Energy recovery and carbon/nitrogen removal from sewage and contaminated groundwater in a coupled hydrolytic-acidogenic sequencing batch reactor and denitrifying biocathode microbial fuel cell

Developing cost-effective technology for treatment of sewage and nitrogen-containing groundwater is one of the crucial challenges of global water industries. Microbial fuel cells (MFCs) oxidize organics from sewage by exoelectrogens on anode to produce electricity while denitrifiers on cathode utilize the generated electricity to reduce nitrogen from contaminated groundwater. As the exoelectrogens are incapable of oxidizing insoluble, polymeric, and complex organics, a novel integration of an anaerobic sequencing batch reactor (ASBR) prior to the MFC simultaneously achieve hydrolytic-acidogenic conversion of complex organics, boost power recovery, and remove Carbon/Nitrogen (C/N) from the sewage and groundwater. The results obtained revealed increases in the fractions of soluble organics and volatile fatty acids in pretreated sewage by 52 ± 19% and 120 ± 40%, respectively. The optimum power and current generation with the pretreated sewage were 7.1 W m^-3 and 45.88 A m^-3, respectively, corresponding to 8% and 10% improvements compared to untreated sewage. Moreover, the integration of the ASBR with the biocathode MFC led to 217% higher carbon and 136% higher nitrogen removal efficiencies compared to the similar system without ASBR. The outcomes of the present study represent the promising prospects of using ASBR pretreatment and successive utilization of solubilized organics in denitrifying biocathode MFCs for simultaneous energy recovery and C/N removal from both sewage and nitrate nitrogen-contaminated groundwater.

Extracellular electron transfer modes and rate-limiting steps in denitrifying biocathodes

Denitrifying bioelectrochemical system provided an alternative technology for nitrogen removal, even power recovery from wastewater, and its nitrogen removal performance and intermediate accumulation were affected by the extracellular electron transfer modes and rate-limiting steps in denitrifying biocathodes. In the current study, the extracellular electron transfer modes and rate-limiting steps for nitrate reduction and nitrite reduction of denitrifying biocathode were investigated through cyclic voltammetry. When the cathode potential swept from 0.003 to - 0.897 V (vs. Ag/AgCl), denitrifiers were indispensable for electrochemical denitrification. Three peak potentials were found in the cyclic voltammogram of denitrifying biocathode, where E₁ (- 0.471 to - 0.465 V) and E₂ (- 0.412 to - 0.428 V) represented respectively nitrate reduction and nitrite oxidation while E₃ (- 0.822 to - 0.826 V) represented nitrite reduction. Nitrate reduction involved the direct electron transfer mode while nitrite reduction involved the mediated electron transfer mode. Intracellular catalytic reaction was the rate-limiting step for nitrate reduction, independent on the electrochemical activity of denitrifying biocathode and the nitrate supply. The nitrate supply posed an effect on the rate-limiting step for nitrite reduction. The mediator transfer was the rate-limiting step for nitrite reduction in the absence of nitrate. But both mediator transfer and intracellular catalytic reaction became the rate-limiting steps for nitrite reduction in the presence of sufficient nitrate.