Main influencing factors of the pollution rate of surface water reverse osmosis water treatment equipment

I. Membrane Material Type (Cellulose Acetate Membrane or Polyamide Membrane)

Common reverse osmosis membranes on the market are categorized by material into: CAB series (cellulose acetate) membrane elements or CPA series (aromatic polyamide) membrane elements. For difficult-to-treat surface water or wastewater systems, CAB membranes are often chosen to replace CPA membranes. The advantages of CAB membranes are their smooth, uncharged surface, which reduces contaminant deposition during use, and microorganisms are less likely to adhere to their surface. Another advantage of CAB membranes is that the feed water can contain 0.3-1.0 ppm free chlorine during operation. Chlorine, as a disinfectant, protects the CAB membrane from harmful bacteria and prevents fouling caused by the growth of microorganisms and algae. CPA membranes themselves are chlorine-resistant, but cannot withstand its oxidizing properties. Therefore, chlorination is required.

II. Water Flux:

Water flux is the amount of water produced per unit effective membrane surface area, expressed in GFD (gallons/square foot/day) or LMH (liters/square meter/hour). The water flux of a reverse osmosis system is directly proportional to the fouling rate. Low water flux means low fouling rate; conversely, increased water flux increases the fouling rate. To reduce water flux, reverse osmosis membrane elements with a larger membrane area can be selected.

If cleaning is required every three months or longer, it indicates that the pretreatment and reverse osmosis system design is reasonable. If cleaning is required every 1 to 3 months, the process needs to be improved or additional equipment added.

III. Crossflow Velocity:

Crossflow velocity is also an important factor affecting the fouling rate of reverse osmosis equipment. The higher the crossflow velocity of the surface feed water and its resulting concentrate on the membrane surface, the lower the membrane fouling rate. This is because the high crossflow velocity of the feed water reduces the sedimentation and accumulation of particulate matter on the membrane surface, and simultaneously accelerates the diffusion rate of high-concentration salts on the membrane surface to the bulk solution, reducing the risk of precipitation of sparingly soluble salts.

IV. Reverse Osmosis Cleaning:

Cleaning and maintenance of reverse osmosis water treatment equipment can effectively prevent equipment fouling. Methods include concentrate recirculation, shutdown low-pressure rinsing, and regular disinfection.

Rinsing and short-term soaking of the RO equipment product water on the membrane elements can inhibit bacterial growth and dissolve or loosen contaminants on the membrane.

Concentrate recirculation can increase crossflow velocity and flush away contaminants on the membrane surface; however, it increases the capacity of the RO feed pump and increases the salt content of the equipment product water.

Shutdown low-pressure cleaning can flush contaminants and concentrate from the membrane elements, preventing microbial growth.

The purpose of regular disinfection is to control microbial growth between cleanings.

V. Reverse Osmosis Membrane Element Structure:

For the purification of well water and surface water using reverse osmosis water treatment equipment, the membrane elements used are mainly spiral-wound membrane elements. Considering the anti-fouling ability of the feed water channel, equipment space requirements, investment and operating costs, and procurement, spiral-wound membrane elements are superior to hollow fiber and plate-and-frame structures. Designing a large membrane area helps to reduce water flux and increase crossflow velocity, inhibiting equipment fouling.