Carbon and nitrogen recovery from biogas slurry via hydrothermal treatment and microbial electrolysis cells assisted anaerobic digestion system

Anaerobic digestion produces substantial biogas slurry comprising abundant nutrients and complex organic matters. In this study, a process combining hydrothermal pretreatment, struvite precipitation and microbial electrolysis cells assisted anaerobic digestion (MEC-AD) was proposed to treat food waste biogas slurry, and the resource and energy recovery efficiencies were evaluated. Both hydrothermal treatment (H) and struvite precipitation (S) essentially increased CH4 production from 9 mL/g COD to 81.9 and 59.9 mL/g COD, respectively. After the combined HS pretreatment, maximum CH4 yield of 227 mL/g COD was achieved in MEC with an applied voltage of 0.8 V. About 23 % of influent COD was converted to CH4, and the residual COD was reduced to 1547 mg/L, greatly saving energy consumption on subsequent wastewater treatment. Overall nitrogen recovery rate of the combined process was 85.8 % and the energy conversion rate was estimated to be 46.2 %. This study demonstrates a novel and economic approach for treating biogas slurry.