RO System Recovery Rate Calculation & Influencing Factors

RO system recovery rate is the percentage of feed water converted from reverse osmosis water treatment to produced water. The higher the recovery rate, the higher the water production of the system, indicating a higher water utilization rate. The recovery rate is related to feed water quality, the environment, the number of membrane elements and their arrangement.

The recovery rate of conventional single RO membrane element is mostly between 12% – 18%, for the 1st stage RO system recovery rate: R1 = 1st stage water production ÷ 1st stage water inlet × 100%, the 2nd stage RO system recovery rate: R2 = 2nd stage water production ÷ 2nd stage water inlet × 100%, and the recovery rate of the whole RO system is not the sum of the two recovery rates, because the concentrate water of the 2nd stage RO system will not be discharged, but will flow back to the 1st stage inlet. So the whole RO system recovery rate calculation formula is:

• R — Recovery rate of RO system
• Q_p — Water output volume of RO system
• Q_f — Feed water volume of RO system

For example, if the feed water volume (Q_f) is 100 gpm and the water output volume (Q_p) is 65 gpm, then the RO system recovery rate (R) is 65 ÷ 100 × 100% = 65%.

• Chemical properties of water quality

  For example, the higher the concentration of calcium ions and magnesium ions in the water, the easier it is to form scale on the surface of the membrane, causing blockage of the membrane element and reducing the recovery rate.

• Membrane element selection & arrangement

  The surface area, material, thickness, etc. of membrane elements vary slightly from brand to brand and industry to industry. The size of the membrane surface area determines the amount of water produced by the membrane system, which leads to differences in the recovery rate of different types of single membranes, but basically does not exceed 18%.

  The recovery rate of the RO membrane system is also affected by the number of membrane elements in series, arrangement and other factors. Membrane elements in series are concentrate water from the previous membrane flowing into the inlet of the next membrane, and concentrate water passes through the RO membrane again, which can improve the recovery rate of the system. However, if all membranes are connected in series to improve the recovery rate, the pressure of the 1st stage membrane will become very large, so several membranes need to be properly connected in parallel to relieve the pressure of the water flow in the 1st stage.

  Therefore, the arrangement of RO membranes has a great influence on the recovery rate of the system in the actual design.

  We give an example of the effect of the number of membrane elements in series on the recovery rate:

  The recovery rate of a single membrane is assumed to be 15% and the feed water volume is 1 gpm.

  
  • Q_f — Feed water volume (gpm)
  • Q_P — Water output volume (gpm)
  • Q_C — Concentrate water volume (gpm)
  • Q_TP — Total water output volume/System water output volume (gpm)

  Recovery rate of a single membrane: (1 × 15%) / 1 × 100% = 15%

  Recovery rate of two membranes in series: (0.85 × 15% + 1 × 15%) / 1 × 100% = 27.75%

  Recovery rate of three membranes in series: (0.85² × 15% + 1 × 27.75%) / 1 × 100% = 38.59%

  Recovery rate of four membranes in series: (0.85³× 15% + 1 × 38.59%) / 1 × 100% = 47.8%

  Recovery rate of five membranes in series: (0.85⁴ × 15% + 1×47.8%) / 1 × 100% = 55.63%

  Recovery rate of six membranes in series: (0.85⁵ × 15% + 1×55.63%) / 1 × 100% = 62.29%

  Recovery rate of ten membranes in series: (0.85⁹ × 15% + 1 × 76.84%) / 1 × 100% = 80.31%

  Recovery rate of twelve membranes in series: (0.85¹¹ × 15% + 1 × 83.27%) / 1 × 100% = 85.78%

  With n membranes in series, the system recovery rate is:

  R = (0.85^n-1 × 15% + 0.85^n-2 × 15% + … + 0.85 × 15% + 15%) / 1 × 100%

  However, this calculation method is ideal, the actual operation will be much less, generally large water treatment equipment to meet the recovery rate of at least 60% – 80%, so at least 6 membrane elements need to be connected in series.

• Working environment

  As a highly efficient separation technology, changes in the operating environment can also have a significant impact on the performance of RO membranes. In general, higher temperatures increase water production; higher system operating pressures increase recovery rates. However, regardless of high temperatures or high operating pressures, it will cause damage to the RO membranes.

• Concentrate water recycle

  Concentrate water recycle is a part of the concentrate water generated by the RO system back to the high pressure pump, mixed with the incoming water and then again into the RO membrane, so the existence of concentrate water recycle will greatly affect the recovery rate of the system.

  

Small reverse osmosis equipment generally has a low recovery rate due to the small amount of membrane elements. To improve the recovery rate, partial recycling of concentrate water is often used, and part of the concentrate water is returned to the inlet again for desalination and recovery. Large reverse osmosis equipment for industrial use can adopt the proper arrangement of membrane elements and multi-stage types to improve the recovery rate due to the large amount of membrane elements.

Although higher recovery rates are generally designed to avoid water waste, higher recovery rates are not always better, as excessively high recovery rates may lead to a decrease in water quality (desalination rate).

Adding pretreatment processes such as activated carbon filter tanks, sand filter tanks, and water softener tanks before the RO system reduces contamination of the RO membrane and also improves the overall recovery rate.