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Short-chain carboxylic acids (SCCAs) production from sewage sludge via anaerobic fermentation is usually restricted by low substrates availability and rapid products consumption. Therefore, the ultrasound (US)-sodium percarbonate (SPC) technique was proposed to effectively break the bottlenecks. Results showed the total SCCAs yield, acetate yield and particulate organics reduction respectively attained 392.8 mg COD/g VSS, 204.6 mg COD/g VSS and 47.4 % under the optimal condition. Mechanistic explorations disclosed that US + SPC largely reduced biodegradation resistances of particulate organics and improved sludge biodegradability. The destruction of spatial structure was the inherent mechanisms for initial solubilization and further degradation of solid-phase sludge. Besides, US + SCP up-regulated hydrolytic and SCCAs-forming enzymes, but downregulated the key enzyme for methanation. Meanwhile, US + SPC altered the microbial structure and stimulated functional microorganism enrichment, well correlated with substrate biotransformation and products output. Overall, this strategy could effectively enhance SCCAs production from WAS and reduce the environmental risk for subsequent sludge disposal.

Recently, phosphorus recovery as vivianite from sludge digestion system has attracted increasing attention because of its high recovery efficiency and economic value. However, the potential impact of vivianite on anaerobic digestion of waste activated sludge remains largely unknown. This study therefore aims to provide such support. Experimental results revealed that the maximal methane yield decreased from 103.55 to 76.55 mL/g volatile solids, with the vivianite level increasing from 0 to 500 mg P/L. Mechanism explorations showed that vivianite caused more substrates remaining in tightly-bound extracellular polymeric substances, and thus suppressed sludge solubilization. In addition, it was observed that hydrolysis, acidiogenesis, acetogenesis and methanogenesis bio-processes were all inhibited by vivianite. Microbial analysis revealed that vivianite significantly decreased the relative abundances of hydrolytic microbes, acidogens and methanogens. Further investigation showed that vivianite benefited sludge agglomeration and can enhance the mass transfer resistance of anaerobic digestion, further supporting the inhibitions on anaerobic digestion.