Harn R/O Systems Blog

How to Calculate Percent Recovery and What Affects it in a Membrane Treatment System?

Posted by Julie Nemeth-Harn on Apr 3, 2018 1:10:00 PM

RECOVERY RATE

Recovery is the amount of water permeated per unit time - usually in gallons per minute (gpm) and expressed as a percentage of the raw water feed flow rate. The design recovery is calculated as follows:

Recovery_Formula_Image (1).jpg

Related Blog: What is the Reverse Osmosis Equipment Manufacturer's (ROEM) Role in Membrane Water Treatment?

Membrane systems operating at 82% recovery will convert 82% of the total raw water input into treated permeate, with the remaining 18% being sent to waste as concentrate (or reject). The recovery rate is monitored using flow meters installed in the permeate and concentrate piping.

NOTE:
Membrane systems are designed to operate at a predetermined flow and recovery rate and should not be operated outside these parameters. Should it become impossible to maintain the required flow rates and recovery or a change is required for some other reason, please promptly notify the Harn R/O Systems technical service department.

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FEED WATER CHEMISTRY

AdobeStock_119840649.jpegThe recovery rate of a membrane system depends mainly on feed water chemistry. Natural well waters can contain various concentrations of ions such as calcium, barium, strontium, fluorides and sulfates plus other low solubility compounds such as silica.

The reverse osmosis process will concentrate these compounds in the concentrate (brine or reject) stream where their concentrations may exceed their solubility limits thus causing precipitation. The highest concentration of salts exists close to the surface of the membrane; (an effect called polarization) here concentration may be 20% higher than in the bulk of the solution in the brine channel. This can lead to the growth of crystals, which can form a scale on the membrane surface. A scaled membrane will show a high differential pressure, reduced permeate flow and poor quality permeate. To achieve higher recovery rates without scaling it is usually necessary to pre-treat the raw feed water by the injection of scale inhibitors and/or other chemicals.

MEMBRANE ELEMENT FLUX RATE

The permeate production rate of a membrane system depends mainly on the available square footage of membrane surface. Accordingly, the design of a membrane system takes into account the membrane flux rate. This is defined as the rate at which a given membrane area is able to produce permeate and is measured in gallons per day per square foot of membrane surface (gfd or sometimes gsfd). Thus, the overall production capacity of membrane system is proportional to the membrane area and therefore the number of membrane elements.

This system was designed to produce permeate water at an average rate of 15 gfd. It is important to design with a conservative flux rate, as an excessive flux rate will accelerate membrane fouling.

An operator may be tempted to increase the membrane feed pressure to increase the permeate flow, but the flux rate will also increase and the system may become more prone to fouling and require chemical cleaning more frequently. To increase the production of an R/O system it is typically necessary to add more membrane surface area.

TRAIN FLOW RATE CONTROL

The recovery rate of a membrane system is regulated using a combination of controls. Ultimately however it is dependent on the ratio of the concentrate and permeate flow rates. The Train supplied has interdependent settings: feed pump speed (controls feed pressure), concentrate flow, permeate flow, and concentrate control valve position. Changing the setting of any one of these will also affect the setting of the other.

These settings are interdependent and adjusting the feed flow/pressure will change the concentrate flow/pressure. The feed flow/pressure is varied by changing the feed pump VFD speed and is used to control the total permeate flow. The concentrate flow is varied by changing the position of the concentrate valve.

Generally water temperature and TDS remain fairly constant day-to-day and as a result very little variation in operating flow rates and pressures should be seen. Over a longer term, variations in feed temperature, feed TDS and any membrane fouling effects may require minor adjustments to the three system flow controls to maintain design parameters.

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Topics: membrane filtration, membrane treatment system, membrane treatment, water treatment, membrane plants, water quality, ro calculations