Research On Removing Cyanobacteria By Pre-pressure Coagulation And Sedimentation Process

As common algae of water source, cyanobacteria outbreak in summer—a large number float on the surface of the water at times. Cyanobacteria are glue group with large-size individual and floating property. Afterbirth with negative charge the cocoons surface of cyanobacteria. The cyanobacteria are extremely difficult to combine with coagulant, thus its coagulation effect is poor. The treatment technologies of cyanobacteria water currently used include pre-oxidation coagulation sedimentation and pre-oxidation by chlorine gas, using 1～2mg/L chlorine or potassium permanganate to take off the afterbirth from the algal cell surface to improve algae coagulation performance. However, the problem lies in that pre-oxidation will take off the afterbirth from the algal cell surface, then break algae cells, algae liquid leak and finally dissolved organic matter, MC-LR, DBPs in water increase, threating the safety of drinking water. The dosage of coagulant is high and the removal efficiency is unsatisfactory, the removal rate after sedimentation of cyanobacteria is just some 80%. Cyanobacteria float and decay in the advection sedimentation tank, and algae will flow into the filter blocking the filter, flushing water consumption, while the flotation process consume large amount of energy.The cyanobacteria float due to buoyancy provided by internel pneumatophore. Research shows that cyanobacteria pneumatophore after pressurized began to break when the pressure is over 0.3MPa. And cyanobacteria pneumatophore break completely when the pressure is over 0.6MPa. A large number of cyanobacteria sink after the pressure reach 0.5MPa. According to the result, the algae removal of coagulation precipitate effect after different pre-pressures, the sedimentation velocity of cyanobacteria floc and the stability of cyanobacteria coagulation sedimentation is studied. The main research results are as follows:(1) Water containing algae shows fine water quality after 0.4-0.8MPa pre-pressured coagulation sedimentation.0.6～0.8MPa pre-pressure coagulation sedimentation has best optimal treatment effect with effluent turbidity of 1.00～0.68 NTU, with the removal rate of chlorophyll-a concentration 97.60%～97.76% and removal rate of CODMn 65.88%～65.88%. Considering the floating phenomenon still exists after 0.4MPa pressure, pre-pressure coagulation sedimentation process in practical application suggested as 0.6-0.8MPa.0.6MPa is a good choice considering energy saving.(2) The sedimentation rate of cyanobacteria floc after 0.4-0.7MPa pre-pressure coagulation sedimentation is significantly faster than that without pressure. After 0.4MPa pressure reinforced coagulation and sedimentation, the coagulation and sedimentation performance of cyanobacteria is slightly worse, a small amount floated, and the sedimentation rate of floc after 0.5MPa pressure is slightly slower than the 0.6,0.7MPa, therefore the pressure range is suggested as 0.6-0.7MPa. After 0.7MPa pressure reinforced coagulation and sedimentation, the proportion of the alga whose u(sedimentation rate)≤0.5cm/min,0.5<u<1.5cm/min,1.5<u≤2.5cm/min, 2.5<u≤3.5cm/min and u>3.5 cm/min is 10.15%,4.03%,5.31%,6.59%,73.92%.(3) The removal efficiency of pre-pressure coagulation sedimentation process to remove turbidity and chlorophyll a was better than pre-oxidation coagulation sedimentation, and pre-pressure coagulation sedimentation can save 50% coagulant and 100% pre-oxidants. Obviously, it reduces the cost of water treatment, and increase minimal investment. The required is just simply transforming of the existing pre-oxidation coagulation sedimentation process.(4) The two parallel tank pressure device with micro power consumption has been invented. The total power consumption of the waterworks is 90 kwh/d whose processing capacity is 50000 tons/day, seen as 1.8 kwh/d of one thousand ton water. According to the electricity consumption, the cost of every ton of water is 0.150 RMB. Congsidering the cost of water of the general system of waterworks is 40(?) RMB/tons of water, the cost of pressure is quite little. The treatment effect and water supply security of pre-pressure coagulation sedimentation process to remove algae are greatly improved compared to traditional pre-oxidation coagulation sedimentation filtration process. And the pre-pressure cancelled the oxidant, avoiding the preoxidation causing the algal toxin leakage and oxidation by-products such as water quality secondary pollution, saving coagulants (aluminum sulfate) of 15～25 mg/L (30%～50%) and reducing the system of water costs 0.1 RMB/m3. Pre-pressure coagulation sedimentation filtration water treatment system has the superiority of high efficiency, safe and environmental, energy saving, easy to implement.