Characteristics of the disinfection by-products (DBPs) and process control techniques of the disinfection process using preozonation and post-chlorination

The pre-ozonation process oxidizes humic acids into simple formic acid thus reduces the formation potentials of THMs and HAAs during the subsequent chlorination process. Laboratory results indicate that with addition of 24.3 mg/l ozone to the sample in a 5-liter closed constant-temperature batch reactor, the ORP value increases from an initial 210 mv to 909 mv for a reaction time of 50 minutes. Meanwhile, reduction of TTHMFP (Total Trihalomethane Formaton Potential) and HAAFP (Haloacetic Acid Formation Potential) reach 80% and 60%, respectively. If the sample is spiked with Bromide ion (Br ^- ) and the reaction is carried out under similar conditions, the TTHMFP is much higher than that in the absence of Br ^- . In contrast, the HAAFP in the presence of Br ^- is only 69% of that in the sample without Br ^- . In order to reduce the HAAFP and TTHMFP to acceptable limits, the reaction should be allow to proceed until its ORP reaches 900 mv. When the ORP value exceeds 900 mv, the resulting THMs and HAAs in the sample after the subsequent chlorination process can be reduced to 0.1 mg/l and 0.06 mg/l, respectively.