Jumat, 22 April 2011

Turbidity, Color, Odor, and Taste in Domestic Water

Ronald E. Hermanson, P.E.

From the clouds to the tap, water contacts and carries many substances, including gases, minerals, and organic matter. Many of these are impurities that interfere with water use by humans. Some of these impurities are completely dissolved; others are solid, suspended particles in the water that cause objectionable cloudiness, color, odor, and taste.

Finely divided, solid particles that absorb or reflect light cause "cloudy water," or turbidity. These particles are generally undissolvable, inorganic mineral matter or organic matter picked up by water flow over and through the ground. Surface water from lakes, streams, and ponds usually has significant amounts of turbidity from surface water runoff, or from bottom deposits stirred up by water movement. Because the earth normally serves as an excellent filter, it is unusual to find significant amounts of turbidity in water from deep wells.

Water drinkers find turbidity objectionable primarily because the physical appearance of "dirty water" is less appealing than clear, sparkling water. Turbidity caused by inorganic minerals is undesirable because its abrasiveness can erode a plumbing system's pipes and fittings, and score its valve seats and washers. Turbidity caused by suspended organic matter is objectionable because it can stain sinks and fixtures, and discolor laundered fabrics.

Dissolved organic matter causes most water color. Surface water usually has some color, and it is sometimes found in well water. This often occurs in areas where swamps or bogs are common. The water picks up colored substances extracted from decaying organic matter. Dissolved organic matter also makes water unpleasant to drink, frequently contributes tastes and odors, and stains surfaces and materials. Even low amounts of organic matter may produce unpleasant "off" tastes and musty odors in drinking water, and foods and beverages prepared with the water.

Hydrogen sulfide gas in water, or sulfur water, produces a rotten egg odor, corrodes plumbing metals, and rapidly tarnishes silver. Even very low concentrations of hydrogen sulfide produce a strong, easily identifiable odor. Minerals dissolved in water at moderate levels add a taste pleasant to most palates; "flat" water appeals to few people. However, too high a mineral concentration gives the water an unpleasant soda or salty taste.

The U.S. Environmental Protection Agency and the Washington Administrative Code for Public Water Supplies has set limits for physical characteristics of water.

Drinking water should contain no impurity that would offend the senses of sight, taste, or smell. Under general use, do not exceed the following limits:
These characteristics are measured by laboratory tests. Although these tests do not directly measure the safety of the water, they are related to consumer acceptance. One unit of turbidity, 15 units of color, and a threshold odor number of 3 are levels at which these characteristics become objectionable to a considerable number of people.

*Many individuals will accept greater concentrations.
**Milligrams per liter (parts per million).

Water Treatment Equipment

Mechanical filters designed to remove solid particles from water are available in different types and sizes. These filters are fine screens that trap solid particles but allow water to pass.

One type uses specially graded sand or other granular synthetic material as filter media in tanks about the same size as household water softeners. This filter is usually sufficient to handle an entire household water supply. It works very well when suspended particles are relatively large or gelatinous. It is not effective when extremely fine particles cause turbidity. The filter must be backwashed periodically to clean its beds, and to flush accumulated matter.

A second type is the cartridge filter. This unit is usually smaller than the tank type. It is often installed to treat water in a single water line. Instead of loose media, this filter uses media formed into a semirigid cartridge. Although some are designed for mechanical cleaning and reuse, most are designed for replacement when they become clogged. This filter cartridge, available in several ratings based on particle size, can remove extremely tiny particles. However, the cartridge has a higher resistance to water flow and can become clogged quite rapidly.

Activated carbon is known for its ability to adsorb soluble organic compounds and certain gases that contribute tastes and odors to a water supply, such as chlorine and hydrogen sulfide. Activated carbon is widely used in granular form in tank-type filters and as finely divided powder in a cartridge. A granular filter must be backwashed periodically; a cartridge must be cleaned or replaced periodically.

If the water to be treated with granular activated carbon contains a high concentration of hydrogen sulfide, the carbon ultimately becomes saturated. Bed treatment with high dosages of household hypochlorite bleach to "burn off" the adsorbed impurities and extend bed life works well. However, the activated carbon bed must be replaced eventually. Chlorine removal consumes some activated carbon. Small amounts must be added to the bed as replacement. Chemical feed pumps may be used to add bleach. Chlorine bleach also oxidizes many impurities.

Reverse osmosis units force water against a semipermeable membrane. This membrane allows some water but few impurities, including dissolved minerals, to pass through. One flow of water enters the unit, but two streams exit. One stream is purified water. The second contains concentrated impurities.

Unfortunately, the large equipment needed in reverse osmosis to treat all domestic water is relatively expensive and generally not feasible. However, a small unit to treat only cooking and drinking water is available. This unit operates continuously and contains a treated-water reservoir. Filtration of the inflow to the unit prevents membrane clogging. The need for filtration depends upon the concentration of suspended particles in the water supply. A reverse osmosis unit, however, needs no regeneration or backwashing, and the membranes last a long time.

Treatment Methods and Equipment Application

Equipment design determines its application to turbidity, color, odor and taste problems. Mechanical filters effectively remove turbidity caused by suspended, solid particles from water not tainted by color, odor and taste. A large, tank-type unit efficiently filters an entire water supply after the water leaves the pressure tank. A cartridge filter in a water line to a specific tap solves the problem of incomplete removal of turbidity. A cartridge filter on specific taps without a tank-type unit is sufficient to purify water used only for cooking and drinking. Activated carbon filters have similar applications: a large, tank-type unit removes turbidity and many tastes and odors from the entire water supply; a cartridge filter purifies an individual water line.

Very high organic matter and rapid saturation of activated carbon may require chlorine oxidation. You can use a chemical feed pump to inject a solution of household bleach into the water pipe between the well pump and the pressure tank. Wire the chemical feed pump to operate simultaneously with the well pump to properly proportion the bleach and water. The pressure tank serves as a mixing vessel, and allows at least some time for organic matter oxidation. Sometimes it is necessary to install extra tanks for more contact time when the organic matter resists oxidation. The chemical feed should provide a chlorine residual of 3 to 5 ppm after it leaves the tank or tanks. An activated carbon filter in the water line will remove this chlorine, which is too high an amount for cooking and drinking, and any precipitated iron or other materials in the water. Water treatment by filtration and chlorination removes iron and disinfects the water.

Hydrogen sulfide is a special case. At very low concentrations, use an activated carbon filter and occasional bleach treatment to extend carbon life.

An iron-removal filter effectively removes low to moderate sulfur concentrations. A chlorination system with activated carbon filtration is the best solution for higher concentrations of hydrogen sulfide.

In all of these applications, the equipment should be installed ahead of a water softener to protect the softener against fouling, and to permit use of unsoftened water in some water lines.

Finally, a small reverse osmosis unit is the most effective if high concentrations of dissolved minerals cause objectionable water taste. This unit is subject to fouling when water contains organic matter, turbidity, iron, and similar contaminants. Prefiltration is the solution. If a water softener is used for hardness removal, the softened water fed to the reverse osmosis unit improves mineral removal performance and extends membrane life.

The best treatment method results from careful consideration of such factors as economics, water quality characteristics, water end-use, water temperature variances, and the inherent limitations of treatment technology. Consult local water treatment representatives before purchase and installation of any water treatment equipment.

Ronald E. Hermanson, Ph.D., P.E., Washington State University Extension Agricultural Engineer, Pullman.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended.
Revised May 1991. Subject code 376. A. EB0994

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