Reverse osmosis is a water purification process in which water is forced by pressure through a semi-permeable membrane. In normal osmosis, water flows from a less concentrated solution through a semi-permeable membrane to a more concentrated solution (see Figure 1). Reverse osmosis uses pressure to reverse normal osmotic flow. Water flows from a more concentrated solution through a semi-permeable membrane to a less concentrated solution (see Figure 2).

Figure 1: Osmosis

Figure 2: Reverse Osmosis

The feedwater to the reverse osmosis system flows over the surface of the membrane and a percentage of the water is forced through by pressure. This is purified water or permeate. The remaining water, concentrate, retains the rejected contaminants and is drained (see Figure 3).

Figure 3: Permeate and Concentrate Flows

The percentage of feedwater that is recovered as permeate, called percent recovery, is typically 50 percent. The other 50 percent is drained.

Two Membrane materials are available:

• Cellulose Acetate (CA or CTA) Membrane - CA/CTA membranes can tolerate chlorine.  Dissolved chlorine will pass through the CA membrane into the purified water.  Systems using CA membranes include pretreatment to chlorinate feedwater to 0.3 - 2.0 ppm.
• Polyamide (PA) Membrane - PA membranes have slightly higher ionic rejection and permeate flow rates than CA membranes.  They have zero chlorine tolerance, so chlorine must be removed from RO feedwater when using PA membranes.

CA membranes are standard.  For animal drinking water, CA membranes are preferred over PA despite lower ion rejections.  They pass dissolved chlorine which tends to keep permeated water bactericidal.  Very high chemical rejections are unnecessary in most laboratory animal applications, but organism content is of paramount concern.

Spiral Wound Membrane Module

Edstrom Industries uses a spiral would membrane module which is, basically, a membrane envelope sealed to a center tube and then rolled up like a scroll.  Spiral wound modules have several advantages over other designs.  They have a very high membrane area per unit volume, and the flat sheet can't break and cause permeate contamination.  Spacers between membrane layers promote turbulent flow to ensure low fouling and longer life (Figure 4 below).

Figure 4: Spiral Wound Membrane Module

Reverse osmosis removes 95-99% of most contaminants including microorganisms, organic compounds, dissolved inorganic compounds, microbial by-products such as endotoxins and pyrogens, and many carcinogenic compounds as shown in the table below. Performance is given as percent rejection or the percentage of the contaminants removed from a given water supply.

Reverse osmosis removes some contaminants more effectively than others. Contact Edstrom Industries, Inc. for more information about the removal of a particular impurity.

The Filtration Spectrum - This chart shows the relationship of contaminant size to filtration process. Reverse osmosis removes materials in the ionic size range and larger. (Chart courtesy of Osmonics, Inc.)