Optimization of the carbon to nitrogen ratio for mainstream deammonification and the resulting shift in nitrification from biofilm to suspension

Abstract

Application of the deammonification process to mainstream wastewater promises energy-efficient nitrogen removal, but has been limited by unwanted activity of nitrite oxidizing bacteria and low anammox activity at moderate temperatures (<20 °C). In the present study, N removal in a mainstream integrated fixed-film activated sludge (IFAS) deammonification process increased by 27% to 73% total inorganic nitrogen (TIN) removal by diverting 10% of the primary effluent flow around the A-stage and directly into the deammonification reactor, thereby increasing the influent C : N ratio from 2.3 to 3.1 g sCOD/g NH₄⁺-N. This change coincided with a dramatic shift in nitrification activity from the biofilm to the suspension, and the increased carbon enabled a higher suspended solids concentration at a realistic solids retention time of 7.3 ± 2.1 days. Anammox biomass and activity was retained over the entire study (>3 years) and was not negatively impacted by the increase in influent carbon. N isotope testing indicated that cross feeding between denitrifiers and anammox played an important role in N removal and that about 53% of N removal was ultimately routed through the anammox metabolism. The reactor temperature was controlled near 20 °C for most of the study, and 72% TIN removal was maintained during a temperature decline down to 12 °C (after which TIN removal reduced to an average of 58% from 12 down to 8 °C). Our work demonstrates the impact of small changes in C : N on performance, population structure, and aggregate type (biofilm vs. floc) in mainstream deammonification bioprocesses and provides a simple approach to control C : N in practice.