Odour Control Strategies in Wastewater Treatment Plants: Activated Sludge Recycling and Oxidized Nitrogen Recycling

Abstract

One of the main concerns associated with wastewater management is the emission of unpleasant odours. The odorous impact derived from wastewater treatment plants (WWTPs) is a source of public environmental complaints in residential areas near these facilities, since odour pollution can cause significant negative effects on the quality of life and the environment. This odorous contamination typically derives from the presence of volatile sulphur compounds (VSCs) and volatile organic compounds (VOCs), some of which have very low odour threshold values, as is the case of H₂S. For this reason, odour prevention strategies in wastewater treatment facilities need further research to find effective and low-cost technologies for the control of malodorous emissions. The purpose of this study was the reduction of the emission of malodorous compounds in WWTPs based on the optimization of the use of by-products derived from wastewater treatment such as the oxidized nitrogen (N-NO_x) from residual streams rich in nitrate (N-NO₃^-) or nitrite (N-NO₂^-) and activated sludge (AS) from the mixed liquor of the nitrification tank or secondary settler. In the experimental tests, gas-tight 2.1 L glass bottles with synthetic septic wastewater were used to evaluate the potential of N-NO_x and AS at different concentrations to biodegrade H₂S, acetic acid and a-pinene as model odorants. Among the most remarkable results, odorant adsorption losses were observed during preliminary abiotic tests (4 h) with concentration losses of 25 % for H₂S and a-pinene, and 7 % for acetic acid. The experiments carried out at different concentrations of AS (0, 10, 25, 50, 100 mg VSS/L) and oxidized nitrogen concentrations (1.5, 5, 7.5 and 10 mg N-NO_x/L) showed an efficient H₂S removal at 7.5–10 mg N-NO_x/L and 50–100 mg VSS/L. However, NO₃^- supported a more effective H₂S abatement than NO₂^-. The concentration of acetic acid showed a slight decrease due to its degradation by microorganisms (from 27 to 23 ppm_v in 4.5 h), concomitantly with the complete biological oxidation of H₂S. Conversely, a-pinene concentrations experienced a similar gradual decrease than in the abiotic tests, with a low influence of NO₃^-, NO₂^- and AS concentrations. Finally, a marked reduction of NO₂^- was observed when increasing AS concentration, suggesting that higher concentrations of NO₂^- compared to NO₃^- are required for complete biological oxidation of odorants during wastewater treatment.