Chloramine Decay and Nitrification in a 2-MG Standpipe
A 2.0-million-gallon welded-steel standpipe (62 ft diameter, 90 ft shell) is the only storage for a neighborhood averaging 0.30 MGD. One 16-inch pipe serves as both inlet and outlet, and the level cycles between 55 and 65 percent of capacity daily. The system is chloraminated, with ammonia dosed to a Cl2:NH3-N weight ratio of 4.3:1 and about 3.4 mg/L total chlorine entering the tank. Over eight weeks, dead-end samples past the tank show total chlorine falling from 2.8 to 0.9 mg/L, free ammonia rising from 0.02 to 0.45 mg/L, nitrite appearing at 0.18 mg/L, and rising HPC counts. The highest TTHM locational running annual average has climbed to 68 ug/L. Nothing at the treatment plant has changed.
1. Explain the chemistry and biology behind the residual loss, compute the tank turnover time, and explain how the tank operating pattern makes it worse.
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Water age. Nominal turnover is 2.0 MG / 0.30 MGD = 6.7 days, already beyond good practice. It is worse than the nominal number: because the level swings only between 55% and 65%, only about 0.2 MG (10% of the tank) is exchanged per day. Much of the water sits for weeks, and the tank stratifies into warm older water over a slow-moving lower layer.
Chemistry. Monochloramine decays on its own, faster at higher temperature, higher pH and longer contact time, and releases free ammonia. Free ammonia at 0.45 mg/L (up from 0.02) is food for nitrifying bacteria that colonize the shell, the piping and the dead-end main.
Biology. Nitrite at 0.18 mg/L is the fingerprint - it only appears where ammonia is being oxidized. Nitrification consumes alkalinity and depresses pH, which accelerates chloramine decay, and the nitrite itself exerts chlorine demand: a self-accelerating loop. Rising HPC confirms biofilm growth.
DBP side. The same long residence explains the TTHM LRAA creeping to 68 ug/L against an 80 ug/L standard.
Rule of thumb. Any distribution sample above about 0.1 mg/L free ammonia or 0.05 mg/L nitrite is telling you nitrification is underway.
2. What operational changes would you make to the tank and the chloramine feed, and what would you measure to know the changes worked?
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Change how the tank is run.
- Widen the operating band sharply (fill toward 90%, draw down to 40-50%) so the daily swing moves many times the current volume.
- Set the altitude valve to fill high and withdraw low to limit short-circuiting.
- Exercise the 16-inch connection and verify the valve strokes fully; a partly open valve can double effective detention.
- Flush the dead-end main past the tank on a schedule until ammonia and nitrite fall, then keep a lower-frequency program.
Fix the chloramine chemistry.
- Move the Cl2:NH3-N weight ratio from 4.3:1 toward about 5:1 (roughly 1:1 molar, the monochloramine stoichiometry) and verify with free and total chlorine analysis that you are making monochloramine rather than dichloramine plus free ammonia.
- Hold a higher total chlorine target at the tank outlet, for example 2.0-2.5 mg/L, so decay still leaves a residual at the far end.
Measure. Daily total chlorine, free ammonia and nitrite at the tank and at three dead ends; monthly TTHM and HAA5 at the worst locational site; periodic HPC. Success is ammonia below 0.1 mg/L, nitrite below 0.05 mg/L and stable dead-end residuals for at least four consecutive weeks.
3. Would you run a temporary free-chlorine burn to reset the system? Support the decision with numbers and state the conditions attached to it.
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Breakpoint arithmetic. Converting free ammonia to nitrogen gas takes about 7.6 mg/L of chlorine per mg/L of NH3-N. At 0.45 mg/L NH3-N that is about 3.4 mg/L consumed, plus demand and a residual - so an entry-point dose on the order of 4.5-5.0 mg/L above current demand, with 1.0-1.5 mg/L free chlorine surviving at the far end.
Case for. Nitrification will not stop while ammonia is present; a three- to four-week burn resets the biofilm and is the standard tool.
Case against, and conditions. The system is already at 68 ug/L TTHM against 80, and free chlorine forms more trihalomethanes and HAAs than chloramine where NOM is present - worst of all at long residence, which is exactly this tank. So:
- Notify the primacy agency and confirm the plan is acceptable before starting.
- Sample TTHM and HAA5 weekly at the maximum-residence site and set a written stopping threshold with management in advance.
- Hold pH and orthophosphate corrosion control steady; free chlorine is harder on lead and copper than chloramine.
- Have a written exit: burn until ammonia is below 0.1 mg/L and nitrite below 0.05 mg/L, then return to chloramine - but only after the water age is fixed, or nitrification returns within months.