Pre-ozonation For Fouling Control During Ultrafiltration Membrane Processing Sewage Effluent

In the recent three decades, ultrafiltration (UF) membrane technology has beenrecognized as the most promising technology for the wastewater treatment and waterreclamation. However, membrane fouling will greatly hinder the widespreadapplication of membrane filtration technology. The water sample was fetched fromthe effluent of pilot-scale SBR operated by key laboratory of water pollution controlcenter. Firstly, ultrafiltration performance and membrane fouling mechanism ofwastewater effluent organic matter (EfOM) were studied. Then, the effect ofpre-ozonation for fouling control during ultrafiltration membrane processing sewageeffluent was ingvestigated. The research work was mainly done from the followingfour aspects: foulants size distribution; solution environment; hydrophobicity/hydrophilicity of EfOM; membrane properties. The scanning electron microscope(SEM)， the attenuated total reflectance-fourier transform infrared spectroscopy(ATR-FTIR) and drop shape analyzer were used to analyze the pollutants on themembrane surface.The ultrafiltration performance of EfOM was studied. When100kDa UFmembrane was used, the reduction of DOC was17%. With the decrease of themembrane’s molecular weight cutoff (MWCO), the DOC reduction only slightlyimproved. The gel permeation chromatography (GPC) and excitation-emission matrix(EEM) results revealed that large molecular weight polysaccharides and proteins canbe effectively removed by the100kDa UF membranes. However, humic substancesin EfOM were only slightly removed during the ultrafiltration process. Comparedwith EfOM, humic substances in NOM were more easily removed by ultrafiltration.Polysaccharides, proteins and colloids in EfOM were easier to be removed by UFmembranes, and were speculated as the main potential foulants for UF membranesprocessing sewage effluent.The actual sewage effluent was classified into particulate EfOM (pretreated byslow sand filters), colloidal EfOM (prefiltered by1.2μm filter), and dissolved EfOM(prefiltered by0.45μm filter). The ultrafiltration performance of the three fractionswas studied separately. Pre-ozonation could all effectively alleviate the UF membranefouling. With the extension of ozone dosage, the membrane flux increasingly improved, the TF and RF weakened, the IF gradually decreased with firstly slightincreasing. The SEM analysis demonstrated that the cake layer formed on themembrane surface seemed very dense and compact. After pre-ozonation, obviousaggregation of foulants on the membrane’s surface can be observed. The FTIR resultspresented that pre-ozonation contributed to a reduction of the amounts of thepolysaccharides, proteins and humic substances on the fouled membrane surface,which effectively alleviate the UF membrane fouling.The effect of solution environment on the UF membrane fouling of ozonatedEfOM was ingvestigated. For the ozonated particulate, colloidal and dissolved EfOM,the presence of Ca2+could further alleviate the UF membrane fouling. With theincrease of Ca2+concentration, the membrane flux increasingly improved. For theozonated colloidal and dissolved EfOM, the TF, RF and IF all decreased with thepresence of Ca2+. As for the ozonated particulate EfOM, the IF decreased, while theRF increased with the presence of Ca2+. The presence of NaCl had no influence onmembrane normalized flux during ultrafiltration membrane processing sewageeffluent. The SEM analysis demonstrated that the aggregation of foulants was moreobvious with the presence of Ca2+, which was due to the bridging interactionsbetween organic molecules.EfOM in sewage effluent can be divided into two groups：the hydrophobic (HP)and the hydrophilic (HL) fractions. The ultrafiltration performance of the two groupswas studied. The results showed that pre-ozonation could all effectively alleviate theUF membrane fouling, the TF and RF weakened, the IF gradually decreased withfirstly slight increasing. For the ozonated hydrophilic fractions, the presence of Ca2+could further alleviate the UF membrane fouling, the TF and IF both weakened, whilethe RF slightly increased. As for the ozonated hydrophobic fractions, the presence ofCa2+and NaCl both led to the reduction of UF membrane flux. The SEM resultspresented that for the membranes fouled with hydrophilic fractions and ozonatedhydrophilic fractions, no obvious foulants could be observed on membrane surfaceafter backwashing with ultrapure water. Obvious foulants could be observed on thesurface of the membranes fouled with hydrophobic fractions, the addition of Ca2+andNaCl both led to the further aggregation of the foulants.The effect of membrane properties on the UF membrane fouling of ozonatedEfOM was ingvestigated. Compared with hydrophobic UF membrane，the EfOM contributed more serious membrane fouling to hydrophilic UF membrane.Pre-ozonation could all effectively alleviate the hydrophilic and hydrophobicmembrane fouling. For different MWCO membranes, whether for tight UFmembranes (20kDa) or loose UF membranes (300kDa), pre-ozonation could alleffectively alleviate the membrane fouling, the TF and RF weakened, the IF graduallydecreased with firstly slight increasing with the extension of ozone dosage.