Effect of hydraulic retention time on electricity generation using a solid plain-graphite plate microbial fuel cell anoxic/oxic process for treating pharmaceutical sewage.

Treatment efficiency and electricity generation were evaluated using a solid plain-graphite plate microbial fuel cell (MFC) anoxic/oxic (A/O) process that treated pharmaceutical sewage using different hydraulic retention times (HRT). Short HRTs increased the volumetric organic loading rate, thereby reducing the MFC performance due to rapid depletion of the substrate (carbon/nitrogen source). The COD removal efficiency decreased from 96.28% at a HRT of 8 h to 90.67% at a HRT of 5 h. The removal efficiency of total nitrogen was reduced from 74.16% at a HRT of 8 h to 53.42% at a HRT of 5 h. A shorter HRT decreased the efficiency in treatment of the pharmaceutical products (PPs), which included acetaminophen, ibuprofen and sulfamethoxazole in an aerobic reactor because these antibiotic compounds inhibited the microbial activity of the aerobic activated sludge in the MFC A/O system. The average power density and coulombic efficiency values were 162.74 mW m and 7.09% at a HRT of 8 h and 29.12 mW m and 2.23% at a HRT of 5 h, respectively. The dominant bacterial species including Hydrogenophaga spp., Rubrivivax spp. and Leptothrix spp., which seem to be involved in PP biodegradation; these were identified in the MFC A/O system under all HRT conditions for the first time using next generation sequencing. Bacterial nanowires were involved in accelerating the transfer of electrons and served as mediators in the SPGRP biofilm. In conclusion, a SPGRP MFC A/O system at a HRT of 8 h gave better removal of COD, T-N and PPs, as well as generated more electricity.