The pretreatment system of reverse osmosis equipment is a core component ensuring the long-term stable operation of membrane elements. Its design must revolve around two core objectives: "fouling control" and "scaling prevention." The effectiveness of the pretreatment system directly affects the lifespan of the membrane elements, the quality of the permeate, and operating costs. Therefore, it must be designed specifically for the raw water characteristics, membrane element type, and system operating parameters.
Raw water quality analysis is the foundation of pretreatment system design. The composition of pollutants varies significantly among different water sources (such as groundwater, surface water, seawater, or wastewater). For example, groundwater may have high iron and manganese content, surface water is characterized by suspended solids, colloids, and organic matter, while seawater is primarily characterized by high salinity, high hardness, and microbial contamination. During the design phase, comprehensive water quality testing is necessary to clarify the types and concentrations of pollutants. For instance, SDI (Sludge Density Index) can be used to assess the risk of colloidal fouling, hardness testing can determine scaling tendency, and COD or TOC analysis can determine organic matter content. This data provides a basis for subsequent process selection, preventing rapid membrane element fouling due to insufficient pretreatment.
For the removal of suspended solids and colloids, multi-stage filtration is crucial. A common process employs a combination of quartz sand filtration, activated carbon filtration, and precision filtration: quartz sand filters remove larger suspended solids, reducing the load on subsequent treatment processes; activated carbon filters remove residual chlorine, organic matter, and some colloids through adsorption, while protecting membrane elements from oxidative damage; precision filters (such as 5μm filter cartridges) act as the final barrier, intercepting tiny particles and preventing them from entering the reverse osmosis system and scratching the membrane surface. For high-turbidity water sources, enhanced processes such as flocculation sedimentation or fiber disc filtration can be added to further reduce the SDI value.
Scale control requires addressing both hardness removal and scale inhibitor addition. If the raw water has high hardness (e.g., excessive calcium and magnesium ion content), hardness ions must be removed through ion exchange softening or lime softening processes to prevent the crystallization of sparingly soluble salts such as calcium carbonate and calcium sulfate on the membrane surface. For incompletely softened water sources, specialized scale inhibitors must be added to inhibit scale crystal formation through dispersion and chelation. Simultaneously, a scale inhibitor with good compatibility should be selected based on the water quality to avoid reaction with flocculants and subsequent precipitation. In addition, adjusting the pH of the influent is also an important method. For example, lowering the pH can reduce the risk of calcium carbonate scaling, but it is necessary to avoid excessively low pH levels that could lead to a decline in membrane element performance.
Microbial contamination control needs to be implemented throughout the entire pretreatment process. Activated carbon filters are prone to bacterial growth and require regular backwashing and the addition of non-oxidizing bactericides. For water sources with high microbial content, such as surface water or seawater, ultraviolet disinfection or ultrafiltration processes should be added to thoroughly remove bacteria, viruses, and algae. System design should minimize dead zones and avoid water stagnation, while employing concentrated water recirculation or regular flushing strategies to prevent microorganisms from adhering to and multiplying on the membrane surface.
The operation and management of the pretreatment system are equally critical. Influent water quality parameters (such as turbidity, SDI, residual chlorine, etc.) need to be monitored regularly, and process parameters adjusted promptly based on changes. Filter cartridges should be replaced and filters cleaned regularly to prevent the pretreatment equipment itself from becoming a source of contamination. A standardized data recording system should be established to judge membrane element fouling trends through changes in indicators such as permeate flow, desalination rate, and pressure difference, and to develop cleaning or replacement plans in advance.
The design of the pretreatment system must balance economy and reliability. While meeting the feed water requirements of membrane elements, the process should be simplified as much as possible to reduce investment and operating costs. For example, for groundwater with good quality, the pretreatment process can be simplified; for water sources with high pollution risk, a multi-barrier design is required to ensure stable system operation. Through a scientifically sound pretreatment system design, the risk of membrane element fouling and scaling can be significantly reduced, extending its service life and ensuring the long-term efficient operation of reverse osmosis equipment.
