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Use of water treatment plant sludge in high-rate activated sludge systems: A techno-economic investigation
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Coagulants such as aluminum sulfate (Al2(SO4)3 (alum)) and ferric chloride (FeCl3) used in water treatment plants (WTPs) led to the generation of sludge that is usually disposed to landfills. However, the utilization of WTP sludge is being encouraged by authorities to achieve sustainable development. This study aims to investigate WTP sludge utilization in a pilot-scale high-rate activated sludge (HRAS) system as a substitute for conventional coagulants. Based on jar tests, the iron sludge was selected for pilot-scale testing due to its superior ability to enhance the treatment efficiency of the HRAS process compared to alum sludge. Iron sludge addition (20.1 ± 1.6 mg dry sludge/L wastewater) slightly improved the removal efficiency of particulate chemical oxygen demand (pCOD) from 74 % to 81 % (p-value: 0.014). Iron sludge addition had a distinct effect on the sludge characteristics of the HRAS process. The average median particle size (d50) increased from 96 ± 3 to 163 ± 14 μm (p-value<0.00) with the addition of iron sludge, which improved the settleability of the HRAS process sludge. However, the biochemical methane potential (BMP) of the HRAS process sludge decreased by 8.9 % (p-value<0.00) after iron sludge addition. In a scenario analysis of WTP sludge use in a hypothetical HRAS plant, the effluent quality index (EQI), an indicator of environmental impact, was calculated and the cost related to the operation (the transfer and landfill disposal of WTP and HRAS process sludge, energy and chemical consumption of the HRAS plant) was estimated. As a result, using WTP sludge in the HRAS plant did not significantly affect the EQI of the plant but decreased overall cost by 11 %. The results showed that the use of WTP sludge as a coagulant in wastewater treatment could achieve mutual benefits for WTPs and WWTPs and have the potential to realize the circular economy model.
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The performance of sludge and the effectiveness of wastewater treatment in various biological treatment facilities are influenced by the morphological features of activated sludge flocs. Currently, these features are assessed qualitatively through visual inspection, without statistical processing of the data. The article presents the results of quantitative determination of the strength of activated sludge flocs. A computerized method has been created to quantify the morphological characteristics of activated sludge flocs by analyzing sludge micrographs using the ImageJ software. Color and contrast indices were used to quantify the strength of the flocs. Photographs of activated sludge from the work of D. Eikelboom were used as standards for determining strength. In the calculations, we used photographs by D. Eikelboom taken at a magnification of 300 times (100 pixels) and 150 times (50 pixels). A laboratory study of the effect of Mg ions on the properties of activated sludge flocs was carried out when controlling the morphology of the flakes using the developed computerized technique. Examining the impact of Mg ions on activated sludge flocs revealed that higher magnesium concentrations in the sludge liquid result in increased linear dimensions (by 59%), floc area (by 81%) and floc volume (by 275%), but concurrently lead to a decrease in the strength of activated sludge flakes (by 21%). The obtained results in general indicated the positive effect of magnesium ion on the sedimentation properties of sludge in the aerotank-settlement system. A scale for quantitative determination of the strength of activated sludge flocs has been constructed. The developed computerized method enhances the precision and detail of visual evaluations of the technological attributes of activated sludge flocs, enabling the detection of even the smallest changes in the morphological characteristics across different wastewater treatment technologies. Keywords: activated sludge, flocs, concentration of magnesium, strength, technological characteristics.
The management of sewage sludge originated from municipal wastewater treatment plants (WWTPs) is an urgent issue. In 2019, the local authority of the Piemonte region started a survey with the aim of collecting recent data concerning wastewater and sludge management in the WWTPs located in its own territory. The survey’s results revealed that 60% of the sludge (51,000 t, as dry substance, d.s.) produced by the local WWTPs was recovered or disposed of outside of the region, and a similar amount of sludge was recovered in agriculture directly or after composting. The increase in the costs to accommodate sewage sludge in recovery or disposal plants, followed to a recent Italian Sentence (27958/2017), and the more and more stringent requirements fixed by lots of European countries for the application of sludge in agriculture, are pushing the Piemonte region authority to re-organize its own network for sludge management, with solutions based onto proximity and diversification. Whether the provisions of the current German legislation are applied in the future also in Italy, approx. 90% of sewage sludge produced into the Piemonte region should be incinerated, with a subsequent step of phosphorous recovery. The new regional plan, according to the Regional Address Deed, should consider a diversification of sludge treatment and recovery practices. On this basis, a need for new plants for around 40,000 t d.s./y could be planned.
One of the key elements in the transformation towards a circular economy (CE) is providing more sustainable practices for resources and waste management. Improvement actions focused on transformation towards a CE should be targeted at all groups of materials and waste. As water is essential for human survival and well-being and plays a significant role in sustainable development (SD), the actions related to the reuse of water and the recovery of raw materials from wastewater and other water-based waste should be taken. The paper presents a proposition for a new CE model framework in the water and wastewater sector, which includes the six following actions: reduction—prevent wastewater generation in the first place by the reduction of water usage and pollution reduction at source; reclamation (removal)—an application of effective technologies for the removal of pollutants from water and wastewater; reuse—reuse of wastewater as an alternative source of water supply (non-potable usage), recycling—recovery of water from wastewater for potable usage; recovery—recovery of resources such as nutrients and energy from water-based waste, and rethink—rethinking how to use resources to create a sustainable economy, which is `free` of waste and emissions. The novelty of the proposed CE model framework is that it presents possible ways of implementing CE principles in the water and wastewater sector, with a strong emphasis not only technological but also organisational and societal changes. Application of the proposed model may help to further transform the European economy to the CE model. Moreover, the indicated model can be significant tool supporting an assessment of local or regional progress towards CE in the water and wastewater sector and further environmental management and planning.
The principle of the conventional activated sludge (CAS) for municipal wastewater treatment is primarily based on biological oxidation by which organic matters are converted to biomass and carbon dioxide. After more than 100 years’ successful application, the CAS process is receiving increasing critiques on its high energy consumption and excessive sludge generation. Currently, almost all municipal wastewater treatment plants with the CAS as a core process are being operated in an energy-negative fashion. To tackle such challenging situations, there is a need to re-examine the present wastewater treatment philosophy by developing and adopting novel process configurations and emerging technologies. The solutions going forward should rely on the ways to improve direct energy recovery from wastewater, while minimizing in-plant energy consumption.
This book begins with a critical overview of the energy situation and challenges in current municipal wastewater treatment plants, showing the necessity of the paradigm shift from removal to recovery in terms of energy and resource. As such, the concept of A-B process is discussed in detail in the book. It appears that various A-B process configurations are able to provide possible engineering solutions in which A-stage is primarily designed for COD capture with the aim for direct anaerobic treatment without producing excessive biosludge, while B-stage is designated for nitrogen removal. Making the wastewater treatment energy self-sustainable is obviously of global significance and eventually may become a game changer for the global market of the municipal wastewater reclamation technology.
The principal audiences include practitioners, professionals, university researchers, undergraduate and postgraduate students who are interested and specialized in municipal wastewater treatment and process design, environmental engineering, and environmental biotechnology.
Water treatment residuals (WTRs) are by-products of the coagulation and flocculation phase ofthe drinking water treatment process that is employed inthe vast majority of water treatment plants globally.Production of WTRs are liable to increase as cleandrinking water becomes a standard resource. One ofthe largest disposal routes of these WTRs was via land-fill, and the related disposal costs are a key driver behindthe operational cost of the water treatment process.WTRs have many physical and chemical properties thatlend them to potential positive reuse routes. Therefore, alarge quantity of literature has been published on alter-native reuse strategies. Existing or suggested alternativedisposal routes for WTRs can be considered to fallwithin several categories: use as a pollutant and excessnutrient absorbent in soils and waters, bulk land appli-cation to agricultural soils, use in construction materials,and reuse through elemental recovery or as a wastewatercoagulant. The main concerns and limitations restrictingcurrent and future beneficial uses of WTRs arediscussed within. This includes those limitations linkedto issues that have received much research attentionsuch as perceived risks of undesirable phosphorousimmobilisation and aluminium toxicity in soils, as well
Conventional activated sludge (CAS) process is one of the most commonly applied processes for municipal wastewater treatment. However, it requires a high energy input and does not promote energy recovery. Currently, high-rate activated sludge (HRAS) process is gaining importance as a good option to reduce the energy demand of wastewater treatment and to capture organic matter for valorizing through anaerobic digestion (AD). Besides, food waste addition to wastewater can help to increase the organic matter content of wastewater and thus, energy recovery in AD. The objective of this study is to evaluate the applicability of co-treatment of municipal wastewater and food waste in a pilot-scale HRAS system as well as to test the minimal hydraulic retention times (HRTs) such as 60 and 30 min. Food waste addition to the wastewater resulted in a 10% increase in chemical oxygen demand (COD) concentration of influent. In the following stages of the study, the pilot-scale system was operated with wastewater solely under the HRTs of 60 and 30 min. With the decrease of HRT, particulate COD removal increased; however, soluble COD removal decreased. The results demonstrated that if the settling process is optimized, more particulate matter can be diverted to sludge stream.
This study performed a parallel comparison of the A-stage (adsorption) and high-rate contact-stabilization (CS) technology for carbon and nutrient redirection, operating both systems at similar sludge retention time (SRT) of 0.16-0.3 d and treating high-strength raw wastewater. Overall at the average 0.22 d SRT condition, both A-stage and CS had similar carbon capture behavior (42-43%) and thus the similar potential for energy recovery. However , the A-stage had better effluent quality (67 mg VSS/L) through the growth of more heterotrophic biomass leading to increased oxidation (22% vs 18%), and increased fraction of nitrogen (26% vs 19%) and phosphorous (36% vs 30%) redirection compared to the CS. At biomass limited conditions and at lower SRT, CS maintained better performance, potentially through a better extracellular polymeric substance management under feast-famine conditions. Full-scale plant energy calculations based on this study s results showed that chemically enhanced primary treatment (CEPT), A-stage, CS, primary treatment + CS and CEPT + CS could all lead to energy neutral plants as enough carbon can be redirected to generate the energy needed to support wastewater treatment. Given the superior performance of CS under lower loading or SRT limitation, the best energy gain (200%) could be potentially reached with a combination of CEPT and CS in series to enhance carbon capture up to 68% in combination with mainstream deammonification.
Conventional activated sludge (CAS) technology has been the most commonly applied technology for treatment of municipal wastewater for more than a century; however, a significant portion of energy content of the wastewater cannot be recovered by this technology. Therefore, different modifications can be applied to the CAS technology in order to increase the energy harvesting from wastewaters. In this paper, physically treated wastewater from a municipal preliminary wastewater treatment plant (WWTP) was treated at a pilot-scale high-rate activated sludge (HRAS) system. The HRAS system was operated under three different hydraulic retention times (HRTs) and the treatment performances were evaluated. Within this concept, HRTs of 130, 95, and 60 min were tested. The results revealed that total chemical oxygen demand (tCOD) removal of 59% was achievable and the effluent total suspended solids (TSS) concentration was 90 mg/L at the HRT of 60 min. Effluent COD and TSS concentrations decreased with the decrease in HRT by means of enhanced flocculation and sedimentation. This is confirmed by the improvement of sludge volume index (SVI) values at decreased HRTs. The calculated mass balance showed that more COD was diverted to sludge stream with decrease in HRT. Diversion of more COD to sludge can improve the energy balance of wastewater treatment by the application of anaerobic sludge digestion. By this way, HRAS process would be a promising technology in the concept of energy efficient wastewater treatment systems.
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A series of pilot-scale studies were performed to compare conventional high-rate activated sludge systems (HRAS) (continuous stirred tank reactor (CSTR) and plug flow (PF) reactor configurations) with high-rate contact-stabilization (CS) technology in terms of carbon recovery potential from chemically enhanced primary treatment effluent at a municipal wastewater treatment plant. This study showed that carbon redirection and recovery could be achieved at short solids retention time (SRT). However, bioflocculation became a limiting factor in the conventional HRAS configurations (total SRT ≤ 1.2 days). At a total SRT ≤1.1 day, the high-rate CS configuration allowed better carbon removal (52–59%), carbon redirection to sludge (0.46–0.55 g COD/g CODadded) and carbon recovery potential (0.33–0.34 gCOD/gCODadded) than the CSTR and PF configurations (28–37% COD removal, carbon redirection of 0.32–0.45 g COD/g CODadded and no carbon harvesting). The presence of a stabilization phase (famine), achieved by aerating the return activated sludge (RAS), followed by low dissolved oxygen contact with the influent (feast) was identified as the main reason for improved biosorption capacity, bioflocculation and settleabilty in the CS configuration. This study showed that high-rate CS is a promising technology for carbon and energy recovery from low-strength wastewaters.
Although the activated sludge process, one of the most remarkable engineering inventions in the 20 th century, has made significant contribution to wastewater reclamation in the past 100 years, its high energy consumption is posing a serious impact and challenge on the current wastewater industry worldwide and is also inevitably linked to the issue of global climate change. In this study, we argued that substantial improvement in the energy efficiency might be no longer achievable through further optimization of the activated sludge process. Instead, we should devote more effort to the development or the adoption of novel treatment configurations and emerging technologies. Of which an example is A-B process which can significantly improve the energy recovery potential at A-stage, while markedly reduces energy consumption at B-stage. Various configurations of A-B process with energy analysis are thus discussed. It appears highly possible to achieve an overall energy gain in WWTPs with A-B process as a core.
While landfilling provides a simple and economic means of waste disposal, it causes environmental impacts including leachate generation and greenhouse gas (GHG) emissions. With the introduction of gas recovery systems, landfills provide a potential source of methane (CH4) as a fuel source. Increasingly revegetation is practiced on traditionally managed landfill sites to mitigate environmental degradation, which also provides a source of biomass for energy production. Combustion of landfill gas for energy production contributes to GHG emission reduction mainly by preventing the release of CH4 into the atmosphere. Biomass from landfill sites can be converted to bioenergy through various processes including pyrolysis, liquefaction and gasification. This review provides a comprehensive overview on the role of landfills as a biorefinery site by focusing on the potential volumes of CH4 and biomass produced from landfills, the various methods of biomass energy conversion, and the opportunities and limitations of energy capture from landfills.
The limited available area for treatment plants in population-dense settlements increases the need for technologies with smaller footprints. The high-rate activated sludge (HRAS) process aims to capture carbon from wastewater by limiting biological assimilation with a high loading rate which enables occupying a smaller footprint. The footprint of the HRAS system can be further reduced by using lamella clarifiers instead of conventional ones. Conventional clarifiers were used in previous studies on the operational parameters of the HRAS system. This study aims to determine the optimum operational conditions in terms of hydraulic retention time (HRT) (75 and 50 min) and dissolved oxygen (DO) concentration (0.2, 0.5, and 0.8 mg/L) of a HRAS system including a lamella clarifier using 124 days data. The best effluent quality and carbon capture were observed at HRT of 75 min and DO concentration of 0.5 mg/L, which was considered the optimum condition with the highest extracellular polymeric substances (EPS) production in the reactor. The high EPS production helped flocs come together and settle faster with the highest carbon capture compared to other operational conditions. Based on the mass balance, 41.7 % of chemical oxygen demand (COD), 34 % of total nitrogen (TN), and 60 % of total phosphorus (TP) in the influent were captured into the sludge stream at the optimum condition. Lower HRT and DO concentration decreased EPS production and led to particulate COD loss through effluent and hampered carbon capture. Furthermore, higher DO concentration caused more carbon loss through oxidation.
Energy-rich sludge can be obtained from primary clarifiers preceding biological reactors. Alternatively, the incoming wastewater can be sent to a very-high-loaded activated sludge system, i.e., a so-called A-stage. However, the effects of applying an A-stage instead of a primary clarifier, on the subsequent sludge digestion for long-term operation is still unknown. In this study, biogas production and permeate quality, and filterability characteristics were investigated in a lab-scale anaerobic membrane bioreactor for primary sludge and A-stage sludge (A-sludge) treatment. A higher specific methane yield was obtained from digestion of A-sludge compared to primary sludge. Similarly, specific methanogenic activity was higher when the anaerobic membrane bioreactor was fed with A-sludge compared to primary sludge. Plant-wide mass balance analysis indicated that about 35% of the organic matter in wastewater was recovered as methane by including an A-stage, compared to about 20% with a primary clarifier.
The water-energy nexus has changed the concept of wastewater treatment plants (WWTPs), which should move from energy consumers into energy neutral or even energy positive facilities. The A/B process aims at achieving self-sufficient energy WWTPs: organic matter is removed in the first step (A-stage) and derived to biogas production whereas autotrophic nitrogen removal is implemented in a second step (B-stage). This work compares two high-rate systems that can be used as A-stage in view of organic matter removal: a continuous high rate activated sludge (HRAS) reactor and a high-rate sequencing batch reactor (HRSBR). Both systems were operated with real urban wastewater at a short hydraulic retention time (2.5 h) and at short sludge retention time (SRT) of 1–2 d to minimize COD mineralization and to maximize organic matter diversion to methane production and, hence, energy recovery. The HRAS showed higher COD removal efficiencies and better energy recovery. On the other hand, the HRSBR was better to avoid undesired nitrification and provided lower COD mineralization for all the SRTs tested (ranging 20–48% for the HRSBR, and 41–58% for the HRAS). Then, the energy as methane recovered per unit of COD degraded was higher in the HRSBR. The HRSBR seems to be a good option, because the solids content in the effluent was similar for both systems and its COD removal efficiency can be further improved by optimizing the SBR cycle configuration.
As the global economy continues to grow, the need for an economic evaluation of wastewater treatment plants (WWTPs) is increasing. Determination of cost functions (CFs) help to assess the costs of WWTP and to be able to reach to the satisfactory financial levels of construction and operation practices in the early phases of a project. In this study, unit capital and operation and maintenance (O&M) costs were calculated by analyzing the real capital and operation and maintenance expenditures of 16 full-scale WWTPs in Istanbul. Besides, the impacts of treatment level and capacity on costs were investigated. The unit total capital cost was found as 0.013 ± 0.004 €/m³ and 0.054 ± 0.009 €/m³ for preliminary and tertiary treatment, respectively, whereas the unit total O&M cost were 0.011 ± 0.007 €/m³ and 0.077 ± 0.021 €/m³ for preliminary and tertiary treatment, respectively. Capital (investment) costs covered 58% of the total cost in preliminary WWTPs, whereas; O&M costs had the highest share (58%) in tertiary WWTPs. The results of this study confirmed that the level of treatment considerably affected the costs of WWTPs. Moreover, the CFs were separately derived for tertiary treatment including A²O with and without digester. The CFs obtained in this study are of utmost importance to be used in the economic evaluation of the planned WWTPs and in the management of existing ones.
The use of coagulants and flocculants in the water and wastewater industry is predicted to increase further in the coming years. Alum is the most widely used coagulant, however, the use of ferric chloride (FeCl3) is gaining popularity. Drinking water production that uses FeCl3 as coagulant produces waste sludge rich in iron. We hypothesised that the iron-rich drinking water sludge (DWS) can potentially be used in the urban wastewater system to reduce dissolved sulfide in sewer systems, aid phosphate removal in wastewater treatment and reduce hydrogen sulfide in the anaerobic digester biogas. This hypothesis was investigated using two laboratory-scale urban wastewater systems, one as an experimental system and the other as a control, each comprising sewer reactors, a sequencing batch reactor (SBR) for wastewater treatment, sludge thickeners and anaerobic digestion reactors. Both were fed with domestic wastewater. The experimental system received in-sewer DWS-dosing at 10 mgFe L-1 while the control had none. The sulfide concentration in the experimental sewer effluent decreased by 3.5 ± 0.2 mgS L-1 as compared with the control, while the phosphate concentration decreased by 3.6 ± 0.3 mgP L-1 after biological wastewater treatment in the experimental SBR. The dissolved sulfide concentration in the experimental anaerobic digester also decreased by 15.9 ± 0.9 mgS L-1 following the DWS-dosing to the sewer reactors. The DWS-doing also enhanced the settleability of the mixed liquor suspended sludge (MLSS) (SVI decreased from 193.2 ± 22.2 to 108.0 ± 7.7 ml g-1), and the dewaterability of the anaerobically digested sludge (the cake solids concentration increased from 15.7 ± 0.3% to 19.1 ± 1.8%). The introduction of DWS into the experimental system significantly increased the COD and TSS concentrations in the wastewater, and consequently the MLSS concentration in the SBR, however, this did not affect normal operation. The results demonstrated that iron-rich waste sludge from drinking water production can be used in the urban wastewater system achieving multiple benefits. Therefore, an integrated approach to urban water and wastewater management should be considered to maximise the benefits of iron use in the system.
Novel wastewater treatment plants (WWTPs) are designed to be more energy efficient than conventional plants. One approach to becoming more energy efficient is the pre-concentration of organic carbon through chemically enhanced primary treatment (CEPT) or high-rate activated sludge (HRAS). This study compares these approaches in terms of energy demand, operational costs, organic micropollutants (OMP), and virus removal efficiency. A CEPT pilot-scale plant was operated at a hydraulic retention time (HRT) of 30 min, and a lab-scale HRAS reactor was operated at an HRT of 2 h and a solid retention time (SRT) of 1 d in continuous mode. A minimum dose of 150 mg/L ferric chloride (FeCl3) was required to achieve a threshold chemical oxygen demand (COD)-to-ammonium ratio below 2 g COD to 1 g of NH4+ -N (fulfilling the requirement for a partial nitritation-anammox reactor), reaching high phosphate (PO43-)-removal efficiency (>99%). A slightly lower COD recovery was attained in the HRAS reactor, due to the partial oxidation of the influent COD (15%). The lower PO43- removal efficiency achieved in the HRAS configuration (13%) was enhanced to a comparable value of that achieved in CEPT by the addition of 30 mg/L FeCl3 at the clarifier. The CEPT configuration was less energy-intensive (0.07 vs 0.13 kWh/m3 of wastewater) but had significantly higher operational costs than the HRAS-based configuration (6.0 vs 3.8 c€/m3 of wastewater). For OMPs with kbiol > 10 L/gVSS·d, considerably higher removal efficiencies were achieved in HRAS (80-90%) than in CEPT (4-55%). For the remaining OMPs, the biotransformation efficiencies were generally higher in HRAS than in CEPT but were below 55% in both configurations. Finally, CEPT was less efficient than HRAS for virus removal. HRAS followed by FeCl3 post-treatment appeared to be a more effective alternative than CEPT for COD pre-concentration in novel WWTPs.
In the last decades, energy scarcity has become an important issue globally. Renewable energy sources have gained importance due to limited fossil fuel reserves and increased concerns on climate change. In this regard, municipal wastewater is a remarkable energy source since huge amounts of wastewater are generated and treated all over the world every day. Conventional activated sludge (CAS) process, which has been in use for more than a century, is the most widely applied treatment method for municipal wastewater. In spite of its reliability and proven success, CAS process suffers from intensive energy requirement and lack of capability to capture a high amount of organic matter from wastewater. In order to recover the energy present in wastewater efficiently, it is crucial to up-concentrate the organics in wastewaters. Several physicochemical and biological processes may be employed for up-concentration of organics and capturing them onto sludge. Capturing of organic matter in sludge phase allows improved energy recovery through anaerobic digestion. This study aims to present a comprehensive evaluation of the current practices applied to up-concentrate organic matter in municipal wastewater. The paper discusses the most frequently used up-concentration methods by addressing lab-scale and full-scale applications with typical operational parameters as well as providing their strengths and constraints. In addition, various up-to-date treatment configurations are introduced in order to provide a future perspective in this field.
The addition of water treatment chemicals has always been considered as a standard operation in water and wastewater treatment. The concentration of chemicals was usually kept to the minimum necessary to achieve a good quality of potable or otherwise treated water. A significant interruption to the status-quo occurred more than 20 years ago after a severe and highly publicized outbreak of Cryptosporidium parvum oocysts. The strategic planning after the outbreak was to shift from physical-chemical to physical treatment methods, such as membrane filtration and UV disinfection. As such, the new procedures were supposed to eliminate the threat of water contamination through a minor addition of chemicals. Such was the mistrust and disappointment with water treatment chemicals themselves.
Indeed, water treatment technologies are now using novel physical treatment methods. Membranes largely replaced granular filtration, and UV is paving the way towards minimization or elimination of the use of classic disinfection chemicals, such as chlorine and its derivatives. Yet, far from the “high-tech” revolution in water treatment technologies actually reducing the use of chemicals, the latter has in fact been significantly increased. The “conventional” chemicals used for pre-treatment, disinfection, corrosion prevention, softening and algae bloom depression are all still in place. Furthermore, new groups of chemicals such as biocides, chelating agents and fouling cleaners are currently used to supplement them. These latter are the chemicals needed to protect the high-tech equipment, to optimize the treatment, and to clean the equipment between uses.
The health effects of the new chemicals introduced into water are yet to be fully established. Typically, a higher treatment efficiency requires effective chemicals, yet these are not always environmentally friendly. It seems obvious that the “high-tech” revolution currently affects the sustainability of water resources, and certainly not in a completely positive way. In short, the adverse effects of the introduction of such a significant amount of treatment chemicals into our sources of water are yet to be evaluated.
This study evaluated high-rate activated sludge treatment across a broad range of short solids retention times (SRT)s (0.5-3d) and found a strong SRT-outcome dependence for performance and subsequent anaerobic degradability of the sludge. Up to 50% total nitrogen, and 35% ammonia removal was also achieved at the longer SRTs, via partitioning rather than reaction. The aerobic SRT significantly affected the anaerobic degradability of the sludge produced (p<0.001), with degradability increasing from 66% to over 80% while reducing the SRT from 3d to 0.5d. This is higher than predicted by conventional models, likely due to additional mechanisms such as adsorption and storage, not included in these.
Green chemists paid much more attention towards the alternative ways to reutilize waste materials instead of its disposal in a non-ecofriendly manner. In this study, drinking-water treatment sludge (DWTS), which is a by-product resulted from drinking water treatment plants, was successfully applied as an adsorbent for Pb(II), Cd(II) and Ni(II) removal from wastewater. The physicochemical characteristics of DWTS were investigated using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and N2 adsorption-desorption isotherms.
The XRD analysis revealed that the DWTS under study consists of quartz and illite phases which had been reported for their adsorption efficiency. Firing of DWTS at 500 °C causes the appearance of albite phase in addition to previous ones which enhances the adsorption capacity of these materials. The influence of different parameters such as firing temperature of DWTS, contact time, pH, DWTS dose and initial metal ions concentration on the adsorption of heavy metal ions and, consequently, on their removal were investigated. DWTS exhibit an adsorption efficiency towards Pb(II) > Cd(II) > Ni(II). The extremely high efficiency of DWTS towards Pb(II) adsorption can nominate it as a specific low-cost adsorbent for Pb ions.
FeCl3 and PACl as coagulants in chemically enhanced primary sedimentation (CEPS) were compared in terms of their efficiencies in removing pollutants from wastewater and their effects on the acidogenic fermentation of CEPS sludge for resource recovery. PACl was found to be more effective than FeCl3 for removing suspended solids by CEPS, with around 20% higher removal efficiency. However, the coagulated Al-sludge experienced more difficulty and had lower efficiency than Fe-sludge in organic hydrolysis and acidogenesis. The batch fermentation results showed that FeCl3 dosed at 10 to 30 mg Fe/Lsewage had little influence on sludge hydrolysis and volatile fatty acid (VFA) production, whereas an obvious inhibitory effect was observed for PACl in organic hydrolysis of the sludge. The specific hydrolysis rate constant (Kh,P) for sludge fermentation decreased from 0.0321 for the sludge without PACl to 0.017 for the Al-sludge obtained at a dosage of 24 mg Al/Lsewage. Compared to the Al-sludge, the Fe-sludge had a much higher VFA yield and significant PO4–P release during the sludge fermentation, which is attributed to the reduction of Fe(III) to Fe(II) under anaerobic conditions and the resulting disintegration of sludge flocs. By simple pH adjustment of the fermented Fe-sludge supernatant, up to 31% of the phosphorus in wastewater can be recovered in the form of vivianite as P fertilizers. VFAs produced in the supernatant are valuable organic carbon resources that can be recovered for beneficial uses.
The main objective of the present review is to compare the existing sewage sludge management solutions in terms of their environmental sustainability. The most commonly used strategies, that include treatment and disposal has been favored within the present state-of-art, considering existing legislation (at European and national level), characterization, ecotoxicology, waste management and actual routs used currently in particular European countries. Selected decision making tools, namely End-of-waste criteria and Life Cycle Assessment has been proposed in order to appropriately assess the possible environmental, economic and technical evaluation of different systems. Therefore, some basic criteria for the best suitable option selection has been described, in the circular economy “from waste to resources” sense. The importance of sewage sludge as a valuable source of matter and energy has been appreciated, as well as a potential risk related to the application of those strategies.
Sludge from drinking water treatment plants is usually disposed of in landfill sites as nonhazardous waste. However, a review of the legislation at European, Spanish, and regional levels identifies uncertainty regarding this type of waste. For example, it is unclear whether the drinking water sludge can be sent to inert landfill sites, which would be less expensive, or whether it could be reused for other purposes. According to Article 5 of Spanish Law 22/2011 on waste and contaminated soil, various physicochemical and biological parameters must be determined before alternative uses can be established. At present, sludge from drinking water treatment plants is wrongly associated with sludge from wastewater treatment plants. In this study, parameters of the sludge produced in drinking water treatment plants that may lead to a new waste status for the same are analyzed. The principal problems that may limit its reuse are its low organic matter and high silica contents.
Current practice of wastewater treatment does not recover the full potential of energy present in wastewater. The potential of using anammox bacteria for autotrophic nitrogen removal combined with a desire for energy optimization brings new attention to the A-stage technology for organic carbon harvesting from municipal wastewater. The goal of this research was to investigate operational conditions of four full-scale A-stage processes and gain insight in the optimal conditions to harvest the maximum amount of organics present in sewage as excess sludge from the A stage. Large differences in removal efficiencies and design aspects were found between the four operational A-stage processes in the Netherlands. Biochemical oxygen demand (BOD) removal efficiencies vary between 40% and 80%, indicating that a good removal efficiency is possible, but that local conditions or design can be very influential. An optimal solid retention time (SRT) for maximal sludge production of 0.3 days was found; a longer SRT resulted in more mineralization of the chemical oxygen demand (COD). SRT control might be an important design aspect for the optimization of A-stage process. A short contact time with a minimum of 15 min and sufficient aeration were found to be optimal for soluble COD removal. Iron addition aided the removal of colloidal/suspended COD by coagulation/flocculation. Sludge flocs formed in the A-stage process are weak and sensitive to anaerobic conditions as well as shear due to, for example, pumping. Besides a good design of the A-stage itself, the further processing of the produced sludge also needs careful attention to optimize the sludge production and energy production.
Wastewater treatment, a great potential alternative to alleviate water shortage, has been attached more and more importance in China, and has been developing very fast. The quantity of wastewater treatment plants in China has increased up to 3272 in June 2013, and has a total handling capacity of 0.14 billion t/day. However, wastewater treatment requires to consuming a lot of energy, and even energy consumption is often the main operation cost of wastewater treatment systems. Thus, it is very necessary to explore energy consumption of wastewater treatment systems and its influential factors, and seek for some possible pathways to save energy and lower cost. In this paper, we investigated the average energy consumption per unit wastewater treatment in Shenzhen, and analyzed the effect of treatment capacity and treatment technology on the energy cost per unit of wastewater treatment. The results showed that the average energy consumption of wastewater treatment plants in Shenzhen was about 0.20±0.06 kWh/t, much less than those in such developed countries as USA, Germany and Japan. This result may be related to the advanced wastewater treatment plants newly constructed and the low water quality requirements of wastewater treatment in Shenzhen. As the key to wastewater treatment, biochemical treatment sub-process consumed 50-70% of total energy cost in wastewater treatment. Secondly, the larger the treat capacity, the lower the energy cost per unit of wastewater treatment was. And the difference of treatment technologies can also significantly affect the energy consumption per unit of wastewater treatment. Finally, labor cost and electricity consumption respectively covered about 30.1% and 26.3% of total economic cost in the three typical wastewater treatment plants in original Shenzhen. Thus, upgrading treatment machines & equipment and improving management level are two effective alternatives to decrease energy consumption and lower total economic cost of wastewater treatment plants in Shenzhen and even China.
Domestic and industrial sludge generated at wastewater treatment facilities is considered a potential biomass source for producing biodiesel. However, transportation of large amounts of sludge from wastewater treatment facilities to a biorefinery is expensive. The objective of this paper is to identify the proper transportation mode to use as a function of the volume shipped and transportation distances. Currently, sludge is mainly shipped by truck and pipeline. We estimated that the fixed and variable cost components of pipeline transportation for a volume such as 480 m3/day and a distance of 100 miles are $0.116/m3 and $0.089/m3/mile, respectively. We estimated the biomass (sludge) transportation cost per gallon of biodiesel, and observed the changes in these costs as a function of distance traveled and volume shipped. The outcomes of this study have the potential to help biofuel plants make better biomass transportation decisions, and consequently reduce the price of biodiesel significantly.
In the present study, feasibility of recovering the coagulant from water treatment plant sludge with sulphuric acid and reusing it in post-treatment of upflow anaerobic sludge blanket (UASB) reactor effluent treating municipal wastewater were studied. The optimum conditions for coagulant recovery from water treatment plant sludge were investigated using response surface methodology (RSM). Sludge obtained from plants that use polyaluminium chloride (PACl) and alum coagulant was utilised for the study. Effect of three variables, pH, solid content and mixing time was studied using a Box-Behnken statistical experimental design. RSM model was developed based on the experimental aluminium recovery, and the response plots were developed. Results of the study showed significant effects of all the three variables and their interactions in the recovery process. The optimum aluminium recovery of 73.26 and 62.73 % from PACl sludge and alum sludge, respectively, was obtained at pH of 2.0, solid content of 0.5 % and mixing time of 30 min. The recovered coagulant solution had elevated concentrations of certain metals and chemical oxygen demand (COD) which raised concern about its reuse potential in water treatment. Hence, the coagulant recovered from PACl sludge was reused as coagulant for post-treatment of UASB reactor effluent treating municipal wastewater. The recovered coagulant gave 71 % COD, 80 % turbidity, 89 % phosphate, 77 % suspended solids and 99.5 % total coliform removal at 25 mg Al/L. Fresh PACl also gave similar performance but at higher dose of 40 mg Al/L. The results suggest that coagulant can be recovered from water treatment plant sludge and can be used to treat UASB reactor effluent treating municipal wastewater which can reduce the consumption of fresh coagulant in wastewater treatment.
This paper describes the implementation of a simulation benchmark for studying the influence of control strategy implementations on combined nitrogen and phosphorus removal processes in a biological wastewater treatment plant. The presented simulation benchmark plant and its performance criteria are to a large extent based on the already existing nitrogen removal simulation benchmark. The paper illustrates and motivates the selection of the treatment plant lay-out, the selection of the biological process model, the development of realistic influent disturbance scenarios for dry, rain and storm weather conditions respectively, the definition of performance indexes that include the phosphorus removal processes, and the selection of a suitable operating point for the plant. Two control loops were implemented: one for dissolved oxygen control using the oxygen transfer coefficient KLa as manipulated variable, the second one for nitrate control in the anoxic zone using the internal recirculation flow rate as manipulated variable. Dynamic simulations for different dissolved oxygen set points illustrate the complex interactions in this plant, and the necessity for a continuous trade off between supplying sufficient oxygen to promote nitrification on the one hand, and the need for low dissolved oxygen concentrations on the other hand to allow sufficient development of phosphorus accumulating organisms. The potential for aeration energy savings in the plant is highlighted based on the dissolved oxygen profiles resulting from open loop simulations with a dynamic dry weather influent scenario. The influence of the dissolved oxygen set point selection on the nitrate control loop performance observed in the simulations further illustrates the need for a plant-wide optimization approach to reach optimal plant performance.
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.
Wastewater treatment including high rate anammox processes have the potential to become energy-neutral or even energy-producing.
This paper attempted to study the feasibility of reusing water treatment works sludge ("alum sludge") to improve particulate pollutant removal from sewage. The main issues focused upon were: (1) the appropriate dosage of the alum sludge, (2) the appropriate operating conditions, and (3) the possible mechanisms for enhancement by adding alum sludge. Actual alum sludge and sewage were applied to a series of jar tests conducted under various conditions. It has been found that both the SS and COD removal efficiencies could be improved by the addition of the alum sludge, which was mainly attributed to the removal of relatively fine particles with a size of 48-200 microm. The appropriate dosage of the alum sludge was determined to be 18-20 mg of Al/L. Increasing the mixing speed or reducing the floc size of the alum sludge enhanced the SS and COD removal and the dispersed alum sludge could remove particulate contaminants with smaller size than the raw sewage. ToF-SIMS evidence revealed that the aluminum species at the surface of the alum sludge were effectively utilized for improving the SS and COD removal. It was postulated that the sweep flocculation and/or the physical adsorption might play key roles in the enhancement of particulate pollutant removal from sewage.
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