Scaling of the EDI module in a reverse osmosis EDI unit is a critical issue affecting the stable operation of the system. The core causes are substandard influent water quality, uncontrolled operating parameters, and insufficient maintenance. Scaling substances mainly include calcium and magnesium scale, silica scale, iron and manganese scale, and microbial films. These substances clog the ion exchange membrane and resin channels, leading to increased permeate conductivity, increased pressure differential, decreased desalination rate, and even permanent module damage. Therefore, a preventative system needs to be built from four dimensions: water quality control, operating parameter optimization, regular maintenance, and standardized operation.
Influent water quality is the source of scaling, and key indicators must be strictly controlled. Calcium and magnesium ions (Ca²⁺, Mg²⁺) are the main components of hard scale such as calcium carbonate and calcium sulfate, and their concentration should be controlled at extremely low levels (e.g., Ca²⁺ < 0.01 mg/L). Silica (SiO₂) easily forms difficult-to-remove silica scale within the EDI module, so SiO₂ must be kept < 0.5 mg/L. Excessive total organic carbon (TOC) will cause organic pollution, so TOC must be maintained < 0.1 mg/L. If the influent hardness exceeds the standard, it needs to be removed in advance by a pretreatment system (such as softening resin or reverse osmosis unit). If the silica content is too high, lime softening or ultrafiltration technology can be used for interception. If organic matter exceeds the standard, it needs to be degraded by activated carbon adsorption or ultraviolet oxidation.
Precise control of operating parameters is the core of preventing scaling. The EDI module is extremely sensitive to recovery rate. High recovery rate will accelerate ion concentration on the concentrate side. When the concentration of sparingly soluble salts in the concentrate exceeds the solubility product, crystallization occurs, forming a scale layer. Therefore, the recovery rate needs to be dynamically adjusted according to the influent water quality to avoid long-term high-load operation. Simultaneously, the matching of current and voltage is crucial. Excessive current will exacerbate the production of hydrogen and oxygen through water electrolysis, leading to drastic fluctuations in local pH and promoting calcium carbonate precipitation; insufficient voltage will fail to effectively drive ion migration, causing ions to accumulate on the membrane surface. Membrane stack voltage (standard value 200-400V DC) needs to be monitored regularly to ensure it fluctuates within a reasonable range. Regular maintenance is a key step in preventing scale accumulation. Chemical cleaning is an effective means of removing existing scale. Targeted cleaning agents should be selected according to the type of scale: calcium and magnesium scale can be cleaned with citric acid or hydrochloric acid circulation; silica scale requires hydrofluoric acid or chelating agents; and organic contamination requires dissolution with alkaline solutions (such as NaOH + NaCl). It is recommended to perform a deep cleaning once per quarter and a simple flush once per month. In addition, the pre-filter cartridge should be replaced regularly (e.g., quarterly) to prevent large particles from entering the EDI module; the electrode plates should be checked for scaling every six months, and deposits should be cleaned promptly; the module should be disassembled annually to check the integrity of the ion exchange membrane, and damaged membranes should be replaced.
Proper operating procedures are the fundamental guarantee for preventing scaling. If the unit is shut down for more than 72 hours, a protective procedure must be performed, including draining the water from the module and injecting antifreeze (such as glycerol) or disinfectant (such as glutaraldehyde) to prevent microbial growth or freezing damage. Long-term operation at low flow rates (<30% of rated value) is strictly prohibited, as low flow rates lead to uneven water distribution and excessively high ion concentrations in localized areas, accelerating scaling. When installing a new module, ensure the concentrated brine sealing ring is located on the inlet side to avoid abnormal increases in product water conductivity or damage to membrane elements during operation.
Preventing scaling of the EDI module in a reverse osmosis EDI unit requires establishing a four-pronged prevention and control system encompassing water quality, parameters, maintenance, and operation. By purifying water at the source, dynamically adjusting operating parameters, performing regular in-depth maintenance, and implementing standardized operating procedures, the risk of scaling can be significantly reduced, module lifespan extended, and the system's long-term stable output of ultrapure water guaranteed.
