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Membrane Technologies for Point-of-Use and Point-of-Entry Applications 625
3.5.4. Recovery Rate
As the feed water enters the RO system, only part of the water comes out as treated water.
Part of the water fed into the system is used to wash away the rejected compounds and goes
down the drain as waste. If not properly designed, only little treated water will be obtained
from large quantities of water fed to the RO systems. Generally, most home RO systems are
designed for 20–30% recovery (i.e., 2–3 gallons of treated water are produced for every 10
gallons fed into the system). In fact, RO systems for household drinking water treatment can
operate at higher recovery rates, but doing so may shorten membrane life.
The recovery rate, or efficiency, of the RO system can be calculated by dividing the
volume of treated water produced by the volume of water fed into the system:
Volume of Treated Water Produced
% Recovery ¼ (2Þ
Volume of Feed Water Used
As described previously, reverse osmosis can occur when the natural osmotic flow
between a dilute solution and a concentrated solution is reversed through the application of
water pressure. If the recovery rate is increased when water pressure remains constant, the
salts in the residual feed become more concentrated and the natural osmotic pressure will
increase until it is as high as the applied feed pressure. The maximum recovery rate usually
depends on the limiting osmotic pressure, the concentration of salts present in the feed water,
and the tendency of salts to precipitate on the membrane.
The device in controlling the recovery rate is the flow regulator, which must be properly
adjusted on the amount of rejection stream. If the flow is slow, the recovery rate is high.
In addition, RO membranes are easily fouled if concentrated impurities are not washed away
quick enough. If the flow is too fast, the recovery rate is low and too much water goes down
the drain.
3.5.5. Maintenance of an RO System
An RO system must be well maintained to ensure reliable performance. The major
component of RO in POU/POE system is not only the RO membrane itself but also other
combined units such as pre- and post-device treatment. Maintenance of the whole system is
important for the RO unit to work in the proper manner. The device might require continuous
operation to maintain peak membrane performance.
The first step to ensure that the system can work properly is to install and operate according
to the manufacturer’s instructions. After installation, retest both the feed water (prior to
treatment) and the treated water at a state-certified laboratory to ensure the system is working
properly and removing the contaminants. The testing is required for both the untreated and
treated water annually. More frequent testing (quarterly or semi-annually) is suggested if high
levels of contaminants are present in the raw water. Results of the water quality from the
testing will help in determining the performance of the RO system, the replacement of
components, and the required maintenance. The maintenance, cleaning, and part replacement
should be performed in accordance with the manufacturer’s recommendations.
Characteristics of feed water to RO system have to be monitored to ensure the quantity and
quality of the purified water. The efficiency of a reverse osmosis treatment unit can be