Abstract
Nitrogen removal and methane production from animal wastewater were investigated in two-phase anaerobic digestion (TPAD) coupled with biological nutrient removal (BNR) process at 35°C. Methane production rate was approximately 0.7 L/L per day at organic loading rate (OLR) of 3.9 gTCOD/L per day in methanogenic up-flow anaerobic sludge blanket (UASB). Conversion efficiency of the removed TCOD in UASB into methane was as high as 72% and overall TCOD removal efficiency in this system was over 97%. Denitrification activities of both methanogenic and acidogenic sludges were evaluated in batch experiments and their specific denitrification rates (SDNRs) were in the range of 0.017–0.024 gNO3−-N/gVSS per day. Denitrification inhibited methanogenesis in the presence of both nitrate and nitrite, while it occurred simultaneously with methanogenesis in the presence of nitrite alone. Partial nitrification of the UASB effluent was successfully performed, while autotrophic denitrification was not observed in the subsequent aerobic/anaerobic biofilters, which resulted in low TN removal. With recirculation of the nitrified effluent to TPAD, nitrogen oxides were completely removed by anaerobic denitrification in the acidogenic CSTR or methanogenic UASB. Overall TN removal efficiency in the TPAD-BNR system was as high as 96%.