This document discusses various methods of disinfection used in water treatment, including the use of disinfectants as chemical oxidants. It provides details on commonly used disinfectants such as chlorine and chloramines. Chlorine is widely used due to its effectiveness and low cost but can form disinfection byproducts. The document explains factors that impact disinfection effectiveness and outlines the concepts of breakpoint chlorination and chlorine residuals. Both advantages and disadvantages are presented for different disinfection methods.

1. Disinfection

2. Background: Current Methods of
Disinfection
• Large-Scale:
– Chlorination
– Ozone
– UV irradiation
• Small Scale:
– Boiling
– Iodine tablets
– Filters

3. Use of Disinfectants as Chemical Oxidants
Oxidation is a chemical reaction where electrons are transferred from one species
(the reducer) to another species (the oxidant).
Disinfectants are used for more than just disinfection in drinking water treatment.
While inactivation of pathogenic organisms is a primary
function, disinfectants are also used oxidants in drinking water treatment for
several other functions:
1. Minimization of Disinfection Byproducts formation : Several strong
oxidants, including potassium permanganate and ozone, may be used to control
DBP
2. Prevention of re-growth in the distribution system and maintenance of
biological stability;
– Removing nutrients from the water prior to distribution
– Maintaining a disinfectant residual in the treated water

4. Continue: Use of Disinfectants as Chemical Oxidants
3. Removal of color: Free chlorine is used for color removal. A low pH is
favored. Color is caused by humic compounds, which have a high potential
for DBP formation
4. Improvement of coagulation and filtration efficiency;
a. Oxidation of organics into more polar forms;
b.Oxidation of metal ions to yield insoluble complexes such as ferric iron
complexes;
c. Change in the structure and size of suspended particles.
5. Oxidation is commonly used to remove taste and odor causing
compounds. Because many of these compounds are very resistant to
oxidation, advanced oxidation processes (ozone/hydrogen
peroxide, ozone/UV, etc.) and ozone by itself are often used to address taste
and odor problems. The effectiveness of various chemicals to control taste
and odors can be site-specific.

5. Continue: Use of Disinfectants as Chemical Oxidants
6. Removal of Iron and Manganese
7. Prevention of algal growth in sedimentation basins and filters:
Prechlorination will prevent slime formation on filters, pipes, and tanks, and
reduce potential taste and odor problems associated with such slimes.
Oxidant Iron (II) Manganese (II)
Chlorine Cl2 0.62 0.77
Chlorine Dioxide, ClO2 1.21 2.45
Ozone, O3 0.43 0.88*
Oxygen, O2 014 0.29
Potassium Permanganate,
KMnO4
0.94 1.92
Source: Culp/wesner/Culp, Langlais et al., 1991
Optimum pH manganese oxidation using ozone is 8-8.5 source Reckhow et al.,

6. Factors affecting disinfection effectiveness
• Time
• pH
• Temperature
• Concentration of the disinfectant
• Concentration of organisms
• Nature of the disinfectant
• Nature of the organisms to be inactivated
• Nature of the suspending medium

7. CT Factor
• One of the most important factors for determining or predicting the germicidal
efficiency of any disinfectant is the CT factor, a version of the Chick-Watson
law (Chick, 1908; Watson, 1908).
• The CT factor is defined as disinfectant contact time, the mathematical
product of C x T, where C is the residual disinfectant concentration measured
in mg/L, and T is the corresponding contact time measured in minutes.
• CT values for chlorine disinfection are based on a free chlorine residual.
• Chlorine is less effective as pH increases from 6 to 9.
• In addition, for a given CT value, a low C and a high T is more effective than
the reverse (i.e., a high C and a low T).
• For all disinfectants, as temperature increases, effectiveness increases.

8. Chlorine
Chlorine has many attractive features that contribute to its wide use in the industry.
Four of the key attributes of chlorine are that it:
• Effectively inactivates a wide range of pathogens commonly found in water;
• Leaves a residual in the water that is easily measured and controlled;
• Is economical; and
• Has an extensive track record of successful use in improving water treatment
operations
There are, however, some concerns regarding chlorine usage that may impact its
uses such as:
• Chlorine reacts with many naturally occurring organic and inorganic
compounds in water to produce undesirable DBPs;
• Hazards associated with using chlorine, specifically chlorine gas, require
special treatment and response programs; and
• High chlorine doses can cause taste and odor problems.

9. Chlorine purposes in water treatment
• Taste and odor control;
• Prevention of algal growths;
• Maintenance of clear filter media;
• Removal of iron and manganese;
• Destruction of hydrogen sulfide;
• Bleaching of certain organic colors;
• Maintenance of distribution system water quality by
controlling slime growth;
• Restoration and preservation of pipeline capacity;
• Restoration of well capacity, water main sterilization; and
• Improved coagulation by activated silica.

10. Chlorine Chemistry
• Chlorine gas hydrolyzes rapidly in water to form hypochlorous acid
• Hypochlorous acid is a weak acid (pH of about 7.5), meaning it dissociates
slightly into hydrogen and hypochlorite ions
• As the germicidal effects of HOCl is much higher than that of OCl-,
chlorination at a lower pH is preferred.

11. Effect of pH on relative amount of hypochlorous acid
and hypochlorite ion at 20 C.

12. Commonly Used Chlorine Sources
Sodium hypochlorite and calcium hypochlorite are the most common sources of
chlorine used for disinfection of onsite water supplies.
• Sodium Hypochlorite (common household bleach)
• Sodium hypochlorite is produced when chlorine gas is dissolved in a sodium
hydroxide solution.
• Clear to slightly yellow colored liquid with a distinct chlorine odor.
• Common laundry bleach - 5.25 to 6.0 percent available chlorine, when bottled.
• Do not use bleach products that contain additives such as
surfactants, thickeners, stabilizers, and perfumes.
• Always check product labels to verify product content and use instructions.

13. Sodium Hypochlorite (common household bleach)
• Higher concentrations of chlorine in sodium hypochlorite solutions are
generally not available.
• Above 15 percent, the stability of hypochlorite solutions is poor, and
decomposition and the concurrent formation of chlorate is of concern
• Sodium hypochlorite solutions are of an unstable nature due to high rates of
available chlorine loss
• Over a period of one year or less, the amount of available chlorine in the
storage container may be reduced by 50 percent or more.
• Solutions more than 60 days old should not be counted upon to contain the full
amount of available chlorine originally in solution
• Swimming pool chlorine - 10.0 to 12.0 percent available chlorine.

14. Sodium Hypochlorite (common household bleach)
• The stability of hypochlorite solutions is greatly affected by
heat, light, pH, initial chlorine concentration, length of storage, and the
presence of heavy metal cations
• These solutions will deteriorate at various rates, depending upon the specific
factors:
1. The higher the concentration, the more rapidly the deterioration.
2. The higher the temperature, the faster the rate of deterioration.
3. The presence of iron, copper, nickel, or cobalt catalyzes the deterioration
of hypochlorite. Iron is the worst offender

15. Commonly Used Chlorine Sources
• Calcium hypochlorite is formed from the precipitate that results from
dissolving chlorine gas in a solution of calcium oxide (lime) and sodium
hydroxide.
Calcium Hypochlorite
• Dry white powder, granules, or tablets - 60 to 70 percent available chlorine
- 12-month shelf life if kept cool and dry - If stored wet, looses chlorine
rapidly and is corrosive.
• A chlorine test kit should be used to check the final chlorine residual in a
prepared chlorine solution to assure that you have the concentration
intended.

16. Which is Best, Sodium Hypochlorite or
Calcium Hypochlorite?
• Sodium hypochlorite is more effective
• This may be associated with the quality of the ground water in the well being treated
rather than with the source of the chlorine itself.
• If there is an abundance of calcium based materials in both bedrock wells. Calcium
hypochlorite already has a high concentration of calcium.
• At 180 ppm of hardness, water is saturated with calcium to the point that it
precipitates out of the solution, changing from the dissolved state to a solid state.
• Introducing a calcium hypochlorite solution into a calcium rich aquifer can cause the
formation of a calcium carbonate (hardness) precipitate that may partially plug off
the well intake.
• Sodium hypochlorite does not have the tendency to create the precipitate.
• If the calcium carbonate concentration in the ground water is above 100 ppm
(mg/l), the use of sodium hypochlorite is recommended instead of calcium
hypochlorite.

17. Typical Chlorine Dosages at Water Treatment Plants

18. ChlorineAdded
Initial chlorine concentration added to water
Chlorine Demand
Reactions with organic
material, metals, other
compounds present in water
prior
to disinfection
Total Chlorine
Remaining chlorine
concentration after chlorine
demand of water
Free Chlorine
Concentration of chlorine
available for disinfection
Combined Chlorine
Concentration of chlorine
combined with nitrogen in the
water and unavailable for disinfection
Chlorine Addition Flow Chart

19. Chlorine residual
• Chlorine persists in water as „residual‟ chlorine after dosing and this helps to minimize
the effects of re-contamination by inactivating microbes which may enter the water
supply after chlorination. It is important to take this into account when estimating
requirements for chlorination to ensure residual chlorine.
• The level of chlorine residual required varies with type of water supply and local
conditions.
• In water supplies which are chlorinated there should always be a minimum of 0.5mg/l
residual chlorine after 30 minutes contact time in water.
• Where there is a risk of cholera or an outbreak has occurred the following chlorine
residuals should be maintained:
– At all points in a piped supply 0.5mg/l
– At standposts and wells 1.0mg/l
– In tanker trucks, at filling 2.0mg/l
• In areas where there is little risk of a cholera outbreak, there should be a chlorine
residual of 0.2 to 0.5 mg/l at all points in the supply. This means that a chlorine residual
of about 1mg/l when water leaves the treatment plant is needed.

20. Combined Chlorine
What is it?
• Free chlorine that has combined with ammonia (NH3) or other nitrogen-
containing organic substances.
• Typically, chloramines are formed .
Where does NH3, etc come from?
• Present in some source waters (e.g., surface water).
• Contamination; oxidation of organic matter
• Some systems (about 25% of U.S. water supplies) actually Add ammonia.
Why would you want to Add ammonia?
• Chloramines still retain disinfect capability (~5 % of FAC, Free Available
Chlorine)
• Chloramines not powerful enough to form THMs.
• Last a lot longer in the mains than free chlorine,
– Free chlorine + Combined chlorine = Total Chlorine Residual
• Can measure “Total” Chlorine
• Can measure “Free” Chlorine
• Combined Chlorine can be determined by subtraction

21. pH Effect on Chlorine
• Chlorine is a more effective disinfectant at pH levels between 6.0 and 7.0, because
hypochlorous acid is maximized at these pH levels
• Any attempt to disinfect water with a pH greater than 9 to 10 or more will not be
very effective.
• The pH determines the biocidal effects of chlorine.
• Chlorine will raise the pH when added to water.
• By increasing the concentration of chlorine, and subsequently raising the pH, the
chlorine solution is actually less efficient as a biocide.
• Controlling the pH of the water in the aquifer is not practical. However buffering or
pH-altering agents may be used to control pH in the chlorine solution being placed
in the well.

22. Temperature Effect on Chlorine
• As temperatures increase, the metabolism rate of microorganisms
increases.
• With the higher metabolic rate, the chlorine is taken into the microbial cell
faster, and its bactericidal effect is significantly increased.
• The higher the temperature the more likely the disinfection will produce
the desired results.
• Virus studies indicate that the contact time should be increased by two to
three times to achieve comparable inactivation levels when the water
temperature is lowered by 10°C (Clarke et al., 1962).
• Steam injection has been used to elevate temperatures in a well and the area
surrounding the Well bore

23. Contact time
• Time is required in order that any pathogens present in the water are inactivated.
• The time taken for different types of microbes to be killed varies widely.
• it is important to ensure that adequate contact time is available before water
enters a distribution system or is collected for use
• In general, amoebic cysts are very resistant and require most exposure.
Bacteria, including free-living Vibrio cholerae are rapidly inactivated by free
chlorine under normal conditions.
• For example, a chlorine residual of 1mg/l after 30 minutes will kill
schistosomiasis cercariae, while 2mg/l after 30 minutes may be required to
kill amoebic cysts.
• Contact time in piped supplies is normally assured by passing the water, after
addition of chlorine, into a tank from which it is then abstracted.
• In small community supplies this is often the storage reservoir (storage tank). In
larger systems purpose-built tanks with baffles may be used. These have the
advantage that they are less prone to "short circuiting" than simple tanks.

24. Breakpoint Chlorination****
• The type of chlorine dosing
normally applied to piped water
supply systems is referred to as
breakpoint chlorination.
Sufficient chlorine is added to
satisfy all of the chlorine
demand and then sufficient
extra chlorine is added for the
purposes of disinfection.
• As the applied Cl2:N ratio
increases from 5:1 to
7.6:1, breakpoint reaction
occurs, reducing the residual
chlorine level to a minimum.
• Breakpoint chlorination results
in the formation of nitrogen
gas, nitrate, and nitrogen
chloride.
• At Cl2:N ratios above 7.6:1, free
chlorine and nitrogen
trichloride are present.
Distilled water and rainwater (no Cl2 demand) will not show a
breakpoint.
Breakpoint

25. Breakpoint- why should we care?****
The importance of break-point chlorination lies
in
the control of:
taste,
odor,
Complaints of “chlorine” odor and “burning
eyes” from pools/ spas that people usually
attribute to over-chlorination is actually due to
chloramines! (i.e. UNDER-chlorination)
and increased germicidal efficiency.
The killing power of chlorine on the right side of
the break point is 25 times higher than that of

26. The “Breakpoint”…another look****
Chlorine is
reduced to
chlorides by
easily
oxidizable
stuff (H2S,
Fe2+, etc.)
Chloramines
broken down
& converted to
nitrogen gas
which leaves
the system
(Breakpoint).
Cl2 consumed
by reaction
with organic
matter. If
NH3 is
present,
chloramine
formation
begins.
At this
point,THM
formation
can occur

27. Advantages and Disadvantages of
Chloramine Use****
Advantages
• Chloramines are not as reactive with organics as free chlorine in forming
DBPs.
• The monochloramine residual is more stable and longer lasting than free
chlorine or chlorine dioxide, thereby providing better protection against
bacterial regrowth in systems with large storage tanks and dead end water
mains. However excess ammonia in the network may cause biofilming.
• Because chloramines do not tend to react with organic compounds, many
systems will experience less incidence of taste and odor complaints when
using chloramines.
• Chloramines are inexpensive.
• Chloramines are easy to make.

28. Advantages and Disadvantages of
Chloramine Use ****
Disadvantages
• The disinfecting properties of chloramines are not as strong as other
disinfectants, such as chlorine, ozone, and chlorine dioxide.
• Chloramines cannot oxidize iron, manganese, and sulfides.
• When using chloramine as the secondary disinfectant, it may be necessary to
periodically convert to free chlorine for biofilm control in the water
distribution system.
• Excess ammonia in the distribution system may lead to nitrification
problems, especially in dead ends and other locations with low disinfectant
residual.
• Monochloramines are less effective as disinfectants at high pH than at low
pH.
• Dichloramines have treatment and operation problems.
• Chloramines must be made on-site.