Watts R4X40-3 - Manual (Page 6)

B. RO OVERVIEW
Reverse osmosis systems utilize semipermeable membrane elements to separate the feed
water into two streams. The pressurized feed water is separated into purified (product) water
and concentrate (reject) water. The impurities contained in the feed water are carried to drain
by the reject water. It is critical to maintain adequate reject flow in order to prevent
membrane scaling and/or fouling.
RO Membrane
Feed Water Product Water
Reject Water
C. PRETREATMENT
The RO feed water must be pretreated in order to prevent membrane damage and/or fouling.
Proper pretreatment is essential for reliable operation of any RO system.
Pretreatment requirements vary depending on the nature of the feed water. Pretreatment
equipment is sold separately. The most common forms of pretreatment are described below.
Media Filter - Used to remove large suspended solids (sediment) from the feed water.
Backwashing the media removes the trapped particles. Backwash can be initiated by time or
differential pressure.
Water Softener - Used to remove calcium and magnesium from the feed water in order to prevent
hardness scaling. The potential for hardness scaling is predicted by the Langelier Saturation
Index (LSI). The LSI should be zero or negative throughout the unit unless approved anti-
scalents are used. Softening is the preferred method of controlling hardness scale.
Carbon Filter - Used to remove chlorine and organics from the feed water. Free chlorine will
cause rapid irreversible damage to the membranes.
The residual free chlorine present in most municipal water supplies will damage the thin film
composite structure of the membranes used in this unit. Carbon filtration or sodium bisulfite
injection should be used to completely remove the free chlorine residual.
Chemical Injection - Typically used to feed antiscalant, coagulant, or bisulfite into the feed water
or to adjust the feed water pH.