Introduction to Disinfection
 In this section we will answer the following question:

What is Disinfection?

Even after wastewater has passed through primary, secondary, and tertiary treatment, microorganisms remain in the water.  Some of these microorganisms are pathogens which could result in the spread of waterborne diseases.  

The final stage in the wastewater treatment process is disinfection, a step which kills the pathogenic bacteria in the water.  After disinfection, the wastewater effluents can be safely released into receiving waters used for water supply, swimming, or shellfishing.  

Types of Disinfection

In the past, wastewater treatment practices 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, the rising cost of chlorine coupled with the chemical's toxicity to fish and other biota and the production of potentially harmful chlorinated hydrocarbons has made chlorination less favored as the disinfectant of choice in wastewater treatment.  

As a result, future wastewater 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.  In addition, ozone will raise the dissolved oxygen level of water.  

The use of both UV light and ozone as disinfectants will be considered individually.  However, since chlorine continues to be used extensively as a disinfectant, we will mainly be concerned with the practice of chlorination.  


In this section we will answer the following question:

Introduction to Chlorination

Chlorination is the application of chlorine to wastewater to accomplish some definite purpose.  In this lesson, we will be concerned with the application of chlorine for the purpose of disinfection, but you should be aware that chlorination can also be used for odor control and for other purposes.  

Chlorine is a well known and economical form of disinfection.  However, chlorination has numerous disadvantages.  Chlorination can produce trihalomethanes, can slow the nervous system in people, can produce chlorinated hydrocarbons which are considered health hazards, and can be corrosive.  

The main types of chlorine used to disinfect wastewater are explained below.  All types of chlorine will kill bacteria and some viruses, E-coli, Salmonella, Typhoid, Cholera, Shingella, and Polio.  But only chlorine dioxide will effectively kill Cryptosporidium, Giardia, protozoans, and some viruses.

Chlorine Gas

Chlorine can be added to the water as a gas.  Once the chlorine enters the water, it produces hypochlorous acid (NaOH) and the disinfectant HOCl.  This is the most concentrated form of chlorine, being 99.9% concentrated.

Chlorine gas is difficult to handle since it is toxic, heavy, corrosive, and an irritant.  

Hypochlorites and Bleaches

Chlorine can also come in a liquid form, known as hypochlorite or bleach.  These liquids work in the same general method as chlorine gas, by producing the disinfectant HOCl.  They are all much less concentrated than chlorine gas.

Hypochlorites have the disadvantage that they may decompose in strength over time while in storage. Temperature, light, and physical energy can all break down the hypochlorites before they are able to react with pathogens in water.  

We will discuss three types of hypochlorites below - sodium hypochlorite, calcium hypochlorite, and commercial bleach.  

Sodium hypochlorite (NaOCl) is up to 12% chlorine. Like chlorine gas, it produces NaOH and HOCl when introduced to water.  

Calcium hypochlorite (Ca(OCl)2), also known as HTH, is also used in water treatment.  This is the type of chlorine which is used in swimming pools.  Calcium hypochlorite is 65-70% concentrated.

The concentration of commercial bleach varies depending on the brand.  Chlorox bleach is 5% chlorine while some other brands are 3.5% concentrated.


When hypochlorites are mixed with ammonia, the result is a chloramine.  The ammonia and chlorine are mixed at a ratio of 4:5 to 1.

Chloramines are weaker than chlorine, but are more stable, so they are often used as the disinfectant in the distribution lines of water treatment systems.  Despite their stability, chloramines can be broken down by bacteria, heat, and light.  

Chloramines are effective at killing bacteria and will also kill some protozoans, but they are very ineffective at killing viruses.  

Chlorine Dioxide

Chlorine dioxide, ClO2, is a very effective form of chlorination since it will kill protozoans,  Cryptosporidium, Giardia, and viruses that other systems may not kill.  In addition, chlorine dioxide oxidizes all metals and organic matter, converting the organic matter to carbon dioxide and water.

Chlorine dioxide is generated on site, and it has the disadvantages that it is very costly and requires a great deal of technical expertise.  The operator must monitor the chlorite in a chlorine dioxide system.  

Chlorine dioxide also has other disadvantages.  It is dangerous around activated carbon, it can produce odors, and it is corrosive.    

Other Methods of Disinfection
In this section we will answer the following question:


Oxygen in the air (O2) is composed of two oxygen molecules.  Under certain conditions, three oxygen molecules can be bound together instead, forming ozone (O3).

Ozone has many advantages as a disinfectant.  It kills all pathogenic organisms by a direct effect on their DNA.  Disinfection occurs 30,000 times faster than with chlorine, so a prolonged contact time is unnecessary.  There is no harmful residual left in the system.

The disadvantages of an ozone disinfection system are a corrosive nature, a high cost for the initial set-up, and a high electricity consumption.  

UV Light

Ultraviolet, or UV, light is light outside the range usually detectable by the human eye.  It can be used to deactivate protozoans so that they can't reproduce and to significantly reduce the bacteria in water.  

The primary disadvantage of UV light is a high operating cost.  In addition, anything which blocks UV light from reaching the water will result in a lack of treatment.