Reverse Osmosis — involves separating water from dissolved solids by forcing the water to pass through a semi-permeable membrane which retains most of the dissolved solids.
As illustrated in Fig. 18-11, this is accomplished by providing sufficient pressure on the system feedwater to overcome the normal osmotic pressure and produce a reasonable flow rate through the membrane. A typical brackish water with a dissolved solids content of 1500 ppmw will have an osmotic pressure of about 15 psi; seawater with a dissolved solids content of about 35000 ppmw has an osmotic pressure of about 350 psi. The applied pressure for brackish water purification is typically in the range of 400-600 psig and for seawater purification, in the range of 800-1000 psig. Recovery of product (desalted) water with reverse osmosis units ranges from 50 to 90% of the feedwater depending upon the feedwater composition, the product water quality requirement, and the number of stages utilized.
Operating costs consist mainly of pumping costs (the pressure drop across the membrane may be from 250 psi to 1000 psi, depending upon dissolved solids content and membrane selection) and membrane cleaning and replacement costs. For water containing from about 250 to 1500 ppmw dissolved solids, an economic comparison of ion exchange and reverse osmosis is frequently necessary to select the more cost effective process. Reverse osmosis has been successfully employed for desalination of seawater. In many cases, the reverse osmosis product water must be treated by one of the ion exchange processes if high quality boiler feed water is required.
A pretreatment system is needed to avoid fouling or excessive degradation of the membrane. Typically, pretreatment will include filtration to remove suspended particles and addition of chemicals to prevent scaling and biological growth. Because the optimum operating temperature for reverse osmosis systems is about 75-80°F, it is frequently desirable to heat the feedwater. This represents an additional operating cost; however, because reverse osmosis is a continuous process which does not require regenerant chemicals, the cost of disposing of the waste water from the reverse osmosis system may be less than that of waste water from an ion exchange unit.
Operating costs consist mainly of pumping costs (the pressure drop across the membrane may be from 250 psi to 1000 psi, depending upon dissolved solids content and membrane selection) and membrane cleaning and replacement costs. For water containing from about 250 to 1500 ppmw dissolved solids, an economic comparison of ion exchange and reverse osmosis is frequently necessary to select the more cost effective process. Reverse osmosis has been successfully employed for desalination of seawater. In many cases, the reverse osmosis product water must be treated by one of the ion exchange processes if high quality boiler feed water is required.
A pretreatment system is needed to avoid fouling or excessive degradation of the membrane. Typically, pretreatment will include filtration to remove suspended particles and addition of chemicals to prevent scaling and biological growth. Because the optimum operating temperature for reverse osmosis systems is about 75-80°F, it is frequently desirable to heat the feedwater. This represents an additional operating cost; however, because reverse osmosis is a continuous process which does not require regenerant chemicals, the cost of disposing of the waste water from the reverse osmosis system may be less than that of waste water from an ion exchange unit.
