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Overcoming Biofouling Problems: Prevention and Treatment in RO Systems

Biofouling is a deadly enemy for RO membranes. Learn about the causes, symptoms (such as extreme pressure drop), prevention strategies with biocides, and effective cleaning procedures to restore the performance of your Reverse Osmosis system.

In the operation of Reverse Osmosis (RO) systems, there is one threat that is the most difficult to overcome and most damaging to performance: Biofouling. Unlike scaling or colloidal fouling, biofouling is living organisms that grow and multiply, forming a sticky and tough slime layer along the water flow, especially on the surface of RO membranes.

This article will discuss in depth the ins and outs of biofouling, from causes and symptoms to comprehensive prevention and treatment strategies.

What is Biofouling and Why is it So Dangerous?

Biofouling is the accumulation and growth of microorganisms (such as bacteria, fungi, and algae) that attach to the surface of RO membranes. These microbes secrete extracellular polymeric substances (EPS) that form a sticky matrix, known as a biofilm.

Why is Biofilm So Problematic?

Biofilm is not just a collection of bacteria. It is a well-protected ‘city’ of microbes. The EPS matrix acts as a shield that:

  • Protects microbes from conventional biocide (disinfectant) attacks.

  • Traps colloidal particles, salts, and other impurities, accelerating fouling.

  • Creates an ideal environment for microbes to continue growing.

The impact on RO systems is significant, causing a decline in production quality, increased operating costs, and premature membrane damage.

Causes and Symptoms of Biofouling: Recognising the Enemy Early On

Main Causes:

  1. Water Source: Surface water (rivers, lakes) has a much higher potential for biofouling than groundwater.

  2. Inadequate Pre-Treatment: Pre-treatment systems (such as multimedia filters and activated carbon) that do not function optimally can become a ‘hotbed’ for microbial growth and actually supply nutrients to the RO system.

  3. Presence of Nutrients: Organic compounds, nitrates, and phosphates in raw water are ‘food’ for microbes to multiply.

  4. Warm Temperatures: Warm water temperatures (above 25°C) accelerate the metabolism and growth of microorganisms.

Visible Symptoms:

  1. Extreme Pressure Drop Increase: This is the most characteristic symptom. The differential pressure between the inlet and concentrate at a single RO stage will increase significantly due to flow being blocked by sticky biofilm.

  2. Decrease in Normalised Permeate Flow: The flow rate of clean water will decrease because the membrane is ‘clogged’ by biofilm.

  3. Increase in Normalised Salt Passage: Because biofilm interferes with flow and salt concentration on the membrane surface, salt rejection decreases, resulting in poorer product quality.

  4. Higher Feed Pressure Requirements: To maintain product flow rate, operating pressure must be continuously increased, which increases energy consumption.

  5. Appearance and Odour: In severe cases, pre-RO filter cartridges will appear slimy and develop an unpleasant odour within a very short time.

Prevention Strategy: Prevention is Better than Cure

Preventing biofilm formation is much easier and cheaper than destroying it once it has formed.

  1. Pre-Treatment Optimisation:

    • Ensure that multimedia and activated carbon filters are working optimally to remove suspended solids and organic matter (TOC).

    • Consider using Ultrafiltration (UF) as a final pre-treatment to physically filter out bacteria and colloidal particles.

  2. Regular Biocide Dosing:

    • Oxidative Biocide: Such as chlorine or ozone. This is very effective at the upstream of the system. IMPORTANT: Chlorine MUST be completely neutralised (with sodium bisulphite/SBS) before entering the RO membrane as composite film membranes are sensitive to oxidants.

    • Non-Oxidative Biocide: (Examples: DBNPA, Glutaraldehyde). These are powerful weapons for biofouling prevention. Non-oxidative biocides can be dosed periodically (e.g., 1-2 times per week) directly into the RO feed water. They work through specific mechanisms to kill microbes and are relatively safe for RO membranes.

  3. Routine Sanitation of the Pre-RO System:

    • Perform regular sanitation of the entire pre-treatment system, including tanks, filters, and pipes. Use a biocide solution (oxidative or non-oxidative) to kill microbes before they have a chance to form a strong biofilm on the RO membrane.

Conclusion: The Key to Success Against Biofouling is Consistency

Biofouling is an ongoing challenge in RO system operations. The key to controlling it lies in a proactive and consistent approach:

  • Monitor system performance closely.

  • Prevent it with optimal pre-treatment and a planned biocide dosing programme.

  • Clean immediately and aggressively using the appropriate cleaning protocol as soon as the first symptoms are detected.

By implementing this strategy, you can extend the life of your RO membranes, maintain operational efficiency, and ensure consistently high product water quality. Consult with a water treatment expert to design a biofouling prevention and cleaning programme specific to your system conditions.