OPERATORS MANUAL
ORION II
Orion II 500-10000 Operators
Page 88 of 95
Manual Part no: DOMAON0100 Issue A
April 2008
9 APPENDIX
9.1 REVERSE OSMOSIS
9.1.1 General
Osmosis is the process which takes place when a solution is separated from a less concentrated
solution by a semi-permeable membrane. The membrane will allow water molecules to pass
through, but not larger molecules, so that it can act effectively like a molecular filter. During osmosis
the water molecules in the more diluted solution will migrate into the more concentrated solution.
This migration can be stopped, and indeed reversed, by the application of a pressure gradient in
opposition to the natural direction of flow. Thus, water molecules can be forced to migrate from the
more concentrated solution to the less concentrated solution.
This process is known as reverse osmosis and it can be used either to concentrate solutions or to
purify water. This is achieved by separating the feed stream, into a concentrate stream containing
most of the dissolved material present in the feed, and a permeate stream which is purified water.
All Reverse Osmosis units must operate with a concentrate stream otherwise the permeate quality
and membrane lifetime will be adversely effected.
Most Reverse Osmosis units operating with potable feedwater convert approximately 75% of the
feed into permeate. Higher and lower conversion may be achievable or advisable depending on the
impurities present in the feedwater and the degree of pre-treatment.
9.1.2 Water Recovery
The 'recovery' of a Reverse Osmosis System is a measure of the proportion of the total input water
that is converted to purified water or permeate. A recovery of 10% means that only 1 part in ten of
the input water is converted to permeate. At 50% recovery, half of the input water is converted. At
75% recovery, three quarters of the input water is converted to permeate.
The recovery on the ORION range is adjustable, and its setting will affect the final water quality. At
high recovery rates, the amount of salts in the concentrate water as it exits the membrane will be
high which will result in a higher level of salts in the permeate.
A balance of the optimum water quality and volume is usually found at recovery rates around 75%.
Single membranes will normally 'recover' only 15-20% permeate from the feed water without being
adversely affected by a build up of contaminants at the membrane surface. However connecting a
number of membranes in series and returning a proportion of the concentrate exiting from the RO
module(s) to the inlet of the pump, the contaminants are removed from the membrane surface by the
increased flow of water across it. This in turn enables the unit to operate at higher recovery rates.
ORION units have two valves which allow the recovery rate to be adjusted, usually to approximately
75% permeate recovery, although lower recovery settings may be recommended on „problem‟ or
„variable‟ waters where extra fouling protection is needed.
9.1.3 Membrane Rejection
The rejection rate of an RO system is a measure of the amount of impurities dissolved in the feed
water that are 'rejected' by the membrane. Membrane rejection is therefore a performance indicator
for the efficiency of the membrane for removing impurities, and is usually measured by the
conductivity of the water, since conductivity is directly related to the total dissolved solids present.
