Chlorination units are the
most common oxidization units sold in North America, they are usually installed
as Point of Entry (POE) systems. They are generally placed at the entrance of
the water system into the house to service all or most incoming water.
Continuous Chlorination --
Effective continuous disinfection requires the chlorine be consistently and
thoroughly mixed with the water and have
sufficient contact time to kill all disease-causing and nuisance organisms.
The most common solution the
Hypochlorinator, uses a positive displacement chemical feed pump to inject a
chlorine solution into the water supply.
Chlorination is effective at destroying or assisting in the filtration of :
- bacteria, parasites
and other organisms
- nuisance iron and sulfate
- oxidizes soluble iron, manganese and hydrogen
so they can be filtered out
- some organics
- bacteria and viruses
Some pathogens such as
Cryptosporidium cysts are not totally eliminated by chlorination. In fact,
Cryptosporidium survivor viability is not impaired by passage through
chlorination, not even chlorine at concentrations straight out of the bleach
bottle. For this reason chlorination of water susceptible to these
pathogens needs to be augmented with a pre-filter of a pore size equal to or less than 3 microns.
The amount of chlorination and
the time required for disinfection depends on the concentration of chlorine,
temperature, and pH of the water, the amount of organic matter in the water, and
the flow rate of the system. Disinfection for most waterborne disease-causing
organisms occurs after 20 minutes of contact time when the pH is between 6 and 8,
and the free available chlorine residual is in the range of 3.0 to 5.0 parts per
Continuous chlorination systems
consist of a chemical metering device that feeds chlorine in sufficient amounts
to kill bacteria. Continuous chlorination is
expensive and requires frequent and intelligent management. Because chlorination
needs to be closely monitored and maintenance is essential, it should not be
considered for disease-causing bacteria control until other alternatives have
been fully explored.
In layman's terms chlorine feed rates
are about 1 cup of 5 percent laundry bleach per 300 gallons of water. This rate
depends on water temperature, pH, and pumping rate. An inexpensive chlorine
residual kit (readily and cheaply available from water supply and pool supply distributors)
can determine if the feed rate should be adjusted up or down to
obtain the proper chlorine residual. A chlorine residual of about 3 to 5 parts per million should
remain to indicate that disinfection is complete. If chlorine taste is a problem, use a
POU carbon filter to remove excess chlorine from drinking water.
Continuous chlorination is
described within this page, for a description of
chlorination follow this link.
Free available chlorine
residual is the amount of chlorine remaining in the water after the chlorine
demand has been met. If the chlorine demand is greater than the amount of
chlorine introduced, there will be no free available chlorine residual.
Unless a chlorine residual is present, adequate amounts of chlorine have not
been added to the water.
Chlorine demand is the
amount of chlorine required to kill bacteria, oxidize iron or other elements
in the water, and oxidize any organic matter that may be present. There is
no easy way to determine the amount of chlorine required—chlorine is added
until the chlorine residual stays within the 3 to 5 ppm range.
Contact time is the amount
of time that the chlorine is present in the water. The combination of
chlorine residual and contact time determines the effectiveness of the
chlorination treatment. The bacterial "kill factor" is defined as
the product of free available chlorine residual and contact time. Thus the
greater the chlorine residual the shorter the required contact time for
Continuous chlorination is a
necessity for surface water supplies such as lakes, springs, ponds or cisterns.
When chlorinating to control disease-causing bacteria, you
need an alarm device that indicates when the chlorine solution supply needs
Because effective organism kill
is a function of contact time, the chlorine solution should be injected into the
water as close to the source as possible. Adequate contact time for
disease-causing bacterial kill depends upon free chlorine residual, water
temperature, water pH (acidity), and the specific organism. Continuous
chlorination typically uses a chlorine residual of 3 to 5 ppm. This level is
considerably higher than that of municipalities (0.2 to 0.5 ppm) where the large
distribution system provides a long contact time. This level of residual
chlorine may cause a taste and/or odor that is objectionable.
At the 3 to 5 ppm level,
adequate contact time for surface water supplies is in the range of 2 to 7
minutes to develop the proper bacterial kill factor. For well water, a contact
time of 2 to 3 minutes is normally sufficient. In any case where disease-causing
bacteria are involved, tests should be conducted after installation and continue
on a regular basis to insure a safe water supply.
The piping in home water systems usually provides
little contact time. The time between the pump and the nearest faucet is usually
less than one minute. A coil of plastic pipe can be used to increase contact time.
The length of pipe required depends on the pipe diameter and the flow rate. For
example, to achieve a 4-minute contact time with a 10 gallon per minute flow
rate, you would need 128 feet of 3 inch nominal diameter pipe or 480 feet of 1.5
inch nominal diameter pipe, enough pipe to fill a bathroom!. Specially baffled
tanks are commercially available to increase contact time.
Some people object to the smell and/or taste of very small amounts
(0.2 to 0.5) of chlorine in public systems This smell is much more
significant at residual rates of 3 to 5 ppm required within a smaller
system. Activated carbon or charcoal filter can be used to remove
the chlorine taste and smell from the water.
For use in the home, chlorine
is readily available as sodium hypochlorite commonly known as household bleach.
This product contains 5 percent available chlorine. Chlorine is also available
as calcium hypochlorite, which is sold in the form of dry pellets with up to 70%
A dry pellet chlorinator are
also available Pellets are injected into the well at a calculated rate. This type of
system uses the well casing as a retention tank, permitting the chlorine to kill
bacteria and oxidize iron and manganese.
Chlorine will not completely
oxidize manganese unless the water is very alkaline (pH is above 9.5).
Suspected carcinogenic compounds, called
from the partial oxidation of organics by chlorine. To remedy
this, activated carbon filtration or reverse osmosis units after the contact
tank and just before the point of use faucet should be a part
of home chlorination systems. Because THMs are very seldom associated with groundwater, they are primarily a
concern where surface water supplies are used. THMs can be removed from drinking water through use of an activated
Shock Chlorination --
chlorination is recommended whenever a well is new, repaired, or tested to be
contaminated. Its use is essential after a flood or entrance of surface water
into the well. Before investing in a
continuous chlorination system, it is wise to try repeated shock chlorination
applications. Our personal well took three shock applications before
testing no longer indicated the presence of biologicals within our water supply.
Chlorination in North
America has a 94 year history. In 1908 Chicago the first, and Jersey city
following shortly thereafter were the first North American cities to
continuously chlorinate their water in response to typhoid fever
There are several sites on the
web with historical background and information on chlorination