Effective control of chlorination and dechlorination at wastewater treatment plants using redox potential

The shortcomings of total chlorine residual as a measure of wastewater disinfection are discussed. The effect of pH is significant in the toxicity of chlorine compounds. Recent investigation to the formation of organic chloramines made the residual measurement technique more difficult to use in wastewater disinfection. Redox potential (or oxidation–reduction potential—ORP) is studied as an alternative way of assessing the efficacy of disinfectants through theoretical consideration and literature review. Experiments performed using wastewater samples showed that ORP is better correlated to the inactivation of coliform than total chlorine residual. Also illustrated is a case study carried out at a municipal wastewater treatment plant in California (U.S.). By replacing conventional methods for establishing chlorine and sulfur dioxide dosage with a system that automatically modulates chemical feed by ORP, the plant reduced chemical use for chlorination and dechlorination by 47 and 62%, respectively, while consistently maintaining compliance with stringent disinfection requirements (2.2 MPN coliform per 100 mL) and chlorine discharge limits (less than 0.1 mg/L).