Reverse Osmosis – What it Is and How it Works

by Ioannis Stellas

What is Osmosis and Reverse Osmosis

Osmosis is the flow of a solvent through a semi permeable membrane from a less concentrated to a more concentrated solution until both solutions are of the same concentration. This flow can be avoided if pressure is exercised from the side of the more concentrated solution. We call this Reverse Osmosis (RO).

            RO is often used for purifying water from salts and other solute substances. It is also commonly used for industrial and chemical purposes where the purity of water is very important.

Aquarium use

            Reverse osmosis is used for purifying water to be added to our aquaria. The water produced this way has zero hardness (carbonate and general) while its pH value is around 6.8-6.9. This water can be used in aquaria hosting species which require soft, slightly acidic water.  This is one of the most natural ways to produce purified water without having to use chemical substances.

            It is worth noting that RO water is not suitable, on its own, for aquarium use; it should be mixed with a quantity of tap water prior to be added to the tank. This is due to the fact that RO water is devoid of electrolytes, salts and trace elements, which are necessary for the right operation of the vital organs of the fishes. Tap water should be treated with a conditioner prior to being mixed with RO water.

            Let’s look at an example of mixing reverse osmosis water with tap water. Suppose the tap water has a carbonate and general hardness of 10 dH. We want to fill a 100 liters aquarium with water that has carbonate and general hardness of 3 dH. To achieve this value, given that the RO unit produces water with zero carbonate and general hardness, we should mix 30 liters of tap water with 70 liters of reverse osmosis water. Another way of producing water with the desirable values without mixing RO with tap water is to use only RO water and add a specified amount of trace elements, salts and electrolytes. To increase the carbonate hardness we can add a specified amount of sodium bicarbonate; to increase the general hardness, we can add a specified amount of magnesium and calcium salts.

            Having explained how RO works I will now look at the components of an RO unit.

Reverse osmosis Unit

            Each RO unit has 3 tubes. One is for feeding tap water through the unit while the other two are used for the outflow of pure water and the outflow of waste water.

            The main element in every RO unit is the membrane, which retains salts, elements and particles existing in tap water. This membrane is quite sensitive; it can easily be destroyed by chlorine, “big” particles as well as water with high temperature. Thus the use of a pre-filter is essential.

            One type of pre-filter used in RO units is described with a penetrability value. Hence manufacturers indicate that particular pre-filters have a penetrability of “x” μm.  These pre-filters retain all the particles in the water that are bigger than x μm thus barring them from reaching the membrane. Naturally, the smaller the x value the better for the membrane, since the membrane has to hold less particles. A usual value for this penetrability is 5 μm. Expensive RO units have pre-filters with penetrability of  1 μm.

            An equally important pre-filter is the carbon one, which removes chlorine in the tap water. Thus only dechlorinated water is allowed through to the membrane. Its function is very important considering that the membrane is chlorine sensitive.

            More expensive RO units might have more carbon pre-filters, such as coconut carbon filters or active carbon filters. This improves pre-filtration thus offering better protection to the membrane. Following pre-filtration the water flows to the membrane.  It is imperative that this water is pressurized; lack of pressure makes reverse osmosis impossible, as indicated.  This requires a minimum tap pressure of at least 35-40 PSI. If the tap pressure is less than 35-40 PSI, the RO unit works inefficiently resulting in larger amounts of water being wasted. The typical output of an RO Unit is one part of of pure water to 3-5 parts of waste water. The waste water is essentially tap water without “big” particles and without chlorine and can be used for cleaning, washing, watering the plants etc.

            The ratio of pure/waste water depends on the water temperature and the flow restrictor of each unit. The volume of pure water is measured in liters/day in a temperature of ~25οC and a specific concentration of dissolved minerals, though for water with temperature less than 25 οC the manufacturers usually provide a table with correction factors for calculating the production volume. It should be noted that if the water has a temperature of 40 οC and above it can destroy the membrane. The flow restrictor determines the ratio of pure/waste water and is usually adjusted to a 1:3 to 1:5 ratio. Some reverse osmosis systems have a valve allowing the water through the membrane from the opposite direction. This cleans the membrane from depositions thus lengthening its life.

            Additional filters that are used after the membrane are carbon filters and deionization (DI) filters. DI filters consist of charged materials which withhold all the positive and negative ions that exist in the water. The result is de-ionized water.

            RO units are often referred to as three or four stage units. This refers to the number of stages of filtration from tap to collection of pure water. The absolute minimum number of stages is two, namely the pre-filter and the membrane.

            As with all equipment RO systems must be inspected and maintained regularly. We must inspect and replace the pre-filters according to the manufacturer’s instructions if we want our membrane to live longer and in order to have as pure water from our system as we can get. Consider that the cost of a membrane is much higher than any pre-filter or filter on our RO system. Sometimes, it is better to buy a whole new RO system than replace everything that is old (i.e. membrane, pre-filters).

            Finally, I would like to mention some of the elements and ions that a RO system can withhold: Calcium, sodium, magnesium, potassium, manganese, iron, aluminium, copper, nickel, cadmium, silver, zinc, the corresponding salts of the elements indicated, ions, bacteria etc.











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