Membrane Fouling And Its Control In A Combined Ultrafiltration System For Water Treatment

The newest standards for drinking water which has been released by ourcountry emphasize the safety of water quality. Conventional treatment processes fordrinking water could no longer fulfill the requirements of increasingly strict qualitystandards. With better quality ensurance, ultrafiltration （UF） technology has beenapplied gradually nowadays. Submerged UF has the features of high waterproductivity, easy in operating and efficient in working, and it has wide prospect.However, membrane fouling seriously constrains the development of UF technology.Pretreatment can effectively control membrane fouling. In this thesis, microflocculation and activated carbon powder （PAC）, which are applied widely in actualoperation, were choosed to combine with submerged UF technique. The mechanismfor fouling in two kinds of process was discussed at first, and then effects of threeoperation parameters, which included dosage, aeration intensity and discharge cycle,on water quality and fouling were studied. A series of optimized operatingparameters were provided, which ensure a good water quality and control thefouling at lowest simultaneouslyAt first membrane fouling speeds for each kind of pretreatment after a stableand long time run were compared. The mechanism of fouling formation under twokinds of pretreatment condition was studied simply. The results showed that whenthe removal rate of DOC for both kinds of pretreatment reached28%, TMP growthrate for micro-flocculation UF prosess was0.47kPa/d, instead, PAC UF processpretreatment was0.27kPa/d, which means the micro-flocculation UF process has alarger membrane fouling speed. A careful chemical cleaning for membranes afterexperiments was did, the left chemical liquids showed that main component ofmembrane fouling was hydrophilic organic compounds with low molecular weight,the liquids also contained a small amount of metal elements, such as Al, Fe, Mn, Zn,V, Cu, Ba, Ti, Ni, Pb, As, Cs. The amount of Al in chemical liquid using inmicro-flocculation UF （18.29mg/L） was obviously higher than that when in PAC UF,which was the main reason for a larger membrane fouling when usingmicro-flocculation UF as pretreatment. A XPS analyzing showed that the maincontents in membrane pollutants were oxide-containing organic compoundscombined small amount of Al and Si. After characterization for organic compoundsin membrane pollutants using3-D Fluorescence spectrum, the results showed that II（amino acids protein substances）, III （fulvic acid substances） were the majorpollutants in micro-flocculation UF process, and IV （soluble biological metabolites）were the major pollutants in PAC UF process. Study of operating parameters on fouling control in micro-flocculation UFcombined process showed that, an addition of3mg/L PACl could strengthen theremoval rates for UV₂₅₄and DOC with28%and24%respectively. Adding morePACl can not only increase the organic components removing, but also decreaseexternal membrane fouling formation. Compared to the inner membrane fouling rate（4.8%） using direct filtration, micro-flocculation UF pretreatment could control thefouling rate under1%. Under the condition of using conventional addition of4mg/L（electric potential closed to zero）, the inner membrane fouling conld be decreased toleast when reduce the addition to3mg/L （negative electric potential）. Increasingaeration could reduce the removal capacity for organic matter, when the ratio of gasto water reaches9:1or12:1, organic compounds can be removed better. Aerationcould also effectively alleviate the membrane fouling, when the ratio of gas to wateris9:1, external membrane fouling could be reduced by54%, and the velocity forinner membrane fouling could be controlled under0.07%（0.2%without aeration）,but a increasing of aeration would not reduce external membrane fouling any more,and even aggravate inner membrane fouling. A extending cycle could reduce theefficiency of organic compounds removal, which might lead to SS accumulation inmembrane tank, thereby increase the cake layer pollution of membrane, this couldbe alleviated by setting discharge cycle to24h.Study of operating parameters on fouling control in PAC UF combined processshowed that this kind of pretreatment could strengthen removal of particulate matter,and adding more PAC could obviously enhance the removal of organic compounds,and the effect was optimum at a dosage of10mg/L, removal rates for COD_MnandUV₂₅₄raised by16%and19%respectively. The addition of PAC accelerated cakelayer fouling formation at first, but a continued addition could decrease cake layer;strengthening aeration would reduce the remaining capacity of membrane forparticulate matter, which could lead to the increasment of out water particulatematter. But a strengthening aeration could increase the amount of suspended PAC,so the removal of organic compounds would be decreased. When the ratio of gas towater reaches12:1, the removal rate for COD_Mnand UV₂₅₄would increase by6%and9%. The aeration would also alleviate the cake layer pollution of membrane byenhancing the shear force of membrane surface, then the velocity for innermembrane fouling could be slow down. The activated carbon adsorption inmembrane tank was saturated when the discharge cycle was extented, thus reducedthe removal rate of organic matter, when the cycle extended to48h and72h, thecorresponding removal rate for COD_Mnand UV₂₅₄reduced by3%and5%respectively. So the discharge cycle of24h was good for the system to keep arelatively high organic compounds removal effect.