1. ENG421 (10abc) – Disinfection
Micro-organisms
Disinfection
Disinfectant Concentration and Contact Time
Disinfection – design considerations
Disinfection – design examples

2. General Water Treatment Technologies (Week 4)
Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

3. Micro-organisms (1 of 4) pathogenic micro-organisms
any micro-organism capable of producing disease
surrogate organisms (e.g. Escherichia coli, E.coli) used to indicate effectiveness of disinfection
Week 1 revision :
most common drinking water health risk
contamination by human or animal excreta and the microorganisms contained in faeces
recent contamination may cause communicable enteric diseases (diseases of the gut)
drinking contaminated water or using it in food preparation may cause new cases of infection
greatest risk of infection to :
infants and young children
people whose immune system is suppressed
the sick
the elderly
Pathogenic (disease-causing) organisms of concern include :
bacteria
viruses
protozoa
diseases caused vary in severity
mild gastroenteritis
severe and sometimes fatal diarrhoea, dysentery, hepatitis, cholera or typhoid fever

4. Micro-organisms (2 of 4) - Week 1 revision (cont) : Giardia (protozoa)
diarrhea, excess gas, stomach or abdominal cramps,
and nausea, causing weight loss and/or dehydration
Cryptosporidium (protozoan)
upset stomach or acute, watery, and non-bloody diarrhoea
nausea/vomiting and abdominal pain

5. Micro-organisms (3 of 4) Week 1 revision (cont) :
Risk of disease from waterborne pathogens influenced by :
• the concentration of pathogenic organisms in the water
• the virulence of the strain
• the per capita intake of contaminated water
• the infectious dose of the particular pathogen
• the susceptibility of individuals
• the incidence of the infection in the community (which will determine the numbers of pathogens being excreted)

6. Micro-organisms (4 of 4)
micro-organisms may be considered as suspended matter :
Bacteria 0.5 – 5 μm
Viruses – 0.01 μm
Protozoans 2 – 15 μm
Most micro-organisms can
attach to other suspended particles
be enveloped by suspended particles
Physicochemical treatment processes for suspended particle removal
remove >98% of micro-organisms
Direct filtration plants
remove up to 99% of micro-organisms
Conventional plants (properly optimised)
remove up to 99.9% of micro-organisms
Even at low concentrations micro-organisms may cause disease
Filter “break-through” may occur
micro-organism concentration (and suspended matter concentration) will rise

7. Disinfection (1 of 10) disinfection process
removes, inactivates, destroys pathogenic micro-organisms
usually final treatment in conventional treatment systems
want to combat potential microbial contamination in distribution lines
treated water leaves plant with residual disinfections
in long transmission lines : add intermediate disinfectant injection
physical means :
ultra-violet light
gamma radiation
X-ray
heat (boiling)
chemical means
chlorine (most widely used)
chlorine dioxide
chloramine
hypochlorite
bromine
iodine
ozone
hydrogen peroxide (perozone)
permanganate

8. Disinfection (2 of 10) Mechanism of disinfection
depends on nature of disinfection and type of micro-organism
four mechanisms :

9. Disinfection (3 of 10)
Considerations in selecting disinfection process

10. Disinfection (4 of 10)
many micro-organisms co-exist in water and have variable resistance to the disinfectants
most resistant bacteria ↓
protozoans
least resistant viruses
within a group, resistance may vary
e.g. protozoans
Cryptosporidium oocysts very resistant to disinfectants
(chlorine, chloramines, ozone)
Giardia cysts much less resistant
so, use surrogate or indicator organisms (E. coli, a bacteria) to measure
effectiveness of disinfectant
but, E. coli may die off before many pathogenic organisms
to determine micro-organism levels in water, “plate counts” are performed regularly

11. Disinfection (5 of 10) Radiation
ultraviolet and gamma waves may be used in small water treatment plants
UV causes irreversible inactivation of DNA at wavelength 255 – 265 nm
all micro-organisms must be exposed to UV
otherwise non-UVed micro-organisms can replicate
very effective for bacteria and viruses
hindered by dissolved and suspended matter in water
interferes with UV radiation reaching micro-organisms

12. Disinfection (6 of 10) Temperature
the effect of temperature may be determined using :

13. Disinfection (7 of 10) Chemical Agents Chlorine
chlorination is proven and reliable technique
chlorine (Cl2) applied to water
as aqueous solution
or diffused directly
in ammonium-free water hypochlorous acid (HOCl) and hypochlorite ion (OCl-) form
composition (ratio) depends on pH and temperature
hypochlorous acid (HOCl) is a better disinfectant than hypochlorite ion (OCl-)
advantages
cost effective
residual chlorine (~ 0.5 mg/L) in water
chlorine is efficient oxidant
improves taste and odour control
prevents algae and slime growth
disadvantage
chlorine reacts with organic compounds in water
→ formation of trihalomethanes (THMs, carcinogenic)
avoid chlorine application in early treatment stages
safer after suspended particle removal (fewer organics)
best choice disinfection for organic free water

14. Disinfection (8 of 10) - Chemical Agents (cont) Chloramines
chloramines are formed by adding chlorine and ammonia (NH3) to water
three chloramines disinfect water
NH2Cl, NHCl2, NCl3
less efficient than chlorine, but very effective
good residual in water
does not form significant quantities of THMs
Chlorine Dioxide
advantages
more effective than chlorine and chloramines for bacteria and viruses
effective at pH 8.5 – 9
minimises lead and other metal corrosion
chlorine dioxide residual maintained in distribution system for some time
disadvantages
higher operating costs than chlorine
formation of chlorate ion (ClO3-) and chlorite ion (ClO2-)
possibly toxic

15. Disinfection (9 of 10) - Chemical Agents (cont) Ozone
very powerful disinfectant
depends on pH and temperature
most widely used after chlorine
advantages
significant reduction in THMs formation
low dosage requirement
may produce ozone on-site
few by-products
disadvantages
expensive (high capital cost)
low solubility of ozone in water
→ bubble ozone through water
unstable, decomposes in water
→ does not provide residual protection
→ use chlorine or chloramine as secondary disinfection (residuals)
aldehydes (by-product) may effect human health
contact time 3 – 10 minutes
low dosage 1 – 2 mg/L

16. Disinfection (10 of 10)
- Chemical Agents (cont)
Ozone

17. Disinfectant Concentration and Contact Time (1 of 7)
Disinfectant Concentration and Contact Time are prime considerations in disinfection
as concentration goes up, micro-organism kill goes up
as contact time goes up, micro-organism kill goes up
low concentration and long contact produces similar results to
high concentration and short contact time
C.t (min.mg/L) : the product of concentration (C, mg/L) and
contact time (t, min) identifies inactivation (kill) for each type
of micro-organism
contact time is not the same as hydraulic retention time
contact time determined using a tracer study
t10 : time taken for 10% of tracer to pass at effluent end is used

18. Disinfectant Concentration and Contact Time (2 of 7)
in general, the longer the contact time the greater the kill (Chick’s Law)

19. Disinfectant Concentration and Contact Time (3 of 7)
Contact Time (cont)
modify Chick’s Law to include disinfectant concentration (Chick-Watson Model)

20. Disinfectant Concentration and Contact Time (4 of 7)
C.t and “log-removal”
disinfection efficiencies are compared in terms of log-removal
log-removal is the absolute value of the logarithmic of the
micro-organism fraction remaining after disinfection
99% inactivation = 2 log-removal
99.9% inactivation = 3 log-removal
99.99% inactivation = 4 log-removal
99.68% inactivation = 2.5 log-removal

21. Disinfectant Concentration and Contact Time (5 of 7)
C.t and “log-removal” (cont)
Performed chlorine is shock addition of sodium hypochlorite, Giardia is a protozoan

22. Disinfectant Concentration and Contact Time (6 of 7)
C.t and “log-removal” (cont)

23. Disinfectant Concentration and Contact Time (7 of 7)
C.t and “log-removal” (cont)

24. Disinfectant Mixing (1 of 3)
disinfectant in liquid form is mixed with water in a rapid mixing arrangement
disinfection takes place in subsequent contact chamber (tank)
disinfectant mixing chambers :

25. Disinfectant Mixing (2 of 3)
disinfectant mixing chambers (cont)

26. Disinfectant Mixing (3 of 3)
disinfectant contact chamber (tank)
must avoid dead spaces, short circuiting and dispersion
→ 1. high length to flow width ratio 70 – 80 : 1
200 : 1 would be ideal (space and cost considerations)
use of baffles helps to reduce tank size
2. water height to flow width ratio < 1

27. Disinfection – design considerations (1 of 2)
factors to consider :
disinfectant dose
mixing arrangements
contact time
contact chamber
feeding systems
ozone generator
dechlorination system
off gas destruction unit
Most systems are very specific to source water characteristics and flow rate
Hydraulic retention time dictated by C.t value
C.t value affected by temperature, pH, micro-organisms
As a result of above considerations, specific design criteria are not followed in disinfection system design

28. Disinfection – design considerations (2 of 2)
As a result of above considerations, specific design criteria are not followed in disinfection system design
Typical values :

29. Disinfection – design example 1 (1 of 2)
Problem 1 :
Determine the required chlorine dose for a relatively clean groundwater source with pH = 8. A 3 log-removal is required. It is 3 km from source to service area and peak flow is 1 m/s in the supply pipe.
Solution :
assume separate contact chamber not required as
groundwater is clean
only virus inactivation is required
assume design safety factor of 0.7 (reduces residence time)
residence time in supply pipe = m x = sec = 35 min
1 m/s

30. Disinfection – design example 1 (2 of 2)
Solution (cont):
for 3 log-removal of viruses, at pH = 8 temp = 5oC
C.t = 6 min.mg/L
C = (6 min.mg/L)/(35 min)
= mg/L
Use 0.2 mg/L dosage at entry point to supply line

31. Disinfection – design example 2 (1 of 4)
Problem 2 :

32. Disinfection – design example 2 (2 of 4)
Solution :

33. Disinfection – design example 2 (3 of 4)
Solution (cont) :

34. Disinfection – design example 2 (4 of 4)
Solution (cont) :

35. References
Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D ), pages 513 – 544
Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H ) , pages 997 – 1042
Kawamura, S., 2000, Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., John Wiley and Sons, New York (TH4538K ), pages 292 – 312
MWH, 2005, Water Treatment Principles and Design, 2nd ed., John Wiley and Sons, New York (TD430 .W ), pages 1035 – 1161
Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E ), pages
Parsons, S.A. and Jefferson, B., 2006, Introduction to Potable Water Treatment Systems, Blackwell, Oxford (TD430 .P ), pages 128 – 139
Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages