Part 4: Wastewater Treatment

Question 1

How much has wastewater consumption increased between 1950 and the present?

Answer 1

85 litres per day to 200 litres per day

Question 2

What are the relative costs of drinking water and sewage treatment?

Answer 2

Drinking water = £ 0.3 m-3

Sewage treatment = £ 0.7 m-3

Question 3

What is the main aim of wastewater treatment?

Answer 3

To reduce nutrient concentrations of C, N and P to normal levels.

Question 4

What is the composition of wastewater (excluding water)?

Answer 4

50% carbohydrates (mostly polymers)
40% protein and urea
10% lipids and their derivatives

Question 5

How much urea does a person produce per day?

Answer 5

40g per day

Question 6

What is BOD?

Answer 6

Biochemical oxygen demand - amount of dissolved oxygen needed by aerobes to break down organic material.

Question 7

How is BOD measured in waste treatment plants.

Answer 7

BOD is measured after 5 days since this represents the first stage of degradation (organics).

Question 8

What is the second phase of sewage breakdown after organics?

Answer 8

Urea breakdown (nitrification)

Question 9

When must sewage be diluted?

Answer 9

If BOD becomes too high or too low.

• too high means too much organics
• too low means there are growth inhibiting compounds such as ammonia at harmful levels

Question 10

What is the BOD of domestic wastewater?

Answer 10

200-300 mg O2 per litre

Question 11

What is COD?

Answer 11

Chemical Oxygen Demand - BOD + additional compounds that are not microbially degraded.

Question 12

How much greater than BOD is COD?

Answer 12

Approximately 2x greater.

Question 13

How is COD measured?

Answer 13

Measured following acidifcation using a strong oxidising agent such as Potassium Dichromate.

Question 14

What is stage 1 of the primary treatment process?

Answer 14

Mechanical filtering of waste. A mesh is used to retain large objects like plastic and also sand etc.

Question 15

What is stage 2 of the primary treatment process?

Answer 15

Sewage is allowed to settle into fractions. Oily scum floats to the top, particulates sink to the bottom.

Question 16

Why must the meat processing and other industries communicate with sewage treatment plants?

Answer 16

Such industries have especially high BOD-demand wastewater, and so have to notify the plant about any unusual flow.

Question 17

What is secondary treatment of sewage?

Answer 17

Settling of waste in an aeration tank. Organics break down or settle and clear water can be removed.

Question 18

Why is activated sludge aerated in secondary treatment?

Answer 18

Aerobic degradation is much more efficient than anaerobic. It also produces less acidity (acidity inhibits growth).

Question 19

What important role do protozoa play in wastewater treatment?

Answer 19

Large numbers of protozoa graze on bacteria, degrading the bacterial population (including pathogens).

Question 20

Under what conditions might the microbial community in an aeration basin collapse?

Answer 20

Inflow of toxic/inhibitory compounds .e.g acidity, ammonia

Question 21

How do sewage managers cope with changes to the bacterial community?

Answer 21

Freeze-dried bacteria can be inoculated into the sewage mixture.

Question 22

How are unusual bacterial assemblages coped with in sewage treatment?

Answer 22

Freeze-dried specialist bacteria are commercially available for halogenated compounds, aromatics etc.

Question 23

What is tertiary treatment of sewage waste?

Answer 23

Final stage of treatment, may be:

• secondary sedimentation of organics
• phosphorous elimination
• disinfection (using UV light or chemical agents)

Question 24

For what is anaerobic digestion used in a sewage treatment plant?

Answer 24

Anaerobic digestion reduces organic content in sludge.

Question 25

How long does anaerobic digestion take in sewage treatment?

Answer 25

10 to 20 days.

Question 26

At what temperature is anaerobic digestion conducted?

Answer 26

Greater than 30 degrees, sometimes at 50+ degrees if thermophiles are used.

Question 27

What is the major end product of anaerobic digestion and for what is it used?

Answer 27

Methane as, used in heating the digester - very efficient process.

Question 28

How much digested sludge is produced per year in UK plants?

Answer 28

1.6 million tonnes.

Question 29

For what is digested sludge from sewage treatment used in the UK?

Answer 29

Agriculture (62%) as a fertiliser, and industrial crops (5%).

Question 30

What proportion of digested sludge is incinerated?

Answer 30

15-20%

Question 31

Why must phosphorous be retained during the waste treatment process?

Answer 31

Phosphorous presents a eutrophication risk.

Question 32

What are the three phases of phosphate elimination in sewage treatment?

Answer 32

1) Pre-elimination - phosphate is precipitated and removed
2) Simultaneous elimination - phosphate precipitated biologically and then removed.
3) Post-treatment elimination - phosphate precipitated from clear effluent.

Question 33

What is the process of chemical phosphate precipitation?

Answer 33

Iron salts (e.g. FeCl2) are used to precipitate phosphorous to FePO4. However, this product is soluble under anoxic conditions and can be an issue in fertiliser.

Question 34

What is the process of biological phosphate elimination?

Answer 34

Some organisms can accumulate phosphate within their cells, lowering the concentration in the water. Genetically modified versions that can further eliminate phosphate have been developed, but cannot cope with aeration basin conditions.

Question 35

Which compounds is the main nitrogen-containing degradation product of organic matter?

Answer 35

Urea, NH2NH2C=O

Question 36

How is urea degraded during sewage treatment?

Answer 36

Urea is bacterially cleaved to carbon dioxide and two ammonia.

Question 37

What acts as an electron donor during degradation of urea?

Answer 37

Organic matter