Study on the influence mechanism of anionic polyacrylamide on acidogenic fermentation of chemically enhanced primary treatment sludge

Chemically enhanced primary treatment (CEPT) sludge, abundant in organic matter, can be transformed into volatile fatty acids (VFAs) through acidogenic fermentation. However, the presence of anionic polyacrylamide (APAM), a commonly used flocculant in CEPT, may impede the fermentation process. This study aimed to investigate the impact of APAM on the acidogenic fermentation performance of CEPT sludge and to reveal its underlying mechanisms. Fermentation experiments showed that APAM at 5 and 10 g/kg TSS significantly decreased VFAs production, whereas 1 g/kg TSS APAM had no significant impact. APAM altered sludge floc properties and microstructure, suppressing the release of organic matter and the activities of key enzymes in the hydrolysis and acidogenesis. At a dosage of 10 g/kg TSS, the degradation rate of APAM reached 60.4 +/- 1.4 %, and its inhibitory effect on fermentation diminished as it degraded. However, the degradation products exerted varying additional effects on different stages of acidogenic fermentation. Metagenomic and metabolomic analyses highlighted that increasing APAM dosage significantly altered the microbial community structure, inhibiting carbohydrate metabolism, amino acid metabolism, membrane transport and lipid metabolism. APAM reduced the abundance of genes associated with extracellular hydrolases and membrane transport, which impacted the hydrolysis and transport of organic matter. Additionally, the abundance of genes encoding key enzymes in pyruvate metabolism and VFAs synthesis was generally downregulated. Future challenges include the development of effective strategies to mitigate APAM inhibition, optimization of fermentation parameters for APAM-containing sludge, and exploration of environmentally friendly flocculant alternatives.