Anoxic/Oxic Treatment Without Biomass Recycle

Abstract

The Anoxic/Oxic (A/O) process involves recirculating mixed liquor between its A and O tanks so that nitrate produced in the O tank can be used to for denitrification with influent COD in the A tank.  Because biomass is recirculated along with nitrate, A/O operation leads to similar microbial communities in the A and O tanks, which may decrease the rates of denitrification and nitrification in each tank.  Here, bench-scale experiments simulated this aspect of the A/O process by exchanging biomass between an anoxic flask and an oxic cylinder at exchange ratios of 0%, 20%, 30%, and 50%.  Nitrification and dentrification rates and the proportion of Live bacteria declined with more biomass exchange:  up to 30% and 50% rate declines, respectively, with 50% biomass exchange.  Phylogenetic analysis documented that the microbial communities became much more similar with biomass exchange, a finding consistent with community composition with a full-scale A/O process in a municipal wastewater treatment plants.  A two-stage vertical baffled bioreactor (VBBR) realized efficient total-nitrogen removal in recirculation without biomass exchange.  Removals of COD and TN were slightly better for the two-stage VBBR than the conventional A/O process, but its hydraulic retention time (HRT) was 55% to 70% of the volume of a conventional A/O process treating the same influent wastewater.  The VBBR was more efficient because its anoxic biofilm was enriched in denitrifying bacteria, while its oxic biofilm was enriched in nitrifying bacteria.

Keywords: Nitrification, Denitrification, biomass cycle, vertical baffled bioreactor, contribution of microbiological community