Unlocking assimilative municipal wastewater treatment and nitrogen recovery using volatile fatty acids as external carbon sources.

Heterotrophic ammonium assimilation (HAA) represents an emerging approach to conventional biological processes by enabling simultaneous removal and recovery of reactive nitrogen and organic matter from municipal wastewater through assimilation as microbial biomass, with the potential advantage of limiting dissimilatory pathways associated with gaseous nitrogen loss and N2O emissions. This study investigated strategies to maximize HAA by activated sludge through the manipulation of process parameters such as dissolved oxygen (DO) concentration and C:N ratio, using volatile fatty acids (VFAs) as external carbon source. Batch experiments were conducted under strict aerobic conditions to determine NH4+-N and COD removal via HAA. At a DO > 6 mg O2/L, the addition of VFAs yielded NH4+ assimilation efficiencies up to 67 and 99 % for C:N 10 and 15, respectively. Based on these results, the process long-term feasibility was evaluated with acetate as carbon source in a bench-scale continuously stirred tank reactor (CSTR), operated for 141 days. At C:N ratio 15 and an average DO of 2.3 ± 1.2 mg O2/ L, nearly complete NH4+-N assimilation (99.1 ± 0.3 %) was achieved with effluent inorganic nitrogen (NH4+-N, NO2--N, NO3--N) and soluble COD concentrations of 1.1 ± 0.4 mg N/L and 53.4 ± 19.5 mg/L, respectively. Microbial community analysis revealed the enrichment of Comamonas, (up to 88.6 %) suggesting a potential role in nitrogen assimilation. Overall, these results demonstrate that effective NH4+ removal can be achieved through HAA by regulating process parameters and supplying VFAs, laying the foundation for circular nutrient management in municipal wastewater treatment.