As water and sewer costs are expected to increase over the next decade, membrane technology will continue to be a viable and economical option in the recovery and reuse of water. This paper describes the mechanism of membrane effected separation (specifically reverse osmosis, ultrafiltration and microfiltration), membranes commercially available, membrane element configurations, and how complete membrane systems are designed. The paper also discusses case histories of a variety of proven applications, as well as areas of high potential. System design, operating parameters and detailed benefits derived from use of membrane systems are also included.
Laboratory, pilot plant and full-scale unit process equipment is presented, as well as the steps required to evaluate feasibility and scale-up to process conditions. Economical evaluation of the process is presented in the case histories. New areas of high potential for payback on water reuse and/or recovery of valuable products, as an added benefit, are also described.
Although a relatively young technology, long term success has been achieved for water recovery in many areas, including process applications. Improvements in equipment and membrane are reducing the cost of these unit processes. Membrane technology will play a major role in all areas of water recovery in the near future.
CROSSFLOW MEMBRANE FlLTRATION
Membrane filtration is the separation of the components of a pressurized fluid performed by polymeric membranes. The openings in the membrane matrices (pores) are so small that significant fluid pressure is required to drive liquid through them; the pressure required varies depending on the size of the pores. Reverse osmosis (RO) membranes have the smallest pores, while microfiltration (MF) membranes have the largest pores, and hence, require the least pressure.
Normal particle filtration has historically not been run in a crossflow design, 'perpendicular flow' may be the most appropriate term, with the solution to be filtered approaching the filter media in a perpendicular direction. The entire influent stream passes through the filter media (except for the particulate matter filtered out). Filter media can be either of the 'depth' type or the 'surface' or 'barrier' type. In this perpendicular flow design, there are only two streams, the influent and the effluent. separation is effected in the micron range or greater; with certain depth filter media achieving as low as a nominal one micron separation.
Crossflow membrane filtration is fundamentally different in design, in that the influent stream is separated into two effluent streams, known as the permeate and concentrate. Permeate is that fraction which has passed through the 'semi-permeable' membrane. The concentrate is that stream which has been enriched in the solutes or suspended solids, which have not passed through the membrane. Membrane is a surface filtration media, which effects separation in the ionic and molecular range, as well as the macromolecular and particle range. The advantage of this design approach is that the membrane media is operated in a continuously self-cleaning mode, with solutes and solids swept away by the concentrate stream, which is running parallel to the membrane. Hence, the term 'crossflow', and the advantage of this approach for separation in the micron, sub-micron, molecular and ionic range.