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Earth and Environmental Science 11–12 · Year 11

Treating turbid water: coagulation, settling and sand filtration measured by turbidity

Module 4: Human Impacts

PracticalLow risk

The idea

Fine suspended particles that will not settle are clumped by a coagulant into flocs that do, and a sand bed traps what is left, the two steps at the heart of a water filtration plant.

Safety card

Low riskAn adult supervises

Hazards

  • alum solution is an eye irritant
  • no water from the practical may be drunk

Controls

  • safety glasses
  • label all beakers not for drinking
  • wash hands

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.

What you need

  • muddy water: 2 g of clay soil stirred into 2 L of tap water and left 10 minutes
  • aluminium sulfate (alum) solution 10 g/L, dropping pipette; three 500 mL beakers
  • turbidity tube (Secchi-type) or a turbidity meter; stopwatch
  • filter column: 2 L bottle with the base cut off, layered gravel, coarse sand, fine sand over cloth

How to do it

  1. Measure the turbidity of the muddy water and pour 400 mL into each of three beakers.
  2. Add 0, 10 and 20 drops of alum solution, stir fast for one minute then slowly for five, and stand for 15 minutes.
  3. Record floc formation and measure the turbidity of the top water of each beaker.
  4. Pour the clearest supernatant through the sand filter and measure the turbidity again.
  5. Compare with Sydney Water’s description of screening, coagulation, filtration and disinfection, and note which steps the classroom did not do (disinfection).

What you should see

The untreated beaker stays cloudy; the alum beakers form visible flocs within minutes that settle to leave clearer water, and turbidity falls step by step: raw, after coagulation, after filtration. The tube reading rises (clearer) at each stage; filtered water is not safe to drink because no disinfection was done, which the learner must state.

What changes

What you change
coagulant dose (drops of alum)
What you measure
turbidity after settling
What you keep the same
  • same raw water
  • same stirring times
  • same settling time
  • same measuring tube

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Clear water is clean water.
  • Filtering through sand removes bacteria and viruses (it removes particles; disinfection removes pathogens).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-11EES11/12-1EES11/12-2EES11/12-3
  • Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-03
  • Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
  2. www.sydneywater.com.au/education/drinking-water/water-quality-filtration.html
  3. www.sydneywater.com.au/education/wastewater-recycling/wastewater-treatment.html
  4. education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf

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