Optimization of activated sludge recycling and oxidized ammonium recycling as odour control strategies in wastewater treatment plants

New odour prevention strategies in wastewater treatment facilities need to be investigated to find effective and low-cost technologies for the control of malodorous emissions. In this study, the potential of activated sludge (AS) and oxidized nitrogen (N-NOx) recycling as an environmentally-friendly and cost-effective strategy for the prevention and minimization of odour nuisance during wastewater treatment was evaluated and optimized using H2S, acetic acid and α-pinene as model odorants in 2.1 L gas tight bottles. This approach uses by-products from wastewater treatment to mitigate odour annoyance. Preliminary abiotic tests showed headspace concentration losses of 25% for H2S and α-pinene, and 7% for acetic acid due to odorant adsorption in 4 h. The experiments carried out at different concentrations of AS (0, 10, 25, 50, 100 mg VSS/L) and oxidized nitrogen (1.5, 5, 7.5 and 10 mg N-NOx/L) revealed an effective H2S removal at 7.5–10 mg N-NOx/L and 50–100 mg VSS/L. Interestingly, NO3− was more effective than NO2− as electron acceptor during the biodegradation of H2S regardless of the AS concentration and N-NOx concentrations. In the presence of dissolved H2S and AS concentrations of 50–100 mg VSS/L, acetic acid was partially metabolized by microorganisms at the end of experiment (from 27 to 23 ppmv in 4.5 h), while α-pinene concentrations experienced a similar gradual decrease than in the abiotic tests. Finally, the assays carried out at concentrations ≥5 mg NO3−/L and 25 mg VSS/L showed a reduction of NO3− to NO2− correlated with the biological oxidation of H2S, which suggested the need to control NO3− supply under sulphur limiting conditions to prevent toxicity problems during wastewater treatment.