The potential of activated sludge recycling (ASR) and oxidized nitrogen recycling (ONR) to prevent the emissions of H2S and acetic acid from the primary settler during domestic wastewater treatment was herein evaluated. The pilot plant consisted of an 8 L primary settler with a 10 L gas-tight headspace coupled to a 11 L nitrification-denitrification activated sludge reactor and an 8 L secondary settler, which were monitored for 175 days. A reduction in the headspace concentrations of H2S and acetic acid by 95 % and 42 %, respectively, was recorded when combining ASR and ONR. Process operation with ASR and ONR supported stable conditions with average organic matter removals of 96 ± 2 %, NO2- concentrations of 24.2 ± 0.4 mg N/L and NO3- concentrations of 9.8 ± 0.4 mg N/L in the effluent, and a biological oxidation of S2- higher than 99 % with average SO42- concentrations of 52 ± 8 mg/L. Interestingly, the sole recirculation of activated sludge to the primary settler without NO3- recycling caused sludge bulking, contributing to increase the concentration of H2S and acetic acid in the primary settler headspace up to 0.99 ± 0.01 ppmv and 2.87 ± 0.12 ppmv, respectively. Sludge bulking also resulted in an increase in the effluent soluble total nitrogen concentration from 5.6 ± 0.1 mg N/L to 50.8 ± 0.2 mg N/L and of NH4+ from 1.3 ± 0.2 mg N/L to 50.7 ± 0.8 mg N/L due to the loss of nitrification under these operational conditions. Overall, the experimental results indicated that ASR and ONR represent cost-effective strategies for the control of malodorous emissions in wastewater treatment plants.
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