Chlorine Disinfection

by Matt Curtis & Erik Johnston

History

Throughout the history of water treatment, chlorination has been synonymous with disinfection. Chlorination was the first method used to disinfect water in approximately 1825 and remained the only widely used method for over 50 years. The following timeline shows a brief history of chlorination in water and wastewater treatment over the past 100 years. The graph to the right of the timeline shows the effect of filtration and then chlorine disinfection on the number of typhoid cases. Filtration dramatically reduced the number of cases of typhoid, and chlorine then also further reduced the number of cases (Data from Davis, 1991.).

TIMELINE

1879

1893

1903

1914

1919

1925

1939

1960

1972

1879

- This marked the first time that chlorine was applied as a disinfectant. William Soper of England treated the feces of typhoid patients before disposal into the sewer. He used chlorinated lime, which was a common form of chlorine used initially. (White, 1972) Return to timeline.

1893

- This date was the first time that chlorine was applied as a disinfectant on a plant scale basis. This application was made at Hamburg, Germany. (White, 1972) Return to timeline.

1903

- This marked the first time chlorine gas was used as a disinfectant in drinking water. This took place in Middlekerke, Belgium. Prior to this date, chlorine was applied through the use hydrated lime, chloride of lime, or bleaching powder. The use of chlorine gas was designed by Maurice Duyk, a chemist for the Belgian Ministry of Public Works. (Pontius, 1990) Return to timeline.

1908

- The first full scale chlorine installation at a drinking water plant in the United States was initiated in this year. This installation took place at the Bubbly Creek Filter Plant in Chicago. This plant served the Chicago Stockyards and was designed by George A. Johnson. The raw water contained a large amount of sewage which was causing sicknesses in the livestock. Johnson implemented chlorine through chloride of lime, and the bacterial content of the water dropped drastically. (Pontius, 1990)Return to timeline.

1910

- C. R. Darnall became the first to use compressed chlorine gas from steel cylinders which is an approach still commonly used today. His installation was in Youngstown, Ohio. His implementation used a pressure-reducing mechanism, a metering device, and an absorption chamber. It was moderately successful, but his setup was only used once. The photo to the right is typical of modern design. (White, 1972) (Photo by Matt Curtis)Return to timeline.

1912

- John Kienle, chief engineer of the Wilmington, Delaware water department, invented another way to apply chlorine to drinking water. He developed a way to push compressed chlorine from cylinders into an absorption tower in which water was flowing opposite the flow of the chlorine. Because the gas flow was opposite the water flow, the chlorine was able to disinfect the water. (Pontius, 1990)Return to timeline.

1913

- An Ornstein chlorinator was installed at Kienle's Wilmington, Delaware water treatment plant. This marked the first time a commercial chlorination system was installed at a municipal water treatment plant. The chlorinator used the same basic premise that Kienle's previous installation did, but the Ornstein chlorinator used both a high and low pressure gauge to more accurately control the amount of chlorine added to the system. (Pontius, 1990) Return to timeline.

1914

- On October 14, 1914, the Department of the Treasury enacted the first set of standards that required the use of disinfection for drinking water. These standards called for a maximum level of bacterial concentration of 2 coliforms per 100 millilters. Because chlorination was the main disinfectant at the time, these standards dramatically increased the number of treatment plants using chlorine. (White, 1972) Return to timeline.

1919

- Two important discoveries were made during this year. Wolman and Enslow discovered the concept of chlorine demand which states that the amount of chlorine needed to disinfect the water is related to the concentration of the waste and the amount of time the chlorine has to contact the water. The other important discovery of 1919 was by Alexander Houston. He discovered that chlorine can also eliminate taste and odor problems in water. (Pontius, 1990) Return to timeline.

1925

- New drinking water standards were enacted that reduced the maximum permissible limit of coliforms from 2 to 1 coliform per 100 millilters. This increased the amount and frequency of chlorination again. (White, 1972) Return to timeline.

1939

- The theory of the chlorine breakpoint was discovered in this year. Chlorine breakpoint theory is discussed in the following section. (White, 1972) Return to timeline.

1960

- A new implementation practice was discovered in this year. The compound loop principle of chlorinator control was implemented, which is the most recent major discovery in chlorine application. (White, 1972) Return to timeline.

1972

- A report entitled "Industrial Pollution of the Lower Mississippi River in Louisiana" was published containing the first evidence of disinfection byproducts in drinking water resulting from organic pollution in source water. (Pontius, 1990) Return to timeline.

As is evident by the dates in the timeline, most of the innovation in chlorination occurred over 70 years ago. Very few innovations or discoveries have been made recently. Most of the current research is being performed in other areas of disinfection. These areas include ozone, chlorine dioxide, and UV radiation. Chlorine is still the most widely used disinfectant in the United States, but other areas of the world are beginning to use other methods of disinfection with increasing frequency. Since chlorine is still widely used, a thorough understanding of how it disinfects and is implemented is important to those interested in water treatment.

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Theory

Disinfection with chlorine is very popular in water and wastewater treatment because of its low cost, ability to form a residual, and its effectivness at low concentrations. Although it is used as a disinfectant, it is a dangerous and potentially fatal chemical if used improperly.

Despite the fact the disinfection process may seem simple, it is actually a quite complicated process. Chlorination in wastewater treatment systems is a fairly complex science which requires knowledge of the plant's effluent characteristics. When free chlorine is added to the wastewater, it takes on various forms depending on the pH of the wastewater. It is important to understand the forms of chlorine which are present because each has a different disinfecting capability. The acid form, HOCL, is a much stronger disinfectant than the hypochlorite ion, OCL-. The graph below depicts the chlorine fractions at different pH values (Drawing by Erik Johnston).

Ammonia present in the effluent can also cause problems as chloramines are formed, which have very little disinfecting power. Some methods to overcome the types of chlorine formed are to adjust the pH of the wastewater prior to chlorination or to simply add a larger amount of chlorine. An adjustment in the pH would allow the operators to form the most desired form of chlorine, hypochlorus acid, which has the greatest disinfecting power. Adding larger amounts of chlorine would be an excellent method to combat the chloramines because the ammonia present would bond to the chlorine but further addition of chlorine would stay in the hypochlorus acid or hypochlorite ion state.

a) Chlorine gas, when exposed to water reacts readily to form hypochlorus acid, HOCl, and hydrochloric acid. Cl₂ + H₂O -> HOCl + HCl

b) If the pH of the wastewater is greater than 8, the hypochlorus acid will dissociate to yield hypochlorite ion. HOCl <-> H⁺ + OCl^-- If however, the pH is much less than 7, then HOCl will not dissociate.

c) If ammonia is present in the wastewater effulent, then the hypochlorus acid will react to form one three types of chloramines depending on the pH, temperature, and reaction time.

Monochloramine and dichloramine are formed in the pH range of 4.5 to 8.5, however, monochloramine is most common when the pH is above 8. When the pH of the wastewater is below 4.5, the most common form of chloramine is trichloramine which produces a very foul odor. The equations for the formation of the different chloramines are as follows: (Reynolds & Richards, 1996)

Monochloramine: NH₃ + HOCl -> NH₂Cl + H₂O Dichloramine: NH₂Cl + 2HOCl -> NHCl₂ + 2H₂O Trichloramine: NHCl₂ + 3HOCl -> NHCl₃ + 3H₂O

Chloramines are an effective disinfectant against bacteria but not against viruses. As a result, it is necessary to add more chlorine to the wastewater to prevent the formation of chloramines and form other stronger forms of disinfectants.

d) The final step is that additional free chlorine reacts with the chloramine to produce hydrogen ion, water , and nitrogen gas which will come out of solution. In the case of the monochloramine, the following reaction occurs:

2NH₂Cl + HOCl -> N₂ + 6HCl + H₂O

Thus, added free chlorine reduces the concentration of chloramines in the disinfection process. Instead the chlorine that is added is allowed to form the stronger disinfectant, hypochlorus acid.

Perhaps the most important stage of the wastewater treatment process is the disinfection stage. This stage is most critical because it has the greatest effect on public health as well as the health of the world's aquatic systems. It is important to realize that wastewater treatment is not a cut and dry process but requires in depth knowledge about the type of wastewater being treated and its characteristics to obtain optimum results. (White, 1972)

The graph shown above depicts the chlorine residual as a function of increasing chlorine dosage with descriptions of each zone given below (Drawing by Erik Johnston, adapted from Reynolds and Richards, 1996).

· Zone I: Chlorine is reduced to chlorides.

· Zone II: Chloramines are formed.

· Zone III: Chloramines are broken down and converted to nitrogen gas which leaves the system (Breakpoint).

· Zone IV: Free residual.

Therefore, it is very important to understand the amount and type of chlorine that must be added to overcome the difficulties in the strength of the disinfectant which results from the wastewater's characteristics.

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Implementation

Water Treatment

The following is a schematic of a water treatment plant (Drawing by Matt Curtis).

In water treatment, pre-chlorination is utilized mainly in situations where the inflow is taken from a surface water source such as a river, lake, or reservoir. Chlorine is usually added in the rapid mixing chamber and effectively prevents the majority of algal growth. Algae is a problem in water treatment plants because it builds up on the filter media and increases the head which means that the filters need to be backwashed more frequently. In addition, the algal growth on the filter media causes taste and odor problems in the treated water. (Reynolds & Richards, 1996)

In the picture to the left, a residual monitor checks the chlorine level in the water leaving the treatment plant. A minimum value is required to prevent regrowth of bacteria throughout the distribution system, and a maximum value is established to prevent taste, odor, and health problems (Photo by Matt Curtis).

Post chlorination is almost always done in water treatment, but can be replaced with chlorine dioxide or chloramines. In this stage chlorine is fed to the drinking water stream which is then sent to the chlorine contact basin to allow the chlorine a long enough detention time to kill all viruses, bacteria, and protozoa that were not removed and rendered inactive in the prior stages of treatment (Photo by Matt Curtis).

Drinking water requires a large addition of chlorine because there must be a residual amount of chlorine in the water that will carry through the system until it reaches the tap of the user. After post chlorination, the water is retained in a clear well prior to distribution. In the picture to the right, the clear pipe with the floater designates the height of the water within the clear well. (Reynolds & Richards, 1996)

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Wastewater Treatment

The following is a schematic of a wastewater treatment plant (Drawing by Erik Johnston).

The final stage of the wastewater treatment process is chlorination, which is used to disinfect the wastewater before it is returned to a river or body of water. Disinfection is necessary to kill any pathogens that may have survived the treatment process and could cause harm to humans or aquatic wildlife. In this stage of the treatment process, chlorine is typically fed into the wastewater stream as it flows to the chlorine contact basin. The chlorine contact basin is a baffled basin which allows the chlorine additional time to react with the wastewater to kill the pathogens.

The following is an animation of a drop of water as it passes through a chlorine contact basin.

The photograph to the left is of the chlorine contact basin at the Blacksburg Wastewater Treatment Plant. (Photo by Matt Curtis)

Following the chlorine contact basin, the wastewater stream is fed a chemical, typically sulfur dioxide, to dechlorinate the water. Dechlorination is essential because chlorine is toxic to aquatic life. The final stage after dechlorination is to again oxygenate the water before it is emitted to the stream so it will contain enough dissolved oxygen to support the aquatic life. This stage of the process is typically done by allowing the wastewater stream to flow over a series of weirs before it is emitted to the water body. (Reynolds & Richards, 1996)

The photograph to the right is of the discharge from the chlorine contact basin. The inlets for the sulfur dioxide addition can be seen as pipes hanging into the water. The turbulent flow of the water is caused by the submerged weirs which oxygenate the water. Also shown are wires for equipment that will monitor the properties of the water as it leaves the chlorine contact basin. (Photo by Matt Curtis)

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References

1. Davis, Mackenzie, L. Introduction to Environmental Engineering McGraw-Hill, Inc. NY, NY. 1991.

2. Pontius, Frederick, Ed. Water Quality and Treatment, 4th Ed. American Water Works Association. McGraw-Hill, Inc. NY, NY. 1990.

3. Reynolds, Tom, Paul Richards. Unit Operations and Processes in Environmental Engineering, 2nd Ed. PWS Publishing Company. Boston. 1996.

4. White, Clifford. Handbook of Chlorination. Van Nostrand Reinhold Company. NY, NY. 1972.

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