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 out of a 200-row EPA table, multiply by the right effective contact time, and verify the plant meets the 3-log Giardia and 4-log virus 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 in under 30 seconds; candidates who don't lose six 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 walks through the five disinfection problem types NCEES tests, ties every formula to its section in the NCEES PE Civil Reference Handbook §6.9.10–§6.9.12 (where the EPA SWTR CT tables are reproduced verbatim), includes two fully solved NCEES-style worked examples (Giardia CT calculation and UV virus dose), and ends with a quick-reference subset of CT values plus the UV-dose table from 40 CFR 141.720 you can scan in 30 seconds on exam day.
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.
Core concepts you must master
The CT product
The fundamental disinfection metric: residual disinfectant concentration C (mg/L) multiplied by effective contact time t (min). Per handbook §6.9.10.4:
where C is the residual concentration measured during peak hourly flow and t10 is the time it takes 10% of the water to flow through the reactor at peak hourly flow. Units: mg·min/L. The 10% (rather than mean residence time) accounts for short-circuiting in real reactors — some water leaves much faster than the average.
Baffling factor and effective contact volume
Tracer studies are the gold standard for t10, but for design and exam problems use:
where θ is the hydraulic residence time (volume / flow) and BF is the baffling factor. Per the EPA LT1ESWTR table reproduced in handbook §6.9.10.4: BF = 0.1 (unbaffled, mixed flow); 0.3 (poor); 0.5 (average); 0.7 (superior); 1.0 (perfect plug flow). The handbook also notes that chlorine-contact-chamber length-to-width ratio of 20:1 to 50:1 is typical good baffling design.
Free chlorine vs. chloramines
Free chlorine (HOCl + OCl−) is fast-acting; chloramines (NH2Cl, NHCl2, NCl3) are slow-acting. EPA SWTR provides separate CT tables for each. Never use the free-chlorine CT table for a chloraminated system — chloramine CT requirements are 50–100× higher for the same target log inactivation. The CT tables are organized by primary disinfectant; pick the right one before you look up a value.
Breakpoint chlorination
The chlorination chart in handbook §6.9.10.1 shows the classic dose-response curve: as Cl2 dose increases, residual rises through three regimes — destruction of chlorine residual by reductants (NH3-Cl2 reactions), formation of chloro-organic and chloramine compounds, destruction of chloramines past the "breakpoint," and finally formation of free chlorine. Past breakpoint, every additional unit of Cl2 dose produces an equal unit of free chlorine residual. The exam-tested calculation is dose-to-residual conversion at a given ammonia concentration, requiring 7.6:1 Cl2:NH3-N stoichiometry to reach breakpoint.
UV dosing
UV disinfection follows a simple intensity × time relationship per handbook §6.9.12.1:
where D is UV dose (mJ/cm2, equivalent to mW·s/cm2), I is the average intensity in the bulk solution (mW/cm2), and t is exposure time (s). UV is highly effective against Cryptosporidium and Giardia (low doses) but requires far higher doses for viruses — adenovirus is particularly UV-resistant. The handbook reproduces the 40 CFR 141.720 UV-dose table, which sets requirements at 22 mJ/cm2 for 4-log Cryptosporidium or 4-log Giardia, but 186 mJ/cm2 for 4-log virus.
Ozone and advanced oxidation
Ozone (O3) is a stronger oxidant than chlorine and effective against Cryptosporidium at far lower CT than chlorine dioxide (handbook §6.9.12 reproduces both Crypto CT tables on page 487). Ozone also produces fewer chlorinated DBPs but does generate bromate when bromide is present. Advanced oxidation processes (AOPs) combine O3/H2O2 or UV/H2O2 to generate hydroxyl radicals — useful for emerging contaminants. AOP detail rarely appears on the exam, but recognition does.
Disinfection byproducts (DBPs)
Free chlorine reacting with natural organic matter produces trihalomethanes (THMs) and haloacetic acids (HAA5), regulated under the EPA Stage 2 Disinfectant/Disinfection Byproducts Rule (D/DBPR) at 80 µg/L (THMs) and 60 µg/L (HAA5) running annual averages. DBP minimization is why utilities switch from free chlorine to chloramines for distribution-system residual, even though chloramine CT is much weaker for primary disinfection.
The 5 types of disinfection problems on the PE WRE exam
Type 1: CT calculation for Giardia inactivation (free chlorine)
Given pH, water temperature, free-chlorine residual, contact-chamber volume, peak flow, and baffling factor — verify whether the plant meets EPA SWTR's 3-log Giardia inactivation target, accounting for any log-removal credit from filtration. Look up CTrequired from the handbook's free-chlorine Giardia table (page 486; six temperature pages, pH 6.0–9.0, residual 0.4–3.0 mg/L). Compute CTcalc = C × t10. Compare and report. Worked below.
Type 2: CT calculation for virus inactivation
Same procedure as Type 1, but the table is much shorter — handbook §6.9.12 reproduces the EPA SWTR virus CT table that gives a single value per temperature row covering pH 6–9. The 4-log virus CT requirement at 0.5 °C is 12 mg·min/L; at 25 °C it drops to 2 mg·min/L. Free chlorine inactivates viruses orders of magnitude faster than Giardia.
Type 3: Required contact time given baffling factor
Inversion of Type 1: given a target CTrequired, residual concentration, and baffling factor, solve for the required tank volume or hydraulic residence time. θ = t10/BF, then V = θ·Qpeak. Common trap: candidates use mean residence time (θ) directly instead of t10.
Type 4: Chlorine dose for breakpoint chlorination
Given an ammonia concentration in the source water, compute the breakpoint chlorine dose using the 7.6:1 mass ratio (Cl2:NH3-N) and add the desired free-chlorine residual. The handbook's chlorination chart visualizes this; on the exam, you'll often see it reduced to a one-line calculation. Watch units — NH3-N is reported as nitrogen, not as ammonia.
Type 5: UV dose for log-inactivation target
Given a UV reactor with average intensity I and a log-inactivation target, look up the required dose Dreq from the handbook UV-dose table (page 488), and solve t = Dreq/I. Compare to the actual residence time. The trap: candidates conflate UV dose with UV intensity. Dose has units of energy per area (mJ/cm2); intensity has units of power per area (mW/cm2). Multiplying intensity by exposure time gives dose.
A worked CT problem
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.
Step 1 — Required disinfection log inactivation. Conventional treatment gives 2.5-log Giardia removal credit per filtration. Required additional disinfection = 3.0 − 2.5 = 0.5-log.
Step 2 — CTrequired for 0.5-log inactivation. From handbook §6.9.12 EPA SWTR Table B-1 (page 486), at 10 °C, pH 7.5, C = 1.0 mg/L: CT for full 3-log Giardia = 134 mg·min/L. EPA permits linear scaling for fractional log inactivation:
Step 3 — CTcalc for the contact chamber. Hydraulic residence time:
t10 = θ × BF = 41.7 × 0.5 = 20.8 min
CTcalc = C × t10 = 1.0 × 20.8 = 20.8 mg·min/L
Step 4 — Compare. CTcalc (20.8) < CTrequired (22.3). The plant does not achieve the required 0.5-log additional Giardia inactivation; total log inactivation = 2.5 (filtration) + (20.8/134) × 3.0 = 2.5 + 0.466 = 2.97 log, just shy of the 3.0-log SWTR target.
Mitigation: increase residual to 1.2 mg/L (CTreq = 137; CTcalc = 1.2 × 20.8 = 25.0 → log inactivation 0.55, total 3.05 log) or improve baffling to "superior" (BF = 0.7 → t10 = 29.2 min, CTcalc = 29.2 → log inactivation 0.65, total 3.15 log). Either fixes the deficit.
A worked UV-dose problem
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?
(a) Required dose and time. From handbook §6.9.12.1 UV-dose table (40 CFR 141.720): 4-log virus inactivation requires Dreq = 186 mJ/cm2.
= 186 mJ/cm2 / 12 mW/cm2 = 15.5 s
(b) Compare. Reactor residence time = 20 s > required 15.5 s → the plant meets the 4-log virus target at the design intensity.
Delivered dose: D = 12 × 20 = 240 mJ/cm2. That same delivered dose is far in excess of the 22 mJ/cm2 needed for 4-log Cryptosporidium and the 22 mJ/cm2 needed for 4-log Giardia — so the same UV reactor easily achieves the protozoa targets. Virus inactivation is the governing UV design criterion in nearly all surface-water plants.
Note: the design intensity used here is the "end-of-lamp-life, fouled-sleeve" condition. New lamps deliver 1.5–2× higher intensity; the design number is the worst-case for the regulatory dose target.
Common errors that cost points
Wrong CT-table column (pH or temperature)
The handbook §6.9.10.4 reproduces the full EPA SWTR Giardia CT table across six temperature blocks (0.5, 5, 10, 15, 20, 25 °C), each with pH columns 6.0 through 9.0 and residual rows 0.4 through 3.0 mg/L. CT values vary by 4–5× across the temperature range and 2–3× across pH. Reading the wrong row or column gives an answer that's correct to procedure but completely wrong in number.
Using free-chlorine CT for a chloraminated system
EPA publishes separate CT tables for free chlorine, chloramines, chlorine dioxide, and ozone. Chloramine CT requirements are 50–100× higher than free chlorine for the same target. If the question says "the plant disinfects with chloramines," do not use the free-chlorine table. The handbook gives the free-chlorine and Cryptosporidium CT-by-chlorine-dioxide and -ozone tables; for chloramine CT, you'd reach into TSS Water Works 2018 or the original EPA SWTR text.
Forgetting baffling factor
Using t10 = θ (i.e., BF = 1.0) for a tank that's actually got "average" baffling (BF = 0.5) overstates contact time by a factor of two. CTcalc ends up doubled — and a plant that actually fails Giardia inactivation looks compliant on paper. Always identify the baffling condition the prompt describes before computing t10.
UV dose vs. UV intensity confusion
Dose is energy per area (mJ/cm2); intensity is power per area (mW/cm2). 22 mJ/cm2 = 22 mW·s/cm2. If the prompt gives intensity in mW/cm2 and you compare it directly to a required dose in mJ/cm2, you've dropped the time dimension and your answer's off by a factor of seconds.
Linear scaling of CT outside permitted range
EPA SWTR allows linear interpolation of CT for fractional log inactivation between 0.5 and 3.0-log Giardia. Below 0.5-log or above 3.0-log, the relationship is non-linear and direct table lookup or kinetic modeling is required. On the exam, fractional-log scaling within the linear range is fair game; outside it, the prompt will specify a different approach.
How to study disinfection for the PE WRE exam
Phase 1 — Concept fluency (Week 1)
Read handbook §6.9.10 (Disinfection) through §6.9.12.1 (UV) end-to-end. Practice navigating the EPA SWTR Giardia CT tables — at minimum, drill finding values at 5 °C, 10 °C, 15 °C across pH 7.0, 7.5, 8.0 with residuals 0.5, 1.0, 1.5 mg/L until cell lookup is automatic. Skim TSS Water Works 2018 Section 4 (water treatment processes) for design-criteria language NCEES might quote.
Phase 2 — Problem-type drills (Weeks 2–3)
Work fifteen problems across the five types: five Giardia CT, two virus CT, three required-time inversions, two breakpoint chlorination, three UV dosing. Time yourself: six minutes per problem. PEwise's Modules 18 and 19 (48+ animated lessons combined) cover disinfection with worked CT-value problems and treatment-train design.
Phase 3 — Multi-concept integration (Week 4)
Solve five problems where you start from raw-water specs (turbidity, pH, temperature, ammonia content) and have to design the full disinfection train: filtration credit, primary disinfection CT or UV dose, chloramine secondary residual, DBP control. That's the realistic Topic 10 question shape — not single-step CT lookup.
Quick reference: CT values and UV doses
CT values for 3-log Giardia inactivation by free chlorine (representative subset)
Values in mg·min/L from EPA SWTR Table B-1 reproduced in handbook §6.9.12 page 486. Full table covers pH 6.0–9.0, residuals 0.4–3.0 mg/L, temperatures 0.5–25 °C. Subset below for residual = 1.0 mg/L:
| Temp (°C) | pH 6.5 | pH 7.0 | pH 7.5 | pH 8.0 | pH 8.5 |
|---|---|---|---|---|---|
| 0.5 | 176 | 210 | 253 | 304 | 365 |
| 5 | 125 | 149 | 179 | 216 | 260 |
| 10 | 94 | 112 | 134 | 162 | 195 |
| 15 | 63 | 75 | 90 | 108 | 130 |
| 20 | 47 | 56 | 67 | 81 | 98 |
| 25 | 31 | 37 | 45 | 53 | 65 |
UV-dose requirements (40 CFR 141.720)
Values in mJ/cm2; full table reproduced in handbook §6.9.12.1 page 488.
| Log credit | Cryptosporidium | Giardia | Virus |
|---|---|---|---|
| 0.5 | 1.6 | 1.5 | 39 |
| 1.0 | 2.5 | 2.1 | 58 |
| 2.0 | 5.8 | 5.2 | 100 |
| 3.0 | 12 | 11 | 143 |
| 4.0 | 22 | 22 | 186 |
Baffling factors (handbook §6.9.10.4)
| Baffling condition | BF | Description |
|---|---|---|
| Unbaffled (mixed flow) | 0.1 | Agitated basin, low L:W ratio |
| Poor | 0.3 | Single/multiple unbaffled inlets, no intra-basin baffles |
| Average | 0.5 | Baffled inlet/outlet with some intra-basin baffles |
| Superior | 0.7 | Perforated inlet baffle, serpentine intra-basin baffles |
| Perfect (plug flow) | 1.0 | Pipeline flow, perforated inlet/outlet baffles |
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.
Final thoughts
Disinfection on the WRE exam rewards engineers who treat the EPA SWTR CT tables as a fluency drill, not a reference. Find the right block (free chlorine, chloramines, chlorine dioxide, ozone). Find the right temperature page. Find the right pH column. Find the right residual row. Read the cell. Multiply by t10 = θ × BF. Compare. Done in five minutes if you've practiced; ten if you haven't. The candidates who pass have practiced the lookup as a separate skill from the calculation. Drill the lookup until it's automatic.
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.
Keep Reading
Activated Sludge & Wastewater Process Design on the PE WRE Exam
Activated-sludge sizing for the PE WRE exam: F/M, SRT, MLSS, aeration tank volume, NRC trickling filter, secondary clarifier — with worked NCEES-style problems.
PE WRE Pump Hydraulics: System Curves, NPSH, and Pump Sizing Problems
Pump-curve / system-curve operating point, NPSH-available vs. NPSH-required, BHP, and lift-station wet-well sizing for the PE WRE exam — with worked examples.
PE Water Resources & Environmental Exam Topics: 2026 NCEES Specs Decoded
Decoded April 2024 NCEES PE WRE spec: all 12 topic areas, question counts, supplied design standards, and what's actually tested through 2026.