Understanding the impact of cationic polyacrylamide on anaerobic digestion of waste activated sludge

The resistance of hydrogenotrophic methanogenic microorganisms to ofloxacin in sludge anaerobic digestion process

Ofloxacin (OFL) is a commonly used antibiotic that can enter wastewater treatment plants and be adsorbed by the sludge, resulting in a high OFL concentration in sludge and affecting the subsequent sludge anaerobic digestion process. However, the micro mechanisms involved in this process have not been thoroughly studied. Therefore, this study focuses on the effect of OFL on the sludge anaerobic digestion of sludge to provide such support. The experimental results showed that the maximal methane yield decreased from 277.7 to 164.7 mL/g VSS with the OFL concentration increased from 0 to 300 mg/L. Additionally, OFL hindered the intermediate biochemical processes of hydrolysis, acidogenesis, acetogenesis, and acetoclastic methanogenesis. However, it promoted hydrogenotrophic methanogenesis process, using H2 as substrate, with the concentration of 300 mg/L OFL was 5.54 fold methane production of that in the control. Further investigation revealed that the negative effect of OFL was likely due to the induction of reactive oxygen species, which led to a decrease in cell activity and interference with the activity of key enzymes. Microbiological analysis revealed that OFL reduced the relative abundance of hydrolysis and acidogenesis bacteria, and Methanosaeta archaea, while increasing the relative abundance of hydrogenotrophic methanogenesis microorganism from 36.54% to 51.48% as the OFL concentration increase from 0 to 300 mg/L.

Molecular and microbial insights towards anaerobic biodegradation of anionic polyacrylamide in oil sands tailings

Anionic polyacrylamide (A-PAM) is widely used as a flocculant in the management of oil sands tailings. Nevertheless, apprehensions arise regarding its potential biodegradation and environmental consequences within the context of oil sands tailings. Consequently, it is imperative to delve into the anaerobic biodegradation of A-PAM in oil sands tailings to gain a comprehensive understanding of its influence on tailings water quality. This work explored the dynamics of A-PAM biodegradation across concentrations: 50, 100, 250, 500, 1000, and 2000 mg/kg TS. The results showed a significant decrease in A-PAM concentration and molecular weight at lower concentrations (50 and 100 mg/kg TS) compared to higher ones, suggesting enhanced degradation efficiency. Likewise, the organic transformation and methane production exhibited dependency on A-PAM concentrations. The peak concentrations observed were 20.0 mg/L for volatile fatty acids (VFAs), 0.07 mg/L for acrylamide (AMD), and 8.9 mL for methane yield, with these maxima being recorded at 50 mg/kg TS. The biodegradation efficiency diminishes at higher concentrations of A-PAM, potentially due to the inhibitory effects of polyacrylic acid accumulation. A-PAM biodegradation under anaerobic condition did not contribute to acute toxicity or genotoxicity. SEM-EDS, FT-IR and XRD analyses further revealed that higher concentrations of A-PAM inhibited the biodegradation by altering floc structure and composition, thereby restricting the microbial activity. Major microorganisms, including Smithella, Candidatus_Cloacimonas, W5, XBB1006, and DMER64 were identified, highlighting A-PAM's dual role as a source of carbon and nitrogen under anaerobic conditions. The above findings from this research not only significantly advance understanding of A-PAM's environmental behavior but also contribute to the effective management practices in oil sands tailings.

Natural flocculant chitosan inhibits short-chain fatty acid production in anaerobic fermentation of waste activated sludge

Chitosan (CTS) serves as an excellent natural flocculant in wastewater purification and sludge conditioning, but its potential impact on anaerobic fermentation of waste-activated sludge is unclear. The current study investigated the role of CTS in short-chain fatty acids (SCFAs) generation via sludge alkaline anaerobic fermentation. The results showed a drastic reduction in SCFA production with CTS, showing a maximum inhibition of 33 % at 6 mg/g of total suspended solids. CTS hindered sludge solubilization through flocculation, and acted as a humus precursor, promoting humus formation, and consequently reduced the amount of available substrates. Further, CTS promoted free ammonia production, posing a challenge to enzymes and cell viability. Additionally, CTS increased the population of Rikenellaceae sp. and weakened the dominance of hydrolyzing and acidifying bacteria. This study deepens the understanding of the potential impact of CTS on anaerobic fermentation and provides a theoretical basis for reducing the risk of polymeric flocculants.

Potential of free nitrous acid (FNA) for sludge treatment and resource recovery from waste activated sludge: A review

The escalating production of waste activated sludge (WAS) presents significant challenges to wastewater treatment plants (WWTPs). Free nitrous acid (FNA), known for its biocidal effect, has gained a growing focus on sludge dewatering, sludge reduction, and resource recovery from WAS due to its eco-friendly and cost-effective properties. Nevertheless, there have been no attempts made to systematically summarize or critically analyze the application of FNA in enhancing treatment and resource utilization of sludge. In this paper, we provided an overview of the current understanding regarding the application potential and influencing factors of FNA in sludge treatment, with a specific focus on enhancing sludge dewatering efficiency and reducing volume. To foster resource development from sludge, various techniques based on FNA have recently been proposed, which were comprehensively reviewed with the corresponding mechanisms meticulously discussed. The results showed that the chemical oxidation and interaction with microorganisms of FNA played the core role in improving resource utilization. Furthermore, current challenges and future prospects of the FNA-based applications were outlined. It is expected that this review can refine the theoretical framework of FNA-based processes, providing a theoretical foundation and technical guidance for the large-scale demonstration of FNA.

Towards sustainable management of polyacrylamide in soil-water environment: Occurrence, degradation, and risk

Polyacrylamide (PAM) possesses unique characteristics, including high water solubility, elevated viscosity and effective flocculation capabilities. These properties make it valuable in various sectors like agriculture, wastewater treatment, enhanced oil recovery, and mineral processing industries, contributing to a continually expanding market. Despite its widespread use globally, understanding its environmental fate at the soil-water interface remains limited. This article aims to provide an overview of the occurrence, degradation pathways, toxicity, and risks associated with PAM in the bioenvironment. The findings indicate that various degradation pathways of PAM may occur in the bioenvironment through mechanical, thermal, chemical, photocatalytic degradation, and/or biodegradation. Through a series of degradation processes, PAM initially transforms into oligomers and acrylamide (AM). Subsequently, AM may undergo biodegradation, converting into acrylic acid (AA) and other compounds such as ammonia. Notably, among these degradation intermediates, AM demonstrates high biodegradability, and the bioaccumulations of both AM and AA are not considered significant. Ensuring the sustainable use of PAM necessitates a comprehensive understanding among policymakers, scholars, and industry professionals regarding PAM, encompassing its properties, applications, degradation pathways, toxic effect on humans and the environment, and relevant regulations. Additionally, this study offers insights into future priority research directions, such as establishing of a reliable source-to-destination supply chain system, determining the maximum allowable amount for PAM in farmlands, and conducting long-term trials for the PAM-containing demolition residues.

Effect of total solids content on anaerobic digestion of waste activated sludge enhanced by high-temperature thermal hydrolysis

Total solids (TS) content may provide a regulatory strategy for optimizing anaerobic digestion enhanced by high-temperature thermal hydrolysis, but the role of TS content is not yet clear. In this study, the effect of TS content on the high-temperature thermal hydrolysis and anaerobic digestion of sludge and its mechanism were investigated. The results showed that increasing the TS content from 2% to 8% increased the sludge solubility and methane production potential, reaching peak values of 26.6% and 336 ± 6 mL/g volatile solids (VS), respectively. With a further increase in TS content to 12%, the strong Maillard reaction increased the aromaticity and structural stability of extracellular polymer substances, decreasing sludge solubility to 18.6%. Furthermore, the decrease in sludge biodegradability and the formation of inhibitory by-products resulted in a reduction in methane production to 272 ± 4 mL/g VS. This article provides a new perspective to understand the role of TS content in the thermal hydrolysis of sludge and a novel approach to regulate the Maillard reaction.

Acceleration of the biodegradation of cationic polyacrylamide by the coupling effect of thermophilic microorganisms and high temperature in hyperthermophilic composting

As a flocculant of sewage sludge, cationic polyacrylamide (CPAM) enters the environment with sludge and exists for a long time, posing serious threats to the environment. Due to the environmental friendliness and high efficiency in the process of organic solid waste treatment, hyperthermophilic composting (HTC) has received increasing attention. However, it is still unclear whether the HTC process can effectively remove CPAM from sludge. In this study, the effects of HTC and conventional thermophilic composting (CTC) on CPAM in sludge were compared and analyzed. At the end of HTC and CTC, the concentrations of CPAM were 278.96 mg kg-1 and 533.89 mg kg-1, respectively, and the removal rates were 72.17% and 46.61%, respectively. The coupling effect of thermophilic microorganisms and high temperature improved the efficiency of HTC and accelerated the biodegradation of CPAM. The diversity and composition of microbial community changed dramatically during HTC. Geobacillus, Thermobispora, Pseudomonas, Brevundimonas, and Bacillus were the dominant bacteria responsible for the high HTC efficiency. To our knowledge, this is the first study in which CPAM-containing sludge is treated using HTC. The ideal performance and the presence of key microorganisms revealed that HTC is feasible for the treatment of CPAM-containing sludge.

Effects of Mixtures of Engineered Nanoparticles and Cocontaminants on Anaerobic Digestion

The widespread application of nanotechnology inevitably leads to an increased release of engineered nanoparticles (ENPs) into the environment. Due to their specific physicochemical properties, ENPs may interact with other contaminants and exert combined effects on the microbial community and metabolism of anaerobic digestion (AD), an important process for organic waste reduction, stabilization, and bioenergy recovery. However, the complicated interactions between ENPs and other contaminants as well as their combined effects on AD are often overlooked. This review therefore focuses on the co-occurrence of ENPs and cocontaminants in the AD process. The key interactions between ENPs and cocontaminants and their combined influences on AD are summarized from the available literature, including the critical mechanisms and influencing factors. Some sulfides, coagulants, and chelating agents have a dramatic "detoxification" effect on the inhibition effect of ENPs on AD. However, some antibiotics and surfactants increase the inhibition of ENPs on AD. The reasons for these differences may be related to the interactive effects between ENPs and cocontaminants, changes of key enzyme activities, adenosine triphosphate (ATP) levels, reactive oxygen species (ROS) production, and microbial communities. New scientific opportunities for a better understanding of the coexistence in real world situations are converging on the scale of nanoparticles.

Anionic polyacrylamide alleviates cadmium inhibition on anaerobic digestion of waste activated sludge

The uncontrolled discharge of industrial wastewater leads to a significant cadmium (Cd) accumulation in waste activated sludge (WAS), posing a serious threat to the steady operation of the anaerobic digestion (AD) system in wastewater treatment plants (WWTPs). Therefore, developing a viable approach to cope with the adverse effects of high-concentration Cd on the AD system is urgently required. This study aims to investigate the potential of using anionic polyacrylamide (APAM), a commonly used agent in WWTPs, to mitigate the adverse effects of Cd in a toxic amount (i.e., 5.0 mg per g total suspended solids (TSS)) on AD of WAS. The results showed that the effectiveness of higher APAM on Cd toxicity alleviation was less than that of lower APAM at the studied level (i.e., the effectiveness order was 1.5 mg APAM per g TSS > 3.0 mg APAM per g TSS > 6.0 mg APAM per g TSS). The moderate supplement of APAM (i.e., 1.5 mg per g TSS) recovered the accumulative methane yield from 190.5 ± 3.6 to 228.9 ± 4.1 mL per g volatile solids by promoting solubilization, hydrolysis, and acidification processes related to methane production. The application of APAM also increased the abundance of key microbes in the AD system, especially Methanolinea among methanogens and Caldilineaceae among hydrolyzers. Furthermore, APAM facilitated the key enzyme activities involved in AD processes and reduced reactive oxygen species (induced by Cd) production via adsorption/enmeshment of Cd by APAM. These findings demonstrate the feasibility of using moderate APAM to mitigate Cd toxicity during AD, providing a promising solution for controlling Cd or other heavy metal toxicity in WWTPs.

Distinct effects of chemical- and bio- flocculants on the sludge acidogenic fermentation for volatile fatty acids production by affecting the acidogenic steps, microbial community structure and metabolic functions

Flocculants play crucial roles in sludge treatment, while the specific impact of chemical and bio-flocculants on sludge anaerobic fermentation was unknown. This study unveiled the contrasting effects of chitosan (CTS) and poly‑aluminum chloride (PAC) on volatile fatty acids (VFAs) generation during sludge fermentation. CTS supplementation resulted in 17.2 % increase in VFAs production, while PAC exposure led to 7.6 % reduction compared to the control. Further investigation revealed that CTS facilitated sludge solubilization and hydrolysis, thus providing sufficient organic substrates for VFAs generation. Additionally, environmental-friendly CTS exposure positively influenced the abundance and activity of functional anaerobes, as well as the expression of genes associated with VFAs biosynthesis. In contrast, PAC exposure resulted in the formation of larger sludge flocs, which hindered WAS solubilization and hydrolysis. Meanwhile, its potential microbial toxicity also impeded the microbial metabolic activity (i.e., genetic expressions), resulting in unsatisfactory VFAs production.

Volatile fatty acid production from primary and secondary sludges to support efficient nutrient management

Enhanced hydrolysis of sludges during fermentation is an important factor to achieve solubilization of complex carbon sources and increase the amount of soluble COD that microorganisms could use as food during biological nutrient removal processes. This research shows that a combination of mixing, bioaugmentation, and co-fermentation can be used to increase the hydrolysis of sludges and enhanced the production of volatile fatty acids (VFA). Mixing of primary sludge (PS) at 350 revolutions per minute (RPM) during fermentation increased the hydrolysis of the sludge and increased the soluble chemical oxygen demand (sCOD) by 72% compared to no mixing. Mixing also increased the production of VFA by 60% compared to no mixing conditions. PS hydrolysis was also evaluated using bioaugmentation with the bacteria Bacillus amyloliquefacients, a known producer of the biosurfactant surfactin. Results showed that bioaugmentation enhanced the hydrolysis of the PS by increasing the amount of soluble carbohydrates and soluble proteins present in the form of sCOD. Methanogenesis experiments performed with co-fermentation of decanted primary sludge (PS) and raw waste-activated sludge (WAS) at 75:25 and 50:50 ratios displayed a decreased in production of total biogas by 25.58% and 20.95% and a reduction on methane production by 20.00% and 28.76% respectively, compared to co-fermentation of raw sludges. Compared to fermentation of the sludges separately, co-fermentation of PS and WAS increased the production of VFA and it was determined that 50:50 was the optimum co-fermentation ratio for production of VFA while reducing the reintroduction of nutrients produced during the fermentation process to BNR processes.

Catalytic hydrothermal carbonization of wet organic solid waste: A review

Hydrothermal carbonization has gained attention in converting wet organic solid waste into hydrochar with many applications such as solid fuel, energy storage material precursor, fertilizer or soil conditioner. Recently, various catalysts such as organic and inorganic catalysts are employed to guide the properties of the hydrochar. This review presents a summarize and a critical discussion on types of catalysts, process parameters and catalytic mechanisms. The catalytic impact of carboxylic acids is related to their acidity level and the number of carboxylic groups. The catalysis level with strong mineral acids is likely related to the number of hydronium ions liberated from their hydrolysis. The impact of inorganic salts is determined by the Lewis acidity of the cation. The metallic ions in metallic salts may incorporate into the hydrochar and increase the ash of the hydrochar. The selection of catalysts for various applications of hydrochars and the environmental and the techno-economic aspects of the process are also presented. Although some catalysts might enhance the characteristics of hydrochar for various applications, these catalysts may also result in considerable carbon loss, particularly in the case of organic acid catalysts, which may potentially ruin the overall advantage of the process. Overall, depending on the expected application of the hydrochar, the type of catalyst and the amount of catalyst loading requires careful consideration. Some recommendations are made for future investigations to improve laboratory-scale process comprehension and understanding of pathways as well as to encourage widespread industrial adoption.

Water-Soluble Synthetic Polymers: Their Environmental Emission Relevant Usage, Transport and Transformation, Persistence, and Toxicity

Water-soluble synthetic polymers (WSPs) are distinct from insoluble plastic particles, which are both critical components of synthetic polymers. In the history of human-made macromolecules, WSPs have consistently portrayed a crucial role and served as the ingredients of a variety of products (e.g., flocculants, thickeners, solubilizers, surfactants, etc.) commonly used in human society. However, the environmental exposures and risks of WSPs with different functions remain poorly understood. This paper provides a critical review of the usage, environmental fate, environmental persistence, and biological consequences of multiple types of WSPs in commercial and industrial production. Investigations have identified a wide market of applications and potential environmental threats of various types of WSPs, but we still lack the suitable assessment tools. The effects of physicochemical properties and environmental factors on the environmental distribution as well as the transport and transformation of WSPs are further summarized. Evidence regarding the degradation of WSPs, including mechanical, thermal, hydrolytic, photoinduced, and biological degradation is summarized, and their environmental persistence is discussed. The toxicity data show that some WSPs can cause adverse effects on aquatic species and microbial communities through intrinsic toxicity and physical hazards. This review may serve as a guide for environmental risk assessment to help develop a sustainable path for WSP management.

The influence of coagulant type on the biological treatment of sewage sludge

Anaerobic digestion (AD) and composting are commonly utilized sludge management methods however, the influence of different coagulant types on these biological processes and their stabilized biomass characteristics have not been fully explained. In this study, the effect of the coagulant used in municipal wastewater treatment on the biological stabilization of sludge was investigated. Fully controlled and monitored small-scale AD and composting bioreactors were utilized. The coagulants tested included an inorganic coagulant (IC), polyaluminium chloride (PAC), and organic coagulants, (OC) polyamine (pAmine) and chitosan (Chit). Overall, the coagulant applied showed a measurable influence on the biological stabilization of sludge. The presence of complex organics compounds from OC in the sludge biomass was found to decrease biomass biodegradability while increasing gas production. During AD, Chit-sludge achieved higher biogas production than pAmine- and PAC-sludges (13 % and 16 %, respectively, in Nm³ CH₄ t^-1VS). In composting, pAmine-sludge achieved the highest feedstock temperature (34-35 °C) and CO₂ gas emissions, followed by Chit- (33 °C) and PAC-sludges (32 °C). Generally, tot-P concentration in PAC-sludge was higher than in pAmine and Chit-sludges both before (20, 17 and 15 g/kg DM, consecutively) and after AD (23, 21 and 20.5 g/kg DM, consecutively), and during the composting (31, 29.5 and 26 g/kg DM, consecutively) process. Tot-N concentrations (g/kg DM) showed a substantial increase after AD (pAmine and PAC ca 50 % and Chit 81 %), while a decrease was observed after composting, specifically in PAC-sludge (PAC 28 %, pAmine and Chit ⁓5 %). The selection of the most suitable coagulant by wastewater treatment facilities depends on the objective of the biological stabilization process. In cases where AD is applied and biogas yield is selected as the target output, the semi-natural OC Chit was found to be the best option among the coagulants tested. Comparably, when the nutrient content of resulting biosolids (AD or composting) is more relevant, it was found that OC-produced sludge contained higher N concentrations, while IC-produced sludge contained slightly higher P concentrations.

Untangling the effect of solids content on thermal-alkali pre-treatment and anaerobic digestion of sludge

Total solids (TS) content is critical for thermal hydrolysis and anaerobic digestion (AD) performance, but its role in thermal-alkaline pre-treatment (TAP) is unclear. Therefore, this study aimed to reveal the key role of TS content in TAP and AD of waste activated sludge. The results showed that the optimum TS content of TAP (at 90 °C for 1 h, pH = 10) was 8 %. Sludge disintegration and methane production increased from 19.7 ± 2.2 % to 34.3 ± 2.9 % and from 167.4 ± 4.2 to 246.0 ± 6.2 mL/g volatile solids, respectively, when TS content were increased from 2 % to 8 %. A high TS content will likely promote sludge disintegration since it will reduce heat loss and improve heating efficiency. Additionally, increasing TS content from 2 % to 10 % minimized the production of intracellular reactive oxygen species by 30.4 ± 0.7 % and increased cell viability by 11.5 ± 2.6 %. In contrast, excessive TS content (i.e., ≥10 %) deteriorated the fluidity of sludge, which prevents it from disintegration. Once TS reached 10 %, the accumulation of ammonia nitrogen and volatile fatty acids reached 812.7 ± 27.4 and 1932.0 ± 5.3 mg/L, respectively, which reduced the activity of acidulase and coenzyme F420 and shifted the archaeal community from acetylotrophic to hydrogenotrophic methanogens. This article provides new insights into the TS content in TAP and AD technology.

Current challenges of high-solid anaerobic digestion and possible measures for its effective applications: a review

The attention that high solids anaerobic digestion process (HS-AD) has received over the years, as a waste management and energy recovery process when compared to low solids anaerobic digestion process, can be attributed to its associated benefits including water conservation and smaller digester foot print. However, high solid content of the feedstock involved in the digestion process poses a barrier to the process stability and performance if it is not well managed. In this review, various limitations to effective performance of the HS-AD process, as well as, the possible measures highlighted in various research studies were garnered to serve as a guide for effective industrial application of this technology. A proposed design concept for overcoming substrate and product inhibition thereby improving methane yield and process stability was recommended for optimum performance of the HS-AD process.

Potassium permanganate pretreatment effectively improves methane production from anaerobic digestion of waste activated sludge: Reaction kinetics and mechanisms

As a powerful oxidizing agent, potassium permanganate (KMnO₄) has attracted widespread interest in sludge treatment and contaminant removal. However, its effect on the anaerobic digestion of waste activated sludge (WAS) is ambiguous. This investigation was designed to provide perspectives into this problem. In comparison with the control, 0.3 g KMnO₄/g TSS pretreatment enhanced the methane production by 78.82 %. Model analysis demonstrated that the KMnO₄ pretreatment enhanced the biochemical methane potential (B₀) of WAS. Mechanistic studies elucidated that the KMnO₄ pretreatment process generated reactive radicals such as ·OH, ·O₂^- and ¹O₂, which contributed to sludge disintegration and biodegradation process of dissolved substances, thus resulting in more substances available for subsequent methane generation. Enzyme activity analysis indicated that KMnO₄ pretreatment facilitated the activities of key enzymes associated with anaerobic digestion to various degrees. Microbial analysis illustrated that the relative abundance of functional microorganisms was significantly elevated after KMnO₄ pretreatment, which was conducive to methane production.

The fate of diclofenac in anaerobic fermentation of waste activated sludge

Diclofenac (DCF), a nonsteroidal anti-inflammatory drug, is one of the most commonly detected pharmaceuticals in wastewater treatment plants. However, the fate of DCF in waste activated sludge (WAS) anaerobic fermentation has not been well-understood so far. This work therefore aims to comprehensively reveal whether and how DCF is transformed in WAS mesophilic anaerobic fermentation through both experimental investigation and density functional theory (DFT) calculation. Experimental results showed that ∼28.8% and 45.8% of DCF were respectively degraded during the batch and long-term fermentation processes. Based on the detected intermediates and DFT-predicted active sites, three metabolic pathways, i.e., chlorination, hydroxylation, and dichlorination, responsible for DCF transformation were proposed. DFT calculation also showed that the Gibbs free energy (ΔG) of the three transformation pathways was respectively 19.0, -4.3, and -19.3 kcal/mol, suggesting that the latter two reactions (i.e., hydroxylation and dichlorination) were thermodynamically favorable. Illumina MiSeq sequencing analyses revealed that DCF improved the populations of complex organic degradation microbes such as Proteiniclasticum and Tissierellales, which was in accord with the chemical analyses above. This work updates the fundamental understanding of the degradation of DCF in WAS anaerobic fermentation process and enlightens engineers to apply theoretical calculation to the field of sludge treatment or other complex microbial ecosystems.

Towards globally relevant, small-footprint dewatering solutions: Optimal conditioner dose for highly variable blackwater from non-sewered sanitation

Globally, the sanitation needs of three billion people are met by non-sewered sanitation. Small-footprint treatment technologies are needed that are appropriate for dense urban areas. Blackwater (BW) (or fecal sludge), contains more than 95% liquid, and dewatering it without conditioning requires large footprints. Chemically-enhanced dewatering with conditioners is a promising option to increase dewatering performance and reduce required footprints. However, before implementation of this solution there is a need for increased knowledge on selection and dosing of conditioners. This study evaluated bio-based and synthetic conditioners (chitosan, tannin-, and starch-based, synthetic with and without poly-acrylamide) with 14 types of BW from five countries. The supernatant after settling with jar-tests was analyzed to quantify optimal dose and dewatering performance. The reduction of total chemical oxygen demand (COD) was >55%, achieved by removal of particulate constituents with mainly soluble COD remaining in the supernatant. A reduction in particulate COD could lead to increased efficiency of soluble COD in supernatant treatment. Bio-based conditioners are as effective as synthetic conditioners, and when performance was variable, it was due to differing properties of TSS, TS, EC and pH. Optimal conditioner dose for synthetic conditioners and chitosan could be predicted using concentrations of total solids (TS) (R² > 0.7), whereas optimal dose for starch- and tannin-based conditioners could be predicted with electrical conductivity (EC) (R² > 0.8), and colloid titration (R² > 0.8). In addition, real-time optical TSS and EC sensors could accurately predict chitosan dose for fresh BW treated at source (R² = 0.97, R² = 0.95). This study validates that use of conditioners for dewatering with highly variable BW can be implemented with real-time measurements for optimal dose, in globally relevant implementations.

Upgrading volatile fatty acids production from anaerobic co-fermentation of orange peel waste and sewage sludge: Critical roles of limonene on functional consortia and microbial metabolic traits

Orange peel waste (OPW) and sewage sludge (SS) valorization for volatile fatty acids (VFAs) production from anaerobic co-fermentation are attractive and feasible. The highest VFAs reached 11996.3 mg COD/L within 10 d at the mass ratio (TS/TS) of 1:1, which was approximately 30-fold of that in sole SS fermentation. The OPW provided plenty of organic substrates and facilitated the fermentation processes by disintegrating SS structure and inhibiting methanogenesis due to the abundant limonene. Also, the OPW feeds reshaped the microbial community and enriched fermentative bacteria, especially those saccharolytic ones (i.e. Prevotella-7). The key genes involved in membrane transport (i.e. ptsG), glycolysis (i.e. pgk), pyruvate metabolism (i.e. ace), and fatty acid biosynthesis (i.e. accA), which are associated with VFAs biosynthesis, were up-regulated in OPW/SS reactors. Overall, it was the increase in bioavailable organic matter and functional microorganisms, and the simultaneous enhancement of metabolic activity that improved the efficient VFAs production.

Uncovering the effect of polyethyleneimine on methane production in anaerobic digestion of sewage sludge

The potential effect of polyethyleneimine as a flocculant on anaerobic digestion of sludge was investigated. Polyethyleneimine above 12 g/kg total suspended solids inhibited the entire anaerobic digestion process including solubilization, hydrolysis, acidification, and methanogenesis. The addition of 24 g/kg total suspended solids polyethyleneimine reduced methane production from 167 ± 5 L/kg volatile suspended solids in the control reactor (without polyethyleneimine) to 141 ± 5 L/kg volatile suspended solids. Polyethyleneimine bound to extracellular polymeric substances, thus enhancing sludge agglomeration and hindering the release of organics. Meanwhile, the reduction of cytochrome C impeded electron transport, consequently curbed direct interspecies electron transfer. The adsorption of carbon dioxide by amine groups also hampered methane conversion. This study elucidated the concept that polyethyleneimine reduces mass transfer in anaerobic digestion, providing new insights into the potential behavior of flocculants in sludge treatment.

Advances in pretreatment strategies to enhance the biodegradability of waste activated sludge for the conversion of refractory substances

Anaerobic digestion (AD) is a low-cost technology widely used to divert waste activated sludge (WAS) to renewable energy production, but is generally restricted by its poor biodegradability which mainly caused by the endogenous and exogenous refractory substances present in WAS. Several conventional methods such as thermal-, chemical-, and mechanical-based pretreatment have been demonstrated to be effective on organics release, but their functions on refractory substances conversion are overlooked. This paper firstly reviewed the presence and role of endogenous and exogenous refractory substances in anaerobic biodegradability of WAS, especially on their inhibition mechanisms. Then, the pretreatment strategies developed for enhancing WAS biodegradability by facilitating refractory substances conversion were comprehensively reviewed, with the conversion pathways and underlying mechanisms being emphasized. Finally, the future research needs were directed, which are supposed to improve the circular bioeconomy of WAS management from the point of removing the hindering barrier of refractory substances on WAS biodegradability.

A Review of Persulfate Activation by Magnetic Catalysts to Degrade Organic Contaminants: Mechanisms and Applications

All kinds of refractory organic pollutants in environmental water pose a serious threat to human health and ecosystems. In recent decades, sulfate radical-based advanced oxidation processes (SR-AOPs) have attracted extensive attention in the removal of these organic pollutants due to their high redox potential and unique selectivity. This review first introduces persulfate activation by magnetic catalysts to degrade organic contaminants. We present the advances and classifications in the generation of sulfate radicals using magnetic catalysts. Subsequently, the degradation mechanisms in magnetic catalysts activated persulfate system are summarized and discussed. After an integrated presentation of magnetic catalysts in SR-AOPs, we discuss the application of persulfate activation by magnetic catalysts in the treatment of wastewater, landfill leachate, biological waste sludge, and soil containing organic pollutants. Finally, the current challenges and perspectives of magnetic catalysts that activated persulfate systems are summarized and put forward.

Polycyclic aromatic hydrocarbons stimulate acidogenesis, acetogenesis and methanogenesis during anaerobic co-digestion of waste activated sludge and food waste

Polycyclic aromatic hydrocarbons (PAHs) have been reported to influence acetic acid production during anaerobic treatment. However, investigations of the impacts of PAHs on the anaerobic co-digestion of waste activated sludge and food waste are limited. Therefore, the effects of PAHs on anaerobic co-digestion were explored in this study. Four kinds of PAHs all exhibited positive contributions to methane production, especially phenanthrene. Mechanism exploration revealed that acidogenesis, acetogenesis, and methanogenesis were improved in the presence of phenanthrene, and acetotrophic methanogenesis had the greatest improvement with 69.4%. Dominant bacteria and archaea related to acetic acid and methane accumulation were changed by phenanthrene. Moreover, extracellular polymeric substances, coenzyme F₄₂₀, and McrA gene copy number were promoted by phenanthrene, which was beneficial for the generation of acetic acid and methane. Overall, this study provides new insights into the role of organic pollutants in the anaerobic co-digestion of solid wastes.

Methane production from peroxymonosulfate pretreated algae biomass: Insights into microbial mechanisms, microcystin detoxification and heavy metal partitioning behavior

This study investigated the methane production potential of algal biomass by anerobic digestion with the addition of peroxymonosulfate (PMS), the removal of microcystin were analyzed and discussed. The microcystin concentration in the collected algal sludge was 1.20 μg/L in the liquid phase and 1393 μg/g in the algal sludge before anaerobic fermentation. The microcystin concentration decreased to 0.20-0.35 μg/L in the liquid phase and 4.16-11.51 μg/g in the sludge phase after 60 days of digestion. The initial PMS dose and residue microcystin concentration could be simulated with a logarithmic decay model (R² > 0.87). Anaerobic digestion could recover energy from algal source in the form of methane gas, which was not affected in the presence of microcystin, and the microcystin removal rate was >99%. Digestion decreased the total contents of Cd and Zn in the liquid phase and increased the total contents of Cr and Pb in the liquid phase. The microbial community and function prediction results indicated that the PMS0.1 system had the highest methane production, which was attributed to the high abundance of Mechanosaeta (40.52%). This study provides insights into microbial mechanisms, microcystin detoxification and the heavy metal partitioning behavior of the algal biomass during methane production.

Electrical stimulation enhancing anaerobic digestion under ammonia inhibition: A comprehensive investigation including proteomic analysis

Microbial electrolysis cell (MEC) coupled anaerobic digestion (AD), named as MEC-AD system, can effectively promote methane production under ammonia inhibition, but the inherent mechanism is still poorly understood. This study comprehensively explored the MEC-AD performance and mechanism under high-concentration ammonia stress including using proteomic analysis. It was found that the methane generation rates in MEC-AD systems were 2.0-2.7 times that of AD ones under 5.0 g/L ammonia stress. Additionally, the experimental conditions for methane generation in MEC-AD systems were optimized using response surface methodology. Further analysis indicates that the activities of acetate kinase and F₄₂₀ were improved, and particularly the direct interspecies electron transfer (DIET) was promoted in MEC-AD systems, as indicated by increased electroactive extracellular polymeric substance, decreased charge transfer resistance, and enrichment of electroactive microbes such as Geobacter on the bioelectrodes. Moreover, proteomic analysis reveals that the DIET associated proteins such as Cytochrome C was up-regulated, and ammonia transfer-related proteins were down-regulated and ammonium detoxification-related proteins were up-regulated in MEC-AD systems. This work provides us a better understanding on the MEC-AD performance especially for the treatment of wastewater containing high-concentration ammonia.

Novel anaerobic digestion of waste activated sludge via isoelectric-point pretreatment: Ultra-short solids retention time and high methane yield

The prolonged reaction period and low methane yield have become a pivotal bottleneck in the anaerobic digestion (AD) of waste activated sludge (WAS), severely limiting its use in bioenergy. This study evaluated the long-term semi-continuous AD of WAS with isoelectric-point (pI) pretreatment in terms of the reaction period, methane yield, material resource recovery, and economic and environmental benefits. The experimental results show that after pI pretreatment, at the same solids retention time (SRT), the average methane yield and methane content in the biogas of WAS increased by 120.9% and 15.5%, respectively. The average methane yield from AD with pI pretreatment at a 5-day SRT was 1.6 times that of the AD without pI pretreatment at a 15-day SRT, indicating that pI pretreatment significantly (P < 0.001) increased the methane yield from the AD of WAS at different SRTs, even at an ultra-short SRT. Statistical analyses of the changes in the concentrations of soluble organic matter in the AD process with and without pI pretreatment confirmed that pI pretreatment enhanced the biochemical reactions related to the transformation of sludge organic matter during the whole AD process, but not the initial organic solubilisation and hydrolysis of the sludge. Furthermore, pI pretreatment recovered the NaHCO₃, Na₂S, and multivalent metals from the sludge, indicating that AD with pI pretreatment efficiently recovered both bioenergy and material resources from the sludge. Economic and environmental analyses of AD with and without pI pretreatment further indicated that with pI pretreatment, the annual economic benefit of AD increases by 162% and carbon emissions decreases by approximately 2.63 × 10⁷ kg CO₂/year compared with conventional AD. These findings serve as an important reference for the development of a novel AD technology for sludge that incorporates second-generation pretreatment and operates at an ultra-short SRT.

Polydimethyldiallylammonium chloride induces oxidative stress in anaerobic digestion of waste activated sludge

The effects and key mechanisms of polydimethyldiallylammonium chloride on anaerobic digestion of waste activated sludge were investigated. Polydimethyldiallylammonium chloride at 38.1 g/kg total solids substantially reduced cumulative methane production from 138.2 ± 5.5 to 49.4 ± 5.0 L CH₄/kg volatile solids added, a reduction of 64.3 ± 0.2%. The quaternary ammonium groups on polydimethyldiallylammonium chloride agglomerated sludge flocs by neutralizing negatively charged amino groups in in extracellular polymeric substances, which hindered the release of organic matter. Quaternary ammonium groups induce oxidative stress by inducing the production of reactive oxygen species, thereby inhibiting the activity of anaerobic digestive enzymes. In addition, quaternary amine groups reduced the abundance of hydrolyzing bacteria, acidifying bacteria, and acetylotrophic methanogens. Oxidative stress could be an underappreciated mechanism that quaternary ammonium groups deteriorate anaerobic digestion, which could be transformative for understanding the potential risks of quaternary ammonium cationic flocculants in biological sludge treatment.

Wet wastes to bioenergy and biochar: A critical review with future perspectives

The ever-increasing rise in the global population coupled with rapid urbanization demands considerable consumption of fossil fuel, food, and water. This in turn leads to energy depletion, greenhouse gas emissions and wet wastes generation (including food waste, animal manure, and sewage sludge). Conversion of the wet wastes to bioenergy and biochar is a promising approach to mitigate wastes, emissions and energy depletion, and simultaneously promotes sustainability and circular economy. In this study, various conversion technologies for transformation of wet wastes to bioenergy and biochar, including anaerobic digestion, gasification, incineration, hydrothermal carbonization, hydrothermal liquefaction, slow and fast pyrolysis, are comprehensively reviewed. The technological challenges impeding the widespread adoption of these wet waste conversion technologies are critically examined. Eventually, the study presents insightful recommendations for the technological advancements and wider acceptance of these processes by establishing a hierarchy of factors dictating their performance. These include: i) life-cycle assessment of these conversion technologies with the consideration of reactor design and catalyst utilization from lab to plant level; ii) process intensification by integrating one or more of the wet waste conversion technologies for improved performance and sustainability; and iii) emerging machine learning modeling is a promising strategy to aid the product characterization and optimization of system design for the specific to the bioenergy or biochar application.

Insights into how poly aluminum chloride and poly ferric sulfate affect methane production from anaerobic digestion of waste activated sludge

Poly aluminum chloride (PAC) and poly ferric sulfate (PFS) are widely used in wastewater treatment and sludge dewatering, resulting in their amounts being accumulated substantially in waste activated sludge (WAS). Till now, however, little information about their influence on WAS digestion is available. This work therefore aims to provide insights into how PAC and PFS affect sludge anaerobic digestion. The experimental results showed that PFS's inhibition to methane production was much severer than PAC, in control reactor (0 mg Al or Fe /g TSS), the maximum cumulative methane production was 152.99 ± 7.18 mL/g VSS, when flocculants concentration increased to 30 mg Al/g TSS or 30 mg Fe/g TSS, the yields decreased to 129.54 ± 6.18 mL/g VSS and 89.52 ± 4.82 mL/g VSS respectively. Mechanism explorations exhibited that protein in WAS could bond with flocculants, which would inhibit protein bioconversion. It was also observed that the apparent activation energy (AAE) of organic solubilisation of PAC and PFS-contained sludge were increased by 38.58% and 18.67% respectively. Meanwhile, compared to the PFS, PAC led to more serious suppression of hydrolysis and acidogenesis processes, with propionic acid used as substrate, PFS inhibit methanogenesis more severely than PAC. Illumina MiSeq sequencing analyses showed that the number of sulfate-reducing bacteria (SRB) enriched obviously in PFS reactor. The results revealed that although PFS reduced methane production more severely than PAC, the reduction was mainly enforced by the activity of SRB but not organic enmeshment. Furthermore, PAC severely suppresses acetotrophic methanogens but PFS depress hydrogenotrophic methanogenesis microorganism mainly. Additionally, malodor control and dewaterability enhancement of digested sludge can be realized with PAC existence. The finding obtained in this study would provide insights into the PFS or PAC-involved sludge anaerobic digestion system and might support the important implication for further manipulate WAS treatment in the future.

Perfluorooctanoic acid triggers oxidative stress in anaerobic digestion of sewage sludge

Perfluorooctanoic acid (PFOA), as a recalcitrant organic pollutant, inevitably enters wastewater treatment facilities and is enriched in settled sludge. However, the potential impact of PFOA on sludge treatment has never been documented. In this study, the effect of PFOA on anaerobic digestion of sewage sludge and its underlying mechanism were investigated through batch and long-term experiments. The presence of PFOA was found to be deleterious for methane production from sewage sludge. 170 mg/kg total solids PFOA reduced the cumulative methane production from 197.1 ± 1.92-159.9 ± 3.10 mL/g volatile solids. PFOA induces the production of reactive oxygen species, which directly leads to cell inactivation and interferes with methane production. PFOA stimulates microorganisms to secrete more extracellular polymeric substances (mainly proteins), which not only hinders the solubilization of organic matter but also down-regulate enzyme activities to inhibit acidification and methanogenesis. In addition, PFOA reduces the diversity of microorganisms, especially the abundance of acid-producing bacteria and methanogens, making the microbial community unfavorable for methane production.

Unraveling the role of polyferric chloride in anaerobic digestion of waste activated sludge

This study explored the effect of polyferric chloride (PFC) as a flocculant on waste activated sludge anaerobic digestion. The results verified that PFC has an inhibitory effect on methane production during anaerobic digestion. PFC with a concentration of 40 g/kg total suspended solids reduced methane production from 195 ± 2.10 to 156 ± 1.50 L/kg volatile suspended solids, a decrease of 20.0 ± 0.09%. PFC released hydroxyl polymers and Fe(III). Hydroxy polymers aggregated sludge flocs and hindered the release of dissolved organic matter. Fe(III) induced dissimilar iron reduction processes to contend with methyl-CoM for electrons, thereby further reducing methane production. In addition, PFC enriched iron-reducing bacteria and reduced the abundance of methanogens, resulting in microbial communities that are not conducive to methane production. This article puts forward innovative insights on the role of PFC in biological sludge treatment, which is expected to guide the flocculant selection during wastewater treatment.

Unveiling the dual faces of chitosan in anaerobic digestion of waste activated sludge

In this study, the roles of chitosan (CTS) in anaerobic digestion of Waste activated sludge (WAS) were investigated. The results show that the methane production potential of WAS is positively correlated with the CTS content. The presence of 30 g/kg total suspended solids CTS increased the cumulative methane production from 215 ± 1.52 to 272 ± 1.83 mL/g volatile suspended solids. The positively charged amino groups in CTS neutralize the hydroxyl and carboxyl groups of extracellular polymeric substances, which reduces the negative charge on the surface of sludge and promotes sludge agglomeration, thereby inhibiting the release of organic matter. CTS also inhibits hydrolysis and acidification by immobilizing hydrolases and acidulase enzymes. However, CTS flocculates humus to avoid its interference with electron transfer, thereby enhancing the activity of coenzyme F420 and methanogenesis. In addition, CTS increases the abundance of methanogens, which also contributes to methane production.

Unveiling the mechanisms of how vivianite affects anaerobic digestion of waste activated sludge

Recently, phosphorus recovery as vivianite from sludge digestion system has attracted increasing attention because of its high recovery efficiency and economic value. However, the potential impact of vivianite on anaerobic digestion of waste activated sludge remains largely unknown. This study therefore aims to provide such support. Experimental results revealed that the maximal methane yield decreased from 103.55 to 76.55 mL/g volatile solids, with the vivianite level increasing from 0 to 500 mg P/L. Mechanism explorations showed that vivianite caused more substrates remaining in tightly-bound extracellular polymeric substances, and thus suppressed sludge solubilization. In addition, it was observed that hydrolysis, acidiogenesis, acetogenesis and methanogenesis bio-processes were all inhibited by vivianite. Microbial analysis revealed that vivianite significantly decreased the relative abundances of hydrolytic microbes, acidogens and methanogens. Further investigation showed that vivianite benefited sludge agglomeration and can enhance the mass transfer resistance of anaerobic digestion, further supporting the inhibitions on anaerobic digestion.

Effect of sodium dodecylbenzene sulfonate on hydrogen production from dark fermentation of waste activated sludge

Sodium dodecylbenzene sulfonate (SDBS), a typical surfactant being widely used in various applications, was highly accumulated in waste activated sludge. To date, however, its effect on hydrogen production from dark fermentation of sludge has not been documented. The work therefore aimed to explore whether and how SDBS affects hydrogen production. Experimental results showed that with an increase of SDBS from 0 to 30 mg/g TSS, the maximal hydrogen yield increased from 2.47 to 10.73 mL/g VSS (without any treatment) and from 13.05 to 23.51 mL/g VSS (under free ammonia pretreatment). Mechanism exploration showed that SDBS lowered surface tension, facilitated organics transfer from solid to liquid. SDBS also destroyed hydrogen bonding networks of protein, promoted macromolecular organics degradation. Besides, SDBS improved the electric charge in organics, then weakened the mutual repulsion, improved adsorb, interact and promoted the availability of reaction sites between anaerobes and organic substances. Enzyme activity analysis showed that SDBS not only improved the activities of enzymes related to hydrolysis and acidification processes, but also inhibited the activities of homoacetogens and methanogens. SDBS presence lowered sludge ORP and created an environment which was helpful to the growth of butyric-type bacteria, thus enhanced butyric-type fermentation, which contributed hydrogen production largely. Microbial community analysis revealed that SDBS existence affected distributions of microbial populations, and increased the abundances of hydrogen producing microorganisms (e.g., unclassified_f_Synergistaceae). PICRUSt2 analysis showed that SDBS reduced hydrogenotrophic methanogens activity for its inhibitory effect on the biotransformation of 5,10-Methenyl-THMPT to 5-methyl-THMPT.

How Does Chitosan Affect Methane Production in Anaerobic Digestion?

The expanding use of chitosan in sewage and sludge treatment processes raises concerns about its potential environmental impacts. However, investigations of the impacts of chitosan on sewage sludge anaerobic digestion where chitosan is present at substantial levels are sparse. This study therefore aims to fill this knowledge gap through both long-term and batch tests. The results showed that 4 g/kg total suspended solid (TSS) chitosan had no acute effects on methane production, but chitosan at 8-32 g/kg TSS inhibited methane production by 7.2-30.3%. Mass balance and metabolism of organic analyses indicated that chitosan restrained the transfer of organic substrates from solid phase to liquid phase, macromolecules to micromolecules, and finally to methane. Further exploration revealed that chitosan suppressed the secretion of extracellular polymeric substances of anaerobes by occupying the connection sites of indigenous carbohydrates and increased the mass transfer resistance between anaerobes and substrates, which thereby lowered the metabolic activities of anaerobes. Although chitosan could be partly degraded by anaerobes, it is much more persistent to be degraded compared with indigenous organics in sludge. Microbial community and key enzyme encoding gene analyses further revealed that the inhibition of chitosan to CO₂-dependent methanogenesis was much severer than that to acetate-dependent methanogenesis.

Unravelling the impacts of perfluorooctanoic acid on anaerobic sludge digestion process

Perfluorooctanoic acid (PFOA) is a type of persistent organic pollutant that has been detected in wastewater treatment systems, subsequently entering the waste activated sludge (WAS) anaerobic digesters. Nevertheless, how PFOA affects the anaerobic digestion of WAS has never been reported till now. In this study, a series of batch digesters were set up to assess the performance of the anaerobic sludge digestion processes with exposures to different levels of PFOA. Experimental results revealed that the increased PFOA concentration (3-60 μg/g-TS) caused the 11.1-19.2% decrease in methane production than the control. Correspondingly, the relative abundances of several key microbes related to acidification (e.g., Longilinea sp.) and methanation (e.g., Methanosaeta sp.) decreased when exposed to PFOA, as demonstrated by microbial community analysis. Further investigations based on modelling and intermediate metabolites analysis confirmed the inhibition of acidification and methanation caused by PFOA, thus decreasing the methane production potential of WAS in anaerobic digestion.

Improving methane production from algal sludge anaerobic fermentation by peroxydisulfate (PDS) pretreatment

This study investigated the potential of improving methane production from algal sludge anaerobic digestion by peroxydisulfate (PDS) pretreatment. The results show that with PDS dosage at 0.02 g PDS/g algal sludge TSS, PDS added system has highest accumulative methane production after 60 days fermentation. The accumulative methane production was 1.08, 1.15, 1.14, 1.13, 1.08, 0.76, and 0.15 times as compared with control, at 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 g PDS/g algal sludge TSS added, respectively. The SCOD in the system was keep increasing with the increment of PDS dosage after 120 min pretreatment. The algal sludge dewatering rate was increased with adding of PDS as pretreatment. The addition of PDS has inhibited the activities of microbes involved in digestion, while the short chain fatty acids production was improved after 3 days digestion. One-substrate model can be used to simulate the methane yield. The hydrolysis rate was decreased after dosing with PDS, while highest actual and predicted accumulative methane yield was occurred at 0.02 g PDS/g algal sludge TSS. Proteobacteria has higher percentage when the PDS was not higher than 0.1 g PDS/g algal sludge TSS, Acetothermia has higher percentage at 0.01 g PDS/g algal sludge TSS. The microcystin-LR (MC-LR) in algal sludge was largely removed after digestion, including the intracellular MC-LR. The higher PDS dosage could cause heavy metal release from algae cell to the digestate during fermentation. The addition of PDS to algal sludge can improve the accumulative methane production and mitigate microcystin concentration.

Cationic polyacrylamide alleviated the inhibitory impact of ZnO nanoparticles on anaerobic digestion of waste activated sludge through reducing reactive oxygen species induced

The enrichment of zinc oxide nanoparticles (ZnO NPs) in waste activated sludge (WAS) has raised concerns about their potential impact on anaerobic digestion of WAS. To date, there is no information regarding how to attenuate the negative effects of ZnO NPs on WAS anaerobic digestion. In this study, it was found that the appropriate amount of cationic polyacrylamide (cPAM) could mitigate the toxicity of ZnO NPs. During short-term exposure, the supplement of 4.0 mg cPAM/g TSS significantly restored biochemical methane potential from 28.6% inhibition to 9.3% inhibition compared with the control digester (P < 0.01). The spiked cPAM promoted the solubilization and acidification stages by weakening the contact between ZnO NPs and anaerobes in anaerobic digestion process, thus providing abundant substance for sequent bio-utilization. In the long-term semi-continues operated reactor, the continuous replacement of cPAM (at 4.0 mg/g TSS) significantly strengthened the recovery of VS destruction rate (20.3% to 26.4%, P < 0.01) and the daily yield of methane (93.5 mL/d to 124.2 mL/d, P < 0.01). Consistent with the restored performance, the application of cPAM increased the total microbial communities and the relative abundances of dominant acidogens and methanogens. Further explorations showed decreased toxicity of ZnO NPs primarily attributed to the decline of reactive oxygen species (ROS) induced by ZnO NPs.

Research on a new cationic polyacrylamide (CPAM) with a cationic microblock structure and its enhanced effect on sludge condition and dewatering

Flocculation is one of the commonly used sludge conditioning methods in water supply plants, which can improve the sludge dewatering performance by reducing the specific resistance of sludge (SRF), decreasing the amount of sludge, and finally lowering the transportation cost and subsequent disposal cost of sludge. Therefore, it is particularly important to develop new and efficient flocculants. In this paper, the template copolymer of acryloxy trimethylammonium chloride (DAC) and acrylamide (AM) was successfully synthesized by microwave-template copolymerization (MV-TP) using sodium polyacrylate (NaPAA) as template. The template copolymer was analyzed by infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance hydrogen spectroscopy (¹H NMR), and scanning electron microscopy (SEM). It was found that this template copolymer had obvious cationic microblock structure. In addition, the test results of association constant (K_M) and polymerization kinetics showed that the MW-TP was assigned to free radical initiated polymerization and the polymerization mechanism was I Zip-up (ZIP). It confirmed the formation of cation fragment structure again. Due to its dense positive charges in this new cationic microblock structure, it greatly improved the functions of electric neutralization, electrical patching, and adsorption bridging. The cationic fragment structure in the template copolymer could help to generate large and dense floc structure and form stable drainage channels. Under external pressure, these large and compact floc structures had greater compressive resistance, which avoided deformation and blockage of drainage channels and voids. It was beneficial to reduce SRF and evidently enhanced sludge dewatering performance.

Enhancing methane production from anaerobic digestion of waste activated sludge with addition of sodium lauroyl sarcosinate

In this study, sodium lauroyl sarcosinate (SLS) was used to promote anaerobic digestion of waste activated sludge for producing methane. It was found maximum cumulative methane production increased from 98.1 ± 3.1 to 166.0 ± 4.3 mL/g Volatile Suspended Solids (VSS) with dosage increasing from 0 (control) to 40 mg SLS/g TSS. But the addition of SLS (>10 mg SLS/g Total Suspended Solids (TSS)) resulted in prolonged lag phase time. Microbiological analysis showed that Syntrophobacter and Syntrophomonas both got enriched in reactors fed with SLS. Furthermore, hydrogenotrophic methanogens genus got more enrichment in contrast to acetoclastic methanogens. Mechanism analysis indicated that addition SLS could decrease surface tension, and promote release of organic matters as well as improve activities of hydrolytic enzymes. Besides, SLS could be nearly degraded completely within 3 days, and its degradation intermediates could be further transformed into methane gradually, thus enhancing methane production eventually.

Enhancing biomethane production and pyrene biodegradation by addition of bio-nano FeS or magnetic carbon during sludge anaerobic digestion

Pyrene exerts toxic effects on methanogens during anaerobic digestion of sludge, thus affecting the efficiency of sludge treatment. This study evaluated the facilitated direct interspecific electron transfer (DIET) between bacteria and methanogens when bio-nano FeS or magnetic carbon is added into anaerobic reactors. Results showed that adding 200 mg/L bio-nano FeS or magnetic carbon clearly reduced the accumulation of short-chain fatty acids and avoided acidification during 25 days of anaerobic digestion. The methane productions were 98.38 L/kg total solid (TS) and 73.69 L/kg TS in the bio-nano FeS and magnetic carbon systems, respectively, which accelerated methane production by 58.1% and 33.4%, respectively, compared with the control system (40.26 L/kg TS). The pyrene removal rates reached 77.5% and 72.1% in the bio-nano FeS and magnetic carbon systems, whereas it was only 40.8% in the control system. Analysis of microbial community structure revealed that methanogens (e.g. Methanosarcina and Methanosaeta) and extracellular electron-transfer bacteria (e.g. Pseudomonas, Cloastridia, and Synergistetes) were enriched in the reactors added with bio-nano FeS or magnetic carbon. This result indicates that the addition of bio-nano FeS or magnetic carbon may promote the activity and growth of microorganisms to improve the efficiency of methane production and pyrene degradation by enhancing DIET.

Enhancing methanogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from interfacial thermodynamics, electron transfer and microbial community

The usefulness of waste activated sludge (WAS) as an energy source is limited by the poor generation efficiency of methane from WAS, which is mainly due to the complex nature of sludge and low abundance of functional microbes. In this study, the interfacial thermodynamics, electron transfer and microbial community of sludge were investigated to reveal the enhancing effects of isoelectric-point (pI) pretreatment on the efficiency of methane generation from WAS. Experimentally, after pI pretreatment, the methane production potential, maximum methane production rate and maximum methane proportion in the biogas increased by 122.2%, 154.4% and 17.4%, respectively, indicating that pI pretreatment enhanced the generation efficiency of methane. Analyses of changes in the solid-liquid interfacial non-covalent interaction energy, electron transfer capacity (ETC) and reductive peak potential values of sludge samples with and without pI pretreatment during a 170-day methanogenic fermentation period revealed that pI pretreatment enhanced the self-driven solid-liquid interfacial hydrophobic attractions of sludge, increased the abiotic driving forces of interfacial enzymatic reactions, promoted the electron transfer efficiency and lowered the barrier of the reduction reaction. It was thus hypothesised that these changes would be responsible for increasing methane production, which was confirmed by the correlation analyses between the interfacial free energy (IFE) and ETC versus daily methane production. Moreover, statistical analyses of the differences between the microbial communities of sludge samples with and without pI pretreatment during fermentation demonstrated that pI pretreatment significantly (P < 0.05) improved the relative abundances of the main functional microbes with respect to hydrolysis, acidification and methanation. A further investigation of the relationships of IFE and ETC with the relative abundances of the main genera of methanogens indicated that the hydrophobic attraction of sludge surface and a high ETC are conducive to the enrichment of hydrogenotrophic methanogens (+29.9%). These findings are expected to provide a conceptual framework for developing second-generation pretreatment methods and provide a methodological reference for revealing the details of the 'black-box' anaerobic digestion process.

Enhanced production of methane in anaerobic water treatment as mediated by the immobilized fungi

Anaerobic digestion of organic waste and wastewater represents an attractive sustainable bio-technology to produce methane as an alternative to fossil energy. In response to improvement of methane production via enhancing methanogenesis, current strategies of the addition of external biological/non-biological materials have to confront either the loss of materials, high cost and/or possible destruction of the microbial community. Here, we report the first case of using immobilized fungi Aspergillus sydowii 8L-9-F02 to optimize the microbial community, achieving remarkable improvement of the methane production in both batch test (1.5 times) and continuous flow operation (1.13-1.31 times). The crucial role of fungi is associated with the stimulation of enrichment of Methanosaeta and Methanobacterium for methanogenesis from 28.2 to 67.4% as well as the improved activity of enzyme F420. Moreover, fungi also increase the content of extracellular polymeric substances, facilitating the formation of bio-aggregates. This work provides a new pathway to enhance methanogenesis during anaerobic digestion of wastewater by using fungi as bio-enhancer.

Hydrothermal pretreatment of sewage sludge enhanced the anaerobic degradation of cationic polyacrylamide (cPAM)

Cationic polyacrylamide (cPAM) is a widely used flocculant to dewater sewage sludge (SS) for high-solids anaerobic digestion (AD), and its degradation is crucial since it would release toxic acrylamide (AM) once entering environment. Hydrothermal treatment (HTT) is an efficient method to enhance the AD efficiency of SS. However, the effects of cPAM on AD of SS and the degradation of cPAM during HTT-AD process have not be studied. The study showed cPAM at 20 mg/g TS increased methane yield of SS from 127.0 to 138.9 ml CH₄/g TS in HTT-AD process, and the biodegradability of cPAM was 76.3%, which was much higher than that (7.4%) without HTT. In HTT-AD process, the enrichment of certain microbes (e.g. Gelria sp.) was observed, which might be related with cPAM degradation. HTT decreased the molecular weight (MW) of cPAM, and resulted in the production of 2-hydroxy-ethyl-trimethylammonium, ammonia, trimethylamine, and ethanol. Methane potential tests of the main HTT products also showed they were easily to be degraded. Overall, HTT-AD integrated process was an efficient method to reduce environmental risk of cPAM as well as increase energy output (biogas), and the study also provided insights into the degradation mechanism of cPAM during HTT.

Mechanistic insights into the effect of poly ferric sulfate on anaerobic digestion of waste activated sludge

Poly ferric sulfate (PFS), one of the typical inorganic flocculants widely used in wastewater management and waste activated sludge (WAS) dewatering, could be accumulated in WAS and inevitably entered in anaerobic digestion system at high levels. However, knowledge about its impact on methane production is virtually absent. This study therefore aims to fill this gap and provide insights into the mechanisms involved through both batch and long-term tests using either real WAS or synthetic wastewaters as the digestion substrates. Experimental results showed that the maximum methane potential and production rate of WAS was respectively retarded by 39.0% and 66.4%, whereas the lag phase was extended by 237.0% at PFS of 40 g per kg of total solids. Mechanism explorations exhibited that PFS induced the physical enmeshment and disrupted the enzyme activity involved in anaerobic digestion, resulting in an inhibitory state of the bioprocess of hydrolysis, acidogenesis, and methanogenesis. Furthermore, PFS's inhibition to hydrogenotrophic methanogenesis was much severer than that to acetotrophic methanogenesis, which could be supported by the elevated abundances of Methanosaeta sp and the dropped abundances of Methanobacterium sp in PFS-present digester, and probably due to the severe mass transfer resistance of hydrogen between the syntrophic bacteria and methanogens, as well as the higher hydrogen appetency of PFS-induced sulfate reducing bacteria. Among the derivatives of PFS, "multinucleate and multichain-hydroxyl polymers" and sulfate were unveiled to be the major contributors to the decreased methane potential, while the "multinucleate and multichain-hydroxyl polymers" were identified to be the chief buster to the slowed methane-producing rate and the extended lag time.

Enhanced technology based for sewage sludge deep dewatering: A critical review

Sludge is an inevitable by product of sewage treatment, and it includes pathogens, heavy metals, organic pollutants and other toxic substances. The components of sludge are complex and variable with extracellular polymeric substances (EPS) being one. EPS are highly hydrophilic and compressible, and make sludge dewatering difficult. Therefore, the development of efficient sludge-dewatering technology is an important means of mitigating rapid sludge growth. At present, the main methods used for sludge deep-dewatering technology are chemical preconditioning with high-pressure filtration and electrical mechanical dewatering. The selection of chemical preconditioning directly determines the final efficiency of the sludge-dewatering process. In this paper, we conduct a comprehensive review of the problems related to sludge dewatering and systematically summarise the impact of different chemical conditioning technologies on the efficiency of sludge dewatering. Furthermore, the characteristics of different enhanced dewatering technologies are evaluated and analysed for their adaptability and final disposal methods. We believe that this review can clarify the chemical conditioner mechanism to improve sludge dewatering, provide reference debugging information for the sludge-dewatering process and promote the development of efficient and environmentally friendly sludge-dewatering technology.