Water Treatment Chemicals

Chemicals serve many purposes in the water treatment process. They remove undesirable substances from water, kill harmful bacteria, help prevent cavities, and help prevent the corrosion of water pipes.

Potassium permanganate (KMnO₄) is an oxidizing agent normally added to the water coming into the plant from the lake. It helps remove iron, manganese, and objectionable tastes and odors. Because the raw water is drawn from below the surface of the lake, dissolved oxygen is low. When the oxygen level is low, iron and manganese are kept in solution. If they are not removed during the treatment process, these metals will cause stains on laundry and plumbing fixtures. When iron and manganese are oxidized, they become insoluble in water and can then be removed by sedimentation and filtration.

Potassium permanganate is also used to oxidize the organic contaminants that cause taste and odor problems. In the past, chlorine was used for these problems, as well as for iron and manganese control. However, research has shown that chlorine plus organic material can intensify some unpleasant tastes and odors and can produce chlorination byproducts such as trihalomethanes. By using potassium permanganate instead of chlorine for pretreatment, chlorine can be added later in the treatment process and be used only for disinfection. 

Alum

Aluminum Sulfate is Al₂(SO₄)₃ and is commonly called filter alum. It is purchased as a liquid at a concentration of 48 percent alum in a water solution. This alum solution is then added to the incoming raw water at the rate of 18-24 milligrams per liter (mg/L). Alums are used in such everyday products as baking powders and deodorants. In water treatment, alum is used as a coagulant, which binds together very fine suspended particles into larger particles that can be removed by settling and filtration. In this way, objectionable color and turbidity (cloudiness), as well as the aluminum itself, are removed from the drinking water.

Lime

Hydrated lime is Calcium Hydroxide or Ca(OH)₂. It is an alkaline compound that is added to the water for pH adjustment. Since filter alum is an acidic salt, it lowers the pH of the water, and lime is used to neutralize this effect. Lime is added between the sedimentation and filtration processes at the rate of 10 - 20 mg/L.

Chlorine

Chlorine is added to water to disinfect it or improve its quality. Chlorine destroys pathogenic microorganisms, oxidizes undesirable elements, such as iron and manganese, and reduces some tastes and odors. After these needs have been met, some residual chlorine remains in the water to protect it from further contamination until it reaches the customer’s tap. Before the use of chlorine, waterborne diseases such as typhoid fever were the source of devastating epidemics. Chlorine is the most important chemical added to water in terms of public health.

Three types of materials are commonly used as a source of chlorine: gaseous chlorine, calcium hypochlorite tablets, and sodium hypochlorite solution. Mainly because of safety concerns, Greensboro switched from chlorine gas to sodium hypochlorite in 1999.

Sodium hypochlorite, NaOCl, is a clear, greenish-yellow solution commonly used in bleaching.  Household bleach usually contains about 5 percent available chlorine. For disinfection of drinking water, a 15 percent sodium hypochlorite solution is used to produce a 1-1.8 mg/L dosage of chlorine in the finished water.

Polyphosphate

Calciquest is a liquid chemical solution containing a long chain polyphosphate polymer. It is designed to isolate iron, manganese, and calcium to keep them in solution. It also prevents corrosion of water lines. Calciquest is added to treated water at the rate of 1.0 mg/L.

Fluoride

Fluoridation is used to maintain fluoride concentrations in drinking water at levels known to reduce tooth decay in children. At optimum levels, fluoride can reduce the incidence of tooth decay among children by 65 percent. The amount of fluoride ingested depends on the amount of water consumption which usually depends on the temperature in a region. The State has recommended control limits of 0.7 to 1.2 mg/L fluoride. Greensboro has established 1.0 mg/L as the optimum fluoride level for this area. Fluoride is received as a 25 percent solution of hydrofluosilicic acid (H₂SiF₆). This solution is fed as a final treatment step before the water enters the distribution system.

Questions?

The City's drinking water laboratory will provide a professional water analysis upon request. If you have questions about your drinking water, call 373-7527.