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Commercial Reverse Osmosis System Questions & Answers

What is the difference between element & system recovery?

Element recovery: This refers to a single membrane element recovery rate. Element recovery = Permeate flow rate of single element / feed flow rate to the single element x 100%.

System recovery: This refers to a cumulative recovery rate. System recovery = cumulative permeate flow rate of membrane elements in a system / feed flow rate to the system x 100%

For example: Suppose there are two parallel pressure vessels and each pressure vessel contains 6 membrane elements. Feed flow to the system is 100 gpm. Since there are two pressure vessels in parallel, feed flow to each vessel is 50 gpm. The first element in each vessel would see 50 gpm of feed. Suppose the first element produces 5 gpm of permeate water and the whole system produces 50 gpm of permeate water. Then, the first element recovery = 5 gpm / 50 gpm x 100% = 10% while the system recovery = 50 gpm / 100 gpm x 100% = 50%

How do I know when to replace elements?

Determining when to replace elements is dependent on the type of problem, whether they are fouled, scaled, or damaged.

Fouled or Scaled Elements
Thin Film membrane elements can be very effectively cleaned due to their tolerance for wide range of pH and temperature. However, if cleaning is delayed, it becomes increasingly difficult to remove foulants or scales from the membrane surface. Cleaning will be more effective if it is tailored to the specific fouling problem.

Knowing when to replace them is dependent upon what you can tolerate. If your system cannot meet the water quality and/or permeate flow specifications even though you change operating conditions or cleanings do not restore the performance, then it is probably a time to replace the membranes.

Some foulants/scales are nearly impossible to clean off: e.g. aluminum, oil, grease, calcium, barium, or strontium sulfate scale, calcium phosphate. So if an element is fouled or scaled with these, it will need to be replaced.

Oxidized or Mechanically Damaged Elements
Such elements cannot be restored as the membrane has been irreversibly damaged. The elements need to be replaced. Elements with moderate telescoping may be still usable.

What is shimming?

The placement of shims on the adapter within the vessel minimizes movement of the individual membrane elements is called "shimming". This reduces wear-out and mechanical abrasion and subsequent o-ring leakage.

Will membranes operate at a temp. greater than 45C (113F)?

Thin Film Membrane Elements are not designed to support the temperature higher than 45C (113F). It does not mean that the elements cannot exceed the maximum temperature limit of 45C. However, a greater potential for element damage exists as the temperature increases above 45C, and the warranty is void. In order to operate or clean at high temperatures, high-temperature or heat-sanitizable elements with different materials of construction are recommended. We currently offer FilmTec heat-sanitizable elements that can be heat-sanitized up to 85C (185F).

What system recovery should I run at?

FAQ Summary:

A general rule is the system can be operated at recoveries of 50% for single stage, 75% for two stage, and 80-85% for three stage systems. % recovery is defined as the ratio of permeate flow to feed flow rate. Parameters such as operating temperature, source of feed, composition of feed, feed concentration, and pH can have an effect on the overall % system recovery and % recovery of individual element. In all cases though, % system recovery is often set to maximize permeate flow while preventing precipitation of super-saturated salts within the membrane system.

What is pressure drop and why is high pressure drop problematic?

Pressure drop is the loss of pressure from the feed end to the concentrate end of a module or a pressure vessel. Under normal operation condition, the pressure drop for a commercial RO membrane element is about 4 to 5 psi (0.3 bar) per element.

(Typical pressure drop through a new home drinking water element is 1 psi at 50 psi feed pressure.)

Pressure drop increases with the extent of fouling. A high pressure drop is problematic because it may lead to telescoping and inefficient operation, and thus a decline in system performance. The maximum allowable pressure drop is 60 psi (4 bar) per 6-element array.

How long can I expect commercial membranes to last in my system?

Membrane life is a function of feed water source, pretreatment, frequency of cleaning, system design, and operating conditions. For economic analysis, a 5 year life is normally used.

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All Content on this site is  intended for informational purposes for experienced water treatment professionals only.  Applied Membranes, Inc. does not assume any liability for any damages caused by the misapplication or misinterpretation of any of the information contained on this website.
Applied Membranes, Inc. 2007