The effect of Mg2+ on digestion performance and microbial community structures in sludge digestion systems

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.

Unveiling the Mechanism of Urine Source Separation-Derived Pretreatment on Enhancing Short-Chain Fatty Acid Yields from Anaerobic Fermentation of Waste Activated Sludge

A novel strategy employing urine wastewater derived from source separation technology, to pretreat waste activated sludge (WAS) for promoting yields of short-chain fatty acids (SCFAs), has been proposed in this study. It was found experimentally that SCFA production could ascend up to 305.4 mg COD/g VSS (volatile suspended solids) with a urine volumetric proportion of 1:2 to the whole reaction system, being 8.8 times that produced in the control. Exploration of the mechanism indicated that WAS disintegration was significantly enhanced due to the synergistic effect of urea and free ammonia (FA). Degradation rates of model organic substrates and measurements of critical enzymatic activities demonstrated that hydrolysis and acidogenesis were inhibited under high urine content (urine proportion of 1:2), while not significantly affected under low urine content (i.e., 1:4), which might be attributed to metal ions existing in urine wastes alleviating the inhibition induced by FA. In contrast, methanogenesis was negatively suppressed by any urine concentration owing to its higher sensitivity to the environmental variations. Shift of microbial population further elucidated the abundance of hydrolytic and acidogenic microbes were enriched in the fermenters with urine addition. The findings provide a new thought for recovering resources from wastes, potentially reducing the pressure of sewage and sludge treatment in wastewater treatment plants.