Worksheet Grade 12 Engineering & Design

Harwick Plant Sedimentation Basin: Volume and Detention Time

A grade 12 engineering worksheet where students find a fictional basin's volume in gallons and its detention time, then test it against the typical 2-to-4-hour range.

Grade: Grade 12
Subject: Engineering & Design
Material: Worksheet
Time: 35 min
Prep: No prep
Materials: Calculator

Standards

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: Items 1 through 5 chain length-width-depth, the 7.48 gallon conversion, and the detention-time formula from cubic feet to hours, the unit-tracking this standard requires.

All materials for HSN-Q.A.1 →

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: Items 6 through 8 evaluate the basin's detention time against the typical 2-to-4-hour range as the plant's flow rises, judging whether the basin still gives particles enough time to settle, the constraint-based solution evaluation this standard requires.

All materials for HS-ETS1-3 →

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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 calculate a rectangular basin's volume in cubic feet and gallons.
Students will calculate detention time and compare it against a typical range as flow changes.

Vocabulary

Sedimentation: Sedimentation is the step where water sits almost still in a big basin so the heavy clumps of dirt can sink to the bottom. The cleaner water on top then moves on to the filters.
Detention time: Detention time is how long water stays inside a tank or basin before flowing out. Water must stay long enough for each treatment step, like settling or disinfecting, to finish its job.
Flow rate: Flow rate measures how much water moves past a point in a certain amount of time, like gallons per minute through a garden hose. Bigger pipes and stronger pushes give bigger flow rates.

Also: basin volume

Teacher overview

Before water is filtered, a treatment plant holds it in a sedimentation basin so that heavy particles have time to settle out. How long the water stays is the detention time, and water typically spends about 2 to 4 hours there, although many state design rules call for at least 4 hours in conventional basins. This worksheet has students find the volume of the fictional Harwick plant's basin, calculate its detention time, and see how that time changes as the plant's flow rises.

Harwick Plant Sedimentation Basin: Volume and Detention Time — Grade 12 Engineering & Design

Name: ____________________________ Date: ______________

Student Worksheet

Use these facts about the fictional Harwick plant. The rectangular basin is 80 feet long, 25 feet wide, with a water depth of 12 feet. Volume = length x width x depth. One cubic foot holds 7.48 gallons. Detention time = volume / flow. Use the typical operating range of 2 to 4 hours. Use 1 MGD = 694 gpm.

Find the basin volume in cubic feet.
Convert the volume to gallons.
The plant runs at 1.5 MGD. Convert that flow to gallons per minute (gpm).
Find the detention time in days, using the volume in gallons and the flow in gallons per day (1.5 MGD = 1,500,000 gallons per day). Round to four decimal places.
Convert the detention time from item 4 to hours. Round to the nearest hundredth.
Does the detention time fall within the typical 2 to 4 hour range?
During peak demand the flow rises to 2.5 MGD (2,500,000 gallons per day). Find the new detention time in hours.
Explain what happens to detention time as flow increases, and why that matters for settling.
Estimate the flow, in MGD, at which the detention time would fall to exactly 2 hours.

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: 24,000 cubic feet.
Volume = 80 feet x 25 feet x 12 feet = 24,000 cubic feet.
Answer: 179,520 gallons.
24,000 cubic feet x 7.48 gallons per cubic foot = 179,520 gallons.
Answer: 1,041 gpm.
1.5 MGD x 694 gpm/MGD = 1,041 gpm.
Answer: 0.1197 day.
Detention time = 179,520 gallons / 1,500,000 gallons per day = 0.11968, which rounds to 0.1197 day.
Answer: 2.87 hours.
0.11968 day x 24 hours per day = 2.872, which rounds to 2.87 hours.
Answer: Yes.
A detention time of 2.87 hours falls between 2 and 4 hours, so it falls within the typical range.
Answer: 1.72 hours.
Detention time = 179,520 gallons / 2,500,000 gallons per day = 0.071808 day; 0.071808 x 24 = 1.723, which rounds to 1.72 hours. This is below the low end of the typical range.
Answer: Detention time falls.
As flow rises, the same basin volume is replaced faster, so detention time falls. Less settling time means fewer particles settle out, which can send more solids on to the filters.
Answer: About 2.15 MGD.
For a 2-hour detention time, the flow needed is 179,520 gallons / (2 / 24 day) = 179,520 / 0.08333 = about 2,154,000 gallons per day, or about 2.15 MGD.

Differentiation

Support

Provide a labeled diagram of the basin with its length, width, and depth marked, and work items 1 and 2 as a class example. Give the formulas and the 7.48 conversion on a printed card.

Extension

Ask students to find the detention time in minutes for the normal flow in item 3, then design a change to the basin (a greater depth or a longer basin) that would keep detention at or above 2 hours even at the peak flow in item 7, and show the calculation that supports it.

ELL supports

Pre-teach "sedimentation," "detention time," and "basin" with a labeled diagram showing particles settling as water moves slowly through. Allow item 8 to be answered with a labeled sketch and a short phrase.