Ashley Bird, Environmental Engineer, Division of Drinking and Groundwater, Ohio Environmental Protection Agency
Karen Mancl, Professor Food, Agricultural and Biological Engineering, The Ohio State University
Water may be safe to drink but may not be very pleasant to use. Contaminants that make the water taste or smell bad or look cloudy or colored are considered aesthetic contaminants. Some water has high levels of dissolved minerals that cause deposits to form in plumbing and interfere with cleaning.
In contrast, most toxic contaminants cannot be detected by smell, taste, or appearance. Aesthetic contaminants may not only make water unpleasant to use, but may cause customers to distrust the quality of their water, rejecting it in favor of better tasting, but less reliable water supplies or more expensive home water treatment or bottled water.
Aesthetic contaminants include:
| Minerals | Metals | Naturally occurring organic compounds |
Chemicals |
|---|---|---|---|
| Total Dissolved Solids (TDS) | Iron | Color | Chlorine |
| Calcium hardness | Manganese | Taste |  |
| Magnesium hardness | |
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| Chloride | |
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| Hydrogen Sulfide | |
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| Sulfate | |
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Iron and manganese discolor laundry and plumbing fixtures, and produce an unpleasant taste in beverages made with the water. Community water systems in Ohio are required to remove iron and manganese from drinking water where present.
Water systems have three options to remove iron and manganese.
| Oxidation followed by filtration | Oxidizing filters | Lime-soda softening |
|---|---|---|
| Oxidizing agents, like air or chlorine, react with iron and manganese dissolved in water forming particles that can be filtered out. The filters must be cleaned by backwashing to remove the trapped particles. | Filters filled with a special oxidizing media, sometimes called greensand, react with the iron and manganese dissolved in water forming particles. The particles are trapped in the filter. Oxidizing filters are cleaned by backwashing and reactivated with a potassium permanganate solution. | The lime/soda softening process, described under hardness, also removes iron and manganese. |
Some public water systems may choose to remove other aesthetic contaminants making the water more pleasant to use and saving the customers the trouble and expense of treating their water in their home or purchasing bottled water.
Hydrogen sulfide gives drinking water a rotten-egg smell. Groundwater supplies in northwest Ohio and in the coal mining regions of Ohio have hydrogen sulfide. It can be removed from water in two ways. Water systems often use both techniques.
| Air stripping | Oxidation followed by filtration |
|---|---|
| Hydrogen sulfide is a dissolved gas and will come out of water when the water is mixed with large volumes of air. Water is usually sprayed over plates constructed of woven wire, perforated materials, or slats. Sometimes the water is sprayed over containers filled with coarse media. Fans force additional air up through the falling water droplets. | Oxidizing agents, like chlorine, react with hydrogen sulfide forming sulfur particles that can be filtered out. The filters must be cleaned by backwashing to remove the trapped particles. |
Hardwater is caused by high levels of calcium and magnesium dissolved in water. Mineral deposits form inside plumbing and in water heaters, and the minerals combine with soaps and detergents to form films and gray deposits. Calcium and magnesium can be removed from water through two types of treatment processes.
| Ion exchange water softeners | Lime/soda softening |
|---|---|
| In public water systems, these are larger versions of a home water softener. Tanks filled with a special resin, called zeolite, are charged with sodium. As water moves through the resin, the calcium and magnesium are exchanged by the resin for sodium. Periodically, the resin must be recharged with sodium in the form of brine (sodium chloride salt). Since sodium is added to the water, the community needs to make an informed decision about adding extra sodium to their drinking water. Federal law requires informing local health departments about adding sodium to water so that they can in turn notify area physicians. | The chemistry of the water is changed to precipitate, to settle and filter out calcium and magnesium from water. First quicklime and soda ash are added raising water pH and causing the precipitate to form and settle out. The resulting water has a high pH and is still saturated with calcium and magnesium. Carbon dioxide is then added to bring the pH back down to normal levels and redissolve the remaining calcium and magnesium. A certain level of calcium and magnesium in water is desirable because it protects the distribution system and customers' plumbing from corrosion. |
Dissolved salts, measured as total dissolved solids (TDS) over 500 mg/l, can give water a salty taste. Much of the salty taste comes from high levels (over 250 mg/l) of chloride. If sulfate levels exceed 250 mg/l, sensitive individuals may experience diarrhea. Salts dissolved in water are difficult to remove. Membrane treatment technologies are available to desalinate water.
Reverse Osmosis (RO)
Reverse osmosis systems have historically been low yielding and high energy consuming, and producers of large volumes of wastewater. Membranes used in RO systems have carried a high replacement cost. Recent advances in low-pressure reverse osmosis and nano-filtration are making this water treatment approach more feasible.
Taste and odors may occur seasonally when leaves and algae blooms in lakes and reservoirs release naturally occurring organic compounds that dissolve in water. The chlorine added to water to kill pathogens can react with these naturally occurring organic compounds creating additional odors and tastes. Public water supplies use two treatment technologies to reduce taste and odors.
| Potassium permanganate | Activated carbon |
|---|---|
| Potassium permanganate is a purple/black crystal. When dissolved in water it acts as a strong oxidizer. Added just after water is brought into the water system, potassium permanganate oxidizes naturally occurring organic compounds to less objectionable compounds. | A black powder (PAC) or granules (GAC) of activated carbon is added to the water early in the treatment process to adsorb naturally occurring organic compounds. The PAC is then settled and filtered out of the water before distribution. GAC may be incorporated into water filters or used in separate contactors after filtration. GAC is monitored as it adsorbs naturally occurring organic compounds and must be replaced when exhausted. |
If water systems use both potassium permanganate and activated carbon, they cannot be added at the same time. At least 15 minutes of reaction time must be provided for the potassium permanganate before the activated carbon is introduced to avoid interference.
High chlorine levels can also lead to customer complaints of taste and odor. Careful adjustment of chlorine levels insures safe water to customers, while minimizing chlorine taste and odor.