| Owing to the terrible raw water quality, the water of Dishui Lake supplied by Dazhihe Lake had been on the status of Low-Middle eutrophication level, and the eutrophication tendency becoming more and more terrible. In order to polish this aquatic- pearl of Dishui Lake, measures of water quality control for meet the demand of sight-seeing function. How to prevent the terrible eutrophication tendency puzzled environmental experts, at the same time, this severe condition attract sufficient attention of the local governor and government. Comprehensive investigation and numerous practical project experience shows that improving supplied water quality, enhancing the fluid of the water body and shorten the cycle for water metathesis are the effective methods to control the bloom of algae and improve the water quality. So, the plan of a water treatment system for terrible quality water is vital to maintain the water quality and meet the need of Dishui Lake as a sight-seeing site.The raw water of this study came from the D-port connecting Dazhihe Lake to Dishui Lake. Two combined processes including biological filter + inclined-tube sedimentation combined process and biological contact oxidizing tank+ air flotation combined process were operated. Pivotal operate parameters were adjusted and every operation conditions were simulated. The water quality such as turbidity, NH3-N, CODMn, algae, chl-a, total nitrogen, total phosphorus, Fe and manganese were measured and compared research were executed comprehensively. Finally, superior water treatment process and its optimal operate parameters were brought forward.This paper is of real significance in water protection for Dishui Lake, furthermore, significant experience accumulated is useful for similar projects home and overseas. This study provided credible reference data for the design and operation of drinking water plant with micro-polluted raw water and biological contact oxidizing tank process. The main results of the study were as follows:(1) In all, biological contact oxidizing tank is superior to biological filter and air flotation is superior to inclined-tube sedimentation not only in pollutants removal efficiency but also in the ability of resist the impactive pollutant load. ExceptTN and CODMn, effluent from biological contact oxidizing tank+ air flotation can achieve standards for II -class criteria of surface water. The pollutants value of the effluent from biological contact oxidizing+ air flotation were lower than that of biological filter + inclined-tube sedimentation especially in algae, chl-a and turbidity. The turbidity value could be reduced to 6NTU by contact oxidizing+ air flotation which met the demand of Dishui Lake as sightseeing site.(2) The main pollutants will be removed by the biological section in both combined processes were ammonia nitrogen, CODMn and algae. A little removal efficiency of ammonia nitrogen and CODMn been gained by physical-chemical section. The main pollutants removed by the physical-chemical section were algae, turbidity, total phosphorus and manganese.(3) Biological contact oxidizing tank+ air flotation is the prepositional combined processes will be applied.(4) There was a accordant influence arose by air-water ratio of biological contact oxidizing tank on CODmju turbidity, algae and chl-a removal, therefore, the air-water ratio should be optimized at 1.0. Air-water ratio didn't take great effect to TP removal in biological contact oxidizing tank. The planned changing of air-water ratio in biological contact oxidizing tank didn't bring expected regular changing to ammonia nitrogen removal efficiency. It is because of the great fluctuation of low ammonia nitrogen concentration in raw water that air-water ratio is not the limiting factor to ammonia nitrogen removal.(5) Hydraulic retention time of biological contact oxidizing tank didn't bring great influence to turbidity, algae, TP and CODMn removal but strongly affected on the ammonia nitrogen removal. The proper hydraulic retention time is 60min.(6) With the increase of hydraulic load in biological filter, the effluent values of CODMn, turbidity, algae, ammonia nitrogen, TP decrease accordingly. Compared to the CODMn removal efficiency, ammonia nitrogen removal efficiency was easier to be interfered by the changing hydraulic load. The proper hydraulic load of biological filter is 25m3/m2.d.(7) To obtain the equivalent pollutants removal efficiencies, the dosage meeting the need of air flotation was less than that of sedimentation tank. Take allfactors into consideration, 30mg/L of coagulant ( Al2(SC>4)3-14H2O)is appropriate for air flotation tank.( 8 ) In all, with the increase of surface load of air flotation tank, the removal efficiencies of turbidity, CODmii, ammonia nitrogen, algae and TP decreased. Take all factors into consideration, 7.5m3/m2-h of surface load is proper.( 9) With the increase of hydraulic load in sedimentation tank, the removal efficiencies of turbidity, CODmii, ammonia nitrogen, algae and TP decreased. According to all factors, coagulant of 30-40mg/L and hydraulic load under 1.5m3/m2h are proper.(10) During this testing, biological media filling rate in biological contact oxidizing tank increased from 30% to 50%, which arose greater influence in ammonia nitrogen removal efficiency than that of CODmd removal efficiency. Biological media filling rate upward of 45% is appropriate to biological contact oxidizing tank.(11) When this suspended biological media are applied in biological contact oxidizing tank for treating raw water, successful performance including startup process and pollutants removal been obtained. The biological media offer good mass transfer effect among air, water and microorganism, furthermore, successful startup process can be achieved under natural microorganism accumulation.
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