How Does Aeration Work?

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.

The Chemical Process Oxidation/Reduction

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.

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 to assure that no water is being used. Then, open a spigot below the pressure tank, capture the water, and measure the number of gallons drawn down from the pressure tank until the well pump turns on. When the pump turns on, immediately close the spigot and 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 multiplied by 60. (Gallons / Seconds) x 60 = Gallons per Minute (gpm) flow. For example, if 16 gallons are drawn down and it takes 90 seconds to build pressure back up, then: 16 divided by 90 = .177. Consequently, .177 x 60 = 10.6 gallons per minute flow rate.

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.

What Methods Are Available to Aerate the Water?

Three common methods to introduce air have been developed, namely, Venturi's, air compressors, and air strippers. Each method is described herein.

1. Venturies (micronizers) are the least expensive method of injecting air, however they can create more problems than they solve. It is necessary to install centuries on the well pump line consequently, all of the water in the building is aerated. Venturies draw air from the atmosphere when the well pump water flows rapidly through the well line. This device drastically restricts the size of the well line to smaller than the diameter of a pencil. This restriction creates major backpressure on the well pump and reduces the flow of water needed to adequately backwash filters. Complaints about low flow and poor performance of product water are possible when centuries are installed. Because all of the water in a facility is aerated before the pressure tank, there is no flexibility with venturies to leave some water lines unaerated if desired. Precipitated iron may clog the well line where the Venturi is installed, the pressure tank and the pressure switch. Another problem with venturies is that jet pumps may not have sufficient water flow to draw air into the water.

2. Air compressors inject air adequately and do not restrict water flow, which is crucial to thoroughly back wash iron filters. Compressors do not interfere with the filter operation and hence, provide superior water quality and quantity in comparison to Venturi's. The location of the air injection point is flexible with the compressor. An injection point just before the aeration tank works well. Air compressors are flexible because the injection point can vary as the need dictates. For residential and commercial customers with high iron and hardness, dealers may install a softener system which will remove the soluble iron then aerate and filter the water to remove sulfur and small amounts of insoluble iron that may pass through the softener. Another illustration is that the air may be introduced following ground water heat pumps or following irrigation systems. These alternative applications are not available with venturies. Compressors are moderately priced and varieties are available in the water treatment industry.

3. Air strippers reduce VOC's in three steps. First, the water and air are thoroughly mixed in a de-pressurized atmosphere. Second, the volatile impurities are discharged through an exhaust. Third, the depressurized vessel is re-pressurized with a jet pump and pressure tank system. The advantage of air stripper systems is that the better brands are capable of removing volatile organic compounds with out using carbon filtration. Carbon filtration removes VOC's and can be less expensive initially but maintenance and landfill regulations may favor air stripper systems. Air strippers are the most expensive aeration systems, however these products are more effective in reducing volatile organic compounds (VOC's) including radon.

( 1 ) WQA Glossary of Terms, p79, 1993 Ed, Harrison & McGowan. 1 00.20

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