FE Environmental Practice Question Solution included after investigation
CASE E-071 · FE

The Treatment Plant That Removed 4,379 lb/day, Not 210

The concentration drop was 210 mg/L, but that was not a mass removal rate. At 2.5 MGD, the plant removed about 4,379 lb/day of BOD.

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Case E-071 engineering failure visual
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2

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3

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4

Fix

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5

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Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A wastewater plant treats 2.5 MGD. Influent BOD is 240 mg/L and effluent BOD is 30 mg/L. Assume the same flow rate at the inlet and outlet and use 8.34 as the lb/day conversion factor for MGD·mg/L. A note reports 210 lb/day removed because 240-30=210. Determine the influent load, effluent load, mass removed per day, and percent removal.

Water Wastewater Environmental Design Verification Root Cause Analysis Safe Correction
E-071
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

A concentration difference was labeled as a mass removal rate.

What was missed

Volumetric flow and unit conversion.

Corrected model

Use Q×C×8.34 for lb/day loading when Q is in MGD and C is in mg/L.

QUICK ANSWER

Using load=Q(MGD)×C(mg/L)×8.34, the influent BOD load is 5004 lb/day and the effluent load is 625.5 lb/day. The plant removes 4378.5 lb/day, and the removal efficiency is 87.5%.

ENGINEERING TAKEAWAY

Mass loading depends on both concentration and volumetric flow. Removal efficiency is based on the ratio of removed concentration or load to the influent value when flow is unchanged.

CASE QUESTIONS

Questions this case should settle.

How do you calculate wastewater contaminant loading in lb/day?

For Q in MGD and C in mg/L, use load=Q×C×8.34.

What is the BOD removal efficiency from 240 to 30 mg/L?

87.5% when inlet and outlet flow are the same.

NEXT FE PRACTICE QUESTION

The Contact Tank That Had Only 21 Effective Minutes

Water Wastewater · 5–10 min

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WHAT YOU’LL LEARN

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  • Strengthen exam-ready thinking through realistic scenarios
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