Worksheet Grade 9 Science

Chlorine Contact Time at Merrowfield: A Grade 9 CT Worksheet

A grade 9 worksheet where students find contact time from basin volume and flow, calculate CT for disinfection, and solve for the residual a plant needs at peak flow.

Grade: Grade 9
Subject: Science
Material: Worksheet
Time: 40 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 6 chain a contact-time calculation and a CT calculation through changing flows, requiring students to carry and interpret units consistently as this standard specifies.

All materials for HSN-Q.A.1 →

HSA-CED.A.1 Common Core Math

Create equations and inequalities in one variable and use them to solve problems. Include equations arising from linear and quadratic functions, and simple rational and exponential functions.

How this material aligns: Item 7 rearranges the CT relationship to solve for the chlorine residual the plant needs at peak flow, the one-variable equation solving this standard describes.

All materials for HSA-CED.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 3 and 6 check the basin's CT against the required disinfection value at normal and at peak flow, and item 8 names the operating changes that would bring a short CT back up to the requirement.

All materials for HS-ETS1-3 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will calculate a contact basin's contact time from its volume and flow rate.
Students will calculate CT as the product of chlorine residual and contact time.
Students will solve for the residual needed to meet a required CT at a higher flow.

Vocabulary

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.
CT value: CT value is a way operators check that water has been disinfected long enough. It is found by multiplying how much disinfectant is in the water by how many minutes the water has been in contact with it.
Disinfection: Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.

Also: contact time, contact basin

Teacher overview

Chlorine needs time in contact with water to kill or inactivate germs, so treatment plants send water through a contact basin before it reaches homes. Operators track a number called CT, the chlorine residual multiplied by the contact time, because a required CT stands for enough disinfection. This worksheet has students calculate contact time and CT for the fictional plant at Merrowfield, then see what happens when the flow speeds up. A common error is to assume that a higher flow gives more disinfection; a higher flow actually shortens the contact time and lowers CT.

Chlorine Contact Time at Merrowfield: A Grade 9 CT Worksheet — Grade 9 Science

Name: ____________________________ Date: ______________

Student Worksheet

Merrowfield (a fictional town) disinfects its water in a chlorine contact basin. Use these numbers for all items:

The contact basin holds 50,000 gallons of water.
Contact time (in minutes) = basin volume (gallons) divided by flow (gallons per minute).
CT = chlorine residual (mg/L) x contact time (minutes).
Merrowfield's operating plan requires a CT of at least 60 mg/L-min for the water it treats.

At normal flow, water moves through the basin at 500 gpm. Find the contact time in minutes.
Operators hold a chlorine residual of 0.8 mg/L. Using your contact time from item 1, calculate the CT.
Compare your CT from item 2 with the required 60 mg/L-min. Does the plant meet the requirement, and by how much?
During peak demand, the flow rises to 800 gpm. Find the new contact time in minutes.
With the same 0.8 mg/L residual, calculate the CT at the peak flow from item 4.
Compare your peak-flow CT from item 5 with the required 60 mg/L-min. Does the plant still meet the requirement? If not, by how much does it fall short?
At the peak contact time from item 4, what chlorine residual, in mg/L, would the plant need to reach a CT of exactly 60 mg/L-min? (Rearrange CT = residual x contact time to residual = CT divided by contact time. Round to the nearest hundredth.)
Explain why a higher flow lowers CT even when the residual stays the same, and give two changes operators could make to bring CT back up to the requirement.

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: 100 min.
Contact time = 50,000 gallons divided by 500 gpm = 100 minutes.
Answer: 80 mg/L-min.
CT = 0.8 mg/L x 100 minutes = 80 mg/L-min.
Answer: Yes, by 20 mg/L-min.
80 mg/L-min is above the required 60 mg/L-min, so the plant meets the requirement with a margin of 80 minus 60 = 20 mg/L-min.
Answer: 62.5 min.
Contact time = 50,000 gallons divided by 800 gpm = 62.5 minutes.
Answer: 50 mg/L-min.
CT = 0.8 mg/L x 62.5 minutes = 50 mg/L-min.
Answer: No, short by 10 mg/L-min.
50 mg/L-min is below the required 60 mg/L-min, so the plant falls short by 60 minus 50 = 10 mg/L-min at peak flow.
Answer: 0.96 mg/L.
Residual = CT divided by contact time = 60 mg/L-min divided by 62.5 minutes = 0.96 mg/L.
Answer: Higher flow shortens contact time, which lowers CT; operators can raise the residual or slow the flow.
CT depends on contact time, and contact time is volume divided by flow. When flow rises, the water spends less time in the fixed basin, so contact time and CT both drop. To restore CT, operators could raise the chlorine residual, reduce the flow through the basin, or add basin volume so water stays longer.

Differentiation

Support

Provide the contact-time and CT formulas already written with labeled blanks for volume, flow, residual, and time. Work item 1 and item 2 together as a pair so students see the two-step order before repeating it for the peak-flow case.

Extension

Ask students to find the highest flow, in gallons per minute, at which the plant can still meet the required CT while holding the same chlorine residual, working backward from the CT requirement to a contact time and then to a flow.

ELL supports

Pre-teach "residual," "contact time," and "CT" with a simple diagram: water enters the basin, chlorine stays in contact for a set time, and CT combines the two. Provide a sentence frame for item 8: "Higher flow lowers CT because ___."