Transitioning Deammonification from Sidestream to Main-Stream Treatment: Long-Term Comparison of Integrated Fixed Film Activated Sludge and Moving Bed Biofilm Reactors with Polyurethane Foam Carriers at Lab-Scale

Abstract

Deammonification, which is based on partial nitritation and anammox (PN/A), is a well- established sidestream treatment for nitrogen removal. However, transferring deammonifi- cation to mainstream wastewater treatment remains challenging due to low temperatures, the need to retain slow-growing anammox bacteria (AnAOB), and their competition for nitrite with nitrite-oxidizing bacteria (NOB) and heterotrophic denitrifiers. This work investigates cubic polyurethane foam carriers to promote growth and retention of AnAOB. A moving bed biofilm reactor (MBBR) and an integrated fixed-film activated sludge (IFAS) reactor were compared over a three-year experimental period at lab-scale. The feasibility of the biofilm carriers for deammonification was first evaluated under sidestream conditions, followed by a stepwise transition to mainstream operational conditions. The impact of operational parameters, including dissolved oxygen concentration, pH value, and aeration strategy, was evaluated with respect to the activity of aerobic ammonium-oxidizing bacteria (AOB), NOB, and AnAOB, as well as nitrogen removal rates. Deammonification reached ni- trogen removal rates of 0.04–0.12 kg N m−3 d−1 (IFAS reactor) and 0.02–0.28 kg N m−3 d−1 (MBBR) at subphases with reactor bulk concentrations above 60 mg NH4-N L−1. Highest nitrogen removal degrees of 77 ± 6% (IFAS) and 76 ± 5% (MBBR) were achieved at reactor bulk concentrations of 96 mg NH4 L−1 and 97 mg NH4 L−1, respectively. Lower concentra- tions triggered NOB activity in both reactors, leading to an increase in nitrate concentration up to 22 mg NO3-N L−1. AOB and AnAOB activities were on average 6-fold higher on the carriers compared to suspended biomass throughout all experimental phases, demonstrat- ing the feasibility of using cubic polyurethane foam carriers for deammonification. This was also confirmed by fluorescence in-situ hybridization (FISH) measurements. Median nitro- gen removal rates over all experimental phases of 0.07 kg N m−3 d−1 for the IFAS reactor and 0.05 kg N m−3 d−1 for the MBBR were achieved, which are comparable to conventional activated sludge systems performing nitrogen removal via nitrification–denitrification. While at lower nitrogen concentrations, the IFAS reactor yielded superior nitrogen removal rates, peak nitrogen removal rates of 0.28 kg N m−3 d−1 were measured in the MBBR con- figuration. However, controlling NOB activity at lower temperatures and concentrations remains a challenge in MBBR and IFAS configurations. In our study, in the IFAS reactor NOB activities were visible on fewer days than in MBBR. At mainstream-like conditions, higher nitrogen removal rates of IFAS (0.09–0.12 kg N m−3 d−1) were achieved compared to the MBBR (0.06–0.09 kg N m−3 d−1). This demonstrates the advantage of the IFAS reactor Water 2026, 18, 1021 https://doi.org/10.3390/w18091021 Water 2026, 18, 1021 2 of 21 in treating mainstream wastewater via deammonification. As an autotrophic nitrogen removal process, the implementation of deammonification in the mainstream of municipal wastewater treatment plants enables enhanced recovery of biogas from sewage organic matter. The latter would otherwise be consumed during the conventional nitrification- denitrification pathway. Consequently, the overall energy balance for wastewater treatment can be improved, contributing to a more environmentally sustainable process