Nitrate in water supplies results from agricultural contamination of ground and surface water that is then used as a water source by the water company. Nitrate levels up to 50 mg/l (as NO3) are allowed in public water supplies.
The dissolved constituents of water used in aquariums can become concentrated due to evaporation. Therefore it is preferable to use water for make-up and topping up that has low levels of nitrate.
Water that has been forced through a reverse osmosis membrane under pressure generally has 95% of its dissolved constituents removed, and this includes nitrate. Water produced by reverse osmosis is ideal for aquariums, since it has no chlorine, very little nitrate and verylow levels of dissolved minerals
Mixed bed ion exchange will remove all dissolved solids from the water but the resin will become exhausted very quickly. Hence this is really only suitable for treating very small volumes of water
There are many different types of ion exchange resin available for different purposes. Nitrate selective resin was developed for drinking water treatment but it is very useful for aquarium owners too. This resin has the appearance of small (usually white) spherical beads about 0.3 to 0.5 mm in diameter. In regenerated form the resin at molecular level is loosely holding harmless chloride ions on the exchange sites. When this resin is brought into contact with water containing nitrate, the resin will preferentially adsorb the nitrate ion onto the exchange site (i.e. it becomes immobilised on the resin) and release the chloride ion into the water. Hence water that is denitrified in this way will have chloride ions in place of nitrate ions but will be similar in overall dissolved solids.
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