Study On The Application Of Micro-flocculation Filtration And Ultrafiltration Combined Process To Treat Micro-polluted Cellared Water

The water resources in the rural areas of northwest China are relatively short,so the rainwater collection cellar water has become an indispensable water resource.Although the water from rainwater harvesting cellars plays an irreplaceable role in solving the production and living water problems of the majority of rural people,there are problems such as excessive organic matter and ammonia nitrogen content,turbidity of water quality,and the tendency to produce bacteria after long storage.Long-term consumption of untreated water from rainwater harvesting cellars can be harmful to the health of local residents.In this study,the water quality characteristics of the rainwater collection cellar water in the northwest rural area were studied,and the process parameters were optimized by using the combined micro-flocculation filtration-ultra-filtration process through artificial simulation of water distribution.It provides reliable technical support for the treatment of rainwater collection cellar water and ensures the water safety in rural areas of northwest China.Firstly,the effect of different PAC dosage on the removal of pollutants in micro-polluted cellar water was investigated,and the optimal flocculant dosage was determined by combining with zeta potential.Three-dimensional fluorescence component analysis as well as principal component analysis were used for the analysis.In the case of the optimum dosage,the effect of different flocculation time and agitation intensity on the removal of pollutants in the microcontaminated cellar water was investigated,and the optimum flocculation time and agitation intensity were determined by combining with the floc size.Finally,the flocculation conditions were optimized by combining the response surface method.Then,the conclusions obtained from the flocculation experiments were combined with the operating conditions of the microflocculation filtration filter,and the effects of different filtration speeds on the removal effect of pollutants and filtration cycle were investigated by using correlation analysis and principal component analysis methods to determine the optimal filtration speed and optimize the operating parameters of the microflocculation filtration filter.Further,experimental studies of three processes（ultrafiltration alone,coagulation-precipitation-ultrafiltration,and microflocculation filtration-ultrafiltration）were conducted to investigate the changes on ultrafiltration membrane fluxes.By analyzing the removal efficiency of pollutants in the stable operation stage of the combined process and combining with the three-dimensional fluorescence analysis of the inlet and outlet water of each unit of the combined process,we investigated whether the effluent water quality of the combined process could meet the standard of domestic drinking water.Finally,a set of micro-flocculation filtration-ultra-filtration water purification device was developed and applied in field experiments to investigate the economic and social benefits of the combined micro-flocculation filtration-ultra-filtration process.The main research results obtained are as follows:（1）Optimal flocculant dosage experiment.Different amounts of PAC were added at 5,10,12,15,20,30 and 35 mg/L,and the water was stirred rapidly at 200 r/min for 2 min and slowly at 50 r/min for 30 min to treat micro-polluted cellar water.The results showed that the PAC dosage was optimal at 15 mg/L,and the removal rates of turbidity,COD_Mn and UV₂₅₄ were88.99%,38.36%and 40.98%,respectively.The effects of different mixing intensity and time on the removal of turbidity,COD_Mn and UV₂₅₄ were investigated,and the changes of floc size during flocculation were also examined.The optimum flocculation time was 12 min and the optimum stirring intensity was 50 r/min,and the floc size was 78.35μm.The optimized flocculation conditions were 14.8 mg/L,12.4 min,and 54.2 r/min by combining with the response surface method.（2）Optimization of microflocculation filtration operation parameters.The relationship between filtration speed,filtration cycle and treatment effect was investigated by conducting microflocculation filtration experiments with filtration speed of 2,4,6,8 and 10 m/h and filtration cycle of 14,18,24,26 and 28 h respectively.The results showed that the optimum microflocculation filtration speed was evaluated by the filtration efficiency factor FN,and the average turbidity of the effluent was 0.9 NTU at a filtration speed of 6 m/h,the filtration period was 24 h,and the head loss of the column reached 140 mm.At this time,the minimum FN value was 0.047 at 6 m/h.The optimum filtration speed was determined to be 6 m/h and the filtration period was 24 h.（3）Experiments of different pretreatment processes to treat micro-polluted cellar water.Compare the effect of three different processes of direct ultrafiltration,coagulation-sedimentation-ultrafiltration and microflocculation filtration-ultrafiltration in treating microcontaminated cellar water to determine the best combination process.The results showed that the average removal rates of turbidity,COD_Mn,UV₂₅₄ and ammonia nitrogen by direct ultrafiltration were 96.86%,19.4%,16.97%and 12.33%.Comparing the removal rates of turbidity by coagulation-precipitation-ultrafiltration and microflocculation filtration-ultrafiltration,we found that the difference between the two processes was only 0.78%,but both were better than direct filtration.However,comparing the removal effects of COD_Mn and UV₂₅₄,coagulation-precipitation-ultrafiltration was better than microflocculation filtration-ultrafiltration,and the removal rates were improved by 2.61%and 2.69%,respectively,and for ammonia nitrogen,the difference between the two was only 0.12%.Experiment of membrane contamination with different pretreatment processes.Comparing the membrane flux changes during the treatment of micro-polluted cellar water by three different processes,the best combination process was determined as micro-flocculation filtration-ultra-filtration process.（4）Experiment of microflocculation filtration-ultrafiltration combined process operation stage to treat micro-contaminated cellar water.Through long-term operation,the effect of the combined microflocculation filtration-ultrafiltration process on the removal of turbidity,COD_Mn,UV₂₅₄ and ammonia nitrogen in the micro-polluted cellar water was investigated.The results showed that the combined process could achieve 98.19%,43.33%,55.67%and 28.59%removal rates of turbidity,COD_Mn,UV₂₅₄ and ammonia nitrogen on average.Combined with the three-dimensional fluorescence spectrum analysis,the removal rates of fluorescence intensity from region I to region V after the combined process treatment were 20.17%,14.96%,22.98%,31.48%and 24.28%,respectively.It shows that the combined process has a significant effect on the removal of fluorescent substances.（5）Field operation experiment.Using the combined micro-flocculation filtration-ultra-filtration process as the technical support,the micro-flocculation filtration-ultra-filtration water purification equipment was developed and field experiments were conducted.The best operating parameters were determined as the filtration rate of 6 m/h,backwashing time of 12 h for microflocculation filtration,30 min for ultrafiltration filtration cycle,and 100 s for ultrafiltration backwashing time,including 50 s for gas washing and 50 s for gas-water washing,etc.By calculating the production and operation costs of the equipment,the total water production cost was 3.27 Yuan/t,which was 34.6%less than the tap water cost.And the equipment has been running for 6 months,the water quality is stable.After the third-party water quality testing,the water quality of the output water all conformed to the Sanitary Standard for Drinking Water for Daily Use（GB5749-2022）.