Study On Enhanced Coagulation And Engineering Application In Xiaowan Water Supply Plant

The temperature and rainfall in the downstream area of the Yangtze River in our country have obvious changes throughout the year.The surface water sources in this area are greatly affected by the external environment.The water quality shows low temperature and low turbidity in winter and turbidity mutation in summer.Meanwhile,contaminated by the domestic sewage discharged from the upstream cities,industrial wastewater and agricultural irrigation along the line,the consumption of oxygen in the raw water is high,especially when the turbidity is abrupt,causing a great impact on the process of water purification plant,which makes it difficult to guarantee the safety of water supply.In this paper,the raw water of Jiangyin Xiaowan water purification plant is taken as the research object to carry out the research of strengthening coagulation process.The characteristics of raw water quality in the downstream area of the Yangtze River are analyzed by analyzing the variation regularity of raw water quality in recent years.And then the coagulation kinetics characteristics and parameters optimization,coagulant types and dosage,the best dosage of coagulant aid and dosage point were studied through batch tests.On this basis,the pilot study of strengthen coagulation efficiency and impact of load capacity were carried out,to provide parameters for the project implementation.The full scale study of strengthening coagulation and the economic analysis was based on the reconstruction project of west bay in Xiaowan water purification plant.Through the analysis of water quality changes of raw water in Xiaowan water plant in recent years,it was shown that the quality of raw water in Jiangyin section of Yangtze River changes greatly throughout the year.Seasonal changes of temperature and turbidity are obvious,and low temperature and low turbidity occur in part of winter.Turbidity in summer is high and changeable under extreme weather.The raw water is weakly alkaline while the pH is around 7.9 throughout the year,with a small change range.The ammonia nitrogen concentration is relatively low,usually below 0.5mg/L.However,the oxygen consumption value is high,about 6.0 mg/L all over the year.The optimum kinetic parameters in the mixing process and flocculation were determined by the coagulation kinetics tests.The optimum speed of the mixing process was 300 r/min and the duration was 45s.The flocculation stage was divided into primary flocculation and secondary flocculation.The optimal primary flocculation speed was 100 r/min,lasting for 5min and the optimal secondary flocculation speed was60 r/min,lasting for 10min.The tests of optimum coagulant,best dosage and dosing point showed that both polymeric aluminum ferric sulfate and polymeric aluminum ferric chloride showed better treatment efficiency than others except for the residual aluminum and residual iron.The best dosage of both were in the range of 60-70 mg/L and the best coagulant aid dosage was 1.0mg/L,dosing at the end of 5min after mixing,that is,immediately after the primary flocculation.The pilot tests of strengthen coagulation studied the dosage of the polymerization of aluminum and aluminum ferric chloride.The amount of coagulant aid and its dosing points were further optimized.The results showed that aluminum ferric chloride had a better removal efficiency for turbidity and the total number of remaining particles,with an effluent water quality of 1.52NTU.However,the polymerized ferric sulphate was better for oxygen removing and the removal rate reached 65%.Both of which reached the best removal efficiency at 65mg/L.When the coagulant and coagulant aid were used in combination,the addition of 1.1mg/L anionic PAM in 15#shaft can control the effluent turbidity,total residual particles and oxygen consumption at 0.3NTU,31/mL and 2.7mg/L or less.The impact load test showed that changes in water quantity and quality will both lead to a decline in effluent quality,but different combinations had different degrees of changes.The quality of effluent treated by the combination of coagulant and coagulant aid was better than that of coagulant alone.The effluent quality of polymerized ferric sulphate+PAM was higher than that of aluminum ferric chloride+PAM combination while facing with impact loading.The amplitude is smaller and the water quality is more stable.When the water volume changes within the range of 0.75-1.25 times of the designed water volume,the turbid water after sedimentation and the total number of remaining particles can be maintained below 0.7 NTU and 70/mL,and the oxygen consumption remained below 3mg/L.When the raw water turbidity was below 250NTU,the water turbidity and the total number of remaining particles can be maintained at 1.0NTU and 100/mL or less and the oxygen consumption remained below 3mg/L.Only when the original water turbidity wass higher than 250NTU,the deterioration of water quality will be aggravated.Based on this,it is determined that the poly-ferric aluminum sulfate is the most suitable coagulant for the raw water.The full scale strengthen coagulation test was carried out based on the reconstruction project of the Western Area of Jiangyin Xiaowan water purification plant.After the design and construction of the building,electrical,structural and automatic control for conventional treatment(mixing,flocculation,sedimentation and filtration),filtration of reuse water pool,clear water tank and discharged sludge water treatment process,the water quality of raw water,the effluent from the sedimentation tank of the reconstruction project and the effluent from the former eastern sedimentation tank were analyzed.As a result,it was found that the combination of polymeric aluminum sulfate+PAM was better than that of polyaluminum chloride alone.The turbidity and residual particles in the effluent were lower than 50% of the original effluent and the removal of oxygen was increased by about 20% at the same time.The quality of the effluent was less fluctuated and the capability of anti-impact load was stronger.The project implementation showed that the total investment of the process for raw water in the downstream area of the Yangtze River was 313,812,300 Yuan.The operation cost was0.47 Yuan/m~3,and the unit operating cost was 0.28 Yuan/m~3.