A systematic study of mitigation strategies to prevent the emission of malodours from primary settlers in wastewater treatment plants

Odor emissions from wastewater treatment plants (WWTPs), primarily driven by sulfur-containing compounds, represent a major environmental and operational challenge that requires effective mitigation strategies. The present study assessed the effectiveness of different strategies of sulfur-borne odor mitigation in the primary settler of a pilot-scale WWTP. The most effective technique was the dosing of the precipitating agent FeCl 3 , which showed removal efficiencies (REs) of 88.5 ± 7.3% for H 2 S and 63.3 ± 3.5% for methyl mercaptan (CH 3 SH), using an optimal Fe (III): S molar ratio of 2:1 based on the influent characteristics of the primary settler. The addition of oxidants (H 2 O 2 , NaClO) in the primary settler demonstrated a low efficiency in mitigating sulfur-containing gas emissions, potentially due to the occurrence of scavenging reactions. However, chemical oxidation with sodium hypochlorite (NaClO) exhibited a strong oxidative capacity, achieving a 98.2 ± 1.7% RE of total dissolved sulfides (S 2− ) at an estimated effective molar ratio of 16:1 (Cl: S). Biological oxidation of odorants induced by nitrate (NO 3 − ) addition and activated sludge recirculation demonstrated moderate efficiency, attributed to the enhanced growth of denitrifying bacteria, which led to a rapid NO 3 − depletion and subsequent recovery of S 2− .