Optimization For Treatment Of Low Turbidity Water In South Taihu Lake Area Using Sludge Recycling Technology

Low coagulation efficiency and high consumption of coagulant are two challenges for low turbidity source water treatment,and sludge recycling enhanced coagulation is generally considered as an effective solution.In the research,the low-turbidity source water of reservoir in the south Taihu Lake area was taken as the research object.Based on the conventional coagulation process,sludge recycling enhanced coagulation to optimize the treatment of low turbidity water was studied.Through the bench-scale experiments and pilot-scale experiments,the enhanced mechanism and influencing factors of the sludge recycling enhanced coagulation technology were investigated,and the safety,stability and applicability of coagulation with sludge recycling were discussed,compared with conventional treatment.The research provided the theoretical basis for optimizing the low turbidity water treatment process and renovating the water plants in the south Taihu Lake area.According to the water quality data of L Reservoir in Huzhou City from 2015 to 2019,the water quality of L Reservoir was typical of low turbidity water.The turbidity and CODMn seasonal fluctuations were obvious,and the concentration in summer and autumn was generally higher than that in spring and winter.Based on the 2015-2019 water quality data,the seasonal Kendall method was used to analyze the past trend of the water quality series,and the Hurst index was calculated by the R/S method to infer that H reservoir turbidity will have an upward trend and CODMn will have no obvious trend in the future.Drinking water treatment should pay attention to the risk of water quality fluctuations.Through the bench-scale experiments and pilot-scale tests in Huzhou,the treatment efficiency and optimization of conventional coagulation processes were investigated.The removal efficiency of poly aluminum chloride(PAC)was better than poly aluminum sulfate(PAS)and poly ferric aluminum chloride(PFAC).With the increase of the coagulant dosage,the turbidity removal efficiency increased and then decreased.The economic dosage of PAC was 15 mg/L,and the corresponding removal efficiencies of turbidity,CODMn and UV254 were 62.0%,31.7%,32.9%,respectively.Adding polyacrylamide(PAM)can effectively improve the removal efficiency,and PAM had a greater effect on PFAC than PAC.The changes of raw water turbidity,pH and organic matter content had a great influence on the effect of coagulation.When treating reservoir water with different turbidity and the dosage of PAC was less than a certain value,the remaining turbidity increased with the increase of the initial turbidity of the raw water.With the increasing dosage,the effect of different initial turbidity gradually weakened and the quality of effluent water tended to be stable.The effect of initial pH on the removal of turbidity and organic matter was related to the hydrolysate of PAC.When the raw water was neutral or weakly alkaline,the removal was better than that under acidic conditions.The higher concentration of CODMn effected the floc morphology and growth rate,resulting the decrease of removal efficiency.The sludge recycling enhanced coagulation effectively used the remaining coagulant and suspended colloidal particles in the sludge of the coagulation sedimentation tank,which improved the removal efficiencies of turbidity and organic matter compared with the conventional coagulation.When 10 mg/L of PAC and sludge with recycle ratio of 6%were added in order,the turbidity,CODMn and UV254 removal efficiencies were 78.8%,41.6%and 35.8%,respectively.The removal efficiencies were better than those of the conventional coagulation in which the dosage of coagulant was 15 mg/L.In addition to improving the removal efficiency,the sludge recycling enhanced coagulation technology can also reduce the coagulation consumption.At the same time,the ability to respond to the fluctuation of raw water quality has been improved.The pilot-scale tests of sludge recycling enhanced coagulation was stable with continuous operation condition.The operating period of the filter tank can reach 48 hours.The treatment effect was good,indicating that the recycyling sludge had good application potential and environmental economic benefits in the treatment of low turbidity water.Through the research of the coagulation mechanism,it can be seen that the fractal dimension of the flocs from the sludge recycling enhanced coagulation technology were larger than those of the conventional coagulation process.The floc structures formed from recycling sludge process were tighter and more regular,indicating a better sedimentation property.The growth rates of the flocs from conventional coagulation process and the sludge recycling enhanced coagulation technology were 43.9 ?m/min and 297.6 ?m/min,respectively.The recycling sludge can increase the particle size of the flocs and shorten the equilibrium time.The recycling sludge process improved the flocs' ability to resist crushing,but the re-flocculation ability of the flocs was significantly reduced.It was speculated that the sweep flocculation played an important role in the recycling sludge process.