The Characteristic Of Combined Fouling And Operation Regulation During Ultrafiltration Of Algae-laden Water

With the increasingly strict standards of drinking water quality and seriously status of water crisis,ultrafiltration(UF)has become one of the most widely used as advanced water treatment technologies due to its high efficiency of solid-liquid separation performance and relatively low energy consumption.In China,more than 50% of large scale drinking water treatment plant employ lakes and reservoirs as source water.however,the seasonal algae bloom caused by the eutrophication could hugely impact the water purification efficiency.On the other hand,the microcystins and various types of odorous substances(including 2-methylisodecyl alcohol and geosmin)secreted from algal cells are mutagenesis,carcinogenesis and teratoeenesis substances.and raise human health problem.If could not be effectively removed,they will pose a great threat to the health of the general public.Based on the size separation principle,UF is an efficient and effective technology for algae-laden water.The severe membrane fouling caused by ultrafiltration treatment of algae-laden water constrains the further promotion of the ultrafiltration process.On the other hand,the ultrafiltration membrane itself has a low removal efficiency for algae-derived organic.This is because the size of soluble organic matter is generally smaller than the size of the ultrafiltration membrane,resulting in the inefficient removal of soluble organics by the ultrafiltration membrane.In addition,at present,domestic and foreign scholars have very few studies on cell disruption during ultrafiltration treatment of algae-laden water.Cell rupture can lead to severe release of intracellular organic substances,thereby worsening water quality.Therefore,it is necessary to pay attention to cell disruption and its related effects during ultrafiltration.Algae cells that exist in the form of granules and algal source organisms that exist in the form of dissolved organic matter are the major pollutants in algae-laden water.algae cells,including live algae cells and algae debris,and algae derived organics,including extracellular organic matters and intracellular organic matters were possibly exisited in the alage-laden water.Therefore,it is necessary to identify the key foulant during UF process as well as the mechanism of the interplay effect,and then put forward corresponding treatment strategies.During the individual fouling test,algae cells(living algae cells and algae debris)could cause more serious flux decline,and algae derived organisms(extracellular and intracellular organic matter)could cause more serious adsorption fouling and irreversible fouling.During the combined fouling test,algae derived organic matter could fill into the gaps in the cake layer formed by algae cells,which could cause much serious membrane fouling.In addition,the intracellular organics could lead to serious flux decline and irreversible fouling.Therefore,during the treatment,the algae rupture should be avoided as well as the intracellular organic release.Based on combined membrane fouling,this paper also investigated the impact of different operating conditions(transmembrane pressure between 50-250 k Pa)on the interaction among ultrafiltration membrane fouling,water purification efficiency and cell rupture,and the impact of membrane fouling reversibility on water purification mechanism.The results showed that the flux hugely decreased with the increase of filtration volume.During this process,the type of fouling changed from membrane pore blocking to cake layer formation,and the transmembrane pressure did not cause severe cell rupture(cell rupture less than 5%).The accumulated cake layer on the membrane surface could improve the water purification efficiency.Due to the compressibility of algae cells,an increasing in transmembrane pressure results in an increased degree of surface cake cake densification,thereby increasing the organic entrapment efficiency.This paper also discussed the effects of ozone and Fe(II)/persulfate pretreatment methods on membrane fouling,water purification efficiency and cell rupture.Ozone oxidation and Fe(II)/persulfate oxidation could cause cell rupture due to free radical oxidation,and ozone oxidation could lead to more severe cell breakage.Fe(II)/persulfate pre-xodation contributes to higher effectiveness of the water purification,especially for the removal efficiency of algal toxins and odorant substances.However,the ozone oxidation pretreatment deteriorates the water quality,especially for odor concentration.This results show that the oxidation selection of Fe(II)/persulfate for degraded algal organics more strongly than the disrupts the cellular.Although ozone oxidation can alleviate the total,the irreversible fouling of the UF membrane adversely increased.On the contrary,the Fe(II)/persulfate oxidation could greatly reduce the total and the irreversible fouling.The results also show that the combined effect of sulfate radical oxidation efficiency and ferric ion coagulation efficiency is the main reason for improving the performance.In this thesis,we first identified the key foulant during the UF treatment of algaeladen water,and and explore the interfacial effects based on combined fouling mechanism.We also found that particle modification effect and steric hindrance effect are the main combined fouling mechanism.On this basis,the pre-filtration effect was further explored by changing the operating pressure of the ultrafiltration and regulating the structure of the cake layer of the ultrafiltration membrane.The effects of ozone oxidation and Fe(II)/persulfate pre-oxidation enhanced ultrafiltration treatment of algae-laden water were comparatively studied.It was found that Fe(II)/persulfate pre-oxidation can effectively enhance the efficiency of ultrafiltration treatment of high algae water,and the effect of ozone pre-oxidation on enhancing the ultrafiltration efficiency is very limited.