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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]

To recover methane from waste activated sludge through anaerobic digestion (AD) is one promising alternative to achieve carbon neutrality for wastewater treatment plants. However, humic acids (HAs) are one of the major compositions in waste activated sludge, and their accumulation performs inhibition effects on AD. This study investigated the potentials of biochar (BC) in alleviating inhibition effects of HAs on AD. Results showed that although the accumulated HAs reduced methane yield by 9.37% compared to control, the highest methane yield, 132.6 mL CH 4 /g VSS, was obtained after adding BC, which was 45.9% higher than that in HA group. Mechanism analysis showed that BC promoted the activities of hydrolase such as protease and α-glucosidase, which were 69.7% and 29.7% higher than those in HA group, respectively. The conversion of short-chain fatty acids was accelerated. In addition, the evolutions of electroactive microorganisms like Clostridium_sensu_stricto_13 and Methanosaeta were consistent with the activitiies of electron transfer and the contents of cytochrome c. Furthermore, parts of HAs rather than all of them were adsorbed by BC, and the remaining free HAs and BC formed synergistic effects on methanogenesis, then both CO 2 reduction and acetoclastic methanogenesis pathways were improved. The findings may provide some solutions to alleviate inhibition effects of HAs on AD. [Display omitted] • The methane yield increased by 45.9% after adding biochar to AD inhibited by HAs. • Biochar alleviated the inhibition effects of HAs on the activities of hydrolase. • Biochar improved electron transfer efficiency and increased cytochrome C content. • Both CO 2 reduction and acetoclastic methanogenesis pathways were improved. • The remaining free HAs and BC formed synergistic effects on methanogenesis. [ABSTRACT FROM AUTHOR]

Promoting methane production performance from waste activated sludge is of great significance for energy self-sufficiency operation of wastewater treatment plants. This study evaluated the methane production enhancement strategies of biochar and quorum-sensing molecules like N-acylated homoserine lactone (AHL) in single-stage and two-stage anaerobic digestion systems. The methane yield increased to 134.9 mL/g volatile suspended solids in single-stage digestion system, which was 41.7 % higher than that of control group, while it in two-stage digestion system was not only lower than that obtained in single-stage digestion system but also less than that observed in groups adding AHL either at the beginning or on the third day of experiment period. The mechanism studies indicated that the single-stage digestion strategy promoted solubilization, hydrolysis, acetogenesis, and methanogenesis stages and improved the conversion and utilization of organic matters with the enhanced syntrophic metabolisms between microorganisms. In addition, the electron transfer activity between microorganisms was promoted, with an increase ratio of 54.7 %, meanwhile, the syntrophic relationships between methanogens such as Methanosarcina and electroactive bacteria such as norank_f__Bacteroidetes_vadinHA17 were promoted. As a result, both acetotrophic and hydrogenotrophic methanogenesis pathways were improved. This study provided some novel approaches for promoting anaerobic digestion performance while using biochar as the additive. [Display omitted] [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] • 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] • 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] • Stepwise alkaline coupled with ammonia stripping promoted SCFAs production from WAS. • The SCFAs yields increase to 323 mg COD/g VSS with HAc and HPr of 62.4%. • Anaerobes related to SCFAs production were enriched greatly with ammonia stripping. • The proposed strategy can simultaneously achieve nitrogen and carbon recovery. To recover resource from waste activated sludge (WAS) is of great significance. This study proposed a promising way, i.e., stepwise alkaline treatment coupled with ammonia stripping, to remarkably enhance short-chain fatty acids (SCFAs) production from WAS anaerobic digestion. The maximal production of SCFAs, with the value of 323 mg COD/g volatile suspended solid, was obtained with first initial pH = 10 adjustment followed by second initial pH = 10 adjustment on third day coupled with ammonia stripping. Mechanistic studies showed that solubilization of both extracellular polymeric substances and cells could be accelerated by stepwise initial pH = 10 adjustment. However, without ammonia stripping, the activities of either acidogens or methanogens could be inhibited by free ammonia formed under alkaline conditions; positively, anaerobes related to SCFAs production were enriched with ammonia stripping. Moreover, the proposed strategy can simultaneously achieve nitrogen and carbon recovery, providing some solutions for the carbon–neutral operation of wastewater treatment plants. [ABSTRACT FROM AUTHOR]