FE Environmental Practice Question Solution included after investigation
CASE E-073 · FE

The Contact Tank That Had Only 21 Effective Minutes

The hydraulic detention time was 30 minutes, but baffling reduced the effective contact time to 21 minutes. The CT value was 31.5, not 45.

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Case E-073 engineering failure visual
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1

Inspect

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2

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3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

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Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A disinfection contact tank has a volume of 0.50 MG and treats 24 MGD. The measured disinfectant residual is 1.5 mg/L. The stated baffling factor is 0.70, defined here as effective contact time divided by theoretical detention time. A note uses the full theoretical detention time and reports CT=45 mg·min/L. Determine the correct effective contact time and CT.

Water Wastewater Environmental Design Verification Root Cause Analysis Safe Correction
E-073
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

Nominal hydraulic detention time was used as effective disinfectant contact time.

What was missed

The stated baffling correction.

Corrected model

Calculate V/Q, apply the defined effective-time factor, then use CT=C×t_effective.

QUICK ANSWER

The theoretical detention time is (0.50/24)×1440=30 min. With a baffling factor of 0.70, effective contact time is 21 min. At a residual of 1.5 mg/L, CT=1.5×21=31.5 mg·min/L.

ENGINEERING TAKEAWAY

When the problem specifies an effective-time factor, calculate theoretical detention time from V/Q, multiply by that factor, then compute CT=C×t_effective.

CASE QUESTIONS

Questions this case should settle.

How is theoretical detention time calculated from tank volume and flow?

Use t=V/Q and convert the resulting time units as needed.

How is CT calculated when an effective contact time is specified?

Use CT=C×t_effective, after applying the stated hydraulic correction to nominal detention time.

NEXT FE PRACTICE QUESTION

The Clarifier System That Needed 12,000 ft², Not 6,000

Water Wastewater · 5–10 min

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