CT Compliance at Peak Flow - the Margin Disappears
A plant disinfects with free chlorine and must meet a state-required CT of 120 mg-min/L for 3-log Giardia inactivation at pH 8.0 and 10 C. The clearwell holds 250,000 gal with a baffling factor of 0.5. Average day flow is 2.0 MGD; peak hour flow is 2.6 MGD. Clearwell-outlet free chlorine is held at 1.5 mg/L. The system's TTHM running annual average is 72 ug/L against an 80 ug/L limit, and HAA5 is 55 ug/L against 60 ug/L. The superintendent's proposal for peak demand is simple: raise the chlorine dose to 2.0 mg/L during peak hours so the math works. The plant has no UV or ozone, and the clearwell cannot be enlarged.
1. Calculate the available CT at average flow and at peak flow. Is the plant in compliance?
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Step 1 - theoretical detention time (V/Q).
Average flow: 2.0 MGD = 1,389 gpm; 250,000 gal ÷ 1,389 gpm = 180 min Peak flow: 2.6 MGD = 1,806 gpm; 250,000 gal ÷ 1,806 gpm = 138.4 min
Step 2 - apply the baffling factor (0.5) to get T10.
- Average: 180 x 0.5 = 90 min
- Peak: 138.4 x 0.5 = 69.2 min
Step 3 - CT = residual x T10.
- Average: 1.5 x 90 = 135 mg-min/L vs 120 required → passes, 12.5% margin
- Peak: 1.5 x 69.2 = 103.8 mg-min/L vs 120 required → fails by 16.2
Answer: the plant is not reliably in compliance. CT is a continuous requirement, not a daily average - it must be met at the peak flow the plant actually experiences. A plant that passes at average flow and fails at peak is out of compliance every time demand peaks, which on a summer afternoon is exactly when it happens.
2. Evaluate the superintendent's proposal to raise the dose to 2.0 mg/L at peak hours.
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It works arithmetically, and it is the riskiest option on the table.
The math checks out: 2.0 mg/L x 69.2 min = 138 mg-min/L, comfortably above 120. Required residual at peak is only 120 ÷ 69.2 = 1.73 mg/L, so 2.0 gives margin.
Why it is still a poor first choice:
- DBP headroom is nearly gone. TTHM is at 72 of 80 ug/L and HAA5 at 55 of 60 ug/L. Both DBP groups rise with chlorine dose and contact time, and the clearwell is the highest-contact point in the system. Raising the dose every peak period pushes both running annual averages toward exceedance - and a TTHM violation triggers monitoring changes and public notice, not just a report entry.
- A higher residual at the clearwell outlet does not survive the trip. Distribution residual is set by decay, not by the clearwell number; dumping chlorine upstream can spike taste and odor complaints while adding little protection at the far end.
- pH 8.0 is already working against you. Free chlorine CT credit is better at lower pH. Dropping to pH 7.5-7.6 buys credit with no extra chemical and less DBP formation than raising the dose.
Better framing: 2.0 mg/L is a stopgap for the next peak day, not a strategy.
3. What alternatives would you evaluate, and what monitoring would you add to prove whichever one you pick is working?
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Options, roughly in order of cost-effectiveness:
- Improve the baffling factor. A formal tracer study on the existing clearwell frequently raises the factor from 0.5 to 0.7. At peak: T10 = 138.4 x 0.7 = 96.9 min; CT at 1.5 mg/L = 145 mg-min/L. That solves the problem with a baffle wall and a study, with zero DBP impact.
- Shift or split the contact volume. Adding a second pass or repurposing an existing basin increases T10 directly.
- Manage peak flow. Level-control or storage to shave the peak hour lowers Q and raises detention time; this is a distribution and operations decision, but it is real credit.
- Lower pH to 7.5. Improves CT credit per mg/L and reduces THM formation versus raising the dose.
- Add UV or ozone for primary disinfection, leaving free chlorine for residual only. Highest capital, but it removes the CT-versus-DBP tradeoff permanently.
Monitoring to prove it:
- Continuous free chlorine and flow at the clearwell outlet, logged so CT is computed at the actual minimum T10, not an assumed one.
- A chlorine decay profile across the clearwell (three or four sample points) to confirm you are not losing residual to demand inside the basin.
- Monthly (or per-schedule) TTHM/HAA5 at the worst-case distribution location during peak season, trended as a running annual average.
- Distribution residual and pressure at the farthest points during peak hour.
- Verify CT against the state table for the actual pH and temperature each month - not the design values.