance municipal sludge disintegration and performance of thermophilic anaerobic digestion (AD). For this purpose, a custom-built CH system simulating MW hydrolysis under identical heating and cooling profiles was used. The effects of three main pretreatment parameters including pretreatment method (CH and MW), heating ramp rate (3, 6 and 11 °C/min) and final temperature (80, 120 and 160 °C) on sludge solubilization and performance of thermophilic batch AD were evaluated. The effects of CH and MW hydrolysis were observed to be similar for sludge disintegration and digester performance (p-value > 0.05), while the effects of final temperature and heating ramp rate were proven to be different (p-value < 0.05). According to the results, it is essential to apply MW and CH pretreatments under identical experimental condition for an unbiased comparison which supports the findings of the author’s earlier study under mesophilic condition. Failing to address this issue explains the significant inconsistency observed among the findings of the previous CH vs. MW comparison studies that were unable to implement identical thermal profiles (between CH and MW) during sludge pretreatment. In comparison with mesophilic AD, thermophilic AD revealed lower biodegradation rate constant at the highest pretreatment temperature tested (160 °C), suggesting its higher sensitivity to the inhibitory effects of thermal pretreatment at the elevated temperatures.
Hydrothermal treatment has demonstrated the ability of improving the dehydration and drying performances of sewage sludge, as well as shown its suitability for producing fuel. On the other hand, because of the abundant nutrient matters in sewage sludge, the produced liquid may be used as the liquid organic fertilizer. In this work, the effect of the hydrothermal treatment on the nutrient behavior in sewage sludge was investigated. The effects of the reaction temperature (180-240°C), and the reaction time (30-90 min) were investigated, and both of solid and liquid products were analyzed individually. The results showed that 40%-70% of nitrogen, 50%-70% of potassium and 10%-15% of phosphorus in sewage sludge could be dissolved into the liquid product, and that the solubilization was highly influenced by the temperature and the reaction time during the hydrothermal process. The hydrothermal treatment can effectively transport nutrient components in sewage sludge into the liquid product.
Microwave (MW) hybrid processes are able to disrupt the flocculent structure of complex waste activated sludge, and help promote the recovery of phosphorus as struvite. In this study, to optimize struvite yield, (1) the characteristics of matter released in MW-hybrid treatments were compared, including MW, MW-acid, MW-alkali, MW-H2O2, and MW-H2O2-alkali. The results showed that selective release of carbon, nitrogen, phosphorus, Ca2 +, and Mg2 + achieved by sludge pretreatment using MW-hybrid processes. MW-H2O2 is the recommended sludge pretreatment process for phosphorus recovery in the form of struvite. The ratio of Mg2 +:NH4+-N:PO43 −-P was 1.2:2.9:1 in the supernatant. (2) To clarify the effects of organic matter on struvite recovery, the composition and molecular weight distribution of organic matters were analyzed. Low molecular weight COD was found to facilitate the removal rate of NH4+-N and PO43-P via crystallization, and the amorphous struvite crystals (< 1 kDa) from the filtered solutions had high purity. Therefore, the present study reveals the necessity of taking into consideration the interference effect of high molecular weight organic matters during struvite crystallization from sewage sludge.
This research paper presented a comparative result on microwave (M) pretreatment and combined microwave-ultrasonic (CMU) pretreatment of mixed sludge. The mixed sludge was composed of 75% primary sludge (PS) and 25% thickened excess activated sludge (TEAS). 0.5 L of mixed sludge was subjected to microwave pretreatment and fed to a semi-batch continuously stirred anaerobic digester at an organic loading rate of 2.75 gCOD/L day. Similarly, 0.5 L of mixed sludge sample in another digester was pretreated in two stages. The removal of TS was 37.7% for M pretreated sludge compared to CMU pretreated sludge of 69.1%. The removal of volatile solids for CMU pretreated sludge was 21% higher than M-pretreated one. The SCOD/TCOD ratio for both M and CMU pretreated sludge was 33% for 15 days of SRT however, percentage change in SCOD/TCOD ratio after 30 days of SRT for CMU-pretreated sludge was 40.6% more than M-pretreated sludge sample due to increased methanogenic disintegration of organics. Maximum percentage of methane produced was 71% for CMU pretreated sludge while it was only 56% for the M pretreated sludge. Nevertheless, the dewaterability measured in capillary suction time (CST) for M-pretreated sludge was better (348 s) compared to CMU-pretreated sludge (398 s) due to higher percentage of fine sludge particles in CMU pretreated sludge. The average particle size and floc size for microwave pretreated digested sludge was much bigger than that of combined microwave ultrasonic pretreated digested sludge. Morphological and structural analysis on SEM and FTIR were conducted to further understand the mechanism of the disintegration process.
This research provides a comprehensive comparison between microwave (MW) and conductive heating (CH) sludge pretreatments under identical heating/cooling profiles at below and above boiling point temperatures. Previous comparison studies were constrained to an uncontrolled or a single heating rate due to lack of a CH equipment simulating MW under identical thermal profiles. In this research, a novel custom-built pressure-sealed vessel which could simulate MW pretreatment under identical heating/cooling profiles was used for CH pretreatment. No statistically significant difference was proven between MW and CH pretreatments in terms of sludge solubilization, anaerobic biogas yield and organics biodegradation rate (p-value>0.05), while statistically significant effects of temperature and heating rate were observed (p-value<0.05). These results explain the contradictory results of previous studies in which only the final temperature (not heating/cooling rates) was controlled.
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Aerobic sewage sludge and dairy manure were pre-treated by conventional heating (CH) (40, 50 and 60°C) followed by microwave enhanced advanced oxidation process (MW/H2O2-AOP) at 100 and 120°C for solids disintegration and nutrient release. Increasing CH and MW temperatures resulted in an increase in soluble chemical oxygen demand concentration and volatile fatty acids in the treated solution. The maximum ratio of soluble to total chemical oxygen demand obtained for sewage sludge and dairy manure was 60% and 22%, respectively. The synergistic effect of microwave irradiation and hydrogen peroxide (H2O2) was more pronounced for sewage sludge at high CH and MW temperatures. Temperature and treatment time played a significant role in releasing orthophosphates. A maximum orthophosphate-to-total phosphate ratio of 55% and 85% was obtained for sewage sludge and dairy manure, respectively. A simple energy balance reveals that the CH step proves to be advantageous during continuous real-time operation of MW/H2O2-AO...
This work experimentally determined the effect of microwave treatment on the disintegration and acidogenesis of waste-activated sludges (WAS) from municipal and coke wastewater treatment plants. Sludge samples (500 g) were heated for 0, 3, 5, 7, 9, 11, and 15 min in a microwave oven (2,450 MHz, 700 W). The degree of sludge solubilization [soluble chemical oxygen demand (SCOD)/COD] increased asymptotically with microwave irradiation time from 1.5% at 0 min to 22.3% at 15 min for WAS from a municipal wastewater treatment plant (WAS(M)) and 1.5% to 5.1% for WAS from a coke wastewater treatment plant of a steel manufacturing industry (WAS(C)). The calcium concentrations in both sludges also increased with microwave irradiation time. The biochemical acidogenic potentials (BAP) increased from 3.70 to 4: 44 gCODL(-1) for WAS(M) and 1.19 to 1.67 gCODL(-1) for WAS(C). The results show that microwave irradiation increases the solubilization and BAP of sludges and that WAS(M) had a higher degree of solubilization and BAP than WAS(C). DOI: 10.1061/(ASCE)EE.1943-7870.0000379. (C) 2011 American Society of Civil Engineers.