Feed water for supply of seawater reverse osmosis plants in general can be divided into three main types:
- Low salinity brackish water with totally dissolved solids (TDS) of up to 5,000 mg/l (ppm)
- High salinity brackish water with TDS in the range of 5,000 to 15,000 mg/l (ppm)
- Seawater with TDS in the range of 25,000 to 40,000 mg/l (ppm)
The quality of the feed water is defined in terms of concentration of suspended particles and saturation levels
of the sparingly soluble salts. The common indicators of suspended particles used in the RO industry are
Turbidity and Silt Density Index (SDI).
The maximum limits are: Turbidity of 1 NTU and SDI of 5. Continuous operation of these values may result in
significant membrane fouling. For long-term reliable operation of the RO unit, the average values of turbidity
and SDI in the feed water should not exceed 0.5 NTU and 3 SDI units, respectively.
Depending on the raw water quality, the pretreatment process may consist of all or some of the following
treatment steps:
- Removal of large particles using a course strainer
- Water disinfection with chlorine
- Clarification with or without flocculation
- Clarification and hardness reduction using lime treatment
- Media filtration
- Reduction of alkalinity by pH adjustment
- Addition of scale inhibitor
- Reduction of free chlorine using sodium bisulphite or activated carbon filters
- Water sterilisation of suspended particles using cartridge filters