Why Does Desalinated Seawater Require Remineralization or pH Adjustment Before Drinking?| Insights by AQUALITEK

Introduction

Seawater desalination plants—especially those based on reverse osmosis (RO)—are capable of producing ultra-pure water by removing more than 99.5% of dissolved salts and impurities.

While this level of purification is ideal for protecting membranes and downstream equipment, it creates a new challenge:

The produced water is often chemically unstable and unsuitable for direct human consumption.

As a result, remineralization and pH adjustment are almost always required before desalinated water can be supplied as drinking water.

This article explains why desalinated water must be conditioned, what risks untreated product water presents, and how mineralization improves safety, taste, and system compatibility.

1. Desalinated Water Is “Too Pure”

1.1 Extremely Low Mineral Content

RO desalination removes:

•Calcium (Ca²⁺)

•Magnesium (Mg²⁺)

•Bicarbonate (HCO₃⁻)

•Trace elements

Typical desalinated water characteristics:

•TDS: < 50 mg/L

•Calcium hardness: near zero

•Alkalinity: almost zero

While this meets purification goals, it creates water that lacks buffering capacity and essential minerals.

2. Low pH and Poor Chemical Stability

2.1 CO₂ Absorption Lowers pH

Desalinated water contains:

•Very little alkalinity

•No natural buffering system

As a result, dissolved carbon dioxide easily lowers pH:

•Typical pH after RO: 5.5–6.5

•Drinking water standards usually require: 6.5–8.5

Low pH water is:

•Corrosive to pipelines and storage tanks

•Aggressive toward metals and concrete

•Unstable during distribution

3. Corrosion Risk in Distribution Systems

3.1 Aggressive Water Attacks Infrastructure

Without mineral stabilization, desalinated water can:

•Leach metals (iron, copper, lead)

•Corrode steel and ductile iron pipes

•Damage concrete reservoirs and linings

This leads to:

•Water quality deterioration

•Increased maintenance costs

•Regulatory non-compliance

Remineralization restores Langelier Saturation Index (LSI) to a safe range, protecting infrastructure.

4. Health and Nutritional Considerations

4.1 Essential Minerals Are Missing

Calcium and magnesium in drinking water contribute to:

•Bone health

•Cardiovascular function

•Electrolyte balance

While food is the primary mineral source, long-term consumption of demineralized water may:

•Reduce dietary mineral intake

•Affect taste acceptance

•Fail to meet public health recommendations

Many national standards now require minimum hardness or mineral content in potable water.

5. Taste and Consumer Acceptance

Desalinated water without conditioning often:

•Tastes “flat” or “empty”

•Feels unnatural compared to natural water sources

Small amounts of minerals:

•Improve mouthfeel

•Enhance palatability

•Increase consumer confidence

Taste quality is a critical factor for public acceptance of desalination projects.

6. Common Remineralization and pH Adjustment Methods

6.1 Limestone (Calcite) Contactors

•Dissolve calcium carbonate

•Raise pH and alkalinity

•Add calcium hardness

6.2 Lime or Caustic Dosing

•Precise pH control

•Fast response

•Often combined with CO₂ dosing

6.3 Blending with Treated Raw Water

•Mixes RO permeate with mineralized water

•Improves stability and taste

•Requires careful quality control

6.4 Magnesium Addition (Optional)

•Enhances health profile

•Improves water balance

•Used in many modern SWRO plants

7. Regulatory and Standard Compliance

Most drinking water standards specify:

•Minimum pH limits

•Corrosion control requirements

•Stability indices

•Recommended mineral ranges

Without post-treatment:

•Desalinated water often fails compliance

•Distribution system risks increase

•Public health concerns arise

Thus, mineralization is a regulatory necessity, not a luxury.

Conclusion

Although seawater desalination produces exceptionally clean water, that purity comes with chemical instability.

Remineralization and pH adjustment are essential to:

•Protect distribution infrastructure

•Ensure regulatory compliance

•Improve taste and acceptance

•Support long-term public health

In potable desalination projects, post-treatment is just as important as membrane performance.