Membrane Bioreactor (mbr) as an Advanced Wastewater Treatment Technology
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Methods to Control Fouling To control the fouling that inevitably occurs in MBR operation, several key parameters can be modified. The most important strategies are concentra- tion polarization suppression, optimization of physical and chemical cleaning protocols, pre-treatment of feed wastewater, and mixed-liquor modification. Fouling related to concentration polarization can be reduced either by pro- moting turbulence or by reducing flux. As mentioned above, both MBR configurations need shear over the membrane surface to prevent this type of fouling. As with most other membrane processes, side-stream MBRs pro- vide shear through pumping, which increase cross-flow velocity, whereas immersed processes employ aeration around the membrane to provide shear stresses. Aeration intensity over the submerged membrane surface is recognized as the key operational parameter in preventing cake formation on the mem- brane surface in the submerged configuration [26–28]. Membrane permea- bility and critical flux have been shown to increase roughly linearly with aeration rate, up to a level above which no further increase is observed. Liu et al. [29] investigated critical aeration intensities for different sludge con- centration and fluxes, drawing a quantitative correlation between those pa- rameters. However, increasing membrane aeration is normally prohibitively expensive. Since membrane aeration contributes significantly to the energy demand, much development has been focused on reducing aeration whilst maintaining membrane permeability. A development has been achieved in aeration efficiency with new jet aeration and cyclic aeration systems. It is of- ten in practice to use different aeration systems for biological system and for Membrane Bioreactor (MBR) as an Advanced Wastewater Treatment Technology 53 membrane fouling control, in order to insure most efficient energy use for both systems demands. Reducing the permeate flux always reduces fouling, but at the same time this strategy demands a more installed membranes, which then contributes to the capital cost of MBR installation. In other words, flux can be maintained below the critical value to ensure stable operation with little or negligible increase in TMP, thus decreasing cleaning frequency and consumption of chemicals, or total installed membrane area can be reduced on the behalf of frequent cleaning. The latter strategy is called intermittent operation. In practice, most submerged MBRs treating municipal wastewater operate at net fluxes of 20–30 L m –2 h –1 with the relaxation period every 10 min and period- ical maintenance chemical cleaning every few months. As mentioned above, pre-treatment of feed wastewater through screening is necessary for both HF and FS membrane modules. HF membranes have a tendency for aggregates of hair and other debris to collect at the top of the membrane elements. These aggregates usually cannot be significantly removed by back-flushing. FS modules are somewhat less prone to such clogging, but they too need a feed-water pre-treatment, though with coarser screens. The properties of the mixed liquor that affect the fouling propensity can also be altered to minimize fouling. The production and release of EPS could be influenced by changing the biological state of biomass, usually by SRT modification as mentioned above, but it is rarely done in practice. To decrease the ESP or SMP concentration, mixed liquor is mostly modified through addition of chemicals. The use of flocculants and coagulants such as alu- minum or ferric chloride has been investigated in an attempt to minimize fouling [10, 30]. Also, the addition of adsorbent reagents such as powdered ac- tivated carbon (PAC) has been found to improve the membrane performance by decreasing the level of organic compounds with potential for membrane fouling [14, 31]. Recently, a commercial product, a cationic polymer-based compound called MPE50 manufactured by Nalco Company has been de- veloped and tested at full-scale to enhance membrane performance. The cleaning protocol is mainly dictated by designed operational net flux as explained above. Usually the protocol suggested by the manufacturer is followed as a guideline, and the majority of the installed plants work in the sub-critical regime. However, cleaning protocol has been studied inten- sively by many researchers where the key parameters of interest are duration and frequency of the cleaning and the back-flush flux. Less frequent, longer back-flushing (600 s filtration/45 s back-flushing) has been found to be more efficient then more frequent but shorter back-flushing (200 s filtration/15 s back-flush) [25]. To optimize the back-flush duration, Smith et al. [32] de- veloped a generic control system based on TMP monitoring. Membrane re- laxation, which is the most common practice for fouling control, encourages the diffusive back-transport of foulants away from the membrane surface, which is enhanced by air scouring. Relaxation allows longer filtration periods 54 J. Radjenovi´c et al. between chemical cleanings, and despite some reports that it may not be a feasible practice for large-scale MBRs [33], it is widely used in practice. Although intensive research has been done on this subject, membrane fouling in MBRs needs further attention in order to understand the complex interactions among biologically active and constantly changing filtration me- dia, hydrodynamic conditions of the filtration process, and the membrane itself. tải về 0.95 Mb. Chia sẻ với bạn bè của bạn:
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