Well water chemistry varies widely between wells and if corrosion is suspected, a simple water test can pinpoint what is making the water corrosive. Acidity in water is expressed by the pH scale, which ranges from 1 to 14. A pH of 7 is considered neutral, with less than 7.0 being acidic and greater than 7.0 being alkaline. In addition to pH, alkalinity, calcium carbonate, total dissolved solids and temperature are measured and can quickly show if acidity is the main cause of the corrosion.
Acidic water is usually caused by dissolved carbon dioxide in rain water, which allows carbonic acid to form. As the rainwater soaks into the ground it ends up as the source for the acidic well water for homeowners. In some cases, mineral acids from mining or naturally occurring mineral acids cause the water to be acidic.
If the water is acidic, or in the range of 4 to 6.8, the water can benefit by having the pH neutralized to a pH of between 7.0 and 7.5. Frequently water that is corrosive is also low in calcium hardness and dissolved solids. This type of water can be made non-corrosive by the simple addition of soda ash. Soda ash, or sodium carbonate, is a non-toxic product that when added to water in the correct dose can raise the pH to 7.0 to 7.5 without making the water harder or affecting the taste or color of the water.
In some cases if there is dissolved iron or manganese in the water, after the pH is raised, the iron and manganese will oxidize and be visible in the water. If the iron is greater than 0.3 ppm and the manganese is greater than 0.05 ppm, a backwashing iron filter should be used after the soda ash is injected.
Soda ash pump systems automatically inject the soda ash solution into the water when the well pump runs. A small pump called a metering pump is used to inject a small amount of soda ash (sodium carbonate) into the water automatically. For home wells the metering pumps are usually wired to turn on and start pumping soda ash solution when the well pump is energized or running.
For best results, allow a few minutes of contact with the water for pH adjustment to occur and the soda ash solution to thoroughly mix in the water by using a contact tank. However, for many residential applications, the existing pressure tank and piping system is enough to provide some mixing, since the pH is neutralized almost instantly.
Soda ash is bought dry, usually in 25 or 50 lb bags and mixed with soft or pure water in the solution tank. When a saturated solution is achieved (approximately 3 pounds per 5 gallons of water, which creates a 10% solution), a solution of between 50 and 500 ppm are injected, depending on the pH, alkalinity and flow rate of the water.
In some cases, instead of dissolved carbon dioxide causing the low pH or acidity, the acidity is caused by mineral acids, either natural or from mining or other industrial wastes. In this type of water, the pH is less than 4.0 then sodium hydroxide (lye) may need to be used with the same type of pump that is used for soda ash. Soda ash works best to neutralize the pH when the pH is between 4 and 6.8.
Another approach to neutralizing acidic water is by the use of a calcite neutralizer filter. These filters will typically raise the pH of the water to 7.0 to 8.0 and add 30 to 100 ppm of hardness depending on the alkalinity and water hardness. Unlike soda ash feeders, these units do raise the hardness of water. They can have certain advantages over soda ash feeders, if there is sediment in the water, or small amounts of iron, in that they also double as self-cleaning filters.
In neutralizer filters, acidic waters slowly dissolve the calcium and magnesium media on contact as the water flows through the filter, raising the pH of the water and increasing the alkalinity. This eliminates the effects of corrosive water chemistries and can help to prevent corrosion of piping and fixtures.
Neutralizer filter tanks are filled with a blend of calcium and magnesium carbonates made from naturally occurring minerals, which dissolve into the water, making it less corrosive. More mineral can quickly and easily be added to the filter tank, typically once per year for most residential applications. No special tools are required.
Generally soda ash feeders are preferred if the water has a pH of less than 5 or 5.5, whereas calcite neutralizers may work better when the pH is between 5.5 and 6.8.
Gerry Bulfin is a licensed water treatment contractor and WQA Certified Water Specialist IV specializing in treating acid well water. He may be contacted through the website www.cleanwaterstore.com or by calling 831-462-8500 or by emailing him here.