How to Effectively Prevent Microorganism and Algae Proliferation in Seawater Desalination Systems?| Insights by AQUALITEK

Introduction

Seawater is a highly biologically active medium.
It naturally contains:

•Bacteria

•Algae

•Plankton

•Biofilm-forming microorganisms

In seawater reverse osmosis (SWRO) systems, biological proliferation is one of the most destructive and difficult-to-control problems. Once biofouling establishes itself, it can lead to:

•Rapid pressure drop increase

•Irreversible membrane fouling

•Frequent chemical cleaning

•Premature membrane failure

The key principle is simple:

You cannot sterilize seawater, but you can effectively prevent uncontrolled biological growth.

This article explains practical, proven strategies to prevent microorganism and algae proliferation throughout a seawater RO system.

1. Understand the Root Cause of Biological Proliferation

Microorganisms and algae proliferate when three conditions coexist:

1.Living organisms (always present in seawater)

2.Nutrients (organic matter, ammonia, phosphates)

3.Favorable conditions (low flow, warm temperature, stagnant zones)

Effective prevention focuses on removing nutrients and eliminating favorable growth environments, rather than trying to kill everything.

2. Proper Seawater Intake Design (First Line of Defense)

Why Intake Design Matters

Many biological problems originate before pretreatment even begins.

Best Practices

•Locate intake away from:

Harbors and wastewater outlets

Shallow coastal zones with algae blooms

•Use deep-water or offshore intakes where possible

•Install coarse screens to block large organisms

A well-designed intake significantly reduces:

•Algae load

•Plankton intrusion

•Seasonal bloom impacts

3. Continuous or Intermittent Oxidant Dosing (Upstream Only)

Purpose

Controlled oxidant dosing is used to:

•Suppress biological activity in intake pipelines

•Prevent biofilm formation before filtration

Common Oxidants

•Sodium hypochlorite

•Chlorine gas

•Ozone (limited cases)

Critical Rule

All oxidants must be completely removed before RO membranes.

Control Measures

•Dechlorination using sodium metabisulfite (SMBS)

•ORP and residual chlorine monitoring

Oxidants are effective only when used upstream and carefully controlled.

4. Optimize Coagulation and Flocculation to Remove Biomass

Why This Is Crucial

Many microorganisms and algae are:

•Too large to pass membranes

•Too small to settle naturally

Optimized coagulation:

•Agglomerates algae and bacteria

•Removes them physically instead of killing them

Key Factors

•Correct coagulant selection

•Proper dosing rate

•Adequate flocculation time

This step dramatically reduces biological load entering filtration units.

5. High-Performance Filtration (Media Filter or UF)

Filtration as a Physical Barrier

Filtration removes:

•Algae cells

•Bacterial clusters

•Organic debris

Recommended Targets

•Turbidity ≤ 0.5 NTU

•SDI₁₅ ≤ 3–5

Ultrafiltration Advantage

UF systems provide:

•Consistent biological removal

•Strong resistance to seasonal algae blooms

•More stable RO operation

6. Avoid Nutrient Accumulation in the System

Even trace nutrients can support biofilm growth.

Key Nutrients to Control

•Dissolved organic carbon (DOC)

•Ammonia

•Phosphates

Control Strategies

•Effective organic removal in pretreatment

•Avoid chemical overdosing

•Prevent coagulant and polymer carryover

7. Eliminate Dead Zones and Stagnant Flow

Why Hydraulics Matter

Microorganisms thrive in:

•Low-flow zones

•Dead ends

•Idle pipelines

Design & Operation Measures

•Avoid oversized pipelines

•Minimize dead legs

•Maintain minimum flushing velocities

•Flush idle units regularly

Good hydraulics are one of the most underrated biofouling control measures.

8. Controlled Use of Non-Oxidizing Biocides (When Necessary)

When to Use

•Severe biological fouling risk

•Warm seawater environments

•Persistent biofouling history

Key Principles

•Use intermittently, not continuously

•Rotate biocides to avoid resistance

•Ensure compatibility with membranes

Biocides are a supportive tool, not a primary solution.

9. Proper Shutdown and Standby Protection

During shutdowns:

•Biological growth accelerates rapidly

Best Practices

•Flush system with clean, low-nutrient water

•Avoid long stagnant periods

•Apply preservation chemicals for long shutdowns

Many biofouling incidents originate during system downtime.

Conclusion

Preventing microorganism and algae proliferation in seawater RO systems requires a system-wide, preventive strategy, not a single treatment step.

Effective control relies on:

•Smart intake design

•Physical removal over chemical killing

•Nutrient reduction

•Hydraulic optimization

•Careful chemical management

When done correctly, these measures:

•Greatly reduce biofouling risk

•Extend membrane life

•Stabilize RO performance

•Lower operating costs

In SWRO systems, biofouling prevention is always more effective—and cheaper—than biofouling removal.