Experiment Research On Algae Monitoring And Removal In Drinking Water Supply System

In recent years,with economic growth and the impact of human activities,more and more lakes and reservoirs gradually developed from the poor nutrition to nutritious. The reservoirs near large and medium-sized cities generally get more serious pollution, and the dominant species is resistant to most of the organic pollution,such as cyanobacteria Microcystis category.Heihe reservoir is the main souce water reservoir. In recent years the TN,TP,and other nutrient concentrations of Heihe reservoir increased year by year,and the number of algae caused a significant increase.So it is urgent to monitor the algae in Heihe reservoir,the main surface source water in Xi'an city.Based on the monitoring of algae,organic and other general indexes of Nanjiao water plant in Xi'an city,this paper analyzed the trend of algae growth and the change of dosage of coagulant and chlorine influenced by algae.Then enhanced coagulation through orthogonal test and single factor experiments was made to find the best enhanced coagulation condition for Xi'an city.Monitoring results showed:Algae species are green algae and diatom from March to May in 2007,of which the dominant species are respectively Limneticus and Melosira,and the peak value of algae amount appears in the end of May,while algae species are cyanobacteria and green algae from June to September,of which the dominant species are respectively Microcystis and Scenedesmus.The average concentration of TN is about 1.54mg/L and the average concentration of TP is about 0.015mg/L and both of them make high TN/TP value which shows that Nitrogen is excessive and Phosphorus is the restrictive factor.The peak value of algae amount is influenced by the concentration of Phosphorus,water temperature and light.Heihe reservoir is now in moderate eutrophication and some eutrophication indicative algae species appear.The eutrophication trend is in evidence.The dosage of coagulant,assist coagulant and chlorine is mainly determined by the temperature and turbidity and algae content.The average removal rate of algae is 78.3%; COD_Mnaverage removal rate is 33.1%;UV₂₅₄average removal rate is 60.6%.The algae peak value in May did not affect because of lower raw water turbidity.Sludge cycle was shortened in July because of high algae content and turbidity.At present the number of algae does not affect filter backwash cycle.Experiment results showed:The sequence of influence factors of turbidity removal is:coagulant dosage＞pH value＞the condition of hydrodynamic mixing＞the kind of coagulants＞chlorine dosage.The sequence of influence factors of UV₂₅₄removal is: coagulant dosage＞pH value＞the kind of coagulants＞chlorine dosage＞the condition of hydrodynamic mixing.Coagulant dosage and the pH value are the most important factors of algae removal.Single factor experiments were made without changing the pH of raw water,and the best PAFC dosage of algae removal is 30 mg / L;the best PAC dosage of algae removal is 35 mg / L.The best option is PAFC dosage of 30 mg/ L and chlorine dosage of 0.6 mg/ L.