CT Shortfall in a Poorly Baffled Clearwell
Your 3.5 MGD surface water plant disinfects with free chlorine and a single unbaffled 200,000-gallon clearwell. At peak flow the theoretical detention time is about 82 minutes, and a tracer study established a baffling factor of 0.30. Under design conditions of 5 degrees C water temperature and pH 7.5, with chlorine residual leaving the clearwell at 0.8 mg/L, the state-required CT for your credited Giardia inactivation is 105 mg-min/L. The state will not reduce the required log credit and will not allow contact time credit for the piping between the clearwell and the first customer. Finished-water TTHM is 62 ug/L against an LRAA limit of 80. Chlorine feed can be pushed to 4 mg/L free, but the distribution system already logs taste complaints.
1. Calculate the CT you are actually achieving and quantify the gap.
Read the worked answer
Work the calculation in the order the regulations define it.
- Theoretical detention time: 200,000 gal / 2,430 gpm (3.5 MGD) = 82.3 min (given).
- T10 = theoretical x baffling factor = 82.3 x 0.30 = 24.7 min. Only the T10 counts.
- CT achieved = T10 x residual = 24.7 x 0.8 = 19.7 mg-min/L.
- Required = 105 mg-min/L, so you are at about 19 percent of the requirement and short by roughly 85 mg-min/L.
Two things to flag in the write-up:
- Use the outlet residual, not the feed residual, since decay happens across the basin; using the higher upstream number overstates CT.
- Everything must be evaluated at peak flow, coldest water, and worst-case pH. Those are the conditions that produced the 105 mg-min/L number, so today's comfortable conditions are not the compliance case.
2. What residual would be needed to close the gap with chlorine alone, and why is that path unattractive?
Read the worked answer
Rearrange CT = T10 x residual.
- Required residual at the clearwell outlet = 105 / 24.7 = 4.25 mg/L.
- Because chlorine decays across the basin, the feed would have to be higher than 4.25 mg/L to leave 4.25 mg/L at the outlet, so the practical dose is well above the 4 mg/L ceiling you identified.
Why that is a bad answer:
- TTHM is already 62 ug/L against an 80 ug/L LRAA. Raising free chlorine several fold at pH 7.5 drives THM and HAA formation further, risking a DBP violation while fixing a CT violation.
- Taste and odor complaints escalate, and customers respond by drinking less treated water, which creates its own distribution water-age problem.
- You would be spending chemical money every day to compensate for a hydraulic deficiency that is fixed once.
The honest conclusion is that chlorine dose is the wrong lever at this site.
3. Propose a compliant strategy and describe how you would verify it rather than assume it.
Read the worked answer
Attack the T10 term first, because it is a multiplier on everything else.
- Baffling: adding baffles to raise the factor from 0.30 to roughly 0.60 to 0.70 lifts T10 to about 49 to 58 min. At T10 of 50 min, the required residual falls to 105 / 50 = 2.1 mg/L, which is achievable without blowing up DBPs.
- Flow: if the clearwell can be operated at a lower peak flow (or a second basin placed in service), T10 rises proportionally. Also check that short-circuiting paths, such as an inlet and outlet too close together, are corrected by the baffle layout.
- If baffling plus a modest residual increase still falls short at 5 degrees C, evaluate UV or ozone for the Giardia credit, which sidesteps the CT-versus-DBP conflict entirely.
Verification:
- Re-run a tracer study after construction; never carry the old baffling factor forward on the assumption that new baffles work as drawn.
- Recompute CT at the new factor for worst-case temperature, pH, and peak flow, and document the calculation.
- Monitor TTHM/HAA5 quarterly to confirm the DBP margin improved rather than eroded, and track taste complaints as the customer-side check.