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Desalination Post-treatment: Neutralization / Remineralization for Drinking water

Why should desalinated water be remineralized?

Reverse Omosis is not a selective ion removal process. After the common 2-pass RO process for Boron Removal, delisalinated water is poor in minerals.

Low mineralized water have few adverse effects:

- High corrosion potential
- Dietary deficiency causing risks of ischaemic heart and cerebrovascular disease (WHO, 2005a; WHO, 2006; Cotruvo, 2006)

The WHO recommends 10 mg/L of Magnesium and 30 mg/L of Calcium for drinking water.

Double-pass SWRO system designed for enhanced Boron removal for dinrking water have a very low mineralization, about 5 to 10 mg/L of Na and 10 to 20 mg/L of Cl. Other ions are below 0.5 mg/L.

Four solutions are widely used to remineralize desalinated water:

Remin. Process	Description	Minerals
1	Blend with 1% clarified seawater + pH neutralization	15 mg/L Mg + 5 mg/L Ca + 125 mg/L Na + 220 mg/L Cl + 25mg/L SO4 pH 7-7.5
2	CO₂ addition + Calcite Limestone (CaCO₃, MgO) percolation + Na₂CO₃	80 mg/L CaCO3 pH 7-7.5
3	CO₂ addition + Dolomite Limestone (CaCO₃, MgCO₃) percolation + Na₂CO₃	80 mg/L CaCO3 + MgCO3 pH 7-7.5
4	Addition of CaCl₂ + NaHCO₃	100 mg/L CaCO3 100 mg/L Na + 50 mg/L Cl pH 7-7.5

Water Quality for drinking and Irrigation water:

Remin. Process	TDS after 2-pass SWRO (Boron Removal Process for Drinking Water)	EC after SWRO +IX (Boron Removal Process for Irrigation water)	SAR after SWRO +IX (Boron removal Process for Irrigation water)
1	350-400 mg/L	0.8-0.9	8
2	50-100 mg/L	0.3-0.4	2.5
3	50-100 mg/L	0.3-0.4	2.5
4	250-300 mg/L	0.6-0.8	6.5

Processes Comparison:

Process	Investment	Operation	Water quality	Ease of operation
1	Very Low	Low	Medium water quality - high sodium chloride content	Easy
2	High	High	Good water quality (very small sodium increase)	Easy
3	High	High	Very Good water quality (very small sodium increase, more magnesium)	Easy
4	Very Low	Low	Medium water quality - high sodium chloride content	Time consuming (chemical dissolution)

Desalination Key Issues

Seawater Intake

Desalination Pretreatments

Desalination Post-treatments

Desalination Polishing steps

Reverse Osmosis Desalination Process

Desalination and Energy Costs

Brine Disposal

Membranes cleaning (CIP)

Storage and Distribution

Containerization

Instrumentation and Control (SCADA)