Study On The Generation Characteristics And Controls Of Chloral Hydrate In Drinking Water In Shenzhen Area

Disinfection by-products (DBPs) is one of drinking water quality safety issues thatcause global concern. In2006, Chinese drinking water quality standard included ChloralHydrate (CH) for the first time, but currently there is very little relevant research aboutCH in China? also no definite conclusion for its influence factors, and lack ofcomprehensive control guidelines only for CH. The characteristics of water quality,water treatment processes, as well as regional, climatic conditions etc. in Shenzhen tendto exist a greater risk for CH exceed to the standard. Because of conventional DBPscontrol methods lack pertinence for CH, it is difficult to achieve the requirements ofcost-effective goals in the drinking water plant production. Therefore, this paper aimedat carriying out research of generation features and control technologies for CH indrinking water on the basis of Shenzhen raw water quality characteristics and watertreatment process conditions to provide technical guidance to prevent or handle thesituaion when CH exceed the standard in shenzhen out-factory water.In this paper, raw water from the Shenzhen Reservoir was the main study object,and the chlorine content, pH value, temperature and other factors on the formation ofCH were discussed to provide guidance for forming control scheme. The control effectson CH of several methods such as enhanced coagulation, powder activated carbon,changing content and type of disinfectant, activated carbon filter etc. were studted. Mainresults obtained are as follows:(1) Using chlorine as the disinfectant under laboratory conditions, chlorine dosage,reaction time, pH value, temperature on the formation of CH were discussed: The yieldof CH enhanced with increasing chlorine dosage; the equilibrium concentrations variedfor different water sources and seasons; CH had the largest formation potential while pHvalue was7.5; the yield of CH enhanced with increasing reaction time, the maximumconcentration obtained at24hours; the concentration of CH changed little when thereaction temperature below20?, but it increased significantly when the reactiontemperature above30?. CH was significant positive correlation with THMs, thePearson coefficient was0.878.(2) Several practicable control methods were selected on the base of existingwaterworks process. Five methods, improvement of chlorine feeding method,chloramine disinfection, using permanganate as pre-oxidation instead ofprechlorination, using powder activated carbon to adsorb CH precursors, enhancedcoagulation were studied in laboratory scale to control CH. Results are shown: Thedosage of prechlorination was a key factor in controlling CH in out-factory water. Generation of CH in permanganate (0.5mg/L) pre-oxidation process was43.7%percentless than that in prechlorination (0.8mg/L) process. Dose of50mg/L of powderactivated carbon was the saturation capacity for absorbing CH precursors,20~30mincontact time can ensure that CH precursors effectively been removed. When the amountof the polyaluminium chloride (PAC) increased to4mg/L, the maximum removal ratereached65.3%. When pH value was7, maximum removal rate of CH precursormaximum obtained,57.6%, with PAC dosing was2mg/L. Adding polyacrylamide(PAM) can help coagulation to remove CH precursor.(3) Using the lab-scale experiment results of CH control mathods as the guidanceof the productive experiments were carried out in the Nanshan water plant.The changesof CH and its precursors in Nanshan water plant during treatment by the currentprocesses were investigated, the effects of each control methods were tested throughapply them to changing the current process. Results are shown: the concentration of CHincreased as the treatment process went on and coagulation precipitation was the keystep in removing of precursor of CH with a removal rate of22.5%. The generation rateof CH (Divide the concentration of CH in out-factory water by the concentration of CHgenerate potential in raw water) in each process from high to low were: powderactivated carbon, prechlorination0mg/L, powder charcoal+potassium permanganate,potassium permanganate, prechlorination0.3mg/L, prechlorination0.6mg/L,the valueswere:5.4%,6.7%,8.4%,14.0%,27.6%,62.2%.(4) The productive experiments were carried out in the Sha Toujiao water plant tostudy the removal effect of CH and its precursor of the activated carbon filter. Resultsare shown: Activated carbon filter can effectively remove the generated CH in water, theaverage removal rate was37.6%, but the concentration of CH increased significantlywhen chlorinated water passed through the sand filter. An average removal rate of25.3%was obtains for the activated carbon filter to removal precursor of CH, whichsignificantly higher than the sand filter's, which was9.6%, and the removal efficiencywas stability.