Lab / experiment Grade 12 Engineering & Design

Build and Test a Bench-Scale Sand Filter: Loading Rate and Clarity

A grade 12 engineering lab where students build a sand filter column, measure its flow, and compute the loading rate in gpm per square foot against the design range.

Grade: Grade 12
Subject: Engineering & Design
Material: Lab / experiment
Time: 55 min
Prep: Moderate prep
Materials: Clear rigid tube or 2-liter bottle with the bottom removed (1 per group), Clean coarse sand (about 2 cups per group), Gravel or small pebbles (about 1 cup per group), Cotton batting or a coffee filter for the support layer, Fine mesh or screen to hold the media at the outlet, 100 mL graduated cylinder (1 per group), Stopwatch or phone timer, Ruler or caliper to measure the column's inside diameter, Muddy water sample (soil stirred into water, about 1 liter per group), Clear cups for the filtered water and a 1-to-5 cloudiness card, Ring stand or a bucket to hold the column upright

Standards

HS-ETS1-3 NGSS

Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.

How this material aligns: In analysis questions 4 and 5 students evaluate whether their built filter's loading rate meets the 2-to-6 gpm per square foot design criterion, the solution evaluation this standard requires.

All materials for HS-ETS1-3 →

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Questions 1 through 4 carry inches, feet, milliliters, and seconds through to gallons per minute per square foot, the multi-step unit conversion this standard targets.

All materials for HSN-Q.A.1 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-17.

Objectives

Students will build a layered sand filter column and run water through it at a measured flow.
Students will compute the filter loading rate in gpm per square foot and compare it against the design range.

Vocabulary

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.
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.

Also: loading rate, media

Teacher overview

A treatment plant's filters are rated by their loading rate, the flow they carry for each square foot of surface. Conventional rapid sand filters are designed to run at about 2 to 6 gallons per minute for each square foot of surface. In this lab, students build a small sand filter column, run water through it, and compute its loading rate in the same units a plant operator uses. They then compare their filter to the real turbidity standard of 0.3 NTU that finished water must meet, and see why clearing visible cloudiness is not the same as making water safe.

Build and Test a Bench-Scale Sand Filter: Loading Rate and Clarity — Grade 12 Engineering & Design

Name: ____________________________ Date: ______________

Procedure

Step 1 (safety): Use only clean tap water and garden soil for the muddy sample. Do not drink or taste any water in this lab. Wipe up spills right away to prevent slipping.

Step 2 (measure the column, 5 min): Measure the inside diameter of your tube with a ruler. Record it in inches. Your teacher may give the class one standard 3-inch tube so every group uses the same diameter.

Step 3 (build the filter, 12 min): Set the tube upright in a ring stand or bucket, outlet down. Place the mesh or screen at the outlet. Add the cotton or coffee filter as a support layer. Add a 2-inch layer of gravel. Add a 4-inch layer of coarse sand on top. The gravel supports the sand and keeps it from washing out.

Step 4 (measure the flow, 10 min): Pour muddy water into the top and keep the water level roughly steady. Once flow is steady, collect the water leaving the outlet in the graduated cylinder. Record the volume in milliliters and the time in seconds. Run three trials.

Step 5 (rate the clarity, 5 min): Before filtering, rate the muddy sample on the 1-to-5 cloudiness card. After filtering, rate the collected water the same way. Record both.

Step 6 (clean up, 3 min): Pour the used media into the waste container your teacher provides. Rinse the cylinder and cups.

Data & Analysis

Trial	Volume collected (mL)	Time (s)	Flow rate (gpm)
1
2
3

Record the clarity rating before filtering and after filtering on the 1-to-5 cloudiness card.

Find your filter's surface area. The column's inside diameter is 3.0 inches, which is 0.25 foot. Use Area = 0.785 x Diameter squared, in square feet. If your tube differs, also compute the area from your own measured diameter.
Find the flow rate in gpm for each of your three trials. Use flow = gallons / minutes, where 1 gallon = 3,785 mL.
Suppose a trial collects 500 mL in 40 seconds. Show the conversion to gallons per minute. Round to the nearest thousandth.
Using the flow from question 3 and your area from question 1, compute the loading rate in gpm per square foot. Round to the nearest hundredth.
Conventional rapid sand filters are designed for 2 to 6 gpm per square foot. Is the loading rate from question 4 within that range? What does that tell you about your filter?
Real plants must produce finished water at or below 0.3 NTU. Your filter clears visible cloudiness, but you cannot measure NTU here. Explain why passing this clarity test does not prove the water is safe to drink.

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: about 0.049 square feet.
Area = 0.785 x (0.25 foot) squared = 0.785 x 0.0625 = 0.0491, which rounds to about 0.049 square feet.
Answer: varies by group's own data.
Each trial's flow equals the collected volume in gallons divided by the time in minutes. Convert mL to gallons by dividing by 3,785, and convert seconds to minutes by dividing by 60. Accept any value consistent with the group's recorded volume and time.
Answer: about 0.198 gpm.
500 mL / 3,785 mL per gallon = 0.1321 gallon. 40 seconds / 60 = 0.6667 minute. Flow = 0.1321 / 0.6667 = 0.1981, which rounds to about 0.198 gpm.
Answer: about 4.04 gpm per square foot.
Loading rate = flow / area = 0.198 gpm / 0.049 square feet = 4.04 gpm per square foot.
Answer: Yes, it is within range.
The rate of 4.04 gpm per square foot falls between 2 and 6, so this bench filter loads at a rate similar to a real rapid sand filter. Accept a group's own value with a correct comparison, since real trials will vary.
Answer: Clarity is not the same as safety.
Accept any sound explanation: the filter removes visible sediment but not the microscopic particles, bacteria, viruses, or dissolved contaminants that make water unsafe. Real plants add disinfection and continuous testing, and only a lab test can confirm the water meets the standard.

Differentiation

Support

Pre-build one demonstration column so groups can copy the layer order. Provide a data table with one trial instead of three, and give the area from question 1 already worked.

Extension

Have groups test a second design, such as a deeper sand layer or an added fine-sand layer, measure its loading rate the same way, and write a short comparison of which design cleared the water better at a similar loading rate.

ELL supports

Provide picture cards for each layer (screen, cotton, gravel, sand) labeled with its name. Allow students to record clarity with the numbered card alone and to show the loading-rate work as a labeled equation.