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
To protect the public health, a public water supply must be free from disease-causing organisms. Disinfection is one step in eliminating pathogens. Chlorine is the most commonly used disinfectant in public water supplies throughout Ohio. Other disinfectants are also being used. All disinfection systems have merits and limitations. Communities must decide which system gives them the merits they want with the limitations they are prepared to live with.
Disinfection is the last step in removing and eliminating pathogens from a drinking water supply. Surface water is first chemically treated, clarified, and filtered to remove particles and microbes. Fortunately, groundwater is often protected with a layer of unsaturated soil that filters out microbes.
The disinfectant is selected first to kill any remaining pathogens. It is not necessary to kill every microorganism (or sterilize) a water supply to disinfect it. The second purpose of disinfection is to provide a residual that continues to kill microbes throughout the distribution system. The residual guards against recontamination. Third, the residual provides a quick indicator of safe water.
Chlorine kills microorganisms, but it takes some time. Chlorine must be in contact with microorganisms for a few minutes to ensure disinfection. The amount of time depends on a number of things.
| More chlorine | Less time |
| Higher water temperature | Less time |
| Higher water pH | More time |
| Cloudy water (high turbidity) | More time |
Contact time is provided in storage tanks called clear wells. Once the necessary contact time is achieved, the water can be distributed to customers. One of the advantages of chlorine is that it continues to disinfect the water as it moves through the distribution system. This residual disinfection helps protect the customer from contamination after the water leaves the treatment plant. Chlorine residuals are also easily measured at taps throughout the system to verify that all of the customers receive safe water.
While chlorine does an excellent job of killing bacteria, it is not effective at killing parasitic cysts like Giardia lamblia or Cryptosporidium. Both of these organisms have caused outbreaks of diarrhea and gastrointestinal cramps in the United States. To overcome this limitation, surface waters from streams, lakes, or reservoirs must be clarified and filtered along with adding chlorine.
Three formulations of chlorine are used in water treatment systems: chlorine gas, liquid hypochlorous acid (bleach), or powdered hypochlorite (HTH). Chlorine gas and HTH must be dissolved in water before or as it is injected into the water system. Each also offers merits and limitations.
| Chlorine Formulation | Merits | Limitations |
|---|---|---|
| Gas | Low chemical cost Strong disinfectant |
Very hazardous
Needs special handling equipment |
| Liquid | Easy to handle Simple injection equipment |
Requires large volume
Loses strength in storage |
| Powdered | Easy to store Simple injection equipment |
Requires mixing equipment
Medium chemical cost Loses strength in storage Forms deposits on equipment |
It is surprising that some people object to disinfection of drinking water. Unfortunately, the disinfectants available today are not perfect and can introduce undesirable side effects.
One common complaint about some disinfectants is that they change the taste of the water or can create an odor in the water. While not unsafe, this interferes with some people's use of the water.
A more critical concern is the potential for some disinfectants to create toxic byproducts. Some disinfectants react with naturally occurring compounds in the water to create toxic combinations. For example:
Naturally occurring organic compounds + Chlorine + Time = THMs
Most surface water supplies contain naturally occurring organic compounds from the decomposition of leaves, algae, and other living things. Over time, these compounds, called precursors, combine with chlorine and form compounds called trihalomethanes (THMs). Some trihalomethanes are suspected carcinogens.
Because disinfectants are chosen for their ability to kill pathogens, they can present a hazard to water system employees and nearby residents. Disinfectants are often purchased in concentrated form to reduce cost and storage space, but these concentrates increase the hazard.
Because no ideal disinfectant exists, a community must make compromises. Listed below are some of the effective disinfectants used in small water systems. The merits and limitations of each are summarized.
| Disinfectant | Merit | Limitation |
|---|---|---|
| Chlorine | Inexpensive Proven technology Easily measured residual Flexible formulation and equipment choices |
Taste and odor concerns Can form byproducts (THMs) Safety hazard with chlorine gas |
| Combined Chlorine (with ammonia) |
Long lasting residual Lower potential for byproducts |
Taste and odor potential Weaker disinfectant Hazardous to tropical fish and dialysis patients Safety hazard with chlorine and ammonia gas |
| Ozone | On-site generation No taste or odor |
Expensive Ozone off-gas hazard to operator No residual in distribution system Can form byproducts (bromates) |
| Iodine | No electricity needed Easily measured residual |
Taste and odor concerns Health concerns of excessive iodine consumption Can form byproducts (iodized THMs) |
| UV | On-site generation No taste or odor |
Electrical cost Careful equipment care and oversight No residual in distribution system Emerging technology |
Don't be discouraged about all of the concerns with a disinfectant. The system designer and operator can both work to reduce or eliminate a limitation. For example, safety concerns can be addressed through:
Taste and odor concerns are another limitation of some disinfectants, especially chlorine. Chlorine taste and odor problems occur when chlorine residuals are higher than necessary. This is a common problem in systems in rural areas with long distribution lines. The system designer and operator can both work to reduce or eliminate a taste and odor limitation through: