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Lab / experiment Grades 5–6 Engineering & Design

Gravel, Sand, and Charcoal: Building a Column Water Filter

Teams build two column water filters in different layer orders, pour standardized muddy water through each, and compare clarity data in a fair, science-fair-ready test.

Objectives

  1. Build a layered column filter from gravel, sand, and activated charcoal, explaining the job each layer performs.
  2. Design and run a fair test comparing two layer orders, controlling variables and collecting clarity data for each.
  3. Explain adsorption in plain language and distinguish it from the straining that gravel and sand perform.
  4. State an evidence-based conclusion, including that filtration alone does not make the collected water safe to drink.

Vocabulary

Filtration
Filtration is the step where water passes through layers of sand, gravel, or other materials that trap the small particles still left in it. It works like a very fine strainer.
Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
Disinfection
Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.

Also: sediment, adsorption, activated charcoal, variable

Teacher overview

Communities that draw drinking water from a river, lake, or reservoir must remove suspended dirt before that water can move on to the rest of treatment, and this lab lets teams build and test a simplified version of that first step. Each team builds two column filters from the same three materials, gravel, sand, and activated charcoal, but stacks the layers in a different order in each one. Both filters then receive the same standardized muddy water, so any difference in how clear the collected water becomes can be traced to one cause: the order of the layers. This is the testable question at the center of the investigation, and it is written up so that the lab can also serve as a science-fair project on its own.

Each layer in the filter has a distinct job. Gravel and sand remove suspended dirt through straining: gravel blocks the largest particles, and sand, with much smaller gaps between its grains, catches finer particles that pass right through the gravel. Activated charcoal works differently. It is manufactured to contain an enormous number of microscopic pores, giving even a small handful a very large internal surface area. As water moves past that surface, some dissolved substances, odors, and coloring stick to it rather than continuing on through the filter, a process called adsorption. Placing charcoal after the coarser straining layers, rather than first, protects its pores from getting coated in mud before it has a chance to do that adsorption work, which is one reason layer order can change how well a filter performs.

Watch for this misconception: students who see cloudy water turn clear after passing through a filter often conclude that the water is now safe to drink. It is not. Filtration removes suspended particles, which is what causes the cloudiness measured in this lab, but it does nothing to kill bacteria or viruses, and it removes only some dissolved contaminants, none of which were ever visible to begin with. Real treatment plants must keep finished water at or below 0.3 NTU, a measure of cloudiness, in at least 95 percent of monthly samples under EPA's Surface Water Treatment Rules, and even water that clear still receives a disinfectant dose, typically chlorine, before it reaches a tap. Address this directly with students before they leave the room: no water filtered in this lab, however clear it looks, should ever be tasted or swallowed.

Gravel, Sand, and Charcoal: Building a Column Water Filter — Grades 5–6 Engineering & Design
Name: ____________________________ Date: ______________

Procedure

Materials (per team of three or four).

  • 2 clear plastic bottles (1-liter or 2-liter), bottoms cut off, OR 2 large clear cups with 4-5 small holes poked in the bottom
  • 2 coffee filters, OR 2 clean cotton cloth squares, OR 8 cotton balls pressed flat (to plug each bottle neck)
  • 2 cups (about 500 mL) coarse gravel or small aquarium gravel, rinsed clean (1 cup per filter)
  • 2 cups (about 500 mL) coarse sand or play sand, rinsed clean (1 cup per filter)
  • 1 cup (about 240 mL) crushed activated aquarium charcoal (1/2 cup per filter)
  • teacher-premixed standardized muddy-water stock, about 8 cups (1.9 L) (recipe below)
  • 2 clear collection cups or jars, one under each filter
  • 1 clear cup to hold a before-filtering sample
  • a clarity card: a page of ordinary printed text, held behind a cup to check readability
  • a 1-cup dry measuring cup and measuring spoons
  • a stopwatch or phone timer
  • safety glasses
  • masking tape and a marker
  • one data recording sheet and pencil per team

Teacher prep, the day of the lab (about 15 minutes before students arrive). Make one large batch of muddy-water stock so every team starts from the same clarity. Mix 3 level tablespoons of sifted potting soil or garden soil into 4 cups (1 liter) of tap water. Stir hard for 30 seconds. Let it rest for 30 seconds so the heaviest grit sinks. Pour off the cloudy water into a large pitcher, leaving the grit behind. Repeat until you have about 8 cups (1.9 liters) of stock per team.

The investigation. This lab compares two filters built from the same materials in a different order. The question is simple: does the order of the gravel, sand, and charcoal layers change how clear the filtered water becomes? The independent variable, the one thing changed on purpose, is the layer order. The dependent variable, what is measured to find the effect, is the clarity score of the filtered water. The controlled variables, everything kept the same in both filters, are the muddy-water stock, the amount of each material, the pour volume, and the pouring speed.

Variable type What it is in this investigation
Independent variable (what we change) Layer order, in the order water passes through: Filter A (gravel, then sand, then charcoal) vs. Filter B (charcoal, then sand, then gravel)
Dependent variable (what we measure) Clarity score (1-5) of the water collected from each filter
Controlled variables (what stays the same) Muddy-water stock, pour volume (2 cups/470 mL), amount of each material (1 cup gravel, 1 cup sand, 1/2 cup charcoal), pour speed, timing of the reading

Fair-testing rules. Change only the layer order between Filter A and Filter B. Keep everything else the same: pour from the same batch of muddy-water stock for every trial, measure out the same amount of each material, pour the same volume each time, and pour at the same steady speed. Run at least two trials of each filter, so one unusual result does not decide the outcome.

Safety. Wear safety glasses while measuring or pouring gravel, sand, and charcoal, since all three create dust. Charcoal can stain skin and clothing; wash hands after handling it. Do not taste or drink any water in this lab, filtered or not, at any point. An adult should cut the bottles, since the cut edges can be sharp. Wipe up spills right away so the floor does not become slippery.

Step 1. Cut the bottom off each of two bottles, or use two clear cups with several small holes poked through the bottom. Label one "Filter A" and one "Filter B" with tape.

Step 2. Turn each bottle upside down so the neck points down. Rest each one inside or over a clear collection cup, so filtered water has somewhere to go.

Step 3. Push a coffee filter, folded cotton cloth, or two cotton balls into the neck of each bottle. This plug should sit loosely enough for water to pass through, but tightly enough to hold the layers above it in place.

Step 4. Build Filter A starting at the plug and working up: pour in 1/2 cup of charcoal, then 1 cup of sand, then 1 cup of gravel on top, so poured water passes through gravel, then sand, then charcoal. Tap the side of the bottle gently after each layer so it settles flat.

Step 5. Build Filter B with the same three materials in the opposite order: 1 cup of gravel first, then 1 cup of sand, then 1/2 cup of charcoal on top.

Step 6. Pour a small sample of the muddy-water stock into a clear cup and rate its clarity with the clarity card before any filtering happens. Record this as the before-score; it applies to both filters, since both start from the same stock.

Step 7. Slowly pour 2 cups (470 mL) of the muddy-water stock into Filter A. Time how many seconds it takes the water to finish draining into the collection cup. A slow, steady pour gives the layers time to work; pouring too fast can push mud straight through.

Step 8. Rate the clarity of the water collected from Filter A with the clarity card. Record the clarity score and the drain time in the data table.

Step 9. Repeat Steps 7 and 8 for Filter B, using a fresh 2 cups (470 mL) of the same muddy-water stock.

Step 10. Gently rinse the gravel, sand, and charcoal from each filter, or rebuild each filter with fresh material, then repeat Steps 7 through 9 for a second trial of both filters.

When the lab is finished, empty each filter's used material into the trash, not down a sink or storm drain, and pour any leftover muddy water down a sink drain only.

Data & Analysis

Each team fills in one data table for both filters and both trials, rating clarity with the same 1-5 scale used for the before-filtering sample:

  • 1 - Opaque. You cannot see the card at all.
  • 2 - Very cloudy. The card is a blur. No letters show.
  • 3 - Hazy. Large shapes show, but they are blurry.
  • 4 - Mostly clear. Most of the printed text can be read.
  • 5 - Clear. The full line of text reads easily.
Filter Trial Clarity before (1-5) Clarity after (1-5) Drain time (seconds)
A (gravel, sand, charcoal) 1
A (gravel, sand, charcoal) 2
B (charcoal, sand, gravel) 1
B (charcoal, sand, gravel) 2

Presenting your results. Make a bar graph with one bar for each filter's average after-score. Label the two bars "Filter A" and "Filter B." Take a photo of the two collection cups side by side right after pouring; a photo shows the difference clearly for a science-fair board. State your conclusion honestly. Say which filter scored higher on average, and by how much. If the two filters scored about the same, or the reversed order did better than expected, say that plainly. A result that does not match a hypothesis is still useful data, and explaining a possible reason for it is part of a strong conclusion.

  1. Write the testable question this lab investigates. Name the independent variable and the dependent variable.
  2. Compare the before-score to the after-score for Filter A. How much did the score change?
  3. Compare the before-score to the after-score for Filter B. How much did the score change?
  4. Which filter had the higher average after-score across its two trials? Use your data to support your answer.
  5. Make a bar graph showing the average after-score for Filter A and Filter B. Describe what the graph shows.
  6. Explain why the charcoal layer can remove things that gravel and sand cannot. Use the word "adsorption" in your explanation.
  7. Explain why every team measured out the same amount of gravel, sand, and charcoal, and poured the same amount of muddy water. What would happen to the fair test if teams used different amounts?
  8. Filtered water from this lab is not safe to drink. Name one more step a real treatment plant uses after filtering, and explain why that step is still needed.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

  1. Answer: The question is whether the order of the gravel, sand, and charcoal layers changes how clear the filtered water becomes.
    The independent variable is the layer order (Filter A's standard order versus Filter B's reversed order); the dependent variable is the clarity score of the filtered water.

  2. Answer: Answers vary with each team's data; the after-score should be equal to or higher than the before-score.
    Filter A's order (gravel, then sand, then charcoal) lets water pass through progressively finer straining before it reaches the charcoal, so most teams should see a clear improvement. Accept any answer that correctly reports the student's own before and after scores and the difference between them.

  3. Answer: 1 to 2 points is a common result for Filter B, a smaller gain than Filter A typically shows.
    Sediment that reaches the charcoal first, before the sand and gravel have a chance to strain it out, can coat the charcoal's pores and reduce how well the rest of the filter works. Accept any answer consistent with the team's own recorded before and after scores and the calculated difference between them.

  4. Answer: Most teams should find Filter A had the higher average after-score, since its order lets water pass through progressively finer straining before reaching the charcoal.
    Accept a Filter B result if the student's own data supports it and the reasoning stays consistent with their recorded numbers.

  5. Answer: The graph should show two bars, with Filter A's bar taller than Filter B's bar in most classes, though the exact heights depend on each team's own averages.
    A correct graph labels both bars, uses a consistent scale from 1 to 5 on the vertical axis, and matches the averages the student calculated from their own data table.

  6. Answer: Gravel and sand remove particles by straining, blocking anything too big to fit through the gaps between grains.
    Charcoal is different: it is full of microscopic pores that give it a large internal surface area, and dissolved substances, odors, and coloring stick to that surface as water flows past, a process called adsorption, rather than passing straight through the way strained particles do.

  7. Answer: Keeping the amounts and pour volume the same makes sure that layer order is the only thing being tested.
    If teams used different amounts of gravel, sand, or charcoal, or poured different amounts of muddy water, a difference in the results could come from those changes instead of from the layer order, and the comparison would no longer be a fair test.

  8. Answer: Filtration removes suspended particles, which is what causes cloudiness, but it does not kill bacteria or viruses and removes only some dissolved contaminants, none of which were visible to begin with.
    Real treatment plants add a disinfectant, typically chlorine, after filtration so that the finished water is safe to drink, not just clear to look at.

Differentiation

Support (below grade level)

Provide two bottles already prepped with the neck plug in place, so teams start directly at Step 4. Reduce the investigation to one trial per filter instead of two, and simplify the data table to the after-scores and drain times only, leaving out the before-score column since both filters share the same starting stock. Pair students so one partner measures materials while the other reads the clarity card and records data on the recording sheet.

Extension (above grade level)

Have advanced teams add a second independent variable while holding layer order constant, such as doubling the depth of the charcoal layer in one filter, and write a paragraph comparing which change, order or depth, produced the bigger difference in clarity. Alternatively, have them calculate a filtration rate in milliliters per second from their drain-time data and compare that rate across all four trials, discussing whether a faster filter is always the better filter.

ELL supports

Provide picture cards for each material (gravel, sand, charcoal) and a labeled diagram of each filter's layer order, so students can match a material's name to its place in the column without relying on written descriptions alone. Allow labeled sketches of the data table's clarity scores in place of full written sentences where language is still developing, and pre-teach "filter," "layer," "adsorption," and "strain" using the actual bottles and materials as visual references.

Extensions

Teams that want to carry this investigation to a school or regional science fair can extend it into a controlled second round: keep the layer order that scored better in Round 1 fixed, and test a different variable, such as the depth of the winning layer or the type of plug material, following the same fair-testing rules already established. If a local water treatment plant, environmental education center, or county extension office offers tours, ask whether its filters use granular activated carbon and where that stage sits within the full treatment process, from intake to the disinfection step that finishes the job this classroom filter cannot do.