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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 …