Lab / experiment Grade 8 Engineering & Design
Ashen Ford Grit Channel: Testing Settling Speed to Set Length
A grade 8 lab where teams time sand, silt, and coffee grounds settling through three water depths, then use the speed to size a wastewater grit channel.
Objectives
- Students will measure the time three materials take to settle through a measured depth of water.
- Students will calculate a settling speed in centimeters per second from depth and time.
- Students will use a settling speed and a channel velocity to find the length a grit channel needs.
Vocabulary
Key words: grit, settling speed, channel velocity, suspended, detention
Teacher overview
The first tank at a wastewater plant is usually a grit channel, a long narrow trough that gives sand, gravel, and coffee-ground-sized debris just enough time to drop to the bottom while the lighter organic material stays suspended and moves on to biological treatment. Getting that balance right is a length problem: water enters at one end at a set speed, and the channel has to be long enough that heavy particles reach the floor before the water leaves. Ashen Ford is a fictional town whose plant is rebuilding its grit channel, and this lab measures the settling speeds that would set that length. Students often expect deeper water to slow a particle down, so the procedure has teams repeat the sand trial at three depths to test that idea against their own data.
Procedure
Safety. Wear goggles for every trial. Use only clean tap water, and never taste or drink any water used in this lab. Pour used water into the bucket rather than the sink, wipe spills immediately, and wash your hands when the lab is over.
Set up the column (8 min). Stand the clear container on a flat surface where it will not be bumped. Using the ruler, mark the outside at 10 cm, 20 cm, and 30 cm above the inside bottom. Fill the container with tap water to the 20 cm mark and let it stand still for one minute so the water stops swirling.
Trial set 1, coarse sand at 20 cm (10 min). Hold a level quarter teaspoon of play sand just above the water surface. Start the timer the moment the sand touches the water, and stop it when most of the grains have reached the bottom. Record the time. Repeat twice more, and record all three times in the data table.
Trial set 2, silt at 20 cm (8 min). Pour off the water and refill to the 20 cm mark, letting it settle again. Repeat the same three trials with a level quarter teaspoon of dry silt or sifted soil. If the silt has not reached the bottom after 60 seconds, record the time as "more than 60 seconds" and move on. Record all three times.
Trial set 3, coffee grounds at 20 cm (8 min). Refill again to the 20 cm mark. Repeat the three trials with a level quarter teaspoon of used coffee grounds. If the grounds have not reached the bottom after 60 seconds, record the time as "more than 60 seconds" and move on.
Trial set 4, changing the depth (12 min). Refill the container to the 10 cm mark and run one sand trial, then refill to the 30 cm mark and run one more. Record both times. These two trials test whether the settling speed you calculated at 20 cm holds at other depths.
Calculate (6 min). For each trial set, average your times, then divide the water depth in centimeters by the average time in seconds to get a settling speed in centimeters per second.
Clean up (8 min). Pour the used water and sediment into the bucket, rinse the container, wipe the station, and return the goggles.
Data & Analysis
| Material and depth | Trial 1 time (s) | Trial 2 time (s) | Trial 3 time (s) | Average time (s) | Settling speed (cm/s) |
|---|---|---|---|---|---|
| Coarse sand, 20 cm | |||||
| Silt, 20 cm | |||||
| Coffee grounds, 20 cm | |||||
| Coarse sand, 10 cm | |||||
| Coarse sand, 30 cm |
- What was your average settling time for coarse sand at the 20 cm depth?
- Calculate the settling speed for coarse sand at 20 cm in centimeters per second, and show the division you used.
- Which of the three materials settled fastest, and which stayed suspended the longest?
- Compare your sand settling speeds at 10 cm, 20 cm, and 30 cm. Did the speed stay about the same? What does your answer say about whether depth changes how fast a grain falls?
- The rebuilt grit channel at the Ashen Ford plant will be 3 feet deep. In a test there, sand settled from the surface to the floor in 3 seconds, while the water moved along the channel at 1 foot per second. How long must the channel be so the sand reaches the floor before the water leaves?
- Suppose the plant built the channel 6 feet deep instead, with sand still settling 1 foot each second and water still moving 1 foot per second along the channel. How long would the channel have to be?
- Using your answers to items 5 and 6, state what happens to the required channel length when the depth doubles, and explain why.
- A grit channel is designed so that light organic material, such as food scraps and paper, stays suspended. Using your result for the slowest material, explain why a plant wants that light material to travel on to the next tank.
- Name one change you would make if you ran this lab again, and say what that change would tell you that this run did not.
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: Varies by team.
Most teams record 2 to 6 seconds for sand at 20 cm. Accept any average consistent with the three trial times the team wrote in the table. - Answer: Varies by team.
The calculation should be 20 cm divided by the team's own average time. A team averaging 4 seconds reports 20 ÷ 4 = 5 centimeters per second. Check that the units in the answer are centimeters per second. - Answer: Sand settled fastest; silt or coffee grounds stayed suspended longest.
Accept either silt or coffee grounds as the slowest if it matches the team's data. Sand grains are dense and relatively large. Silt particles are much finer, and the finest can stay suspended for many minutes. Coffee grounds are larger but only a little denser than water, so they also fall more slowly than sand. - Answer: The speeds stayed about the same.
Accept any set of three speeds within roughly a centimeter per second of one another, and any explanation stating that depth changes the time but not the speed. A deeper column takes longer because the grain has farther to fall, not because it falls more slowly. - Answer: 3 feet.
Sand needs 3 seconds to settle. In 3 seconds the water carries it 3 seconds × 1 foot per second = 3 feet along the channel, so the channel must be at least 3 feet long for the sand to reach the floor before the water exits. - Answer: 6 feet.
At 1 foot per second of settling, sand needs 6 seconds to fall 6 feet. In 6 seconds the water carries it 6 × 1 = 6 feet along the channel, so the channel must be at least 6 feet long. - Answer: The required length doubles.
Doubling the depth doubles the settling time, and the water keeps moving at the same speed during that longer time, so it travels twice as far before the grain lands. Length and depth change together as long as the channel velocity stays fixed. - Answer: Organic material belongs in the next treatment stage, not in the grit.
Grit is washed and hauled to a landfill, while the organic material is food for the microbes in the plant's biological tanks. If that light material settled with the sand, the plant would pay to haul away material it needs and would send weaker wastewater on to treatment. A slow material in the lab shows how a particle can ride the flow past the end of a channel. Accept any response making that separation clear. - Answer: Accept any specific change with a matching reason.
Strong responses name running more trials to steady the average, using a taller column to time longer falls more precisely, warming or cooling the water to test temperature, or testing a mixed sample of sand and grounds together to see whether they separate as cleanly as they did alone.
Differentiation
Support
Pre-mark the containers at 10, 20, and 30 cm before class so teams begin timing immediately, and give these students a data table with the depth column already filled in and the division for item 2 written as "depth ÷ average time = speed." Assign a timing partner so one student watches the container while the other runs the stopwatch.
Extension
Have students test whether a grain's starting position matters by releasing sand at the water surface in one trial and just below the surface in another, using the same depth. Ask them to report both speeds and explain which release point better matches how grit actually enters a channel from an incoming sewer pipe.
ELL supports
Demonstrate one full trial while naming each step, and post a picture sequence of the lab steps above the sink. Provide a word bank with "settle," "suspended," "depth," "speed," and "grit," pairing each word with a simple drawing, and allow item 3 to be answered by ranking the three materials in a numbered list rather than in sentences.