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Back to Basics Guide To Disinfection with Chlorine



To address the need for basic but useful information, the American Water Works Association (AWWA) has developed a series of Back to Basics guides on all aspects of water operations. In addition to telling you what and how things should be done, the guides explain why. These explanations will help you make judgment calls in situations where it is impossible to cover all possible variation in the instructions.

The series is intended for small water systems and entry-level operators. The series is not intended as "The last word" on any of the topics. Instead, it is intended to be used with proper consideration for local, state, provincial, and federal regulations.

The series should be used as a guide to better management and operation for water system that have had difficulty finding information that is user-friendly and easy to read. Each guide offers other sources of information to direct you to more in-depth discussions and related topics.

AWWA recommends that the reader use appropriate expert – a lawyer, accountant, engineer, or regulator – Whenever required. Self-education is the important first step to improved management and operations, and there is an almost equally important second step. The second step should always be an increased awareness that justified and timely use of an expert is the best way to avoid unnecessary mistakes, liabilities, and expenses. The back to Basic guides are designed to help and encourage you to better understand what it takes to run a water system, why it is necessary to undestake it, who can help, and how to stay in charge.

Each guide addresses just one aspect of small water system. This allows you to focus on one topic at time, choosing only those guides that apply to your system and your situation. As you read the information, think about how it applies to your system. Highlight the text with a marker and make notes in the margin. The guides are punched so you can store them in three-ring binder.

Thanks are extended to the author, James A. Olson, and to technical reviewers Gary Logsdon, Ken Kerri, and Jerry Connell.


A solution feeder injects a set amount of chlorine directly into the main water supply.


One major concern in providing safe drinking water is controlling waterborne diseases. Considerable time and expense have been invested in developing methods for the disinfection and protection of public water supplies.

The methods used to disinfect water are heat (not very practical for system-wide use, but the best method for short-them, individual use); radiation (sunlight and ultraviolet light, both have major drawbacks when the flow is larger than for one household and neither provides a measurable chlorine residual in the distribution system); and chemical disinfection (bromine, iodine, ozone, chlorine, and chlorine compounds).

This guide reviews chlorination practices for small water systems, those that serve between 10 and 1000 taps, because chlorination is the most widely used disinfection process. It is important to remember that the people in charge of the water system – volunteer or paid – are the most important link in providing the barrier between disease –causing pathogens and safe water. Only the efforts of all persons involved in the operation of a water system will allow utilities to continue to maintain the delivery of safe, potable water.


Chlorine is an industrial chemical that is identified by the symbol Cl2. Pure chlorine is never found in nature but is often produced for industrial uses. At normal temperatures and pressures, chlorine is a heavy, yellow-green gas with a sharp, irritating odor. Chlorine is stored and shipped as a liquid under pressure in steel containers. If a containers leaks, liquid chlorine could be present for a very short time. Liquid chlorine is an amber-colored, oily fluid; under normal conditions, it evaporates (changes to a gas) very rapidly . Chlorine can be combined with other chemicals to produce products that are stable and exist as solids or liquids at normal temperatures and pressures.

Types of Chlorine

The three forms in which small system usually handle chlorine are chlorine liquid and gas (in steel cylinders); calcium hypochlorite (a solid or powder); and sodium hypochlorite (a liquid).

Chlorine gas

Chlorine gas is approximately 2.5 times heavier than air. Because it is heavier than air it sinks in most cases and accumulates in low spots. It is yellow –green in color ands has a strong, penetrating odor. Liquid chlorine will change to a gas very rapidly under normal atmospheric condictions. One part of liquid chlorine will form 459 parts of gas when it evaporates. The chlorine gas used to disinfect water is shipped and store as a liquid in silver-colored, pressurized cylinders (usually referred to as gas cylinders).

There sizes of gas cylinders are commonly used to store chlorine gas they are 100-lb (limited in use) and 150-lb cylinders and to containers. The 150-lb cylinders are used by most large water treatment plants. Ton containers are used by most large water treatment plants. All types of chlorine containers bear a green or white label that reads "liquid chlorine __ Nonflammable Compressed Gas". This label should always be left on the cylinder.

A 150-lb cylinder contains 150-lb (68 Kg) of chlorine. At normal ambient temperatures, about 80 percent of the volume of the cylinder is liquid chlorine and the other 20 percent is chlorine gas. Because the gas is lighter than the liquid, it is always at the top of the container. Cylinders

Courtesy of the Chlorine Institute, Inc.

should be stored and transported in an upright position so that the gas is at the value end of the cylinder.
All pressurized chlorine containers have fusible plugs. For safety reasons, the plugs have a melting point between 158° and 165° F (70° and 74° C) to allow the container to vent to the air to relieve pressure and to prevent the cylinders from rupturing in case of fire or exposure to high temperatures. The fusible plug is located in the cylinder valve on 100- and 150-lb cylinders. Store chlorine cylinders in a cool, ventilated area to prevent accidental pressure buildup and release.
Chlorine gas is a hazardous substance and should be handled with care. The following list gives some general safety precautions that should be taken when handling 100-or 150-lb chlorine cylinders:

Figure 2 Chlorine cylinder on scale. Courtesy of Capital
Controls Company.

                    Figure 3 Standar cylinder                        Figure 4 Standar cylinder
                    valve: poured-type fusible                      valve: screwed-type fusible
                    plug. Courtesy of the Chlorine               plug. Courtesy of the Chlorine
                    Institute, Inc.                                           Institute, Inc.

- Check each cylinder for serious dents, cuts, or gouges that might decrease the wall thickness of the container.
- Look for signs of corrosion or pitting that might be serious enough to weaken the cylinder.
- Check for obvious bulges
- Put your hand on each cylinder to see if it is hot to the touch. This could indicate internal moisture contamination. If there appears to be internal moisture, return the cylinder to the manufacturer or supplier
-Visually inspect the valves and fusible plugs.

Calcium hypochlorite

Calcium hypochlorite (ca(OCI)2) is a dry, white or yellow-white, granular material produced from the reaction of lime and chlorine. When dissolved in water it releases active chlorine. Calcium hypochlorite is commercially available in granular powdered, or tablet forms. These products are dissolved in water to form a liquid before they are used to disinfect water.

While calcium hypochlorite is not as dangerous as chlorine gas, it should be handled according to the recommended procedures. The following is a short list of precautions than should be taken when working with this solid form of chlorine.

Sodium hypochlorite

Sodium hypochlorite (NaOCI) is a water-based solution of sodium hydroxide and chlorine. This form of chlorine is purchased in containers of various size in varying concentrations of up to 15 percent by weight. Household bleach is sodium hypochlorite in a 3_4 percent concentration. The following is a list of precautions to take when working with this liquid form of chlorine:

Disinfecting water by chlorination

The purpose of disinfection in water treatment is to kill disease-causing organisms (pathogens) that may be present. There are several types of disease-causing organism; each type has unique characteristics, such as size, that influence their resistance to chlorine.

Types of pathogens.

Bacteria are microscopic organisms. Individual bacteria are usually so small they are invisible to the naked eye. Therefore, it is impossible to tell if bacteria are present simply by looking at a water sample. Most bacteria are harmless, but a few types may cause serious illness and even death. Some bacterial diseases transmitted by drinking contaminated water include typhoid fever, dysentery, and cholera.
Viruses are very small, much smaller that bacteria, and they, too, can because serious illness. Infectious hepatitis and poliomyelitis are two viral diseases that can be transmitted in contaminated drinking water.
Protozoa are microscopic animals too small to be detected by the human eye but larger that bacteria. Among the waterborne diseases they cause are amoebic dysentery and glardiasis.
These three types of organism (virus, bacteria, and protozoa) differ in size and in their resistance to chlorine. All of these organism are called pathogens, pathogens are defined as disease-causing organisms.

Chlorination to kill pathogens

Chloritation __ the addition of chlorine to water __ is the most common form of disinfection practiced in the United State and Canada today. When properly understood and correctly operated, the chlorination process is a safe, practical, and effective way to destroy pathogens in water.
Chlorine demand. Chlorine is an extremely active chemical and will react with many other compounds. When chlorine is added to water, it will first react with common inorganic compounds present in most water sources, such as hydrogen sulfide and ferrous iron. This is a rapid, almost instantaneous, reaction. No disinfection occurs at this stage. As more chlorine is added, the chlorine reacts with the ammonia and organic matter in the water to form chloroorganic compounds, such as chloramines. These newly formed compounds have a disinfecting action that is quite slow. The initial reactions form a part of the chlorine demand of water. Once the initial chlorine demand has been met, further chlorine addition will start to build up a chlorine residual that will accomplish the rest of the disinfection in the distribution system. The point at which the initial chlorine demand is met is called the chlorine breakpoint.
Chlorine residuals There are several types of chlorine residuals that can be measured in treated water. Free chlorine residual is your disinfection safety margin and includes chlorine in the form of hypochlorous acid (HOCI) is the most effective disinfectant form of chlorine. Combined chlorine residual includes chlorine in three chloramine forms are produced when chlorine reacts with and combines with ammonia products in the water. Combined chlorine residual requires up to 100 times the contact time or at least 25 times the chlorine concentration to be as effective as free chlorine residual in desinfecting water. Total chlorine residual is the sum of the free and combined residuals.

Chlorine – residual test kits measure the free available chlorine and the total chlorine residual. The combined available chlorine residual is the difference between free and total chorine residual. For example, if the free chlorine residual is measured as 1 mg/L and the total chlorine residual as 3 mg/L the difference of 2 mg/L is the combined chlorine residual.
Maintaining a 0.2 mg/L free chlorine residual in the water system is an acceptable atart to providing safe drinking water.

How much to use?.

The amount of chlorine that is added to a system is the chlorine dosage must be high enough to satisfy the chlorine demand and maintain a chlorine residual sifficient to kill pathogens through to the last tap of the distribution syatem (see dosing guidelines on page 7).

Several physical qualities of water affect the chlorine dosage. Changing water temperatures requires change in the amount of chlorine to be used. The pH or acidity of the water also affects the amount of chlorine needed to carry out disinfection. The best results in chlorination occur when the water supply has a pH of 7.5 or less. The turbidity (a measure of the suspended matter) of the water can affect the effectiveness of chlorinations. Pathogens that are encased in suspended matter may be protected from contact with the chlorine. To effectively treat water with high turbidity it is necesary to increase the amount of chlorine added to the water, extend the time that the pathogens are exposed to the chlorine residuals, or a combination of both of these actions.

Contact time

The length of time that the chlorine is allowed to be in contact with water before the water reaches the first tap in the distribution system is called the chlorine contact time. Chlorine needs a period of time in contact with the pathogens to provide effective disinfection. Proper desinfection could take 10-60 min. This contact tcan be provided in a holding tank and/or in the distribution system.

Table 1 the gives the contact times for 99.9 percent Giardia removal with 0.6 mg/L chlorine residual. Tables of this type are available for all water conditions; the appropiate charts for your water system should be on hand at your water treatment site.

Regulatory requirements .The US Enviromental Protection Agency (USEPA) has put into affect the Surface Water Treatment Rule (SWTR), which sets treatment standards to inactive 99.9 percent

Table 1 Contact Times (CT) for 99.9 Percent Giardia Removal With 0.6 mg/L
Chlorine Residual
of the water                                        pH of the Water
                              6.0            6.5             7.0                 7.5             8.0                    8.5
                                              Required Contact Time CT
5° C                      100            120            143                171            204                   244
10° C                      75              90            107                128            153                   183
15°C                       50              60              72                  86            102                   122
20°C                       38              45              54                  64              77                     92

of the Giardia Cysts in a water supply, based on studies of the effects of chlorine on other pathogens, if the standards for treating Giardia are met, other pathogens will also be inactivated. Operators of water

chlorinations system should become familiar with the chart that the USEPA has made available for determining the proper chlorine dosages. * there are many reasons why general rules do not hold up when treating water, but a free chlorine residual of at least 0.6 mg/L in the water leaving the treatment site, wich is usually recommended to ensure a free residual of 0.2 mg/L at the last tap on the distribution system, is a very good desinfection practice.

The amount of chlorine aqdded to water to ensure proper desinfection should take into account the physical qualities of the water being treated and any regulatory requirements.

Determining dosage and dosage rate

The dosage of chlorine is the amount of chlorine applied to a specific quantity of water. Dosage is determinined by adding the chlorine demand to the residual that will be maintained. Dosage is expessed as milligrams per litre (mg/L).

chlorine dosage       =        chlorine       +          chlorine
       mg/L                             mg/L                       mg/L

The dosage rate is usually expressed as pounds per 24 hours and is determined by multiplyng the dosage in milligrams per litre by 8.34 (the weight of 1 gal [3.8 L] of water) by the flow rate in millions of gallons per day (megalitres per day).

(mg/L) mgd) (8.34 lb/gal) = lb/day.

Example. Assume a free residual of 0.2 mg/L is to be maintained in water having a chlorine demand of 0.4 mg/L.

What dosage of liquid chlorine will be required for a system that treats 500,000 gal/day (2 ML/day)?

dosage = demand + residual = milligrams per litre

0.4 mg/L+0.2 mg/L=0.6 mg/L.

How many pounds of chlorine per day will be required?

Dosage rate = dosage x weight of water x flow rate = pounds/day
               0.6 mg/L x 8.33 x 0.5 mil gal = 0.25 lb/day.

Choices of chlorine feed equipment

When choosing between a gas or a solution feeder, your decision should be based on the fallowing:

By answering these questions you have taken the first stepo toward selecting a gas or solution feeder. A profesional can then help you make the final decision on the type of feeder that’s best for your system.

Things To Know About Gas and
Solution Feeders

  • Solution feeders are used for smaller flows (100,000 gpd  or less).

  • Solution feeders require regular, sometimes daily, mixing and filling of the solution bucket.

  • Solution feeders tend to clog and may require more time in maintenance and upkeep.

  • Upkeep and maintenance of solution feeders are simpler and  potentially less hazardous than for a gas chlorination unit.

  • The increased cost for a gas feeder may be offset by reduced maintenance and manpower costs.

  • If a system has periods of high chlorine demand, for example, during spring runoff or other seasonal events, a gas feeder may be more effective in handling this "peak" demand.

  • If the chlorine injection points is located where access is a  problem, chlorine gas delivery may not be accomplished in a safe manner.

  • If chlorine gas in considered, the purchase price should include the price of a self-contained breathing apparatus to be used in times of emergency.

Gas cholination and how it works

The most commonly used gas system is one which water and chlorine are mixed under vacuum and the resulting chlorine solution is then mixed with the remainder of the water supply. A chlorine gas system should include the fallowing:

Maintenance. Some general maintenance tips for any gas chlorination system include;

Figure 8 Schematic of a Gas Chlorinator. Courtesy of Capital Controls Company.

Solution feeder and how it works

The solution feeder works by means of a chemical feed pump, generally a diaphragm pump that can be operated either electrically or by water. A bucket to mix a chlorine solution is also needed. The feed pump then injects a set amount of chlorine into the main water supply (see figure opposite page 1).

A good chlorine solution feed system should include the fallowing:


Two basic types of tests are required to determnine the effectineness of desinfection. One is the chlorine residual test, which determines if chlorination is performing properly. Only one method is currently approved for measuring chlorine rewsidual under USEPA drinking water regulations. This is the DPD method. This method is a simple and quick way to measure chlorine residual. It takes lees than 5 min to complete the test.


Figure 9 Several types of DPD fiel test kits

Residual chlorine measurements should be taken at least once each day on public water system. It is important that the water samples be taken from different areas of the distribution system. This will ensure that there is a chlorine residual throughout the entire system. The date, location, and results of these tests should be recorder in the system log book.

The second test to determine the effectivevess of desinfection is the microbiological contamination test, which indicates if the treatment has successfully eliminated pathogens from the water.

To ensure that drinking water meets safety standards, treatment system are required to perform regular microbiologicalk sampling and tasting of the water being supoplied. Generally, reports of these tests must be submitted to state drinking water agencies.

Each treatments system should check with its local public health authority to establish wich tests must be made, how often the test should be made, where to submit the test results, and which laboratories are approved to run the tests.

Because biological water tests are usually run under laboratory conditions, it is unlikely that a small water treatment plant would be equipped or staffed to do this type of testing. In most cases the operator would be responsible for collecting proper samples and delivering them to a certified laboratory.

Collecting water samples is not difficult, but it is very important that samples are taken properly and handled correctly so that they arrive at the certified laboratory in good condition. Detailed instruccions for taking samples laboratory in good condition. Detailed instructions for taking samples for microbiological tests are given in Pocket Guide to Water Sampling


Field study training program

Kerri, Kent, small system Operation and maintenance vol 1. California State University, Sacramento (2d ed, 1987).

Sampling Guide

Pocket guide to water sampling _ Microbiological contaminats AWWA Denver (1990).

Water Supply Operations Series

Introduction to Water Quality Analyses vol.4 AWWA. Denver (1982).
Introduction to water treatment vol 2. AWWA, DENVER (1984)


Water Distribution Operator Training Handbook AWWA, Denver (1976).

Actualizado el 26/Mayo/98
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