In our previous article, we identified the four major types of reverse osmosis (RO) membrane fouling. Identification is only the starting point. What truly determines the long-term stable operation of a system is a full-process prevention and control strategy spanning design, operation and cleaning. Industry practice shows that a systematic prevention and control system centered on the "whole-process, multi-barrier" concept — covering enhanced pretreatment, in-line chemical inhibition, regular chemical cleaning and operation management optimization — is the key to ensuring the economical and efficient operation of RO systems.

I. Pretreatment: Block Fouling Before It Reaches the Membrane

Pretreatment is the first barrier against fouling. For particulate fouling, the stable operation of multi-media filters and ultrafiltration (UF) systems is critical: once breakthrough or fiber breakage occurs, colloidal particles will directly enter the RO system. The Silt Density Index (SDI) should be consistently maintained below 5; if the threshold is exceeded, the pretreatment chain must be inspected immediately.

For inorganic scaling, antiscalant dosing is the most direct measure. Antiscalants can relatively increase the solubility of sparingly soluble salts, prevent calcium carbonate, calcium sulfate and other salts from crystallizing on the membrane surface, and also reduce the risk of iron ions clogging membrane micropores. However, antiscalants are not a panacea: the dosage must be dynamically adjusted according to water quality stability indices (such as the Langelier Saturation Index, LSI). When the recovery rate is too high, even excessive antiscalant can barely fully inhibit scaling at the tail end of the system.

For organic and biological fouling, the management of activated carbon filters is particularly critical. Once activated carbon is saturated with adsorption, it not only loses its ability to remove organic matter, but also becomes a breeding ground for microorganisms. Controlling feed water Total Organic Carbon (TOC) below 2 mg/L is an effective baseline for inhibiting bacterial growth.

II. The "Chemical Defense Line" in Operation: Disinfection and Scale Inhibition

Once biological fouling forms, it is extremely difficult to clean. Therefore, continuous inhibition during operation is far more important than remedial action afterwards.

The use of biocides requires strategic planning. Conventional oxidizing biocides (such as sodium hypochlorite) dosed in the pretreatment section must be completely neutralized with a reducing agent (such as sodium bisulfite) before entering the RO membranes, to prevent oxidative damage to membrane elements. During operation, it is more recommended to use non-oxidizing biocides (such as DBNPA, isothiazolinone) for periodic shock dosing, at a reference dosage of 15–25 mg/L. These agents can penetrate biofilm slime to achieve killing and stripping effects. Experience shows that alternating oxidizing and non-oxidizing biocides can effectively reduce microbial resistance.

Judging the timing of cleaning is also evidence-based. When the normalized permeate flow drops by 10%–15% compared with the level after the last cleaning, or the inter-stage differential pressure increases by 10%–15%, the cleaning procedure should be initiated, without waiting for the system to deteriorate severely.

III. Chemical Cleaning: Targeted Treatment with Combined Solutions

The core principle of cleaning is "alkaline first, then acidic". Alkaline cleaning removes organic fouling and biofilm, while acidic cleaning dissolves inorganic scale.

Typical alkaline cleaning formula (for reference only; formula varies with specific operating conditions): 1.5% sodium tripolyphosphate + 0.05% sodium dodecyl benzene sulfonate, adjusted to pH 12.5 with caustic soda, and circulated for about 10 hours. In practical applications at reclaimed water plants, this formula has maintained the normalized membrane flux retention rate at 90% and reduced cleaning agent costs by nearly 70%. For acid cleaning, 1.5% citric acid solution is commonly used, with pH controlled at 2–3; cleaning for about 1 hour can effectively remove calcium carbonate and metal oxide scales.

Special reminder: the temperature of the cleaning solution should generally not exceed 35–40°C, and the pH should be controlled within 2–12 for composite membranes. After cleaning, the system must be thoroughly flushed with permeate water to avoid chemical residue.

IV. Monitoring and Closed-Loop Management: Let Data Drive Decisions

For prevention and control strategies to be truly implemented, continuous operation monitoring is indispensable. Operators are recommended to establish a "standardized parameter log" to record the normalized values of three core indicators daily: permeate flow, salt rejection and inter-stage differential pressure. Once a trend change is detected, the distribution of differential pressure across stages can be used to preliminarily determine whether fouling is concentrated at the head or tail end, so as to adjust pretreatment or initiate cleaning in advance and prevent fouling from accumulating to an irreversible level.

There is no "silver bullet" for membrane fouling prevention and control. It is a systematic project spanning from pretreatment to cleaning, from chemical agents to data management. Only when every barrier is properly implemented can reverse osmosis membranes shift from "frequent clogging" to "long-cycle stable operation".