CT Is Short, DBPs Are Close to the Limit, and Ammonia Is in the Water

Volume: 20 x 40 x 10 = 8,000 ft3 x 7.48 = 59,840 gal

Flow: 2.5 MGD / 1,440 = 1,736 gpm

Detention: 59,840 / 1,736 = 34.5 min theoretical; t10 = 34.5 x 0.5 = 17.2 min

CT: use the residual at the end of the contactor, 0.80 mg/L

• CT = 0.80 x 17.2 = 13.8 mg-min/L

Against 47 mg-min/L:

• Fraction of required CT = 13.8 / 47 = 0.29
• Log inactivation achieved = 3 x 0.29 = about 0.9 log, against a 3-log requirement

Conclusion: the plant is short by roughly a factor of 3.4 and is not achieving the required inactivation, even before considering that pH 7.6 sits above the tabulated range and that the table value must be adjusted upward.

Two things that must be checked before you accept this number:

• Is the 0.80 mg/L free or total? With 0.15 mg/L ammonia as N, the breakpoint demand is about 0.15 x 7.6 = 1.14 mg/L of chlorine. If the measured residual is largely monochloramine, the free-chlorine CT credit collapses; chloramine CT requirements are roughly 20 times higher than free chlorine for the same log credit, which would make the shortfall far worse.
• Is peak hourly flow really the design condition? CT must be demonstrated at peak hourly flow, the worst case, not at average day.

1. Improve contactor hydraulics (usually cheapest and fastest).

• Adding baffles, a serpentine path, or a diffuser wall could raise the baffling factor from 0.5 toward 0.8-0.9. At bf = 0.85, t10 = 29.3 min and CT at 0.80 mg/L = 23.4 mg-min/L. Helpful, but still short of 47 on its own.
• Constraint: civil work and a shutdown window; must re-verify t10 by tracer test rather than assume the new factor.

2. Raise the free chlorine residual.

• At 2.0 mg/L leaving the clearwell and bf = 0.85: CT = 2.0 x 29.3 = 58 mg-min/L, which clears 47 with margin.
• Constraint: this is the same lever that raises TTHM and HAA5. At 72/80 and 48/60 ug/L you have almost no headroom; raising the residual without reducing precursors is likely to put you into exceedance. It also raises the risk of taste and odor complaints and of nitrification if any part of the system is chloraminated.

3. Add contact volume (clearwell expansion or a second basin).

• To reach CT 47 at 0.80 mg/L and bf 0.5 you would need t10 = 58.8 min, i.e. theoretical detention of ~118 min, roughly 204,000 gal, about 27,000 ft3. That is a capital project.
• Constraint: capital cost and site space, plus the water age increase works against DBP control.

4. Reduce precursor and demand upstream.

• If TOC in this groundwater is low, the gain is small; but lowering pH to 7.0-7.2 both strengthens the free chlorine CT credit and shifts THM formation down. Check the LSI and corrosion control first.
• Constraint: corrosion control and the Lead and Copper Rule.

5. Add a non-chemical barrier (UV) for Cryptosporidium/Giardia credit.

• Lets you stop chasing ever-higher chlorine CT and directly reduces the DBP pressure.
• Constraint: capital, O&M, and the fact that UV provides no residual.

Ranking for this plant: fix hydraulics first (cheap, immediate), then evaluate pH adjustment, then consider UV versus clearwell expansion, and treat raising the residual to 2.0 mg/L as a short-term compliance bridge only if DBPs are watched weekly, not as the plan.

Why it does not fix CT: chloramines are a much weaker primary disinfectant. The CT requirement for 3-log Giardia with chloramine is on the order of 20 times the free chlorine value, so a plant already at 13.8 mg-min/L of free chlorine credit is nowhere near a chloramine-based 3-log credit. Chloramines are a residual maintenance tool, not a compliance fix for inactivation. If you convert, you must first achieve the required inactivation with free chlorine (or UV), then convert downstream of that point.

Also note: the incoming 0.15 mg/L ammonia means you are already partly forming chloramines unintentionally whenever the chlorine-to-ammonia ratio drifts. That is a plausible reason the residual readings look erratic and is worth confirming with free-versus-total chlorine measurements before you design anything.

Operational program required alongside a chloramine conversion:

• Confirm the target ratio and control it. Typically a chlorine-to-ammonia-N weight ratio around 4:1 to 5:1 for monochloramine; monitor it continuously or at least daily, because a ratio drift creates free ammonia that feeds nitrification or dichloramine that causes taste complaints.
• Nitrification monitoring program: total chlorine, free ammonia, nitrite, and (ideally) heterotrophic plate count at representative dead ends and at storage tanks, weekly in warm months. Establish an action level, for example nitrite above 0.05 mg/L or a falling total chlorine to ammonia ratio at a fixed site.
• Storage turnover plan: chloraminated water cannot sit. Every tank needs a fill-and-draw cycle and, in most cases, a mixer.
• A written breakpoint or chlorine-burn procedure: a planned, temporary switch back to free chlorine to knock down nitrifiers, with public notice through the state as required, and a plan for the taste and odor that follows.
• DBP monitoring at the highest-LRAA sites during and after conversion, since some systems see HAA5 rise even as TTHM falls; HAA5 at 48 ug/L against a 60 ug/L MCL is your tighter constraint here, not TTHM.
• Elastomer and materials check: some gaskets and seals degrade differently with chloramines.
• Operator training and recordkeeping: the conversion changes every residual target in the system, and the state will want the rationale, the setpoints, and the monitoring results documented.