Study On Purification Efficiency And Membrane Fouling Of Biological Slow Filtration Integrated Ultrafiltration Process In Micro-polluted Water Treatment

With the rapid economic growth,the surface water environment has been damaged severely.As a consequence,a large number of micro-polluted water bodies appeared,resulting in“water-quality water shortages”which greatly aggravated China’s water resource shortage situation.The treatment and further reuse of micro-polluted water is of far-reaching significance for improving the water environment and alleviating the tension of water resources.Ultrafiltration（UF）process,called“the third generation water treatment process”has attracted much attention due to its advantages of high solid-liquid separation efficiency,good disinfection performance,and no secondary pollution.Nevertheless,UF has little effect on removing organic matter and confronts severe membrane fouling during long-term operation,thereby constructing combined process is generally necessary.Considering the characteristics of micro-polluted water with high concentration of organic matter and ammonia,this study designed a biological slow filter（BSF）filled with activated carbon and zeolite filters as a pretreatment process,and then constructed a BSF-UF system for micro-polluted water treatment.The water purification efficacy and characteristics of BSF were investigated by means of the constant flux continuous operation test and constant pressure batch ultrafiltration test.The antifouling performance of BSF-UF system and corresponding membrane fouling mechanism were demonstrated.The water purification efficacy and anti-fouling ability of BSF-UF system,coagulation-ultrafiltration system(CUF1 system referred to coagulant dosage of 0.1 mmol·L^-1 Al³⁺and CUF2 system referred to 0.5 mmol·L^-1 Al³⁺)and UF system were compared and combed.Besides,the membrane fouling mechanism of each system was analyzed and discussed.The research results and conclusions are as follows:（1）The pollutants removal characteristics and mechanisms of BSF varied with depths.In the first half of the BSF,relying on the synergistic effect of adsorption and biodegradation,removal rates of Total Organic Carbon（TOC）and Specific Ultraviolet Absorbance（SUVA）reached 70.7%and 87.9%respectively.Macromolecular biopolymers（30 k Da<MW<80 kDa）,medium and low molecular weight humus organics（1 kDa<MW<10 kDa）,and other small molecular organics（MW<1 kDa）can be effectively removed.Besides,combined adsorption of zeolite and nitrification of nitrifying bacteria,the removal rate of ammonia reached 53.0%.In the middle quarter,there was no obvious removal of organic matter under anoxic state.The phosphate increased slightly due to the release of phosphorus by the polyphosphate bacteria in this section.In the last quarter,the dissolved oxygen increased owing to atmospheric reoxygenation in the vicinity of the water surface of BSF.Meanwhile,the phosphate content dropped.At the same time,the content of macromolecular biopolymer in the water rebounded,and the concentration of TOC in effluent increased.On the whole,BSF showed a significant removal efficiency for organic matter and ammonia,but the removal of the biopolymer didn’t live up to the expectation.（2）Both the constant-flux continuous experiment and the constant-pressure batch UF experiment followed the same trend that the ability to remove the organic matter among all systems can be ranked as BSF-UF system>CUF2 system>CUF1 system>UF system.Under constant flux continuous operation mode,the removal of TOC by BSF-UF system reached77.9%,which was significantly higher than that of UF system（i.e.11.0%）.In the constant pressure batch UF experiment,the TOC removal of BSF-UF system reached 65.0%,which was significantly higher than 9.3%of the UF system,17.0%of the CUF1 system,and 35.0%of the CUF2 system.Under the two experimental modes,ultraviolet-visible spectrum（UV-Vis）,size-exclusion chromatography（SEC）and excitation emission matrix（EEM）results followed the same trend that BSF-UF system had significant removal effects on all kinds of organic matters.CUF1 and CUF2 systems can merely remove macromolecular biopolymers and high molecular weight humics,while they exhibited poor removal ability on medium and low molecular weight organic matters.Scarcely can UF system intercept organic matters except for macromolecular biopolymers.（3）Under constant-flux continuous operation mode,the results of five hydraulic cleaning cycles（32 days）revealed that the anti-fouling ability of the BSF-UF system is significantly superior to that of UF system.And the mitigation effect of the total membrane fouling and irreversible membrane fouling on BSF-UF system became increasingly pronounced with the increased operation period.Contents of soluble microbial products（SMP）accounted for the absolutely largest proportion of the accumulated extracellular polymers（EPS）on the membrane surface of two systems,so it determined the total membrane pollution degree.The SMP content accumulated on the membrane surface of BSF-UF system was only about 60.0%that of the UF system,hence the greatly alleviated membrane fouling.Whereas the contents of tight-bound extracellular polymers（TBEPS）and loose-bound extracellular polymers（LBEPS）in two systems were nearly the same,indicating BSF pretreatment cannot effectively reduce the activated biomass accumulated on the membrane surface.Consequenctly,the long-term biological membrane fouling cannot be resolved radically.At the initial stage of the operation,the clogging of protein instead of humics in membrane pores led to the close degree of irreversible membrane fouling of two systems,considering a neglectable amount of humics adsorbed in membrane pores.With the increase of the operating cycle,the increasing accumulation of humics in the membrane pores of UF system led to the more severe irreversible membrane pollution of UF system in contrast with BSF-UF system.（4）In the constant-pressure batch UF experiment,CUF2 system showed outstanding ability to alleviate membrane fouling,and its final flux was about 4.3 times that of the UF system.In contrast,BSF-UF system and CUF1 system merely slightly improved membrane fouling compared to the UF system.Analysis of the foulants accumulated on membrane surface and in membrane pores revealed humics had neglectable effect on the initial membrane fouling.The deposition of biopolymers on the membrane surface and in membrane pores to varying degrees in various systems directly led to different degrees of membrane fouling.The accumulated amount of polysaccharide on the membrane surface was more than ten times that of protein,which dominated the final membrane fouling.The research can provide new reference of BSF-UF system’s application in micro-polluted water treatment,and provide new insights for ultrafiltration fouling.