A disinfection choice for public and private water systems.
Purification of drinking water containing microbiological contamination requires some form of disinfection treatment to kill or render microbiological organisms harmless.
Of the available disinfection treatment methods for private water systems, chlorination in the most commonly used.
Private Drinking Water
Nationwide, there are approximately 170,000 public water systems (PWS) protected by focused teamwork at the federal, state and local levels. A PWS is defined as a system for the provision to the public of water for human consumption through pipes or other constructed conveyances, if such a system has at least 15 service connections or regularly serves at least 25 individuals for at least 60 days a year. Under The Safe Drinking Water Act, a clear partnership is established between the EPA and the states to protect drinking water supplies and sources. Currently in the United States, 283.7 million people obtain their water supply from public water systems while 42 million have private drinking water supplies.
Typically, private drinking water supplies are drawn from the ground water through wells, but some households also use water from streams or cisterns. Private systems, while not federally regulated, are vulnerable to biological contamination from sewage, improper well construction and poor-quality water sources. The EPA encourages households to take special precautions to ensure the protection and maintenance of their private drinking water supplies.
Precautionary Measures and Testing
In households, water used for drinking and cooking should be free of microorganisms that cause illnesses such as gastroenteritis, typhoid fever, cholera and dysentery. To detect contamination problems early, the EPA recommends that private water supplies be tested annually for nitrate and coliform bacteria. Homeowners should test for other potential contaminants such as radon and pesticides if a problem is suspected. When repairs or improvements are made to a private well, testing also is recommended.
The biological quality of drinking water is determined by tests for coliform group bacteria. The presence of coliform typically indicates disease-causing microorganism contamination. The standard for coliform bacteria in drinking water is less than 1 coliform colony per 100 milliliters of sample (<1:100 ml).
If a contaminant is detected, test results from laboratories include the concentration of the contaminant and an indication if the concentration exceeds a drinking water quality standard. The initial presence of coliform requires retesting the water supply. The retested sample should be analyzed for fecal coliform organisms. If positive, prompt action is required.
Conditions Causing Contamination
Coliform bacteria may contaminate well water from the following conditions.
- Loose or worn seal on the drilled and driven well.
- Defective, too short or inadequately sealed casings in the drilled and driven well.
- Cracked or loose-fitting cover on a dug well.
- Defective wall lining or cracked concrete apron of a dug well.
- Repair to well structure of submerged pump.
- Flooding of the well from hurricanes, floods, heavy rainfall or other natural disasters.
- Breakdown and repair of a septic tank system.
- Wells drilled into fractured rock formations.
- Wells located in areas where ground waters are subject to continuous contamination from outside sources.
The addition of chlorine to a water supply readily combines with chemicals dissolved in water, microorganisms, plant material, odors and colors. Chlorine that is “used up” by these components comprises the chlorine demand of a treatment system. Sufficient amounts of chlorine must be added to a water supply to meet the chlorine demand and provide residual disinfection. Free or residual chlorine is the amount of the disinfectant that does not combine with components in the water, and the breakpoint is the point in which free chlorine is available for continuous disinfection.
Prior to installing a permanent means of chlorine disinfection onto a well, a homeowner must be sure that contamination originates from the ground water itself and is not a temporary condition. When the “one time only” addition of chlorine to a water supply—known as shock chlorination—does not eliminate bacteriological problems, a permanent means of continuous chlorination is required (Table 1).
Permanent and continuous chlorination of a private water supply can be done through a chlorine pump, solid feed unit, aspirator or suction device. Unlike public water systems that utilize chlorine in the gaseous form, a private water system uses liquid chlorine or dry chlorine depending on the chosen method of chlorination.
In 1979, the EPA adopted a trihalomethane (THM) regulation limiting the allowable level of this carcinogenic disinfection byproduct in drinking water. The maximum contaminant level for total THMs in drinking water is 0.10mg/L. THMs are chemicals that are formed, primarily in surface water, when naturally occurring organic materials combine with free chlorine. Since groundwater rarely has high levels of organic materials such as humic and fulvic acids, chlorinated private wells contain much lower levels of THMs.
Despite the popularity of chlorination, the treatment method has limitations when attempting to disinfect private wells that are heavily contaminated and possess protozoan parasites such as Cryptosporidium parvum and Giardia lamblia. Ultraviolet (UV) disinfection and reverse osmosis (RO) filtration both have proved effective at inactivating specific protozoan. Both methodologies purify water without the addition of harsh chemicals or the need to handle hazardous materials.
UV disinfection is the process where microorganisms are exposed to UV light at a specified intensity for a specific period of time. This process renders the microorganism to be considered “microbiologically dead.” UV light penetrates the cell wall of the microorganism affecting the DNA by fusing the Thyamine bond within the DNA strand, which prevents the DNA strand from replicating during the reproduction process. This fusing of the Thyamine bond is known as forming a dimerase of the Thyamine bond. If the microorganism is unable to reproduce/replicate then it is considered to be “microbiologically dead.” While providing a 99.99 percent inactivation of bacterium and viruses, UV will have no effect on water chemistry.
RO filtration uses a semipermeable membrane that enables the water being purified to pass through while contaminants remain behind. Traditionally, osmosis refers to the attempt to reach equilibrium by dissimilar liquid systems trying to reach the same concentration of materials on both sides of a semipermeable membrane. Reversing the osmotic process is accomplished by applying pressure to stop the natural osmosis process, creating RO. RO removes virtually all organic compounds and 90 to 99 percent of all ions from the processed water. In addition, RO can reject 99.9 percent of viruses, bacteria and pyrogens.
Alternative methods of treatment for private water supplies such as UV and RO do not provide a residual effect like chlorination. Without a residual, the regrowth of contaminants further down in the distribution system becomes possible.
Chlorination generally is an inexpensive treatment method and proven to be effective against a broad spectrum of pathogens. Although it has shown itself to be effective against waterborne bacteria and viruses, it provides only some degree of protection against protozoan agents. Nevertheless, a private water supply should utilize a treatment system that kills or neutralizes all pathogens in the water through an automatic, simply maintained and safe process. Chlorination remains the most popular choice of treatment for private water supplies by homeowners.