Treatment methods for continuous disinfection of water

Treatment methods for continuous
disinfection of water
Continuous Chlorination
Municipal water treatment plants throughout the United States continuously add chlorine
to ensure that their water is free of bacteria. Chlorination treatment systems are
basically comprised of a feed system that injects a chlorine solution (sodium
hypochlorite) or dry powder (calcium hypochlorite) into the water ahead of a storage
tank. Most chlorinators use positive displacement feed pumps to meter the chlorine into
the water. Other units may utilize suction-type chlorinators or pellet droppers to deliver
the chlorine. The raw water entering the chlorinator should be perfectly clear or free of
any suspended sediment or cloudiness in order for the chlorine to effectively kill the
bacteria. A sediment filter is routinely installed ahead of the chlorinator to remove small
amounts of suspended material. The chlorine that is injected into the water is
consumed as it kills bacteria. The chlorine is also consumed by impurities in water like
iron, hydrogen sulfide, and organic materials.
The amount of chlorine needed to kill bacteria and oxidize all of the impurities in the
water is known as the chlorine demand. Thus, the total amount of chlorine that must be
injected into the water will depend on the chlorine demand of the raw water. Other
water characteristics like pH and temperature will also affect the amount of chlorine that
must be injected into the water. The goal of continuous chlorination is to provide enough
chlorine to satisfy the chlorine demand and still allow for approximately 0.3 to 0.5
milligrams per liter of residual chlorine in the water. This residual chlorine is then
available to kill bacteria that may enter the water after the chlorinator.
The time required for the chlorine to kill bacteria is known as the contact time. The
required contact time will vary depending on water characteristics but a general rule-ofthumb is to provide approximately 30 minutes of contact time. Standard pressure tanks
are usually not large enough to provide sufficient contact time so a larger intermediate
holding tank may need to be installed. Sufficient contact time can also be achieved by
running the water through coiled pipes. Contact time requirements can be shortened by
increasing the chlorine dose (superchlorination) but this may require the addition of a
carbon filter to remove the objectionable chlorine taste and odor. Continuous
chlorination treatment systems require significant maintenance. Chlorinators must be
routinely checked to ensure proper operation and chlorine supplies must be periodically
replenished, usually on a weekly basis. Both liquid and solid forms of chlorine are
poisonous and irritants that must be handled according to specific safety measures.
Ultraviolet Light
Ultraviolet (UV) light has become a popular option for disinfection treatment because it
does not add any chemical to the water. The unit consists of a UV light bulb encased by
a quartz glass sleeve. Water is irradiated with UV light as it flows in a thin film over the
glass sleeve. The untreated water entering the unit must be completely clear and free
from any suspended sediment or turbidity to allow all of the bacteria to be irradiated by
the light. A sediment filter is often installed ahead of the UV unit to remove any
sediment or organic matter before it enters the unit. The quartz glass sleeve must also
be kept free of any film. Overnight cleaning solutions can be used to keep the glass
sleeve clean or optional wipers can be purchased with the unit to manually clean the
glass. The unit requires electricity and will cause a small but noticeable increase in your
electric bill (about $2-4/month).
The disadvantage of this system is that it only kills bacteria inside the unit and does not
provide any residual disinfectant for bacteria that may survive or be introduced into the
plumbing after the UV light unit. Maintenance requirements are minimal for UV units but
the light bulb will slowly lose intensity over time and will require replacement about once
a year. Some units come equipped with a UV light intensity sensor that can detect
when the bulb is not emitting sufficient UV light. These sensors add to the initial cost of
the unit but may pay for themselves in increased bulb life. The UV light treatment
system is not recommended when the total coliform test result exceeds 1000 cfm/100
ml or the fecal coliform test exceeds 100 cfu/100 ml.
Other Options
There are numerous other treatment processes that can be used to disinfect water.
They are not recommended for continuous disinfection for a variety of reasons.
Boiling water for about one minute effectively kills bacteria. This method is frequently
used to disinfect water during emergencies or while camping. Boiling is time and energy
intensive, however, and only supplies small amounts of water. It is not a long-term or
continuous option for water supply disinfection.
In recent years, ozonation has received more attention as a method for treating water
quality problems including bacterial contamination. Like chlorine, ozone is a strong
oxidant that kills bacteria but it is a much more unstable gas that must be generated
onsite using electricity. Once the ozone is produced, it is injected into the water where it
kills the bacteria. Ozonation units may be much more costly than chlorination or UV
light systems. However, they may be useful where multiple water quality problems must
be treated such as disinfection in combination with removal of iron and manganese.
Iodine has been used in the past much like chlorine to continuously disinfect water.
Iodination is no longer considered a permanent disinfection option due to health
concerns related to long-term exposure to low levels of iodine residual in water. The
U.S. Environmental Protection Agency now recommends iodination only for short-term
or emergency disinfection.
A Practical Approach to Calculating Dose Values for Water Disinfection
UC-Davis Vegetable Research and Information Center
Postharvest Chlorination - Basic Properties and Key Points for Effective Disinfection
UC-Davis Division of Agriculture and Natural Resources
Introduction to Oxidation-Reduction-Potential as the Standard of Postharvest Water
Disinfection Monitoring
UC-Davis Vegetable Research and Information Center
Water Disinfection with Ozone - Science and technology of ozone as a disinfectant
Virginia Tech University
Wash Water Sanitation: How Do I Compare Different Systems?
Davis Fresh Technologies, LLC
Adapted from “Treating Coliform Bacteria in Drinking Water”, Bryan R. Swistock,
Extension Associate, William E. Sharpe, Professor of Forest Hydrology
Paul D. Robillard, Associate Professor of Agricultural Engineering, Penn State
Cooperative Extension Publication