Nitrification in a Chloraminated Distribution Zone
A 3.5 MGD distribution system uses chloramines. The treatment plant feeds chlorine and ammonia at a 4.5:1 Cl2:N weight ratio, targeting 0.9 mg/L total chlorine. In August, total chlorine at the entry point is 0.95 mg/L and free ammonia is 0.08 mg/L. At a mid-zone sample point 3 miles out, total chlorine is 0.35 mg/L, free ammonia is 0.22 mg/L, and nitrite is 0.18 mg/L. A 1.5 MG storage tank has a turnover of 5 days. HPC at the mid-zone is 180 CFU/mL. A new subdivision with 1.2 miles of 8-inch main was added last year and is on a dead end. The utility has had two customer taste complaints.
1. Do these data confirm nitrification? Explain the evidence and what additional samples you would collect.
Read the worked answer
The mid-zone data confirm nitrification. Total chlorine fell from 0.95 to 0.35 mg/L, free ammonia rose from 0.08 to 0.22 mg/L, and nitrite appeared at 0.18 mg/L; HPC of 180 CFU/mL is a further indicator. In a chloraminated system, nitrifiers consume ammonia and create nitrite, which exerts a chloramine demand and depresses total chlorine. The 5-day tank turnover and the new dead-end main provide the stagnation and warm water that nitrifiers need. The entry-point ratio is not necessarily wrong, but the residual has decayed in the zone; adding more ammonia would feed nitrifiers, not fix the problem. Confirm with a second set of samples at the tank outlet and at the dead end, plus free ammonia, nitrite, and HPC. Check for a free-chlorine spike that could have broken the chloramine ratio.
2. What operational response would you use in the affected zone, and what are the risks?
Read the worked answer
Breakpoint chlorinate temporarily in the affected zone. Raise free chlorine to 1.5–2.0 mg/L at the mid-zone for several days, coordinated with the plant, then flush the dead-end main and the new subdivision to remove nitrite and biofilm. Do not blend free chlorine and chloramine carelessly; switch the plant to free chlorine for the flush period if the system can tolerate it, and notify customers of taste and odor. Increase tank turnover by cycling the 1.5 MG tank and avoiding short-circuiting; if the tank is stratified, mix it or lower the level. After nitrite is below 0.05 mg/L and total chlorine holds, return to chloramine with a target ratio and a free ammonia target below 0.1 mg/L. Do not overdose ammonia to chase residual. Monitor for THMs if free chlorination lasts more than a few weeks.
3. What monitoring plan would you use to catch a recurrence early?
Read the worked answer
Sample daily at the entry, tank outlet, mid-zone, and dead end for total chlorine, free ammonia, nitrite, and HPC until stable, then reduce to twice weekly. Add nitrite and free ammonia to routine distribution monitoring in warm months. Track temperature and tank turnover; nitrification risk rises above 20°C and with turnover longer than 3–5 days. If nitrite exceeds 0.1 mg/L or total chlorine falls below 0.5 mg/L at a routine site, trigger flushing and a zone chlorination response. Inspect the new 8-inch main for dead ends and add a blow-off or loop. Keep records of customer taste complaints. Report nitrite results as operational data, not as a compliance MCL, unless the state requires it. After the event, review the ammonia-to-chlorine ratio and the tank mixing design.