Texas, TCEQ, Wastewater Treatment, Class D
Free Detention Time Practice: Texas Class D Wastewater
Free detention time practice problems for the Texas Class D wastewater operator exam. You practice detention time in hours and minutes, including the unit changes questions bury inside it. Every problem uses fresh numbers, checks your answer, and shows all ten worked steps with nothing skipped. Progress saves on your device.
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See one problem worked in fullEvery step, from the question to the rounded answer
You practice detention time in hours and minutes, including the unit changes questions bury inside it. This one works detention time in hours from gallons and daily flow from start to finish. Every number traces to the question, a named conversion, or the line before it.
1. What you know
Basin volume: 100000 gal. Flow through it: 400000 gpd.
2. What you need
How long the water stays inside, in hours.
3. Match the units
Gallons over gallons-per-day leaves days. One day has 24 hours, so the last multiply moves the answer into hours.
4. Choose the formula
Volume divided by flow tells how much of a day the water spends inside, and multiplying by 24 restates that day fraction in hours.
hours = (gallons / gpd) x 24
5. Put in the values
Put each labeled value into the formula before doing any arithmetic.
hr = 100000 / 400000 x 24
6. Track the units
Gallons divide out first, leaving days. Then days divide out, leaving hours.
gal / (gal/day) -> day, day x hr/day -> hr
7. Work it
One operation, one line.
100000 / 400000 = 0.25
0.25 x 24 = 6
8. Round at the end
The rule here: round hours to one decimal place at the end. 6 becomes 6 hr.
9. Check the answer
6 hours should sit below 24 unless the basin holds more than a full day of flow. The unit left is hours, which is what the question asked for.
10. What it means
Detention time is contact time: how long disinfectant actually touches the water, or how long solids get to settle before the water moves on.
Formulas and where each number comes fromThe formula for each shape, plus every conversion and constant
Detention time in hours from gallons and daily flow
hours = (gallons / gpd) x 24
Volume divided by flow tells how much of a day the water spends inside, and multiplying by 24 restates that day fraction in hours.
Detention time in minutes from cubic feet and gpm
minutes = (cubic feet x 7.48) / gpm
The flow is counted in gallons, so first restate the tank in gallons, then divide by the gallons leaving each minute.
Where each number comes from
- One day has 24 hours. Definition of the day. Exact.
- One cubic foot holds 7.48 gallons. 1 gallon is defined as 231 cubic inches. 1 cubic foot is 12 x 12 x 12 = 1728 cubic inches. 1728 divided by 231 = 7.4805, used as 7.48.
Where this fits on the official examThe TCEQ subjects and math scope this module serves
- The official subject list this module serves
- Twenty official Need-to-Know subject categories, from Statutes and Rules through Math Problems (incl. Permits, Design Criteria, Safety, Laboratory, Collection, Pumping, treatment stages, Process Control, Disinfection, Sludge Digestion, Biosolids, Effluent Disposal).
- Source: TCEQ Wastewater Operator Exam Knowledge ('What You Need to Know for a Class D Wastewater Treatment Plant Operator Exam' table.)
Last reviewed 2026-08-01. Facts link to their official source.