Lab / experiment Grade 11 Science
Lindmere's Taste-and-Odor Problem: A Carbon Dose Adsorption Lab
Grade 11 students dose colored water with activated carbon at four levels, compare it with a sand control, and turn their removal data into a full-scale carbon feed rate.
Objectives
- Students will measure how color removal changes as activated carbon dose increases.
- Students will compare adsorption onto activated carbon with adsorption onto an equal mass of sand.
- Students will convert a bench-scale dose into a full-scale carbon feed rate in pounds per day.
Vocabulary
- Activated carbon
- Activated carbon is a special charcoal full of microscopic holes that grab and hold chemicals as water flows past. It is the same material used in many home water filter pitchers.
Also: adsorption, dose, surface area, removal efficiency
Teacher overview
When an algae bloom releases taste-and-odor compounds into a reservoir, the compounds themselves are not a health hazard, but the treated water can smell like soil or mold, and customers complain within hours. Utilities answer with powdered activated carbon, a material whose internal pore structure gives a single gram an enormous surface for dissolved molecules to stick to. This lab has students dose colored water at four carbon levels, run a sand control at the highest mass, and use the color that remains to build a dose-response curve. The classroom test is a simplified analog of a plant jar test rather than a certified measurement. One misconception to address before students start: adsorption is not filtration. The carbon does not strain molecules out by size; it holds them on its surface, which is why the mass of carbon matters more than the size of its grains and why doubling the dose does not double the removal.
Procedure
Step 1. Safety and setup: wear goggles, and do not taste or drink anything used in this lab. Crushed carbon produces a fine dust, so pour it slowly and wipe spills with a damp towel rather than blowing them away. Wash hands at the end. Suppose your group works for the fictional Lindmere Water Treatment Plant, which needs a carbon dose recommendation by the end of the period.
Step 2. Make the stock solution: add 8 drops of blue food coloring to 1 liter of water and stir until the color is even. This stands in for a reservoir carrying a taste-and-odor compound.
Step 3. Build a comparison scale. Label four cups 100, 75, 50, and 25. Put 60 milliliters of stock in the cup labeled 100. In the others, combine stock and clean water to make 60 milliliters at three-quarters, one-half, and one-quarter strength. Set them on white paper. These are the standards you will match filtrates against.
Step 4. Label five treatment cups: 0.0 g, 0.5 g, 1.0 g, 2.0 g, and Sand. Measure 100 milliliters of stock into each.
Step 5. Weigh the carbon to the nearest 0.1 gram and add it to the matching cup. Weigh 2.0 grams of sand into the cup labeled Sand. The 0.0 g cup receives nothing and is your untreated control.
Step 6. Stir every cup for 15 seconds, then repeat the stir every 2 minutes for a total contact time of 10 minutes. Start the timer when you first stir.
Step 7. At 10 minutes, pour each cup through its own coffee filter into a clean, labeled cup. Use a fresh filter each time so carbon from one dose does not treat another.
Step 8. Hold each filtrate against the white paper next to the comparison scale and record the standard it most closely matches as the percent color remaining. Two group members should judge independently and agree on a value before recording it.
Step 9. Dispose of the used carbon and sand in the trash, not the sink. Pour the filtrates down the drain with running water and rinse all glassware.
Data & Analysis
| Treatment | Carbon or sand mass (g per 100 mL) | Dose (mg/L) | Percent color remaining | Percent removal |
|---|---|---|---|---|
| Untreated control | 0.0 | 0 | ||
| Carbon, low | 0.5 | |||
| Carbon, medium | 1.0 | |||
| Carbon, high | 2.0 | |||
| Sand control | 2.0 |
- Convert the low carbon dose of 0.5 grams per 100 milliliters to milligrams per liter, showing each conversion step.
- Using your own readings, calculate the percent removal for the 1.0-gram carbon cup. Percent removal equals 100 minus the percent color remaining.
- Compare the 2.0-gram carbon cup with the 2.0-gram sand cup. State which removed more color, and explain what that difference says about the attraction between the dye molecules and each solid surface.
- Plot percent removal on the vertical axis against carbon dose in milligrams per liter on the horizontal axis for your three carbon cups plus the untreated control at zero. Describe the shape of the curve between the lowest and highest doses.
- Using your graph, estimate the carbon dose in milligrams per liter that would give 90 percent color removal, and state whether that estimate is an interpolation between your points or an extrapolation beyond them.
- The fictional Lindmere plant treats 2.5 MGD and feeds powdered activated carbon at 22 mg/L during a taste-and-odor episode. Using pounds per day equal to concentration in mg/L times flow in million gallons per day times 8.34, calculate the pounds of carbon fed per day.
- Your bench doses ran in the thousands of milligrams per liter while the plant in item 6 feeds tens of milligrams per liter. Give two reasons the bench test needs so much more carbon per liter than the plant does.
- Identify one source of error in the color comparison in Step 8, and state whether it would make your measured percent removal look too high or too low.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: 5,000 mg/L.
0.5 gram per 100 milliliters is 5 grams per liter, because 100 milliliters goes into 1 liter ten times. 5 grams per liter x 1,000 milligrams per gram = 5,000 mg/L. - Answer: the percent removal for the student's own 1.0-gram cup.
Accept any value equal to 100 minus the percent color remaining that the group recorded. Most groups should find the 1.0-gram cup between the 0.5-gram and 2.0-gram results, and a value that falls outside that order signals a filtering or labeling error worth checking. - Answer: The carbon removed far more color than the sand.
Sand offers a small, largely smooth surface, while activated carbon is riddled with pores that give a gram of it an enormous internal surface, and that surface attracts the dye molecules strongly enough to hold them out of solution. Accept any answer that ties the difference to the amount of available surface and to a stronger attraction between the dye and the carbon than between the dye and quartz sand. - Answer: a curve that rises steeply and then flattens.
Accept any description of diminishing returns: removal climbs quickly between zero and the low dose, then gains less between the medium and high doses. Groups whose points fall in a straight line should be asked whether their comparison scale had enough steps to detect the flattening. - Answer: the dose the student's own curve gives for 90 percent removal.
Accept any dose read consistently from the group's plotted curve, with the correct label. If the group's highest measured removal is below 90 percent, the estimate lies beyond the data and must be called an extrapolation; if 90 percent falls between two measured points, it is an interpolation. - Answer: 458.7 pounds per day.
22 mg/L x 2.5 MGD x 8.34 = 458.7 pounds of carbon per day. Working stepwise: 22 x 2.5 = 55, and 55 x 8.34 = 458.7. - Answer: Short contact time and a far stronger test solution.
Accept any two sound reasons. The bench test gives only 10 minutes of contact in a cup that is stirred intermittently, while a plant provides longer contact with continuous mixing before the carbon settles or is filtered out. The food coloring is also present at a far higher concentration than a real taste-and-odor compound, which people can smell at concentrations too small to see, so the bench test asks the carbon to hold much more material per liter. - Answer: a stated error source with its direction.
Accept any genuine source with a correct direction, such as carbon fines passing through the filter and darkening the filtrate, which makes removal look too low; judging color under a colored light or against a tinted background, which can push the reading either way if the direction is explained; or leaving carbon in contact during filtration, which adds treatment time and makes removal look too high.
Differentiation
Support
Prepare the stock solution and the four comparison standards in advance so groups begin at the dosing step, and give the dose column of the data table already converted to milligrams per liter. Provide a worked percent-removal example using values from a different trial directly above item 2.
Extension
Have groups run a second 2.0-gram carbon cup with only a 2-minute contact time, compare it with the 10-minute result, and explain what the difference shows about how fast adsorption reaches equilibrium. Ask them to state what that finding means for a plant deciding where in the treatment train to add carbon.
ELL supports
Pair adsorption, dose, contact time, and removal with a labeled sketch showing dye molecules sticking to a pitted carbon grain and sliding past a smooth sand grain. Let students record readings as numbers in the table and answer items 3, 7, and 8 with the frames "The carbon removed more because ___" and "This error would make my removal look ___ because ___."