Study On The Controls Of Chloral Hydrate In Water Purification Process

Chloral hydrate(CH) is the next prevalent disinfection by-product(DBP) with genotoxicity and teratogenicity in addition to trihalomethanes and haloacetic acids. Due to the megathermal and rainy climate and the frequent outbreak of algae in Southern Area perennially, CH yields during drinking water treatment easily exceed the limit of 10 μg/L. However, there are no cost-effective methods to control CH in practice and literature. The formation and removal of CH of polar organic compounds in different treatment processes were studied, aiming to provide guidance to the establishment of control scheme. The enhanced coagulation and GAC-sand dual media filter were used to control CH for different water sources and treatment process in Southern Area.The results on formation and removal of CH of hydrophilic and hydrophobic materials in conventional and ozone-biological activated carbon(O3/BAC) processes are as follows: the removal rate of CH and its precursors by the coagulation-sedimentation process was the largest, but the removal rate in the sand filter unit was still low. The organic matter in raw water was mainly consist of hydrophilic materials, the next were hydrophobic and weakly hydrophobic materials. Algae, particulate organic matter and large molecule hydrophobic materials were primarily removed by the coagulation-sedimentation process, while hydrophilic substances were removed by the BAC process. There was higher risk for CH yields reaching the limit of 10 μg/L in conventional process. The most cost-effective measures to control CH and its precursors in conventional process were enhanced coagulation sedimentation process and improving the removal by sand filter.The results of change of coagulation condition during coagulation are as follows: when the p H was between 6.0 and 7.0, the new coagulant had a better effect on the removal of CH precursors than polymeric aluminum chloride(PAC). On the other hand, the findings were opposite as the p H was between 7.0 and 8.5. The CH yields were minimum by the treatment of both, as the p H was 7.0. The new coagulant had a better effect on removal of CH precursors than PAC at low dosages of 2 to 6 mg/L. The removal rates of CH precursors by PAC and new coagulant were 63.20% and 69.60%, respectively, under the optimum dosage of 6 mg/L. PAC had a better effect on removal of CH precursors than the new coagulant at high dosages of 6 to 10 mg/L. PAM had a better effect on removal of CH precursors than HCA.The results of adsorption in combination with coagulation are as follows: powdered activated carbon could be used to remove CH precursors by adsorption alone or in combination with coagulation. As the optimum dose of powdered activated carbon was 30 mg/L, the removal rates to CH yields for one day’ cultivation(CH1d) and CHFP were 57.49% and 75.21%, respectively. Compared to coagulation alone, the removal rates of CH1 d and CHFP by powdered activated carbon in combination with coagulation were increased by 14.26% and 18.49 %, respectively. Powdered activated carbon and bentonite had a good effect on CH adsorption.The results of pre-oxidation in combination with coagulation are as follows: CH precursors were removed by KMn O4, Cl O2, H2O2 and ultrasonic combined with coagulation to the highest extent. The optimum concentration of KMn O4, Cl O2, H2O2 were 0.4 mg/L, 0.5 mg/L, 3.0 mg/L, respectively. The optimum parameters for ultrasonic were as follows: ultrasonic frequency was 40 KHz、ultrasonic power was 70 W and lasted time was 15 s. Compared to coagulation alone, the removal rates of CH1 d and CHFP by KMn O4, Cl O2, H2O2 and ultrasonic combined with coagulation were increased by 9.67% and 2.36%, 12.18% and 2.15%, 6.37% and 5.89%, 4.34% and 5.84%, separately. The CH yields increased by the combination of Cl2, O3 and O3/H2O2 with coagulation, and the risk was greater when O3/H2O2 combined with coagulation.GAC-sand dual media filter was employed to remove CH and its precursors, and the results are as follows: the filter had a good effect on the removal of CH and its precursors, and the mean concentrations of CH and its precursors in effluent were below 1.00 μg/L and 15.20 μg/L, separately. The mean removal rate of CH and its precursors by the filter were 99.11% and 41.38%, respectively. Under the optimum layer of GAC and sand of 1.0m and 0.4m, respectively, the removal rate of CH precursors was 42.37%. The optimum gas-water ratio of the aerated GAC-sand dual media filter was 0.14, removal rate of CH precursor could increased by 4.80%. 8 m/h was the optimum rate of filter, with the corresponding removal rate of 44.63% to CH precursors.