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%

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.

Thin Film Membrane Elements are not designed to support the temperature higher than 45°C (113°F). It does not mean that the elements cannot exceed the maximum temperature limit of 45°C. However, a greater potential for element damage exists as the temperature increases above 45°C, 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 85°C (185°F).

Loss of salt rejection and loss of permeate flow are the most common problems encountered in reverse osmosis (RO) and nanofiltration (NF). Plugging of the feed channels associated with pressure drop increase is another typical problem. If the rejection and/or the permeate flow decreases moderately and slowly, this may indicate a normal fouling which can be handled by proper and regular cleaning.

An immediate decline in performance indicates a defect or misoperation of the plant. In any case, it is essential that the proper corrective measure is taken as early as possible because any delay decreases the chance of restoring the plant performance – apart from other problems that might be created by an excessively low permeate flow and/or too high permeate TDS.

A prerequisite for early detection of potential problems is proper record keeping and plant performance normalization. This includes proper calibration of all instruments. Without accurate readings it might be too late before a problem is detected and corrected.

Once a performance decline has been identified, the first step in solving the problem is to localize the problem and to identify the cause(s) of the problem. The first step is to evaluate the performance and the operation of the system. This can be done using the data of the record keeping logsheet or of some additional on-line measurements. Then some checks and system tests should be made. If the system data is not sufficient in determining the cause(s) and to recommend corrective action, one or more membrane elements must be taken from the plant and analyzed. Element performance analysis includes non-destructive and destructive analysis. Finally, corrective measures are taken to restore the plant performance and to avoid future problems.

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.

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.

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.