Nitrification in a Tank That Barely Turns Over

Tank turnover:

• Nightly volume pumped: 2.0 MGD x 6 h / 24 h = 0.50 MG per day
• Turnover: 1.5 MG / 0.50 MG per day = 3.0 days, i.e. about 72 hours from full to empty

System-wide storage age:

• 2.4 MG / 1.4 MGD = 1.7 days of storage in the system

Which number matters: the system average of 1.7 days looks acceptable and is the number that gets reported, but it is an average, and averages hide the problem. The 1.5 MG tank is a plug-flow-ish vessel that takes three days to cycle, so water leaving the tank at the end of a cycle has been sitting for up to 72 hours, and the far end of the distribution system downstream of it is seeing water even older than that once travel time is added.

That is the mechanism behind everything in the sample: in a chloraminated system, long residence time lets the monochloramine residual decay, releasing free ammonia, which is exactly what the 0.45 mg/L free ammonia shows. Nitrifiers then oxidize that ammonia to nitrite (0.18 mg/L, more than triple the 0.05 mg/L action level), the residual falls further (0.20 mg/L), and HPC climbs to 340 CFU/mL. The numbers are a coherent chain, not four separate problems.

Within days (operational, no capital):

• Flush the dead-end main and the other known low-flow sites. This removes the stagnant, high-nitrite water immediately and gives you a clean baseline. It is a holding action, not a fix.
• Change the tank operating band to force a deeper cycle. Instead of cycling between, say, 20 and 24 ft, cycle between 8 and 22 ft. Turnover at 0.50 MG/day against a 1.5 MG tank is the core problem, and a deeper draw does not change the volume, so pair this with more pumping hours spread across the day to raise daily throughput.
• Verify the chlorine-to-ammonia ratio at the plant. Some of the free ammonia at the far end is decay product, but some may be ratio drift at the point of application. Measure free and total chlorine and free ammonia at the plant, and trim to hold a consistent monochloramine residual.
• Sample the nitrification profile, not just the one dead end: plant effluent, tank inlet, tank outlet, and three or four distribution sites at increasing distance. You need to know whether the tank or the mains are driving it.

Within weeks:

• Install or commission a tank mixer. Mixing eliminates stratification and short-circuiting, so the tank behaves as a mixed reactor instead of a stale plug.
• Consider a controlled chlorine burn. Temporarily switch the system to free chlorine to oxidize the ammonia and knock down the biofilm, with the residual carried high enough for a full system turnover. Watch DBPs closely (see the third task; this is the measure with a real DBP cost) and follow the state's notification requirements.

Within months:

• Look at the dead-end main itself. A 4-inch main at the end of a large system may need looping, upsizing, or a permanent automatic flusher. Nitrification is a hydraulic problem before it is a chemistry problem.

Expected effects: flushing drops nitrite within hours; a deeper, longer tank cycle plus mixing should bring free ammonia below 0.1 mg/L and hold total chlorine above 0.5 mg/L at the dead end; a chlorine burn knocks down HPC and biofilm but the effect regresses within months if water age is not fixed. Do not accept a chlorine burn as the whole answer.

Assessment: a burn at 2.5 mg/L free chlorine held for long enough to turn the system over is a substantial increase in oxidant exposure, and it will raise TTHM and HAA5. The current results, TTHM 61 ug/L and HAA5 44 ug/L, are within the 80/60 MCLs but not far below them; HAA5 at 44 has only about 26 percent headroom, and HAA5 typically climbs with higher pH and higher chlorine dose. There is also a seasonal reality: DBP formation peaks in warm weather, so a burn run in July at 2.5 mg/L is a different risk than the same burn in March.

Conditions I would attach:

• Schedule it outside the warm season if the operational situation allows. Nitrite at 0.18 mg/L over the action level is a priority, but if it is October, waiting three weeks buys you a much safer DBP window.
• Stage the residual. Start at 1.5 to 2.0 mg/L free and hold it for a full system turnover, rather than dumping 2.5 mg/L in on day one. The goal is biofilm contact time, not a peak number on a log sheet.
• Monitor DBPs during, not after. Run TTHM/HAA5 at the highest-LRAA sites at the midpoint and at the end of the burn. Define a stop level in advance, for example HAA5 above 55 ug/L or TTHM above 70 ug/L, that triggers ending the burn early.
• Watch for the operational side effects. Free chlorine will strip the monochloramine residual that some of the system is used to; expect taste and odor complaints, and prepare the customer messaging and the state notification.
• Protect the sample sites. During the burn, your nitrification monitoring parameters change meaning; keep recording free chlorine, total chlorine, ammonia, and nitrite at the same sites so the before/after comparison is valid.
• Document the plan. The state will want the rationale, the residual targets, the start and end dates, and the DBP results in the operating record.

Bottom line: the burn is worth doing, but not at 2.5 mg/L, not without DBP stop levels, and not in place of fixing the tank turnover and the dead-end hydraulics.