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Aiming to strengthen the performance of waste activated sludge digestion, pretreatment is a prerequisite for keeping operations within an industrially acceptable time- frame. In this study, the performance of microaeration coupled with thermophilic (MT) pretreatment on waste activated sludge solubilization and acidification was investigated. The results showed that the maximum soluble organics concentrations, including soluble proteins and carbohydrates, reached 2290 mg COD/L in 24 h when the ventilation rate and temperature were 0.05 vvm and 70 °C. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) test on pure-culture (E.coli) and scanning electron microscopy analysis indicated that MT pretreatment effectively destroyed microbial cell wall and resulted in an increase in soluble proteins. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis revealed that MT pretreatment reduced the diversity of the bacterial community during pretreatment. Along with the dissolution of a large quantity of organic matter, microbial species such as Lactococcus and Methyloversatilis bloomed at the end of acidification period, which contributed to SCFAs production. This study revealed that MT pretreatment facilitated the WAS hydrolysis efficiency and enhanced SCFAs (3149 mg COD/L with fermentation for 96 h) production, especially for acetic acid (50%) accumulation, which provides a new perspective for the application and recovery of sludge resources.

Challenges are always proposed when pursuing more energy and resource recovery from waste activated sludge via the anaerobic digestion (AD) process. Recently, microbial electrolysis has been integrated with AD and has been proven to enhance sludge conversion and system stability. This study investigated the effect of organic load on fermentation and biogas production in a bioelectrolysis-assisted AD reactor. Four different organic loads of alkaline waste sludge from 6 g/L to 14 g/L were investigated for their effects on the methanogenesis rate, substrate metabolism, electrochemical performance, and contribution. The results showed that the integrated system had a stronger tolerance to organic loads than the traditional anaerobic system. When the sludge concentration reached 14 g/L, the methanogenic rate, total methane yield, and SS removal rate significantly increased, reaching 47.1 mL/d, 96.2 mL/gVSS, and 71.6%, which were 1.27, 2.08, and 1.28 times those of the control, respectively. A high organic load was beneficial to the overall methanogenic rate but prolonged the fermentation period. Under a low organic load, the energy efficiency of the system deteriorated because the power loss increased and the electrochemical contribution rate was less than 50%. This result suggests that the integrated system can work and improve the overall energy yield from a high organic load of wasted sludge digestion. [ABSTRACT FROM AUTHOR]

Methane production rate (MPR) in waste activated sludge (WAS) digestion processes is typically limited by the initial steps of complex organic matter degradation, leading to a limited MPR due to sludge fermentation speed of solid particles. In this study, a novel microbial electrolysis AD reactor (ME-AD) was used to accelerate methane production for energy recovery from WAS. Carbon bioconversion was accelerated by ME producing H 2 at the cathode. MPR was enhanced to 91.8 gCH 4 /m 3 reactor/d in the microbial electrolysis ME-AD reactor, thus improving the rate by 3 times compared to control conditions (30.6 gCH 4 /m 3 reactor/d in AD). The methane production yield reached 116.2 mg/g VSS in the ME-AD reactor. According to balance calculation on electron transfer and methane yield, the increased methane production was mostly dependent on electron contribution through the ME system. Thus, the use of the novel ME-AD reactor allowed to significantly enhance carbon degradation and methane production from WAS. [ABSTRACT FROM AUTHOR]

To recover resource from waste activated sludge (WAS) presents great roles on developing carbon-neutral operation of wastewater treatment plants. This study was conducted to evaluate SCFAs production potential from anaerobic fermentation of WAS by a novel strategy, i.e., low temperature assisting potassium ferrate (PF) treatment. Results showed that the maximal SCFAs production reached 217 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS) with the shortened fermentation time of six days, and the sum of acetic and propionic acids reached 128 mg COD/g VSS. Meanwhile, the potentially recovered carbon source, in terms of SCFAs, soluble polysaccharides and proteins, reached 437 mg COD/g VSS. The mechanism study indicated that both solubilization and hydrolysis steps were accelerated, supporting by the highest accumulations of soluble organics and ammonia nitrogen, while methanogenesis step was inhibited, supporting by the lowest methane production. In addition, it is noted that low temperature treatment facilitated organics release from extracellular polymeric substance (EPSs), with the removal efficiencies of 51.1% for polysaccharides and 44.3% for proteins, while PF facilitated cell lysis. Interestingly, compared with low temperature followed by PF treatment, PF followed by low temperature treatment performed better on facilitating disintegration of both cells and EPSs, with the highest soluble polysaccharides and proteins of 439 and 1845 mg COD/L. Also, the acid-producing bacteria was greatly enriched, with total percentage of 29.5%. The findings of this study may provide some potential solutions for SCFAs production enhancement from WAS. [Display omitted] • PF followed by low temperature treatment improved SCFAs production from WAS. • The potentially recovered carbon source increased to 437 mg COD/g VSS. • Low temperature treatment promoted EPSs removal while PF facilitated cell lysis. • PF-T enhanced solubilization and hydrolysis steps but inhibited methanogenesis. • PF-T treatment facilitated the enrichment of acid-producing bacteria. [ABSTRACT FROM AUTHOR]

Abstract: The short-chain fatty acids (SCFAs) accumulated in waste activated sludge (WAS) fermentation was adopted as an alternative extra carbon source for biohydrogen production in microbial electrolysis cells (MECs). WAS was pretreated by bi-frequency ultrasonic and the highest SCFAs were accumulated at 3rd day. Three groups of tests were conducted in single chamber MECs for H2 production under different SCOD concentrations. SCOD removals were up to 60% at diluted influent, but reduced to 50% at original concentration. Highest H2 yield was 1.2 mL H2/mg COD at 2-fold dilution with 155% energy efficiency. Results showed that >90% of acetate and ∼90% of propionate were effectively converted to hydrogen, and next were n-butyrate and n-valerate (at dilutions), but <20% of iso-butyrate and iso-valerate were converted. The overall biohydrogen recovery in this study was 120 ml H2/g VSS/d. This work shows a possibility of cascade utilization of WAS fermentation liquid and H2 generation in MEC. [Copyright &y& Elsevier]

Sludge lysate is an unavoidable and refractory liquid produced from the waste activated sludge hydrothermal pyrolysis, which contains plenty of hazardous refractory organic compounds and value-added organic resources. Here, the proof of concept for an integrated strategy that couples technically compatible pretreatment to microbial electrolysis assisted AD (ME-AD) system is investigated for sludge lysate treatment and resource recovery. The pretreatment process shows a positive effectiveness on the ME-AD by reducing the organic load and inhibitory matters, which promote the residual refractory organic compounds (Maillard reaction products and humic acid-like substances) and carbon sources further biodegradation and bio-transformation. Combining membrane separation with ME-AD increased not only both the yield and purity of methane to 268.76 mL CH 4 /g COD and 98%, respectively, but also the recovery of 70.0~82.4% crude proteins (9.1 ± 0.5 g/L) from sludge lysate. Alternatively, the alkaline precipitation combined with ME-AD enhanced the recovery efficiency of short-chain fatty acids (SCFAs). The visible decreasing in the unpleasant color of the effluents was observed, implying that the degradation of harmful refractory organic was almost eliminated in sludge lysate. This strategy is worthy to be developed in WWTP for sludge lysate treatment with considerable bio-resources recovery and refractory organics removal. [Display omitted] • Sludge lysate was treated for resource recovery, including crude proteins and biogas. • Pretreatment showed a positive effect on the subsequent ME-AD process. • Maillard reaction products were effectively degraded in the ME-AD process. • Methane production rate of 268.76 mL/gCOD was achieved by membrane separation with ME-AD. [ABSTRACT FROM AUTHOR]

Biochar has been used to enhance methane generation from anaerobic digestion through establishing direct interspecific electron transfer between microorganisms. However, the microbial communication is still inadequate, thereby limiting further methane production improvement contributed by biochar. This study investigated the roles of quorum-sensing molecules, acylated homoserine lactone (AHL), in anaerobic digestion of waste activated sludge aided by biochar. Results showed that the co-addition of separated biochar and AHL achieved best methane production performance, with the maximal methane yield of 154.7 mL/g volatile suspended solids, which increased by 51.9%, 47.2%, 17.9%, and 39.4% respectively compared to that of control, AHL-loaded biochar, sole AHL, and sole biochar groups. The reason was that the co-addition of separated biochar and AHL promoted the stages of hydrolysis and acidification, promoting the conversion of organic matters and short-chain fatty acids, and optimizing the accumulation of acetate acid. Moreover, the methanogenesis stage also performed best among experimental groups. Correspondingly, the highest activities of electron transfer and coenzyme F420 were obtained, with increase ratios of 33.2% and 27.2% respectively compared to that of control. Furthermore, biochar did more significant effects on the evolution of microbial communities than AHL, and the direct interspecific electron transfer between fermentative bacteria and methanogens were possibly promoted. [Display omitted] • The strategies of biochar and AHL regulating anaerobic digestion were optimized. • The co-addition of separated biochar and AHL increased methane yield by 51.9%. • AHL-loaded biochar obtained less methane yield than either sole AHL or biochar. • The activities of electron transfer were the highest with addition of biochar and AHL. • Methanogenic communities shifted to mixotrophic methanogens like Methanosarcina. [ABSTRACT FROM AUTHOR]

Persulfate oxidation (PS) is widely employed as a promising alternative for waste activated sludge pretreatment due to the capability of generating free radicals. The product differences and microbiological mechanisms by which PS activation triggers WAS digestion through multiple modes need to be further investigated. This study comprehensively investigated the effects of persulfate oxidation activated through multiple modes, i.e., ferrous, zero-valent iron (ZVI), ultraviolet (UV) and heat, on the performance of sludge digestion. Results showed that PS_ZVI significantly accelerated the methane production rate to 12.02 mL/g VSS. By contrast, PS_Heat promoted the sludge acidification and gained the maximum short-chain fatty acids (SCFAs) yield (277.11 ± 7.81 mg COD/g VSS), which was 3.41-fold compared to that in PS_ZVI. Moreover, ferrous and ZVI activated PS achieved the oriented conversion of acetate, the proportions of which took 73% and 78%, respectively. MiSeq sequencing results revealed that PS_Heat and PS_UV evidently enriched anaerobic fermentation bacteria (AFB) (i.e., Macellibacteroides and Clostridium XlVa). However, PS_Ferrous and PS_ZVI facilitated the enrichment of Woesearchaeota and methanogens. Furthermore, molecular ecological network and mantel test revealed the intrinsic interactions among the multiple functional microbes and environmental variables. The homo-acetogens and sulfate-reducing bacterial had potential cooperative and symbiotic relationships with AFB, while the nitrate-reducing bacteria displayed distinguishing ecological niches. Suitable activation modes for PS pretreatments resulted in an upregulation of genes expression responsible for digestion. This study established a scientific foundation for the application of sulfate radical-based oxidation on energy or high value-added chemicals recovery from waste residues. [Display omitted] • Effect of multiple activated persulfate oxidation on sludge digestion was studied. • Heat and UV activated persulfate effectively promoted the accumulation of SCFAs. • ZVI and Ferrous activated persulfate were beneficial for methane production. • Microbial community structures were altered by multiple persulfate activation modes. • Suitable activation modes for persulfate upregulated the critical genes expression. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Na+ + Lysozyme pretreatment increased the yield of SCFAs in WAS by 7.6 times. • A synergistic effect was occurred between Na+ and lysozyme in promoting cell lysis. • Na+ + Lysozyme pretreatment significantly shortened the hydrolysis and acidification reaction times. • 87% of the carbon source recovery in the liquid phase can be achieved after PAC conditioning. Augmentation of the anaerobic treatment efficiency of waste activated sludge (WAS) to achieve better resource utilization is an effective approach for reducing carbon emissions and minimizing solid waste generation. However, resource recovery from WAS is often limited by the insufficient release and hydrolysis of organic matters within microbial cells. To overcome these issues, a novel Na+ and lysozyme co-pretreatment method has been used in this study to accelerate the production of short-chain fatty acids (SCFAs) by promoting cell lysis. Experimental results have revealed that the maximal SCFAs production of 302.8 ± 47.8 mg COD/g VSS is achieved with 10 % w/w lysozyme and 0.34 mol/L Na+ co-pretreatment on day 2, which is 7.6-fold higher than that of the control. Mechanism exploration has also exposed that Na+ and lysozyme exhibit a pronounced synergistic effect in cell lysis, where 1 + 1 > 2. The maximum sludge disintegration degree (DD M) and SCFAs conversion efficiency have been achieved by combined pretreatment, reaching 68.5 % and 74 %, respectively. The structure of extracellular polymeric substance (EPS) including protein content and looseness are also modified by Na+ or Na+ + Lysozyme pretreatment, which further confirm the impact of Na+ on the permeability of EPS. The carbon sources generated through Na+ and lysozyme pretreated WAS can be efficiently recovered through PAC conditioning, with a high recovery rate of 87 %. The Na+ and lysozyme reagents implemented in the present study are all non-toxic and cost-effective, thereby promising a green and efficient solution for the carbon recovery and separation from the WAS. [ABSTRACT FROM AUTHOR]

Biochar produced from bio-wastes has been widely used to promote the performance of anaerobic digestion. Waste activated sludge (WAS) is considered as a kind of popular precursor for biochar preparation, but the abundant resources in WAS were neglected previously. In this study, the roles of biochar prepared from raw, pretreated, and fermented sludge on anaerobic digestion were investigated. That is, parts of carbon sources and nutrients like polysaccharides, proteins, and phosphorus were firstly recovered after sludge pretreatment or fermentation, and then the sludge residuals were used as raw material to prepare biochar. The methane yield improved by 22.1% with adding the biochar (AK-BC) prepared by sludge residual obtained from alkaline pretreatment. Mechanism study suggested that the characteristics of AK-BC like specific surface area and defect levels were updated. Then, the conversion performance of intermediate metabolites and electro-activities of extracellular polymeric substances were up-regulated. As a result, the activity of electron transfer was increased with the presence of AK-BC, with increase ratio of 21.4%. In addition, the electroactive microorganisms like Anaerolineaceae and Methanosaeta were enriched with the presence of AK-BC, and the potential direct interspecies electron transfer was possibly established. Moreover, both aceticlastic and CO 2 -reducing methanogenesis pathways were improved by up-regulating related enzymes. Therefore, the proposed strategy can not only obtain preferred biochar but also recover abundant resources like carbon source, nutrients, and bioenergy. [Display omitted] • Biochar from raw, pretreated, and fermented sludge residuals were optimized. • Methane yield increased by 22.1% with biochar from alkaline pretreated sludge. • Alkaline pretreated sludge biochar increased electron transport activity by 21.4%. • Both aceticlastic and CO 2 -reducing methanogenesis pathways were improved. • A green and sustainable strategy for final disposal of sludge was proposed. [ABSTRACT FROM AUTHOR]

A huge production of waste activated sludge (WAS) has been a burden for wastewater treatment plants (WWTPs) with high disposal cost and little benefit back to wastewater purification. The short-chain fatty acids (SCFAs) produced by a short-term acidogenic fermentation of WAS before methane production have been proven to be a high-quality carbon source available for microbial denitrification process. The dual purpose of full recovery of fermentation liquid products and facilitating disposal of residual solid waste necessitate an efficient solid-liquid separation process of short-term fermentation liquid. The transformation and loss of various soluble carbon sources between solid and liquid are very important issues for carbon recovery efficiency when combining short-term fermentation and sludge dewatering in WWTPs. Here we testified the three conventional preconditioning coagulants, Polyferric Sulfate (PFS), Poly Aluminum Chloride (PAC) and Polyacrylamide (PAM), to improve the efficiency of subsequent solid-liquid separation. The results show that conversion yield of SCFAs in the liquid phase of sludge after short-term fermentation was 195 mg COD/g VSS, when using the coagulants PFS, PAC, and PAM for recovery, the recovery ratio was 79.5%, 82.0%, and 85.9%, respectively, while the dewaterability could be improved after preconditioning short-term fermentation sludge. The complexation of Al3+/Fe3+ in metal coagulants with carboxyl groups of SCFA demonstrated by Density Functional Theory calculation led to small part of soluble carbons co-migration to the solid phase, mainly a loss of high molecular weight organic compounds (carbohydrate, proteins, humic acids), while the application of PAM had little impact on carbon recovery. Economic calculations further showed PAM preconditioning short-term fermentation liquid of WAS could achieve higher recovery benefits. [Display omitted] • Soluble carbon recovery for denitrification is carried out from waste activated sludge. • The technology is optimized by combining short-term fermentation and sludge dewatering in WWTPs. • Conventional coagulants were applied for carbon source recovery. • The transformation and loss of various soluble carbon sources were evaluated. • Potential mechanism of the loss of Short-chain fatty acids was revealed. [ABSTRACT FROM AUTHOR]

Methane recovery from bio-wastes is one promising renewable energy to relieve the energy crisis faced around the world. Zeolite is one of attractive exogenous additives for promoting methane production from anaerobic digestion due to its potentials in regulating microorganism enrichment and anaerobic environment. However, the poor electron mediated ability limited its positive effects. Hence, this study hypothesized that the electron mediated ability of zeolite would be improved by loading magnetite, thereby generating more positive impacts on the performance of sludge anaerobic digestion. Compared to control group, magnetite modified zeolite (MZ) effectively improved methane yield by 37.4%. The reason was that the electron mediated ability and adsorption capacity of zeolite were improved by loading magnetite, facilitating the degradation of refractory organics and the conversion of volatile fatty acids. Moreover, the activities of cytochrome c and F420 increased by 33.3% and 29.8% with MZ additive, contributing to stimulating efficient electron transfer process. In addition, the electro-active genera were enriched, such as Bacteroidetes_vadinHA17 , Methanobacterium , and Methanosaeta , and the direct interspecies electron transfer was possibly established for methanogenesis with the help of MZ. This study will provide valuable references for the application of zeolite in anaerobic digestion. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Immobilized CB and NF electrode can increase methane yield by 38%-94%. • Applying no voltage to the electrodes is more conducive to methane production. • Syntrophic methane production was enhanced by immobilized electrode. • Voltage weakens anode methanogenesis but strengthens cathode DIET/IHT methanogenesis. Introduction of voltage or conductive materials have been proven to be effective strategies for enhancing the stability and efficiency of sludge digestion for methane recovery. This experiment simulated a two-phase anaerobic system, consisting of an acid-producing phase with alkali pretreatment and a methane-producing phase with immobilized electrodes and voltage enhancement. Various effective and comparative experiments were conducted to investigate the crucial roles of immobilized electrodes like carbon brush (CB) and nickel foam (NF), electroactive biofilm and voltage during the sludge methane-producing phase. The experimental results indicate that directly application of immobilized CB and NF electrode to promote sludge anaerobic digestion achieves better performance than additional voltage supply. The electrodes can increase methane production by 38% to 94%, whereas under additional voltage supply, methane production only increases by 22%–53% compared to the control. Through the analysis of key transformations in organic substrates and changes in microbial community structure, the inherent advantages of electrode materials in promoting the syntrophic methanogenesis processes are confirmed. Compared to suspended sludge, the types and relative abundance of typical syntrophic methane-producing microorganisms, Methanosarcina , and acetogenic bacteria, Syntrophomonas on the electrode surface significantly increase by 8.5% and 1.7%. This process is notably superior to the methane generation process dominated by hydrogenotrophic methanogensis after applying voltage. The results indicate that CB and NF are suitable conductive materials to promote anaerobic methane recovery, and their effectiveness surpasses that of applying voltage. These research outcomes provide new insights for the development of novel anaerobic digestion enhancement systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biochar prepared by iron-rich fermented sludge improved methane yield by 23.4%. • The electro-activity properties of EPS were up-regulated by prepared biochar. • Electroactive microorganisms were enriched and DIET might be established. • CO 2 -reduction and acetoclastic methanogenesis pathways were both promoted. • The proposed route realized stepwise resource recovery and final disposal of WAS. Anaerobic biological treatment of waste activated sludge (WAS) is proven as a low-carbon footprint technology with dual advantages of bio-fuel recovery and sludge stabilization. However, sludge residue after anaerobic biological treatment has been a serious obstacle to build waste-free cities. In this study, sludge residue, iron-rich sludge residue, and iron-rich fermented sludge residue were selected as the raw materials of biochar and the roles of prepared biochar in anaerobic digestion of WAS were investigated. Results indicated that the prepared biochar improved methane production and biochar derived from iron-rich fermented sludge residue (PFF-BC) obtained the best performance, with an increment of 23.4%. Mechanism analysis illustrated that the structural characteristics of PFF-BC were updated, including iron species, defect levels, and specific surface area, which played positive roles in the conversion of intermediate metabolites. Meanwhile, the compositions and electro-activity properties of extracellular polymeric substances were both stimulated by PFF-BC. Correspondingly, the activities of electron transport system and cytochrome c increased by 23.4% and 36.7%, respectively. For microbial communities, the abundances of electroactive microorganisms (such as Anaerolineaceae , Syntrophomonas and Methanosaeta) increased and direct interspecies electron transfer might be established, improving CO 2 -reduction and acetoclastic methanogenesis pathways. These findings will provide important references for stepwise resource recovery and final disposal of WAS. [ABSTRACT FROM AUTHOR]

Adding conductive materials for enhancing methane production from biomass has caused widespread attention. However, the discharge of digestive residues inevitably causes the loss of conductive materials, which not only weakens the roles of conductive materials but also increases the operating cost. This study aims to reveal the potentials of recycled magnetic biochar (MBC) in sludge anaerobic digestion. Results suggested that the direct utilization was better than the indirect utilization. Compared to raw MBC group, methane production further increased by 6.5–7.0 % in multi-cycle MBC groups. The mechanism analysis illustrated that the recycled MBC accelerated the conversion of intermediates. Besides, the recycled MBC promoted the content of redox-active substances and the activity of intracellular electron transfer systems. In addition, the acetoclastic methanogenesis pathway was improved in recycled MBC biofilms while the hydrogenotrophic methanogenesis was reduced. More interestingly, the microbial communities in biofilms presented directional evolutions with the increased cycle numbers of MBC. Correspondingly, the dominant direct interspecies electron transfer patterns changed from Geobacter & Methanosaeta to Clostridium & Methanosarcina , and the electron mediators converted to MBC and c-type cytochrome. The findings provided new insights into reutilization of conductive materials in anaerobic digestion. [Display omitted] • Direct utilization is a better way to recycle MBC compared to indirect utilization. • Multi-cycle MBC increased methane production by 6.5–7.0% compared to raw MBC. • Intracellular and extracellular electron transfers were all promoted by recycled MBC. • Microbial communities in biofilms showed directional evolution with recycled MBC. • Methanogenesis pathways and electron mediators were regulated by recycled MBC. [ABSTRACT FROM AUTHOR]

Potassium ferrate (K 2 FeO 4) has been extensively employed to promote short-chain fatty acids (SCFAs) production from anaerobic fermentation of waste activated sludge (WAS) because of its potent oxidizing property and formation of alkaline hydrolyzed products (potassium hydroxide, KOH and ferric hydroxide, Fe(OH) 3). However, whether K 2 FeO 4 actually works as dual functions of both an oxidizing agent and an alkalinity enhancer during the anaerobic fermentation process remains uncertain. This study aims to identify the contributions of hydrolyzed products of K 2 FeO 4 on SCFAs production. The results showed that K 2 FeO 4 did not execute dual functions of oxidization and alkalinity in promoting SCFAs production. The accumulation of SCFAs using K 2 FeO 4 treatment (183 mg COD/g volatile suspended solids, VSS) was less than that using either KOH (192 mg COD/g VSS) or KOH & Fe(OH) 3 (210 mg COD/g VSS). The mechanism analysis indicated that the synergistic effects caused by oxidization and alkalinity properties of K 2 FeO 4 did not happen on solubilization, hydrolysis, and acidogenesis stages, and the inhibition effect caused by K 2 FeO 4 on methanogenesis stage at the initial phase was more severe than that of its hydrolyzed products. It was also noted that the inhibition effects of K 2 FeO 4 and its hydrolyzed products on the methanogenesis stage could be relieved during a longer sludge retention time, and the final methane yields using KOH or KOH & Fe(OH) 3 treatment were higher than that using K 2 FeO 4 , further confirming that dual functions of K 2 FeO 4 were not obtained. Therefore, K 2 FeO 4 may not be an alternative strategy for enhancing the production of SCFAs from WAS compared to its alkaline hydrolyzed products. Regarding the strong oxidization property of K 2 FeO 4 , more attention could be turned to the fates of refractory organics in the anaerobic fermentation of WAS. [Display omitted] • Dual functions of potassium ferrate on anaerobic digestion of WAS were evaluated. • The highest SCFAs production was observed with hydrolyzed products. • No dual functions were achieved on solubilization and hydrolysis stages. • Potassium ferrate performed more inhibitions on methanogenesis at initial phase. • Potassium ferrate may not an alternative strategy compared to alkaline strategy. [ABSTRACT FROM AUTHOR]

Anaerobic digestion is widely employed for the treatment of waste activated sludge (WAS) due to its advantages like simultaneous energy recovery and sludge stabilization, promoting carbon-neutral operation of wastewater treatment plants. Natural zeolite, a low-cost and eco-friendly additive, has the potential to improve methane production from anaerobic digestion. This study investigated the effects of natural zeolite on anaerobic digestion when the substrate was WAS. It was found that methane production potential in response to natural zeolite was dosage-dependent. The optimal dosage was 0.1 g zeolite/g volatile suspended solids (VSS), with a methane yield of 181.89 ± 6.75 mL/g VSS, which increased by 20.1% compared to that of the control. Although the methane yields with other dosages of natural zeolite were higher than that of control, they were lesser than that with 0.1 g zeolite/g VSS. Natural zeolite affected transfer and conversion of proteins much more than polysaccharides in liquid phase and extracellular polymeric substances. In anaerobic digestion, natural zeolite had with little effects on WAS solubilization, while it improved hydrolysis, acidification, and methanogenesis. The dosages of natural zeolite did have significant effects on bacterial communities in biofilm rather than suspension, while the archaeal communities in biofilm and suspension were all greatly related to natural zeolite dosages. The developed biofilms promoted richness and functionality of microbial communities. The syntrophic metabolism relationships between methanogens and bacteria were improved, which was proved by selective enrichment of Methanosarcina , Syntrophomonas , and Petrimonas. The findings of this work provided some new solutions for promoting methane production from WAS, and the roles of natural zeolite in anaerobic digestion. [Display omitted] • Effects of zeolite on anaerobic digestion of WAS was dosage dependent. • Methane yield increased by 20.1% with additive dosage of zeolite of 0.1 g/g VSS. • Zeolite promoted hydrolysis, acidification, and methanogenesis stages. • Zeolite improved syntrophic metabolism between methanogens and bacteria. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe anode achieved 101–743% higher of hydrogen productivity than other iron sources. • Fe anode led complete removal of phosphate within 2 d (vs. 0–39.4% in other groups). • More crystalline vivianite particles were observed in EF system assisted by Fe anode. • Homo-acetogens (78.0%), EAB (8.0%) and DIRB (2.3%) were enriched in Fe anode group. • Correlation and syntrophy between functional microbes and metabolites were explored. Electro-fermentation (EF) is a promising technology for extracting valuable metabolites from waste biomass, while its effectiveness for phosphorus recovery has received little attention. In this study, we investigated the impact of different iron sources, i.e., FeCl 3 , FeOOH, Fe 2 O 3 , zero-valent iron (ZVI), a built-in Fe anode, and stainless-steel mesh (SSM), on concurrent biohydrogen and vivianite (Fe 3 (PO 4) 2 ·8H 2 O) recovery from sludge fermentation liquid (SFL) in an EF system. Results indicated that the Fe anode group achieved the highest hydrogen productivity of 17.7 mmol/g COD at 5 d, which was 101–743% higher than that of other iron sources. The utilization efficiency of short-chain fatty acids (SCFAs) peaked at 79.1% within 5 d, which was 1.2 folds higher than that of Control (without iron addition). Moreover, the phosphate removal efficiency reached 100% within 2 d and 5 d in Fe anode and SSM group, respectively, while the other groups achieved only 0–39.4% removal. SEM and XRD analyses demonstrated the existence of vivianite particles in the recovered products in the groups of FeCl 3 , FeOOH and Fe anode. Electrochemically active bacteria (EAB), e.g., Geobacter , Comamonas , and Desulfovibrio , accounted for 3.6–60.2% of all groups. Geobacter , Azospira , Comamonas , and Desulfovibrio , which are also considered dissimilatory iron reducing bacteria (DIRB), were enriched to 0.1–40.7% in all groups. Acetoanbacterium and Acetoaerobium , typical homo-acetogens, were enriched to 78.0% in the Fe anode group, while was only 9.0–29.3% in other groups. Correlation and molecular ecological network (MEN) analyses of the functional microbial consortia further indicated the intertrophic interaction. This study provides a theoretical basis for synchronous hydrogen and phosphorus recovery from WAS in the further industrial implementation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Citric acid promoted hydrogen corrosion of zero valent iron for methane production. • Zero valent iron and citric acid increased methane production potential by 63.8%. • Hydrogen accumulation with zero valent iron and citric acid were the highest. • Ferrous ions increased to 2.32 times higher than that with solo zero valent iron. • Hydrogenotrophic methanogens enriched while acetotrophic methanogens reduced. Anaerobic digestion of waste activated sludge (WAS) for biofuel recovery is one key step for developing energy-self-sufficient and carbon–neutral wastewater treatment plants. Although zero valent iron (ZVI) is an alternative additive to promote methane production from WAS, low hydrogen corrosion of ZVI is one important limitation. This study explored whether methane production potential could be further enhanced while using citric acid (CA) to accelerate hydrogen corrosion of ZVI. Results showed that the maximum methane production reached 92.77 mL/g volatile suspended solid (VSS) with ZVI of 30 mM and CA of 6 mM, which increased by 63.8% compared to that with solo ZVI. Mechanism analysis demonstrated that the proposed strategy could accelerate the solubilization of WAS, increase the accumulation of short-chain fatty acids (SCFAs), and optimize the composition of SCFAs. Meanwhile, both the efficiencies of hydrogen yield and production rate obtained with ZVI and CA were the highest. The accumulation of hydrogen with ZVI and CA reached 5 mL/g VSS during the 9-hour period, which was 2.42 times higher than that with solo ZVI, and the hourly hydrogen production rate was 2.47 mL/g VSS−1h−1, which was higher than those with either solo CA or ZVI. Thus, the effectiveness of 1 + 1＞2 was obtained. Correspondingly, the concentration of ferrous ion with ZVI and CA increased to 2.32 times higher than that with solo ZVI. In addition, the microbial community analysis indicated that hydrogenotrophic methanogens were predominated with ZVI and CA. These findings may provide some new solutions for promoting the role of nano-ZVI in enhancing methane production from anaerobic digestion. [ABSTRACT FROM AUTHOR]

[Display omitted] • Methane yield increased by 22.08% with particle size of 0.2–0.5 mm at the same dosage. • Lag phase of anaerobic digestion of waste activated sludge was shortened with zeolite. • Zeolite promoted hydrolysis, acidification, and methanogenesis stages. • Zeolite regulated the evolutions of microbial communities in suspension and biofilm. Anaerobic digestion has been proved as one promising strategy to simultaneously achieve resource recovery and environmental pollution control for biosolid treatment, and adding exogenous materials is a potential alternative to promote the above process. This study investigated response mechanisms of anaerobic digestion of waste activated sludge (WAS) to particle sizes of zeolite. Results showed that the methane production reached 186.75 ± 7.62 mL/g volatile suspended solids (VSS) with zeolite of the particle size of 0.2–0.5 mm and the additive dosage of 0.1 g/g VSS, which increased by 22.08% compared to that in control. Mechanism study revealed that zeolite could improve hydrolysis, acidification, and methanogenesis stages. Rapid consumption rates of soluble polysaccharides and proteins were observed, correspondingly, the accumulations of short-chain fatty acids (SCFAs) were enhanced, and the compositions of SCFAs were optimized. Moreover, the activities of F420 increased by 28% with zeolite, and the syntrophic metabolism between bacteria and methanogens were promoted. [ABSTRACT FROM AUTHOR]

Integrated microbial electrolysis with anaerobic digestion is proved to be an effective way to improve methanogenesis efficiency of waste activated sludge (WAS). WAS requires pretreatment for efficient improvement of acidification or methanogenesis efficiency, but excessive acidification may inhibit the methanogenesis. In order to balance these two stages, a method for efficient WAS hydrolysis and methanogenesis has been proposed in this study by high-alkaline pretreatment integrated with microbial electrolysis system. The effects of pretreatment methods and voltage on the normal temperature digestion of WAS have also been further investigated with emphasis on the effects of voltage and substrate metabolism. The results show that compared to low-alkaline pretreatment (pH = 10), high-alkaline pretreatment (pH > 14) can double the SCOD release and promote the VFAs accumulation to 5657 ± 392 mg COD/L, but inhibit the methanogenesis process. Microbial electrolysis can alleviate this inhibition effectively through the rapid consumption of VFAs and speeding up of the methanogenesis process. The optimal methane yield of the integrated system is 120.4 ± 8.4 mL/g VSS at the voltage of 0.5 V. Enzyme activities, high-throughput and gene function prediction analysis reveal that the cathode and anode maintain the activity of methanogens under high substrate concentrations. Voltage positively responded to improved methane yield from 0.3 to 0.8 V, but higher than 1.1 V is found to be unfavorable for cathodic methanogenesis and results in additional power loss. These findings provide a perspective idea for rapid and maximum biogas recovery from WAS. • Microbial electrolysis contributed the methanogenesis in high-alkaline pretreated sludge. • 0.5 V was the optimal and economical for accelerating methanogenesis. • High voltage could reduce the enrichment and activity of cathodic methanogens. • Anodic microorganisms facilitated the syntrophic methanogenesis process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Recent advances in roles of HA in AD of WAS were systematically reviewed. • Both substrate types and operation conditions can affect the evolution of HA. • HA works as surfactant, electron transfer mediator, and terminal electron acceptor. • Carbon materials present great potentials in alleviating inhibition effect of HA. • Perspectives for revealing the multi-functional roles of HA in AD were proposed. Waste activated sludge (WAS), the major by-product of wastewater treatment plants, has been hugely produced with the increase of wastewater treatment capacity. Anaerobic digestion (AD) is one promising technology to simultaneously achieve resource recovery and sludge stabilization. However, refractory organics contained in WAS can affect AD. Regarding the high contents, complex structures, and variable compositions of humic acid (HA) in WAS, the potential effects of HA in AD of WAS should be urgently assessed. This review highlights the current advances focusing on the fates, roles, and inhibition mitigation strategies of HA in AD of WAS, and concludes the underlying mechanisms and perspectives. The sources of HA contain the original and secondary ways, and both substrate types and operation conditions can affect evolution of HA. In AD of WAS with the presence of HA, positive effects are observed on solubilization, acidogenesis, and acetogenesis stages, while positive and negative effects are observed on hydrolysis and methanogenesis stages. This is because HA can not only work as surfactant, electron transfer mediator, and terminal electron acceptor, but also can regulate the structure and function of microbial communities, as well as the activities of enzymes. To alleviate the toxic effects of HA on methanogens, inhibition mitigation strategies such as extraction, pretreatment, and metal ions have been employed, and carbon materials present great potentials. Based on recent findings, the dynamic evolution and transformation pathways of HA should be identified. Then, multi-functional roles (i.e., positive and negative effects) of HA on electron transfer pathways, syntrophic metabolism networks, and functional microorganisms should be revealed. And finally, a green, sustainable, and low-cost strategy is still needed for alleviating inhibition effect of HA in AD. [ABSTRACT FROM AUTHOR]

[Display omitted] • Recent advances for establishing DIET in AD of WAS were systematically reviewed. • How to establish potential DIET in AD of WAS was analyzed and discussed. • The potential advantages of AD-based on DIET were summarized and discussed. • Challenges and perspectives for establishing DIET in AD of WAS were proposed. To realize energy self-sufficiency demand is a key component of carbon–neutral goal of wastewater treatment plants (WWTPs). Waste activated sludge (WAS), as the by-product of WWTPs, is produced in large quantities. Considering that WAS is a collection of organics, anaerobic digestion (AD) is a promising technology for methane recovery from WAS. However, conventional interspecies hydrogen transfer is vulnerable to environment changes, which can lead to instability or failure of AD. The recent finding of direct interspecies electron transfer (DIET) provides an alternative strategy to promote both the methane production potential and rate of WAS through an efficient and stable electron transfer method. This review highlights the current advances in relevant AD-based DIET for methane production from WAS, including the pathways and advantages for establishing DIET in the AD of WAS, and concludes with the current challenges in theory and application. The three main strategies for establishing DIET are conductive e-pili, conductive materials, and external voltage. These strategies not only present the advantages of shortening the lag phase and promoting the potential and rate of methane production but can also enhance intermediate degradation and provide buffering effects to inhibitors. Additionally, the role and function of microbial community structures can achieve directional regulation. Based on recent findings, theory shortages include unlocking syntrophic partners, disclosing cellular structure features, and exploring sophisticated models, while sustainable pretreatment strategies, applicable technical equipments, and comprehensive assessments may potentially limit DIET in engineering applications. This review aims to provide a comprehensive platform for establishing DIET in the AD of WAS and the carbon–neutral goal of WWTPs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Potassium ferrate followed by alkali-stripping strategy was used to treat sludge. • The potentially recovered carbon source reached 384 mg COD/g VSS. • The potentially recovered nitrogen was about 8.71 mg NH 4 +-N/g VSS. • The short-chain fatty acids production increased rather than reduced with stripping. Waste activated sludge (WAS) is a potential resource to achieve carbon–neutral goal of wastewater treatment plant. However, the solubilization is always the rate-limiting step for resource recovery from anaerobic digestion of WAS. This study reported a novel strategy, i.e., potassium ferrate (PF) followed by alkali-stripping treatment, to achieve short-chain fatty acids (SCFAs) and nitrogen recovery from WAS. Results showed that whether the stripping process was conducted under alkaline condition or not, the SCFAs production potential was increased rather than reduced. The promoted SCFAs production was due to the accelerated solubilization and hydrolysis stages but the inhibited methanogenesis stage. The SCFAs yield reached 258 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS), and the carbon source, including SCFAs, soluble polysaccharides and proteins, reached 384 mg COD/g VSS. The potentially recovered nitrogen was about 8.71 mg NH 4 +-N/g VSS. This work may provide some new solutions for enhancing resource recovery from WAS. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synergisms weren't formed with co-additive of BC and NM. • The methane yield decreased by 24.7 and 38.3% compared to that in BC and NM. • The direct interspecies electron transfer contributed by NM was reduced by BC. • The NM adsorbed on BC were converted to nano-hematite. • The electron-active microorganisms were reduced at suspended and attached stages. To realize energy-self-sufficient operation of wastewater treatment plants is a key step for carbon–neutral goal, and conductive materials have been widely used to build direct interspecies electron transfer (DIET) to promote methane production from waste activated sludge (WAS). However, the role and significance of co-additive of biochar (BC) and nano-magnetite (NM) have not been reported, and whether synergistic effects can be formed or not is still unknown. This study investigated the effects of co-additive of BC and NM by stepwise and simultaneous strategies on anaerobic digestion. Non-synergistic rather than synergistic effects were observed. The methane production reduced to 122.4 mL/g volatile suspended solids in simultaneous co-additive strategy, which was 24.7 and 38.3% lower than that in sole BC and NM, respectively. Differently, methane productions in stepwise additive strategies were higher than that in sole BC but lower than that in sole NM. Mechanism studies indicated that although NM accelerated WAS solubilization and hydrolysis, BC adsorbed soluble organics, reducing available substrates for methanogens at suspended stage. Also, extracellular polymeric substances were decreased, weakening electrochemical functions of redox-active shuttles. Besides, the DIET contributed by NM was reduced by the adsorption of BC to NM, and the NM adsorbed on BC were converted to nano-hematite, which presented inhibitions to methanogenesis. Moreover, the electron-active microorganisms, such as Clostridium and Methanobacterium , were all reduced at suspended and attached stages with the presence of BCs, limiting potential contributions of DIET on methane production improvement. This work may provide some new insights for additive strategies of BC and NM for methane production enhancement from WAS. [ABSTRACT FROM AUTHOR]

[Display omitted] • Freezing-low temperature treatment promoted SCFAs production from WAS. • The potentially recovered carbon source increased to 321 mg COD/g VSS. • Both solubilization and hydrolysis steps of WAS were accelerated. • Macellibacteroides, Paraclostridium and Romboutsia were mainly enriched. • A shortened sludge retention time inhibited methanogenesis but a longer promoted. This study proposed a novel and high-efficiency strategy, i.e., freezing followed by low-temperature thermal treatment, to significantly promote short-chain fatty acids (SCFAs) production from waste activated sludge compared to traditional freezing/thawing treatment. The maximal production of SCFAs was 212 mg COD/g VSS with a shortened retention time of five days, and the potentially recovered carbon source, including SCFAs, soluble polysaccharides and proteins, reached 321 mg COD/g VSS, increased by 92.1 and 28.3% compared to sole freezing and thermal treatment. Both the solubilization and hydrolysis steps of WAS were accelerated, and the acid-producing microorganisms, such as Macellibacteroides , Romboutsia and Paraclostridium , were greatly enriched, with a total abundance of 13.9%, which was only 0.54% in control. Interestingly, the methane production was inhibited at a shortened retention time, resulting in SCFAs accumulation, whereas it was increased by 32.0% at a longer sludge retention time, providing a potential solution for energy recovery from WAS. [ABSTRACT FROM AUTHOR]

• Coupling SO 4 ∙‾ oxidation with io-SRB mediation enhanced acetogenesis from WAS. • SO 4 ∙‾ and ∙OH were the key radicals responsible for coupled Fe(VI)/S(IV) oxidation. • io-SRB mediation promoted the organics transformation on the basis of SO 4 ∙‾ oxidation. • SO 4 ∙‾/io-SRB altered the structure of WAS fermentative acidogenic microbiome. • Synergistic relationships among io-SRB, hydrolyzing bacteria and acidogens were revealed. Acetate (HAc) production via acetogenesis is a promising biorefinery approach for waste activated sludge (WAS); however, it is hampered by the thermodynamic constraints of the bioconversion of 3–5 carbon atom short-chain fatty acids (SCFAs). Sulfate radical (SO 4 ∙‾)-based advanced oxidation is regarded as an appropriate candidate for accelerating WAS fermentation. In this study, we enriched an incomplete-oxidative sulfate reducing bacteria (io-SRB), combined with SO 4 ∙‾ oxidation (generated by potassium ferrate (PF) and sodium sulfite (Na 2 SO 3)), to boost WAS acetogenesis. Generated sulfate during SO 4 ∙‾ oxidation served as the necessary substrates for io-SRB metabolism. A proof-of-concept based on experimental data for the whole process is presented. Results confirmed that the PF + Na 2 SO 3 + SRB test achieved the maximum SCFAs generation (4261 ± 210 mg COD/L with 60.9 ± 0.5% HAc) over the PF + Na 2 SO 3 test without io-SRB mediation (2521 ± 109 mg COD/L with 50.6 ± 0.3% HAc). Particle size analysis and fluorescence spectroscopy indicated that PF + Na 2 SO 3 oxidation had positive effects on accelerating soluble organics release. SO 4 ∙‾ was the key radical, playing the most important role, as indicated by electron paramagnetic resonance and radical scavenging analysis. X-ray photoelectron spectroscopy revealed that io-SRB mediation further promoted the transformation of polysaccharides and proteins into carboxylic acids, based on SO 4 ∙‾ oxidation. Moreover, 79% Fe(VI) was reduced to Fe(III), and most S(IV) was converted to SO 4 2−, approximately 40% of which was metabolized by io-SRB consortium. Clearly, SO 4 ∙‾ oxidation and io-SRB stimulation significantly altered the composition of the key microbiome, with fermentative acidogenic bacteria predominating. The possible synergistic relationships among io-SRB, hydrolyzing bacteria and acidogens were revealed by molecular ecological network analysis. This study provides new insights into the improvement of value-added bio-metabolite recovery from SO 4 ∙−-based WAS fermentation. [ABSTRACT FROM AUTHOR]