Waste water treatment

Question 1

State the stages of treating waste water

Answer 1

screening and grit removal

• sedimentation to produce sewage sludge and effluent
• anaerobic digestion of sewage sludge
• aerobic biological treatment of effluent.

Question 2

Describe how waste water is treated

Answer 2

In the first stage, the sewage is screened by passing though a mesh. This removes solids and pieces of grit

The sewage is now allowed to settle in large sedimentation tanks. This produces a liquid effluent and a semi-solid sludge which sinks.

The sludge is taken away and digested by anaerobic bacteria. In the absense of oxygen, these bacteria produce biogas which can burned for electricity

At the end, the digested sludge can be used as fertilisers for farming.

The liquid effluent contains large amounts of organic molecules and harmful microorganisms. Both of these need to be reduced before the water is returned to the environment. Air is bubbled through the liquid effluent. This allow aerobic bacteria to multiply.
In the presence of oxygen, the aerobic bacteria digest the organic molecules and the harmful microorganisms

After this stage, the liquid effluent can be safely discharged into nearby rivers or the sea

Question 3

What is waste water

Answer 3

Water that is flushed away down toilets, drains from baths, toilets, agricultural waste.
This water goes into sewers and is treated at a sewage treatment plant.

Question 4

What does waste water contain

Answer 4

Waste water contains large amounts of organic molecules e.g. from urine and faeces

Contains harmful microorganisms such as bacteria

Question 5

Comment on how easily potable water can be produced

Answer 5

The easiest way to produce potable water is to use ground water from aquifers.
Usually this is safe to drink, once it has been treated with chlorine. (Aquifers can sometimes be polluted e.g. with fertilisers from farms therefore the water from aquifers needs to be tested carefully)

We can make potable water directly from waste water such as sewage. However this takes many purification steps. Therefore this is only done in places where water is scarce.

Salt water needs to be desalinated to produce potable water. Desalination requires a lot of energy and is expensive