Improvement Of Water Quality Treated By Filtration Tank Under The Condition Of Influent With Low-turbidity

At present, with the progress of the coagulation and sedimentation, water qualityin many water treatment plants has been improved significantly. Sometimes theturbidity of water from the sedimentation tank can be as low as1NTU. Theoperation and design of the filter are different from previous experience in this lowturbidity water conditions. Considering the above issues and phenomena, this thesisfocused on the optimal operation and the optimum design of the filter in order toreduce the investment for the filter construction and to improve the effectiveness offilter operation.In this paper, the method of comparative experiments was used to examine theeffect of L/d₁₀in a filter on the filtration performance. With increase of L/d₁₀value,lower turbidity of the filtered water can be reached and also the filtration cycle canbe longer which means more clean water can be gotten for a filtration cycle.However, the head loss of the filter layer increased with the increasing of L/d₁₀.From the point view of technology and economy, L/d₁₀=1200should be taken.Affects of filter aid, taken polyacrylamide （PAM） and polyferric sulfate （PFS）,on performance of the filter were also studied. Under the conditions of low turbidityabout1.5NTU, and L/d₁₀=1200, and the filtration rate8m/h, the optimum dosageof PAM is8μg/L while PFS is15μg/L. By compare the performance, one can findthat turbidity from the filter with PAM as the filter aid was much lower and alsomuch stabler than that of filter with PFS. The duration with high water productionrate is much longer than that of hat of filter with PFS. The head loss was within anacceptable range.The effects of the filtration rate on the performance of filter were also studied.Results shows that the filtration rate varied from8-12m/h affects the water qualityslightly under the conditions of low turbidity about1.5NTU, and the optimumdosage of PAM about8μg/L, as well as L/d₁₀=1200. Therefore, in this context, an appropriate increase of the filtration rate will increase the amount of the producedwater while the water quality was not affected yet. From the point view of waterquality and water quantity, the filter rate can be taken as12m/h.Under the conditions of low turbidity about1.5NTU, and the optimum dosage ofPAM about8μg/L, as well as L/d₁₀=1200, by mathematical regression analysismethod combined with the actual measured data, mathematical equations werederived theoretically between the filter layer head loss and filter effluent turbidity,which ish ’0.405I nc’1.054. If the turbidity in the effluent of a filter is requiredlower than0.5NTU, the corresponding maximum head loss is0.78m, therefore, thefilter head should take0.8-1.0m, rather than the original standard1.5-2.0m.