1. 1
   A sewage treatment works receives waste water from homes and businesses in a large town, and processes it through several stages before releasing effluent back into a nearby river.

   (a)

   The first stage of treatment involves passing the waste water through screens and grit removal tanks. What is the main purpose of this first stage?

   [1 mark]

   • ATo remove large solid objects and grit from the waste water
   • BTo kill all bacteria present in the waste water
   • CTo evaporate the water and leave behind dissolved salts
   • DTo add oxygen to the waste water

   (b)

   After screening and grit removal, the waste water passes into large sedimentation tanks. What are the two products formed at this stage?

   [1 mark]

   • AChlorine gas and oxygen
   • BPotable water and grit
   • CAmmonia and nitric acid
   • DSewage sludge and effluent

   (c)

   Describe what happens to the sewage sludge produced during sedimentation, and explain why this treatment is necessary before the sludge can be disposed of safely.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A factory is comparing the cost of obtaining clean water from three different sources for its manufacturing process: ground water from a nearby borehole, treated waste water from the local sewage works, and desalinated sea water from the coast.

   (a)

   Which source is generally the easiest and cheapest to treat to produce usable water?

   [1 mark]

   • ASea water
   • BWaste water
   • CGround water
   • DAll three are equally easy to treat

   (b)

   The factory's effluent (treated waste water) undergoes aerobic biological treatment before being released. What does 'aerobic' mean in this context?

   [1 mark]

   • AThe treatment takes place in the absence of oxygen
   • BThe treatment takes place in the presence of oxygen
   • CThe treatment uses only chemical reagents, no microorganisms
   • DThe treatment removes all dissolved salts

   (c)

   Explain why it is generally easier and cheaper to obtain usable water from ground water than from waste water, in terms of the treatment steps required.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   During a period of drought, a regional water authority is investigating whether treated waste water could be used to supplement supplies of potable water, as an alternative to expanding desalination capacity.

   (a)

   Describe the three main stages of waste water treatment that occur after screening and grit removal, in the order they take place.

   [3 marks]

   (b)

   Evaluate the relative ease of obtaining potable water from waste water compared with obtaining it from ground water and from salt (sea) water, given that the authority must also consider cost and energy use.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A historic industrial city built its sewage system over a century ago, and the original waste water treatment works discharged untreated sewage directly into the river, causing major outbreaks of disease among residents downstream. The city later constructed a modern treatment works using screening, sedimentation, anaerobic digestion and aerobic biological treatment.

   (a)

   Discuss how each stage of the modern treatment process addresses the problems caused by the original untreated discharge.

   [6 marks]

   (b)

   City planners are now considering whether the digested sewage sludge from the modern treatment works could be sold to local farmers as a soil fertiliser, rather than being disposed of in landfill. Discuss the potential benefits and risks of this proposal.

   [6 marks]

   Total for question 4: 12 marks

End of questions