Why Is Boron Removal from Seawater So Challenging?| Insights by AQUALITEK

Introduction

As seawater desalination becomes a critical solution for freshwater scarcity, boron removal has emerged as one of the most technically challenging aspects of reverse osmosis (RO) system design and operation. Unlike salts such as sodium chloride, boron behaves very differently in seawater and exhibits poor rejection by conventional RO membranes.

This article explains why boron is difficult to remove from seawater and provides a best-practice overview of the most effective boron removal technologies currently used in seawater desalination plants.

Why Is Boron Removal from Seawater So Difficult?

1. Boron Exists Mainly as Undissociated Boric Acid

In seawater (pH ≈ 8.0–8.2), boron is present primarily as boric acid (H₃BO₃) rather than as a charged ion.

•Boric acid is electrically neutral

•RO membranes reject ions more effectively than neutral molecules

•As a result, boric acid passes through standard RO membranes more easily

This fundamental chemical behavior is the core reason boron removal is difficult.

2. Small Molecular Size

Boric acid has a very small molecular size, close to the pore size of RO membranes. Even high-rejection seawater RO membranes struggle to block it completely, especially at normal operating pressures.

3. Stringent Boron Limits in Drinking Water

Many international standards impose strict boron limits:

•WHO guideline: ≤ 2.4 mg/L

•EU & some national standards: ≤ 1.0 mg/L

•Agricultural irrigation water: often ≤ 0.5 mg/L

Raw seawater typically contains 4–6 mg/L of boron, meaning standard single-pass RO is insufficient.

4. High Salinity Limits Operating Flexibility

Raising pressure or recovery excessively to improve boron rejection increases:

•Energy consumption

•Scaling risk

•Membrane fouling

•Equipment stress

This limits how aggressively boron can be removed in a single RO pass.

Best Treatment Methods for Boron Removal from Seawater

1. Two-Pass RO with Interstage pH Adjustment (Best Overall Solution)

How it works:

•First pass: Conventional seawater RO removes most salts

•pH is increased (typically to 9.0–10.5)

•Second pass RO removes boron more effectively

Why it works:

•Higher pH converts boric acid into borate ions (B(OH)₄⁻)

•Borate ions are charged and easily rejected by RO membranes

Advantages:

•Proven and widely adopted

•High reliability

•Suitable for large municipal plants

Disadvantages:

•Higher CAPEX and OPEX

•Requires chemical dosing and pH control

✅ Industry standard for potable desalination

2. High-pH Single-Pass RO (Limited Application)

How it works:

•Operating seawater RO at elevated pH (≈ 9.5–10)

Advantages:

•Simpler system design

•Lower capital cost

Limitations:

•Severe scaling risk

•Increased membrane fouling

•Limited boron removal efficiency

⚠️ Typically only used for non-potable or blended water applications

3. Boron-Selective Ion Exchange Resin (Polishing Step)

How it works:

•Uses boron-specific chelating resins

•Applied after RO as a polishing unit

Advantages:

•Very high boron removal efficiency

•Can achieve ultra-low boron levels

Disadvantages:

•Resin regeneration required

•Sensitive to water quality

•Higher operational complexity

✅ Best suited for small flows or high-purity water requirements

4. Membrane Blending Strategies

How it works:

•Mix low-boron permeate with higher-boron streams

•Achieves target concentration without full removal

Advantages:

•Reduced system cost

•Lower energy demand

Disadvantages:

•Limited flexibility

•Not suitable for strict drinking water standards

5. Emerging Technologies (Currently Limited Use)

•Specialized boron-rejecting membranes

•Forward osmosis hybrid systems

•Advanced electrochemical separation

⚠️ These are still not mainstream due to cost, reliability, or scale limitations.

Best Practice Recommendations

Conclusion

Boron removal from seawater is challenging due to its neutral chemical form, small molecular size, and strict regulatory limits. Conventional RO alone is insufficient to meet modern drinking water standards.

Among all available technologies, two-pass RO with interstage pH adjustment remains the most effective, reliable, and widely accepted solution for boron removal in seawater desalination. Selecting the right approach depends on water quality targets, system scale, and operational priorities.