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Technologies K–10 · Years 5–6

Designing a water filter from gravel, sand and charcoal

Design and Production (NSW Science and Technology K–6, 2017); Design and Technologies: Processes and production skills, Investigating and defining (ACARA v9)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Layers of gravel, sand and charcoal remove particles from muddy water, making it clearer, and the layer order and grain size trade clarity against flow speed, but clear water is not necessarily safe water.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

  • Drinking filtered water
  • Soil microbes, including tetanus
  • Cut plastic edges
  • Charcoal dust

Controls

  • Filtered water is never drunk and is poured onto garden beds
  • Gloves when handling soil and muddy water; hands washed; cuts covered
  • An adult cuts the bottles and tapes the cut edges
  • Charcoal rinsed before use to remove dust

Note

No heat. Record the activity on Primary School RiskAssess (riskassess.com.au).

What you need

  • 1 × 2 L PET bottle per group, cut by an adult so the top inverts into the base as a funnel
  • Cotton wool or a coffee filter, washed gravel (5 to 10 mm), coarse sand, fine sand, rinsed activated charcoal (aquarium grade)
  • 1 L of muddy water: 1 L tap water stirred with 2 heaped tablespoons of garden soil and left to settle for 1 minute
  • 1 turbidity tube (a clear tube at least 60 cm long with a black-and-white disc pattern at the base and a cm scale)
  • 1 L measuring jug, timer, gloves

How to do it

  1. Measure the muddy water in the turbidity tube: pour it in until the disc pattern just disappears and record the depth in cm.
  2. Design the layers on paper: order, thickness and reasons.
  3. Build the filter and pour in 500 mL of muddy water. Time how long the water takes to pass through.
  4. Measure the filtered water's volume and its turbidity-tube depth.
  5. Change one design choice (layer order, sand grain size, or layer thickness), predict the effect, rebuild and test again.
  6. Evaluate both designs against the class criteria: clarity, flow time and cost of materials.

What you should see

The filtered water is clearer: the disc pattern stays visible through a deeper column than for the muddy water. The first water through may be cloudy until loose fine particles wash out. Finer sand gives clearer water but a slower flow; charcoal removes some colour and smell. The water is still not safe to drink: filters like this do not reliably remove bacteria, viruses or dissolved chemicals. For comparison, the Australian Drinking Water Guidelines set 5 NTU as the aesthetic limit at the tap and describe less than 1 NTU as desirable at the time of disinfection with chlorine, which a classroom filter cannot confirm. The learner knows it worked when the turbidity-tube depth rises after filtering and the redesign changes clarity or flow in the predicted direction.

What changes

What you change
one filter design choice (layer order, grain size or thickness)
What you measure
turbidity-tube depth (cm) and flow time
What you keep the same
  • the same muddy water
  • 500 mL poured
  • the same bottle size
  • the same turbidity tube

Common misconceptions

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

  • Clear water is safe to drink (microbes and dissolved chemicals can be invisible).
  • The more layers the better (extra fine layers can slow the flow to a trickle).
  • Charcoal kills germs (it holds some chemicals on its surface; it does not disinfect).

Curriculum references

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

  • Science and Technology K–6 Syllabus (2017), NESA. The syllabus taught in 2026; the 2024 syllabus replaces it from 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.ST3-2DP-TST3-7MW-T
  • Science and Technology K–6 Syllabus (2024), NESA. Implementation from 2027, so this code describes the future syllabus. Code read from the outcomes page on 2026-09-22.ST3-DDT-01
  • Australian Curriculum v9AC9TDE6P01AC9TDE6P02AC9TDE6P03AC9TDE6P04AC9TDE6K05

Sources

The pages the author read to write this activity.

  1. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.teachengineering.org/activities/view/cub_environ_lesson06_activity2
  4. guidelines.nhmrc.gov.au/australian-drinking-water-guidelines/part-5/physical-chemical-characteristics/turbidity

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