Mechanism And Efficacy Of Removing Microcystis By Utilizing Silver Carp In Presedimentation Tank In Waterworks

High concentration of Microcystis in raw water cause negative effect on the operation of conventional drinking water processes and drinking water quality. In order to reduce algae load of conventional drinking water treatment and other processes for algae removal, the biological pretreatment of algae-laden raw water by stocking filter-feeding silver carp was proposed in waterworks. Since 2005 the experiment was conducted in the presedimentation tank in a waterworks in Tianjin.Algae species in raw water were identified to comprehensively understand the algae changes and removal characteristics. 7 phyla, 43 classes, 68 species of algae was determined based on the analysis on the phytoplankton community structures in Yinluan raw water. In spring green algae was the predominant species in Yinluan raw water, and the mean value of algae concentration was 189.4×104cells/L, while in summer cyanobacteria was the predominant species, and the mean value of algae concentration was 2724.8×104cells/L (the highest value was 5933×104cells/L). In summer Microsystis flos-aquae had very high proportion of 80.9% in the raw water. In autumn green algae became the predominant species again. Water temperature is key factor that influences the phytoplankton community structure.After silver carp were stocked in the presedimentation tank at the density of 65g/m3, phytoplankton community structure had remarkable influence on removal efficacy of algae in Yinluan raw water. Because silver carp cannot inhibit the growth of green algae, the algae concentration were 2.54 times more in effluent than in influent. Although in summer the concentration of green algae rose, the algae removal reached to 55.3% due to the effective removal of Microsystis flos-aquae by silver carp. However in autumn the algae concentration were more in influent than in effluent. Contast test indicated that stocking silver carp led to the effective removal of Microsystis flos-aquae. It was also found that silver carp stocking had not remarkable influence on turbidity and pH in the presedimentation tank, and the silver carp filter-feeding on algae may contribute to the transfer and removal of phosphorus.Revised regression equations of algae removal efficacy could accurately predict the Microsystis flos-aquae removal efficacy, and the kinetic model was able to clearly describe the mechanisms of the change of algae in the presedimentation tank stocked by silver carp. Kinetic model analysis showed that enough stocking density of silver carp, dominance of Microcystis in influent and suitable HRT makes the biological pretreatment of utilizing silver in presedimentation tank possible.Algae removal characteristics can be fully played when the biological pretreatment of stocking silver carp in presedimentation tank and conventional processes are combined. The biological pretreatment is suitable for Microsystis flos-aquae removal, and conventional processes can effectively remove other algae. Algae removal of prechlorination unit are stable and reliable, while the coagulation-sedimentation units have a better removal for green algae than cyanobacteria, and the high filter rate of algae is important to ensure low-level algae concentration in water.In summary, the biological pretreatment technology of stocking silver carp in presedimentation tank not only has a stable and good Microcystis removal performance and clear operation mechanism, also has the advantages of no energy and chemicals consumption, no greenhouse gas emissions, and has a bright future of practical application.