Optimization Of The Drinking Water Pretreatment And The Combined Membrane Filtration Process

With the promulgation and implementation of Water Quality Standard for Urban Water Supply by Ministry of Construction in 2005,the total number of detection indexes was 103,in which the amounts of the conventional ones were 42. In this standard,the detection of trace organic pollutants was emphasized.In 2006 the Ministry of Health issued the National Hygienic Standard for Drinking Water.It can be seen that the water quality standard is stricter and stricter,while the raw water quality has been deteriorating in recent years.Nowadays most of the waterworks in our country are still using the traditional water purification process that is coagulation-sedimentation-filtration-disinfection,which mainly removes the turbidity,chroma and bacteria etc.,with the removal efficiencies of ammonia nitrogen and dissolved organic pollutants low.Even in the practice of the ozone/activated carbon(O₃/AC) advanced treatment process,which has been under popularization and application in China,there are some problems such as the incapability of lowering the turbidity further,the poor biological stability in the effluent,the incomplete removal of algae in the effluent at the algae eruptive period and the hidden troubles causing by red worm breeding.So developing the effective combined process to produce drinking water with good quality is of great urgency.As the pretreatment processes of membrane treatment,the contaminant removal efficiencies of four combined processes were first discussed,which were preozonation -coagulation sedimentation-granular activated carbon(GAC)-sand filtration, coagulation sedimentation-post-ozonation-GAC-sand filtration,preozonation-coagulation sedimentation-activated carbon filtration and coagulation sedimentation -post-ozonation-activated carbon filtration.Then the contaminant removal efficiency and operation parameters of membrane for advanced treatment in the drinking water were investigated.From the results above,a new combined process for drinking water treatment was proposed,which is preozonation-coagulation sedimentation-GAC-sand filtration-membrane filtration.And this process has been applied to the reconstruction project for advanced treatment of drinking water in Qingtai Water Plant of Hangzhou.The results show that:1.For the removal of organic compounds,activated carbon filter was more effective than the GAC-sand filter,while the turbidity removal efficiency was higher in the latter one.And there was little difference between the two filters in the removal of other idexs.The organic compound removal efficiency of filter in preozonation process was higher than that in post-ozonation process,and the total removal efficiencies of organic compounds in the two processes were close.2.In the GAC-sand filter,the organic compound removal efficiency has the tendency of decreasing,while the removal efficiency of ammonia nitrogen experienced the process of increasing-decreasing-increasing.The time needed for biofilm formation was 15 to 20 days at the temperature of 20℃.In the GAC-sand filter,the activated carbon with the thickness of 350mm in the upper layer could remove 80%contaminants.3.In the water treated by GAC-sand filter column,the particle sizes of most granules,about 94.4%,were less than 10μm and the ratio of granules at the particle size of 10-25μm was 5.5%,with 0.1%granules having the particle size more than 25μm.Referring to granules with particle size less than 10μm,the majority of them had particle size less than 5μm.After back washing,as the turbidity changed little, with the amount of granules a lot,the particle counter could be used to strengthen the GAC-sand filter management.There were small changes in the pollutant removal efficiency in the GAC-sand filter in a cycle,with Mn removed less after back washing. After being operated for 10min,the water quality of the initial filter water in GAC-sand filter was the poorest.It would become better then and be stable at the time of 20-30min.So the discharge pipe for initial filter water could be neglected in the practical project.4.Membrane filtration could remove organic compounds and ammonia nitrogen with low efficiencies,whereas much of the turbidity and biomass could be eliminated, with flux having little effect on it.After membrane filtration Fe existing in the water would be at a low level,nevertheless,affected by the pretreatment Mn could not be removed efficiently,and the alkalinity and residual chlorine could be almost the same.5.With the comparison study of different fluxes,the most appropriate designing flux was recommended as 120LMH.And compared with the operation with reflux, that without reflux was more suitable.However,when water quality is poor,the operation with reflux could be considered sometimes.6.The TMP could not be influenced significantly by temperature until it came to be lower than 15℃.With the water temperature going down,the absolute TMP increment in a cycle of chemical cleaning would increased,and the relative TMP increment compared with that before the cleaning was increased.During the operation,the absolute TMP increment was 0.18-0.73bar and the relative TMP increment was 20-72.28%.The TMP was 1.11-1.8bar before cleaning with 0.89-1.1bar after.And the decrease of turbidity was favor for slowing down the increase of TMP.7.Comparing two hydraulic cleaning methods,which are gas washing 1.5min+washing 0.5min and gas washing 1.0min+washing 0.5min,it can be found that in the initial several cycles after the chemical cleaning,the two methods had almost the same effect on TMP.However,after 6 cycles of hydraulic cleaning,the TMP under the condition of gas washing 1.0min+washing 0.5min was apparently higher than that with gas washing 1.5min+washing 0.5min.The average TMP under the former condition was 1.12bar,with 1.10bar detected under the latter one,about 1.8%higher.8.For the small chemical cleaning,the way of pickling and alkali washing was recommended,with the cycle of 24h and the concentration of the cleaning fluid at 300-400ppm.If the water quality is good,the cycle of the pickling could be prolonged properly.The chemical cleaning with sodium hypochlorite and citric acid in this experiment was satisfactory.9.The differences in the pollutant removal efficiencies of four different types of membranes used in this experiment were not significant,and they had good adaptability to the influent,suitable for the source water used in this experiment.But compared with others,the adaptability of membrane 3 was worse,especially when the UV₂₅₄ value was high in the winter with the flux hardly increased.In the running process,operation differential pressure in the inside-out membrane was apparently lower than that in outside-in membrane.10.The waters with and without chlorine were studied separately.It is found that when the water with some residual chlorine,which was effluent of granular activated carbon(GAC)-sand filter added with chlorine(residual chlorine 0.1-0.3mg/L),was used as the influent,the membrane system was run better than that without chlorine, which behaved as the pressure difference increased slowly at the same running condition.In the membrane system,the operation cost was mainly used to pay for the power consumption and medicine consumption.But compared with the latter one, the power consumption cost much more.