LAUSR

Abstract Synergistic treatment via thermophilic microaerobic digestion and microcurrent was adopted to release the internal carbon source from excess sludge, and the effects of the microcurrent on sludge digestion and carbon source release were investigated. Compared with the control group where no current was applied, the digestion supernatant in the tested R1-45 °C reactor maintained a relatively lower phosphate concentration. The optimal technical parameters included a digestion temperature of 45 °C, a current intensity of 175 mA, and a digestion time of 96 h; the concentrations of soluble chemical oxidation demand (SCOD), total nitrogen (TN), and total phosphate (TP) for the test group were 12 189, 903, and 105 mg L−1, respectively, and the ratios of SCOD/TN and SCOD/TP reached 13.5 and 116.1. The sludge flocs were disaggregated into smaller particles upon thermophilic treatment coupled with microcurrent application, and abundant organic matters were released into the digestion supernatant, resulting in a rapid increase in the SCOD. The hydroxyl radicals produced by the microcurrent could directly oxidize fatty acids into end molecules, and it also facilitated regeneration of NAD+ (nicotinamide adenine dinucleotide) through the oxidation of NADH; the orthophosphate in digestion supernatant could be continuously absorbed to synthesize ATP, accompanied by accumulation of acetic and propionic acids. In addition, synergistic treatment accelerated the production of humic acid-like substances and played an important role in the transformation of soluble microbial products.