Disinfection for the purpose of this discussion is to control bacteria, viruses and cysts in potable water for residential systems. The control of the bacteria and viruses is done with either chlorine or Ultra Violet (U.V.) lights. Ozone will not be discussed because at present there is no good way to control or monitor it in a residential system. Cysts only found in surface water supplies (lakes, streams, some shallow wells, etc...) are usually controlled by mechanical filtration.

Chlorination top

Chlorination is the only method recognized by the EPA and the State of Maine for the control of bacteria and viruses in public water supplies. It provides for residual disinfection in the distribution lines. The residual provides an added margin of safety for any time there might be contamination introduced into the distribution line after the treatment plant or system. The contamination in the distribution lines could be introduced through small leaks in the line or a cross connection due to incorrect plumbing in buildings connected to the line.

Chlorination with a solution feed pump top

Chlorination is usually accomplished by injecting a chlorine solution with a positive displacement solution feed pump. The pump is designed to come on every time the well pump starts. The amount of chlorine introduced is controlled by the solution pump stroke and or frequency of stroke. The water is then sent to a retention tank that is sized to allow at least 10 minutes of retention. The concentration of free chlorine is to maintained at .4 ppm - .8 ppm. The concentration of the chlorine is to be measured after passing through the retention tank. Only free chlorine needs to be measured.

To monitor the system the solution levels and the free chlorine needs to be checked and or tested on a regular basis. The chlorine can be easily tested with a test kit like those used for pools. The solution levels requires only a visual inspection. The safest protocol is to check solution levels and test the chlorine every day to be sure the system is working. The chlorine levels should be checked at a point of use in the distribution system. In a residential situation this means at one of the faucets. If the system has been provided with dechlorination (such a carbon filter), the chlorine concentration must be checked just before the equipment providing the dechlorination.

Chlorination with dry pellet well chlorinators top

Chlorine can be introduced directly into the well. This is usually done with devices that drop dry pellets of chlorine ( sodium hypochlorite) into to the well. The chlorinator comes on every time the well pump does. A bottle of chlorine tabs is inverted with the mouth over the top of a horizontal rotating disk. The disk has holes in it. The holes in the disk pick up chlorine tablets as it passes under the mouth of the bottle and moves them to a hole that leads to the well. The amount of chlorine is usually controlled by changing the number of openings in the disk that moves the tablet.

There are several advantages to this type of system.

1. The well provides extra retention, however, using this system to control pathogenic bacteria will require the use of a retention tank to provide for a known amount of retention.
2. The homeowner doesn't have to mix solutions.
3. In cases where iron bacterial slime is severe, the slime is controlled providing for additional pump life.

Sizing Retention Tanks top

Retention tanks are sized based on the expected continuos peak flow. Once the peak flow is determined, install a flow restrictor equal to this flow to be sure the flow does not exceed that for which the system is sized. In most cases, this will be assumed to be 5 gallons per minute (see "Sizing the System"). If one wishes to maintain a retention time of 10 minutes then the tank must be sized to hold the amount of water used in 10 minutes. At 5 gallon per minute flow the tank needs to have a capacity of at least 50 gallons ( see example).

Example - 10 minutes X 5 gallons/minute = 50 gallons

Since there will be some mixing in the tank, the tank will have to be larger than 50 gallons. In a true continuous flow situation, the tank would need to be sized at three times this capacity (150 gallons) or there would have to be baffles installed in the tank to prevent mixing and to provide for adequate retention. In a residential situation, there is never true continuous flow so the tank is sized to take care of the largest volume at any peak usage (such as filling a washing machine or morning showers). The smallest standard tank size that will hold at least 50 gallons is a 80 gallon tank. An 80 gallon size would allow for the total volume of water that is likely to be used at the peak flow of 5 gallons per min.

What margin of safety does this give? If we assume there has been no water used for 10 minutes, at the beginning of the use there is 80 gallons of treated water in the tank and during the use there will be another 27 gallons (divide 80 by three to allow for mixing) of water made. This will provide a total volume of treated water of 107 gallons during any true continuos use of 5 gallons/min, a very unlikely occurrence.

If watering the lawn or other long term uses need to be considered then, increase the size of the retention tank and make sure to install flow restrictors on the system.

UV top

If designed properly, a system with an U.V. light can provide a very safe and user friendly. The light does not, however, provide for any residual disinfection in the distribution system. This lack of residual means that extra care must be taken in installation to make sure the distribution system is thoroughly disinfected during the installation process. U.V. lights alone should not be used on any system that there is a potential for contamination in the distribution system such as underground lines or cross connections with other supplies. Cross connection is not uncommon in cottages on lakes where there maybe a backup supply of water provided from a second source such as the lake.

The water the U.V. light is treating must be relatively clear and colorless to allow the light to pass through it to kill the bacteria and viruses. The water should be free of any materials that might build up on the surface of the bulb and could decrease the output from the bulb. Algae, iron, manganese and hardness are the most common materials that could cause this problem.

There are 3 different levels of safety that can be provided with U.V. lights. They are as follows:

1. U.V. light with an inspection hole to determine if the light is on.
2. U.V. light with a photo sensor that will automatically determine if the light is on or off. If the light is not on, an alarm will sound and or the water will be shut off with a solenoid valve.
3. U.V. light with a transmitance detector that will not only determine if the light is on but will also measure the intensity of the light after it has passed through the water. If the light that transmitted through the water is not sufficient, the detector will shut the water off to the system until the problem is corrected.

If a U.V. light is used by itself (no other disinfection), the type described in #3 above should be used. If chlorination is used with retention then the U.V. light system described in #2 above would be satisfactory because it is used as part of a redundant system. The combination of chlorination and U.V. light would be a good choice for any situation where residual is needed and or redundancy is required.

Filtration for cysts top

Filtration of cysts is only required where the source of water is a surface supply such as a lake, pond, or stream. In most cases, the filtration is only practical at the point of use. This is because the filtration is so fine that large volumes of water will plug the filter quickly. Only filters or filter systems rated and tested for cyst removal should be used.

There are a number of systems on the market that are NSF rated for the removal of cysts. All of these filters are of the cartridge type. In most cases, these systems have carbon in them to remove the chlorine found in public water supplies. Units with carbon should only be used on a system that has chlorine residual to make sure there is no bacterial build up in the filter.

There are cartridges that do not have carbon that have been tested and rated for cyst removal. These cartridges are not whole systems and require the selection of a cartridge, sump system and faucet if used as a point of use system.