1.11 Waste Water Treatment

Question 1

What is domestic waste?

Answer 1

Waste from showers, sinks, toilets. As soon as this water gets flushed down the drain, it goes to the sewers and the sewage treatment plants.

Question 2

How does water come from agricultural systems?

Answer 2

• Nutrient run-off from fields
• Animal waste

Question 3

How does water come from industrial sources?

Answer 3

• Factors that make and use chemicals

Question 4

Sewage Treatment: (4)

Answer 4

• Screening: removes anything big like twigs or plastic bottles. This is often done by passing the sewage through a mesh that only lets the sewage itself get through.
• Let the sewage sit in a settlement tank for a while so it can undergo sedimentation. This is where the heavier solid bits in the mixture sink to the bottom to form sludge, and we are left with a lighter part called the effluent at the top. We can then separate this into 2 different tanks. They key different is that the effluent is under aerobic conditions, and the sludge is under anaerobic conditions.
• Next, air is pumped air into the effluent tank which encourages aerobic bacteria (bacteria that needs oxygen to survive) to grow and break down any organic matter (aerobic digestion). We keep the sludge tank sealed so there’s little or no oxygen. The organic matter is broken down by anaerobic digestion. As long as they are given time, they will undergo digestion. For the effluent, this means that the water is now safe and we can release it back into the environment. However, for the sludge, the anaerobic digestion produces methane, which can be captured then burned as an energy source, while the remaining digested waste can be used as a fertiliser as it is rich in nutrients.
• In some cases, waste water can contain toxic substances which have to removed by additional stages. For example, by adding chemicals to precipitate any metals or using ultraviolet radiation.

Question 5

Is treating waste water easier or harder than desalinating salt water?

Answer 5

Easier

Question 6

True or false? Waste water is usually returned to the environment without processing.

Answer 6

False. Waste water should always be processed before entering the environment to minimise pollution.

Question 7

Brief steps in the treatment of sewage (4)

Answer 7

1) Screening - The sewage is passed through gratings and meshes to remove anything large.
2) Sedimentation - The sewage is left to sit in a settlement tank, so that the heavier particles settle at the bottom as sludge, while the lighter particles settle at the top as effluent.
3) Aerobic digestion - Air is pumped through the effluent to supply the bacteria with oxygen.
4) Anaerobic digestion - The sludge is sealed in a container to prevent the entry of air, which ensures anaerobic respiration.

Question 8

Which gaseous product does the anaerobic digestion of sludge produce?

Answer 8

Methane