MC8: Waste water treatment, water purification, and waterborne microbial diseases

Question 1

What is the water cycle?

Answer 1

The cyclic exchange of water between the atmosphere and biosphere

Question 2

Desribe the water (hydrological) cycle.

Answer 2

Water precipitates as rain and falls to Earth

It is returned by evaporation to the air

Human communities interact with the hydrological cycle by drawing water for drinking and other purposes, and by returning waterwater

Question 3

Define ‘wastewater’

Answer 3

Liquid derived from domestic sewage or industry that cannot be discharged into lakes/streams due to health, economic, environmental, or aesthetic reasons

Question 4

What is the most important single measure available to ensure public health?

Answer 4

Water purification

Question 5

How are microbes used in water purification?

Answer 5

They are used to identify, remove, and degrade

Question 6

Name two pathogens associated with water

Answer 6

Vibrio cholerae, Salmonella

Question 7

Where does Vibrio cholerae occur?

Answer 7

Naturally in the plankton of fresh, brackish, and salt water

Question 8

What symptoms does cholera cause?

Answer 8

Exhaustive diarrhoea, vomiting

Question 9

How many Vibrio cholerae must be injected to cause cholera in a normal healthy adult?

Answer 9

100 million

Question 10

What disease does Salmonella enterica cause?

Answer 10

Typhoid

Question 11

How is water tested for the contamination of human sewage?

Answer 11

Coliform test

Question 12

What is the current statutory test for human faeces in water?

Answer 12

The presumptive coliform test

Question 13

Describe the MPN method of coliform test

Answer 13

​MPN

• 15 Durham tubes innoculated with samples of the water:
  
  • 5 with 10 mL
  • 5 with 1 mL
  • 5 with 0.1 mL
• They are placed upside down in tubes of MacConkey agar solution and incubated at 35°C for 24 hours
• After 24 hours, the tubes are both examined for gas production
• Gas production = positive presumptive result
• All positive presumptive cultures are used to innoculate tubes of brilliant freen lactose bile broth, which are then incubated for 48 hours at 35°C
• A positive test result is gas production which is used to determine MPN

Question 14

Describe the MF method of coliform test

Answer 14

• A membrane filter is placed on a filter support
• The water sample is filtered through the membrane
• Membrane filter removed and placed in plate containing the appropriate medium
• Incubated for 24 hours
• Typical coliform colonies

Question 15

What bacteria are used for coliform tests in the USA?

Answer 15

• Enterococcus faecalis* (90-95%)
• Enterococcus faecium* (5-10%)

Question 16

What is BOD?

Answer 16

Biochemical oxygen demand

• Water passing through the ground picks up nutrients
• Carbon run-off accelerates microbial respiration
  
  • O₂ becomes depleted
• Microbial oxygen consumption creates a biochemical oxygen demand
  
  • High BOD accelerates heterotrophic respiration which can cause a massive die-off of fish and other aquatic animals

Question 17

How is BOD used to test for microbial load?

Answer 17

• The O₂ content of the water under test is measured (reading 1)
• It is measured again five days later after being stored in a sealed bottle (reading 2)
• BOD₅ = reading 1 – reading 2 (mg/L = ppm)
• This is an indirect measure of organic material
• The maximum concentration permitted in sewage works discarded into rivers is 20 ppm BOD₅

Question 18

What are the four stages of wastewater treatment?

Answer 18

1. Preliminary treatment: removes solid debris
2. Primary treatment: fine screens and sedimentation tanks remove insoluble particles
3. Secondary treatment: microbial decomposition of organic content
4. Tertiary (advanced) treatment: chlorination or other chemical applications to remove pathogens

Question 19

What are the goals of wastewater treatment?

Answer 19

• Kill pathogens
  
  • Many die outside the human body anyway as this is their niche and outside it they are less competitive
  • Chlorination (hypochlorite: OCl^–) or irradiation with UV to produce ozone and kill remaining pathogens
• Removing organic material
  
  • Oxidation of organic compounds is a microbial process

Question 20

List the processes and microbes associated with primary wastewater treatment

Answer 20

Wastewater → grates and screens → settled

Processed water still has a high BOD so most plants have secondary treatment

Question 21

List the processes and microbes associated with secondary wastewater treatment

Answer 21

1. Complex polymers (polysaccharides, lipids, proteins) are hydrolysed by microbial enzymes (anarobic bacteria)
2. Monomers (sugars, fatty acids, amino acids) are fermented by fermentative acetogenic bacteria
3. Acetate, hydrogen gas, and carbon dioxide gas undergo methanogenesis by menthanogenic archaea

Question 22

How do trickle filters work?

Answer 22

• Bed of stones or plastic
• Trickle wastewater through
• Develop film of attached organisms
• Bacteria/fungi/protozoa/worms (grazers)
• Ratio of 8:1:1:2
• Mainly Gram negative bacteria
  
  • Pseudomonas
  • Zoogloea
  • Sphaerotilus
    *

Question 23

How does activated sludge work?

Answer 23

• Aerate settled sewage to get ‘activated floc’
• Selects for organisms that flocculate and sediment
  
  • e.g. Gram negative bacteria
    
    • Pseudomonas
    • Zoogloea
    • Archromobacter
  • e.g. ciliates
    
    • Vorticella
• Complex engineering, sensitive to shock, economical with space

Question 24

What is water purification?

Answer 24

The process of removing undesirable chemical and biological contaminants from raw waiter, with the aim to produce water fit for a specific purpose – usually for human consumption

Question 25

What are the three main methods of water purification?

Answer 25

• Physical
  
  • ​Filtration
  • Sedimentation
• Biological
  
  • Slow sand filters
  • Activated sludge
• Chemical
  
  • Flocculation
  • Chlorination
  • UV