FE Environmental Practice Question Solution included after investigation
CASE E-079 · FE

The Incinerator That Achieved 99.9%, Not 99%

An outlet of 0.025 kg/hr from a 25 kg/hr hazardous constituent feed means only 0.1% remains. The destruction/removal efficiency is 99.9%.

▶ INTERACTIVE CASE FILE • 5–10 MIN • CLICK TO INVESTIGATE

Inspect, commit, prove, fix, and sign off.

Case E-079 engineering failure visual
HOW THIS WORKS

Think like
a real engineer.

Follow the investigation process used in the field — in five guided steps.

1

Inspect

Review the problem, diagram, and evidence.

2

Commit

Choose your hypothesis.

3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

Sign Off

See the full debrief and key takeaways.

Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A hazardous-waste thermal treatment unit receives 25 kg/hr of a target organic constituent. The measured constituent mass rate in the treated exhaust is 0.025 kg/hr. Using DRE=[(Win-Wout)/Win]×100%, a note reports 99% because it treats 0.001 as 1%. Determine the correct DRE.

Solid Hazardous Waste Environmental Design Verification Root Cause Analysis Safe Correction
E-079
INVESTIGATION WORKSPACE WORK THROUGH THE CASE — YOUR CHOICES MATTER.
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DEBRIEF (LOCKED)

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Find the root cause, confirm the fix, and see how this connects to the exam.

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🔒Exam takeawayComplete the case
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

A decimal fraction was converted to percent incorrectly.

What was missed

A fraction of 0.001 equals 0.1%, not 1%.

Corrected model

Use DRE=[1-Wout/Win]×100%.

QUICK ANSWER

Wout/Win=0.025/25=0.001, which is 0.1% remaining. Therefore DRE=[1-0.001]×100=99.9%.

ENGINEERING TAKEAWAY

Compute the outlet-to-inlet mass-rate fraction first, convert it correctly to percent, and subtract from 100% to obtain destruction/removal efficiency.

CASE QUESTIONS

Questions this case should settle.

What is the DRE formula?

DRE=[(Win-Wout)/Win]×100%.

What outlet rate corresponds to 99.99% DRE from a 25 kg/hr inlet?

0.0025 kg/hr or less.

NEXT FE PRACTICE QUESTION

The Mixed Waste Batch That Was 5.67%, Not 4.5%

Solid Hazardous Waste · 5–10 min

Continue Practice →
WHAT YOU’LL LEARN

Real failures. Lasting skills.

Each case is designed to build the judgment, analysis, and confidence you need for engineering exams — and beyond.

  • Apply core engineering concepts to real-world problems
  • Practice structured troubleshooting and analysis
  • Strengthen exam-ready thinking through realistic scenarios
COMMON QUESTIONS
Do I need prior knowledge to use these cases?

Basic subject familiarity helps, but every case is designed to teach through the investigation itself.

How long does a case take?

Most cases are designed for a focused 5–10 minute investigation.

Are the cases aligned with engineering exams?

Each case is mapped to a verified exam, subject, topic, and misconception before publication.

What happens after I complete a case?

The sealed debrief unlocks with the root cause, corrected reasoning, fix, and takeaway.