New insight into enhanced short-chain fatty acids production from waste activated sludge through pretreatment of cation exchange resin coupled NaCl addition

Abiotic and biotic roles of metals in the anaerobic digestion of sewage sludge: A review

Anaerobic digestion (AD) is a promising technique for sludge treatment and resource recovery. Metals are very important components of sludge and can have substantial effects on its complex nature and microbial activity. However, systematic reviews have not addressed how metals in sludge affect AD and how they can be regulated to improve AD. This paper comprehensively reviews the effects of metals on the AD of sludge from both abiotic and biotic perspectives. First, we introduce the contents and basic characteristics (e.g., chemical forms) of intrinsic metals in sewage sludge. Then, we summarise the main mechanism by which metals influence sludge properties and the methods for removing metals and thus improving AD. Next, we analyze the effects of both intrinsic and exogenous metals on the enzymes and microbial communities involved in anaerobic bioconversion, focusing on the types, critical concentrations and valence states of the metals. Finally, we propose ideas for future research on the roles of metals in the AD of sludge. In summary, this review systematically clarifies the roles of metals in the AD of sludge and provides a reference for improving AD by regulating these metals.

Revealing the mechanism of novel nitrogen-doped biochar supported magnetite (NBM) enhancing anaerobic digestion of waste-activated sludge by sludge characteristics

Anaerobic digestion (AD) is a promising technology in waste treatment and energy recovery. However, it suffers from long retention time and low biogas yield. In this study, novel nitrogen-doped biochar supported magnetite (NBM) was synthesized and applied to enhance the AD of waste-activated sludge. Results showed that NBM increased cumulative methane production and SCOD removal efficiency by up to 1.75 times and 15% respectively at 5 g/L compared with the blank. NBM enhanced both hydrolysis and methanogenesis process during AD and the activities of α-glucosidase, protease, coenzyme F₄₂₀ and electron transport system were increased by 19%, 163%, 104% and 160% respectively at 5 g/L NBM compared with the blank. NBM also facilitated the secretion of conductive protein in extracellular polymeric substances as well as the formation of conductive pili, leading to 3.18-7.59 times higher sludge electrical conductivity. Microbial community analysis revealed that bacteria Clostridia and archaea Methanosarcina and Methanosaeta were enriched by the addition of NBM, and direct interspecies electron transfer might be promoted between them. This study provides a practical reference for future material synthesis and its application.

Novel nitrogen-doped biochar supported magnetite promotes anaerobic digestion: Material characterization and metagenomic analysis

Although different conductive materials have been applied to anaerobic digestion, there has not been a material that can really combine their merits and make up their shortcoming from each other. In this study, a novel nitrogen-doped biochar supported magnetite (Fe₃O₄@N-BC) was synthesized. Various material characterizations confirmed that nitrogen atoms were successful doped into the biochar and magnetite precipitated on its surface. 5 g/L Fe₃O₄@N-BC achieved the highest promotion of cumulative CH₄ production by 1.75 times compared with the blank group. Further metagenomic analysis revealed that Fe₃O₄@N-BC could increase the gene abundances of pilA, MmcA, Fpo, Rnf and HdrEd in bacteria Clotridium, Pseudomonas and Syntrophomonas and archaea Methanosarcina. Redundancy analysis showed that it was electrical conductivity and electron exchange capacity that were the key physicochemical characteristics for Fe₃O₄@N-BC to facilitating direct interspecies electron transfer. This study provides a reference for future conductive material synthesis and its application for anaerobic digestion.

Metagenomic analysis reveals the size effect of magnetite on anaerobic digestion of waste activated sludge after thermal hydrolysis pretreatment

Although magnetite has been widely investigated in anaerobic digestion (AD), its role in the practical AD of waste-activated sludge (WAS) after thermal hydrolysis pretreatment (THP) and its size effect remain unclear. In this study, magnetite with four different particle sizes was added during the AD of WAS after THP. With the reduction of magnetite particle size, cumulative methane production was increased, while the optimal dosage of magnetite decreased, with 0.1 μm magnetite at an optimal dosage of 2 g/L achieving the highest cumulative methane production increase of 111.97 % compared with the blank group (without magnetite). Smaller magnetite particles increased α-glucosidase and protease activities, coenzyme F₄₂₀ concentration, and electron-transport system activity (20.30 %, 173.02 %, 60.39 % and 158.08 % higher respectively than the blank group). The size of magnetite also influenced the establishment of direct interspecies electron transfer (DIET) during AD. Based on the analysis of the pilA gene abundance, magnetite with a large particle size could promote the formation of e-pili in syntrophic electroactive bacteria (Clostridium, Syntrophomonas, and Pseudomonas) and methanogens (Methanospirillum), thereby completing electron transfer. However, small-sized magnetite particles stimulated DIET by enhancing the secretion of conductive proteins in extracellular polymeric substances and membrane-bound enzymes (Fpo) in Methanosarcina.

Reconsidering operation pattern for cation-exchange resin assistant anaerobic fermentation of waste activated sludge: Shorting residence period towards dosage-reduction and anti-fouling

Short-chain fatty acids (SCFAs) generation through anaerobic fermentation has been regarded as a promising pathway to achieve carbon recovery and economic benefits in waste activated sludge management. Despite the cation exchange resin (CER) assistant anaerobic fermentation strategy has been previously reported for enhancing anaerobic fermentation, the overlarge CER usage and serious CER pollution have limited its engineering application. This study provided a reconsideration for the operation pattern modification. Through 4-day anaerobic fermentation with CER residence period shrinking to 1 day, 40.9% sludge VSS solubilization and reduction were achieved, triggering a considerable sludge hydrolysis rate of 28.4%. Thereby, SCFAs production was improved to 264.8 mg COD/g VSS. Such performances were approximately 80.2-87.8% of those with conventional CER residence period (8 days). The organic composition distribution and parallel factor analysis demonstrated that similar biodegradability and utilizability of fermentative liquid were achievable with various operation patterns. Compared with the conventional operation pattern, the modified operation pattern with shortened CER residence period (1 day) also displayed satisfying anaerobic fermentation efficiency and numerous engineering bene fits, e.g. decreased CER usage, reduced engineering footprint, relieved CER fouling, and increased operation convenience. The findings might provide sustainable development for CER assistant anaerobic fermentation strategy and enlighten the direction of anaerobic fermentation process.

Impact of divalent cations on lysozyme-induced solubilisation of waste-activated sludge: Perspectives of extracellular polymeric substances and surface electronegativity

Lysozyme hydrolysis can accelerate waste-activated sludge (WAS) solubilisation, which can significantly shorten the process and promote the efficiency of anaerobic digestion. This study investigated the impact of divalent cations on lysozyme-induced solubilisation of WAS. The performance of lysozyme pretreatment was dramatically inhibited by Mg²⁺ and Ca²⁺. Compared to the control group, the amount of net SCOD, protein, and polysaccharides released to the supernatant were reduced by 36.6%, 44.7%, and 35.8%, respectively, in the presence of divalent cations. The extracellular polymeric substance (EPS) matrix became tightly bound, resulting in fewer proteins and polysaccharides being extracted from loosely-bound EPS (LB-EPS) with divalent cations, which was detrimental to the solubilisation of WAS. Divalent cations decreased the surface electronegativity of sludge particles and prolonged the adsorption of lysozymes by sludge flocs. More than 16.6% of total lysozymes remained in the liquid phase of WAS after 240 min Mg²⁺ and Ca²⁺ strengthened the binding among proteins and polysaccharides and promoted the intermolecular cross-linking of polysaccharides. The EPS matrix formed a dense spatial reticular structure that blocked the transfer of lysozymes from the EPS matrix to the pellet. As a result, the lysozymes accumulated in LB-EPS rather than hydrolysing the microorganism's cell wall. This study provides a new perspective on the restriction of WAS pretreatment with lysozymes and optimises the method of lysozyme-induced solubilisation of WAS.

Insights into carbon recovery from excess sludge through enzyme-catalyzing hydrolysis strategy: Environmental benefits and carbon-emission reduction

This study introduced the excellent improvement of enzyme cocktail (lysozyme and protease) on hydrolysis efficiency and the role of reducing carbon emission as an alternative carbon source. The best dosing method after optimization was to add four parts of lysozyme at 0 h and one part of protease at 1 h. The extracellular proteins and polysaccharides increased by 118% and 64% respectively under the optimal dosing mode. Enzyme cocktails reduced more organic matters and extended the distribution of sludge particles in the small particle size part. The enzymatic-treated sludge could reduce 21.09 kg CO₂/t VSS if utilized to replace methanol for denitrification carbon source. Enzyme cocktails did better in enhancing both solubilization and hydrolysis than single enzymes under the optimal method. This study will provide a more integrated and comprehensive system for enzymatic pretreatment and new insight into the enzymatic pretreatment enhancing hydrolysis and reducing carbon emission.