Effect of zero-valent iron (ZVI) and biogas slurry reflux on methane production by anaerobic digestion of waste activated sludge.

This study aimed to improve anaerobic digestion (AD) efficiency through the addition of zero-valent iron (ZVI) and biogas slurry. This paper demonstrated that methane production was most effectively promoted at a biogas slurry reflux ratio of 60%. The introduction of ZVI into anaerobic systems does not enhance its bioavailability. However, both biogas slurry reflux and the combination of ZVI with biogas slurry reflux increase the relative abundance of microorganisms involved in the direct interspecific electron transfer (DIET) process. Among them, the dominant microorganisms Methanosaeta, Methanobacterium, Methanobrevibacter, and Methanolinea accounted for over 60% of the total methanogenic archaea. The Tax4Fun function prediction results indicate that biogas slurry reflux and the combination of ZVI with biogas slurry reflux can increase the content of key enzymes in the acetotrophic and hydrotrophic methanogenesis pathways, thereby strengthening these pathways. The corrosion of ZVI promotes hydrogen production, and the biogas slurry reflux provided additional alkaline and anaerobic microorganisms for the anaerobic system. Their synergistic effect promoted the growth of hydrotrophic methanogens and improved the activities of various enzymes in the hydrolysis and acidification phases, enhanced the system's buffer capacity, and prevented secondary environmental pollution. PRACTITIONER POINTS: Optimal methane production was achieved at a biogas slurry reflux ratio of 60%. Biogas slurry reflux in anaerobic digestion substantially reduced discharge. ZVI addition in combination with biogas slurry reflux facilitates the DIET process.

Effect of thermal hydrolysis pretreatment on the occurrence of N-acyl homoserine lactone during anaerobic digestion of waste activated sludge.

This study sought to investigate the relationship between N-acyl homoserine lactones (AHLs) and methanogenic microorganisms, focusing on endogenous AHLs in the anaerobic digestion (AD) process. By analyzing waste activated sludge (WAS) samples, we examine the changes in microbial communities and the AHLs-methanogens connection. The Mantel test and Spearman correlation analysis were conducted to gain novel insights into the AD process. Our findings demonstrate that thermal hydrolysis pretreatment (THP) modifies AHL concentrations during AD, thereby enhancing methanogenic bacteria activity and regulating social interactions among microorganisms. In the Eth group (AD of THP samples labeled Eth), Methanobacterium and Methanosarcina accounted for over 80% of the methanogenic bacteria, with correlation coefficients greater than 0.5 between these bacterial taxa and N-hexyl-l-homoserine lactone (C6-HSL) and N-enanthyl-l-homoserine lactone (C7-HSL).

Characteristics of digested sludge-derived biochar for promoting methane production during anaerobic digestion of waste activated sludge.

This study investigated a novel method for enhancing methane production during anaerobic digestion of waste activated sludge with digested sludge-derived biochar (DSBC). Using response surface methodology, the following process conditions for DSBC synthesis were optimized: heating rate = 13.23 °C/min, pyrolysis temperature = 516 °C, and heating time = 192 min. DSBC significantly enhanced the methane production by 48 % and improved key coenzyme activity that accelerated the bioconversion of organic matter while promoting the decomposition and transformation of volatile fatty acids. Consequently, the lag period of methane production was shortened to 4.89 days, while the average proportion of methane greatly increased to 73.22%. Thus, DSBC could facilitate efficient methanogenesis in the anaerobic system by promoting electron transfer between syntrophic partners through the charge-discharge cycle of surface oxygen-containing functional groups. The study provides a reference for the resource utilization of anaerobic sludge residues and efficient anaerobic methanogenesis from sludge.

Investigating the synergic role of asynchronous dosed protease and lysozyme for facilitating excess sludge solubilization and acidogenic fermentation.

Improving the anaerobic fermentation (AF) efficiency of excess sludge (ES) is essential for attaining biosolid minimization, stabilization, resource recovery, and carbon-emission reduction. Along these lines, here, the synergistic mechanism of protease and lysozyme for enhancing hydrolysis and AF efficiency with better recovery of volatile fatty acids (VFAs) was thoroughly investigated. Single lysozyme was capable of reducing the zeta potential and fractal dimension when dosed into the ES-AF system, which was beneficial for increasing the contact probability between proteases and extracellular proteins. Moreover, the weight-averaged molecular weight of the loosely-bound extracellular polymeric substance (LB-EPS) reduced from 1867 to 1490 in the protease-AF group, which facilitated the penetration of EPS by the lysozyme. The soluble DNA and extracellular DNA (eDNA) of the enzyme cocktail pretreated group increased by 23.24 % and 77.09 %, and the cell viability decreased after 6-hour hydrolysis, demonstrating a better hydrolysis efficiency. Remarkably, the asynchronous dosed enzyme cocktail pretreatment was proven a better strategy to enhance both the solubilization and hydrolysis processes since the synergistic effect of these two enzymes can exclude the mutual interference. As a result, the VFAs were increased by 1.26 times higher than the blank group. The underlying mechanism of an environmental-friendly and effective strategy was examined to promote ES hydrolysis and acidogenic fermentation, which was beneficial for the recovery of VFAs and carbon-emission reduction.

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.