Frequently Asked Questions about Reverse Osmosis Membranes and Ion Exchange Resins (111-120)

111. What is the maximum operating pressure of Dow FILMTEC elements?

This depends on the specific element model, for example:

TW1.8”, 125psi (8.6 bar)

TW＞1.8”, 300psi (21 bar)

BW30/NF, 600psi (41 bar)

SW, approximately 1000psi (69 bar)

112. How often should an RO system be cleaned?

Generally, the RO system should be cleaned when the normalized flux decreases by 10-15%, or the system desalination rate decreases by 10-15%, or the operating pressure and inter-stage pressure differential increase by 10-15%. The cleaning frequency is directly related to the level of system pretreatment.

When SDI15 < 3, the cleaning frequency may be 4 times per year;

When SDI15 is around 5, the cleaning frequency may need to be doubled.

However, the cleaning frequency depends on the specific conditions of each project site.

113. What is SDI? Currently, the best and most effective technique for evaluating the potential for colloidal fouling in the feed water of an RO/NF system is to measure the Sedimentation Density Index (SDI, also known as the fouling index). This is a crucial parameter that must be determined before RO design and must be measured regularly during RO/NF operation (2-3 times daily for surface water). ASTM D4189-82 specifies the standard for this test. The feed water specification for membrane systems requires an SDI15 value ≤ 5. Effective pretreatment techniques for reducing SDI include multi-media filters, ultrafiltration, and microfiltration. Adding polyelectrolytes before filtration can sometimes enhance the physical filtration capabilities and reduce the SDI value.

114. What is the difference between constant-speed and variable-speed drive pumps?

RO/NF systems typically use constant-speed drive pumps because membrane system operation is generally very simple, and once the required flow rate and pressure are determined, the pump's operating curve remains constant. Constant-speed pumps are well-suited for projects where the feed water salinity is almost constant and the water temperature is constant or fluctuates very little, typically within a temperature range of less than 5°C. Projects with significant variations in influent temperature or salinity, as well as those with high salinity influent such as seawater desalination and sub-seawater desalination projects, must use variable-speed centrifugal pumps. Using variable-speed pumps is energy-efficient; although the initial investment is higher, energy consumption is lower, operating costs are lower, and the impact on the power grid and system is smaller.

115. Should reverse osmosis or ion exchange processes be used for general influent?

Under many influent conditions, both ion exchange resins and reverse osmosis are technically feasible. The choice of process should be based on economic comparison. Generally, the higher the salinity, the more economical reverse osmosis is; the lower the salinity, the more economical ion exchange is. Due to the widespread adoption of reverse osmosis technology, the combination of reverse osmosis + ion exchange, multi-stage reverse osmosis, or reverse osmosis + other deep desalination technologies has become a recognized more technically and economically reasonable water treatment solution. For further information, please consult a water treatment engineering company or a Dow representative.

116. Could you introduce some highly effective bactericides and disinfectants?

Membrane systems are often susceptible to fouling, with biofouling caused by bacteria being the main cause. Biofouling creates a breeding ground for other impurities and exacerbates the fouling problem. Symptoms of membrane fouling include a decrease in permeate flow rate under constant feed flow conditions, or the need to increase operating pressure to maintain a constant permeate flow rate, which also reduces the desalination rate. When these symptoms appear and biofouling is suspected, users often attempt to "regenerate and revive" the membrane, requiring the use of biocides. Biocides must meet the following requirements:

Membrane compatibility

Rapid action

Low cost

Easy to transport and store, with high stability and ease of use

Does not permeate the membrane into the permeate side

Broad-spectrum control effect

Must be biodegradable

Must comply with existing and upcoming regulations

2,2-Dibromo-3-nitropropionic acid (DBNPA) best meets these requirements, demonstrating excellent effectiveness in preventing biofouling in membrane systems.

117. What is the maximum permissible flux of Dow FILMTEC elements? The maximum allowable flux depends on the influent water quality; detailed specifications can be found in the typical flux specifications in the design guidelines.

118. Why are fouling factors (FF) and temperature included in ROSA design software?

This is to simulate different permeate flux distributions within the system. Under conditions of high influent temperature and high fouling factors (corresponding to less polluted water sources), the permeate flux distribution within the system will become more uneven. By artificially inputting different fouling factors (FF) and temperature conditions based on engineering experience, the potential fouling tendency of certain elements caused by excessively high permeate flux can be analyzed. Under conditions of low temperature and low fouling factors (corresponding to more polluted water sources such as wastewater and seawater), the highest required system operating pressure will be simulated.

119. How many years can membrane elements typically be used?

The lifespan of a membrane depends on the membrane's chemical stability, the element's physical stability, cleanability, influent water source, pretreatment, cleaning frequency, and operational management level. Based on economic analysis, it is typically 5 years or more.

120. Has Dow approved the compatibility of chemicals used in water treatment systems containing Dow FILMTEC membrane elements?

No. Chemical compatibility assessment is the responsibility of the chemical supplier.