Understanding the Impact of Reducing Agent Overdosing on RO System Performance

Understanding the Impact of Reducing Agent Overdosing on Reverse Osmosis (RO) System Performance

Reverse osmosis (RO) systems are crucial for many water treatment processes. However, their efficiency can be compromised by operational issues, one significant challenge being the improper dosage of reducing agents. Many operators inquire about the correct dosage for reducing agents or the ideal Oxidation-Reduction Potential (ORP) for RO feed water. While essential for neutralizing oxidizers like chlorine, overdosing reducing agents can lead to severe operational problems, particularly microbial fouling.

Why is Reducing Agent Dosage Critical in RO Systems?

Reducing agents, commonly sodium bisulfite, are added to RO feed water primarily to remove residual chlorine, which can damage RO membranes. The generally recommended dosage is 2 to 5 times the residual chlorine value in the feed water.

The Problem: Widespread Overdosing and its Causes

Operational data often reveals that reducing agents are frequently dosed far beyond the actual requirement – sometimes 30 to 50 times the necessary amount. This overdosing typically stems from two main issues:

Lack of Awareness: Operators may be unsure of the correct dosage calculation and continue using initial commissioning rates, which don't account for variations in water flow or chlorine levels.

Inaccurate Monitoring: Faulty or poorly calibrated ORP meters can provide misleading readings, prompting operators to increase dosage excessively to meet target ORP levels.

Consequences of Overdosing: Increased Pressure and Microbial Fouling

Excessive addition of reducing agents directly contributes to:

Increased Differential Pressure: A primary indicator of fouling is a noticeable rise in the pressure difference across the RO membranes.

Microbial Contamination: Overdosing fosters the growth of microorganisms, often observed as a slimy, transparent, yellowish substance.

RO systems inherently provide favorable conditions for microbial growth: nutrients (trapped organic matter), suitable temperature and pH, and a large surface area for attachment. When microbial fouling occurs, bacteria proliferate on the membrane surface and feed spacers, obstructing flow, reducing water production (permeate flux), and rapidly increasing differential pressure.

The Link Between Reducing Agent Overdose and Biofouling

Overdosing reducing agents like sodium bisulfite creates an anaerobic (low oxygen) environment by reacting with dissolved oxygen in the feed water. This environment is ideal for specific types of bacteria, particularly Sulfate-Reducing Bacteria (SRB).

SRBs are microorganisms that utilize sulfur compounds for energy. They can reduce sulfates, sulfites (like the dosed sodium bisulfite), and thiosulfates into sulfides, notably hydrogen sulfide (H₂S), known for its "rotten egg" smell, especially in anaerobic conditions. The excess sodium bisulfite acts as a food source, fueling rapid SRB proliferation within the RO system.

While systems with higher dissolved oxygen might experience biofouling without a noticeable odor, low-oxygen systems are prone to developing unpleasant smells due to H₂S production. Regardless of the odor, significant microbial growth inevitably leads to membrane blockage, reduced permeate flow, and increased operating pressure.

Control Strategies to Prevent Overdosing Issues

To mitigate microbial fouling caused by reducing agent overdose, implement the following control measures:

Accurate Dosing: Regularly (e.g., daily) measure the residual chlorine in the RO feed water. Adjust the reducing agent dosage to be 2-5 times this measured chlorine level.

Reliable Monitoring: Ensure ORP meters are correctly calibrated and functioning properly to provide accurate feedback on water conditions. Avoid relying solely on potentially inaccurate ORP readings to control dosage.

System Audits: Periodically review chemical dosing protocols and system performance data (differential pressure, permeate flow) to identify potential overdosing issues early.

Consider Alternatives (If applicable): Evaluate alternative dechlorination methods like activated carbon filters if persistent overdosing issues cannot be resolved through dosing control.

Conclusion

While reducing agents are necessary for protecting RO membranes from chlorine damage, overdosing poses a significant threat to system performance through microbial fouling. Understanding the link between excess reducing agents (like sodium bisulfite), anaerobic conditions, and SRB growth is crucial. By implementing accurate dosing based on residual chlorine levels and ensuring reliable monitoring, operators can prevent costly fouling issues, maintain optimal water production, and extend membrane life.

Explore our customized RO systems or high-performance RO membranes to enhance your system stability.

Need expert advice? Contact the STARK technical team for a dosing audit and optimization support.