Texas, TCEQ, Wastewater Treatment, Class D
Free Chlorine Dosage Practice: Texas Class D Wastewater
Last reviewed 2026-08-01. Facts link to their official source.
Free chlorine dosage practice problems for the Texas Class D wastewater operator exam. You practice dose, demand, and residual, the three-way relationship behind disinfection questions. 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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One full worked example, every step shown
You practice dose, demand, and residual, the three-way relationship behind disinfection questions. This example works residual from dose and demand from start to finish. Every number traces to the question, a named conversion, or the line before it.
1. What you know
Chlorine dose: 2.8 mg/L. Chlorine demand: 2.1 mg/L.
2. What you need
The chlorine residual left in the water, in mg/L.
3. Match the units
Dose, demand, and residual are all in mg/L, so the subtraction needs no conversion.
4. Choose the formula
The water uses up the demand first, so whatever is left of the dose is the residual still protecting the water.
residual = dose - demand
5. Put in the values
Put each labeled value into the formula before doing any arithmetic.
mg/L = 2.8 - 2.1
6. Track the units
Subtracting like units keeps the unit. Milligrams per liter stays.
mg/L - mg/L -> mg/L
7. Work it
One operation, one line.
2.8 - 2.1 = 0.7
8. Round at the end
The rule here: round mg/L to one decimal place at the end. 0.7 becomes 0.7 mg/L.
9. Check the answer
0.7 mg/L must land between zero and the dose, and it does. The unit left is mg/L, which is what the question asked for.
10. What it means
The residual is what your test kit should read at the sample tap. A residual near zero with a normal dose points at rising demand in the water.
The formulas in this module
Residual from dose and demand
residual = dose - demand
The water uses up the demand first, so whatever is left of the dose is the residual still protecting the water.
Dose needed from demand and target residual
dose = demand + residual
The dose must first feed what the water uses up, then still leave the residual you want, so the two add.
Demand from dose and measured residual
demand = dose - residual
Whatever part of the dose did not survive as residual was used up by the water, and that used-up part is the demand.
Where this fits in the official exam
- 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.)