Exercise set

Wastewater Disinfection CT, UV Dose, and Effluent Release Exercises

Solved wastewater disinfection exercises for contact time, CT, chlorine feed, residual decay, UV dose, UVT derating, analyzer bias and effluent release gates.

Branch: Environmental Engineering
Content: Exercise set
Updated: Jul 03, 2026
Revision: v1.0.0 · reviewed

These exercises focus on the disinfection and effluent-release barrier in wastewater systems. They cover effective contact time, chlorine CT, chlorine feed, dechlorination, residual decay, UV dose, UV transmittance, lamp aging, peak-flow derating, validated flow, analyzer bias, microbial log-removal evidence and release gates.

Assume simplified screening calculations unless an exercise states otherwise. Field release requires validated hydraulic conditions, tracer or baffling evidence, residual analyzer calibration, grab checks, UV intensity records, UVT measurements, lamp status, maintenance records, alarms, bypass status and permit reporting basis.

Release Evidence Notes

Disinfection evidence must tie dose to hydraulics and water quality. A contact basin does not provide credit from nominal volume alone; a UV reactor does not provide credit from lamp-on status alone.

Chlorine CT evidence should state residual, effective contact time, peak flow, baffling or tracer basis, temperature or pH assumptions when relevant and whether dechlorination affects the downstream sample.

UV evidence should state validated flow, intensity, exposure, UV transmittance, fouling, lamp age, sensor calibration and whether the operating point is inside the validated envelope.

Engineering Boundary Notes

These calculations do not replace permit-specific disinfection rules, microbial validation, tracer testing, reactor validation, public-health review or laboratory QA. They are screening exercises for disinfection release readiness.

Common Release Mistakes

using nominal basin volume instead of effective contact time;
using a residual reading without checking analyzer bias or sampling location;
treating UV lamps as acceptable because they are energized;
ignoring UV transmittance, sleeve fouling or lamp aging;
releasing effluent while bypass, invalid data or peak-flow derating is unresolved.

Scenario Map

Scenario	Exercises	Primary check	Engineering decision
Chlorine CT	1, 2, 3, 4, 5, 12, 13	contact time, CT, residual, feed, dechlorination, decay and bias	Decide whether chlorine credit is defensible.
UV dose	6, 7, 8, 9, 10	dose, UVT, lamp age, peak flow and validated flow	Decide whether UV credit is inside the validated envelope.
Effluent release	11, 14, 15, 16, 17	short-circuit, log removal, bypass, evidence and monitoring	Decide whether disinfection data support discharge.
Release gate	18	all-of disinfection release	Decide whether effluent release can close.

Exercise 1: Effective Contact Time

A chlorine contact basin has volume $640\ \text{m}^3$ , peak flow $0.42\ \text{m}^3/\text{s}$ and baffling factor $0.55$ . Compute effective contact time.

Solution

Nominal time:

t_n=\dfrac{640}{0.42}=1524\ \text{s}=25.4\ \text{min}

Effective time:

t_e=0.55(25.4)=14.0\ \text{min}

Engineering Comment

The disinfection calculation should use effective time, not nominal basin volume.

Plausibility Check

Baffling factor near one half should reduce a $25$ minute nominal time to about $14$ minutes.

Exercise 2: Chlorine CT

Effective contact time is $14.0\ \text{min}$ and measured residual is $2.4\ \text{mg/L}$ . Compute CT.

Solution

CT=2.4(14.0)=33.6\ \text{mg min/L}

Engineering Comment

CT is only credible if residual is measured at the correct location and the basin hydraulics are valid for the flow.

Plausibility Check

A few milligrams per liter over about fifteen minutes gives a CT in the thirties.

Exercise 3: Residual Needed for Target CT

Required CT is $36\ \text{mg min/L}$ and effective contact time is $14.0\ \text{min}$ . Compute required residual.

Solution

C=\dfrac{36}{14.0}=2.57\ \text{mg/L}

Engineering Comment

Increasing residual can improve CT but may increase dechlorination demand or byproduct risk.

Plausibility Check

The required residual is slightly above the current $2.4\ \text{mg/L}$ because current CT is slightly below target.

Exercise 4: Chlorine Feed Capacity

Peak flow is $0.42\ \text{m}^3/\text{s}$ and chlorine dose is $6.0\ \text{mg/L}$ . Compute chlorine feed in kg/h.

Solution

Convert flow:

Q=0.42(3600)=1512\ \text{m}^3/\text{h}

Feed:

\dot{m}=1512(6.0)\ \text{g/h}=9072\ \text{g/h}=9.07\ \text{kg/h}

Engineering Comment

Feed capacity should include maximum validated flow, dose setpoint, standby equipment and chemical strength.

Plausibility Check

Thousands of cubic meters per hour at a few grams per cubic meter gives kilograms per hour.

Exercise 5: Dechlorination Dose

Effluent residual before dechlorination is $1.8\ \text{mg/L}$ . Required dechlorination chemical ratio is $1.4\ \text{mg chemical/mg chlorine}$ . Flow is $1512\ \text{m}^3/\text{h}$ . Compute chemical feed.

Solution

Chemical concentration:

C_c=1.8(1.4)=2.52\ \text{mg/L}

Feed:

\dot{m}=1512(2.52)=3810\ \text{g/h}=3.81\ \text{kg/h}

Engineering Comment

Dechlorination must be part of the release package if residual chlorine has an aquatic toxicity limit.

Plausibility Check

The chemical feed is lower than chlorine feed because the concentration is lower.

Exercise 6: UV Dose Screen

UV intensity is $2.8\ \text{mW/cm}^2$ and exposure time is $16\ \text{s}$ . Compute UV dose.

Solution

D=2.8(16)=44.8\ \text{mJ/cm}^2

Engineering Comment

This simple product is a screen. Validated UV reactors use more detailed dose-response and hydraulics.

Plausibility Check

Several milliwatts per square centimeter over several seconds gives tens of millijoules per square centimeter.

Exercise 7: UVT Derating

Base UV dose is $44.8\ \text{mJ/cm}^2$ . Current UV transmittance derating factor is $0.78$ . Compute delivered dose.

Solution

D_d=44.8(0.78)=34.9\ \text{mJ/cm}^2

Engineering Comment

Low UVT can push a reactor below validated dose even when lamp power is unchanged.

Plausibility Check

A derating factor below one should reduce the delivered dose by about one fifth.

Exercise 8: Lamp Aging Factor

Delivered dose after UVT derating is $34.9\ \text{mJ/cm}^2$ . Lamp aging factor is $0.90$ . Compute aged delivered dose.

Solution

D_a=34.9(0.90)=31.4\ \text{mJ/cm}^2

Engineering Comment

Lamp aging and sleeve fouling should be tracked before the plant reaches the low-dose alarm.

Plausibility Check

A $10\%$ aging penalty takes the dose from mid-thirties to low-thirties.

Exercise 9: Peak-Flow Dose Derating

At normal flow, aged delivered dose is $31.4\ \text{mJ/cm}^2$ . Peak flow is $1.25$ times normal flow. Estimate peak-flow dose assuming dose is inversely proportional to flow.

Solution

D_p=\dfrac{31.4}{1.25}=25.1\ \text{mJ/cm}^2

Engineering Comment

Wet-weather hydraulic peaks can invalidate UV credit even when normal-flow dose is acceptable.

Plausibility Check

Increasing flow by one quarter reduces dose by one fifth.

Exercise 10: Validated Flow Limit

A UV reactor is validated up to $0.52\ \text{m}^3/\text{s}$ . Current peak flow is $0.58\ \text{m}^3/\text{s}$ . Compute exceedance.

Solution

\Delta Q=0.58-0.52=0.06\ \text{m}^3/\text{s}

Percent exceedance:

\dfrac{0.06}{0.52}=11.5\%

Engineering Comment

Operating above validated flow should trigger flow limitation, bypass rules, additional reactors or hold conditions.

Plausibility Check

The excess is small in absolute terms but more than ten percent of the validated limit.

Exercise 11: Contact Basin Short-Circuit Screen

Tracer testing shows $T_{10}=12.5\ \text{min}$ while nominal residence time is $25.4\ \text{min}$ . Compute the baffling factor.

Solution

BF=\dfrac{12.5}{25.4}=0.492

Engineering Comment

Tracer evidence should replace assumed baffling when available.

Plausibility Check

The first ten percent of tracer exits at about half the nominal time, so the baffling factor is about $0.5$ .

Exercise 12: Chlorine Residual Decay

Initial residual is $2.4\ \text{mg/L}$ . Decay rate is $0.035\ \text{mg/L/min}$ for $14$ minutes. Estimate final residual.

Solution

Decay:

\Delta C=0.035(14)=0.49\ \text{mg/L}

Final residual:

C_f=2.4-0.49=1.91\ \text{mg/L}

Engineering Comment

Residual at the compliance point can differ from residual at the injection or basin inlet.

Plausibility Check

The residual falls by about half a milligram per liter, not by several milligrams.

Exercise 13: Analyzer Bias Guard

Online residual analyzer reads $2.4\ \text{mg/L}$ . Grab check is $2.1\ \text{mg/L}$ . Compute analyzer bias.

Solution

b=2.4-2.1=0.3\ \text{mg/L}

Relative to grab:

\dfrac{0.3}{2.1}=14.3\%

Engineering Comment

A positive bias can make CT appear acceptable when true residual is lower.

Plausibility Check

The online value is visibly higher than the grab value, so the bias is positive.

Exercise 14: Log-Removal Evidence

Influent indicator count is $12000\ \text{CFU/100 mL}$ and effluent count is $80\ \text{CFU/100 mL}$ . Compute log reduction.

Solution

LR=\log_{10}\left(\dfrac{12000}{80}\right)=\log_{10}(150)=2.18

Engineering Comment

Microbial samples validate outcomes but are usually too slow to control real-time dose.

Plausibility Check

Reducing by a factor of $100$ is two logs; a factor of $150$ is slightly more than two logs.

Exercise 15: Effluent Bypass Fraction

During a peak event, $1200\ \text{m}^3$ receives validated disinfection and $90\ \text{m}^3$ bypasses the validated reactor. Compute bypass fraction.

Solution

f_b=\dfrac{90}{1200+90}=0.0698=7.0\%

Engineering Comment

Bypassed water should be reported and evaluated separately; it cannot be hidden inside blended effluent.

Plausibility Check

Ninety cubic meters is less than one tenth of total flow, so about seven percent is plausible.

Exercise 16: Disinfection Evidence Completion

The release package requires peak flow, tracer basis, residual analyzer check, grab residual, chlorine feed, dechlorination feed, UV intensity, UVT, lamp status, bypass status and effluent sample. Nine of eleven records are complete. Compute completion.

Solution

C=\dfrac{9}{11}=81.8\%

Engineering Comment

Completion below ninety percent is weak when missing records could be UVT, bypass or analyzer checks.

Plausibility Check

Nine of eleven is a little over four fifths.

Exercise 17: Valid Monitoring Hours

A reporting day has $24$ hours. Valid disinfection monitoring is available for $22.5$ hours. The target is $95\%$ . Determine whether the target is met.

Solution

f=\dfrac{22.5}{24}=93.75\%

The target is not met because:

93.75\%<95\%

Engineering Comment

Missing monitoring during a high-flow interval is more serious than missing monitoring during stable low flow.

Plausibility Check

The plant missed $1.5$ hours; $5\%$ of a day is only $1.2$ hours, so it fails.

Exercise 18: Disinfection Release Gate

A release gate requires CT at least $36\ \text{mg min/L}$ , UV dose at least $30\ \text{mJ/cm}^2$ , no bypass, evidence completion above $90\%$ and monitoring validity above $95\%$ . Current values are CT $33.6$ , UV dose $31.4$ , bypass $7.0\%$ , evidence completion $81.8\%$ and monitoring validity $93.75\%$ . Decide release status.

Solution

UV dose passes. CT, bypass, evidence completion and monitoring validity fail:

33.6<36

81.8\%<90\%

Release status:

\text{hold}

Engineering Comment

Effluent release should hold because the chlorine, bypass and evidence barriers are not closed.

Plausibility Check

An all-of disinfection gate fails when several required barriers fail even if UV dose is acceptable.

Validation Package Checklist

Chlorine CT uses effective contact time, verified residual, peak-flow basis and correct sampling location.
UV dose evidence includes intensity, UVT, lamp age, fouling, validated flow and alarm state.
Bypass and invalid monitoring periods are preserved as exceptions, not averaged away.
Effluent release separates disinfection evidence from upstream biological treatment and clarifier performance.

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