Chung-Hak Lee

Membrane filtration characteristics in membrane-coupled activated sludge system -- the effect of physiological states of activated sludge on membrane fouling

The effect of sludge physiology on membrane fouling was investigated in a membrane-coupled activated sludge (MCAS) system. A series of ultrafiltrations were performed to assess the flux behaviors according to foaming potential, solids retention time (SRT), growth phase and nutrient condition of the activated sludge. The foaming sludge showed greater flux decline than the non-foaming sludge. The extraordinary increase, that is, more than 100 times in membrane fouling for the foaming sludge, was attributed to the hydrophobic and waxy nature of the foaming sludge surface, which was confirmed by a comparison with relative hydrophobicity. Membrane fouling tendency was increased as SRT decreased. A greater flux decline was observed at the endogenous phase than at the log growth phase. The activated sludge acclimated to the nitrogen deficient substrates produced less extracellular polymeric substances (EPS) and exhibited higher flux than the control activated sludge. The quantitative measurements of EPS content in order to estimate the extent of membrane fouling in various activated sludges showed that, in any physiological states of activated sludge, the higher the content of EPS the activated sludge had, the greater the membrane fouling proceeded. The EPS content of activated sludge is suggested as a probable index for the membrane fouling in a MCAS system.

Comparison of the filtration characteristics between attached and suspended growth microorganisms in submerged membrane bioreactor

An attached growth bioreactor was designed to minimize the effect of suspended microorganisms on membrane fouling in submerged membrane bioreactor. Comparison of mixed liquor from attached and suspended growth systems was made to elucidate major factors giving rise to different filtration characteristics. Unexpectedly, the rate of membrane fouling of the attached growth system was about 7 times higher than that of the suspended growth system despite similar characteristics of soluble fraction from the two reactors. Filtration performance proved to depend on the concentration of mixed liquor suspended solids (MLSS). Better filtration performance with suspended growth was explained by the formation of dynamic membranes with suspended solids. A series of analyses such as hydraulic resistance, specific cake resistance, scanning electron microscope, and atomic force microscope were carried out to elucidate the different filtration characteristics of the two systems.

Membrane fouling mechanisms in the membrane-coupled anaerobic bioreactor

Abstract This study focused on the membrane fouling mechanisms during the longtime operation of a membrane-coupled anaerobic bioreactor (MCAB) system designed for the treatment of alcohol-distillery wastewater. This system provided interesting information on anaerobic digestion and membrane performance associated with the fouling mechanisms in the membrane bioreactor. Enhanced COD removal was achieved with the complete retention of biomass either inside the anaerobic reactor or on the membrane surface. Membrane fouling was mainy attributed to external fouling, which was closely related to the movement of cell population to the membrane surface and inorganic precipitation at the membrane surface. The major composition of the inorganic foulant was identified as MgNH 4 PO 4 ·6H 2 O (struvite), whose deposition together with the microbial cells attached at the membrane surface played a significant role in the formation of the strongly attached cake layer limiting membrane permeability. The struvite precipitation/deposition mechanisms were examined thoroughly in relation to the chemical composition of the influent wastewater and the subsequent anaerobic decomposition in the membrane bioreactor. The conceptual resistance-in-series model was applied to assess the fouling characteristics.

Effects of membrane fouling on solute rejection during membrane filtration of activated sludge

To assess the relationship between solute rejection and membrane fouling in a MBR system, membrane filtrations of activated sludge in different physiological states were carried out with ultrafiltration membranes. Regardless of the physiological states of the activated sludge (foaming, bulking, pin-point floc, exponential growth, endogenous phase, normal state sludge), the hydrophobic membrane (PM30) always showed greater solute rejection than the hydrophilic membrane (YM30). To investigate the key factors affecting solute rejection, the cake layer resistances (Rc) and the fouling resistances (Rf) were measured. The Rc and Rf values for the PM30 were always higher than for the YM30 and the Rc prevailed over the Rf in all cases. The solutes rejection by the adsorption onto/into the membrane was relatively small. This suggests that the cake layer deposited on the membrane surfaces played an important role in the solute rejection, i.e. dominant solute removals were attributed to the adsorption and/or sieving onto the cakes. Consequently, the difference in solute rejection efficiency between hydrophilic and hydrophobic membranes was mainly due to the degree of sieving and/or adsorption onto the cakes deposited on the membrane, and partly due to adsorption into membrane pores and the surfaces.

Analysis of CaSO4 scale formation mechanism in various nanofiltration modules

Scale formation of sparingly soluble salts has a significant effect on flux decline in nanofiltration (NF) system. This study focuses on the elucidation of the different mechanisms of scale formation according to membrane modules in NF system. In unstirred batch NF, flux decline was mainly due to surface (heterogeneous) crystallization, while in crossflow NF, fouling was attributed to both surface and bulk crystallization. However, the extent of contribution of each crystallization to the fouling depended on the NF modules. The bulk (homogeneous) crystallization followed by crystal deposition on the surface of the nanofilter played a major role in flux decline in the spiral wound module whereas surface blockage due to the surface crystallization does in the tubular module. When an on-line microfiler was introduced to prevent crystal deposition during the concentration run, flux improvement was pronounced only in case of the spiral wound module, whereas it was negligible in case of the tubular module. This was because the microfilter could only remove crystals formed in the retentate through the bulk crystallization which is the dominant fouling mechanism in the spiral wound module. A modified resistance-in-series model was applied to assess the fouling characteristics of each NF module based on the bulk and the surface crystallization. The greatest extent of the fouling due to surface crystallization in tubular module was attributed to its highest concentration polarization modulus compared with the other two modules at the same crossflow rate. ©1999 Elsevier Science B.V. All rights reserved.

Effect of calcium ion on the fouling of nanofilter by humic acid in drinking water production

Abstract The influence of the charge of humic acid (HA) on the fouling of nanofilters was investigated as a function of pH and concentration of calcium ions. The charge of the humic acid as well as the zeta-potential of the membrane surface were measured to elucidate the mechanism of humic acid deposition on the membrane surface. The negative charge of humic acid as well as the negative zeta-potential of the membrane surface fouled with humic acid increased with higher pH. As a result the further deposition of humic acid on the membrane surface was expected to decrease with higher pH because of larger repulsive forces. With moderate calcium ion concentration, however, the adsorption of humic acid onto the membrane surface decreased until neutral pH and then increased again. It was attributed to calcium ion bridging between two free functional groups of humic acids. Calcium ion rejection decreased in the presence of humic acid deposited on membrane surface. This could be explained by the effect of humic acid on the net charge of the membrane. The addition of calcium chelating agent, EDTA, improved flux, especially at alkaline conditions. The diagram of Ca–EDTA vs pH was used to explain the flux improvement.

EFFECT OF OPERATING CONDITIONS ON CaSO4 SCALE FORMATION MECHANISM IN NANOFILTRATION FOR WATER SOFTENING

The eAect of the operating condition, such as transmembrane pressure and crossflow velocity on the scale formation pathways, was investigated in nanofiltration (NF) of a CaSO4 solution. The examination of scanning electron microscopy of a fouled membrane surface provided strong evidence that there are two mechanisms in NF fouling (e.g., scale formation on the membrane surface) in nanofiltration; surface and bulk crystallization. The hydrodynamic operating conditions determined which fouling mechanism was dominant. The fouling due to surface crystallization augmented with an increase in operating pressure, but reduced with an increase in flow velocity. Concentration polarization was found to be responsible for this type of fouling. On the other hand, the extent of fouling by bulk crystallization appeared to be the most important at intermediate crossflow velocity and higher operating pressure (e.g., higher flux) because both secondary nucleation eAect and deposition probability depends on crossflow velocity and permeation rate. An experimental and theoretical method for diAerentiating one mechanism from another was explored in order to elucidate the dominant fouling mechanism according to the hydrodynamic conditions. 7 2000 Elsevier Science Ltd. All rights reserved Key words—nanofiltration (NF), calcium sulfate, membrane fouling, scaling, bulk crystallization, sur- face crystallization

Comparison of the filtration characteristics of organic and inorganic membranes in a membrane-coupled anaerobic bioreactor

Comparison of filtration characteristics of organic and inorganic membranes was made in terms of physicochemical properties of the membrane materials, cake layer formation, backflushing and backfeeding effects in a membrane-coupled anaerobic bioreactor. For the inorganic membrane, struvite (MgNH4PO4 x 6H2O) was found to have accumulated inside the membrane pore and plays a key role in flux decline. For the organic, however, a thick cake layer composed of biomass and struvite formed on the membrane surface, thus causing a major hydraulic resistance. In order to mitigate flux decline for both membranes, backflushing and backfeeding modes were examined. With acidic (pH 2.0) backflushing, the flux was approximately doubled for the organic membrane. However, unexpectedly a negative effect was observed for the inorganic membrane. An alkaline backflushing instead of acidic backflushing gave rise to a flux improvement by a factor of two without any negative effect, even for the inorganic membrane. The backfeeding mode gave rise to a much higher flux compared with the normal mode in both types of membrane, although the flux returned to the same level as that with the normal mode after 6 days for the inorganic membrane. The differences between the two types of membranes were explained by membrane morphology, a ligand exchange reaction as well as a surface charge effect.

Hydrodynamic behavior of anaerobic biosolids during crossflow filtration in the membrane anaerobic bioreactor

Abstract Anaerobic biosolids deposition on the membrane surface has a significant effect on flux decline in membrane-coupled anaerobic bioreactors (MCAB). The aim of this study was to investigate the hydrodynamic behavior of anaerobic biosolids during crossflow ultrafiltration of digestion broth in the MCAB. A theoretical approach to predict flux decline was made considering the biosolids transport mechanism based on hydrodynamics and surface interactions. The continuous size reduction of biosolids offered an exponential flux decline at the initial stage, as the biomass cake layers being formed by the deposition of smaller particles with back-transport velocities less than the flux. Since flux recovery could not be achieved readily on the way of ultrafiltration due to irreversible biofouling, the biosolids movement toward the membrane surface should be controlled at the beginning of the MCAB operation. The optimal operating condition which prevents biosolids deposition onto the membrane surface could be predicted by the evaluation of critical flux.

Characteristics of microfiltration membranes in a membrane coupled sequencing batch reactor system

Factors affecting filtration performance were investigated in a sequencing batch reactor (SBR) coupled with a submerged microfiltration module. Special bioreactors for aerobic and anoxic phases were specifically designed in order to differentiate the effect of dissolved oxygen (DO) from that of mixing intensity, on membrane filterability. When the filterability of a submerged microfilter was examined at each SBR phase, DO concentration, as well as mixing intensity proved to have a major influence on the membrane performance regardless of the SBR phase. A higher DO concentration resulted in a slower rise in TMP, corresponding to less membrane fouling, which was investigated in depth through a series of analyses including resistance measurements and compressibility of the cake layer as well as particle sizes as a functions of DO for both aerobic and anoxic phases in SBR.