BIRM is a cost-effective filter medium for reducing iron and manganese in water. It can be used in gravity-fed or pressurised water treatment systems. Birm is a filter medium based on aluminosilicates coated with manganese dioxide, and it acts as an insoluble catalyst for the reaction between dissolved oxygen and iron compounds. In groundwater, dissolved iron is usually present as ferrous bicarbonate because of excess free carbon dioxide, and it cannot be filtered. Acting as an insoluble catalyst for the reaction between dissolved oxygen and iron compounds, Birm oxidises ferrous (divalent) iron to ferric (trivalent) iron, producing ferric hydroxide, which precipitates and can easily be removed by filtration.
Birm's physical characteristics make it an excellent filter medium that is very easy to clean by backwashing, which removes the precipitate. Birm is not consumed when removing iron, which makes it extremely cost-effective compared with other iron removal methods.
Larger particles are filtered out at the top of the filter media, while very fine particles are retained in the lower layers of the filter. Iron occurs in water in various forms: as divalent ions (Fe2+), trivalent ions (Fe3+), iron bound bacteriologically or organically, colloidal and inorganically bound iron, or combinations of all these forms.
Divalent iron is very easy to detect: take a sample at the source and keep it in a glass container for 15÷30 minutes. At first the water is clear and colourless, but over this time it gradually changes colour, turning yellow or even brown, depending on how much iron in the water has been oxidised by the air it came into contact with.
Trivalent iron and bacteriologically or organically bound iron are immediately visible in a water sample taken at the source. Since it is already oxidised, it appears as a yellowish, orange or reddish colour. After a relatively short time these iron compounds precipitate and settle at the bottom of the sample container.
Colloidal and inorganically bound iron is the hardest to remove from water. This type of iron also appears as a yellowish colour, but it does not change as the sample is exposed to air. Over time these iron compounds remain suspended in the sample container.