Membrane Fouling Controlled By Ozonation In Ozone+MBR Process

As a new highly effective water treatment technologies, Membrane bioreactor (MBR) has many advantages such as small footprint, good water quality, high organic load, low sludge production, easily start, can be realized modular and so on. It receives people's attention increasingly. However, the existence of membrane fouling bring a series of problems such as fast increasing of filtration resistance, serious weaken of membrane flux,short frequency of replacement and clean membrane module. Membrane fouling has become the bottleneck for the widely used of MBR. Therefore, how to control the membrane fouling has been the focus of the current research about MBR.Ozone, as a highly efficient and no secondary pollution strong oxidants, has been used widely in membrane filtration systems. In our laboratory's previous study of sludge reduction by ozonation, we found that the way adding proper amount of ozone to the MBR, can achieve the reducing of sludge as well as the mitigating of membrane fouling, so in this study, it couples ozone with MBR in order to study the impact of ozone to membrane fouling. The contains mainly including four parts:①test of membrane function in distilled water;②test of optimization about ozone dosage;③test of optimum operating conditions;④comparing MBR and the ozone +MBR two sets of systems on long-run, analysis of the mitigating of membrane fouling and the effect of sludge reductionThe results show that:(1) The slope of membrane module 1 and membrane module 2 about the relationship between specific flux and membrane pressure is K1=0.104, K2=0.119 respectively, it means that these two membrane modules have a strong comparability, which is the foundation of the subsequent research in membrane fouling(2) In the test of optimization about ozone dosage, the highest specific flux can reach up to 0.96LMH/kPa on the condition of ozone dosage was 32.23mg.l-1 and the reaction time was 15min. the characteristics of sludge mixture change a lot after ozonization the concentration of MLSS basically shows a downward trend, the viscosity changes little, the concentration of TOC,polysaccharide and protein in the sludge increases with the large of dosage concentration and the long of reaction time. The value of SOUR and pH decrease a little, but it doesn't affect the survival of microorganisms. All these results have provide theoretic support for mitigating membrane fouling in ozone +MBR process.(3) In the optimization test of operating conditions, It investigated the effect of membrane flux by three operating parameters including suction pressure, aeration, the ratio of stop/run. the results showed that:①the best suction pressure≤0.03MPa, when more than 0.03 MPa, the decay rate of flux increases fast;②the optimal ratio of pumping suction/stop was 9:1;③the optimal aeration quantity is 100 L. h-1. when at aeration capacity of 200 L.h-1 the membrane flux was not significantly better than the 100 L. h-1,this may thanks to the circular structure of reactor and the microporous aeration structure, it will produce small,uniform size bubble, accelerate the mass transfer and give full play of aeration without the formation of dead angle.(4) Running two systems parallel for 175 days, it found the SF values in three stages of 175 days shows the same regulation that the membrane fouling in ozone +MBR process developed rapidly at first, and later slowly, compared with the MBR process.(5) It investigated fouling resistance composition of the third stage's membrane modules which have run for 44 days. The results showed:①the membrane fouling in MBR process is more serious than ozone +MBR process. in MBR, the total resistance reached 21.08×1012m-1,the rate of cake layer resistance Rc, the irreversible resistance Rp, natural resistance Rm is 56.31%,24.86%,18.83%, while ozone+MBR process is only developed to 17.95×1012m-1, The rate of Rc,Rp, Rm is 40.84%,36.66%, 22.62%;②Whether in ozone +MBR or MBR process, cake layer resistance Rc is always the main resistance in all kinds of fouling;③Compared with the MBR process, in ozone +MBR process cake layer resistance Rc is relatively reduced,but the irreversible resistance Rp was increased.(6) The experiments by different cleaning methods to restore flux further showed that cake layer fouling is the majority part in membrane fouling whether in ozone +MBR or MBR process, meanwhile, through the analysis of membrane surface pollutants by FTIR it shows the main components of organic pollution is EPS which mainly contain polysaccharide and protein. Ozone +MBR process can migitate inorganic fouling such as Fe, Ca deposition..(7) Without regard to loss of sludge in membrane cleaning and sampling, both of two systems didn't discharge sludge during a long period of 175 days of experiment operation, the sludge concentration of control reactor increased by 7458mg·L-1 from 5282mg·L-1 to 12740mg·L-1. But the sludge concentration of ozone +MBR reactor just increased by 3388mg·L-1 from 5310mg·L-1 to 8698mg·L-1. The increased amount of sludge in ozone +MBR process only account for 45.4% of the MBR process, it meant that the ozone +MBR process did have the effect of reducing sludge.(8) The average removal rates on COD, NH3-N and TP in ozone+MBR process can reach 90%,95.8%,62.1% respectively. The water quality of membrane effluent in ozone +MBR process is slightly worse than MBR process, however the impact is only a little.By adding ozone directly to MBR, the research found that with the proper amount of ozone, the ozone +MBR process can achieve not only reducing the sludge, but also mitigating membrane fouling in the condition of ensuring the effluent's water quality, this research will provide some reference for the ozone +MBR process whether it is used for sludge reduction or controlling membrane fouling.