Study On Technology Of Sea Water Quality Control For Domestic Use

According to the actual demand of seawater demonstration project, technology ofseawater quality control for domestic use was studied. Relationships betweendisinfectant dosage and the removal of BOD5, total count of bacteria and density ofcoliform were investigated by using sodium hypochlorite and chlorine dioxide as thedisinfectant. The research result is as follow: The higher removal efficiency oncoliform in the seawater can be achieved by using chlorine dioxide as disinfectantthan sodium hypochlorite under the same dosage and reaction time, the removalefficiency of coliform can be achieved about 97.89% by using sodium hypochloritewith dosage of 3.0mg/L, and it can be achieved about 99.94% by chlorine dioxideunder the same dosage. The removal efficiency of total number of bacteria in theseawater is higher by using chlorine dioxide as disinfectant than sodium hypochlorite,the removal efficiency of total count of bacteria in the seawater can be achieved about99.96% by chlorine dioxide using with dosage of 5.0mg/L, and the residual count ofbacteria is 470CFU/L. It can be achieved about 99.92% by sodium hypochlorite underthe same dosage, the residual count of bacteria is 1000CFU/L. The higher BOD5removal efficiency in the seawater can be achieved by using chlorine dioxide asoxidant than sodium hypochlorite under the same dosage and reaction time, the BOD5removal efficiency can be achieved about 94.5% by using chlorine dioxide withdosage of 3.0mg/L, and the residual BOD5 is 0.79mg/L. It can be achieved about92.9% by sodium hypochlorite under the same dosage, and the residual BOD5 is0.84mg/L. Chlorine dioxide is better than sodium hypochlorite in both organicpollutants and microbe removal. If the seawater quality in intake area is good,disinfection technology is needed only to remove microbe in the seawater, and shouldguarantee residual chlorine in the end of pipe network to be more than 0.2 mg/L torestrain the growth of alga, and enough dissolved oxygen in the pipe could beguaranteed, so anaerobic environment will not be developed in the pipeline. Seawatershould be aerated during pre-treatment process to increase dissolved oxygenconcentration when for domestic use if the water quality in intake sea area is not sogood.Onsite monitoring of the seawater supply system finds that DO concentration inthe seawater will reduce gradually during the pipe transportation, especially from thewater intake pump station to the treatment plant, because the length of pipe andtransportation time are relative long, the ability of oxygen refilling is greatlydecreased and oxygen consumption speed of the microbe in the seawater is relativehigh, which would cause a evident reduction of DO concentration in the seawater.Through investigation and research, comprehensive analysis was made on thecommon used anti-biological fouling technologies, conclusion can be made as follows:The distribution of inshore fouling organisms is affected by factors such as season,temperature, salinity, and water current smoothness etc. In seawater utilization project,the sodium hypochlorite production by seawater electrolysis is the most economicaland effective method for preventing biological fouling caused by sea organisms. Theelectrolysis method should be also used for anti-biological fouling of domestic usedseawater. In order to control biological fouling effectively, the residual chlorine levelin seawater should not be less than 0.5 mg/L. For continuous chlorine dosing, theinitial dosage should be controlled at 1.5 ～2.0 mg/L, and residual chlorineconcentration should be 0.5～1.0 mg/L. For intermittent dosing, the residual chlorinelevel should be monitored carefully, dosing should be carried out when there is notenough residual chlorine, with dosage of 2～4 mg/L and dosing time of 20 min.