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Corrosion

Neutralizing Filters
Page 3

Corrosion Control

From the above, it is apparent that corrosion is not a simple problem, and in most systems, is related to more than one factor. Further, it is impossible to completely halt corrosion where water contacts metals, but we do have some methods which will reduce corrosion.

Where the prime cause of corrosion is acidity as indicated by low pH values, the obvious answer is to neutralize that acidity. One of the simplest ways is to install a neutralizing filter, which contains materials such as calcite, (calcium carbonate) or magnesia (magnesium oxide). As the water passes through a bed of such materials, the carbonic acid is neutralized and a small amount of the bed is dissolved. This is essentially the same reaction which occurs when acidic waters trickle through limestone strata in the ground, and small amounts of hardness are added to the water. "Overfeeding" does not occur, because as the water is neutralized, the dissolving action automatically stops.

Neutralizing filters must be backwashed periodically because they do serve as mechanical filters to remove solid particles from the water. Further, particles of material at the top of the bed ultimately become so small that they tend to clog the bed. Thus, they must be removed by backwashing. From time to time additional material must be added to the bed to replace that which is dissolved.

An alternative method of neutralizing acid water is to feed a solution of soda ash (sodium carbonate) to the water supply with a chemical feed pump. The chemical feed pump may be wired to operate in conjunction with the well pump, and thus good proportioning of the soda ash to the water flow is obtained. By introducing the soda ash solution ahead of the pressure tank, good mixing and neutralization are obtained. The feed rate of the feed pump is usually adjusted to produce a treated water pH of 7.5 to 8.0.

Where acidity is the only problem, the neutralizing filters are usually the best approach. However, where the water contains much iron, or disinfection of the water is desired, the chemical feed pump is often used since hypochlorite bleach and soda ash may be mixed in a single solution and fed into the water system with the same pump unit. When the prime causes of corrosion are high concentrations of dissolved minerals, which increase its electrical conductivity, or dissolved oxygen, there are no feasible and economical methods of removing these materials from small private water systems. The pickup of dissolved oxygen from the air in the pressure tank can be reduced by the use of one of the pressure tank designs which incorporate either a flexible "membrane" or a floating disc to minimize the water are exposed to the air.

Although it is not feasible to remove dissolved minerals and oxygen from the water, two types of materials are available to control their corrosive actions. Several types of food grade polyphosphate compounds and silicate compounds are available which can be fed into the water system for corrosion control. In action, these materials lay down very thin films on the interior metal surfaces, thus minimizing the water to metal contact and reducing the rate of corrosion. Since the films do slowly re-dissolve, the feeding of the materials should be maintained at the proper levels. At the beginning of a feeding program, old corrosion deposits may be loosened and flushed through the system, and this often appears to make a "red water" problem worse, and higher than normal feed rates may be required until the system is reasonably clean and the film established. Then the feed rate can usually be reduced just to maintain the protective film.

Polyphosphate compounds can be added to the water either as dry soluble crystals or as a solution. The compound is fed into the water either as dry soluble crystals or as a solution. The compound is fed into the water either by a chemical feeder or a chemical feed pump. The chemical feeder is a tank-type unit which is installed so that a portion of the water flow passes through the tank and, in a manner determined by the technical design of the feeder, the solution formed by the compound is added to the water. Chemical feeders utilize one or both of the feed-rate control factors of solubility of the chemical compound and flow rate through the dispenser. For a greater degree of feed-rate control, some chemical feeders incorporate pressure differential devices and/or precision orifices within the feeder itself. Others are designed to be used with a valve in the main water line to create slight resistance to flow, to force some water flow through the feeder tank. Sometimes this valve is incorporated directly into the feeder design.

All chemical feeders must be refilled with the chemical compound periodically to replace that which has been dissolved.

The polyphosphate compound in liquid solution can also be added to the water by means of a chemical feed pump, which can be set to inject a specific amount of the solution from the chemical feeder tank into the water at regular intervals. Highly soluble silicate compounds in solution used for corrosion control can also be fed into the water by means of chemical feed pumps.

Thus, several approaches may be used to control corrosion in household water systems. Acid waters may be neutralized with filters designed for that purpose, or soda ash may be fed with chemical solution feeders. Chemicals such as polyphosphates and silicates may be fed to form protective films in the water systems. For further corrosion control, water heaters should be set only as high as necessary, and temperatures above 140F avoided. Even when modifying plumbing systems, avoid the use of dissimilar metals where possible.

NOTE: The determination of which treatment method is best should be made only after careful consideration of many factors such as economics, water quality characteristic, the end use to which the water is to be put, temperature variances of the water to be treated, the inherent limitations of the available treatment technology, and others. This determination can best be made by your local water treatment representatives and they should be consulted prior to the purchase and installation of any water treatment equipment.

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