Pressure Pretreatment And Combined Process For Raw Water Containing Algae In Drinking Water Treatment

About 20%of the raw water in drinking water plants of our country comes from closed water bodies such as lakes and reservoirs.However,the eutrophication of lakes and reservoirs has caused serious pollution of the source water by algae.The water safety of residents is threatened.Existing technologies have a poor removal effect when processing algae in raw water,or the process may cause the algae cells to rupture,causing secondary pollution.Microcystis is the main algae species that leads to eutrophication of the water body,and its vesicles are important organs for regulating the floating of algal cells.This dissertation proposes a method of pretreatment of algae-containing raw water by using pressure to destroy vesicles in drinking water plants.The water purification efficiency of a combined process based on pressure pretreatment is explored for different water qualities.A water quality survey was conducted on the X reservoir in S city from April to October 2019.It is found that the average monthly algae concentration in the reservoir reaches the highest in August at 60μg/L（calculated as chlorophyll a,the same below）and the lowest in April at about 38μg/L.Microcystis can account for up to 62.9%in the distribution of algae species.Therefore,the effective removal of microcystis is critical in the drinking water treatment process.The algae removal effect and parameters of pressure pretreatment are researched in this dissertation.The result shows that pressurization has little effect on non-Microcystis,and Microcystis can effectively settle after pressurization.The maximum algae removal effect can be achieved when the pressure increases to 0.7MPa.Pressurized pretreatment can greatly improve the treatment efficiency of coagulated sediment for algae-containing raw water.Comparing with no pressure pretreatment,the algae removal rate increases from 55%to 91.7%,and the turbidity removal rate increases from 50%to 85%.At the same time,the optimal dosage of PAC can be reduced by 60%,and the precipitation time is shortened by 50%.Taking the X reservoir water as the test raw water,the combined process of drinking water treatment based on pressurized pretreatment is studied through the whole process test for different water quality conditions.Algae concentration and organic matter（in terms of oxygen consumption,the same below）content are two important indicators of drinking water quality.The research result shows that as for low algae and low organic matter raw water,pressurized pretreatment+conventional process and chemical pretreatment+conventional process can both achieve good water purification effect.As for low algae and high organic raw water,pressurized pretreatment is superior to chemical pretreatment.As for high algae and low organic raw water,pressurized pretreatment+conventional process can achieve good results,while the effluent by-products of chemical Pretreatment+conventional process exceed the standard.High algae and high organic matter raw water is the current difficult problem in terms of drinking water treatment.This dissertation proposes a two-stage coagulation and precipitation combination process based on pressure pretreatment,which is pressurization/coagulation precipitation/Chemical pre-oxidation/coagulation precipitation/filtration.Pressurization/coagulation precipitation can remove 35%of oxygen consumption and 90%of algae.After ClO₂ pre-oxidation/coagulation precipitation,the removal rate of oxygen consumption reaches 89%,and the removal rate of algae reaches 100%.The use of pre-pressurization followed by ClO₂ pre-oxidation treatment method can greatly reduce the production of by-product chlorite to within 0.5 mg/L,which is lower than the 0.7 mg/L standard.