Study On The Effect Of Preoxidation On Algae Control In A Reservoir In The South Of China

In recent years,rapid development of the society,coupled with the annual heat and humidity in Southern Area,result in serious water pollution and algae blooms in the water which has brought difficulties to the water treatment.The raw water sampled from Shenzhen Shiyan reservoir was taken as the research object in which Cyanobacteria sp.such as Pseudoanabaena sp.and Oscillatoria sp.were the dominate algal,aiming to select highly efficient,safe,and economical pre-oxidation method,and compared UV pretreatment with two traditional chemical pre-oxidants such as KMnO₄ and ClO₂.The results of permanganate oxidation are as follows:low concentration(0 to1.5mg·L^-1)of permanganate had no significant effect on the inactivation of algae cells.High concentration(5mg·L^-1)of permanganate would damage the algae cell structure,and the total number of algae cells had decreased by 28%.Permanganate could effectively inhibit the growth and reproduction of algae cells.The study also found that permanganate can effectively reduce the concentration of organic matter in water,but permanganate oxidation on cell can cause the release of intracellular matters,and the extracellular concentration of 2-MIB and GSM would increase on the permanganate dosage of 1.5mg·L^-1.Both the concentration of extracellular DOC and total fluorescence of DOM showed the trend of decreasing-increasing-decreasing with the increase of oxidation time.The contents of aromatic protein-like substances and soluble microbial product-like substances increased in DOM on oxidation by 5mg·L^-1of permanganate for 10min.Permanganate could effectively enhance the coagulation on algae cells and organics removal,and the optimal dosage of PAC is 3mg·L^-1,while the optimal dosage of permanganate is 0.6mg·L^-1.The results of chlorine dioxide oxidation are as follows:low concentration(0 to1.5mg·L^-1)of chlorine dioxide had a significant effect on the inactivation of algae cells,but wouldn’t damage the algae cell structure into pieces,so the total number of algal cells decreased by only 13%on oxidation by 1.5mg·L^-1 of chlorine dioxide for20 min.The study also found that the chlorine dioxide can damage the algae cell to release intracellular matters,and the concentration of extracellular DOC,MIB and GSM would increase.Chlorine dioxide can effectively inhibit the growth and reproduction of algae cells.In addition,chlorine dioxide could effectively enhance the coagulation to dislodge algae cells and organics,and the optimal dosage of PAC is3mg/L,while the optimal dosage of chlorine dioxide is 0.6mg/L.The results of UV pretreatment are as follows:ultraviolet light would kill algae by destroying the nucleic acid substance of algae cells,and with the increase of ultraviolet irradiation intensity and dose,the mortality of algae increased,but the structure of algae cells maintained good integrity.Ultraviolet irradiation could damage the algae cell to release intracellular matters,and the concentration of extracellular DOC would increase,but ultraviolet irradiation could reduce the pH,turbidity,COD_Mn,UV₂₅₄ and ammonia nitrogen of raw water to a small extent.It also could be effective to remove taste and odor.The concentration of 2-MIB would decreased respectively by 9.4%and 18.5%,while the concentration of GSM would decreased respectively by 44.8%and 63.3%on ultraviolet irradiation by 700mJ/cm²and 1400mJ/cm².Compared with permanganate and chlorine dioxide,ultraviolet irradiation could be more effective in longer time in algae inhibition,after ultraviolet irradiation algae began grow and reproduce faster at the 7th day.In addition,ultraviolet irradiation could effectively enhance the coagulation and sedimentation,and the optimal dose of ultraviolet irradiation is 400 mJ/cm².The cost of per cubic metre water by three different pretreatments are as follows:200mJ/cm² ultraviolet irradiation>0.6mg·L^-1ClO₂>0.6mg·L^-1 KMnO₄.The cost of chlorine dioxide was lowest,but chlorine dioxide would seriously damage the algae cells to release intracellular matters.The effect of algae inhibition and reduction of odor by ultraviolet irradiation was the best,but the cost of ultraviolet irradiation is much higher than the cost of chemical oxidants.It believes that the optimization of permanganate pre-oxidation is most suitable for this kind of water quality.