REVERSE OSMOSIS ENERGY REQUIREMENTS WITH VARIATIONS IN INTAKE FEED SALINITY

PRESSURE REQUIRED

Energy is required to run the pumps that pressurize the incoming salty water and cause the fresh water component to pass through the very fine pores of the semi-permeable membranes. The pressure required is the sum of two components, that which is required to just equal the osmotic pressure (at which no flow would occur accross the membranes) and an additional pressure to cause a desireable flow rate through the membranes. The total pressure used is normally approximately two times the osmotic pressure.

The osmotic pressure is roughly proportional to the salt concentration (salinity) which varies from near ocean level concentrations, about 35,000 parts per million by weight (ppm) to as low as only a few thousand ppm at the upper reaches of San Pablo Bay when diluted by fresh water run-off from the Sacramento River during and following rainy seasons.

The osmotic pressure may can easily be determined using one of several on-line calculators. Click here for one of these. Simply scroll down to the data table and enter the salt concentration of your choice, or simply choose either Sea Water or Brackish Water to view results similar to the conditions that the MMWD will have during dry and wet seasons. The osmotic pressure is given in units of bar (barometric pressure). Simply multiply this by 14.7 psi per standard barometric pressure to obtain the osmotic pressure in pounds per square inch (psi).

ENERGY REQUIRED

The theoretical minimum energy required (which occurs when the upstream pressure is just equal to the osmotic pressure, and under which condition there is no flow across the membrane) is only about 2 Kilowatt-hours per 1000 gallon of pure water product. However, practical systems require a higher pressure to cause a reasonable flow rate through the membrane, requiring much more power per unit of product. The proposed MMWD system would require about 10 kWh/1000 gal during wet periods when the salt content of the feed water is lower and about 14 kWh/1000 gal during the dry season when the salt content of the feed water approaches that of ocean water. (Ref: MMWD Environmental Impact Report, Chapter 5.)

See Bay Salinity for a discussion of the varying salinity of the San Francisco Bay.