sewage treatment - tertiary treatment

Question 1

What does tertiary treatment involve?

Answer 1

it involves the removal of phosphorus and nitrogen compounds from effluents.

Question 2

what does the water contain before going through this treatment?

Answer 2

The water contains dissolved nitrates from organic waste and dissolved phosphates from washing powders

Question 3

The water contains dissolved nitrates from organic waste and dissolved phosphates from washing powders.
What happens if these are left in the water?

Answer 3

If left in the water these substances can cause eutrophication

Question 4

What is the removal of nitrogen affected through?

Answer 4

The removal of nitrogen is affected through the biological oxidation of nitrogen from ammonia to nitrate (nitrification), followed by denitrification, the reduction of nitrate to nitrogen gas.

Question 5

What is nitrification?

Answer 5

ammonia to nitrate

Question 6

What is denitrification?

Answer 6

nitrate to nitrogen gas

Question 7

How is nitrogen gas disposed of from the water?

Answer 7

nitrogen gas is released into the atmosphere and thus removed from the water

Question 8

Where do phosphates come from?

Answer 8

house hold detergents (washing powders or washing-up liquids)

Question 9

Where do nitrates come from?

Answer 9

nitrates come from organic materials in sewage

Question 10

How do nitrates and phosphates cause pollution?

Answer 10

They cause pollution by eutrophication

Question 11

How are phosphates removed from the water? (3)

Answer 11

by precipitation, e.g. by adding aluminum sulfate.
phosphates precipitate out of the water as aluminum phosphate

Al^3+ + PO4^3- —> AlPO4↓

they can also be removed by adding iron (|||) chloride. This removes the phosphates as iron (|||) phosphate

Lime may also be used to precipitate phosphates. (calcium carbonate)

Question 12

How are the precipitates formed (to remove the phosphates) taken out of the water?

Answer 12

the insoluble phosphate compounds are allowed to settle out and are removed before the final effluent is discharged into waterways

Question 13

What ions must be added in order to remove the phosphates?

Answer 13

calcium or aluminium or chlorine ions

Question 14

How may nitrogen be present in the water?

Answer 14

in the form of ammonia, nitrite compounds, nitrate compounds or organic compounds containing nitrogen

Question 15

Is it cheap/expensive or easy/hard to remove nitrogen?

Answer 15

The removal of all the nitrogen compounds is difficult and expensive

Question 16

how is are nitrogen compounds removed from the water? (2)(hence why it is expensive)

Answer 16

denitrifying bacteria are used to convert nitrate ions to nitrogen gas.

Another method involves the use of ion exchange to remove nitrate ions from the effluent but this is expensive.

Question 17

What is the effluent placed into when removing nitrogen?

Answer 17

the effluent is placed in a tank that has no free oxygen and the bacteria (denitrifying bacteria) take the oxygen atoms from the nitrate ions, converting them into nitrogen gas

Question 18

Why is tertiary treatment not always carried at all sewage treatment plants?

Answer 18

because it is expensive

Question 19

define tertiary treatment of sewage

Answer 19

tertiary treatment of sewage is a process involving the removal of phosphorus compounds by precipitation and the removal of nitrogen compounds by biological and ion-exchange methods