How do reverse osmosis membranes work and what affects quality and production?
Posted by Administrator on 11/7/2009
Osmosis is the flow from a high concentration of water to a low
concentration of water. To help understand the flow of water imagine a
sealed filled water balloon with a hole in it what happens to the water
inside? The water quickly leaves balloon because of the concentration
of water inside the balloon is higher than outside which makes the water
wants to equalize the concentration of water.
Reverse osmosis is quite the opposite. The flow of water is from a
low concentration to a high concentration. Imagine an empty balloon if
you're filling the water balloon with a hose then you are using the
water pressure and forcing water against its natural equalization
A reverse osmosis (ro) membrane is simply a thin semi-permeable layer
that separates two solutions. A ro membrane is a type of physical
separation that is capable of separating molecules down to 1/10,000
micron. Since the size of the pores on the membrane is so small, it
requires pressure to force water through. Most molecules are too large
to pass through a reverse osmosis membrane but small enough for some
salts, sugars and water molecules to pass through. Rejection rates of ro
membranes average around 96-98% under ideal conditions (250 ppm
softened tapwater, 77 °F (25 °C), 50 psig (3.4 bar), and 15% recovery).
TDS levels, temperature, pressure and recovery rates are all things
that affect the product water quality of reverse osmosis membrane.
Effect of Pressure
Feed water pressure affects both the product water production and the
rejection rates of RO membranes. The increase of feed water pressure
directly increases the water production.
Rejection rates also increase when pressure is increased but will plateau.
Effect of Temperature
Temperature has a direct linear effect to production rates. As
temperature increase, water production increases almost linearly because
of the higher diffusion rates of water through the membrane. Rejection
rates are actually lowered when temperature rises. This is due to a
higher diffusion rate of salt across the membrane.
Effect of Salt Concentration or TDS
TDS inversely affects the pressure required for reverse osmosis which
in turn affects the production rates. If feed water was constant and
TDS increases then the production rate decreases because of the osmotic
Osmotic pressure is the pressure and potential energy required to
force water to move against its natural direction across a
semi-permeable membrane. Every 100 ppm (parts per million) in TDS
equals 1 psi (pounds per square inch). The higher the TDS, the more
pressure required to force through the membrane.
Effect of Recovery Rates
Recovery rate refers to the amount of product water being produced
which is controlled by the flow restriction on the waste line. Most reverse osmosis systems
are sized with a sized flow restrictor will have a product to waste
ratio of 1 to 4 which is a recovery rate of 25% this is made
purposefully as a sales point to produce more product water but lower
the rejection rate. For example, the proper size for a 50 GPD membrane
is a 15% recovery rate or a 1 to 6.7 ratio. Lowering the recovery rates
will increase the rejection rate and improve the quality of water.
Raising the recovery rates will cause the quality of the product water
to decrease and will affect the required driving pressure needed for
reverse osmosis to take effect.
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