Drinking Water Disinfection on the PE WRE Exam: CT Values, Chlorine, and UV
CT calculations, chlorine and chloramine disinfection, UV dosing, and DBP control for the PE WRE exam — with worked NCEES-style problems and EPA SWTR reference tables.
You've solved Manning's-equation problems in your sleep. Then a drinking-water disinfection question lands on your PE Water Resources exam — flow rate, free-chlorine residual, pH, water temperature, baffling factor — and you have to find the right CT value in a large EPA table, multiply by the right effective contact time, and verify the plant meets its Giardia and virus inactivation targets under the Surface Water Treatment Rule. The math is one-step. The lookup is what kills the clock. Candidates who pass have practiced finding the right table cell fast; candidates who don't lose minutes paging through cross-tabulations.
Drinking-water disinfection lives under NCEES Topic 10 — Drinking Water Distribution and Treatment, which carries 6–9 questions on the 80-question PE Civil WRE exam per the April 2024 specification. Sub-topic 10H ("Disinfection, including disinfection byproducts") is where CT calculations and UV dosing problems live. Pair that with sub-topic 10F (coagulation and flocculation) and 10G (membrane processes and media filtration), and you're looking at 2–4 questions per form on water-treatment design.
This post covers the five disinfection problem types NCEES tests, names where each one lives in the NCEES PE Civil Reference Handbook §6.9.10–§6.9.12 (where the EPA SWTR CT tables are reproduced), and includes two NCEES-style example problems (a Giardia CT verification and a UV virus dose).
Why drinking water disinfection matters on the PE WRE exam
The April 2024 NCEES PE Civil WRE specification puts Drinking Water Distribution and Treatment at 6–9 questions, and Topic 11 (Wastewater) adds another 7–11 — meaning combined, water and wastewater treatment account for 13–20 of the 80 questions. Disinfection is one of two areas where the handbook supplies extensive tabulated data (the other is the Metcalf & Eddy activated-sludge design table). When the handbook has tables, NCEES tends to test the lookup — they want to see whether you can navigate the reference under time pressure, not whether you can re-derive the kinetics.
The supplied references on exam day are the NCEES PE Civil Reference Handbook (with EPA SWTR CT tables reproduced in §6.9.12) and TSS Water Works 2018 (the design-criteria standard published by the Great Lakes—Upper Mississippi River Board, supplied as a searchable PDF). Between those two documents, you have everything you need for any disinfection question NCEES can ask — provided you know which document holds which piece.
What the exam tests
At a high level, the exam tests whether you can confirm that a disinfection process achieves the pathogen inactivation it needs to — reasoning about contact time, dose, and the credit a given process earns. The framework is shared with other kinetic problems on the exam; the test is recognizing which standard applies and reading the right values into it.
Here are a couple of the problem types you'll face. The honest test: could you carry each one all the way to a defensible answer, in about six minutes, on exam day?
Worked example 1 — Giardia CT verification. A conventional water-treatment plant (granted 2.5-log Giardia removal credit per handbook §6.9.12) operates at 10 °C, pH 7.5, with a free-chlorine residual of C = 1.0 mg/L in the contact chamber. Contact-chamber volume V = 50,000 gal; peak hourly flow Qpeak = 1,200 gpm. The contact chamber has "average" baffling (BF = 0.5). Verify whether the plant meets the EPA SWTR 3-log overall Giardia inactivation requirement.
Solution path: Required disinfection log inactivation → Compare.
Problems exactly like this are worked step by step on video in the PEwise PE Water Resources course — every calculation, every unit conversion, and where the wrong answer choices come from.
Worked example 2 — UV dose for 4-log virus inactivation. A surface-water treatment plant uses UV as primary disinfection. The UV reactor has average bulk intensity I = 12 mW/cm2 at end-of-lamp-life conditions. The plant must achieve 4-log virus inactivation. (a) What's the required exposure time? (b) If the reactor's hydraulic residence time at peak flow is 20 seconds, does it meet the requirement?
To see problems like this worked to the final answer on video, head to the PEwise PE Water Resources course.
The full methods behind these — the relationships, the procedures, and the mistakes that quietly cost points — are taught step by step in PEwise's PE Water Resources course, with animated worked problems rather than a wall of formulas.
Master CT Lookups Under Time Pressure
PEwise's PE WRE course drills CT-table navigation across temperature, pH, and residual until you find the right cell in 30 seconds. When the lookup is automatic, you spend the six-minute clock on the calculation, not on paging through 200 rows.
Connecting this to your overall PE WRE exam strategy
Drinking-water disinfection is one piece of Topic 10's 6–9 questions. The same mass-balance and unit-conversion reasoning carries through coagulation/flocculation dosing, sedimentation overflow rates, and filtration loading rates — Topic 10 is a tightly connected design topic. Once you've mastered CT calculations, the parallel skills land naturally on the wastewater side: see our activated-sludge design post for the F/M and SRT mass-balance work that mirrors the CT-product reasoning here. For the broader Topic 10 + Topic 11 structure, the PE WRE topics decoded post walks the full April 2024 NCEES spec topic by topic with PEwise module mapping.
Master Drinking Water Disinfection with PEwise
PEwise's Modules 18 and 19 (48+ animated lessons combined) cover sedimentation, coagulation/flocculation, filtration, disinfection (chlorine, chloramines, UV, ozone), and DBP control — with worked CT-value problems and treatment-train design. Course author Mahdi Bahrampouri, Ph.D., Civil Engineer and Co-Founder of PEwise, built the curriculum directly against the NCEES April 2024 PE WRE specification and the EPA SWTR / TSS Water Works 2018 standards.
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