In its passage from the clouds to the tap, water contacts many different substances, including gases, minerals and organic matter, and water has the ability to carry many of these substances as impurities which interfere with the use of water by man. Some of these impurities are in true solution, completely dissolved in the water, while other impurities are simply solid particles suspended in the water. Those materials which cause turbidity, color, taste and odor are obviously objectionable.
“Cloudy water” is due to the presence of turbidity or finely divided solid particles, which can absorb or reflect light. These particles may be inorganic mineral matter which does not dissolve, or organic matter, picked up as the water flows over and through the ground. The earth normally serves as an excellent filter, and thus it is unusual to find significant amounts of turbidity in water from deep wells. Surface water from lakes, streams and ponds usually has significant amounts of turbidity derived from surface water run-off, or from bottom deposits stirred up by movement of the water.
Turbidity is objectionable for several reasons. Most obvious is the fact that no one enjoys drinking “dirty water.” We much prefer clear, sparkling water over that which is cloudy. The physical appearance of foods and beverages is a major factor in our enjoyment of these necessities.
If the turbidity is due to inorganic minerals, it can have an abrasive effect on a plumbing system, causing physical wear or erosion on pipe and fittings, and can score valve seats and washers. If the turbidity is due to suspended organic matter, it can cause stains of sinks and fixtures, and discolor laundered fabrics, much like iron.
Color in water is most often due to dissolved organic matter. It too makes the water unpleasant to drink, often contributes tastes and odors, and causes staining of surfaces and materials touched by the water. Surface water usually has some color, but in contrast to turbidity, color is sometimes found in well waters. This usually occurs in areas where swamps or bogs are common, and the water picks up colored substances extracted from the decaying organic matter.
Organic matter in water may not only contribute turbidity or color, but may also produce unpleasant tastes or odors. Even when the amount of organic matter is very low, “off” tastes and musty odors may be found. Again, these tastes and odors affect drinking water, as well as the foods and beverages prepared with the water.
Two special cases exist in the field of tastes and colors. The minerals dissolved in water do add taste to water, and at moderate levels, the tastes are pleasant. Distilled water which contains virtually no minerals tastes “flat” to most persons, and we do not enjoy drinking such water. On the other hand, too high a mineral concentration gives the water unpleasant “soda” or “salty” tastes, and interferes with the flavors of food and beverages.
The other special case occurs when hydrogen sulfide gas is present in the water. So-called “sulfur water” not only produces the obnoxious “rotten egg” odor, but is corrosive to plumbing metals and causes rapid tarnishing of silver. Even very low concentrations of hydrogen sulfide are detectable because of the strong odors.
Water Treatment Equipment
Mechanical filters are specifically designed for the removal of solid particles from the water, and are available in several different types and sizes. These filters simply act as fine screens, which trap solid particles but allow the water to pass through.
One type of filter uses specially graded sand or other granular synthetic material as the filter media, in tanks with the same range of sizes as household water softeners. These filters are capable of filtering substantial flow rates, and usually are used to filter the entire household water supply. They will work very well when the suspended particles are relatively large or gelatinous in nature, but are not effective in some cases where the turbidity is due to extremely fine particles. These must be backwashed periodically to clean the beds and to flush the accumulated matter down a drain.
A second type of mechanical filter is the so-called “cartridge” filter. These units are usually smaller than the tank type filters, and are often installed to treat the water in a single line. Instead of the loose media in the tank type filters, these filters use media which is formed or shaped into more or less rigid cartridges. Although some of the cartridges are designed for mechanical cleaning and reuse, most are designed for replacement when they become clogged with accumulated solids.
These filter cartridges are available in several ratings based on the sizes of particles to be removed, and are capable of removing particles down to extremely tiny sizes. However, the cartridges with extremely small filtering pores have higher resistance to water flow, and can become clogged quite rapidly if high concentrations of large particles are present in the water.
Activated carbon is a unique material known for its ability to absorb soluble organic compounds and certain gases, such as chlorine and hydrogen sulfide, all of which may contribute tastes and odors to a water supply. Thus activated carbon filters are widely used in water treatment, both in the form of granular tank-type filters, and as finely divided powders incorporated into cartridges. The granular filters must be backwashed periodically, and the cartridges cleaned or replaced from time to time.
If the water to be treated with granular activated carbon contains a high concentration of organic matter or hydrogen sulfide, the carbon ultimately becomes saturated and can adsorb no more of these impurities. Some success has been reported in treating the beds with high dosages of household hypochlorite bleach to “burn off” the absorbed impurities, and extend the life of the bed. However, in time it usually becomes necessary to replace the activated carbon bed.
If the activated carbon is used for the removal of chlorine, some of the carbon is actually consumed in the process. In such cases, small amounts of carbon must be added to the bed to replace that which is lost.
Chemical feed pumps may also be used in some and odor cases in the treatment of color, taste and problems. A feed of an excess of chlorine, usually in the form of household hypochlorite bleach solution, is capable of oxidizing many of the impurities.
A relatively new process, called “reverse osmosis,” has been developed in recent years. In this process, the water to be treated is forced against a semi-permeable membrane. This membrane allows some of the water but few of the impurities, including dissolved materials, to pass through. A small amount of the water containing the rejected impurities flows to waste. Thus one flow of water enters the unit, and two streams come out; one a stream of water with most of the impurities removed, and a second containing the concentrated impurities.
Unfortunately, large equipment of this type is relatively expensive, and thus it is not generally feasible to use it to treat all of the water in a home. However, small units which treat only the water to be used for cooking and drinking are available for household use. These units operate continuously, and contain a reservoir for the treated water. To avoid clogging of the membrane in the reverse osmosis unit, filtration of the water to the unit is often recommended. However, no regeneration or backwashing is required, and the membranes do have extended lives.
Treatment Methods-Equipment Application
Let us look at the ways that these pieces of equipment may be used to treat the problems of turbidity, color, odor and taste.
If the problem is simply one of suspended solids or turbidity, with no evidence of color, odor or taste, one of the mechanical filters may be used to remove the solid particles. Depending on the specific circumstances, a large tank-type unit may be used to filter the entire water supply after the water leaves the pressure tank. If the turbidity is of such a nature that complete removal is not achieved, a cartridge filter might be used in the water line to a specific tap. Alternately, if the turbidity is not a major problem, and really is objectionable only in water used for cooking and drinking, the cartridge filter may be used on specific taps and the tank-type unit may not be necessary.
Similar approaches may be used with activated carbon filters. The large tank-type units may be used to remove turbidity and many tastes and odors from the entire water supply, or the cartridge filters may be used in individual water lines.
If the load of organic matter is very high, and the activated carbon becomes saturated rapidly, chlorine can be used to oxidize the organic matter. A chemical feed pump can be wired to operate with the well pump, and a solution of household hypochlorite bleach fed into the water line between the pump and the pressure tank. The pressure tank serves as a mixing vessel and allows at least some time for the oxidation of the organic matter. In some unusual case, it is necessary to install extra tanks for more contact time when the organic matter resists oxidation. The chemical feed is adjusted to provide a chlorine residual of 3 to 5 ppm after the tank(s).
Since this amount of chlorine would be objectionable for cooking and drinking, and the water could contain any precipitated iron or other materials, an activated carbon filter may be used in the water line to remove both the precipitated matter and the excess chlorine. Treating the water by filtration and chlorination has the advantage of removing iron as well as disinfecting the water.
Hydrogen sulfide in the water is a special case, as noted above. At very low concentrations, an activated carbon filter may be used, and occasional treatment with bleach will extend the life of the carbon. For low to moderate “sulfur” concentrations, an iron removal filter may be effective in correcting the problem. For higher concentrations, the chlorination system with activated carbon filtration is the best solution for hydrogen sulfide removal.
In all of these cases, the equipment should be installed ahead of a water softener, if one is used, to protect the softener against fouling, and also to permit the use of unsoftened water in some water lines.
Finally, if the taste is due to high concentrations of dissolved materials, the small reverse osmosis units are recommended. Since these units too are subject to fouling if the water contains organic matter, turbidity, iron, and similar contaminants, pre-filtration is often recommended. If a water softener is used for hardness removal, the softened water should be fed to the reverse osmosis unit to improve the mineral removal performance and extend the life of the membrane.
IMPORTANT: The determination of which treatment method is best should be made only after careful consideration of many factors such as economics, water quality characteristics, 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.