Successful establishment of denitrification-enhanced anaerobic digestion with nitrate as an exogenous stimulant for effective hydrolysis and methanogenesis of N, N-dimethylformamide-containing wastewater

A new strategy of denitrification-enhanced anaerobic digestion (DEAD) was successfully established for effective anaerobic digestion (AD) of N, N-dimethylformamide (DMF) by using nitrate as an exogenous stimulant. It was the first time that stable anaerobic treatment of high-strength DMF-containing wastewater was realized in this study, and verified by a lab-scale up-flow anaerobic sludge blanket (UASB). Synthetic wastewater with a constant DMF concentration of approximately 2000 mg/L and an organic loading rate (OLR) of approximately 3.0 kg COD/m3/d was effectively treated during a 350-day long-term operation. The UASB initially realized a rapid start-up and maintained a high DMF removal with stable methane production for two months due to the enrichment of DMF-hydrolyzing bacteria (DHB) contained in the seed sludge. However, hydrolysis of DMF was gradually weakened due to the continuous decaying of DHB, and these DHB were unable to proliferate anaerobically. Dosing nitrate to the UASB resulted in a significant recovery of both DMF removal and methane production. Since DHB like Paracoccus are also known as denitrifying bacteria, dosage of nitrate significantly stimulated and enhanced their growth and DMF-hydrolyzing ability under the condition of simultaneous denitrification and methanogenesis (SDM), facilitating the establishment of the DEAD system: Nitrate stimulates DHB to perform heterotrophic denitrification to maintain the proliferation by competing intermediates with methane-producing archaea (MPA), while the rest of intermediates can also be consumed for methanogenesis. After long-term of adjustment, a molar ratio of approximately 1.59 for nitrate to DMF (N/D ratio) could realize an effective removal of 98.8% for the anaerobic degradation of DMF with stable methane production rate of 0.60 L/L. This study introduces a new direction to enhance the methanogenic degradation of degradation-resistant organic matters based on the new concept of DEAD.