Characterization Of Disinfection By-products During Pre-chlrorination And The Control Technologies

In potable water treatment, preozonation has been applied throughout the world due to its important role in sterilization, bleaching, algae removal as well as flocculation and the improvement in finished water quality by reducing filtered water turbidity and the formation of disinfection by-products (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs). Recently, ozone applied in water supply treatment has become a research focus in China, but most of the former studies about ozonation were mainly focused on lab and pilot-scale tests and little attention was taken on the treatment effects of preozonation in full-scale plants. It is of great significance to study the characterization of DBPs during preozonation and the removal technlogies, find the optimal balance point among water quality indicators such as bromate, carbonaceous DBPs (C-DBPs) and nitrogenous DBPs (N-DBPs) to provide a theoretical basis and practical experience in optimizing water treatment processes and improving the safety of drinking water quality.The source water in Tianjin was used in this contribution to learn the effects of preozonation on DBPs formation. The CT value of preozonation process was optimized and the effects of preozonation on C-DBPs and trichloronitromethane (TCNM) formation were investigated. The following main conclusions can be drawn:The lab-scale experiments of the effects of CT value on the DBPs formation during preozonation showed that the UV254of the effluents after preozonation dropped a lot; dissolved organic carbon (DOC), trihalomethanes formation potential (THMFP) and haloacetic acids formation potential (HAAFP) also decreased under different preozonation CT values. The bromate level in each effluent was also within the maximum contaminant level (MCL) set by Standards for Drinking Water Quality of China (GB5749-2006). The initial ozone dosage of1.35-2.0mg/L and the contact time of10minutes were determined as the optimal CT value (13.5-20.0mg/L·min) during the preozonation of raw water used in this study. In practical preozonation process of waterworks, lower initial ozone dosage (1.35-1.5mg/L) can be used with reagard to the raw water with low temperature and low turbidity. As to the raw water in summer with high temperature and high amounts of algae, the initial ozone dosage can be appropriately increased to2.0mg/L. Chemical and biological safety of drinking water can be guaranteed under such treatment conditions.The research on the characterization of DBPs formation under the optimal preozonation CT value showed that, for the experimental raw water, molecular weight (MW)>30kDa fraction was the main precursor of THMs and HAAs. The THMFP and HAAFP were about25.14%and40.36%of the raw water, respectively. MW10-30kDa fraction was the most reactive precursor to the formation of both THMs and HAAs. For the effluent of preozonated water, MW<1kDa fraction had the main contribution to the formation of THMs and HAAs. MW1-3kDa fraction was the most reactive precusor to the formation of THMs, while the MW3-10kDa fraction was the most reactive precusor to the formation of HAAs. Preozonation caused the oxidation of large molecules and altered the MW distribution towards smaller molecules.With the addition of potassium bromide solution to the raw water, it concluded that when the concentration of bromide in the experimental raw water was143.521μg/L, there was10.402μg/L bromate generated in the preozonated water under the optimal CT value. Thus, it was determined that the concentration of bromate in the preozonated water was probably beyond the MCL of Standards for Drinking Water Quality of China when the concentration of bromide in the Luan river water reached285μg/L. The study on optimizing the preozonation process under such bromide concentration in the raw water showed that making preozonation contact time20min, adjusting the pH of the raw water to6.0and adding a certain concentration of ammonia can achieve dual-purpose for the removal of DBPs formation potential and the minimization of bromate generation.Through the experiments of factors affecting the trichloronitromethane formation potential (TCNMFP) during the preozonation of amino acid solutions, it concluded that the increases in TCNMFP of glcine, lysine and typtophane were positively related to the increase in initial ozone dose. TCNMFP of each preozonated AA sample rised with the reaction time lasting. The effect of pH on TCNMFP of AAs by preozonation was compound-specific. More TCNMFP was tested in all of the four AAs samples in the presence of nitrite. But as the concentration of NO2-increasing to higher levels, the overall extent of formation enhancement was not so sigificant.The study of the effect on TCNMFP during the preozonation of practical raw water showed that, for the experimental raw water, dissolved organic nitrogen (DON) fraction of MW>30kDa was the main precursor of TCNM. MW10-30kDa fraction was the most reactive precursor to the formation of TCNM. For the effluent of preozonated water, MW<1kDa fraction had the main contribution to the formation of TCNM. The effect of preozonation on the increase of TCNMFP may be due to the formation potential of this DON fraction. The fractions of MW3-10kDa, MW10-30kDa and MW>30kDa were all among the most reactive precusors to the formation of TCNM. TCNMFP increased dramatically after preozonation of the raw water. When the ozone dosage was low, TCNMFP reduced by a low level and with the increase of ozone dosage, TCNMFP got increased. By extending the reaction time, TCNMFP first decreased and then increased. The TCNMFP had the minimum value after5min’s reaction. The TCNMFP of preozonated water had a sharp increase as the pH rised. Strongly acidic environment (pH=4) could help control TCNM formation during preozonation process. When the raw water was under critical bromide concentration condition, in the premise of ensuring bromate content was not excessive, less or no ammonia could be added in the raw water to control the formation of TCNM.