Drinking water treatment and disinfection

Drinking Water Treatment

Disinfection of drinking water is vital to protect public health from waterborne diseases.

Water treatment and disinfection is such an important part of public health that the US Centers for Disease Control and Prevention (CDC) has called it one of the ten greatest public health achievements of the 20th century.

To make untreated source water (e.g., lake and river water) safe to drink, water must be carefully treated using a combination of physical and chemical processes.  Water treatment consists of removing organic material, particles, bacteria, and other contaminants and disinfection to kill or inactivate microorganisms.  

Our water treatment process includes: coagulation and flocculation, sedimentation, filtration, and disinfection. The first step, coagulation and flocculation, consists of adding a chemical to the water that allows particles to bind together forming larger particles called “floc.” The floc then settles out in the sedimentation process. After the floc has settled, water then moves through the filtration process, which is where we begin primary disinfection to kill and inactive microorganisms. Our primary disinfectant is free chlorine in the form of liquid bleach.  We then add fluoride for dental health; make sure the pH of the filtered water is at the proper level; and add a phosphate compound to inhibit corrosion of lead and copper. The water then flows to a large tank where additional chlorine is added to ensure proper disinfection.  As the water leaves the plant, ammonia is added to the water and combines with chlorine to form "chloramines."  This is our secondary disinfectant, the purpose of which is to maintain a disinfectant residual as water travels through our network of pipes and storage tanks from the water plant to our customers.  Since 2002, we have used chloramines as our secondary disinfectant all months except March.  During March we use chlorine as both our primary and secondary disinfectant.

For more information about water disinfection and the different types of disinfectants available, see the section below on water disinfection.

More information about Water Treatment is available at: US CDC

Drinking Water Disinfection Options

Disinfection of drinking water is vital to protect public health from waterborne diseases. 

The water treatment process includes two disinfection steps: primary disinfection and secondary disinfection. The purpose of primary disinfection is to kill and inactivate microorganisms, such as bacteria, viruses and protozoa. Options for primary disinfectants include chlorine (free chlorine), chloramines, chlorine dioxide, ozone, and ultraviolet light (UV). Secondary disinfection is then used to keep the water safe and prevent microbial contamination or bacterial regrowth, as water travels through the distribution system. There are minimum levels of disinfectant required depending on which type of secondary disinfectant used. Ozone and UV do not provide a disinfectant residual and therefore, do not protect distribution systems from bacterial regrowth. This leaves chlorine, chloramines, and chlorine dioxide as the only options for secondary disinfection.

Chemical disinfectants can react with naturally present organic matter water to form disinfection byproducts (DBPs), which may pose health risks. DPBs are regulated by the U.S Environmental Protection Agency (EPA). The EPA has set maximum contaminant levels for trihalomethanes (THMs), haloacetic acids (HAAs), chlorite, and bromate.

Water treatment plants must balance proper disinfection with minimizing the formation of DBPs. Strategies used to control DPBs include optimization of the removal of naturally occurring organic matter and selecting a disinfectant that minimizes DPB formation.

Chlorine

Chlorine is a disinfectant used to treat drinking water.  Chlorine is a slightly stronger disinfectant than chloramines.  Chlorine is less chemically stable than chloramines and is often used at several points in the treatment process because the levels drop over time.  Chlorine readily reacts with organic material present in all water to produce disinfection byproducts.

Chloramines

Chloramines is a disinfectant used to treat drinking water. It is formed by carefully adding ammonia to water containing chlorine.  Chloramines provide long-lasting protect`tion from bacterial regrowth because it is more stable than chlorine and does not break down quickly.  Additionally, chloramines do not readily react with organic material and produce lower levels of regulated disinfection byproducts than chlorine. 

More information about disinfectants is available at:

How does OWASA kill germs and bacteria in drinking water?

OWASA treats water with chlorine at several points in the treatment process at the water treatment plant.  Eleven months of the year, we convert the chlorine to chloramines by carefully adding ammonia as the final step in treating our water before sending it into the water system.  Chloramines are an effective and long-lasting disinfectant for killing bacteria and germs.  Chloramines produce lower levels of a group of regulated substances called disinfectant byproducts, which can be harmful at high levels over a lifetime of exposure, than chlorine.

In March, we use chlorine as our secondary disinfectant. This switch is part of a routine program to maintain the water system and ensure a high level of disinfection year round in accordance with recommendations from the NC Department of Environmental Quality.

OWASA has used chloramines for water disinfection in our distribution system since 2002.

For more information, see the sections above on water treatment and disinfectants.

Are chloramines and chlorine in our water safe?

Yes, OWASA water treated with chloramines or chlorine is safe to drink. We carefully control the disinfectant levels in our water to meet the standards in the Federal Safe Drinking Water Act. We routinely monitor the chloramine and chlorine levels throughout the water system. OWASA complies with all State and Federal drinking water standards. 

How can I remove or neutralize chlorine in the month of March, when OWASA disinfects only with chlorine and the water then has chlorine taste or odor?

OWASA does not recommend removing chlorine, except for special uses including dialysis and in fish/amphibian tanks, because chlorine kills bacteria.  However, we recognize that some customers may wish to remove chlorine because of sensitivities.

If you would like to remove the chlorine from the drinking water, you may do so with several methods, including:

  • filtering the water with an activated carbon filter such as a Brita pitcher filter,
  • letting the water sit uncovered in the refrigerator for a day or so,
  • boiling the water for one minute to cause the chlorine to evaporate, or
  • adding a few lemon slices to a pitcher of water. The lemon has ascorbic acid, which will naturally dechlorinate the drinking water. 

How can I remove chloramines?

OWASA does not recommend removing chloramines, except for special uses including dialysis and in fish/amphibian tanks, because chloramines kill bacteria. However, we recognize that some customers may wish to remove chloramines because of sensitivities.

To effectively reduce chloramine levels in drinking water, select a filter, filtration system, or treatment system that has been tested specifically for chloramine reduction under NSF/ANSI Standard 042.

Can water with chlorine or chloramines affect rubber plumbing components such as toilet flappers and washers in water fixtures?

All rubber components have useful lifetimes and require periodic replacement.  For example, expansion tanks for hot water heaters typically have a five-year warranty.  Selecting rubber plumbing components intended for use in water treated with chloramines will increase the useful lifetime of:

  • toilet flappers and
  • supply lines and flexible connectors or hoses.

When replacing  rubber components, be sure to choose a model designed for use in water systems with chloramines.  Note that “chemical resistant” and “chlorine resistant” are not the same as chloramine resistant.  Suggested materials and products we have verified with manufacturers are listed below.

Toilet flappers

  • Chlorazone® is a proprietary rubber which has been tested for chloramine resistance and is used in some Korky® products as well as some specialty flappers for specific brand toilets. Products made of Chlorazone® are labeled as such and as chloramine resistant. Chlorazone® toilet flappers are warrantied for five years compared to the one-year warranty of other flappers.
  • At this time, we are not aware of other toilet flappers or toilet flapper materials that have been tested for chloramine resistance.

Flexible connectors and supply lines

PVC is generally considered non-reactive to chlorine and chloramines.  Flexible connectors and supply lines with PVC cores are more resistant to chloramines than those made with cores of other materials.  Hardware store on-line databases typically allow customers to limit searches by material, including core material.  The following manufacturers make flexible connectors and supply lines with PVC cores, but they also may make some with other materials, so be sure to check thoroughly:

  • Fluidmaster
  • BrassCraft
  • Eastman
  • Watts

Expansion tanks

Expansion tanks are installed on water heaters to protect the heater from the pressure produced as water is heated and expands.  Expansion tanks contain an air bladder that is compressed as the pressure increases.  The air bladder is made of flexible butyl rubber, which is sensitive to both chlorine and chloramines.  When the bladder fails, black material may be visible in the hot water from showers, faucets, etc. and the hot water heater will not be protected from thermal expansion pressure.  Most expansion tanks are warrantied for five years. 

Do chloramines contribute to dezincification of brass?

Current research has not found chloramines to contribute to dezincification of brass.  A 2011 study of chemical and physical factors associated with real plumbing systems found that chlorine and chloramines slightly inhibit dezincification.

Dezincification is the selective release of zinc from brass.  It is a type of corrosion that can affect high-zinc-content brass.  Direct connections of different types of metals, water velocity, and higher temperatures can facilitate dezincification of susceptible brass.  Water chemistry can contribute to dezincification of susceptible brass.  OWASA drinking water is classified as non-aggressive with respect to dezincification on the basis of its chloride level, alkalinity and pH.

In 2009, all major US plumbing codes were updated to address dezincification.  Plumbing codes now require that brass fittings used with plastic plumbing (e.g., PEX) meet NSF/ANSI Standard 14.  This standard includes testing brass fittings containing greater than 15% zinc for dezincification susceptibility.  NSF/ANSI Standard 14 was further updated in 2012.

Do chloramines cause the release of lead from brass or other materials?

Current research has not found a relationship between chloramines and release of lead from brass or other metals.

Some plumbing fixtures and fittings within homes may contain lead.  Fittings and fixtures containing lead should be considered a potential source of lead in drinking water.  In 2014, the Safe Drinking Water Act reduced the maximum allowable lead in all plumbing materials and fixtures from 8% to 0.25%.

OWASA has an effective corrosion control program which includes properly managing the drinking water chemistry and adding a phosphate compound to inhibit the release of lead from household plumbing. The phosphate forms a protective coating inside pipes and fixtures to control corrosion in our public water system and in private plumbing.

For more information about lead in drinking water and how OWASA minimizes corrosion and the release of lead, please click here.

What precautions should kidney dialysis patients take before using our tap water?

Both chlorine and chloramines must be removed from the water used in kidney dialysis machines. Medical clinics that perform dialysis are responsible for purifying the water that enters the dialysis machines.

We have informed the overall community including dialysis clinics and medical facilities about the need to remove chloramines from water used in dialysis machines.  We also provide annual notices about our switch to chlorine during the month of March. Customers with home dialysis equipment should contact the equipment manufacturer or dealer for information about whether any adjustments in operation or maintenance of the equipment are needed.

Do I need to treat my tap water before I use it in an aquarium with fish or amphibians?

Yes. The chlorine and chloramines in our drinking water are toxic to fish and amphibians.  

We disinfect our water with chlorine in the month of March and with chloramines in the rest of the year. These two types of disinfectants must be dechlorinated differently. Chemical treatment or additives are available at most pet stores. For more information on treating water for your aquarium, please contact a pet or fish supply store.

How can I get more information?

We invite you to:

  • call our Water Treatment Plant Laboratory staff at (919) 537-4228,
  • send an e-mail to WTPLaboratory@owasa.org,
  • visit or write a letter to us at 400 Jones Ferry Road, Carrboro, NC 27510, or
  • send us a fax at (919) 968-4464.