Insights into the impact of metals in waste activated sludge on its anaerobic digestion: From perspectives of physical structure and organic matter composition

The utilization of chemical phosphorus removers introduces metal fractions into sludge, significantly disrupting its anaerobic digestion (AD) performance. Our previous study explored ascorbic acid's (vitamin C, VC) high efficiency in iron separation, shedding light on Fe's influence on AD. Introducing citric acid (CA), a non-selective metal complexant provides the possibility to mitigate Fe's impact by comparing the digestive performances of CA-treated and VC-treated sludge samples. This comparison aimed to discern the distinct effects of various metals contained in the sludge on the digestion process. Findings revealed a negative correlation between reduced metal content and increased methane production, emphasizing Fe's primary influence, followed by Ca, Mg, and Al. Employing chemical and molecular biological analyses, the study delved deeper into the limiting effects and intrinsic mechanisms of anaerobic organic matter transformation involving metals. It was discovered that reducing iron content in sludge curtailed the proliferation of iron-reducing bacteria. The downregulation of metals also spurred gene expression in hydrolysis, acid production, and methanogenic metabolic pathways. These insights offer a more precise understanding of how different metals affect sludge digestion performance, offering valuable guidance for operational enhancements in anaerobic digestion.