Nitrification in a Dead-End Chloraminated Main

Working diagnosis: nitrification. In a chloraminated system, nitrifying bacteria oxidize free ammonia and consume monochloramine in the process. The signature is exactly what the samples show:

• Total chlorine decays from 0.62 to 0.09 mg/L along the main while free ammonia rises from 0.03 to 0.38 mg/L. In a simple decay situation, ammonia would stay bound or fall with the chloramine; it does not climb.
• Nitrite at 0.09 mg/L as N is a direct product of ammonia oxidation and is essentially absent in a healthy chloraminated main.
• Conditions favor it: warm water (21°C), long detention time in 4 miles of dead-end main, a tank that turns over only every six days, and no blow-off to move water.

Ruling out alternatives:

• Simple chlorine demand / sediment: would not raise free ammonia or produce nitrite.
• Overfeed of ammonia (ratio error at the plant): would show elevated free ammonia at the plant, not just at the dead end.
• Coliform regrowth from biofilm: plausible and probably happening, but it is a consequence here, not the root cause - HPC and coliforms follow the loss of disinfectant residual.

The coliform-positive result is a symptom of lost residual, but it still has to be handled on its own schedule.

Sampling compliance first: collect repeat samples within 24 hours at the original site and at one upstream and one downstream location within five service connections. Verify the positive with the lab, confirm holding time and chlorine neutralization were correct, and check whether this is a second positive in the month (which triggers a Level 1 assessment). One total-coliform-positive routine sample with negative repeats and no E. coli does not by itself trigger an assessment, but it does require the repeats and full documentation.

Operational response, in order:

1. Flush the dead end to turn the water over and restore residual. Start at the extremity and work back - but flushing alone is a temporary fix in a 4-mile dead end.
2. Confirm the plant is holding the target chlorine-to-ammonia ratio and that its analyzers are calibrated. Shift the ratio slightly toward chlorine if ammonia leaving the plant is creeping up.
3. Consider a temporary free-chlorine burn in the affected zone if the piping and the DBP picture allow it - this resets the biofilm but has to be planned, not improvised, and requires monitoring for TTHM/HAA5.
4. Increase tank turnover and cycling to reduce water age.
5. Step up monitoring in the affected area: total chlorine, free ammonia, nitrite, and nitrate at several points, plus coliform samples, until residuals hold.

Notify the primacy agency if a repeat is positive or if you cannot maintain residual.

The real problem is water age, not chlorine dose. Four miles of unbranching main plus a tank cycling every six days will produce nitrification in almost any chloraminated system in summer. Raising the dose alone just feeds the bacteria more ammonia and risks DBPs.

Options, ranked:

• Install a blow-off and a programmed automatic flushing program at the dead end with enough volume to turn the main over on a schedule - the cheapest fix and usually the highest-yield one.
• Loop the main or interconnect with an adjacent zone to create flow-through.
• Increase tank cycling and, if the hydraulics allow, lower the tank's operating range to cut storage age.
• Add a booster chloramination or chlorine station if turnover cannot be improved.
• If nitrification proves uncontrollable, evaluate converting that pressure zone to free chlorine year-round.

Verification: establish a baseline map of total chlorine, free ammonia, nitrite, and nitrate at 6–8 points on the branch before and after the change. Success looks like total chlorine holding above your target at the extremity, free ammonia staying below about 0.1 mg/L, nitrite at or near zero, and no further coliform positives - sustained through the next August, not just the next week. Report the results to the agency as part of your monitoring plan review.