Water that has been through a Reverse Osmosis filter, is very pure filtered. Reverse Osmosis water is water that is pumped under high pressure through a membrane in reverse flow direction to that in osmosis. A part of the water passes through, and is then virtually without contaminants. In this manner one can make demineralized water also known as demi-water. Demi-water is water from which all the salts have been removed.
In the greenhouse horticulture an RO unit is often used for the purification of groundwater. RO water usually does not contain any oxygen and depending on the area it may contain methane. Methane gas can pass the RO membrane. Methane may cause a slimy accumulation which can clog the irrigation system. By aerating the RO water in a small reservoir with an Oxydent Airlift, all the volatile compounds can be removed and oxygen is added to the water.