C14 : 14.3 - Treating waste water

Question 1

list some sources of waste water

Answer 1

• Domestic uses : washing-up dishes, showers, baths,cooking, toilets
• Farming : Agricultural waste from animal farms and nutrient run-off which is collected from fields - > abundance of waste water

Question 2

why do Both domestic and agricultural sewage needs to be processed and if done otherwise?

Answer 2

to remove organic matter, harmful microbes, particulates and toxins -> ensures safe to return to environment (eg freshwater sources like lakes/rivers)
- otherwise, rivers polluted and it poses a health risk

Question 3

When you run water down a drain, where does it pass through

Answer 3

sewers and then finally to sewage treatment plants

Question 4

why would industrial waste water ( eg Haber process) need to be treated

Answer 4

removal of organic matter and harmful chemicals - so additional treatment required

Question 5

Describe and explain the steps in sewage treatment

Answer 5

• Screening and grit removal : removes large solid objects (eg twigs, plastic bags) and grit
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• Primary sedimentation tanks : water allowed to stand still => heavier, suspended solids settle and sink to bottom - called sewage sludge, whilst lighter matter which is also known as effluent, floats to the top
• large paddle rotate and push sludge to center of tank => piped to storage for further treatment
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• Aerobic digestion : effluent is removed and treated by biological aerobic digestion
• air is pumped into water => encourages breakdown of organic matter and microbes by aerobic bacteria
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• Final settling tanks : fine solids and useful bacteria allowed to settle => can be recycled back to aerobic digestion or to sludge treatment.
• treated waste water safe enough to discharged back to environment
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• Organic matter in Sludge waste is digested anaerobically by bacteria => releases methane gas => which is main gas in natural gas => source of energy (piped gas supply) or electrical
• sludge can be dried and used as fertiliser / used as source of renewable energy => however, energy has to be supplied to heat sludge

Question 6

what should we do if there are toxic substances in the waste water and give e.g

Answer 6

• important to use additional phases of treatment
• e.g. using membranes, adding additional chemicals e.g. to precipitate metals out of solution, and also U.V. radiation, ozone, chlorine (although it can form toxic organic compounds once in environment)

Question 7

Sewage water is often treated in areas where there is little ___________ available + and give one pro of it

Answer 7

• Sewage water is often treated in areas where there is little freshwater available
• Though this process is longer than processing and preparing freshwater, it uses less energy than the desalination of salt water

Question 8

Waste water comes from many ____________ ______________ so the process of waste water treatment is ___________ and involves various ______________.

Answer 8

Waste water comes from many different sources so the process of waste water treatment is complex and involves various stages.