Membrane softening : the concept and its application to municipal water supply

Abstract In 1976, a Florida-based equipment manufacturer conceived the idea of using modified reverse osmosis membrane instead of lime softening to treat waters with low total dissolved solids. The growth of the application was slow at first, but in 1989, this blossomed to over 125 mgd of facilities planned, designed or being built in Florida. Membrane softening (MS) is a desalination process since it removes dissolved inorganics as well as organics from the feed water and the product water complies with present SDWA regulations. MS is particularly cost effective in treating ground water in Florida which is high in hardness and natural organics such as humic and fulvic acids. The MS process is similar to conventional reverse osmosis (RO) except three stages are employed for 90% recovery, operation is at 90 to 120 psi with a booster pump on the third stage; and pretreatment is greatly simplified. Capital costs for MS facilities are less than $0.90 per USgpd capacity (over 30 USmgd capacity) and O&M costs are in the order of $0.40 per 1,000 US gallons. Utilizing blending of waters treated by MS and other techniques further enhances the application of MS. Desalination by MS of waters below 1,000 ppm TDS extends the economical treatability range of desalination for drinking water supplies. It is a technology whose time has come.