During the operation of edi electric desalination equipment, microbial contamination of the feed water is a key factor affecting equipment performance and product water quality. Microorganisms (such as bacteria, algae, and fungi) multiply on the membrane surface or resin layer, forming biofilms that lead to decreased membrane flux, reduced desalination rates, and even irreversible damage. To effectively control this type of contamination, a systematic solution needs to be built from multiple dimensions, including pretreatment optimization, chemical dosing, operating parameter control, equipment structure maintenance, and daily management.
The pretreatment stage is the first line of defense against microbial contamination. EDI equipment typically uses reverse osmosis (RO) pure water as feed water. If the pretreatment system fails to effectively remove microorganisms and their breeding grounds (such as organic matter and suspended solids), residual microorganisms will directly enter the EDI module. Therefore, pretreatment needs to employ multi-stage filtration (such as sand filtration, carbon filtration, and security filtration) combined with ultrafiltration (UF) technology to reduce the microbial load through physical interception. For example, ultrafiltration membranes can have pore sizes as low as 0.01 microns, intercepting most bacteria and colloidal particles, providing low-risk feed water for subsequent EDI treatment.
Chemical dosing is an important means of inhibiting microbial activity. Adding appropriate amounts of bactericides (such as sodium hypochlorite or chlorine dioxide) or oxidants during the pretreatment stage or before EDI influent can kill residual microorganisms. However, the ion exchange membranes in the EDI module are sensitive to oxidants, and excessive addition may lead to oxidative degradation of the membrane material. Therefore, after adding oxidants, it is necessary to neutralize residual oxidizing substances by adding reducing agents (such as sodium bisulfite) to ensure that the water entering the EDI is neither biologically active nor oxidizing. Furthermore, for biofilm formation, periodic chemical cleaning can be performed using alternating acid-base cleaning or specialized biological cleaning agents (such as peracetic acid) to remove microbial residues from the membrane surface and flow channels.
Proper control of operating parameters is crucial for preventing microbial contamination. Maintaining appropriate transmembrane pressure differential (TMP) and flow rate can prevent the formation of "dead zones" or stagnant areas in the system, which, due to slow water flow, easily become breeding grounds for microbial aggregation and reproduction. The design of EDI equipment must fully consider the flow channel structure and water flow distribution. Optimizing the layout of the desalination and concentrate chambers can increase flow velocity and reduce dead zones. Simultaneously, parameters such as system differential pressure, permeate flow, and conductivity should be monitored regularly. Increased differential pressure, decreased permeate flow, or shortened cleaning cycles may indicate a risk of biofouling, requiring timely intervention.
The anti-fouling design of the equipment structure and materials is fundamental to long-term stable operation. The EDI module adopts a modular structure, with the desalination chamber filled with anion and cation exchange resins, and the concentrate and desalination chambers separated by selective ion exchange membranes. The membrane material must possess high selective permeability, low adsorption, and antifouling properties to reduce microbial adhesion. Furthermore, strict control of equipment sealing is essential to prevent external microorganisms from invading the internal flow channels due to leakage. For example, employing a double-sealed structure or regularly checking the condition of the seals can reduce the risk of microbial contamination.
Daily maintenance and management are crucial for the continuous prevention and control of microbial contamination. Regularly replacing pretreatment system consumables (such as filter cartridges and ultrafiltration membranes) prevents excessive accumulation of microorganisms on their surfaces. Regularly checking water quality indicators (such as residual chlorine and SDI values) allows for timely assessment of pretreatment effectiveness. When equipment is shut down for extended periods, periodic circulation or pressure maintenance measures should be implemented to prevent microbial growth in a stagnant state. For example, running the equipment for several hours weekly or maintaining a system filled with qualified water can inhibit microbial growth.
Controlling influent microbial contamination in EDI electric desalination equipment requires a comprehensive approach throughout the entire process, including pretreatment, chemical dosing, operational control, equipment design, and daily management. By employing multi-stage pretreatment to retain microorganisms, chemical dosing to inhibit activity, optimizing operating parameters to reduce stagnation, using anti-fouling membrane materials, and rigorous daily maintenance, a multi-layered protection system can be constructed to ensure long-term stable equipment operation and that the produced water meets quality standards.
