13. Waste Water And Sewage Treatment

Question 1

How and In what way does the input of organic matter, such as sewage, affect the O2 content of a river or stream?

Answer 1

The product of secondary treatment can be discarded into waterways but high in phosphorous and nitrates that may lead to increased growth of microbes and reduced O2

Question 2

Biological oxygen demand is used as a measure of water quality. What does it measure and how does it do this?

Answer 2

• determines the levels of biological degradable material present
• the test relies on the amount of oxygen required by microbes to remove (degrade) these organic compounds
• measures the decrease in oxygen concentrations following the oxygen dependent oxidation of organic compounds by bacteria
• high BOD indicates high levels of organic material in water tested

Question 3

How does the microbial action of the secondary processing stage of wastewater treatment remove pathogenic organisms?

Answer 3

• removes 90-95% BOD and many bacterial pathogens
• biological oxidation process carried out by micro organisms
  
  • converts organic to inorganic and reduces BOD
• micro organisms aggregate and form a settleable floc structure

Question 4

Part of the sewage treatment process can be used to fuel the plant. How Is this done?

Answer 4

• Sludge from the treatment process is digested in three 38m high egg shaped anaerobic sludge digesters.

Question 5

Protozoan parasites and eggs are removed at what stage of wastewater treatment to remove pathogenic organisms?

Answer 5

• Tertiary treatment stage
  
  • the filtration at this stage is effective at removal of protozoan parasites

Question 6

Various nutrient levels remain high in the wastewater treatment process. Name these nutrients and detail how they may be removed.

Answer 6

Nitrogen
- chemical removal = treatment with a strong base (NaOH) converts ammonium ions (nH4+) to ammonia gas (NH3)
- biological removal = reduction of ammonia to nitrate then from nitrate to nitrogen gas via nitrification bacteria (Nitrobacter and Nitrosomous)

Phosphate
- precipitation as insoluble salts
-> calcium chloride addition forms insoluble calcium phosphate
-> addition of ferric chloride forms insoluble ferric phosphate

Question 7

Septic tanks are a combination of aerobic and anaerobic processes. Detail these processes.

Answer 7

Anaerobic septic treatment
- only suitable for small volumes of water due to slow retention time
- effluent flows to soil absorption field
- need pumping to maintain efficiency -> otherwise blockages and leaks occur and maintenance is needed
- require different reactor design for larger industrial use

Question 8

Why do home septic tanks frequently fail to work properly?

Answer 8

• an overload of the septic system with more water than it can handle
• incorrect maintenance or not enough maintenance
• incorrect installation

Question 9

Draw a flow diagram detailing the stages of wastewater treatment

Answer 9

Primary treatment -> physical process
Secondary treatment -> removes 90-95% BOD and many bacterial pathogens
Tertiary treatment -> products of secondary treatment can be discarded into waterways, physiochemical removal of turbidity caused by presence of minerals
Advanced treatment

Question 10

Why is it important that the bacteria utilised in the secondary stage of water treatment form a normal floc?

Answer 10

Normal floc settles well whereas bulking sludge does not settle