Low Sludge Research Articles

Improving the quality of produced water discharged in open waters by incorporating a microbiological treatment on operating offshore platforms

Produced water is the water that comes along with oil during the reservoir extraction and is the main subproduct of the petroleum industry, accounting for 3 to 5 times the global production of oil. The produced water separated from the oil still contains some oily fractions in its composition, making it necessary for some treatment before its discard (most common practice adopted on offshore platforms) or reuse. The traditional method of treatment is based on the physical separation of oil and water, driven by their density difference. This type of treatment is unable to remove dissolved or small diameter particles dispersed, which turned out to be an issue in face of some new legislation worldwide that lowered the maximum amount of total oil and grease (TOG) the water can have for its discharge. Biological treatment is a widespread method for treating wastewater in several different industries due to its relative simplicity and low operational costs. Nevertheless, as it is a treatment that normally requires large-volume vessels, it is not used on offshore platforms due to a lack of space for such high dimension equipment. The objective of this research was to carry out a literature review using the article and patent search bases to find technical options that allow the use of biological treatment for water produced on offshore oil production platforms. Next, propose a model to incorporate biological treatment and estimate the performance of this produced water treatment model. An arrangement was proposed to convert a cargo tank of an operating offshore oil platform into a microbiological reactor. The conversion could be useful to treat the produced water that has a concentration of oil above the maximum limit allowed to discharge or to promote the additional extraction of a fraction of oil that could not be removed by the conventional treatment plants. The objective of this research was to perform a literature review to find technical options that could allow the use of biological treatment for the produced water in offshore oil-producing platforms, to propose a model to incorporate the biological treatment, and to estimate the performance of this model treating produced water. After researching articles and patents in the literature, it was seen that the Upflow Anaerobic Sludge Blanket (UASB) was the best type of reactor to be incorporated into an offshore platform. The reactor was chosen since it had good performance in treating produced water anaerobically and low sludge production. The UASB reactor most resembled the arrangement of a cargo tank, which was a paramount factor for the feasibility of the process's success. It is expected that the cargo tank converted to an UASB could remove at least an extra 30% of the total oil and grease (TOG) when compared to the traditional physical removal process existent for the produced water. However, it is still necessary to perform some laboratory tests to better predict the removal rate that would be observed in using this microbiological treatment with the high salinity-produced water from pre-salt Brazilian fields.

Regeneration of Used Lube Oil by using Solvent Extraction Method

Regenerated or recycle of oil is economical process and reduce the environmental pollution produce from the used lube oil. Regenerated of used oil with solvent extraction shows the yield up to 90 % which is more than the oil regenerated from the different acid. In the solvent extraction we need separate column for solvent recovery. The major benefit of solvent extraction process gives higher yield and high quality product compares with acid treatment process and also reduce the sludge generation problem that produce by acid method. That will be reduce the degree and nature of contamination, environmental/health risks associated with disposal. With help of Activated charcoal/alumina or silica gel can removes the color and various impurities presents in the recycled used oil by using different acids. The major drawback to the acid/clay method is difficulty of removal of clay sludge. MEK are 62%, 68% and 72% at feed to solvent ration 1:2, 1:4 and 1:6 settling time 24 hr., extraction 30-40 mins at temperature of 35-45 OC. As per analysis the optimum feed to solvent ration will be 1:4 in which yield of oil and its properties are satisfactory for further use of oil. As per analysis with MEK solvent regenerated oil gives highest yield or recovery of lube oil from waste oil. As per analysis the optimum feed to solvent ration will be 1:4 in which yield of oil and its properties are satisfactory for further use of oil. Solvent extraction process cost reducing, less sludge producing and high yield process. All there solvents have better yield and high recoverability by simple distillation process. By experimental analysis it’s clear that the solvent extraction process for regeneration of lube oil is easy, simple, cost effective and low sludge producing. Additionally use of alumina or silica gel are beneficial for removal of impurities and color from regenerated oil.

Densification of activated sludge for better settling performances: experimental characterization in batch column and model parameters calibration.

Conventional activated sludge (CAS) and densified sludge obtained using hydro-cyclone selective wasting were compared at a full-scale water resources recovery facility. The densified tested sludge, containing around 30-50% of aerobic granules, showed enhanced settleability with low and stable sludge volume index (SVI) compared to CAS, which suffered recurrent filamentous bulking. Further in-depth batch settling tests were carried out using a 40 cm diameter column fitted with ultrasonic transducers to monitor both sludge blanket height and vertical velocity profiles. Hindered settling and compression parameters were calibrated from the experiment for latter modelling use. Test sludge displayed more than doubled settling velocities compared to CAS, with hindered settling velocities remaining >3 m·h-1 even at high solids concentrations of 6.85 g·L-1. The compression regime was attained at much higher critical concentration for the test sludge. It also displayed enhanced thickening properties, with concentrations obtained after 30 min of settling being 20.9 and 8.5 g·L-1 respectively for test and control sludge. This allows for a substantial reduction of recirculation rates in practice. These results open perspectives in optimizing existing plant operation as well as clarifier design and modelling using densified sludge.

Efficient aerobic granular sludge production in simultaneous feeding and drawing sequencing batch reactors fed with low-strength municipal wastewater under high organic loading rate conditions

To establish rapid and efficient aerobic granular sludge (AGS) production for augmentation strategy to mainstream activated sludge process, AGS production was evaluated under high organic loading rate (OLR) conditions in a side-stream simultaneous feeding and drawing sequencing batch reactor (SFD-SBR) fed with low-strength municipal wastewater. Specifically, three SFD-SBRs were operated under different OLRs (≥ 1.3 kg-tCOD m−3 d−1) with a 100% volume exchange ratio (VER). Rapid granulation (SVI30 < 80 mL g−1 and granule fraction of ~ 70%) was achieved within 40 days in all reactors. However, an excessively high OLR (2.0 kg-tCOD m−3 d−1) tended to labilize the AGS settleability (SVI5 of ~ 120 mL g−1) despite the larger AGS size due to the outgrowth of fluffy flocs and filamentous bacteria. Furthermore, after granulation, application of a long settling time (10 min) to maintain a low sludge blanket level during the feeding period was key for stable and efficient AGS production (0.17 g-SS g-tCOD−1) under a high OLR of 1.6 kg-tCOD m−3 d−1 compared to AGS production with a short settling time (0.10 g-SS g-tCOD−1). Taken together, this study demonstrated that application of low selection pressure and moderate up-flow linear velocity enables stable and efficient AGS production in SFD-SBRs fed with low-strength municipal wastewater under high VER and OLR conditions.

Optimization of Fenton process for concurrent COD removal and lower sludge production: Process intensification and impact of reagents dosing mode

The versatile applications of the advanced oxidation processes (AOPs) are considered promising options for wastewaters treatment. Among AOPs, the classical Fenton process (CFP) is well recognized in literature due to its notable advantages. But the drawbacks associated with the CFP are the main barrier to its extensive applications. The primary process's drawbacks are the high sludge production and high reagents requirement, making the process unsustainable. Hence, the present study aims to overcome these drawbacks and improve the process's performance. The reagents (Fe2+ and H2O2) dosage were adopted as the variable parameters and varied throughout the experiments at fixed pH 3 and a reaction time of 45 min. The experiments were performed in three different stages; stage I (S–I: single-step reagents dosing mode), stage II (S-II: two-step reagents dosing mode), and stage III (S-III: three-step reagents dosing mode) and landfill leachate was taken as a sample. The dosing mode of reagents was found to be a crucial influencing factor to improve the overall Fenton process's performance by effectively utilizing the hydroxyl radicals (•OH) and avoiding the scavenging reactions. The two-step reagents dosing mode was found better, i.e., 52 ± 2% chemical oxygen demand (COD) removal and 40 ± 2 mL sludge production compared to S–I and S-III with approximately 10–15% extra COD removal and low sludge production at fewer reagents dosage (Fe2+ dosage 798 mg/L and H2O2 dosage 2098 mg/L). The lesser sludge production reduces secondary environmental pollution, and low reagents dosage makes the process cost-effective. The optimization conditions were obtained by response surface methodology (RSM), and high coefficient of determination (R2 > 0.95) values confirm the adequacy of the models obtained. Furthermore, few experiments were performed on the sludge reusability, and valuable remarks were highlighted. Overall, the purpose of this study is to enhance the CFP's performance by overcoming its drawbacks.

Automated Monitoring of a High-Speed Flocculation Flat-Bottomed Sludge Blanket Clarifier Pond during Drought and Flood Conditions

The occasional rains that occur during dry seasons often stir up the bottom sediment of reservoirs, which leads to high turbidity and electrical conductivity in raw waters received by water utility companies. A newly developed real-time precision multi-layer sensor (RPMS) system was used to monitor a high-speed flocculation flat-bottomed sludge blanket clarifier (HFSBC) pond in real time to solve the water quality problems caused by drought and flood conditions. The RPMS is capable of monitoring the thickness of a sludge blanket; if the coagulation and sedimentation processes of the HFSBC are not working well, the sludge blanket will be thin and have a low sludge concentration. Conversely, if the HFSBC is working properly, the sludge blanket will have a thick and highly concentrated layer of sludge. Any heavy metals that are not removed by water treatment processes will enter the water supply network, which will result in poor water quality for end users. Against the backdrop of intensifying climate change, the enhancement of automated monitoring systems and adaptation processes will be an important part of efforts to minimize and resolve acute changes in water quality.

Techno-Economic Evaluation of Ozone Application to Reduce Sludge Production in Small Urban WWTPs

In Chile, small wastewater treatment plants (WWTPs) (treatment capacity of less than 4,800 m3/d) are normally not designed with consideration for the potential valorization of generated sludge. For this reason, they are generally operated at high solids residence times (SRT) (15 d) to promote the decay of biomass, promoting less sludge production and reducing the costs associated with biomass management. Operation at high SRT implies the need for a larger activated sludge system, increasing capital costs. The implementation of a sludge-disintegration unit by ozonation in future WWTPs could enable operation at an SRT of 3 d, with low sludge generation. In this work, we evaluate how the implementation of a sludge-ozonation system in small WWTPs (200–4000 m3/d) would affect treatment costs. Four scenarios were studied: (1) a current WWTP operated at an SRT of 15 d, without a sludge ozonation system; (2) a WWTP operated at an SRT of 15 d, with a sludge-ozonation system that would achieve zero sludge production; (3) a WWTP operated at an SRT of 3 d, with a sludge-ozonation system that would provide the same sludge production as scenario 1; (4) a WWTP operated at an SRT of 15 d, with a sludge-ozonation system that would achieve zero sludge production. Economic analysis shows that the treatment costs for scenarios 1 and 2 are similar, while a reduction in cost of up to 47% is obtained for scenarios 3 and 4.

UASB Performance and Perspectives in Urban Wastewater Treatment at Sub-Mesophilic Operating Temperature

UASBs present several advantages compared to conventional wastewater treatment processes, including relatively low construction cost facilities, low excess sludge production, plain operation and maintenance, energy generation in the form of biogas, robustness in terms of COD removal efficiency, pH stability, and recovery time. Although anaerobic treatment is possible at every temperature, colder climates lead to lower process performance and biogas production. These factors can be critical in determining the applicability and sustainability of this technology for the treatment of urban wastewater at low operating temperature. The purpose of this study is the performance evaluation of a pilot-scale (2.75 m3) UASB reactor for treatment of urban wastewater at sub-mesophilic temperature (25 °C), below the optimal range for the process, as related to biogas production and organic matter removal. The results show that, despite lower methane production and COD removal efficiency compared to operation under ideal conditions, a UASB can still achieve satisfactory performance, and although not sufficient to grant effluent discharge requirements, it may be used as a pretreatment step for carbon removal with some degree of energy recovery. Options for UASB pretreatment applications in municipal WWTPs are discussed.

A review on electrocoagulation process for the removal of emerging contaminants: theory, fundamentals, and applications.

Electrocoagulation (EC) is an excellent and promising technology in wastewater treatment, as it combines the benefits of coagulation, flotation, and electrochemistry. During the last decade, extensive researches have focused on removal of emerging contaminants by using electrocoagualtion, due to its several advantages like compactness, cost-effectiveness, efficiency, low sludge production, and eco-friendness. Emerging contaminants (ECs) are micropollutants found in trace amounts that discharging into conventional wastewater treatment (WWT) plants entering surface waters and imposing a high threat to human and aquatic life. Various studies reveal that about 90% of emerging contaminants are disposed unscientifically into water bodies, creating problems to public health and environment. The studies on removal of emerging contaminants from wastewater are by global researchers are critically reviewed. The core findings proved that still more research required into optimization of parameters, system design, and economic feasibility to explore the potential of EC combined systems. This review has introduced an innovative collection of current knowledge on electro-coagulation for the removal of emerging contaminants.

How to Produce an Alternative Carbon Source for Denitrification by Treating and Drastically Reducing Biological Sewage Sludge.

Minimizing the biological sewage sludge (BSS) produced by wastewater treatment plants (WWTPs) represents an increasingly difficult challenge. With this goal, tests on a semi-full scale Thermophilic Alternate Membrane Biological Reactor (ThAlMBR) were carried out for 12 months. ThAlMBR was applied both on thickened (TBSS) and digested biological sewage sludge (DBSS) with alternating aeration conditions, and emerged: (i) high COD removal yields (up to 90%), (ii) a low specific sludge production (0.02–0.05 kgVS produced/kgCODremoved), (iii) the possibility of recovery the aqueous carbon residue (permeate) in denitrification processes, replacing purchased external carbon sources. Based on the respirometric tests, an excellent biological treatability of the permeate by the mesophilic biomass was observed and the denitrification kinetics reached with the diluted permeate ((4.0 mgN-NO3−/(gVSS h)) were found comparable to those of methanol (4.4 mgN-NO3−/(gVSS h)). Moreover, thanks to the similar results obtained on TBSS and DBSS, ThAlMBR proved to be compatible with diverse sludge line points, ensuring in both cases an important sludge minimization.

Membrane bioreactors for hospital wastewater treatment: recent advancements in membranes and processes.

Discharged hospital wastewater contains various pathogenic microorganisms, antibiotic groups, toxic organic compounds, radioactive elements, and ionic pollutants. These contaminants harm the environment and human health causing the spread of disease. Thus, effective treatment of hospital wastewater is an urgent task for sustainable development. Membranes, with controllable porous and nonporous structures, have been rapidly developed for molecular separations. In particular, membrane bioreactor (MBR) technology demonstrated high removal efficiency toward organic compounds and low waste sludge production. To further enhance the separation efficiency and achieve material recovery from hospital waste streams, novel concepts of MBRs and their applications are rapidly evolved through hybridizing novel membranes (non hydrophilic ultrafiltration/microfiltration) into the MBR units (hybrid MBRs) or the MBR as a pretreatment step and integrating other membrane processes as subsequent secondary purification step (integrated MBR-membrane systems). However, there is a lack of reviews on the latest advancement in MBR technologies for hospital wastewater treatment, and analysis on its major challenges and future trends. This review started with an overview of main pollutants in common hospital waste-water, followed by an understanding on the key performance indicators/criteria in MBR membranes (i.e., solute selectivity) and processes (e.g., fouling). Then, an in-depth analysis was provided into the recent development of hybrid MBR and integrated MBR-membrane system concepts, and applications correlated with wastewater sources, with a particular focus on hospital wastewaters. It is anticipated that this review will shed light on the knowledge gaps in the field, highlighting the potential contribution of hybrid MBRs and integrated MBR-membrane systems toward global epidemic prevention.

A review of the bioelectrochemical system as an emerging versatile technology for reduction of antibiotic resistance genes.

Antibiotic contamination and the resulting resistance genes have attracted worldwide attention because of the extensive overuse and abuse of antibiotics, which seriously affects the environment as well as human health. Bioelectrochemical system (BES), a potential avenue to be explored, can alleviate antibiotic pollution and reduce antibiotic resistance genes (ARGs). This review mainly focuses on analyzing the possible reasons for the good performance of ARG reduction by BESs and potential ways to improve its performance on the basis of revealing the generation and transmission of ARGs in BES. This system reduces ARGs through two pathways: (1) the contribution of BES to the low selection pressure of ARGs caused by the efficient removal of antibiotics, and (2) inhibition of ARG transmission caused by low sludge yield. To promote the reduction of ARGs, incorporating additives, improving the removal rate of antibiotics by adjusting the environmental conditions, and controlling the microbial community in BES are proposed. Furthermore, this review also provides an overview of bioelectrochemical coupling systems including the BES coupled with the Fenton system, BES coupled with constructed wetland, and BES coupled with photocatalysis, which demonstrates that this method is applicable in different situations and conditions and provides inspiration to improve these systems to control ARGs. Finally, the challenges and outlooks are addressed, which is constructive for the development of technologies for antibiotic and ARG contamination remediation and blocking risk migration.

Challenges and development directions of membrane bioreactors operated on passenger ships in international shipping

In membrane bioreactor (MBR) technology, the activated sludge method is integrated with the separation of solid particles by ultrafiltration (UF). The technology ensures a high effluent quality, a shortened hydraulic retention time and a long sludge age that promotes slowly growing microorganisms and low sludge production. These advantages and the modular construction mean that MBRs have started to treat wastewater generated on passenger ships to adjust the treatment systems to the International Convention for the Prevention of Pollution from Ships. The aim of this paper is to present operational aspects of MBRs treating wastewater generated on ships, which are different from the aspects of MBR operation on land. This paper describes the consequences of separate treatment of gray wastewater (from showers, washing machines and kitchens) and black wastewater (from toilets), and of discontinuous flow of wastewater resulting from very high variability in the passenger numer and the use of the MBR as a ship ballast element. The possibility of introducing a water recovery technology using the existing infrastructure on passenger ships as well as the hybrid UF/reverse osmosis technology is presented. The findings demonstrated that gray effluent may be reused for marine main engine cooling jackets of high and low temperature, ship boilers or ship laundry.

MgAl-LDH@Fe3O4 based heterogeneous electro-Fenton process for the concentrated liquor of gas field wastewater treatment: Optimization, mechanism, and stability

The concentrated liquor of gas field wastewater (CL-GFW) obtained by cryogenic multi-effect distillation is high-concentration organic wastewater. The MgAl-LDH@Fe3O4 particles were successfully synthesized as the catalysts of the heterogeneous electro-Fenton (EF) system to effectively degrade refractory organic pollutants in the CL-GFW. The particles broadened the pH range of the EF process under optimal conditions. The highest COD removal efficiency could achieve 88.18% after 4 h with pH of 5. The mesoporous particles were systematically studied via a series of characterization techniques. The heterogeneous degradation mechanisms were explored by radical quenching experiments, variation of H2O2 and Fe ion concentrations, and X-ray photoelectron spectroscopy (XPS) analysis. The results indicated that the synergistic effects of the active free radical oxidation, enhanced catalysis, and surface complexation contributed to the degradation of recalcitrant organics in the heterogeneous EF process. Electrocatalysis and homogeneous EF trials exhibited lower COD degradation rates of 45.68% and 62.86% comparing with the heterogeneous EF. The representative refractory organics before and after the heterogeneous EF reaction were identified by GC-MS and HPLC-MS methods. The excellent stability and reusability of the particles in the heterogeneous EF system were verified through five cyclic experiments with low sludge production and metal leaching rate.

Recent advancements in the removal/recovery of toxic metals from aquatic system using flotation techniques

The paramount cause of water scarcity is pollution, which is becoming a massive issue since the last century. Besides, it is evident that water pollution is the main cause of emerging contaminants that are left untreated from industries, can cause serious threats to humans and biota as well. One of the best ways in remediating pollutants and finding a way for generating useable water is to use this contaminated water after the necessary treatment. Heavy metals are of major concern in treatment because of their toxicity, non-biodegradability, carcinogenicity, and they can cause inevitable damages even at low concentrations. In this review article, available different flotation techniques are discussed to address this issue. Flotation tends to be one of the promising techniques that have shown a high scope because of its high produce, low sludge formation, and ease of operation. From the several pieces of literature, it can be inferred that the flotation process can be conducted in one step, and that does not need any expensive materials. Further, this paper deliberates the versatility of each process in disclosing its advantages, limitations, further scope of research and fills the loopholes in the process for better effectiveness. Overall, flotation is a highly probable as well as effective treatment technology to eradicate noxious pollutants present in wastewater and thus helps to compromise environmental and social sustainability.

Organic matter and total nitrogen removal from wastewater using a pilot-scale membrane-aerated biofilm reactor

A pilot-scale membrane-aerated biofilm reactor (MABR) modified with Polyvinyl Alcohol (PVA) gel bio-carriers containing denitrifying bacteria was operated with the purpose of enhancing total nitrogen removal efficiency from actual domestic wastewater. MABR system was operated under 6, 9 and 12 h of hydraulic retention times (HRT) in different time periods. Addition of the bio-carriers increased nitrification and total nitrogen removal efficiencies by maximum of 11% and 18% at 12 h of HRT respectively. Maximum of 88% of nitrification and 79% of total nitrogen removal efficiencies were reported in the PVA gel modified MABR. Organic matter removal performance was mostly maintained above 90% with insignificant improvements. In addition to this, natural pH increase observed after PVA gel addition paves a new way for potential cost reduction in industrial scale MABR applications. Low sludge volume index around 80 mL/g indicated rapid settling characteristics of produced sludge.

Phosphorus and carbon solubilization strategies for wastewater sludge valorisation

Phosphorus and carbon solubilisation from wastewater sludges is a difficult task due to specific sludge characteristics and low biodegradability of sludges. This work evaluates operational strategies to solubilise phosphorus and/or carbon from different municipal sludges in a bioacidification reactor operated at low sludge retention time (SRT) compared to conventional digestors, with the objective of valorising sludge and recovering resources onsite of a water resource recovery facility (WRRF). The individual impact of three municipal sludges, three organic loading rates (OLR), two SRTs, two temperatures and thermal hydrolysis pre-treatment was assessed through a well-designed experimental methodology. Bioacidification provided a maximum 79% of PO43--P/Ptot and 29% of CODsol/CODtot solubilisation ratios, which was an increase of approximately 60% and 26%, respectively, compared to a non bioacidified sludge. Similar performance was achieved for the three different sludges and OLRs used in this study under SRT of 4 days and 35 °C. Kinetic tests elucidated that lower SRT (2 days) and temperature (20 °C) negatively impacted P release due to biological fermentation decrease. Iron and polyphosphate release was impacted by present microbial community, while calcium release was more dependent on pH. Organic phosphorus, nitrogen and carbon solubilisation was limited by low sludge hydrolysis and increased only after a thermal-alkaline sludge hydrolysis pre-treatment (pH 9, 80 °C for 1 h) was combined with the bioacidification. This study demonstrates the high potential of the bioacidification process to valorise sludge and provides recommendations for its implementation onsite based on the individual requirements of each WRRF.

Denitrification performance and mechanism of biofilter constructed with sulfur autotrophic denitrification composite filler in engineering application

Sulfur autotrophic denitrification (SAD) is a promising technology due to its low cost and low sludge production. Based on previous studies on SAD materials as well as the denitrification mechanism of SAD technology, this study constructed two biofilters with a sulfur autotrophic denitrification composite filler (SADCF) to investigate the application potential of SAD technology. The feasibility of a SADCF-based biofilter was demonstrated, with a maximum nitrate volume load of 0.75 kg N/(m3·d) and low accumulation of nitrite and ammonium. In addition, an improved backwashing method (air–water backwashing) was obtained by comparing two different backwashing methods. Furthermore, some iron reducing bacteria (0.4% Geothrix) along with a rapid proliferation of the main sulfur-oxidizing bacteria (23.0% Thiobacillus and 27.7% Ferritrophicum) were found under real-world operating conditions. Overall, the results of this study provide a case reference for the operation of SADCF-based biofilters and the application of SAD technology in engineering.

Effect of the famine phase length on the properties of aerobic granular sludge treating greywater

Abstract Sequencing batch reactors (SBR) treating high-strength greywater need an aerobic granular sludge (AGS) with good properties, such as a low sludge volume index (SVI) and high settling velocities and substrate uptake rates to yield short settling and aeration stages. To promote the formation of stable granular sludge, the length of the famine phase could be a key factor. In this regard, the effect of the duration of this variable on the AGS properties was assessed by comparing a gradual versus an abrupt reduction of the famine phase in two SBR treating greywater. The initial average famine phase of 3.3 h was gradually reduced to 0.3 h over 20 weeks in one reactor, and abruptly in another one. This condition induced filamentous outgrowth, as well as the deterioration on the properties of the sludge; the effect was more accelerated when the famine periods were abruptly shortened. In both cases the reduction on the famine periods induced increased organic loading rates, which led to degranulation events when it was higher than 2.5 g-chemical oxygen demand g-volatile suspended solids−1 d−1. Afterwards, the biomass adapted to this situation, by forming new small-filamentous aggregates with similar SVI to that of the stable AGS formed with the longest famine period.

Alkaline thermal pretreatment of waste activated sludge for enhanced hydrogen production in microbial electrolysis cells

Resource utilization of waste activated sludge (WAS) has become a mainstream development direction. Alkaline thermal pretreatment (TPT) was found to greatly promote the bioaccessibility and biodegradability of the sludge. The organic matter including soluble chemical oxygen demand (SCOD), soluble carbohydrate, soluble protein and volatile fatty acids (VFAs) after low temperature (90 °C) pretreatment was 4.8%–65.9% higher than that after high temperature (180 °C) pretreatment. These increasements could be contributed by the alkaline treatment condition and the longer treatment time. The alkaline condition reduced the resistance of cell wall to the temperature. The pretreatment time at 90 °C was two times of that at 180 °C, allowing more organic matter to be released. But the total energy consumption of low temperature pretreatment (2580.7 kJ/L) was 30.5% lower than that of high temperature pretreatment (3711.8 kJ/L). The sludge fermentation liquid (SFL) was then employed as the substrate in microbial electrolysis cells (MECs), and the utilization efficiency of acetic acid was the highest (74.9%–83.2%). The hydrogen yield using low temperature pretreated sludge was 0.44 m3/(m3·d), which was higher than that of using high temperature pretreated sludge (0.31 m3/(m3·d)). These results suggested that alkaline TPT at 90 °C was an effective way to hydrolyze sludge and further enhance hydrogen production in MECs.