Aeration reduces or removes numerous impurities from contaminated water, especially in cases of high iron or hydrogen sulfide content. Air affects water both chemically and physically, as dissolved minerals are oxidized, and volatile organics are stripped from the water. Unwanted elements that are removed chemically from the water go through three steps: Oxidation, Precipitation, and Filtration.
Oxidation/Reduction
Oxidation is the loss of electrons from the reducing agent. 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 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.
Precipitation
One of the requirements for 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“x54“ 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.
Filtration
Precipitated iron is filtered successfully with a variety of filter media. It may be necessary to experiment to determine which filter media works best in that area. Dealers can avoid future service problems by initially sizing the filtering system properly. 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. The proper well water flow rate is determined by counting the gallons drawn down and the time between cut in and cut off cycle of the well pump. To do this, one must allow the well pump to build up to full pressure. Close the main shut off valve to the building. Measure the number of gallons drawn down from the pressure tank until the well pump turns on. Time the period it takes for the well pump to recover, that is, between cut in and cut out. The formula for determining the flow rate is gallons drawn down that were measured above, divided by the seconds required for recovery, then multiply by 60.
Considering that the backwash rate for many filter media should be twice the filter rate, it may be necessary to install a second filter in parallel to accommodate the service flow. In the illustration above where eight gallons is determined as the flow, that flow will be adequate to backwash many filter media in an eight to ten inch diameter tank. However, the service flow for many common filter media in eight to ten inch diameter tanks is about four or five gallons per minute. Hence, forcing eight gallons per minute through a single ten-inch tank may pass iron. In order to govern the flow of water evenly through tanks installed in parallel, flow restrictors may be installed on the outlet side of the filter tanks.
(Gallons / Seconds) × 60 = Gallons per Minute (gpm) flow
For example, if 8 gallons are drawn down and it takes 60 seconds to build pressure back up, then: 8 divided by 60 = .133. Consequently, .133 × 60 = 8 gallons per minute flow rate.