In recent years, however, stricter environmental legislation and the growing costs of wastewater treatment have increased the attractiveness of the high and reliable degree of purification that can be achieved with membrane bioreactor. However, the problem of membrane fouling is as old as membrane filtration itself.
In this study, the phenomenon of membrane fouling was extensively studied at various microbial conditions, influent conditions, airflow rates, sludge retention times and soluble organic concentrations. Moreover, the pilot-scale membrane bioreactors were operated to prove the results obtained from lab-scale reactor.
The role of extracellular polymers on microbial surface characteristics and on adhesion of microorganisms to membrane surface was investigated on chapter 3. The surface charge was linearly increased with the increase of carbohydrate to protein ratio in the EPS (EPSc/EPSp), which weaken the bonding force between microorganisms and multivalent cations. Higher negative surface charge of microorganisms also induced the easy adhesion to the membrane surface which was negatively-charged and hydrophilic. As the result, the portion of cake resistance to total resistance increased with the increase of negative charge of membrane surface. The ratio of EPSc/EPSp could be used as the indicator of potential biofilm formation on the membrane surface.
The investigation of characteristics of microorganisms and their soluble microbial products (SMP) at various influent conditions showed that the sludge concentration was proportional to the organic concentration of the influent and sludge retention times (SRT). On the other hand, total amount of EPS and the composition of EPS were not so much different at different influent conditions. The concentration of soluble microbial products (SMP) was similar despite of influent COD concentration, however, the hydrophobicity or specific UV absorbance was inversely proportional to the influent COD concentration. The SRT of 20 days and the F/M ratio of 0.2 - 0.3 could minimize EPSc/EPSp and SMP production. To the contrary of previous investigations, the concentration of suspended solids in the mixed liquor was not strongly correlated with membrane fouling. The EPSc/EPSp was the important factors on membrane fouling, that total resistance was increased with decreasing EPSc/EPSp. This phenomenon was also observed in the experiment of the resistance induced by the soluble fractions. The decrease of COD:N ratio gave the increase of the effluent DOC, protein concentration and hydrophobicity and would expect severe membrane fouling.
In the experiments for the fates and inhibition of SMP in MBR, the production of SMP was averaged 4.2% of influent carbon in SRT of 15 days. In long SRT, MBR showed accumulation of SMP in mixed liquor to 15.3 mg/l and then decreased by acclimated microorganisms, which could decompose high molecular weight of SMP. Despite of the decrease of DOC in mixed liquor, the membrane resistance was increased by high molecular, hydrophobic and protein-like matters. The respirometry showed that in long SRT, microorganisms were capable of decomposition of SMP up to 26.8 % of COD. It could be also observed in the enumeration of active biomass, which was almost stable despite of total cell number.
Considering above results, pilot scale MBR process was operated to remove organics and nitrogen in the wastewater. During the operation, the suggested process gave the excellent treatability without concern of temperature. However, the efficiency of nitrogen removal was greatly affected by the change of hydraulic retention time induced by membrane fouling. Stirred cell test showed that soluble fraction of mixed liquor was responsible to membrane fouling, rather than suspended fraction. In the real situation, the amount of protein or EPSc/EPSp could be also regarded as the one of important factors affecting membrane fouling.
