Air affects water chemically and physically. Chemically, the dissolved minerals are oxidized. Physically, volatile organics are “stripped” from the water.
Unwanted elements that are removed chemically from the water go through three steps: Oxidation/Reduction, Precipitation, and Filtration.
By definition, according to the Water Quality Association’s Glossary of Terms, “oxidation is the loss of electrons from the reducing agent (which is said to have been ‘oxidized’ in the process). Since electrons carry negative charges, oxidation results in an increase of positive valence.” Oxidation reduces the number of electrons orbiting an element causing the element to bond with oxygen, which has an attraction for those electrons. Hence, oxidation/reduction.
For example, iron is most commonly found in its soluble state as ferrous bicarbonate, Fe (HCO3)2. Ferrous iron has a positive two valence. As ferrous iron is oxidized, the number of electrons is reduced and the iron develops a valence of positive three, ferric hydroxide, Fe(OH)3. To fully aerate iron, the amount of dissolved oxygen present must be at least 15% of the total amount of iron present. When dissolved oxygen is sufficiently present the iron and oxygen bond together. Soluble ferrous bicarbonate may be completely oxidized and changed to the insoluble ferric hydroxide precipitate, Fe (OH) 3, except when the water is acidic. The insoluble ferric hydroxide is commonly described as “red water”. When iron is fully oxidized in alkaline water, iron readily precipitates.
One of the requirements to successful precipitation is to provide sufficient contact time for the oxygen and minerals to react. For iron removal, it is generally best to have an aeration tank, which provides contact time and a vent to expel excess air. That tank should be about the same size as the filter tank(s). Well-designed aeration tanks are constructed so that a pocket of air is maintained at the upper one-third to one-half portion of the tank height. An inlet diffuser allows the water to spray in the pocket of air. Depending upon the chemical properties of water, a 10″ x 54″ aeration tank works well for iron and hydrogen sulfide, (H2S), on most residential applications. H2S levels in excess of 10 mg/l will require larger aeration tanks.
Precipitated iron is filtered successfully with a variety of filter media. Sometimes it is necessary to experiment to determine which filter media works best in your area. Sizing the filtering system properly will avoid much grief. While it is possible to add filters to existing systems, it’s easiest if you get it right the first time.
The flow rate of the water at the pressure tank should be measured accurately because many filter media require approximately twice the backwash flow rate as the service flow rate. Timing how long it takes to fill up a measured bucket is an inaccurate method of attaining flow rates.
TrueWaterQuality carries a variety of Aeration/Oxidization solutions such as the Entipur Aircycle which uses natural air oxidization process to precipitate iron, manganese and hydrogen sulfide odor. This system can remove higher amounts of Iron/Manganese as well as remove rotten egg smell that often accompanies high iron water.
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