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How do reverse osmosis membranes work and what affects quality and production?

Posted by 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 tendencies.

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 pressure difference.

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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