Why Can’t Seawater RO Permeate Be Used Directly as Drinking Water?| Insights by AQUALITEK

Introduction

Seawater reverse osmosis (SWRO) is one of the most advanced and reliable technologies for converting seawater into freshwater.
From a desalination perspective, the produced permeate often meets — or even exceeds — purity standards.

However, pure water produced by seawater RO systems is generally not suitable for direct human consumption.

This is not a limitation of RO technology, but rather a result of human health requirements, water chemistry balance, and regulatory standards.

1. Seawater RO Permeate Is Too Low in Minerals

1.1 Near-Zero Mineral Content

SWRO membranes remove:

•Sodium and chloride

•Calcium and magnesium

•Potassium and bicarbonate

•Trace minerals and salts

•Typical characteristics of SWRO permeate:

•TDS: < 20–50 mg/L

•Calcium hardness: ~0

•Alkalinity: extremely low

While this level of purity is excellent for industrial use, it is not ideal for drinking water.

1.2 Minerals Are Essential for Human Health

Drinking water is an important source of:

•Calcium (bone health)

•Magnesium (cardiovascular function)

•Potassium and trace elements

Long-term consumption of demineralized water may:

•Reduce mineral intake

•Disturb electrolyte balance

•Increase the risk of mineral deficiencies

This is why most international drinking water guidelines recommend minimum mineral content.

2. RO Permeate Has Unstable and Low pH

2.1 Natural Acidity of RO Water

Seawater RO permeate typically:

•Has very low alkalinity

•Absorbs CO₂ easily from air

•Exhibits a pH between 5.0 and 6.5

This slightly acidic nature causes several problems.

2.2 Corrosion Risk in Distribution Systems

Low-mineral, low-alkalinity water is chemically aggressive and may:

•Corrode pipelines and storage tanks

•Leach metals such as iron, copper, or lead

•Cause taste and color issues

Even if the RO water itself is clean, secondary contamination can occur after treatment.

3. Poor Taste and Mouthfeel

3.1 Flat and Unpleasant Taste

Taste perception is strongly influenced by dissolved minerals.

RO permeate often tastes:

•Flat

•Bland

•Slightly acidic

Consumers commonly describe it as:

“Empty” or “unnatural”

3.2 Consumer Acceptance Matters

For municipal or commercial desalination projects:

•User acceptance is critical

•Taste complaints can undermine public trust

•Remineralization significantly improves palatability

4. Drinking Water Regulations Do Not Accept Pure RO Water

4.1 Minimum Water Quality Standards

Many international standards require:

•Minimum hardness

•Minimum alkalinity

•Stable pH range (usually 6.5–8.5)

Examples include:

•WHO drinking water guidelines

•EU Drinking Water Directive

•National standards in the Middle East and Asia

Pure RO water does not meet these requirements without post-treatment.

5. Remineralization: A Mandatory Post-Treatment Step

5.1 Common Remineralization Methods

To make seawater RO permeate suitable for drinking, systems typically include:

•Limestone (calcite) contactors

•Dolomite filters

•Lime or calcium hydroxide dosing

•CO₂ injection for pH control

•Blending with treated surface or groundwater

5.2 Objectives of Remineralization

Post-treatment aims to:

•Restore calcium and magnesium

•Increase alkalinity and buffering capacity

•Stabilize pH

•Improve taste

•Protect downstream infrastructure

6. Why “Pure” Is Not Always “Better” for Drinking Water

In industrial applications, ultra-pure water is ideal.
For human consumption, however, balanced water chemistry is far more important than absolute purity.

Drinking water must be:

•Chemically stable

•Nutritionally appropriate

•Pleasant to drink

•Safe throughout the entire distribution network

Seawater RO permeate alone does not satisfy these conditions.

Conclusion

Although seawater RO systems produce extremely pure water, this water cannot be used directly as drinking water due to:

•Lack of essential minerals

•Low and unstable pH

•Corrosive behavior

•Poor taste

•Non-compliance with drinking water standards

Therefore, remineralization and pH adjustment are indispensable steps in all seawater desalination plants designed for potable water supply.

The goal of desalination is not just salt removal, but producing healthy, stable, and acceptable drinking water.