Much of membrane research in the past decade has centered on reverse osmosis (RO) and the control of scaling, fouling and troubleshooting in RO. The popular acceptance of RO has paved the way for other crossflow membrane technologies that hold great promise for commercialization. The increased use of microfiltration in industrial applications, however, presents new opportunities and challenges for chemical suppliers and end-users alike. Because of its versatility, robust nature, and small footprint, microfiltration could well replace, or at least augment, traditional equipment including clarifiers, media filters, lime softeners, ion exchange units, evaporators, and distillation units. Among the current uses of microfiltration are raw water clarification for municipal and industrial use, RO pretreatment, and general wastewater treatment, primarily for suspended solids removal. We have successfully combined novel polymeric precipitants with microfiltration for the treatment of metal-containing wastewater in the electronics and microelectronics industries. These applications highlight the potential for polymer-assisted microfiltration. Details of the chemical treatments and potential for future uses will be presented.
Over the past decade, crossflow membrane technology has become a common part of numerous processes. The best known crossflow technology is reverse osmosis (RO) which is commonly used to prepare water for boiler feed and processes requiring high purity water such as those in the pharmaceutical and microelectronics industry. It is also used for seawater desalination to produce potable water. RO separates feed streams into a concentrate and a permeate stream using the mechanisms of size exclusion and diffusion through the membrane. It is capable of removing ionic impurities from waste streams. Other types of membrane filtration include ultrafiltration (UF) and microfiltration (MF). UF and MF separate streams by a sieving mechanism to produce a concentrate and a permeate stream. UF and MF membranes are categorized by the type of membrane material and the molecular weight cutoff (MWCO), which indicates the size of retained particles. In conjunction with RO and other conventional treatments, these membrane technologies are used in the production of such diverse goods as concentrated fruit juice, cold pasteurized alcoholic beverages, dairy products, soy sauce and soft drinks. All crossflow membrane systems have certain features in common. For example, all cross flow systems have 3 liquid streams: the feed stream, the permeate stream (liquid and certain dissolved or suspended materials which pass through the pores of the membrane), and the concentrate stream. Depending on the use, the concentrate stream may either be recycled, sent to drain as waste, or become the final product of interest. Likewise, the permeate stream may either be further purified, used as is, or disposed of. For example, in the production of ultrapure water for pharmaceutical use, permeate from one RO unit may be sent to a second or even a third unit for further purification. The concentrate water in this case would be treated as waste. On the other hand, in a dairy plant, the concentrate from milk whey may be retained while the permeate stream is delegated for disposal.
POTENTIAL FOR WASTE APPLICATIONS
Pressure driven membrane processes are becoming cost competitive with conventional processes due to continued improvements in membrane materials and understanding of membrane operation.' A study by the Freedonia Group 2 analyzes the membrane markets and predicts significant growth over the next several years (see Table 1 ). According to the study, water and wastewater applications make up