This document discusses disinfection methods used to treat water. It lists the members of the Disinfection Group and explains that disinfection is necessary to kill pathogens in water and prevent waterborne diseases. The document then discusses various pathogens and the diseases they cause. It explains that the goal of disinfection is to reduce microorganisms to a safe level and lists some requirements of effective disinfectants. The document summarizes various disinfection methods including chlorine, bromine, iodine, ozone, ultraviolet light, and their advantages and disadvantages.

1. Disinfection Group 2013
1

2. GROUP MEMBERS
 Miss C Chinamano
 Mr T Matsvayi
 Mr B Makuwe
 Mr O Mazvimbakupa
 Mr P Chatambudza
 Mr T Nyikayaramba
 Mr G Dube
 Mr E Majange
2

3. When water comes out of filter plants, it may contain
bacteria and other micro-organisms, some of which may be
pathogenic. It is therefore necessary to disinfect water to
kill disease causing micro-organisms and thus to prevent
water borne diseases. Disinfection follows filtration. When
the aim is to kill all the micro-organisms whether harmful
or not, the process is called sterilisation. The aim of
disinfection is to reduce the number of micro-organisms to
a safe limit. Disinfection requires complicated mechanisms
that need attention of skilled operators to avoid breakdown
and incorrect dosage.
3

4. Pathogen Disease Caused
Bacteria:
Anthrax anthrax
Escherichia coli E. coli infection
Myobacterium tuberculosis tuberculosis
Salmonella salmonellosis, paratyphoid
Vibrio cholerae cholera
Viruses:
Hepatitis Virus Hepatitis A
Polio Virus polio
Parasites:
Cryptosporidium cryptosporidiosis
Giardia lamblia giardiasis
4
The table shows some of the harmful micro-
organisms we are worried about.

5.  To kill pathogens that are still present since the treatment
processes kill approximately 99% of the pathogens. (90% is
a precautionary measure for water that is safe to drink.
 To prevent the possibility of re-growth of micro-organisms
in storage and distribution systems.
 Disinfection can be broken down into 2 categories namely:
 Primary disinfection- this is a chemical oxidation process
undertaken at a treatment facility which inactivates
pathogens at the source water. Common technologies
involve chlorine, monochloramine, chlorine dioxide, ozone
ultra violet light in terms of their effectiveness against
various pathogens.
5

6. Secondary disinfection- occurs throughout the distribution
system prevents bacterial growth. The purpose of a
secondary treatment is to maintain the water quality
achieved at a water treatment plant throughout the
distribution system up to the tap. Secondary disinfection
provides a final barrier against microbial contamination.
Common technologies include chlorination as well as
chloramination.
6

7. 1 The disinfectant should be effective and quick in killing the
micro-organisms potentially present in water within the contact
time available, the range of temperatures encountered and the
anticipated fluctuations in the composition, concentration and
condition of water being treated.
2 The disinfectant should be readily available at reasonable cost.
3 It should be safe to handle, transport, apply and control.
4 It should not render the water toxic aesthetically or otherwise
for its intended use.
5 It should be readily soluble in water at the concentration
required.
6 It should be tasteless and odourless.
7 It should be able to persist in residual concentrations as a
safeguard against recontamination.
7

8. A disinfectant either destroys or inactivates the pathogens
by way of the following four mechanisms:
1 Damages or destroys the cellular structure of micro-
organisms.
2 Alters the cell permeability.
3 Interferes with growth by changing the colloidal nature of
cell protoplasm.
4 Inactivation of critical enzyme systems responsible for
energy yielding metabolism.
8

9. Various methods of disinfection can be broadly classified
under (a) physical methods and (b) chemical methods.
1 Physical methods: physical methods include the
following:
Disinfection by heat: boiling of water.
Disinfection by light: sunlight is a natural disinfectant.
Irradiation by ultra-violet rays intensifies disinfection
especially when treating large quantities of water.
2 Chemical methods: these include the following:
-Oxidising chemicals: halogens such as chlorine, bromine
and iodine.
Ozone and other oxidants such as potassium permanganate
and hydrogen peroxide.
9

10.  Metal ions such as silver and copper ions.
 Alkalis and acids: pathogens do not last long in highly
alkaline (ph>11) or highly acidic (ph 3) waters. The
destruction of bacteria by caustic lime is an example.
 Other chemicals such as surface active chemicals (soaps,
synthetic detergents), phenols, alcohols, ammonium
compounds but the most common ones are oxidising
agents of which chlorine is the mostly used chemical.
10

11. When bromine reacts with water the following reaction
takes place
BrCl+H2O →HOBr+HCl
The disinfectant is (HOBr) hypobromous acid
Though the disinfectant is as reliable and flexible as
chlorine
Hypobromous acid react with ammonia to give the
following reactions
11

12. HOBr+NH3→NH2Br+H2O
HOBr+ NH2Br→NHBr2+H2O
HOBr+NHBr2→NBr3+H2O
Advantages
 Needs less contact time than chlorine
 Breakdown to harmless chloride and bromide salts
Disadvantages
 Not yet proven a disinfectant
12

13. Was usually used during the 1914 -1918 war
React with water to form the following equation
I2+H2O→HIO+I+ + H+
HIO→H+ + IO-
I2 and HIO are the disinfectants
Iodine does not readily react with organic matter so it is
more effective than chlorine and bromine for infected
waters
13

14.  does not react with ammonia
 can be easily stored and applied simply
 not affected by pH over the normal range of potable water
 reacts only with organic impurities
14
 impact colour and taste
 produces allergy reactions
 costs more than chlorine
 high concentrations required

15.  Ozone is a toxic blue gas that exists as an allotropic form
of oxygen (O3).
 It is highly unstable as a gas and boils at 1120C
 It is highly soluble however the solubility is affected by
the low partial pressure so it is difficult to obtain a
concentration of ozone which is greater than 1 to 2 mg/l
 Ozone is formed by passing O2 or air through an electrical
discharge of 5000 to 20000 volts at 50 to 500 Hz.
Ozonation can produce 25g of O3 per m2 of air.
15

16. The process is
Air + high electrical voltage→ionised oxygen + heat
O2→2O2
Ionized oxygen + non-ionised oxygen→ozone
2O+2O2→2O3
Disinfection process
O3+H2O→HO3+OH-
HO3+OH-→2HO2
O3+H2O→HO+2O2
HO+HO2→H2O+O2
16

17. H2O and HO are the disinfectants and have oxidising
powers the bacteria is killed directly by cell lysis
17
Advantages
 Does not produce dissolved solids and no residuals
 Less environmental impacts
 Not affected by ammonia or pH
Disadvantages
 measurement of ozone is very difficult
 suitable for high quality waters with low turbidity
 does not contain a residual
 it is expensive to produce because of high
demand of electricity

18.  this type of dis infection inactivates the micro organisms
 uv radiation lies between a wavelength of 15 to 400nm
 however spores cysts and algae are hard to in activate
using uv light
 uv light can be created by using a low to medium
pressure mecury vapour arc/cathode discharge lamps
 the turbidity and colour in water reduces the intensity of
radiation
18

19.  does not change the chemical composition of water
 no handling of chemicals
 no danger of overdosing
 no addition of taste or colour to the treated water
 has limited contact time
 little maintenance required
19

20.  it has no residual effect
 it is more costly than chlorine
 Only low turbidity waters or rather high quality waters
can be effectively treated
20

21.  What really happens the instant chlorine is added to
water.
 When added to water, chlorine reacts rapidly to form
Hypoclorous acid, hydrogen and Chloride ions
(effectively dissolved Hypocloride ions).
 CL₂ + H₂O ↔ HCLO + H⁺ +CL⁻ (pH ˂ 4)
 Gaseous Chlorine drops to zero at a pH = 4, the
Hypoclorous acid may then dissociates at a pH approx
10.
 HCLO ↔ H⁺ + OCL⁻ (pH range of 5 – 10 )
21

22. Where a solution of sodium or calcium hypoclorite
is used they dissociate as follows,
Ca(OCL)₂ ↔ Ca²⁺ + 2OCL⁻ (calcium hypoclorite)
NaOCL ↔ Na⁺ + OCL⁻ (sodium hypochlorite)
for equilibrium the pH range should be 5 – 10.
The Sodium or Calcium ions will also affect the hydrogen
ion concentration. Sodium and calcium hypoclorite
therefore have identical reactions in water as chlorine, but
the presence of the metal ions affects the concentration of
hydroxide ions, resulting in an increased pH value, whereas
chlorine gas lowers the pH value. The reactions between
chlorine and ammonia is also very impotant in
22

23. chlorination, they remove hypochlorus acid and the
compounds formed are disinfectants used to provide
residual disinfection in the distribution systems.
23

24. Chlorine or hypochlorous acid reacts with the ammonium
ion to successively replace the hydrogen atoms with
chlorine.
NH₃ + HOCL ↔ NH₂ CL + H₂O
(monochloramine)
NH₂ CL + HOCL ↔ NHCL₂ + H₂O (dichloramine )
NHCL₂ + HOCL ↔ NCL₃ + H₂O (trichloramine)
24

25. The relationships between the amount of the three
chloramine depends on the pH value and the ammonia
concentration in the water. Trichloramine forms only at
very low pH values, while the other two prevail during
water purification. Dichloramine being the more powerful
bactericide, however, chlorination of water containing
ammonia also leads to the production of nitrogen gas
2NHCL₂ + HOCL → N₂ + 3HCL + H₂O
25

26. Nitrogen produced converts hypoclorite to hydrochloric
acid, nitrate production follow but at a minor reaction.
Hypochlorus acid and hypochlorite ion together form “ free
chlorine “, the chloramines are known as “ combined
chlorine
26

27. Chlorine is introduced to water through various direct and
indirect methods. Direct methods are those in which
chlorine is introduced as a pure element normally in liquid
or gaseous state. Indirect methods are those in which the
chlorine is introduced through chlorine-containing
compounds. In whichever way the chlorine is introduced to
water, certain general reversible reactions takes place in the
water producing hypo-chlorous acid and hypo-chlorite ions
and these are the one that take place in the disinfection.
27

28. Generally, chlorine is applied to water in one of the
following forms:-
 Bleaching powder or hypochlorite.
 Chloramines
 Chorine dioxide
 Free chlorine gas/liquid
28

29. Bleaching powder
This is chemically calcium hypo-chlorite Ca(OCl)2 and is
chlorinated lime, containing about 33.5% of
chlorine(Punmia,2010). The process of chlorination using
hypo-chlorites in called hypo-chlorination. When
introduced to water calcium hypo-chlorite reacts as
follows:-
Ca(OCI)2 +H2O ↔ 2HOCI+Ca(OH)2
in the same way in which Sodium hypo-chlorite reacts that
is:-
NaOCI+H2O ↔ HOCI+NaCI
29

30.  Bleaching powder is however unstable and it losses
strength during storage or exposure to air. For this cause
bleaching powder is only used on small installations or
under emergency.
 Commercially compounds such as High Test Hypo-
chlorites(HTH), Pittcide, Pittchlor, Hoodchlor etc are
used instead of bleaching powder. High Test Hypo-
chlorites having an available chlorine content of 65-70%
are more stable easily soluble, free flowing and non
hygroscopic.
 Hypochlorites are applied to water as a solution by means
of a hypochlorite feeding apparatus.
30

31. Chloramines
These are compounds of chlorine and ammonia. In the
treatment ammonia is added to water just before chlorine is
applied and the following reactions takes place.
H2O +CI ↔ HOCI+HCI
NH3+HOCI ↔ H2O+NH2CI
NH2CI+HOCI ↔ H2O+NHCI2
NHCI2+HOCI ↔H2O+NCI3
31

32. The formation of a particular type of chloramine is
dependent upon the pH range of the water. Although
dissolves quickly in water it does not diffuse easily hence it
should be mixed with help of mechanical means for at least
20 minutes to 1hour before the application of chlorine.
Ammonia may be applied in liquid or gaseous state or as
ammonium sulphate or as ammonium chloride.. The
disinfection reactions are much slower than with chlorine
alone and therefore a longer contact period is provided
before the water is used.
32

33.  It is more effective than chlorine lone. Its bactericidal
effects persists for a longer period.
 Avoids tests and odour especially those due to phenols.
 The quantity of chlorine required becomes less especially
if organic matter is present in large quantities.
 Water treated with this causes less irritation to eyes and
nose and hence it is suitable for treating waters for
swimming pools.
 There is no danger of overdose.
33

34.  Chlorine is added to water in either gaseous or liquid
form. Chlorine gas is a greenish-yellow poisonous gas ith
a typical odour. When chlorine gas is subjected to a
pressure of 7kg/cm2 it is converted to liquid. Hence liquid
chlorine is stored and supplied in metal containers under
a pressure of 10.5kg/cm2.
 The chlorine doses depends on organic matter present in
the water to be disinfected, ph of water, amount of carbon
dioxide present in the water, temperature and time of
contact.
34

35. Chlorine dioxide is an unstable gas and therefore is
produced at the point of use by passing chlorine through
sodium chlorite(NaCIO2) in the following reaction.
2NaCIO2+CI2 ↔ 2NaCI+2CIO2
It is highly effective in control of certain tastes and odour
problems.
It is entirely harmless in aqueous solution.
35

36.  Plain Chlorination
 Pre-Chlorination
 Double or multiple chlorination
 Breakpoint Chlorination
 Super Chlorination
 Dechlorination
36

37.  This is the application of Chlorine to relatively
plain or raw water supply as it enters the
distribution system.
 Involves also the chlorination of raw waters in tanks
or reservoirs to check weed growth organic matter
,algae and bacteria. Chlorination removes colour
and odour from water.
37

38. Application of Chlorine to water before its treatment
,especially before the filtration and sedimentation. This reduce
the amount of coagulant required because of the oxidation of
organic matter.
 Pre-chlorination reduce coagulant quantity needed
 Reduce bacteria load on filters
 Controls algae and planktons in basins and filters runs
 Eliminates taste and odour
Residual Chlorine
For satisfactory disinfection , pre chlorination is done so
as to maintain 0.3-0.4 mg/l free available chlorine.
38

39. It is the application of chlorine to water after its treatment
.It is the standard form of chlorination. Dosage should give
residual chlorine before water enters the distribution system.
Post Chlorination protects against contamination from cross
connections.
39

40.  Double refers to the application of chlorination at two
or more points in the purification process. Double
chlorination is resorted to, in which chlorine is
applied before water enters the sedimentation tanks
and after it leaves the filter plants.
40

41. 41

42.  Chlorine is applied to water two actions take place one
after the other:
 Kills bacteria and disinfection is affected ,and
 It oxidizes the organic matter.
 Chlorine reacts with ferrous and sulphate ions when
added with water,reacts with ammonia forming
chloramines and chloramines. This result in an increase in
Chlorine that closely follow the applied chlorine dose,
with the difference due to other reactions (Binnie et
al.,2013).
 After all the ammonia is converted to dichloramine the
above equations takes place which involves the formation
of HCI acid and the chloride ion, neither which are
detected as free residual chlorine (Binnie,2013).
42

43.  This explains the decrease of the second curve
which shows the residual chlorine
concentration decreasing with increased
chlorine dose ,with apparent anomalous effect
being associated with the formation of
nitrogen.
 After the completion of the reaction of
dichloramines to form trichloramines or
nitrogen, the chlorine residual increases as the
chlorine dose increases.
43

44.  The point at which the chlorine residual starts to increase
again is referred to as the break point.
 Prior to the breakpoint ,chlorine is present predominantly
as combined chlorine ,after the breakpoint free chlorine
predominates.
 The Chlorine demand of any given water is the amount of
chlorine required to take the reaction to the break point. It
has to be determined by experiment.
44

45. The application of chlorine at or slightly higher
than the breakpoint concentration will have the
following advantages.
 It will remove taste and odour
 It will have adequate chlorine removal
 It will leave a desired chlorine removal
 It will complete the oxidation of ammonia and
other compounds
 It will remove colour due to organic matter
 It will remove manganese
45

46.  This is the application of Chlorine beyond the stage
of break point. It destroys odours and test
resulting from chloro products formed from the
decomposition products of algae and algae.
 Super-chlorination can be done at any point or
points of chlorination.
 Usually applied after filtration, Super chlorination
follows longer retention or contact time of 30-60
minutes.
 Super chlorination can be adopted when they is an
epidemic in the local community, when water is
liable to sudden fluctuations in chlorine demand
due to high organic impurities and or when water
contains cysts E. Histolytica an organism causing
amoebic bacteria.
46

47.  Dechlorination is a process that removes
excess chlorine from water before distribution
to the consumers to avoid chlorine tastes.
Achieved by aeration and or chemicals .
 Dechlorination should be done in a method
that Residual Chlorine remains in water.
47

48.  contact time
 concentration and type of chemical
 intensity and nature of physical agent
 temperature of water
 pH of water
 types of organisms
 turbidity
 Presence of metallic compounds
48

49. For a given concentration of disinfectant, the longer the
contact time, the greater the kill. For a clear bright water a
contact time of 30 minutes may be suitable using a dosage
of 0.2-0.5 mg/l. The rate of bacterial kill is directly
proportional to the number of living organisms remaining at
specified time and can be given by Chick’s law:
Where: N = number of organisms at time t remaining or
living
t = time
k = constant, time-1
Integrating at t=0 and t= t
49

50. 50

51. Increasing temperature results in more rapid kill and
reduces the disinfection time because many reactions are
accelerated by an increase in temperature.
Increase in temperature:
 lowers surface tension
 increases acidity
 decrease viscosity
 diminishes adsorption

52. pH of water
Increasing pH reduces effectiveness of chlorine. The
effecting disinfecting compound hypochlorous acid is
formed in greater quantities at low pH than at high pH
values. Of great interest is vegetative bacteria, bacterial
spores and amoebic cysts is affected by pH value.
Disinfection is a complex process that is difficult to model,
given the wide range of
variables that affect its efficiency. In practice, simple design
criteria that give adequate reassurance of effective
disinfection are applied; the effectiveness of disinfection is
verified using microbiological indicator parameters,
notably the absence of coliform and E. coli organisms in
the treated water.
52

53. Cntp = k const
Where:
C =concentration of disinfectant
n =constant
tp =time required to effect a constant % kill
Thus the required degree of disinfection can be achieved by
a high dose for short contact time or lower dose for a longer
contact time.

54. Metallic compounds such as iron and manganese in
solution in the water utilises large amounts of chlorine to
convert these into their higher stages of oxidation which are
insoluble in water hence iron and manganese must be
removed.
54

55.  Organic and inorganic matter reacts with most oxidising
disinfectants and reduces their effectiveness e.g.
sulphates, nitrites and ferrous salts.
 Their demand must first be met before bactericidal action
commences.
 Turbidity reduces effectiveness of the disinfectant by
absorbing and protecting entrapped bacteria.

56. advantages disadvantages
Cheap & readily available as gas,
liquid or powder
chorine is a poisonous and toxic
gas
high solubility 7000mg/l Corrosive, requires special
alloys/non metal conduits are
needed)
leaves a residual in solution requires careful handling,
operation and storage
toxic to most micro organisms
the vapour is irritant
removes iron and manganese and
ammonia nitrogen during
oxidation
strong oxidising agent, reacts
with most elements and
compounds
destroys taste and other odour
compounds
gives rise to taste and odour
in the presence of phenols

57. Chlorine usage in the treatment of 25000 m3/day is
9kg/day. The residual chlorine after 10minutes contact
time is 0.2mg/L. Calculate the dosage in milligrams per
litre and the chlorine demand of the water
Solution:
Water treated per day = 25000 m3 = 25x106 L/day
Chlorine consumed per day = 9kg = 9x106 mg/day
therefore chlorine used per litre of water = 9x106
25x106
=0.36mg/L
57

58. Also , residual chlorine = 0.2mg/L
chlorine demand = 0.36-0.2= 0.16mg/L
58

59. Results of chlorine demand test on a raw water are given
below. Determine the break-point dosage and the chlorine
demand.
59
Sample no. Chlorine dosage
mg/L
Residual chlorine
after 10mins
contact.(mg/L)
1 0.2 0.18
2 0.4 0.34
3 0.6 0.48
4 0.8 0.46
5 0.9 0.27
6 1.0 0.18
7 1.2 0.38
8 1.4 0.58
9 1.6 0.78

60. From the curve, beak-point occurs at point D, at which the
applied chlorine = 1.0mg/L
Therefore
break point dosage = 1.0mg/L
chlorine demand at break-point = 1.0-0.18
= 0.82mg/L
It is observed that since the slope of curve C is 450, the
chlorine demand (=0.82mg/L) remains constant after the
break-point, since all additional chlorine added after point
D appears as free chlorine
60

61. 61