Maui School Garden Network
November 28, 2020
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How Homeowners on Well Water Use Calcite (Crushed Marble) to Neutralize Corrosive Well Water

Author: Administrator
Crushed marble, also known generically as calcite, is a naturally occurring calcium carbonate media that is crushed and screened. It can be used inexpensively to neutralize acidic or low pH waters to a neutral, less corrosive condition.

For homes that use well water, one of the most common causes of copper pipe corrosion is low pH, also known as acidic water. A low pH is water with a pH of less than 7.0 pH. Signs of acid water are corrosion of fixtures, leaking water heater tanks, pinhole leaks, blue staining (from copper pipes) or rust staining (from iron pipes).

Common causes for acidic water are acid rainfall due to atmospheric carbon dioxide and other airborne pollutants, runoff from mining spoils, and decomposition of plant materials.

One of the advantages of calcite or crushed marble is its self-limiting property. When used properly, it corrects pH only enough to reach a non-corrosive condition. It does not overcorrect. Upon contact with the crushed marble, acidic waters slowly dissolve the calcium carbonate to raise the pH thereby reducing the potential leaching of copper, lead and other metals found in typical plumbing systems.

Calcite typically has a density of 100 pounds to one cubic foot and is near white in color. The mesh size most commonly used in neutralizer filters is 16 x 40. The composition of this natural mineral is 95% calcium carbonate and 5% magnesium carbonate.

To correct low pH on a home water system, calcite is used in a filter tank similar in shape to a water softener or iron filter tank. Common tank sizes for homes are 10" to 13" in diameter by 48" to 54" in height. A filter gravel support bed is typically used beneath the calcite.

Depth of the calcite bed will vary but a minimum bed depth of 24" is recommended. Periodic backwashing will prevent packing, reclassify the filter bed and maintain high service rates.

The service flow rate recommended is 2 to 6 gallons per minute per square feet of calcite filter bed area. For example, a 13" tank is about 1 square foot. Even though the service flow rate is 2 to 6 gallons per minute, much higher intermittent flow rates can be realized and adequate neutralizing of the water still take place.

If the neutralizer filter is run continuously or for several hours at one time, say in filling a storage tank to serve multiple homes, the service flow rate of 2 to 6 gallons per minute should be used. For homes however a peak flow rate of 15 to 20 gallons per minute per square foot can be realized.

The backwash flow rate recommended is 10 to 12 gallons per minute. Depending on pH, water chemistry and service flow, the mineral bed will have to be periodically replenished as the Calcite is depleted.

In addition to being an excellent acid water neutralizer, this type of calcite is also an excellent filter media and can remove sediment and particles down to the 30 micron range in size. During the backwash cycle, the trapped sediment is flushed to drain, eliminating problems with pressure drop through the filter bed.

Calcite media is best used when the water is low in hardness and the pH is from 5.9 to 6.9. If the pH is less than 5.9 and especially if the pH is less than 5.0, magnesium oxide is often blended in with the calcite. Depending on the water chemistry magnesium oxide is added at a ratio of 10% to 30%, to 90% to 70% crushed marble.

Calcite will raise the hardness level of water although often no water softener is needed, because the increase in hardness is minimal. Some waters are soft to begin with and after treatment with calcite will be moderately hard, but not hard enough to warrant a water softener. Since water chemistry and temperature vary, it is hard to predict exactly how much harder the water will become, but most users find they do not need a water softener after installing a calcite neutralizer filter.


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