Research On The Removal Efficiency Of Daphnia In Water Supply

According to the operation process in a water plant in Southern China, investigate the factors affecting removal efficiency of daphnia. Experiments demonstrate that the activity of daphnia is the key factor affecting the removal efficiency in the sedimentation tank. The living bodies couldn't be removed because they can move freely in the tank. Meanwhile the dead ones can settle automatically, they are apt to combine with flocs and accelerate settling velocity. However, the density of daphnia approach to the water, some dead bodies still couldn't settle for interference of other reasons. Daphnia can penetrate the filter depends on the influent water density. Moreover, they may breed in the filter and cause the sand filter effluent daphnia density out of control. The GAC filter can provide a favorable environment for daphnia, so improper operation may lead to largely breed of daphnia, and the effluent daphnia concentration would increase dramatically.Selecting a suitable inactivation chemical is the primary problem regarding to reduce the concentration of daphnia in the tap water. We have established a criterion which is applied in water plant to evaluate the daphnia dead or not. Based on this criterion, we have discussed the inactivation effects of chemical such as chlorine,chlorine dioxide, chloramines , saline, ammonia and ozone on the daphnia . Experiments demonstrate that all these chemicals can successfully inactivate daphnia. Dosage, contact time, and organic concentration of raw water are the key factors affecting inactivation efficiency. Except ammonia need to function with basic, other chemicals can function well under pH between 6 to 8. We should choose suitable chemical, dosing point, and dosage according to specific situation of different water plants. According the plant situation, adopt chloride as the daphnia inactivation chemical, dose chloride in the influent pump station above 1.0 mg/L, and the same to reclaim pond.When the daphnia breed in the sand filter, backwash the filter with chlorine solution, keep the effluent residual chloride above 1.0 mg/L. Once the daphnia breed in the GAC filter, stop the GAC filter, and immerse the bed with high strength of saline water in order to inactivate daphnia. Backwash the filter until no daphnia in effluent and the residual chloride meet the water quality standards.