Overdosing Reducing Agents in RO Systems: Biofouling Risks & Solutions | STARK

Reverse osmosis (RO) systems rely on chemical dosing to protect membranes from damage, especially from residual chlorine. One of the most commonly used additives is a reducing agent, such as sodium bisulfite (SBS), which neutralizes chlorine before it can degrade polyamide membranes.

However, in many RO operations, reducing agents are overdosed due to manual estimations, poor calibration, or operator caution. While the intention is to safeguard the membrane, this practice can inadvertently create ideal conditions for biofouling and microbial growth inside the membrane elements.

This article explores how excessive use of reducing agents contributes to anaerobic conditions, promotes microbial contamination—especially by sulfate-reducing bacteria (SRB)—and ultimately affects RO performance. We'll also discuss strategies to optimize dosing and prevent long-term system degradation.

Why Reducing Agents Are Used in RO Systems

Chlorine is commonly used in municipal and industrial water sources for disinfection. However, polyamide-based RO membranes are highly sensitive to chlorine damage. Even low concentrations (as little as 0.1 ppm) can cause irreversible membrane degradation, reducing salt rejection performance and system lifespan.

To address this risk, reducing agents such as sodium bisulfite (NaHSO₃), sodium metabisulfite, or sodium thiosulfate are dosed into the feedwater prior to the membrane. These chemicals neutralize residual free chlorine through redox reactions, ensuring the protection of membrane surfaces.

The dosage is typically calculated based on the measured free chlorine concentration, with a recommended stoichiometric ratio of 1.5–2.0 mg/L of sodium bisulfite per 1.0 mg/L of chlorine. However, many systems rely on fixed or excessive dosing as a safety margin—a practice that may lead to unintended consequences when not properly monitored.

The Hidden Risk: Overdosing and Its Real Impact

While reducing agents serve a protective role, excessive dosing—especially without chlorine present—can create unintended side effects. One of the most overlooked risks is the development of anaerobic conditions inside the RO system's piping and membrane elements.

When excess sodium bisulfite is introduced into water that no longer contains chlorine, it consumes the dissolved oxygen through residual redox activity. This deoxygenated environment becomes an ideal breeding ground for anaerobic microorganisms, especially sulfate-reducing bacteria (SRB) and other biofilm-forming species.

Over time, these microbes colonize the inner surface of membrane elements, forming slime layers and increasing differential pressure (ΔP) across membrane vessels. In severe cases, this biofouling leads to taste and odor problems in the permeate water, reduced flow rates, and even irreversible membrane damage.

Ironically, the very chemical intended to protect the membrane may be accelerating its decline—if improperly dosed and left unchecked.

Anaerobic Conditions and SRB Growth

Once excess reducing agents deplete the dissolved oxygen in feedwater, the system environment becomes increasingly anaerobic. This shift is particularly problematic in sections of the RO system where water stagnates or flow is intermittent, such as pre-treatment tanks, membrane housings, or dead zones in piping.

In these low-oxygen areas, sulfate-reducing bacteria (SRB) find ideal conditions to proliferate. These microbes use sulfate (SO₄²⁻) as an electron acceptor, producing hydrogen sulfide (H₂S) as a metabolic byproduct. The results are both chemical and operational:

• Formation of dark slime layers on membrane surfaces and feed spacers
• Hydrogen sulfide gas generation, leading to rotten egg odors and corrosion risks
• Accelerated membrane fouling and decline in permeate quality

SRB-related contamination is particularly insidious because it can persist unnoticed for weeks or months before causing sudden system performance collapse. By the time differential pressure or flow loss is detected, significant membrane damage may have already occurred.

Symptoms and Detection of Biofouling

One of the earliest signs of biofouling caused by overdosing of reducing agents is an abnormal increase in differential pressure (ΔP) across the membrane elements. As biofilm accumulates on membrane surfaces and feed spacers, water flow resistance rises—forcing the high-pressure pump to work harder.

Common Indicators of Biofouling

• Gradual increase in ΔP across stages or pressure vessels
• Decreased permeate flow despite stable feed pressure and conductivity
• Unusual odors (e.g., sulfur or rotten egg smell from H₂S)
• Color changes or slime observed in disassembled elements

If undetected, biofouling can spread and lead to irreversible damage to membrane surfaces. Monitoring ORP (oxidation-reduction potential), tracking ΔP trends, and performing routine membrane autopsies can help catch problems before they escalate.

How to Optimize Reducing Agent Dosage

Preventing the risks of overdosing requires a shift from manual estimation to precision-controlled dosing strategies. The goal is to introduce just enough reducing agent to neutralize chlorine—no more, no less.

Best Practices for Optimized Dosing

• Use ORP sensors: Inline oxidation-reduction potential (ORP) monitoring offers real-time feedback on redox levels, helping avoid excessive dosing when no chlorine is present.
• Calibrate dosing pumps regularly: Ensure peristaltic or diaphragm pumps deliver accurate, stable flow based on feedwater flow rate and chlorine concentration.
• Install automatic dosing controllers: PLC-based chemical injection systems adjust SBS dosage based on actual feedwater parameters, minimizing waste and risk.
• Test for residual chlorine post-injection: A low but measurable trace indicates complete neutralization without surplus.

Proper dosing not only protects membranes from chemical attack but also preserves the microbiological stability of the entire RO train. This results in longer membrane life, fewer cleanings, and more predictable system performance.

Conclusion: Precision Dosing Protects Your RO System

While reducing agents are essential for chlorine removal in RO systems, overdosing can unintentionally trigger microbial contamination and operational decline. Anaerobic conditions fueled by excess sodium bisulfite promote biofouling, pressure loss, and irreversible membrane damage.

To ensure optimal system performance, chemical dosing must be approached scientifically—not intuitively. By implementing ORP monitoring, calibrated pumps, and intelligent control systems, plant operators can avoid hidden fouling risks and extend membrane service life.

At STARK Water, we specialize in customized water treatment solutions for RO system stability, dosing optimization, and long-term performance protection.

Need help diagnosing a fouling issue or upgrading your pretreatment strategy? Contact our technical team today for expert support.