Self-Assessment Questions (SAQs) for Study Session 5

Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions.

SAQ 5.1 (tests Learning Outcomes 5.1 and 5.2)

Match the following words to their correct definitions.

Two lists follow, match one item from the first with one item from the second. Each item can only be matched once. There are 21 items in each list.

  1. defluoridation

  2. sustainable

  3. water utility

  4. coagulant

  5. coarse screens

  6. coagulation

  7. sedimentation

  8. pre-chlorination

  9. filtration

  10. fluoridation

  11. water treatment

  12. disinfection

  13. resilience

  14. mechanised

  15. flocculation

  16. fine screens

  17. flocculant

  18. microstrainer

  19. residual chlorine

  20. aerated

  21. contact time

Match each of the previous list items with an item from the following list:

  • a.settling of solids

  • b.the process by which harmful substances are removed from water so that it is safe for human consumption

  • c.the ability to withstand stress or a natural hazard

  • d.the organisation that is responsible for producing and distributing drinking water

  • e.able to be maintained at its best for many years

  • f.chlorination before the main treatment stages of the water purification process

  • g.separation of solids from a liquid

  • h.the process whereby the size of particles increases as a result of particle combining together

  • i.the addition of fluoride

  • j.a rotating drum with a stainless steel fabric with a stainless steel fabric with a mesh size ranging from 15 µm to 64 µm

  • k.the neutralisation of the electrical charge of particles by using a coagulant

  • l.steel bars that have a spacing of 5–15 cm

  • m.the amount of chlorine left after all the pollutants have reacted with it

  • n.the elimination of micro-organisms that can cause disease

  • o.the duration for which the water undergoing treatment is exposed to a disinfectant

  • p.supplied with air

  • q.a chemical that assists the process of flocculation

  • r.where machines are used to carry out a function

  • s.removal of excess fluoride from water

  • t.a chemical used in water treatment to neutralise the charge on fine particles

  • u.steel bars with a spacing of 5–20 mm

The correct answers are:
  • 1 = s,
  • 2 = e,
  • 3 = d,
  • 4 = t,
  • 5 = l,
  • 6 = k,
  • 7 = a,
  • 8 = f,
  • 9 = g,
  • 10 = i,
  • 11 = b,
  • 12 = n,
  • 13 = c,
  • 14 = r,
  • 15 = h,
  • 16 = u,
  • 17 = q,
  • 18 = j,
  • 19 = m,
  • 20 = p,
  • 21 = o

SAQ 5.2 (tests Learning Outcome 5.2)

Imagine you have to inform the local population of the new water treatment plant that is to be built in their town. Draw a simple flow diagram to show the different stages of treatment, and write one or two sentences to describe what happens and why at each of the stages.

Answer

Your diagram should look something like Figure 5.9.

Figure 5.9  The seven-stage process of water treatment.

Stage 1  Screening removes large floating and suspended solids, which can damage equipment in the plant, or block pipework.

Stage 2  Aeration expels acidic gases such as carbon dioxide and hydrogen sulphide, and gaseous organic compounds that can give an unpleasant taste to the water. Aeration also oxidises iron and manganese to their solid form so that they can be removed, thereby eliminating bad flavour and staining. If excess algae are present in the raw water, pre-chlorination is carried out. The chlorine also oxidises compounds that cause taste and odour.

Stage 3  Coagulation neutralises the negative electrical charge in particles in the water, which enables the particles to come together to form flocs. Flocculation results in large flocs forming.

Stage 4  The large flocs formed during flocculation settle out in the sedimentation stage, leaving largely clear water.

Stage 5  Any solids remaining in the water after sedimentation are removed during filtration.

Stage 6  The water is then chlorinated to kill any pathogenic micro-organisms present.

Stage 7  Supplementary treatment, such as the addition of fluoride, or the reduction of the fluoride level, then takes place.

SAQ 5.3 (tests Learning Outcome 5.3)

Recalling your study of the wastes produced during water treatment, assign the different wastes to the management options shown below.

Management optionWaste
Sent to landfill                                                                                                                              
Recycled
Discharged to sewer
Reused
Taken to a sewage treatment plant

Answer

You should have identified the following wastes for each option.

Management optionWaste
Sent to landfillCoarse screenings; fine screenings (if no sewer present); sludge from the sedimentation tank
RecycledPlastic chemical drums
Discharged to sewerFine screenings; backwash from the rapid gravity sand filter
ReusedWooden and cardboard packaging
Taken to a sewage treatment plantSludge from the sedimentation tank

SAQ 5.4 (tests Learning Outcome 5.4)

Which of the following statements do not contribute to the attainment of sustainability and resilience in a water treatment plant? Give reasons for your choice.

  • a.using locally available materials
  • b.making sure the plant is protected from natural hazards
  • c.using a diesel generator for running the pumps and compressors
  • d.using simple systems where possible
  • e.using the cheapest equipment that is in the market.

Answer

c.  Using a diesel generator for running the pumps and compressors will not contribute to sustainability and resilience. Diesel is a non-renewable source of energy and will run out in time. It is better to use a renewable source of energy, for example electricity generated by solar or wind power.

e.  Using the cheapest equipment that is on the market will also not contribute to sustainability and resilience. The cheapest equipment is often the least robust and is likely to fail before long.

SAQ 5.5 (tests Learning Outcome 5.5)

Gideon, a new urban WASH worker, needs guidance on how to calculate the water requirement and service reservoir size in a new area that is to be developed in a town that already has a population of 150,000. In this new area, the population will be 30,000, and there will be three new health centres. The three health centres together will treat 250 people a day. There will also be a day school for 1500 pupils.

Draw up the calculations to show Gideon how it’s done.

Answer

Using data from Tables 5.1 and 5.2, the water requirement each day for the extra population will be 80 litres x 30,000 = 2,400,000 litres, or 2400 m3.

The three health centres will need 135 litres x 250 = 33,750 litres a day, or 33.75 m3 a day.

The day school will need 18.5 litres x 1500 = 27,750 litres a day, or 27.75 m3 a day.

The total additional water requirement will be 2400 + 33.75 + 22.75 = 2461.5 m3.

With allowance for leakage, etc. the water requirement will be 2461.5 m3 x 1.15 = 2830.7 m3, say, 2831 m3.

The service reservoir has to hold 2831 m3 x 1.5 = 4246.5 m3, say, 4247 m3.

Thus, the water requirement in the new area would be 2831 m3 per day, and the size of service reservoir required would be 4247 m3.

Summary of Study Session 5