Effect of HRT and MLSS on THM precursor removal in the activated sludge process

Abstract

This study was conducted to evaluate the effect of hydraulic retention time (HRT) and mixed liquor suspended solids (MLSS) on trihalomethane (THM) precursor removal in the activated sludge process and to determine the effect of chemical properties of the organic components on trihalomethane formation potential (THMFP). A laboratory-scale reactor for the activated sludge process was used with synthetic domestic wastewater as influent. Experimental conditions, HRT and MLSS, were varied and the effluents were evaluated in terms of DOC and THMFP. Hydrophilicity, hydrophobicity and chemical functional groups were analyzed to determine the chemical parameters that may affect THMFP. Optimum removal of THMFP was achieved at HRT 24 h and at MLSS of ca. 2500 mg l −1. Due to the organic metabolites produced in the activated sludge floc, better DOC or THMFP removal was not observed at longer HRTs and larger MLSS. Decrease in DOC did not result in comparable removal of THMFP. This was attributed to two factors: the higher reactivity of the hydrophobic organic carbon (18.5 μg THMFP mg −1 C) compared to the hydrophilic organic carbon (6.2 μg THMFP mg −1 C) and the nonremoval of the remaining hydrophobic THM precursors in HRTs longer than 8 h. The activated sludge process preferentially removed the hydrophilic organic substances which was shown to have less potential to form THM. The activated sludge process effluents exhibited similar chemical characteristics compared with various types of treated industrial wastewaters determined in a previous study. The activated sludge process effluents contained larger amounts of the hydrophilic organic substances compared to the hydrophobic fractions. Larger amounts of the carboxylic and phenolic-OH functional groups were determined in the hydrophilic fractions than in the hydrophobic fractions. The equation obtained for THMFP prediction based on bulk parameters and specific functional groups of the treated industrial wastewaters was also found to be applicable on the activated sludge process effluents. The equation suggests that bulk THMFP was influenced by UV 260 and organic nitrogen of both the hydrophilic and the hydrophobic fractions, and phenolic-OH of the hydrophilic fractions.