The Impact of Excessive Reducing Agent Addition on Reverse Osmosis
The impact of excessive reducing agent addition on Reverse Osmosis (RO) systems is a very practical and important issue in RO operation and maintenance. Many operators mistakenly believe that "more is better," but this is not the case.
The core role of reducing agents (using SBS as an example) in Ro Systems is to eliminate residual chlorine and other oxidizing substances in the feed water, protecting the oxidant-sensitive polyamide composite membrane. The reaction formula is:
NaHSO₃ + HClO → NaHSO₄ + HCl
This reaction is a precise stoichiometric reaction. Excessive addition means exceeding the stoichiometric amount required to react with oxidizing substances in the feed water, resulting in the presence of free sulfite ions in the system. It is these free reducing substances that bring about a series of negative consequences.
1. Proliferation of Microbial Contamination
This is the most serious and common consequence of excessive addition. Sodium sulfite itself is a nutrient source (carbon and sulfur source) for bacteria. When excessive SBS is present in the system, it provides a continuous "food" source for aerobic and anaerobic bacteria (such as sulfate-reducing bacteria) in the water.
When the biodegradable organic carbon (BCOD) in the RO feed water is insufficient, excess sulfite can be utilized by specific bacterial groups, leading to a biofilm growth rate increase of over 30%. In an RO system with a long-term SBS overfill of more than 50%, the replacement cycle of the security filter cartridge is shortened from the standard 1-2 months to 2-3 weeks, and the filter cartridge surface becomes slippery with a noticeable biological odor.
Microbial contamination directly causes a rapid increase in inter-stage pressure differential within the system. Within 3-6 months, the pressure differential increase may be 0.1-0.3 MPa higher than the standard control, doubling the cleaning frequency.
2. Sulfite Scale Formation
Dissolved oxygen in the water oxidizes some sulfite (SO₃²⁻) into sulfate (SO₄²⁻). Sulfate ions combine with calcium ions (Ca²⁺) commonly found in water to form calcium sulfate (CaSO₄, gypsum scale), which has low solubility.
Under conditions of pH > 8, the oxidation rate of sulfites is significantly accelerated. For an RO system with a recovery rate of 75%, adding an excess of 10 ppm SBS may result in an additional 40-50 ppm of SO₄²⁻ on the concentrate side.
While the LSI (Langrillly Index) or S&DSI (Steve and Davis Index) on the concentrate side does not cause scaling with calcium carbonate, the ion product of calcium sulfate ([Ca²⁺] × [SO₄²⁻]) may exceed 80% of its solubility product (Ksp), greatly increasing the risk of scaling. Once calcium sulfate scale forms, it is far more difficult to clean than calcium carbonate scale.
3. Creating a reducing environment for corrosion in the system
Excessive reducing agent maintains a highly reducing environment throughout the RO system. This can damage the passivation film (usually an oxide film) on the surface of metal components in the system (such as high-pressure pumps, pipelines, membrane housings, etc.), transforming them into a more reactive and easily corroded state.
In a strongly reducing environment with an ORP (oxidation-reduction potential) consistently below -200mV, the corrosion rate of carbon steel can be several times higher than in a slightly oxidizing or neutral environment (ORP 50-200mV).
This corrosion typically involves both uniform corrosion and pitting corrosion, which, in the long run, can shorten the lifespan of core equipment such as high-pressure pumps and may lead to metal ion (such as Fe²⁺) contamination of the Ro Membrane.
4. Irreversible Degradation of Membrane Performance
Although polyamide membranes are susceptible to chlorine, prolonged exposure to excessive reducing agents can also affect the membrane polymer itself. Some studies suggest that excessive SBS may cause slight hydrolysis or structural changes in the membrane polymer.
In systems with strictly controlled SBS dosage, the annual decline rate of the standardized desalination rate of the RO membrane may be less than 3%; however, in systems with long-term excessive dosage, this value may reach 5%-7% or even higher.
This decline in desalination rate is usually irreversible and cannot be restored by chemical cleaning. This means that the effective lifespan of the membrane element is shortened.
Correct Dosage Control and Recommendations
To avoid the above problems, the dosage of the reducing agent must be precisely controlled.
1. Scientific Calculation of Dosage:
Theoretical Dosage: Removing 1 ppm of residual chlorine requires approximately 1.8-3.0 ppm of SBS (depending on water quality and pH). A 3:1 weight ratio (i.e., 3 ppm SBS : 1 ppm residual chlorine) is typically used as a safety factor.
Total Dosage = (Residual Chlorine Value × 3) + Safety Margin (typically 0.1-0.5 ppm)
2. Key Monitoring Indicators:
ORP is the most direct and effective online indicator for controlling SBS dosage. For most RO membranes, it is recommended to control the RO feed water ORP value between +100mV and +200mV. This range ensures effective removal of residual chlorine while preventing the system from entering a highly reducing environment.
It is essential to ensure that the ORP value remains below the upper limit specified by the membrane manufacturer (usually +250mV), but an excessively low ORP value should not be pursued.
3. Regular Testing and Validation:
Regularly test the residual chlorine in the RO feed water using DPD residual chlorine test kits to ensure it is 0. This is the "gold standard" for verifying whether SBS dosage is sufficient.
Simultaneously, monitor biofouling trends using microbial indicators (such as total bacterial count and ATP testing). If an abnormal increase is detected, the first step should be to investigate whether excessive SBS dosage has been added.