Cleaning Water With Flocculation

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.

- 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.

1. 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.
2. Give each group seven clear cups. Draw a fill line about three-quarters up each cup and label cups 1–7.
3. 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.
4. Shake the source water to resuspend solids and fill cups 1–5 to the fill line. Note initial appearance.
5. After the heaviest particles settle briefly, add 1 mL of the assigned flocculant to cups 2–5.
6. Stir only cups 3 and 5 for several seconds using separate stirrers to avoid cross contamination.
7. Observe cups 1–5 for about 5 minutes and record changes in clarity and any visible flocs or settled layers.
8. Re-shake the source water and fill cups 6 and 7 to the fill line.
9. Add 1 mL lemon juice to cups 6 and 7 and stir to acidify the water.
10. Add 1 mL alum to cup 6 and 1 mL polymer to cup 7. Stir both with separate stirrers.
11. Observe cups 6 and 7 for about 15 minutes, recording floc formation, settling behavior, and clarity compared to the control.
12. As a class, compare which conditions improved solids removal: flocculant type, stirring vs no stirring, and neutral vs acidic pH.
13. Conclude by ranking treatments from most to least effective based on observed settling and clarity.

Flocculation - MITK12Videos:

📄 Things That Matter to Flocculants - Teach Engineering: https://www.teachengineering.org/activities/view/usf_flocculant_activity01

• 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.

• 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.

• 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.)