Causes and Solutions for Seven Abnormal Phenomena in Ultrapure Water Reverse Osmosis Systems

I. Scaling

The raw water contains metal ions such as calcium, magnesium, barium, and strontium, as well as anions such as carbonate, sulfate, and silicate. During reverse osmosis operation, water continuously permeates through the membrane element to form pure water, while salts are retained on the concentrate side, leading to continuous salt concentration. When the concentration of slightly soluble salts (calcium carbonate, calcium sulfate, silica, barium sulfate, etc.) exceeds their saturation solubility, they will crystallize and precipitate on the membrane surface and flow channel mesh, gradually accumulating to form a scale layer. Meanwhile, problems such as excessively high system recovery rates, insufficient scale inhibitor dosage or mismatched reagent selection, poor pretreatment softening effect, sudden changes in raw water quality, and incomplete low-pressure flushing can further exacerbate the scaling rate. In addition, uneven dosing and localized retention of concentrated water can also cause localized supersaturation and scaling.

II. Oxidation Phenomenon

The core cause of oxidation is the presence of excessive oxidizing substances on the influent side. The most common are residual chlorine and hypochlorite. In addition, excessive levels of ozone, hydrogen peroxide, and potassium permanganate can also cause oxidative damage. Improper control of the ultrapure water system's pretreatment disinfection process, insufficient or ineffective reducing agents (failing to completely neutralize residual chlorine), excessive disinfectant dosage, uneven mixing of disinfectants leading to localized excessive oxidant concentrations, aging system pipelines breeding microorganisms, and excessive addition of oxidizing bactericides can all cause oxidative damage to the membrane element's functional layer, resulting in membrane pore structure deformation and loss of density.

III. Membrane Rupture

Mechanical pressure overload is the primary cause. Sudden pressure rises and falls during system start-up and shutdown, excessive instantaneous pressure differences during high-pressure operation, and frequent water hammer impacts can tear membranes and damage the sealing structure. Improper equipment installation and maintenance, such as misaligned membrane elements, uneven tightening, and mixing of new and old membrane elements, can lead to uneven stress during operation and cause damage. Membrane shell aging and cracking, and long-term use causing hardening, deformation, and detachment of sealing rings can also contribute. Furthermore, uncontrolled chemical cleaning parameters, such as excessively high cleaning solution concentration, excessive temperature, and excessive soaking time, can corrode the membrane material, increasing membrane brittleness and causing cracking and damage. Foreign objects entering the membrane channels can scratch or puncture the membrane under high pressure, also leading to localized rupture.

IV. Contamination

Pretreatment system failure is the core cause. Impurities in the raw water, such as humic acid, grease, suspended colloids, silt, algae, and bacteria, are not completely filtered and directly enter the reverse osmosis membrane system. Organic contamination primarily originates from natural organic matter in the raw water, industrial residual grease, and cleaning agent residues; colloidal contamination mainly consists of the adsorption and accumulation of clay and high-molecular-weight colloidal particles in the water; microbial contamination is due to long-term low-temperature operation, intermittent shutdowns, and the growth of bacteria and biofilms in pipe dead zones, with microbial metabolic products adhering to the membrane surface; inorganic contamination is formed by the deposition of fine suspended impurities and metal oxides. Simultaneously, the system's lack of long-term cleaning and insufficient flushing frequency allow pollutants to continuously accumulate and solidify, forming stubborn contamination.

V. Telescope Effect

Excessive system operating pressure differential and high-pressure impacts are the main contributing factors. The transmembrane pressure differential far exceeds the rated standard, subjecting the membrane element to axial thrust beyond its fixed limit. Equipment installation defects, such as missing, misaligned, or damaged thrust rings inside the membrane housing, prevent the membrane element from being fixed in place; insufficient matching between the membrane element and the housing, and excessive installation gaps. Frequent system start-ups and shutdowns, and violent water flow fluctuations, cause repeated axial impact forces on the membrane element, leading to gradual displacement, tensile deformation, and sealing structure failure over time.

VI. Compaction Phenomenon Prolonged high-pressure, overload operation is the core contributing factor. To ensure permeate flow, the system maintains ultra-high transmembrane pressure differentials for extended periods. The dense membrane surface layer and support layer are continuously subjected to pressure compression, causing pores to gradually shrink and collapse, resulting in membrane structure compaction. Undersized equipment selection, insufficient permeate load design, and prolonged full-load or overload operation accelerate membrane compaction and aging. Furthermore, frequent pressure fluctuations and start-up/shutdown shocks repeatedly compress the membrane structure, exacerbating the compaction process. High-temperature operating environments also contribute to accelerated membrane material shrinkage and compaction, shortening membrane element lifespan.

VII. Concentration Polarization Concentration polarization is a common mass transfer anomaly in reverse osmosis operation. It refers to a situation where the salt concentration on the concentrate side of the membrane surface is significantly higher than the concentration in the bulk water flow, causing a localized increase in osmotic pressure. This significantly reduces membrane separation efficiency and exacerbates scaling and fouling risks.

The seven abnormal phenomena in ultrapure water reverse osmosis systems are interconnected and mutually causal. Concentration polarization accelerates scaling and fouling, high-pressure conditions trigger the telescope effect and compaction, and oxidation and damage directly lead to system failure. The core of stable system operation lies in "prevention first, precise operation and maintenance." By optimizing pretreatment processes, standardizing operating parameters, establishing a periodic cleaning and inspection system, and strictly controlling fluctuations in water quality and operating conditions, various abnormal phenomena can be effectively avoided. For existing failures, it is necessary to accurately determine the cause and take targeted measures to minimize equipment wear, ensure the quality of ultrapure water production, and extend the service life of reverse osmosis membrane elements.