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

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.