Effects Of Pre-ozonation On Disinfection By-products For Micro-polluted Raw Water

The pre-chlorination process can effectively control pathogenicmicroorganisms and the cost was very low. Therefore, the pre-chlorination processwas widely used in water supply plants in the world. However, during thepre-chloriantion process, the natural organic matter（NOM） in raw water reacted withchlorine to form disinfection by-products（DBPs）, of which trihalomethane（THMs）and haloacetic acids（HAAs） were considered carcinogenic. Therefore, newtechnology of pre-oxidation is required, to decrease DBPs and to keep the dringkingwater safe. Pre-ozonation process is a promising pre-oxidation process that are beingstudied.The source water in Tianjin was used in the lab-scale study. The effect ofpre-ozonation on DBPs was studied by changing the operational parameter; afull-scale study was also placed in a water supply plant in Tianjin. It was carried outduring different water quality periods and with different source waters. Theperformance of pre-ozonation on raw water was studied, and the effect ofpre-ozonation on DBPs was expecially estimated. The different effects between thepre-chlorination and pre-ozonation were investigated. It is significative to study theapplication of pre-ozonation in full-scale plant.The lab-scale research on pre-ozonation showed that the formation potential oftrihalomethanes （THMFP） and haloacetic acids（HAAFP） decreased by increasingthe ozone dosage, but the high concentration of ozone might lead to the formation ofbromate. When the reactiion time was low, THMFP and HAAFP increased by a lowlevel, while by extending the reaction time, THMFP and HAAFP were reduced.With a low reaction time, bromate was formed rapidly. When the raw water wasacidic, the concentration of THMFP and HAAFP got larger by increasing pH; whenalkaline, the result was opposite. There was a great formation of bromate when thewater was alkaline.As to the Luan River water in summer with high amounts of algae, the full-scale study showed that the removal rates of pre-ozonation on algae,Chlorophyll a, UV₂₅₄and SUVA were45.76%,44%,38.21%and32.07%,respectively. Ozone decomposed the high molecular weight（MW） organic mattersinto lower MW ones, such as organic matters with MW between3kDa and10kDaor between1kDa and3kDa. After pre-ozonation, about12.43%of THMFP and15.06%of HAAFP were reduced. The removal of THMFP was attributable to thereduction of the formation potential of Chloroform and Bromodichloromethane;while the removal of MCAA, DCAA and TCAA contributed to the decrease ofHAAFP. The concentration of bromate was below6μg/L. Formaldehyde wasformed to some extent, and then removed effectively by the follow-up processes. Inthe final effluent, the concentration of formaldehyde was33.7μg/L. The otherprocesses following pre-ozonation gave a further treatment and the final effluentshowed a good water quality, which suggested an excellent treatment of the waterwith high alage.With regard to water in winter with low temperature and low turbidity, thefull-scale study showed that the removal rates of pre-ozonation on algae,Chlorophyll a, UV₂₅₄and SUVA were34.45%,34.35%,38.21%and32.07%,respectively. After pre-ozonation, about30.95%of THMFP and33.61%of HAAFPwere reduced. The removal of THMFP was attributable to the reduction of theformation potential of Chloroform and Bromodichloromethane; while the removalof DCAA and TCAA contributed to the decrease of HAAFP. No bromate wasdetected from the effluent after pre-ozonation. Formaldehyde was formed to someextent, and then removed effectively by the follow-up processes, in the final effluent,the concentration was70.12μg/L. The other processes following pre-ozonation gavea further treatment, and the final effluent showed a good water quality, whichsuggested an excellent treatment of the water with low temperature and low turbidity.The performance of the processes also showed an excellent ability to treat differentsource waters.Comparing the effects of the two different pre-oxidation processes,pre-chlorination and pre-ozonation, to treat raw waters in different water qualityperiods, it showed the same results that pre-ozonation had a better effects to treat alage, colony, UV₂₅₄, SUVA, THMFP and HAAFP. The treatment processes withpre-ozonation produced drinking water with better water quality. The pre-ozonationprocess may reduce the demand of coagulating agent and chlorine, remove theDBPFP and control the formation of by-products of ozonation.In conclusion, pre-ozonation had a better effects to treat alage, colony, UV₂₅₄,SUVA, THMFP and HAAFP, compared to pre-chlorination. The processes appliedin the water supply plant with pre-ozonation had a good ability to treat raw waters indifferent water quality periods or from different water sources. The water quality offinal effluent met the drinking water standards. The DBPFP was reduced and theformation of by-products of ozonation was controlled effectively. The processeswith pre-ozonation had an excellent effect to treat the source waters in Tianjin.