Water for Schools
Menu

Lesson plan Grade 10 Careers & CTE

Sizing a Chlorine Contact Tank: An Operator Math Lesson

A 50-minute grade 10 careers lesson where students take the operator's role at the fictional Maple Glen plant to compute basin volume, detention time, and chlorine feed.

Objectives

  1. Students will calculate a rectangular basin's volume in cubic feet and convert it to gallons.
  2. Students will use the detention-time formula to find how long water stays in a contact tank.
  3. Students will apply the pounds formula and judge whether the chlorine residual stays within the regulatory maximum.

Vocabulary

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.
Chlorine residual
Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.

Also: contact tank, pounds formula

Teacher overview

At a drinking water plant, chlorine needs time in contact with the water to disinfect it, so plants build a contact tank that holds the water long enough before it enters the distribution system. In this lesson students step into the operator's role at the fictional Maple Glen Water Treatment Plant and work the same numbers an operator checks: the tank's volume, the detention time, and the chlorine feed rate. A common misconception this lesson corrects is that a bigger dose alone means safe water; disinfection depends on both the dose and the contact time, and the residual left in the water must still stay under the regulatory maximum.

Procedure

Hook (5 min): Ask students why a plant cannot simply add chlorine at the last pipe before a house. Draw out the idea that chlorine needs time to work, then introduce the contact tank as the place that time is provided. State the day's task: check the math on a contact tank at the fictional Maple Glen plant.

Direct instruction (12 min): Model the method with a practice basin that is different from the handout, a fictional basin 50 ft long, 15 ft wide, and 10 ft deep at a flow of 1.0 MGD. Show volume = length x width x depth = 50 x 15 x 10 = 7,500 cubic feet; then gallons = 7,500 x 7.48 = 56,100 gallons. Show detention time = volume in gallons divided by flow, times 1440 minutes per day: 56,100 / 1,000,000 x 1440 = about 81 minutes. Then model the pounds formula: chlorine (lbs/day) = dose (mg/L) x flow (MGD) x 8.34.

Guided practice (13 min): With the class, begin the Maple Glen tank on the handout. Work items 1 and 2 together (volume in cubic feet, then gallons), checking that every student carries the correct units. Pause for a quick thumbs-up or thumbs-down check: can students state which number is the flow and which is the volume before dividing? Re-teach if more than a few thumbs are down.

Independent practice (15 min): Students complete handout items 3 through 8 on their own or in pairs, showing each step. Circulate and watch for the common error of dividing flow by volume instead of volume by flow.

Closure / exit ticket (5 min): On an index card, students answer one question: "If the Maple Glen flow doubled but the tank stayed the same size, would the detention time go up or down, and why?" Collect and scan for the reasoning that detention time falls because volume is fixed while flow rises.

Sizing a Chlorine Contact Tank: An Operator Math Lesson — Grade 10 Careers & CTE
Name: ____________________________ Date: ______________

Student Handout

You are an operator at the fictional Maple Glen Water Treatment Plant. Its chlorine contact tank is a rectangular basin 60 ft long, 20 ft wide, and 12 ft deep. The plant flow is 2.0 MGD. Use these facts: one cubic foot holds 7.48 gallons; detention time in minutes = volume in gallons / flow in gallons per day x 1440; chlorine (lbs/day) = dose (mg/L) x flow (MGD) x 8.34; one gallon of water weighs 8.34 pounds. The maximum residual disinfectant level for chlorine is 4.0 mg/L.

  1. Find the volume of the contact tank in cubic feet.
  2. Convert your answer from item 1 to gallons.
  3. Write the plant flow of 2.0 MGD as gallons per day.
  4. Calculate the detention time in the tank, in minutes, to the nearest whole minute.
  5. The operator doses chlorine at 2.0 mg/L. Using the pounds formula, how many pounds of chlorine per day does the plant feed?
  6. The target chlorine residual leaving the tank is 1.0 mg/L. What is the chlorine demand (dose minus residual), in mg/L?
  7. The residual leaving the tank is 1.0 mg/L. Is that residual below the 4.0 mg/L maximum residual disinfectant level? Explain in one sentence.
  8. If the plant flow doubled to 4.0 MGD while the tank stayed the same size, about how long would the detention time be, and why?

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: 14,400 cubic feet. Volume = 60 x 20 x 12 = 14,400 cu ft.
  2. Answer: 107,712 gallons. 14,400 cu ft x 7.48 gal/cu ft = 107,712 gallons.
  3. Answer: 2,000,000 gallons per day. 2.0 MGD means 2.0 million gallons per day = 2,000,000 gal/day.
  4. Answer: About 78 minutes. Detention = 107,712 / 2,000,000 = 0.05386 day; 0.05386 x 1440 = 77.6, which rounds to 78 minutes.
  5. Answer: 33.36 lbs/day. Chlorine = 2.0 mg/L x 2.0 MGD x 8.34 = 33.36 pounds per day.
  6. Answer: 1.0 mg/L. Chlorine demand = dose - residual = 2.0 - 1.0 = 1.0 mg/L.
  7. Answer: Yes. The maximum residual disinfectant level limits the chlorine residual in the water delivered to customers, not the dose fed at the plant, and 1.0 mg/L is well below 4.0 mg/L.
  8. Answer: About 39 minutes. Detention time = volume / flow, and the volume stays 107,712 gallons while the flow doubles, so the time is cut in half, from about 78 minutes to about 39 minutes.

Differentiation

Support

Provide a partially worked handout with item 1 already completed and the detention-time formula written as a labeled fill-in-the-blank, so students focus on carrying units rather than recalling the setup. Allow a calculator for every step.

Extension

Ask students to find what basin depth would be needed to hold the same detention time if the plant flow rose by half, keeping the length and width the same, and to show the calculation that supports their answer.

ELL supports

Pre-teach "contact tank," "detention time," and "chlorine residual" with a labeled diagram of a rectangular basin showing length, width, and depth. Provide a sentence starter for item 8: "The detention time would ___ because ___."