Watts Pure Water PWR4021 Series, Installation, Operation And Maintenance Manual

4

C. Pretreatment

The RO feed water must be pretreated in order to prevent mem-

brane 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 antiscalants 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 antiscalants, coagulant, 

or bisulfite into the feed water or to adjust the feed water pH.

Prefilter Cartridge -

 Used to remove smaller suspended solids and 

trap any particles that may be generated by the other pretreatment. 

The cartridge(s) should be replaced when the pressure drop across 

the housing increases 5 - 10 psig over the clean cartridge pressure 

drop. The effect of suspended solids is measured by the silt density 

index (SDI) test. An SDI of five (5) or less is specified by most mem-

brane manufacturers and three (3) or less is recommended.

Iron & Manganese -

 Iron should be removed to less than 0.1 ppm. 

Manganese should be removed to less than 0.05 ppm. Special me-

dia filters and/or chemical treatment is commonly used.

pH -

 The pH is often lowered to reduce the scaling potential.

Silica:

 Reported on the analysis as SiO2. Silica forms a coating on 

membrane surfaces when the concentration exceeds its solubility. 

Additionally, the solubility is highly pH and temperature dependent. 

Silica fouling can be prevented with chemical injection and/or reduc-

ing the recovery.