Occurrence of retinoic acids and their metabolites in sewage and their removal efficiencies by chemically enhanced primary treatment and secondary biological treatment

Sewage treatment plants (STPs) are important in densely populated megacities like Hong Kong to control the release of harmful pollutants from households and industries into the receiving water bodies and maintain water quality for supporting various beneficial uses. This study investigated the occurrence of the teratogenic retinoic acids (RAs) and their oxidative metabolites in sewage and sludge of six selected STPs that treat about 87% of all sewage in Hong Kong annually, and compared the removal efficiencies of these compounds from sewage between two major sewage treatment processes, i.e., chemically enhanced primary treatment (CEPT) and secondary (biological) treatment. The total concentrations of the studied RAs in influent, effluent and sludge from the six selected STPs were found between 21.5 and 33.1 ng/L, 12.0–20.4 ng/L, and 4.33–7.02 ng/g dry weight, respectively. The compounds were dominated by all-trans-RA and 13-cis-RA, together accounting for 46.9–65.6%, 38.4–56.7%, and 62.8–82.8% of the total RAs in influent, effluent and sludge, respectively. The studied RAs could not be satisfactorily removed by both treatment processes with removal efficiencies ranging from 25.4% to 47.4% only, without significant difference in their removal between CEPT and secondary treatment. Based on the calculated hazard quotients of all-trans-RA equivalents (0.248–0.521), the treated effluents from all the six STPs exhibited medium ecological risks to the receiving coastal environment. Therefore, continuous monitoring of these compounds and enhancement of treatment technologies of STPs shall be considered in the future to improve the removal efficiencies of these compounds.