FE daily problem for October 6, 2026 (Clarifier overflow rate and detention time)
This is the FE practice problem that Engineer Exam Lab members worked on October 6, 2026. The topic is clarifier overflow rate and detention time, from the FE Environmental exam specification. Try it with the FE Reference Handbook open before you read on. The worked solution below gives the handbook section and page, the equation, every substitution with units, a size check, and the mistake behind each wrong answer.
The problem
Daily problem · FE Environmental, Clarifier overflow rate and detention time
A circular clarifier 90 ft in diameter treats a flow of 7.63 MGD. Find the overflow rate in gpd/ft².
Enter your answer in gpd/ft², 3 significant figures.
Handbook: Environmental Engineering, Clarifier, FE Reference Handbook 10.6
Commit to an answer before you scroll. Writing the given values with their units first is the fastest way to spot what the question really asks for.
Worked solution, step by step
- Answer: 1,200 gpd/ft²
- Given: D = 90 ft, Q = 7.63 MGD
- Find: overflow rate vo (gpd/ft²)
- Handbook: Environmental Engineering, Clarifier (overflow rate), page 345
- Equation:
vo = Q/A_surface, with A = πD²/4
Substitute, keeping the units on every value:
A = πD²/4 = π(90 ft)²/4 = 6,362 ft²Q = 7.63 MGD = 7,630,000 gal/dvo = Q/A = (7,630,000 gal/d)/(6,362 ft²) = 1,200 gpd/ft²
Result: 1,200 gpd/ft², 3 significant figures.
Check: Units (gal/d)/ft² = gpd/ft²; 1 MGD = 1,000,000 gal/d.
Common wrong answers and the mistake behind each
A fill-in question has no options to compare, so the usual wrong answers come from a slip in the setup. Each value below is what you get from one specific mistake:
- 300 gpd/ft²: used πD² instead of πD²/4, so the area is 4 times too large and the rate 4 times too small.
- 160 gpd/ft²: converted the flow to ft³/d before dividing, which gives ft/d, not gpd/ft².
- 4,800 gpd/ft²: put the radius into πD²/4, so the area is 4 times too small and the rate 4 times too large.
Where this is in the FE Reference Handbook
This problem uses Environmental Engineering, Clarifier (overflow rate), page 345 of FE Reference Handbook 10.6. The handbook is a free PDF in MyNCEES, and on exam day you search the same document on screen, so practice finding the section with the search box instead of from memory. Search for the name of the quantity you need, not the symbol, and check the page heading before you copy an equation.
How to approach problems like this
A fill-in question gives you nothing to compare against, so the habits that protect you are carrying units through every line, converting once at the start, and rounding only at the end. Enter the value in the units the question names. Before you submit, check the size of the result against the situation and confirm that the units reduce correctly; most wrong fill-in answers differ from the right one by a factor that a unit check would catch. Keep one more significant figure in the intermediate steps than you report in the answer, so rounding in the middle of the work does not move the last digit. If the problem mixes unit systems, pick one at the start, convert every given value into it, and write the converted values down before you substitute anything.
Whatever the result, log it. A short error log with one line per missed problem (the topic, the mistake named in the solution, and the handbook page) shows within a few weeks which habits cost you points, and it turns review time into targeted practice instead of rereading.
For ten more problems on this topic, each with a full solution, see the clarifier overflow rate and detention time practice set. Every past daily problem is in the daily problem archive, one page per day.
Frequently asked questions
Is this a real FE exam question?
No. It is an original problem that our problem engine generated from the formulas in the FE Reference Handbook and checked by code before it was posted. Real exam questions are confidential, and the agreement every examinee accepts forbids sharing them.
Why does this problem appear here a day late?
Members of Engineer Exam Lab get each problem first, in the morning, and the worked solution the same evening. The problem and its solution come to this public archive the next day.
How should I use the worked solution?
Compare your work with it line by line, not only the final answer. If you picked a wrong answer, find the note that matches it above; each one names the exact mistake, which is the habit to fix before exam day.
Sources
- NCEES FE Reference Handbook 10.6 (free PDF in MyNCEES). Retrieved October 3, 2026.
- NCEES FE Environmental CBT exam specifications (PDF). Retrieved October 3, 2026.