SECTION 2-4
EQUIPMENT DESCRIPTION
FLOWMAX RO MANUAL REV: B 05/1999
USFilter
United States Filter Corporation
water through the semi-permeable membrane in the opposite direction, leaving behind the
dissolved ions and the suspended solids. In the Reverse Osmosis process, the water that passes
through the membrane is commonly referred to as permeate, or product, water. The water that
remains behind the membrane along with the dissolved and suspended solids is referred to as the
reject, brine, or reject water.
The table below gives some approximate osmotic pressure values as an example of how much
pressure must be applied to the solution with a greater ionic concentration before any water will
pass through a membrane into a solution containing a lesser ionic concentration.
SODIUM CHLORIDE CONC.
APPROX. OSMOTIC PRESSURE
100 PPM
1 PSIG
1,000 PPM
11 PSIG
10,000 PPM (brackish water)
110 PSIG
35,000 PPM (sea water)
350 PSIG
It is impractical to convert all of the water processed to product water for several reasons
including the osmotic pressure which develops as a result of the concentration of the dissolved
ions which accumulate on the one side of the membrane, and the inability to keep the membrane
free from suspended solids that would foul the surface of the membrane if not removed.
The rate of product water passage (or productivity) through the membrane is referred to as the
flux rate and is generally expressed in "gallons per square foot of membrane surface per day" or
in "gallons per day per reverse osmosis cartridge". The flux rate of a particular membrane is
generally limited by several factors including: temperature, operating pressure, and the surface
flushing action to keep the membrane surface free of suspended solids.
The rate of recovery of feed water converted into product water is generally expressed in the
form of a percentage, with the ratio of product water being expressed as the recovery rate. For
example if the feed flow rate to the RO unit is 200 GPM and the product rate from the RO unit is
150 GPM the rate of recovery would be expressed as 75%. The remaining 50 GPM did not pass
through the membrane. It is referred to as the reject water and would be discarded (i.e., not
recovered).
Membrane rejection is an expression of the ability to restrict the passage of dissolved ions
through the membrane, and is generally expressed as a percentage. That is, if the feed water to
the membrane contains 100 PPM of dissolved solids, and the resulting product water contains
only 2 PPM of dissolved solids after processing, the resultant dissolved ions rejection rate is
98%. The opposite of this expression is also sometimes used and is known as the salt passage
rate, which, for our example, would equal a rate of 2%. A membrane rejection rate of 100% is
not practical due to imperfections in the membrane and the construction of the membrane
element.
Practical Applications
Reverse osmosis becomes practical for water treatment when synthetic, semi-permeable
membrane material is packaged in a suitable membrane element. Typically the elements are
