Cleaning Water With Flocculation
Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★☆☆ Minimal safety procedures required
Categories: Water and Solubility, Pollution and Conservation
Alternative titles: Cleaning Water With Alum
Summary
Students clean real pond or river water using two flocculants, aluminum sulfate and a polymer clarifier. They compare how stirring and lowering pH with lemon juice affect how fast particles clump and settle.
Procedure
- Collect about 1 liter of turbid water per group from a nearby pond or stream, including a little bottom sediment. Cap and bring to class.
- Prepare two stock flocculant solutions in labeled containers: dissolve a small pinch of aluminum sulfate in about 10 mL tap water; dilute a small amount of pool clarifier polymer in about 10 mL tap water.
- Give each group seven clear cups. Draw a fill line about three-quarters up each cup and label cups 1–7.
- Record in journals what each cup will test: 1 control; 2 alum not stirred; 3 alum stirred; 4 polymer not stirred; 5 polymer stirred; 6 alum plus lemon juice; 7 polymer plus lemon juice.
- Shake the source water to resuspend solids and fill cups 1–5 to the fill line. Note initial appearance.
- After the heaviest particles settle briefly, add 1 mL of the assigned flocculant to cups 2–5.
- Stir only cups 3 and 5 for several seconds using separate stirrers to avoid cross contamination.
- Observe cups 1–5 for about 5 minutes and record changes in clarity and any visible flocs or settled layers.
- Re-shake the source water and fill cups 6 and 7 to the fill line.
- Add 1 mL lemon juice to cups 6 and 7 and stir to acidify the water.
- Add 1 mL alum to cup 6 and 1 mL polymer to cup 7. Stir both with separate stirrers.
- Observe cups 6 and 7 for about 15 minutes, recording floc formation, settling behavior, and clarity compared to the control.
- As a class, compare which conditions improved solids removal: flocculant type, stirring vs no stirring, and neutral vs acidic pH.
- Conclude by ranking treatments from most to least effective based on observed settling and clarity.
Links
Flocculation - MITK12Videos:
📄 Things That Matter to Flocculants - Teach Engineering: https://www.teachengineering.org/activities/view/usf_flocculant_activity01
Variations
- Try several doses of each flocculant to find an optimal amount for your local water.
- Replace lemon juice with a mild base such as baking soda solution to test the effect of higher pH.
- Test natural flocculants such as crushed moringa seeds or chitosan and compare to alum and polymer.
- Add a simple measurement, such as a Secchi tube or ruler behind the cup, to semi-quantify clarity changes.
- Repeat with water from different sites or after rain to see how source water changes results.
Safety Precautions
- Treat all field-collected water as nonpotable; do not taste or touch face while working and wash hands after the activity.
- Label all solutions clearly and keep alum, polymer, and lemon juice out of eyes and mouth; avoid splashes.
- Use separate stirrers for different chemicals to prevent unintended reactions.
Questions to Consider
- Why did stirring change the outcome for the same flocculant and dose? (Stirring increases particle collisions so more microflocs form, which then settle.)
- Why might alum work differently from a polymer clarifier? (Alum neutralizes particle charges, while polymers bridge particles together; their mechanisms respond differently to water chemistry.)
- How did lowering pH with lemon juice affect alum performance, and why? (Acidic conditions can shift aluminum species and reduce charge neutralization at this dose, slowing floc formation.)
- Which treatment produced the clearest water fastest, and what evidence supports your choice? (State the cup number and specific observations such as visible flocs, settled layer height, and clarity.)
- If you increased the dose of polymer or alum, what problems might appear? (Overdosing can restabilize particles or create tiny, fragile flocs that do not settle well.)