Study On Water Purification Characteristics And Operational Stabilization Of Combined Filtration Technology Constructed By Bank Filtration And Nanofiltration

In the context of continuous improvement of the water environment,the use of the most simplified and appropriate process to achieve safe treatment of drinking water is the goal of water purification technology development.The bank filtration(BF)/membrane filtration combined filtration technology,which couples the natural biodegradation of BF and the high-efficiency interception of membrane filtration,is a green water purification process that emphasizes the natural properties of drinking water,does not use chemical pretreatment,and has low carbon and low consumption.Therefore,this paper used BF and nanofiltration(NF)processes to construct a combined filtration.The effectiveness and mechanism of BF/NF combined filtration in the water purification process were systematically investigated.The characteristics and mechanisms of stable operation and resistance to water quality impact of the combined filtration technology were explored.Further study the stability measures during the operation of the combined filtration technology to improve the applicability of the technology.The BF is the first barrier in the combined filtration technology.BF as a natural water treatment technology,relying on sediment layer and aquifer sediment media,can remove28.1-41.4% of organic matter under different seasonal water quality conditions.However,by comparing the organic removal effect of BF with different compositions,it can be seen that BF removed organic matter from water while humus-like organic matter in the filter media was also gradually precipitated.In addition,BF was bioselective and superficially active for the removal of organic matter in water.The BF system can remove 74.5% of biodegradable dissolved organic carbon(BDOC)in water by biological action,of which more than 60% of organic matter removal can be achieved only through the superficial 5-10 cm of highly active area.When combined with membrane filtration technology,on the one hand,it strengthened the removal of BDOC from water,making the effluent BDOC concentration less than 0.2 mg/L and greatly enhancing the biological stability of water quality.On the other hand,BF decomposed some small-molecule organics while decomposing large-molecule organics in the water,alleviating the accumulation of organics on the membrane surface,and reducing membrane fouling.However,while BF reduced NF organic membrane fouling,it also increased the accumulation of extracellular polymer(EPS)on the membrane surface.43.1% of membrane fouling originated from biofilm fouling.Overall,under the condition of good water quality,the effect of BF on NF membrane pollution control was still better than that of traditional technology,and the combined filtration technology is technically reasonable.As a water purification technology that takes water directly from the river for treatment,the combined filtration will face complex and variable water quality.During the year-round cycle,the BF fraction exhibited significant stabilization characteristics for organic removal as raw water quality deteriorated or improved.Water quality changes could change the proportion and total amount of BDOC in the water,affecting the biological metabolism in the active layer of the BF surface,thus enhancing the performance of the BF fraction for organic removal when the water quality deteriorates,and failing to significantly improve the effluent quality when the water quality improves.In long-term operation,water quality changes did not significantly affect the microbial diversity and composition in the active layer.For trace organics in water,the BF removed trace organics through the whole process of adsorption and degradation,which was little affected by water quality changes,and combined with the retention performance of NF,the combined filtration could remove up to 74.09% of sulfadimethylpyrimidine.Moreover,water quality changes can enhance the removal of trace organics by affecting NF membrane fouling and mitigating the concentration polarization and membrane enrichment of organics at the NF membrane surface.Unlike the stability of BF to water quality changes,the NF responded significantly to membrane fouling development and fouling characteristics when water quality changes,as changes in the percentage of macromolecular organics in water can mitigate NF membrane fouling while reducing the percentage of unfavorable energy consumption.In the combined filtration technology,BF and NF showed diametrically opposed characteristics during water quality changes but rely on the stable characteristics of the natural treatment process microbial community,so that the combined filtration technology can maintain the stability of effluent water quality and membrane filtration energy consumption under variable water quality conditions throughout the year,providing the system with resistance to water quality shock loading capacity.The chemical cleaning process can have a stable effect on NF and improve the efficiency of the combined filtration technology.The impact of chemical cleaning process on NF was cross-cycle.Preferably using chelating agent or surfactant cleaning,can restore 90.5-94.2% permeability in a single cycle,and reduce 44.9-53.8% irreversible pollution accumulation in multi-cycle operation.The chemical cleaning process changed the fouling characteristics of the membrane surface after cleaning by the reaction of the agent and the membrane surface foulants,which can inhibit the adhesion of protein and polysaccharide pollutants in the water to the membrane surface and alleviated up to 44.1%of the initial membrane fouling in the next cycle.However,for the bio-fouling problem brought by BF,the chemical cleaning process of all types of agents cannot effectively restore the filtration performanc.The operating efficiency of NF in the complete cycle after chemical cleaning was also different.By affecting the non-electrostatic effect of membrane surface and contaminant distribution,the removal rate of target organics can be effectively improved after cleaning with suitable agents.Meanwhile,chemical cleaning can reduce 4.54% of membrane filtration unfavorable energy consumption ratio.Through chemical cleaning,the stabilization of the membrane filtration part can be realized,so as to improve the operating efficiency of the combined filtration without increasing energy consumption,chemical consumption and environmental impact.This paper explored the mechanism and synergistic effect of BF and NF in the water purification process from the combined filtration technology and proved that the combined filtration significantly improved the biological stability and safety of drinking water under the premise of green operation.Aiming at the operating characteristics of the two parts of BF and NF,a stable operation mechanism of combined filtration technology was proposed.The stabilization technology of combined filtration based on the NF chemical cleaning process was proposed to improve the energy consumption conditions.The research results will provide technical support for the broader exploration and application of the BF/NF combined filtration technology.