Enhanced primary treatment for net energy production from sewage – The genetic clarification of substrate-acetate-methane pathway in anaerobic digestion

Organic matters such as cellulose are key components in municipal sewage, and its degradation consume significant aeration energy in biological process. Pre-harvesting organic particles by chemical enhanced primary treatment (CEPT) could prevent energy consumption while increase bioenergy potential in anaerobic digestion (AD). This study provides biological proof of an “substrate-acetate-methane pathway” that may result in energy positive practice in sewage treatment. When applying CEPT sludge in AD, a stable methane yield at 0.42 m³/kg VSS_in was achieved. Meanwhile, the results of mass balance suggested that for biogas conversion of CEPT sewage sludge, cellulose and protein degradation played a key factor at low organic loading operation (2 gVSS/L-day, SRT = 15 days), while lipid degradation was more significant at high loading operation (3 gVSS/L-day, SRT = 20 days). Microbial analysis revealed high abundance of cellulolytic bacteria, cellulosome related genes, and methanogens; and low abundance of syntrophs, suggesting effective degradation and uptake of amorphous cellulose to acetate. An unexpected result showed that cellulosome genes were identified from two high abundance species of Methanosarcina. In line with the reduced total volatile fatty acids and propionate (to 1,200 and 600 mg/L, respectively), such cellulolytic and methanogenic synergy implies unique syntrophs bypass for bioenergy production. With the combination of CEPT sludge and AD, the net energy of sewage treatment could increase from −0.36 kWh to 0.06 kWh per cubic meter wastewater treated, including reduced energy consumption in aeration process.