Chlorination Notes, adapted from a chlorination course.

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Chlorination
Random notes on chlorination
and related topics
Alexandre Marques | 16-03-2019

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Contents
1. Brief history of chlorination............................................................................................................3
2. Disinfection meaning ......................................................................................................................3
3. Three common water disinfectants................................................................................................3
4. Chlorine element ............................................................................................................................4
5. Water chlorination..........................................................................................................................4
6. Shock chlorination...........................................................................................................................4
7. Benefits of chlorine use ..................................................................................................................4
8. Health effects of chlorine exposure................................................................................................5
9. Risk of chlorine exposition and what to do ....................................................................................5
10. Waterborne pathogens...................................................................................................................5
11. Waterborne diseases ......................................................................................................................5
12. Waterborne virus............................................................................................................................6
13. Cryptosporidium and Giardia analysis ............................................................................................6
14. Common indicator organisms.........................................................................................................7
15. Bacteriophages ...............................................................................................................................7
16. Virions .............................................................................................................................................7
17. Criteria to evaluate a water disinfection system............................................................................7
18. Water Quality..................................................................................................................................8
19. Chemistry of chlorination ...............................................................................................................8
20. Definitions.......................................................................................................................................9
21. Chloramines ..................................................................................................................................10
22. Breakpoint chlorination ................................................................................................................10
23. Chlorinating chemicals..................................................................................................................11
24. Sodium hypochlorite properties...................................................................................................11
25. Calcium hypochlorite properties ..................................................................................................11
26. Health effects of hypochlorite exposure ......................................................................................12
27. Uses for chlorine in industry (other than water treatment).........................................................12
28. Chlorine generation ......................................................................................................................12
29. Oxidation processes......................................................................................................................12
30. Requirements for a chlorine room design....................................................................................13
31. Chlorine leaks and corrective measures.......................................................................................13
32. Chlorine dioxide production .........................................................................................................14
33. Ozone............................................................................................................................................14
34. Disinfection by-products...............................................................................................................15
35. Trihalomethanes...........................................................................................................................16
36. Bacteriological monitoring............................................................................................................15
37. Public notice..................................................................................................................................16
38. Natural organic matter removal ...................................................................................................16
39. Quality control ..............................................................................................................................17
40. Some water treatment chemicals.................................................................................................17

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Due to the nature of these notes, some regulations and procedures presented only apply to
the USA territory.
1. Brief history of chlorination
Chlorine was discovered in 1774 (officially, but possibly earlier). By around 1785 it started
being used to bleach cotton. In 1835 it started being used to remove odour from water. In
1890, only, it was found that chlorine was an effective water disinfectant.
In 1810 chlorine was recognised as an element (and not a chemical compound). In the early
1900s started being used to disinfect water in Belgium, Great Britain and North America. In
1939 the chlorine breakpoint was discovered. In 1972 disinfection by-products where
identified.
Due to its long history and worldwide use chlorine is nowadays one of the most popular,
reliable and well-known water disinfectant.
2. Disinfection meaning
Disinfection is the process of killing or neutralizing pathogens. Pathogens are organisms
harmful for human health. Neutralizing pathogens means making then unable to reproduce,
so they become harmless.
Water can, potentially, widespread diseases as typhoid, cholera, polio, hepatitis or
cryptosporidium, for instance. Disinfection makes water safe to drink.
As it is not feasible, cost-effective, neither quick enough testing for every possible pathogen,
indicator organisms are used instead. These organisms exist in large number where
pathogens are likely to be. They are easier to identify in the lab and their absence indicates
the water is free of pathogens.
The absence of indicator organisms is not enough to prove the wholesomeness of the water
and all the disinfection process must respect the established guidelines.
3. Three common water disinfectants
Three commonly used water disinfectants are chlorine, chloramines and chlorine dioxide.
While disinfection makes water much safer to use and drink, there are still some risks
(especially when regulations not followed). Water with more chlorine than the prescribed
cause eye and nose irritation and stomach discomfort (when drunk).
The health effects of excessive chloramines in water are similar to chlorine: eye and nose
irritation and stomach discomfort; chloramines in excess can additionally cause anaemia.
Chlorine dioxide in excess may affect children (and foetuses) nervous system. It might cause
anaemia as well.

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4. Chlorine element
• Symbol: Cl
• Atomic number: 17
• Colour: Green (when concentrated)
• Discoverer: Carl Wilhelm Scheele
• Molecular weight: 70.9
• Vapor density: 2.5
• Melting point: -101 oC
5. Water chlorination
Water chlorination is disinfecting water using chlorine. Disinfecting is neutralizing pathogens.
Chlorine can be used in different forms and the ones commonly used in water treatment are:
gas (Cl2), liquid (sodium hypochlorite: NaClO) and solid (Calcium hypochlorite: Ca(ClO)2). All
these 3 forms in water are converted to hypochlorous acid (HOCl), which is the main
disinfectant in chlorination.
6. Shock chlorination
Shock chlorination is the process of adding (and mixing) a huge quantity of sodium
hypochlorite in water to reduce algae and microorganisms content. It is used to treat
swimming pools and wells. The water should only be used (for swimming or drinking) when
chlorine concentration is below 3 ppm (3 mg/l).
7. Benefits of chlorine use
Chlorine is one of the most widely used drinking water disinfectants due to its properties,
several available forms and history.
Chlorine is a strong oxidizer and very effective killing (or neutralising) pathogens in general
(with exception to cryptosporidium and, to some extent, Giardia). It also kills other
microorganisms that would grow in mains network and water storage tanks and also oxidizes
other chemicals and minerals (iron and manganese); it improves water taste and odour.
Chlorine is available to use in the pure form as a gas (compressed liquefied gas in drums),
liquid as sodium hypochlorite or solid as calcium hypochlorite tablets. Once in water, all these
three forms produce the same chemical compounds and work in a very similar way.
Being one of the first water disinfectants used, its effectiveness is well proved and potential
by-products well studied and known.

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8. Health effects of chlorine exposure
Chlorine is highly reactive and can cause severe frostbite and chemical burns. It will react with live
tissue, especially the wet tissue. It irritates and burns particularly the eyes, the respiratory track and
lungs, but also the skin. When inhaled in high concentrations (or for long periods) it can be fatal.
9. Risk of chlorine exposition and what to do
In normal conditions people is never exposed to chlorine. It only happens in exceptional and
accidental conditions.
In these situations, people should leave the area with chlorine. If outside, wind socks or wind
sleeves indicate the direction of wind and people should go against the wind. If indoors, as
chlorine is denser than air and cumulate at ground level, people should go upstairs whenever
possible.
In case of contact with chlorine, clothing must be removed and body washed with water and
soap.
10. Waterborne pathogens
Pathogens are microorganisms that cause disease. They can be bacteria, virus or protozoa
(tiny animals). Generally, they cause intestinal illness for short time (days). But in extreme
situations, they may kill.
The main source of waterborne pathogens are faeces (which explains the use of coliforms,
and particularly E. coli, as contamination indicators); they live in animal intestines. So, people
get contaminated when in contact, via mouth, with even minute amounts of pathogens.
These pathogens, coming from animals or wastewater, eventually reach water streams. If this
water is drunk without proper disinfection, the host is infected. People can be also
contaminated when food is washed with contaminated water or manipulated with unwashed
hand.
11. Waterborne diseases
There are many waterborne diseases caused by pathogens. Some of the most well-known or
common these days:
• Cholera: Historically, it was the cause of many pandemics and till a serious problem
mainly in Africa. It is caused by bacteria. The symptoms are vomiting, diarrhoea,
dehydration and, if not treated, death.
• Hepatitis A: This is caused by a virus. The symptoms are abdominal pain, vomiting,
weight loss, enlarged liver and may last up to 4 months.

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• Cryptosporidium: This is caused by protozoa; still a challenge for drinking water
treatment as it is chlorine resistant. It also causes stomach pain and diarrhoea during
a few days. May kill the most vulnerable people.
• Salmonellosis: Cause by the bacterium Salmonella. Symptoms are diarrhoea or
vomiting.
NB: Symptoms here refer to both symptoms and signs, for simplicity.
12. Waterborne virus
Comparatively to bacteria or protozoa, viruses are much smaller (<0.1 µm). They are more
persistent in the environment and, due to the smaller size, they can go further into he ground
and contaminate ground water.
Detecting viruses existing in the environment is possible, but costly, time consuming and
technically demanding. For these reasons using indicators would be more practical, thus using
coliphages as indicator viruses for faecal contamination has been proposed. Coliphages are
viruses that infect, and replicate in, coliform bacteria.
Two indicator viruses have been proposed: Somatic and F-specific coliphages. The limitation
of the first one is they exist in both faecal-contaminated and uncontaminated water, making
them a poor indicator. The second one only reproduces in warm conditions (likely from warm-
blooded animal or sewage), so it is a better candidate.
Coliphages successfully represent the survival and transport of viruses in the environment,
but their correlation with pathogenic viruses is still to be demonstrated.
They can be removed from water by coagulation and filtration or neutralized by disinfection.
13. Cryptosporidium and Giardia analysis
To test water for Cryptosporidium and Giardia presence, it is followed the Method 1623 (US
EPA 1999c). Ten litres of water are sampled following Myers and Sylvester (1997) procedures.
Equipment used must be sterilized. First, the equipment is washed with soap and warm tap
water to remove bigger particles and then washed with distilled water. Following that, the
equipment is submerged in 12% concentration hypochlorite during 30 min, before being
rinsed 3 times with sterilized water. This disinfection step if preferably done indoors, when
possible.
The 10 litres sampled are stored in a manufacturer sterilized container and sent to the
laboratory in a cardboard.
In the laboratory, the 10 litres sampled are filtered, the filtrate is removed from the filter with
a solvent and then centrifuged to concentrate the Cryptosporidium oocyst and Giardia cysts.
These cysts are after magnetised to be separated from the other filtrated particles with a

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magnet. The separated cysts are then stained with a fluorescent chemical. Finally, the cysts
are observed on microscopy to confirm their nature and, to finish, the oocysts and cysts are
counted.
14. Common indicator organisms
The most common indicator organisms used to evaluate drinking water or wastewater are
coliforms, a class of bacteria.
In the UK, total coliforms and E. coli are used at the end of drinking water treatments plants
and Enterococci is used at customers tap (The Water Supply Regulations).
15. Bacteriophages
Bacteriophages are a type of virus that infects bacteria. They may have a lytic cycle, lysogenic
cycle, or both. The ones with lytic destroy the bacteria immediately after replicating the
virion. Then these bacteriophages need to find another host to replicate again. Lysogenic
cycle means the virus lives inside bacteria for a while. If the bacteria reproduce itself, the
offspring will be contaminated. When the conditions in the bacteria cell worsen, then the lysis
happens.
There are specific receptors on the surface of the bacteria (flagella included) where the virus
attaches himself. The virus can only infect bacteria with matching receptors. Some virus can
inject the genetic material through the membrane of the infected bacteria. Others need to
first destroy part of the cell membrane to then introduce the genetic material.
16. Virions
Virions are virus that are completely functional and able to infect living tissue. Their
biomolecules are: genetic material (DNA or RNA), single or double stranded, nucleoprotein
capsid, sometimes an envelope and have, usually, receptor enzymes or proteins to bind or
enter into the host.
17. Criteria to evaluate a water disinfection system
The six criteria to evaluate a disinfection system are: safety, effectiveness, cost, complexity of
use, environmental effects, and water characteristics (flow and quality).
• Safety: Most of the disinfectants are strong oxidizers, so the disinfection system must
be design to be safe and operators must be trained. The system should be safe to the
environment as well, even is case of spills or accidents.

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• Effectiveness: The effectiveness of the system is naturally another major criterion and
any system that does not meet the disinfection requirements is not an option.
• Cost: The cost that will be determinant to select among safe and effective systems.
The cost is also relative to the scale of the system. Hypochlorite tablets are cost
effective for wells ad swimming pools, while for large water treatment plants (100+
megalitres/day) chlorine gas is the cheapest option.
• Complexity: Some systems require extensive training and safety measures, as chlorine
gas, while other systems are relatively safe and do not much training or safety
measures, as calcium hypochlorite (tablets).
• Environmental effects: Eventually, the treated water will end up in the environment,
either after used by people or released from treatment plants. Obviously, it is our
interest to protect the environment.
• Characteristics of the water: The flow can be constant, low, high, or changing quickly
between extremes. The system must be effective for the respective(s) flow(s).
Additionally, some systems are more effective for determined water quality (for
example UV light requires lower turbidity than chlorine for effective disinfection) or
more effective for determined pathogens (for example, UV light neutralizes
cryptosporidium, a chlorine-resistant protozoa). So, the system must be specific for
the job.
18. Water Quality
The water quality can be broken down into 3 broad categories: the microbiological quality
(bacteria, viruses, protozoa), the chemical quality and, not less important, the physical
aesthetic quality (turbidity, colour, taste, odour and appearance).
19. Chemistry of chlorination
When chlorine, in its different forms, is added to the water it does not react with pathogens
but some intermediate reactions happen.
When chlorine gas is added to the water:
𝐶𝑙2 + 𝐻2 𝑂 → 𝐻𝑂𝐶𝑙 + 𝐻𝐶𝑙
(chlorine + water → hypochlorous acid + hydrochloric acid)
When sodium hypochlorite added to the water:
𝑁𝑎𝑂𝐶𝑙 + 𝐻2 𝑂 → 𝐻𝑂𝐶𝑙 + 𝑁𝑎(𝑂𝐻)
(calcium hypochlorite + water → hypochlorous acid + sodium hydroxide)

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When calcium hypochlorite added to the water:
𝐶𝑎𝑂𝐶𝑙 + 𝐻2 𝑂 → 2𝐻𝑂𝐶𝑙 + 𝐶𝑎(𝑂𝐻)
(calcium hypochlorite + water → [2x] hypochlorous acid + sodium hydroxide)
All forms produce hypochlorous acid (HOCl) which is the desired disinfectant when adding
chlorine. Hypochlorous acid still converts and exists in balance with hypochlorite ion (OCl-).
This balance is pH an temperature dependent (but mainly pH).
𝐻𝑂𝐶𝑙 ⇔ 𝐻+
+ 𝑂𝐶𝐿−
(hypochlorous acid ⇔ hydrogen + hypochlorite ion)
Hypochlorite ion has also some disinfecting power, but it is 100x weaker than HOCl. At about
pH 7.5 there is the same amount of HOCL and OCl-. So, disinfection is much less effective at
high pH (pH>8). At pH lower than 7.5 the disinfection is more effective (more HOCl in
solution), but the risk of forming disinfection by-products also increases. At very low pH
(pH<3) there is also Cl-, which is a disinfectant as well, but this is not much relevant in water
treatment as these are not usual pH values.
20. Definitions
• Chlorine demand: Chlorine consumed oxidizing impurities in water, becoming
unavailable for disinfection. There is free chlorine once the chlorine demand is
satisfied.
• Free chlorine: Chlorine in the form of hypochlorous acid (HOCl) and hypochlorite ion
(OCl-). Chloride ion (Cl-) also relevant at low pH. These are the forms available to
disinfect, excluding chloramines (combined chlorine).
• Combined chlorine: Chlorine reacts with ammonia to form chloramines. These
chloramines are combined chlorine. They are disinfectants, weaker than HOCl, but
more stable.
• Total chlorine residual: Chlorine concentration in water available for disinfection,
after chlorine demand satisfied. It is free chlorine and combined chlorine together.
May be called total chlorine or chlorine residual, as well.
• Pre-chlorination: Addition of chlorine at the beginning of the water treatment. It
controls biological life and removes taste and odour.
• Breakpoint chlorination: It means adding chlorine to the water up to the point where
the chlorine demand has been satisfied. Any chlorine added after this point will be
free chlorine.

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21. Chloramines
Chloramines are normally used for secondary disinfection. When primary filtration is
performed using chlorine, then ammonia can be added to for chloramines.
Ammonia + Hypochlorous acid → Chloramines + Water
The amount and type of chloramine depends on the ratio ammonia to chlorine and pH.
There are some reasons to use chloramines. They are more stable than HOCL, so they can
keep a residual disinfectant in water longer and up to the extremities of the mains network.
Another big reason is they do not for THM or HAA. Furthermore, it is better than chlorine
controlling coliforms and biofilm growth.
There are also some disadvantages. It is a weaker disinfectant than chlorine and needs an
extra chemical dosing (ammonia). Other disadvantage is chloraminated water is not suitable
for dialysis (and chloramines need to be removed or a different water source used).
22. Breakpoint chlorination
It means adding chlorine to the water up to the point where the chlorine demand has been
satisfied. Any chlorine added after this point (chlorine breakpoint) will be free chlorine.
When chlorine is added to the water, the first things it will oxidize are minerals (Fe2+,
Mn2+…), forming insoluble oxides (stage not shown on this figure). These reactions are quick,
so as long as there are non-oxidised minerals in the water there will be no chlorine residual
(or Free chlorine or Available chlorine).
The second thing free chlorine will react with is ammonia (or other nitrogen compounds), if
available. They combine to form chloramines (combined chlorine). Initially they form
monochloramines (the desired chloramines for disinfection)
chlorine breakpoint

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From 5:1 to 7:1 (chlorine:ammonia) monochloramines are destroyed to form di- and
trichloramines and nitrogen gas is released to the atmosphere. Adding more chlorine actually
reduces chlorine residual, during this stage.
On the last stage, more chlorine added results in more Free chlorine. The free chlorine is
mainly HOCL and OCl- (their proportions are pH dependent). Formation of THM is possible.
23.Chlorinating chemicals
23.1. Sodium hypochlorite
Sodium hypochlorite is safer than chlorine gas and requires less training and safety measures
to be used. Being liquid, relatively simple to dose. On the downside, it naturally degrades over
time and it is more expensive than chlorine gas.
23.2. Calcium hypochlorite
Calcium hypochlorite is the safest form of chlorine. It is very stable, providing a reliable dosing,
and easy to use. On the other hand, the precipitated solid may compromise the chemical
dosing and only cost-effective for small scale (wells and swimming pools).
24. Sodium hypochlorite properties
Sodium hypochlorite is clear and slightly yellow. It is a strong oxidant and increases the pH of
the water.
The main limitation of sodium hypochlorite is its instability. It degrades over time. The
chlorine evaporates from the solution and salts are formed blocking the pipework. For this
reason, it needs to be used shortly after delivery. Heating or sunlight can only accelerate de
hypochlorite disintegration.
It can be formed by electrolyzing brine water or by adding chlorine gas to caustic soda (NaOH).
25. Calcium hypochlorite properties
Calcium hypochlorite is solid (commercialized in powder or tablet) and stable. Being solid, it
is safer than chlorine gas or liquid hypochlorite and its stability makes the dosing more reliable
than sodium hypochlorite. It is less corrosive than liquid chlorine. Calcium hypochlorite has
lower pH than liquid hypochlorite, making disinfection more effective. Because it is relative
safe, it requires relatively little training and safety measures to be used. Highly cost-effective
at small scale.

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26. Health effects of hypochlorite exposure
Hypochlorite releases toxic gases that can irritate the eyes, airways and lungs. It can also
cause cough, chest pain and pulmonary oedema. In contact to skin it causes damage by
liquefaction necrosis. The good side is hypochlorite salts are not considered carcinogenic to
humans.
27. Uses for chlorine in industry (other than water treatment)
Chlorine is used to make PVC (polyvinyl chloride). It is a polymer. PVC can be used for a range
of products from windows to drinking water pipelines.
Chlorine is also needed to make polyurethane, solvents, chemicals for paper treatment and
other chemicals.
28. Chlorine generation
There are different ways to produce chlorine. A common way is from brine (salty water)
electrolysis. The chemical formula for the reaction is:
2𝑁𝑎𝐶𝑙 + 2𝐻2 𝑂
𝐷𝐶 𝑝𝑜𝑤𝑒𝑟
→ 𝐶𝑙2 + 𝐻2 + 2𝑁𝑎𝑂𝐻
(sodium chloride + water → chlorine + hydrogen + sodium hydroxide)
It is needed 33 amps and 1.67 kg of salt to produce 1 kg of chlorine. Hydrogen and sodium
hydroxide are the by-products of chlorine production.
28.1. Parts of a chlorine generator
• DC power supply: A direct current power supply with strong amperage is needed.
• Power controller: Controls the power supplied to the generator
• Cathode compartment: Pressurized water is injected in this compartment. Hydrogen
and Sodium hydroxide are released from here.
• Another compartment: It is filled with brine (salty water) to a certain concentration.
Chlorine gas is formed in (and removed from) this compartment.
29. Oxidation processes
Generally speaking, the way they kill microorganisms is by destroying their cell membrane.
Once the membrane is broken, microorganisms can no longer function and die.
29.1. Oxidizing agents
• Sodium hypochlorite: It I the main constituent of household bleach, used to disinfect
surfaces in general. An be used to disinfect swimming pools and drinking water as
well.

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• Chloramines: It results from the reaction of HOCl with ammonia (NH3). Used in
drinking water treatment as a secondary disinfectant because it lasts longer and
produces less disinfection by-products. Not good for primary disinfection as it is a
relatively weak disinfectant and would require an extremely long contact time I the
contact tank.
• Chlorine dioxide: Used to disinfect drinking water. Produces less disinfection by-
products than chlorine.
• Hydrogen peroxide: Used in hospitals to disinfect or sterilize surfaces. Leaves no long-
term residues. Can be used on its own or in combination with other chemicals. (I saw
it installed in 2 water treatment plants, but was never used.)
• Performic acid: Formed by reacting hydrogen peroxide with formic acid. It is a
powerful perorganic acid used for disinfecting the feet of swimming pool users,
among other uses.
30. Requirements for a chlorine room design
Chlorine is a great disinfectant, but also a dangerous chemical. A chlorine storage room must
meet a range of conditions ensure effective and reliable dosing, but also for safety reasons.
• Heating: Chlorine rooms must be kept at 15 oC, minimum, to ensure the pressurized
chlorine is in the gas form when leaving the chlorine drum and through the pipework.
To avoid any possibility of chlorine condensation in chlorinators, the chlorinators
room (or area) should be kept at 3-5 oC higher than chlorine room.
• Ventilation: Chlorine rooms must have powerful exhaust ventilation. It should be able
to renew all the chlorine room air within one minute.
• Safety: Chlorine rooms must be alarmed, have strong doors and properly locked,
making sure no unauthorized person gets access to the inside of the building. Remotely
operated shutoff valves (ROSOV) are installed to isolate the chlorine drums, if needed. They
work with compressed air, so can work even in case of power loss. Safety shower must be
available.
• Ground floor installation: If there is a choice, chlorine drums should be stored on
ground floor. As chlorine is denser than air, storage on ground floor will minimise the
gas dispersion and environmental and health risks.
31. Chlorine leaks and corrective measures
In case of a minor leak (<5ppm - gas sensors will tell), I should call 2 toxic gases trained fitters
to find and fix the leak. Ammonium hydroxide can be used to locate the source of the leak, as
it forms a dense white smoke in contact with chlorine. Ventilation helps clearing the area.
In case of a major leak (>5ppm) a dedicated emergency response team must be called to deal
with the leak. In the meantime, I should evacuate everyone from the area, evacuate
neighbourhood if at risk and inform site supervisor. Everyone entering the contaminated area

14. 14
must wear the adequate personal and protective equipment (PPE). These events may need
to be reported to the Health & Safety Executive (HSE) or other organisms.
32. Chlorine dioxide production
Chlorine dioxide (ClO2) is not the most popular choice as disinfectant, but it is more reactive
than chlorine and does not produce trihalomethanes (THM). As it cannot be compressed to
be transported, it needs to be produced on-site.
There are 3 proven ways to produce ClO2 efficiently. One way is by adding sodium chloride
(NaClO2) to chlorinated water (with Cl2 gas). It forms chlorine dioxide. Another method is by
combining sodium hypochlorite (NaOCl) and hydrochloric acid (HCl). The la method uses
sodium chlorate (NaClO3) and sulfuric acid (H2SO4).
33. Ozone
Ozone (O3) is likely to be the strongest oxidizing agent used for water treatment. Liquid ozone
is very unstable and explosive, so it cannot be stored or transported. It needs to be produced
on site from air or, more often, from oxygen. The oxygen (O2) under high voltage (10000 to
20000 volts) converts to Ozone with an efficiency of about 10%.
3𝑂2
𝐸𝑛𝑒𝑟𝑔𝑦
→ 2𝑂3
In use, the ozone converts to oxygen molecules (O2) and oxygen atoms (O).
𝑂3 → 𝑂2 + 𝑂
It is the highly unstable and reactive oxygen atom that will oxidise and work as disinfectant.
It is very effective in water treatment and can used at the beginning of the treatment works,
killing algae and bacteria, destroying pesticides and then helping all the downstream stages,
or at the end of the treatment helping the disinfection stage. The needed contact time is short
(1-2 min). The mixture can be promoted by ozone diffusers in a contact tank or by a static
mixer.
The ozone release to the air is burnt on site, releasing oxygen only.
2𝑂3 → 2𝑂2
Other ozone advantages: does not produce trihalomethanes (THM) or haloacetic acids (HAA),
removes turbidity in some conditions and inactivates cryptosporidium (to some extent or
higher dose than needed for other ends). As mentioned before, it also kills algae and destroys
pesticides.
The main disadvantages are the formation of other disinfection by-products, as bromate (in
the presence of bromide), the inability to give residual disinfectant and the need to have
ozone produced on-site (adding cost and maintenance). Although being a powerful

15. 15
disinfectant, it usually does not replace other disinfection methods that provide some
residual protection.
34. Bacteriological monitoring
The test used to count total coliforms (or more precisely colony forming units – CFU) and E.
coli is the Colilert test. It is relatively simple and gives results within 18-24 hrs of incubation
at 35 oC with the Colilert reagent. As on the picture below, the cells with coliforms turn yellow
and the ones with E. coli specifically turn fluorescent under UV light. The result is given in
CFU/100 ml of sample.
Another test used is the SimPlate. After incubation, the wells with coliforms will change colour
and the ones with E. coli will become fluorescent (similar to previous method).
35.Disinfection by-products
Water disinfection has proved to be by far to be much better for public health than its risks,
but there are some that have been closely monitored. When disinfectants are added to the
water they do not react only with pathogens, but react also with other chemicals and
sometimes forming undesired products, called disinfection by-products.

16. 16
The Disinfectants and Disinfection By-product Rule (DBPR) states the limits for
trihalomethanes (THM), haloacetic acids (HAA) and others, as well as the calculation method.
36.Trihalomethanes
Trihalomethanes (THM) are disinfection by-products (DBP). They are a group of 4 chemicals:
chloroform, bromodichloromethane, dibromochloromethane, and bromoform; also referred
as Total Trihalomethanes (TTHM). The issue associated with THM is they are potentially
carcinogenic, among other health issues. Thus, their presence in the water must be minimised
and prevented.
They result form the reaction of chlorine with organic and inorganic matter. The best way to
prevent the formation of trihalomethanes is preventing the existence of their precursors.
Organic and inorganic matter can be removed before disinfection by coagulation or filtration.
Coagulation will be more effective at pH 4-5 and ferric coagulants better than alum ones. The
more effective these processes are, the less THM can be formed. Additionally, clean water
will automatically require less chlorine for an effective disinfection, reducing the other
precursor. Chlorine can be further reduced by pre-oxidation using different a method (but
potentially at the cost of different DBP).
The limit for TTHM in drinking water is 100 µg/l (100 ppb) in the UK and 80 µg/l (80 ppb) in
the US.
37. Public notice
A public notice is needed when a treatment plants fails a treatment or a prescribed
concentration or value (PCV). It is also needed when there is a failure in quality monitoring or
testing.
A public notice will be needed in case of exceedance in nitrates concentration, total coliforms
or epidemics, for instance. Users will be informed about the issue and advise about action
needed (ex: to boil water before use…).
38. Natural organic matter removal
Natural organic matter (NOM) reacts with chlorine (or other disinfectants) producing the
undesired disinfection by-products (DBP). Thus, NOM should be removed prior to disinfection
to avoid DBP. Ways to remove NOM:
• Coagulation and clarification: NOM are naturally removed by this largely used
process. But this process can be optimized for NOM removal by optimizing pH and
increasing inorganic coagulant dosing.
• Adsorption: Activated carbon is used to adsorb organic matter in general on its
extremely large surface area (~1000m2/g).

17. 17
• Membranes: Membrane have been showing effective to remove many contaminants,
including NOM.
39. Quality control
The quality control starts at the sampling point. Containers must be approved and samplers
(at least for drinking water) must be qualified and authorised personnel.
In the laboratory, all procedures must follow known and reliable methods. Personnel must be
highly trained and the equipment must meet high standard of consistency an precision.
Operating procedures are documented and followed.
40. Some water treatment chemicals
• Limestone: Calcium carbonate (CaCO3)
• Gypsum: calcium sulphate dihydrate (CaSO4·2H2O).
• Activated carbon: Also known as activated charcoal.
• Iron sulphate: Could be Ferrous sulphate [FeSO4], but for water treatment it is used
Ferric sulphate, or Iron (III) sulphate [Fe2(SO4)3], due to its higher positive charge.
• Salt: Sodium Chloride (NaCl).