Answers to common questions about reverse osmosis membrane and ion exchange resin (111-120)

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

This depends on different component models, for example:

  • TW1.8”, 125psi (8.6 bar)
  • TW>1.8”, 300psi (21 bar)
  • BW30/NF, 600psi (41 bar)
  • SW, about 1000psi (69 bar)

112. How often should the RO system be cleaned?

In general, when the standardized flux drops by 10-15%, or the system desalination rate drops by 10-15%, or the operating pressure and pressure difference between sections increase by 10-15%, the RO system should be cleaned. The frequency of cleaning is directly related to the degree of system pretreatment,
When SDI15<3, the cleaning frequency may be 4 times per year;
When SDI15 is around 5, the cleaning frequency may be doubled
But the frequency of cleaning depends on the actual situation of each project site.

113. What is SDI?

At present, the best technique to evaluate the possible colloidal pollution of RO/NF system influent water is to measure the sedimentation density index (SDI, also known as fouling index) of influent water, which is an important parameter that must be determined before RO design. During the operation of the RO/NF, periodic measurements must be made (for surface waters measured 2 to 3 times a day). ASTM D4189-82 specifies the test standards. The water inlet regulation of the membrane system is that the SDI15 value must be ≤5. Effective techniques for reducing SDI pretreatment include multi-media filters, ultrafiltration, and microfiltration. Adding polydielectric before filtration can sometimes enhance the above-mentioned physical filtration and reduce the SDI value.

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

RO/NF systems usually use a constant speed to drive the water pump, because the operation of the membrane system is generally very simple and once the required flow and pressure are determined, the pump’s operating curve will be constant. The choice of constant speed pump is very suitable for projects where the salt content of the inlet water is almost constant and the water temperature is constant or the temperature fluctuation is small, usually the temperature change range is less than 5 ℃. When the temperature of the inlet water changes greatly or the salinity of the inlet water fluctuates greatly, and high salinity inlet water such as seawater desalination and sub-seawater desalination projects, variable speed drive centrifugal pumps must be used. Variable speed pumps can save energy. Although the investment is high, the energy consumption is low, the operating cost is low, and the impact on the power grid and the system is small.

115. Should the reverse osmosis process or the ion exchange process be used for influent?

Under many water inlet conditions, it is technically feasible to use ion exchange resin or reverse osmosis. The choice of process should be determined by economic comparison. In general, the higher the salt content, the more economical the reverse osmosis. The lower the amount, the more economical the ion exchange. Due to the widespread popularity of reverse osmosis technology, the combined process of reverse osmosis + ion exchange process or multi-stage reverse osmosis or reverse osmosis + other deep desalination technologies has become a more technically and economically more reasonable water treatment program. To understand, please consult a water treatment engineering company or a Dow representative.

116. Can you tell us about high-efficiency fungicides and disinfecting drugs?

Membrane systems are often susceptible to fouling, and biological contamination caused by bacteria is the main reason. Biological pollution forms a hotbed enriched with other impurities and exacerbates the pollution problem. The symptoms of the membrane after pollution are that under constant water flow conditions, the water production volume decreases or in order to maintain the water production volume unchanged, the operating pressure must be increased. At the same time, the desalination rate will also be reduced. When the above symptoms occur and biological contamination is suspected, users always try to “regenerate and recover” the membrane. This requires the use of fungicides, which have the following requirements for fungicides:

  • Compatible with membrane
  • Fast action
  • Cheap
  • Easy to transport and store, with high stability and easy to use features
  • Will not enter the water production side through the membrane
  • Has broad-spectrum control
  • Must be biodegradable
  • Must comply with existing and upcoming regulations

It is 2,2-bisbromo-3-nitrilopropionamide (DBNPA) that can satisfy the above conditions very well, and it has a very good effect in preventing biological pollution of the membrane system.

117. What is the maximum allowable flux of Dow FILMTEC components?

The maximum allowable flux depends on the quality of the influent water. For detailed regulations, please refer to the typical flux regulations in the design guidelines.

118. Why should the pollution factor (FF) and temperature be included in the ROSA design software?

This is to simulate the distribution of different produced water fluxes in the system. Under the condition of high inlet water temperature and high pollution factor (corresponding to the water source with lower pollution), the produced water flux distribution in the system will tend to be more uneven. Through empirical input of different pollution factors (FF) and temperature conditions, it is possible to analyze the potential pollution tendency of certain components due to excessively high water production flux, while at low temperatures and low pollution factors (corresponding to higher pollution Under the condition of water sources such as waste water, seawater, etc., the highest required system operating pressure will be simulated.

 119. How long can the membrane element be used for?

The service life of the membrane depends on the chemical stability of the membrane, the physical stability of the element, the cleanability, the water source of the inlet, the pretreatment, the cleaning frequency, the operation management level, etc. According to economic analysis, it is usually more than 5 years.

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

No. Chemical compatibility judgment is the responsibility of chemical suppliers.

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