A small-system operator asked Florida Gateway College what processes could be used to remove or mitigate taste and odors in drinking water.

Taste and odors are the most widespread causes of customer complaints. Although they do not pose health risks, the public relations difficulties that result from taste and odor make treatment important. Treatment involves a taste and odor control program that includes monitoring for problems and performing preventive maintenance on the system. In some cases, complex treatment and special equipment are required.

The best approach to treat taste and odor problems is prevention. Algal blooms in reservoirs can be prevented by using copper sulfate, while algae growing on walls of plant basins can be removed through shock treatment with chlorine. It’s also important to prevent anaerobic conditions in reservoirs, the distribution system, and the sedimentation basins, as that can lead to hydrogen sulfide formation. In the distribution system, periodic flushing and adequate chlorine residuals should keep the pipes odor-free.

An integral part of any taste and odor control program is testing the water using the Threshold Odor Test and the Flavor Profile Analysis. These are more subjective than tests for other water parameters because they depend on human perception. However, while it is difficult to perform the tests objectively, they provide valuable information on the cause of the problem, how concentrated the problem chemical is, and how to treat it.

The Threshold Odor Test helps determine the amount of odor present. The water is diluted with odor-free water and is smelled. The dilutions continue until no odor can be discerned. The last dilution at which odor is detected determines the Threshold Odor Number (TON). If several people independently perform this test, the averaged TON can be relatively accurate.

The Flavor Profile Analysis uses a panel of trained judges who taste the water and list which tastes they can detect. Since the tastes are described carefully, this analysis can help in determining which chemicals are at the source of the problem.

These tests can help find the source of a particular problem or as part of routine monitoring. To find the source of a problem, the water should be tested at various locations from the source water to the customer’s tap. Past records can help predict seasonal variations in taste and odor so that problems can be prevented before they reach the customer. Records of past treatment methods can make it much easier to determine which methods should be effective.

The choice of an active treatment method for taste and odor problems depends on the cause of the problem. Active treatment may involve plant process optimization, chemical or mechanical oxidation, air stripping or adsorption.

Optimizing plant processes

Chlorine smells are among the most common problems reported by water customers and are among the simplest to treat. The problem can be corrected by optimizing the chlorine dosage. If the tastes and odors are associated with color and turbidity, or with floating algae, optimization of the coagulation and flocculation, sedimentation and filtration processes should take care of the problem. Optimization is often the simplest and most economical treatment for taste and odor, since the equipment is already in place.

Air stripping

Volatile compounds can sometimes be removed from water using aerators that strip the compounds out. This technique is usually better for controlling odors than tastes, and it is very effective at removing hydrogen sulfide. Trihalomethanes can be removed using aeration, if the aeration follows chlorination. However, in that setup, the operator must be aware that passing air through treated water can add contamination.

Oxidation

Oxidation is another common method to remove tastes, odors, and trihalomethane precursors. It can be mechanical, using an aerator, or chemical, by addition of chlorine, chlorine dioxide, potassium permanganate or ozone. Aerators in general are effective in removing only tastes associated with iron and manganese. In other cases, chemicals must be used.

Chlorine is the most-used chemical for oxidation. The chlorine dosage must be greater than is used for disinfection — a method called superchlorination. After superchlorination has removed the taste and odor, the excess chlorine must be removed using granular activated carbon. Chlorination can deal with fishy, flowery, or grassy odors and with hydrogen sulfide and iron. However, it can make some problems worse, especially those caused by phenols. 

Other chemicals used for oxidation include chlorine dioxide, potassium permanganate and ozone. Potassium permanganate is used to treat organic contaminants, while chlorine dioxide does well against phenolic and algal tastes. Ozone is a strong oxidant that will treat more problems than chlorine, and it lacks the objectionable byproducts.

Adsorption

Adsorption occurs when Van der Waal’s forces pull contaminants out of the water and stick them onto the surface of another material. The material, known as the adsorbent, has a large surface area and large pores, enabling removal of large amounts of contaminants.

Several materials can be used as adsorbents. The most widespread is activated carbon, formed when carbon from wood, coal, peat or nut shells is exposed to heat in the absence of oxygen. The popularity of activated carbon stems from its lack of specificity — it can treat many taste and odor problems.

The other types of adsorbents, synthetic resins and activated alumina, are typically used as filter media. Activated alumina is used to remove excess fluoride as well as arsenic and selenium. Synthetic resins can remove trihalomethanes. However, synthetic resins are costly, and their use is still in developmental stages.

Activated carbon

Powdered activated carbon (PAC) and granular activated carbon (GAC) have advantages and disadvantages. In general, PAC is used more often for its low initial cost and the flexibility of dosage, which allows concentrations to be adjusted to changing contaminant levels. However, PAC has a high operating cost if used continuously. It cannot be regenerated, produces large amounts of sludge, and can break through filters to cause dirty-water complaints. In addition, dust from PAC particles makes handling difficult.

GAC becomes a more economical choice in larger systems or where taste and odors must be controlled continuously. The disadvantages include a high initial cost to buy the filter or contactor, and the tendency of GAC filters to grow bacteria.

To cure its taste and odor problem, traced to iron, the small system water plant decided to use an aeration process as the raw water entered the plant. The aerator consisted of a series of plates with holes that allowed the water to drop from one plate to the next, oxidizing the iron so that it could be filtered out of the stream.

ABOUT THE AUTHOR

John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.