Worksheet Grade 12 Science

Disinfection CT and Baffling at the Whitcombe Clearwell

A grade 12 science worksheet on chlorine CT: students find contact time from clearwell volume and flow, apply a baffling factor, and check CT against a target.

Grade: Grade 12
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: Students carry units through the volume-over-flow, baffling-factor, and CT calculations in items 1 through 8, the multi-step unit reasoning this standard expects.

All materials for HSN-Q.A.1 →

HSN-Q.A.2 Common Core Math

Define appropriate quantities for the purpose of descriptive modeling.

How this material aligns: Items 3, 7, and 8 have students define and report CT in milligram-minutes per liter, choosing the quantity that describes disinfection strength, as this standard requires.

All materials for HSN-Q.A.2 →

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 9 and 10 weigh two competing ways to hold the required CT at peak flow, adding baffle walls versus raising chlorine residual, against the 4.0 mg/L maximum residual safety constraint, the prioritized trade-off 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 2026-07-23.

Objectives

Students will calculate a clearwell's contact time from its volume and the plant flow rate.
Students will apply a baffling factor to find effective contact time and compute the CT value.
Students will compare an achieved CT to a required CT and adjust residual to keep a safety margin.

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.
Clearwell: A clearwell is a large storage tank at a treatment plant that holds finished water right after it comes out of the filters. It gives the disinfectant enough time to work before the water is pumped out to homes and businesses.

Also: contact time, baffling factor

Teacher overview

Chlorine needs time in contact with water to inactivate pathogens, and operators track that as a CT value: the chlorine residual multiplied by the effective contact time. This worksheet uses the fictional Whitcombe treatment plant, whose clearwell holds finished water while chlorine acts on it before the water enters the distribution system. Students calculate contact time from the clearwell volume and flow, reduce it with a baffling factor that accounts for short-circuiting, and compare the achieved CT to a target. A frequent mistake is using the full clearwell detention time as if all the water stayed the design time. Real basins let some water slip through faster, which is exactly what the baffling factor corrects.

Disinfection CT and Baffling at the Whitcombe Clearwell — Grade 12 Science

Name: ____________________________ Date: ______________

Student Worksheet

The Whitcombe clearwell (a fictional facility used for this worksheet) has these values:

Clearwell volume: 250,000 gallons
Normal plant flow: 1,800 gpm
Baffling factor: 0.5 (average baffling)
Chlorine residual leaving the clearwell: 1.2 mg/L
Required CT for the target inactivation: 45 mg/L-min

Use these relationships for every item:

Theoretical detention time (min) = clearwell volume (gal) / flow (gpm)
Effective contact time (min) = theoretical detention time x baffling factor
CT (mg/L-min) = chlorine residual (mg/L) x effective contact time (min)

Calculate the theoretical detention time of the clearwell at the normal flow of 1,800 gpm. Round to the nearest whole minute.
Apply the 0.5 baffling factor to find the effective contact time at normal flow. Round to the nearest whole minute.
Calculate the achieved CT at normal flow, using the 1.2 mg/L residual. Round to the nearest whole mg/L-min.
Does the achieved CT from item 3 meet the required CT of 45 mg/L-min? Answer yes or no.
During peak demand the flow rises to 3,000 gpm. Calculate the new theoretical detention time. Round to the nearest whole minute.
Apply the 0.5 baffling factor to find the effective contact time at peak flow. Round to the nearest whole minute.
Calculate the achieved CT at peak flow, with the residual still at 1.2 mg/L. Round to the nearest whole mg/L-min.
At peak flow, the operator raises the chlorine residual to 1.5 mg/L to rebuild the safety margin. Calculate the new CT at peak flow. Round to the nearest whole mg/L-min.
Baffle walls could raise the baffling factor from 0.5 to 0.7. Explain how that change would affect the effective contact time and the CT without adding any more chlorine.
The residual in item 8 stays well below the maximum residual disinfectant level of 4.0 mg/L. Explain why raising residual to boost CT is limited by that maximum.

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: 139 minutes.
250,000 gallons / 1,800 gpm = 138.9, which rounds to 139 minutes.
Answer: 69 minutes.
138.9 minutes x 0.5 = 69.4, which rounds to 69 minutes.
Answer: 83 mg/L-min.
CT = 1.2 mg/L x 69.4 minutes = 83.3, which rounds to 83 mg/L-min.
Answer: Yes.
The achieved CT of 83 mg/L-min is greater than the required 45 mg/L-min, so the clearwell meets the target with margin to spare.
Answer: 83 minutes.
250,000 gallons / 3,000 gpm = 83.3, which rounds to 83 minutes.
Answer: 42 minutes.
83.3 minutes x 0.5 = 41.7, which rounds to 42 minutes.
Answer: 50 mg/L-min.
CT = 1.2 mg/L x 41.7 minutes = 50.0 mg/L-min. This still clears the 45 mg/L-min target, but with little margin.
Answer: 63 mg/L-min.
CT = 1.5 mg/L x 41.7 minutes = 62.5, which rounds to 63 mg/L-min. The higher residual restores the margin lost to the faster flow.
Answer: A higher baffling factor raises effective contact time and CT.
Going from 0.5 to 0.7 means a larger share of the clearwell's detention time counts as real contact time, so effective contact time and the CT both rise. Better baffling adds disinfection capacity through basin design rather than through more chemical.
Answer: Residual is capped by the 4.0 mg/L maximum for chlorine.
CT can be raised by increasing either contact time or residual, but residual is capped by the maximum residual disinfectant level of 4.0 mg/L, a health-based limit on chlorine itself, judged as a running annual average of distribution-system samples. Higher doses also form more disinfection byproducts, which are regulated separately. Beyond that limit, operators must gain CT through contact time or baffling instead.

Differentiation

Support

Provide the three relationships on a printed card and complete items 1 through 3 together as a worked example before students take on the peak-flow items. Accept unrounded values, and give a filled sketch of the clearwell with the flow arrow labeled.

Extension

Ask students to find the highest peak flow at which the clearwell, keeping the worksheet's residual and baffling factor, still just meets the required CT target. They should set up the CT relationship and solve for the flow rather than guess.

ELL supports

Pre-teach "contact time," "baffling factor," and "residual" with a simple diagram showing water entering one end of a clearwell and leaving the other. Allow students to answer items 9 and 10 with a short labeled list rather than full sentences.

Extensions

Have students look up whether their local plant uses a clearwell or separate contact basins, and write a short question for the utility asking how it keeps CT on target when demand spikes on a hot afternoon.