Disinfection

Disinfection

Before water treatment became common, waterborne diseases could spread quickly through a population, killing or harming hundreds of people. The table below shows some common, water-transmitted diseases as well as the organisms (pathogens) which cause each disease. More information on water-borne pathogens can be found in ENV 108.

Pathogen	Disease Caused
Bacteria:
Anthrax	anthrax
Escherichia coli	E. coli infection
Myobacterium tuberculosis	tuberculosis
Salmonella	salmonellosis, paratyphoid
Vibrio cholerae	cholera
Viruses:
Hepatitis Virus	Hepatitis A
Polio Virus	polio
Parasites:
Cryptosporidium	cryptosporidiosis
Giardia lamblia	giardiasis

The primary goal of water treatment is to ensure that the water is safe to drink and does not contain any disease-causing microorganisms. The best way to ensure pathogen-free drinking water is to make sure that the pathogens never enter the water in the first place. However, this may be a difficult matter in a surface water supply which is fed by a large watershed. Most treatments plants choose to remove or kill pathogens in water rather than to ensure that the entire watershed is free of pathogens.

Pathogens can be removed from water through either physical or chemical processes. Some other treatment processes, notably sedimentation and filtration, can remove a large percentage of bacteria and other microorganisms from the water by physical means. Storage can also kill a portion of the disease-causing bacteria in water.

This page will be concerned with disinfection, which is the process of selectively destroying or inactivating pathogenic organisms in water, usually by chemical means. Disinfection is different from sterilization, which is the complete destruction of all organisms found in water and which is usually expensive and unnecessary. Disinfection is a required part of the water treatment process while sterilization is not.

Testing and Requirements

The goal of disinfection is to remove or inactivate all disease-causing organisms in water. However, testing for each type of pathogen individually would be costly and inefficient. Instead, operators focus on three indicators of pathogen removal efficiency. The first two have been discussed in previous lessons - Giardia and viruses. The third test, total coliform, is the most frequently used indicator of disinfection efficiency.

Coliform bacteria are often found in the guts of warm-blooded animals such as humans, but can also be found in plants, soil, water, or air. It is relatively simple to test for the number of coliform bacteria found in water, and their presence indicates that other pathogenic bacteria are also likely to be present. If disinfection removes all of the coliforms from the water, then the operator can safely assume that the other disease-causing microorganisms have also been removed.

The standards for the removal of Giardia and viruses are 99.9% and 99.99%, respectively. After disinfection, standards for total coliform require that water should have 0 coliforms per hundred millimeters of water sampled. If less than 40 samples of water are tested per month, then no more than one sample can test positive for coliform bacteria. If forty or more samples are taken more month, then no more than 5% of the samples can be positive.

Types of Disinfection

The table below summarizes eight disinfection processes.

Disinfection Method	Disinfection Process	Advantages	Disadvantages	Uses
Chlorine	chemical reaction with pathogens	a small dose kills bacteria rapidly; residual can be maintained	in some cases, chlorination can cause the formation of trihalomethanes	widespread use to disinfect water; also used in color, taste, and odor removal, improving coagulation, and killing algae.
Iodine	chemical reaction with pathogens	good disinfectant	high cost; harmful to pregnant women	emergency treatment of water supplies; disinfecting small, non-permanent water supplies
Bromine	chemical reaction with pathogens		handling difficulties; residuals hard to obtain; supply is limited	very limited use, primarily for treating swimming pool water
Bases (sodium hydroxide and lime)	chemical reaction with pathogens		bitter taste in the water; handling difficulties	sterilize water pipes
Ozone	chemical reaction with pathogens	good disinfectant; better virucide than chlorine; oxidizes iron, manganese, sulfide, and organics; removes color, odor, and taste	high cost; lack of residual; storage difficulties; maintenance requirements; safety problems; unpredictable disinfection; no track record	disinfection; treating iron and manganese, helping flocculation, removing algae, oxidizing organics, removing color, treating tastes and odors
Ultraviolet	UV light causes biological changes which kill the pathogens	lack of dangerous by-products	lack of measurable residual; cost of operation; turbidity interferes with disinfection	small or local systems and industrial applications
Ultrasonic	sound waves destroy pathogens by vibration		very expensive
Heat	boiling water for about five minutes will destroy essentially all microorganisms	simple, requires little equipment	very energy intensive; expensive	Individuals may boil their water for household quantities of water when quality of water is questionable

In the past, water treatment plants have principally relied on the use of chlorine for disinfection. The prevalent use of chlorine has come about because chlorine is an excellent disinfecting chemical and, until recently, has been available at a reasonable cost.

However, chlorine has several disadvantages. Chlorine is becoming more expensive and has been shown to be toxic to fish and other biota. In addition, chlorine can combine with organic substances in water to produce trihalomethanes, which are suspected of causing cancer.

As a result, future water treatment may see an increased use of ozone or ultraviolet (UV) light. Both types of treatment are effective disinfecting agents and leave no toxic residual. The table below lists some of the factors which may influence the choice of treatment method in a new plant.

	Chlorine (Gas or Hypochlorite)	Chlorine Dioxide	Chloramine	Ozone	Ultraviolet
Produces trihalomethanes?	yes	no	yes	sometimes	no
Produces other troublesome byproducts?	yes	yes	yes	yes	sometimes
Impacted by lime softening?	yes	no	yes	no	yes
Impacted by turbidity?	somewhat	somewhat	somewhat	somewhat	yes
Meets Giardia removal standards?	no	yes	no	yes	no
Meets Cryptosporidium removal standards?	no	no	no	yes	no
Meets virus removal standards?	yes	yes	no	yes	yes
Operator skill level	low	high	low/medium	high	medium
Applicable to large utilities?	yes	yes	yes	yes	no
Applicable to small utilities?	yes	yes	yes	yes	yes

You may note that many of the disinfection methods do not meet standards for Giardia, Cryptosporidium, and virus removal. This does not mean that these disinfection methods cannot be used. When used in conjunction with filtration, all of the disinfection methods can be used to meet removal standards.