Disinfection

Question 1

What are some desired disinfectant characteristics?

Answer 1

1. Toxic to microorganisms
2. Soluble in water and cell tissues
3. Stable for storage prior to use
4. Non-toxic to higher life forms
5. Effective over the expected temperature and pH range
6. Should be able to penetrate surfaces
7. Available in large enough quantities @ reasonable price
8. Safe to handle
9. Leave a measurable residual

An ideal disinfectant does not exist - cannot achieve all 9 desired characteristics.

Question 2

What are some conventional disinfectants?

Answer 2

• chlorine
• chloramines
• chlorine dioxide
• ozone

Question 3

What is primary disinfection?

Answer 3

Applying a disinfectant at a treatment plant to kill or inactivate pathogens.

Primary disinfectant - responsible for microbiological kill/inactivation

Question 4

What is secondary disinfection?

Answer 4

Applying a disinfectant to water as it enters the distribution system, to prevent biofilm growth on pipe walls and protect the water from intrusion.

Secondary disinfectant - maintain measurable residual in distribution system of primary disinfectant

Question 5

What is combined chlorine?

Answer 5

residual existing in combination with ammonia (chloramines) or organic nitrogen compounds

Question 6

Why are dichloramine and trichloramine undesirable?

Answer 6

Because of their taste and odour

Question 7

What is the optimal pH for chlorine disinfection?

Answer 7

pH = 6-7

Question 8

What is free chlorine?

Answer 8

Sum of residual chlorine in water [HOCl] + [OCl-]

Question 9

What is total chlorine?

Answer 9

Total chlorine = free chlorine + combined chlorine

Question 10

What happens after more chlorine dosage is added after the breakpoint?

Answer 10

Most of the chloramines are oxidized and additional chlorine addition creates an equal residual.

Question 11

How are disinfection by-products (DBPs) formed?

Answer 11

Chlorine + precursors (natural organic matter) = DBPs

Question 12

What are two examples of DBPs?

Answer 12

Trihalomethanes (THMs) and Haloacetic Acids (HAAs)

Question 13

What are the main steps to the alternative design to reduce DBP formation?

Answer 13

1. Improve precursor removal prior to chlorination
2. Move point of chlorination
3. Use alternative disinfectant

Question 14

How do you improve precursor removal?

Answer 14

Optimize chemical coagulation

- add powdered activated carbon (PAC) to adsorb precursors

Question 15

What factors should be considered when selecting primary disinfectants?

Answer 15

1. Effectiveness (pathogen reduction)
2. Potential to form DBPs
3. Cost

Question 16

What are some examples of alternative primary disinfectants?

Answer 16

1. Ozone
2. Chlorine dioxide
3. Ultraviolet (UV) light

Question 17

Disinfection must provide a minimum of?
___ log inactivation for Giardia
___ log inactivation for viruses
___ log inactivation for Cryptosporidium

Answer 17

0.5 log inactivation for Giardia
2.0 log inactivation for viruses
No additional inactivation for Cryptosporidium

Question 18

What is the difference between removal and inactivation?

Answer 18

removal - physical removal via coagulation/flocculation/sedimentation/filtration

inactivation - kill or inactivate (unable to reproduce) pathogens

Question 19

What is C (in CT value)?

Answer 19

C = disinfectant residual concentration (mg/L)

Question 20

What is T (in CT value)?

Answer 20

T = contact time during which the disinfectant is present (min)