Best Guide: How to Determine if an RO System Is Contaminated with Biological Substances—and How to Handle It| Insights by AQUALITEK

Introduction

RO membranes are highly sensitive to biological pollutants. When bacteria, algae, fungi, or biofilms enter the system—especially through pretreatment failures or warm stagnant water—they multiply rapidly and attach to membrane surfaces.

If not properly controlled, biological contamination can cause severe fouling, shorten membrane lifespan, and increase operating costs. Early detection and timely treatment are essential.

1. How to Determine if an RO System Has Biological Contamination

Biological fouling has some clear and measurable indicators. The following are the most reliable signs operators should monitor daily.

1.1 Rapid Increase in Differential Pressure (ΔP)

This is the most direct and common sign.

•ΔP across the membrane increases gradually or suddenly

•Pressure rise occurs mainly at the front stage

•System requires more pressure to maintain the same flow

Reason: Biofilm accumulates on the feed side, restricting flow.

1.2 Decrease in Permeate Flow, but Conductivity Remains Normal

Unlike scaling or chemical fouling, biological fouling:

•Reduces permeate output

•Does not significantly affect salt rejection in early stages

This is an important diagnostic difference.

1.3 Slippery or Gel-Like Material Found During Disassembly

When a membrane is removed, biological contamination is obvious:

•Slimy, gelatinous, sticky layers

•Typically transparent, white, yellow, or brown

•Sometimes foul odor

This confirms biofouling.

1.4 Sudden Fouling After Long Idle Time

Systems left idle with stagnant water—especially warm conditions—are prone to bacterial growth.

If ΔP rises quickly after startup, biological growth is highly likely.

1.5 High Bacterial Count in Feed Water

If pretreatment fails, bacteria easily enter the RO system.

Signs include:

•Increase in total bacteria count

•Poor performance of multimedia filter or activated carbon filter

•Malfunctioning dosing system (e.g., sodium bisulfite)

1.6 Unusual Odor in Concentrate or Permeate

Biological fouling may produce:

•Musty smell

•Rotting odor

•Earthy or fishy smells

This is often detected even before performance drops.

2. What Causes Biological Contamination in RO Systems?

Common reasons include:

•Activated carbon filters without regular sterilization

•Failure of chlorine residual control

•SBS overdosing (removes chlorine too early)

•Long-term stagnation of water

•Warm operating environments (>25°C)

•Poor CIP disinfection practice

•Incorrect preservation of spare membranes

Understanding the root cause prevents recurrence.

3. How to Handle Biological Contamination Effectively

Once biological fouling is suspected, prompt action is needed. The following steps provide a reliable and industry-standard approach.

3.1 Perform a Biological CIP (Chemical Cleaning)

Use a disinfectant-compatible cleaning solution such as:

•Alkaline detergent with surfactants (for organic + biofilm removal)

•Chlorine-free biocidal cleaner (compatible with polyamide RO membranes)

•Hydrogen peroxide (only in pretreatment, not RO membranes)

•Non-oxidizing biocides

Cleaning procedure:

1.Rinse RO system

2.Circulate cleaning solution for 30–60 minutes

3.Soak for 2–6 hours

4.Repeat circulation

5.Rinse thoroughly with RO permeate or softened water

Avoid oxidizing agents like chlorine at the RO stage—they permanently damage membranes.

3.2 Disinfect the Pretreatment System

Biofouling usually originates upstream.

Key steps:

•Sterilize multimedia and carbon filters

•Replace activated carbon if biofilm is severe

•Clean cartridge filter housings

•Flush pipelines with disinfectant

•Inspect SBS and chemical dosing systems

3.3 Increase Flushing Frequency

Frequent flushing reduces biofilm adhesion, especially in:

•Warm seasons

•Low-flow periods

•Systems with intermittent operation

3.4 Optimize Chlorine Control

Critical rules:

•Pretreatment requires stable residual chlorine

•RO membrane must never be exposed to chlorine

•Sodium bisulfite dosing must be accurate

•Online ORP monitoring is recommended

3.5 Lower Feed Water Temperature if Practical

Biological growth increases dramatically above 25°C.

3.6 Plan a Preventive Biocide Dosing Program

Industrial RO systems often dose:

•Isothiazolinone

•DBNPA

•Glutaraldehyde (upstream only)

Dosing is periodic—e.g., once weekly or biweekly.

3.7 Membrane Preservation for Long-Term Shutdown

If a system is idle for >24–48 hours:

•Apply a membrane storage solution (biocide-based)

•Avoid leaving the membrane wet but untreated

This prevents reinfection.

4. What Happens If Biological Contamination Is Not Addressed?

Consequences include:

•Rapid membrane fouling

•Permanent damage to membrane surface

•Higher operating pressure (energy cost rises)

•Membrane blockage leading to element replacement

•Poor permeate quality

•Shorter membrane lifespan

•Unscheduled downtime

In severe cases, entire RO trains must be cleaned or replaced.

Conclusion

Biological contamination is one of the fastest-growing threats to RO performance, but it is highly manageable with early detection, proper disinfection, and a strong preventive strategy. By monitoring ΔP, flow rate, and visual signs, operators can respond quickly and avoid severe operational losses.