Frequently Asked Questions about Reverse Osmosis Membranes and Ion Exchange Resins (31-40)

31. When to choose low-energy or ultra-low-energy membrane elements?

Dow FILMTEC's low-energy membrane elements are the LE and LP types, and the ultra-low-energy type is XLE. These are for customers whose primary goal is to achieve high desalination and permeate flow rates while minimizing energy consumption. For example, our ultra-low-energy XLE consumes only 50% of the energy of the corresponding standard BW product, but has a lower desalination rate. Please consult a Dow representative or refer to the product specification.

32. What is the relationship between feed water TDS and conductivity?

When you obtain the feed water conductivity value, you must convert it to a TDS value so that it can be entered into the software design. For most water sources, the conductivity/TDS ratio is between 1.2 and 1.7. For ROSA design, a ratio of 1.4 is used for seawater and a ratio of 1.3 is used for brackish water, which usually yields a good approximate conversion rate.

33. How can I tell if the membrane has become fouled?

The following are common symptoms of contamination:

 Decreased permeate flow rate at standard pressure

 Operating pressure must be increased to achieve standard permeate flow rate

 Increased pressure drop between feed and concentrate

 Increased weight of membrane elements

 Significant change in membrane removal rate (increase or decrease)

 When the element is removed from the pressure vessel, water is poured onto the feed side of the upright membrane element. The water does not flow through the membrane element but overflows from the end face (indicating complete blockage of the feed channel).

34. When is it necessary to test the membrane system, and how?

A decrease in the system's desalination rate may be a uniform change across the elements, or it may be limited to a few membrane elements at the beginning or end. It may be a failure in every pressure vessel of the entire system, or it may be limited to only a few pressure vessels. Therefore, it is necessary to measure the TDS value of the permeate from a single pressure vessel, as well as the performance of a single membrane element. Please refer to the relevant chapters in this manual for information on membrane element performance testing methods.

35. How to prevent microbial growth inside the original packaging of membrane elements?

When the preservative solution becomes turbid, it is very likely due to microbial growth. Membrane elements protected with sodium bisulfite should be inspected every three months. If the protective solution becomes cloudy, the element should be removed from the sealed storage bag and re-immersed in fresh protective solution (1% by weight of food-grade sodium bisulfite, unactivated by cobalt) for approximately 1 hour. The element should then be resealed and drained before repackaging.

36. What is the weight of Dow FILMTEC membrane elements?

Dow FILMTEC New Element Weight Summary Table

* Note: Wet element weight is after draining.

The degree of fouling can be judged by the weight of the used element; some severely fouled membrane elements may more than double in weight.

37. How can the energy consumption of the membrane system be reduced?

Using low-energy membrane elements (LP, BWLE, or ultra-low-energy membrane elements (XLE)) is sufficient, but it should be noted that their desalination rate is slightly lower than that of standard membrane elements.

38. Should air be purged from the membrane system before startup?

We require that air be purged at low pressure and low flow rate before starting the membrane system. The operating pressure is between 0.2 and 0.4 MPa. During low-pressure flushing and venting, all concentrate and permeate should be drained into the sewer. If the pressure rises too quickly, and air remains in the membrane element and pressure vessel, additional impact forces will be generated in the direction and radial direction of water flow within the element, which may severely rupture the membrane sheath.

39. How are Dow wet elements protected?

Wet elements are protected with 1% (by weight) sodium bisulfite, while ultrapure water polishing membrane elements are protected with 500 ppm (by weight) sodium bisulfite.

40. What are the requirements for pressure vessels?

Pressure vessels must be designed to comply with American Society of Mechanical Engineers (ASME) standards, preferably with ASME standard certification, or at least should be manufactured according to ASME standard requirements.