Disinfection performance of mixed oxidant, NaClO, and Ca(ClO)2 in anaerobic effluents: effects on microbial quality, physicochemical properties, and trihalomethane formation

Abstract The disinfectant action of the mixed oxidants (MIOXs) was compared with that of sodium and calcium hypochlorite (NaClO and Ca(ClO) 2 ) in the post-treatment of anaerobic effluent. The following responses were evaluated: microbiological reduction (total coliforms and Escherichia coli ); disinfection byproducts formation (total trihalomethanes—THM 4 , i.e., the sum of chloroform (TCM), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform (TBM); and changes in physicochemical parameters: pH, temperature, redox potential, conductivity, chemical oxygen demand, ammonia nitrogen (N-NH 4 + ) and ions: sodium, calcium, magnesium, potassium, chloride, chlorate, chlorite, nitrite, nitrate, sulfate, and phosphate. The experiment was conducted using a jar test system for 20 min at 100 rpm with a dosage of 2 mg of Cl 2 ·L −1 . Microbiological results indicated greater efficiency in the removal of total coliforms for MIOX, with results showing a log reduction from 6.23 log 10 MPN per 100 mL to 1.68 ± 0.13, 1.94 ± 0.08, and 2.53 ± 0.39 log 10 MPN per 100 mL for Ca(ClO) 2 , NaClO, and MIOX, respectively. THM 4 formation was predominantly TCM, with levels below the limit of detection for DBCM, BDCM, or TBM. The increases in electrical conductivity stood out among the physicochemical parameters, being more pronounced for MIOX. This result suggested that MIOX may have promoted greater ionic transport in the aqueous matrix, thereby enhancing disinfection. MIOX also demonstrated greater efficiency in oxidizing nitrogen compounds. This study suggests that mixed oxidants can exert a combined effect on the removal of nitrogen compounds, thereby improving the treatment of anaerobic wastewater containing N-NH 4 + .