CT Compliance Versus DBP Headroom
Your 6.0 MGD surface water plant disinfects with free chlorine added at the clearwell inlet. The clearwell holds 400,000 gallons and tracer testing gives a baffling factor of 0.30. At the average flow of 3.0 MGD you measure 1.2 mg/L free chlorine at the clearwell outlet, at pH 7.9 and 12°C. The state's CT table for your conditions (3-log Giardia inactivation) requires CT = 64 mg/L·min. Your peak-hour flow is 4.5 MGD. Meanwhile your Stage 2 LRAA is 78 µg/L for TTHM at the worst location (MCL 80) and 55 µg/L for HAA5 (MCL 60), and you are already on quarterly DBP monitoring.
1. Calculate available CT at average flow and at peak-hour flow. Are you in compliance?
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Detention time. Clearwell volume ÷ flow, with units kept straight (1 MGD = 1,000,000 gal/day).
At 3.0 MGD: 400,000 ÷ 3,000,000 = 0.1333 day = 192 min At 4.5 MGD: 400,000 ÷ 4,500,000 = 0.0889 day = 128 min
T10. Multiply by the baffling factor of 0.30:
Average: 192 × 0.30 = 57.6 min Peak: 128 × 0.30 = 38.4 min
CT available = residual × T10:
Average: 1.2 × 57.6 = 69.1 mg/L·min ≥ 64 → compliant Peak hour: 1.2 × 38.4 = 46.1 mg/L·min < 64 → not compliant
CT must be met at peak hourly flow, not average. On paper your plant looks fine because the average case passes by about 8%. In reality, every peak hour of the day is a treatment technique shortfall of about 28%. That is the finding that matters, and it is invisible if you only calculate at average flow.
2. Propose at least three ways to close the peak-hour gap, weigh each against your DBP situation, and recommend one.
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Option 1 - Raise the chlorine residual. You need 64 ÷ 38.4 = 1.67 mg/L at peak, up from 1.2. Simple and cheap to implement, but your TTHM LRAA is already 78 against an MCL of 80. Pushing residual up 40% on water that is already forming DBPs at that rate is likely to put you into a TTHM violation. Reject as a primary fix.
Option 2 - Improve clearwell baffling. Going from a baffling factor of 0.30 to 0.50 gives T10 = 128 × 0.50 = 64 min at peak, so CT = 1.2 × 64 = 76.8 mg/L·min at the existing residual. This buys compliance with no additional chlorine and therefore no additional DBP formation. Capital cost is real (baffle walls, inlet/outlet rearrangement, tracer re-testing), but it addresses the actual defect: your clearwell is poorly baffled and you are throwing away two thirds of the volume you already own. Recommend.
Option 3 - Reduce TOC ahead of chlorination. Enhanced coagulation or a higher coagulant dose to improve TOC removal lowers DBP precursor load, which creates headroom to run a slightly higher residual if needed. This is the right companion to Option 2 and may be required anyway.
Option 4 - Move or split the chlorine dose point. Adding chlorine downstream (for example at the filter effluent) shortens the time precursors are in contact with free chlorine, but you must still satisfy CT somewhere - check that any relocation keeps the required T10 and does not simply move the same contact time.
Option 5 - Chloramination. Attractive for DBP control but brings nitrification management, a different CT table, and monitoring obligations. Not a 30-day fix.
Recommendation: baffling improvements plus TOC removal. Raise residual only as a documented interim measure, with DBP samples collected immediately to prove you are not trading a treatment technique violation for an MCL violation.
3. What do the current TTHM and HAA5 numbers trigger in terms of monitoring and compliance, independent of the CT problem?
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Operational evaluation level. 0.8 × MCL is 64 µg/L for TTHM and 48 µg/L for HAA5. Any single sample above those values triggers increased monitoring (quarterly) - you are at 78 and 55, above both, so quarterly monitoring is required regardless of any reduced-monitoring schedule you might otherwise qualify for. You are already quarterly, so the practical effect is that you cannot drop back to annual and you should expect the state to look closely at your operational evaluation.
LRAA math. The LRAA is the locational running annual average - for each monitoring location, the average of the last four quarters at that site, not a system-wide average. A good result elsewhere does not rescue the worst site. Your 78 is the worst site, which is the one that counts.
Violation. A TTHM LRAA above 80 or an HAA5 LRAA above 60 is a violation. You are at 98% of the TTHM MCL and 92% of the HAA5 MCL - one bad quarter at that site puts you over. Any exceedance requires public notice on the schedule set by the state.
The two problems interact: the easy fix for CT (more chlorine) is the wrong fix for DBPs. That tension is the whole judgement call in this case, and the record should show you evaluated it rather than picking the cheap lever.