Membrane solid-liquid separation

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

A biological step is always necessary to remove the carbonaceous fraction from the influent wastewater suspended biomass treatments are the most common. These entail long SRTs (>25-30 d), and compartmentalization of the biological reactor is necessary for the removal of recalcitrant compounds. Furthermore, as many micro-pollutants tend to adsorb/absorb to the biomass flocks, efficient solid/ liquid separation can greatly improve their removal from wastewater and, at the same time, guarantee consistently good effluent quality. MBRs have been suggested for this purpose by many authors [9, 58, 80, 93], some of whom found that ultrafiltration (UF) membranes are more efficient than MF membranes [9, 93]. 

Yamamoto, K., Hiasa, M., Mahmood, T. and Matsuo, T. (1989) Direct solid-liquid separation using hollow fiber membrane in an activated sludge aeration tank. Water Science and Technology, 21 (4/5), 43—54. 

The contributions of Dr. Joseph D. Henry (Alternative Solid/Liquid Separations), Dr William Eykamp (Membrane Separation Processes), Dr. T. Alan Hatton (Selection of Biochemical Separation Processes), Dr. Robert Lemlich (Adsorptive-Bubble Separation Methods), Dr. Charles G. Moyers (Crystallization from the Melt), and Dr. Michael P. Thien (Selection of Biochemical Separation Processes), who were authors for the seventh edition, are acknowledged. 

MF may be used to remove these heavy metals provided pretreatment chemicals are added to precipitate the metals to particles of filterable size. The chemical pretreatment step is crucial since it will affect the performance of the membrane and the resultant sludge volume as well as the contaminant removal efficiency. Reduction/oxidation, absorption/oxidation, and/or catalytic reactions are utilized along with pH adjustment to provide the optimum precipitation. Although conventional methods of waste water treatment may use a similar pretreatment chemistry, the final solid/liquid separation by gravity settling is usually not as effective as membrane filtration. 

Cell recycle fermentors consist of two main units a vessel where the biomass is allowed to grow, and a membrane separation unit (as in Figure 7.40). Vessels are usually designed to insure a uniform concentration of nutrients and pH throughout the whole volume. Due to complete mixing, process control and stability of the microbial slurry are not difficult to achieve.88 After anaerobic stabilization, when the biomass is well developed, the reactor biomass is pumped to the UF unit where solid-liquid separation occurs. The sludge is flushed back to the reactor. In most cases, the flow rate of nutrient feed is kept equal to the permeate flow rate thus keeping a constant liquid level in the anaerobic reactor. 

There are presumably a variety of reasons why soluble polymers are less often used as supports or ligands for catalysis. The most likely reason is that there is a perception that recovery of a soluble polymer is e3q>erimentally more difficult than recovery of an insoluble cross-linked polymer. This perception stems from the mistaken belief that a soluble polymer can only be isolated as a viscous, intractable gooey material. However, as is demonstrated in the examples below, this is not true. Indeed, many soluble polymers can be easily isolated as tractable solids. Moreover, even in cases where the soluble polymer is not a tractable soUd or where a solid/liquid separation is deemed less desirable, soluble polymers can often be separated from low molecular weight products either on the basis of size (membrane fQtration) or on the basis of their selective solubility in one phase of a biphasic mixture. 

In addition, some scale-up works need apparatus that are operated for preparative purposes as well, along the lines of the kilo lab, but in a flexible environment not focused exclusively on batch processing as the kilo lab is. Examples of such apparatus are fluid bed crystallizers, hydroclones for the evaluation of that method of solid/liquid separation, lyophili-zation cabinets with special vial sampling capabilities, intermediate scale membrane processing assemblies, etc. An area well suited for such testing purposes is not only highly desirable, but often facilitates preparative work by processing methods not within the scope of the kilo lab. Such an area should be reasonably open for the manipulation of portable equipment, with ample walk-in hoods and tall California racks, well distributed utilities, portable measurement panels for recorders, fiowmeters and the like. 

K. Yamamoto, M. Hiasa, T. M ah mood, T. Matsuo, Direct Solid-Liquid Separation Using Hollow Fiber Membrane in... 

Appropriate membrane technologies in the context of solid-liquid separation are microfiltration and the use of the more open membranes in ultrafiltration. Other membrane processes, including the pressure-driven processes of hyperfihration and reverse osmosis, are concerned primarily with the removal of dissolved species fi om a solvent and shall not be considered. The boundary between the finer end of microfiltration and the coarser end of ultrafiltration is not sharp, and ultrafiltration is used for fine colloid-liquid separation. The start of the regions of ndcrofiltratian, ultrafihration and hyperfiltration occurs, approximately, with the fihration of particles of diameter 10, 0.1 and 0.005 rm, respectively. 

Hollow-fibre membrane modules are similar to the capillary type described above, but with fibres of outside diameters ranging from 80 to 500 pm. It is usual to pack a hollow-fibre module with many hundreds or thousands of these fibres, thus membrane area per unit volume is extremely hi. It should be apparent that filtration using hollow-fibre modules is only realistic with process fluids prefiltered to prevent fibre blockage fins limits the technology and it is applied mainly in UF. Also used in uhrafiltration is a spiral-wound membrane module which is often compared to a Swiss roD. The membrane and a spacer are wound round a former, with an appropriate permeate spacer flow is introduced and removed from the ends. This module design is not appropriate for solid-liquid separation, even when filtering colloids, because of the possibility of flow channel blockage and so it will not be discussed any finther. 

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