1. Dr. Prabha Joshi, Asst. Professor Department: B.E. Civil Engineering
Disinfection of Water
Dr. Prabha Joshi, Asst. Professor
Department: B.E. Civil Engineering
Subject: Environmental Engineering - I
Semester: VI
Teaching Aids Service by KRRC Information Section

2. Key Terms Cleaning Disinfection Sanitation Sterilization
Removal of Visible physical dirt and stains. A clean surface is defined as being free from soil (e.g. food residues), free from bad odours, be non-greasy to the touch and have no visible oxidation (e.g. rust).
Disinfection
Removal of harmful bacteria / microbes. A sanitized clean surface is defined as a clean surface that is substantially free from pathogenic microorganisms and undesirable numbers of spoilage microorganisms.
Sanitation
Process in which most or nearly all micro organisms (whether or not pathogenic) killed through use of chemicals, heat, ultraviolet rays e.g. Milk is disinfected by heating up to 100degree C for atleast 10 sec. to kill most microbes (but not necessarily their spores) to make it more stable than pasteurized milk
Sterilization
Total destruction of all microorganisms ( whether or not pathogenic) and their spores, usually through the use of drastic methods such as concentrated toxic/ non toxic chemicals (Chlorine, formaldehyde, glutar-aldehydes, etc.), very high temperatures, or intense radiation. A sterilized item cannot support life in any from.

3. WHO on alternative disinfectants: Iodine, silver, copper, quaternary ammonium compounds
none of them are considered suitable for long-term use to disinfect drinking water
Iodine is difficult to deliver to water and can cause adverse health effects
However, iodine, either dissolved in water or in the form of an iodinated exchange resin, has been used for short-term water treatment
Silver and copper are difficult to deliver to water and are only bacteriostatic.
Quaternary ammonium compounds are limited in availability, costly and not effective against viruses and parasites.

4. Ozone: O3
Very strong oxidant (very low C▪t values) but has no residual disinfection power
Generated by passing high voltage through the air between two electrodes
More expensive than chlorination but does not produce trihalomethanes which are suspected carcinogens
Widely used in Europe, limited use in U.S.

5. Ozone (advantages and disadvantages)
Highly effective against all type of microbes
Disadvantages
Expensive
Unstable (must produced on-site)
High toxicity
High chemical hazards
Highly sensitive to inorganic and organic loads
Formation of harmful disinfection by-products (DBP’s)
Highly complicated maintenance and operation
No lasting residuals

6. Ox idant Advantages Disadvantages Chlorine Chloramines
Strong oxidant
Persistant residual
Chlorinated by -
products
Taste and odor problems
pH influences effectiveness
Chloramines
No trihalomethane
formation
Weak oxidant
Some organic halide
Taste, odor, and growth problems
Chlorine dioxide
Relatively persistant
residual
No trihalomethane prod.
No pH effect
Total organic halide formation
ClO3 and ClO2 by products On
site generation required
Hydrocarbon odors possib le
Ozone
No trihalomethane or
organic halide formed
No taste or odor prob.
Little pH effects
Coagulant aid
Some by products
biodegradable
Short half
life
Energy intensive
Some by products biodegradable
Complex
generation
Corrosive

7. Ultraviolet irradiation
has been used in wastewater disinfection for more than 50 years
Increased interest after the discovery of its remarkable effectiveness against Cryptosporidium parvum and Giardia lamblia in late 1990’s

8. Ultraviolet irradiationC G T
DNA
physical process
energy absorbed by DNA
pyrimidine dimers, strand breaks, other damages
inhibits replication UV

9. UV Disinfection
Optimum ultraviolet light wavelength range for germicidal effect: 250 nm nm
Low pressure mercury lamps emit nm
Damages microbial/viral DNA and viral RNA by blocking nucleic acid replication
Does not produce toxic by-products
Higher costs than chemical disinfection, no residual disinfection

10. UV disinfection (advantages and disadvantages)
Very effective against bacteria, fungi, protozoa
Independent on pH, temperature, and other materials in water
No known formation of DBP
Disadvantages
Not so effective against viruses
No lasting residuals
Expensive

11. Disinfectants Bromine
Bromine Bromine can be used for the disinfection of swimming pool and cooling tower water. It is not used for the disinfection of drinking water. What are the characteristics of bromine? Bromine has the atomic number 35. Like chlorine, it is a halogen and it easily reacts with other elements. In nature bromine can only be found in compounds. These combinations are called bromides. Bromides are used to obtain pure bromine and to produce bromine products. After fluorine, bromine is the most reactive element. It reacts with many different substances, is very corrosive and destructive on organic material. Bromine is a bleach. It is poisonous in fluid form and bromine vapor is destructive for the human skin, eyes and respirational tract.

12. Bromine can easily be dissolved in water (35 g per L water), carbon disulfide and other organic solutions.
When added to water, bromine forms hypobromous acid.
Hyprobromous acid is a weak acid. It partly dissociates to form hydrogen ions and hypobromite ions. T
he rate of hypobromous acid and hypobromite ions is determined by the pH value of the water. When the pH value is between 6.5 and 9 both hypobromous acid and hypobromite ions can be found in water. 
If water contains ammonia nitrogen, bromamines will be formed (NH2Br, NHBr2 and NHBr3). For disinfection bromamines are as effective as hypobromous acid.

13. Disinfection with bromine
In the United States bromine has been used since the 1930's for the disinfection of water. Bromine substances are disinfectants and can be used as an alternative for chlorine. In swimming pools, bromine is used against the formation and growth of algae, bacteria and odours in swimming water Bromine can be applied in fluid form or in a mixture. When bromine is applied in fluid form, the following equilibrium is established:
Br2 + 2H2O « HOBr + H3O+ + Br-  HOBr + 2H2O « OBr- + H3O-
This equilibrium strongly depends on the pH value. At the pH value that is usually found in swimming pools, bromine is mainly present as hypobromous acid (HOBr). Bromine has to be used combined with an oxidizing agent (for example chlorine or ozone).

14. Table 1: influence of the pH on the formation of hypobromous acid.
Hypobromous acid (HOBr)
hypobromite ion (OBr-)
% bromine as HOBr pH
% bromine as OBr-
100
6,0
0,0
99,4
6,5
0,6
98,0
7,0
2,0
94,0
7,5
83,0
8,0
17,0
57,0
8,5
43,0

15. Silver as a Disinfectant…
Silver is used as a bacteriostatic agent for point-of-use or household water treatment by storing water in vessels composed of silver or passing water through porous or granular filter media impregnated with silver
Many microbes including viruses, protozoan cysts, oocysts, and bacterial spores, are not inactivated at silver concentrations employed for point-of-use drinking water treatment
Bacteria may develop silver resistance
Therefore, silver is not recommended for routine disinfection of household water

16. Key Words
Chlorine Residual- Measurable chlorine remaining after the demand is satisfied.
Chlorine Demand- The amount of chlorine destroyed by reaction with Fe, Mn, turbidity, organics, and microorganisms in the water.
Chlorine Demand= Chlorine Dose- Chlorine Residual
Free Chlorine Residual- point past breakpoint where
HOCl (hypochlorous acid) and OCL- (hypochlorite ion) form.
25 times more powerful than combined chlorine for disinfection
Combined residual chlorine: chlorine combined with organics or ammonia.
NH2Cl (monochloramine) and
NH(Cl)2 (dichloramine)
N(Cl)3 (nitrogen trichloride)

17. Key Words
Breakpoint chlorination: The point at which near complete oxidation of nitrogen compounds is reached . Any point beyond breakpoint is mostly free chlorine (HOCL and OCL-)
C•T (concentration and contact time): Effectiveness of chlorination is dependant on chlorine concentration and contact time.
Sterilization: The destruction of all living things in a sample
Disinfection: The removal or inactivation of disease causing (pathogenic) organisms

18. Chlorine Disinfection Mechanisms
Oxidation of membrane-bound enzymes for transport and oxidative phosphorylation
Oxidation of cytoplasmic enzymes
Oxidation of cytoplasmic amino acids to nitrites and aldehydes
Oxidation of nucleotide bases
Chlorine substitution onto amino acids
DNA mutations
DNA lesions
Rittmann, B. E. and P. M. Huck (1989). "Biological Treatment of Public Water Supplies." Critical Reviews in Environmental Control 19(2):

19. Chick’s Law
The death of microorganisms is first order with respect to time
Thus, the remaining number of viable microorganisms, N, decreases with time, t, according to:
where k is an empirical constant descriptive of the microorganism, pH and disinfectant used.
Integrating with respect to time, and replacing limits (N = No at t = 0) yields:

20. Kinetics of Disinfection
Inactivation is a gradual process involving a series of physicochemical and biochemical steps. Inactivation is described by the equation:
Nt/N0 = e-kt
Where:
N0 = number of microorganisms at time = 0
Nt = number of microorganisms at time = t
k = a decay constant (1/time)
t = time
Ideally, inactivation follows first-order kinetics (blue line), but often non-ideal behaviors occur resulting from clumping of cells or multiple hits of critical sites before inactivation

21. Concentration and Contact Time
Effectiveness of chlorination depends primarily on the concentration used and the time of exposure
Disinfectant effectiveness can be expressed as a C ▪ t value where:
C = disinfectant concentration
t = time required to inactivate a 99% of the population under specific conditions
The lower the C ▪ t, the more effective the disinfectant
In general, resistance to disinfection is in the following order:
vegetative bacteria < enteric viruses < spore-forming bacteria < protozoan cysts

22. Current increase in waterborne diseases:
Giardiasis
Cryptosporidium
Infectious Hepatisis ( viral infection )
Protozoa

23. Scales of the Embedded Virus
Location
Deactivation rate
Dispersed
Very fast
Inside cell with disrupted cell wall
Slow
Inside intact cell
Very slow
1000 nm
1 mm
Virus particles are about 20 nm, HOCl are about 0.2 nm
Viruses and bacteria are embedded within organic matter in turbid water

24. Mass Transport and Chlorine Protection
Chlorine must diffuse through cell contents to reach virus
Organic material inside the cell reacts with chlorine before it gets to the virus

25. Scale this up to a Fecal Aggregate
Turbid water could easily have organic particles that are 10 or even 100 mm in diameter
The amount of organic matter in a small particle and the slow diffusion would provide long term protection for embedded pathogens
10 mm

26. Getting the Right Dose: WHO on Chlorination
Chlorine compounds usually destroy pathogens after 30 minutes of contact time, and free residual chlorine (0.2–0.5 mg per liter of treated water) can be maintained in the water supply to provide ongoing disinfection.
Several chlorine compounds, such as sodium hypochlorite and calcium hypochlorite, can be used domestically, but the active chlorine concentrations of such sources can be different and this should be taken into account when calculating the amount of chlorine to add to the water.

27. Getting the Right Dose: WHO on Chlorination
The amount of chlorine that will be needed to kill the pathogens will be affected by the quality of the untreated water and by the strength of the chlorine compound used.
If the water is excessively turbid, it should be filtered or allowed to settle before chlorinating it

28. Chlorine Reactions Cl2 + H2O  HOCl + HCl- HOCl  H+ + OCl-
Charges +1 -2 +1 -1
Cl2 + H2O  HOCl + HCl-
HOCl  H+ + OCl-
The sum of HOCl and OCl- is called the -
Hypochlorous acid
Hypochlorite ion
free chlorine residual

29. Chlorine and pH HOCl  H+ + OCl- low
HOCl is the more effective disinfectant
Therefore chlorine disinfection is more effective at ________ pH
Dissociation constant is
HOCl and OCl- are in equilibrium at pH 7.5
low

30. Ammonia Reactions NH3(aq) + HOCl  NH2Cl+ H2O-3 +1 -3 +1 +1
NH3(aq) + HOCl  NH2Cl+ H2O
Combined chlorine
NH2Cl + HOCl  NHCl2+ H2O
Rittmann, B. E. and P. M. Huck (1989). "Biological Treatment of Public Water Supplies." Critical Reviews in Environmental Control 19(2):
Substitution reactions…
The combined chlorine maintains its oxidizing potential

31. Removal of ammonia by chlorination
Breakpoint Chlorination: The addition of chlorine in such amount that it Oxidizes the organic matter, reducing matters and free ammonia in raw water. And leaves free residual Cl which disinfect Water.
Removal of ammonia by chlorination
Oxidizing equivalents of chlorine are consumed -3 +1 -1
2NH3(aq) + 3HOCl  N2+ 3Cl- + 3H2O
Rittmann, B. E. and P. M. Huck (1989). "Biological Treatment of Public Water Supplies." Critical Reviews in Environmental Control 19(2):

32. Generalized curve obtained during breakpoint chlorination

33. Break Point Chlorination

34. Advantages of Breakpoint Chlorination
It will remove taste and odour of water.
The desired residual chlorine will remain in water.
It will remove organic matter and Mn.
It will complete oxidation of Ammonia from water.

35. Does Chlorine Completely oxidize organic matter?
4HOCl + CH4  CO2 + 2H2O + 4Cl- + 4H+
Oxidation states
Carbon in organic matter (-4)
Carbon in carbon dioxide (+4)
Chlorine in HOCl (+1)
Chloride (-1)
Therefore should take 4 moles of chlorine (Cl2) per mole of organic carbon
23.6 g chlorine/g organic carbon

36. Common Disinfectants in Water Treatment
Chlorine
Chloramines
Chlorine dioxide
Ozone
Ultraviolet light

37. Chlorine Most commonly used disinfectant
In water chlorine undergoes the following reaction:
Cl2 + H2O HOCl + HCl
HOCl H+ + OCl-
HOCl and OCl- is defined as free available chlorine
HOCl more effective than OCl- due to lack of charge
Presence of HOCL and OCl- is determined by pH
In drinking water 1 mg/L of chlorine for 30 min is generally sufficient to reduce bacterial numbers. In wastewater with interfering substances up to mg/L may be required

38. Interfering Substances
Turbidity can prevent adequate contact between chlorine and pathogens
Chlorine reacts with organic and inorganic nitrogenous compounds, iron, manganese, and hydrogen sulfide.
Dissolved organic compounds exert a chlorine demand
Knowing the concentrations of interfering substances is important in determining chlorine dose

39. Chloramines Chloramines are produced by combining chlorine and ammonia
NH HOCl NH2Cl + H2O monochloramine
NH2Cl + HOCl NH2Cl2 + H2O dichloramine
NH2Cl2 + HOCl NCl3 + H2O trichloramine
breakpoint reaction
Used mainly as secondary disinfectants, e.g., following ozone treatment, when a residual in the distribution system is needed

40. Chloramines (advantages and disadvantages)
Less corrosive
Less toxicity and chemical hazards
Relatively tolerable to inorganic and organic loads
No known formation of DBP (disinfection byproducts)
Relatively long-lasting residuals
Disadvantages
Not so effective against viruses, protozoan cysts, and bacterial spores

41. General Characteristics
Chlorine Dioxide: ClO2
General Characteristics
Highly soluble in water (10x more than Cl2)
Volatile & subject to photo-decomposition
Unstable at high concentrations (>15%) and under pressure ~ must be generated onsite
2NaClO2 + Cl ClO NaCl
Oxidizes, does not chlorinate, using selective one- electron transfer
Does not react with ammonia
Unaffected by pH over broad range (4-8) 41

42. Chlorine dioxide (advantages and disadvantages)
Very effective against all type of microbes
Disadvantages
Expensive
Unstable (must produced on-site)
High toxicity
2ClO2 + 2OH- = H2O + ClO3- (Chlorate) + ClO2-(Chlorite) (in alkaline pH)
High chemical hazards
Highly sensitive to inorganic and organic loads
Formation of harmful disinfection by-products (DBP’s)
No lasting residuals

43. Chlorine Advantages
Chlorine provides a strong residual in the distribution system.
Chlorine can be easily converted to chloramines which also provide a strong residual and do not produce by-products.
Chlorine is easy to apply. Can use it in liquid, solid, or gas form
Chlorine is a relatively inexpensive disinfecting agent.
Chlorine is effective at low concentrations.

44. Chlorine Disadvantages
When chlorine reacts with organic material its' concentration is reduced and trihalomethanes(THM's), haloacetic acids (HAA5), chlorite (when chlorine dioxide is used) and bromate (when ozone is used) are disinfection-by-products (DBP's). These compounds are carcinogenic.
Chlorine provides poor Cryptosporidium and Giardia control.
Effectiveness varies depending on turbidity, [ammonia], pH, etc.
Chlorine is a dangerous and potentially fatal chemical if used improperly

45. TREATMENT OF WATER - CONTD
Methods of chlorination
Plain chlorination
Pre-chlorination
Post chlorination
Double chlorination
Breakpoint chlorination
Super chlorination
De chlorination

46. Chlorination levels
If a system does not allow adequate contact time with normal dosages of chlorine, superchlorination followed by dechlorination (chlorine removal) may be necessary. Superchlorination: It is adopted during outbreak of epidemics and in heavily polluted waters. It provides a chlorine residual of mg/l, 10 times the recommended minimum breakpoint chlorine concentration. Retention time for superchlorination is approximately 5 minutes.

47. Dechlorination: Dechlorination is the process of removing residual chlorine from disinfected wastewater prior to discharge to the distribution system. Sulfur dioxide is most commonly used for dechlorination . Other methods of dechlorination are uses of Sodium thiosulphate and Sodium biosulphate and prolonged storage and absorption on charcoal.
Prechlorination: Application of chlorine prior to any unit treatment process. This helps in controlling biological growth in raw water pipes and increasing the efficiency of further treatment process.
Post Chlorination: application of chlorine to treated water before it enters in the distribution system. This helps to maintain residual chlorine in treated water.
Re- Chlorination: When distribution system is long and complex, the residual chlorine of 0.2 mg/l may not be available at the consumer end. To achieve this stage-wise application of chlorine in distribution system is carried out and is called re-chlorination.

48. TREATMENT OF WATER - CONTD
Plain chlorination →Treatment in which only chlorination is done
Used when the water is more or less pure with less turbidity (20-30mg/lit) → Lake water
Dosage → 0.50mg/lit
Pre-chlorination →
Applied before filtration/ even sedimentation
Reduce load on filter bed
Reduce the odour, colour, algae, organic impurities
Applied such that mg/lit as residual

49. TREATMENT OF WATER - CONTD
Post chlorination →
Ordinary method
Applied to the filtered water
Applied such that mg/lit as residual
Double chlorination →
Pre and post chlorination is done simultaneously