Advances in Water Treatment and Enviromental Management

WATER TREATMENT TECHNOLOGIES FOR THE NINETIES 161they could remove everything. They can operate without chemical addition to water,are reliable, compact and easy to automate. Their major disadvantages are stilltheir rather high capital and operating costs, the fact that they are prone to foulingwhich requires often a high level of pretreatment and regular chemical cleaning.They may also have a rather important reject stream whose disposal may createproblems.Membranes can be made in a wide variety of shapes: fibers, tubes, flat sheets in theform of spiral wound modules or plate and frames. They can have an almost unlimitedrange of porosity—or molecular weight cut-offs—and the development of syntheticorganic chemistry gives hopes of great improvement in the membrane composition.This will certainly result in lower operating pressures, decreased fouling, betterresistance to disinfectants, biodegradation etc… and altogether a much longer lastingperiod. The new composite membranes with sophisticated coatings have now littleto do with the original cellulose acetate membranes and the progress will likelyaccelerate in the coming few years.Several outside forces are pulling the membrane technology toward drinking waterapplications. The first and primary force is the development of the market.Desalination plants using reverse osmosis membranes are being built around theworld. In addition an important market is being developed in Florida where plantsfor a total capacity of roughly 200 MOD are already in operation and where atleast an additional 100 MOD will be built every year. Membranes in Florida arenot only used for desalination but more and more for organics removal (THMFP’s)together with softening. Looser membranes as opposed to conventional R.O.—inthe range of nanofiltration and ultrafiltration—are thus being installed and makethe treatment more and more cost effective as compared to conventional coagulationflocculation and lime precipitation. Total costs for a plant of 0,5 MOD withmembranes are in the range of 1.5 $71000 gallons as compared to more than 2.$/1000 gallons for lime softening. This difference decreases, however, when thesize of the plant increases. Membranes are now moving northward and westwardthroughout the United States, but Florida will remain indeed the nest for membraneproliferation in the drinking water sector. Altogether it is predicted that the totalmarket for membranes in water treatment will be in the range of $500 million inyear 1996.Another reason for the progresses of membrane technology is the development ofconsiderable international research programs such as the Aquarenaissance programin Japan and the Eureka program in Europe and several projects funded by theAWWA research foundation in the U.S. The “Eureka” program in particular, fuelsthe development of ultrafiltration and microfiltration membranes at Lyonnaise desEaux, which are capable of meeting the costs of conventional filtration technologieswith a whole load of technical advantages. Plants are already in operation and ifthese membranes can take advantage of the window of opportunity opened by theSurface Water Treatment Rule of the SDWA in the USA. They would bring then oneof the most important revolution in water treatment practices.An interesting feature of membranes is in their ability to constitute a reaction bycombining the separation with a reaction in the recirculation loop. A whole world ofmembrane reactors can thus be envisioned: absorption reactors with powderedactivated carbon in MF membranes, oxidation reactor, carbonate precipitation andiron precipitation reactors. The best hope being in the membrane/bioreactors whichwould significantly open the door to industrial applications of biotechnology towater treatment.