A Thesis submitted in partial requirements for the degree of fulfillment Master of of the Science, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 1990

This stud y investigated the occurrence of reaction products with public health significance when chlorine dioxide disinfection is adopted in place of the current chlorination at Bangkhen Water Treatment Plant . An earlier study at the plant (GHAZALI,1989) h ad revealed a threat to public health in the future from continued chlorination due to the formation of excessive amounts of trihalomethanes (THMs ) in drinking water. The THMs, arise from t h e reaction of chlorine with a variety of organic compounds present in the water a nd, have toxic effects. The present study involved monitoring the resulting levels o f THMs, chlorates , chlorites a nd residual chlorine dioxide in the untreated water and after adding the disinfectant. Chlorine dioxide doses of 1 to 10mg/L were applied to filtered unchlorinated water from Bangkhen Water Treatment Plant. The organic content of t h e water was on average about 2.4mg/L , as the total organic carbon (TOC ) . The trihalomethanes (THMs ) were measured by the head space - gas chromatographic method. Chlorine dioxide residuals were monitored with the simple iodometric method . Chlorates and chlorites were quantified u sing iodometric titrations while varying the pH in a de-aerated medium. Neither bromine nor THMs were found in t he filtered water from the treatment plant . After application of chlorine dioxide dioxide, no THMs were detected in t h e treated water . Very l ow levels of chlorites and chlorates ( 0.1mg/L) were observed in t he finished water even after 48 hours of contact after the dosing of chlorine dioxide in excess of 10mg/L. The stability of the chlorine dioxide was evident from the observed ( - 0 .lmg/L) residuals at the end of 48 h ours contact time. From t h is study it can b e seen that chlorine dioxide at doses of 1mg/L to 1 .2mg/L constitutes a viable alternative for Bangkhen Water Treatment Plant to the present chlorination for curbing THMs and adequate disinfection of t h e water.