Chloramine Residual Collapse on a Dead-End Street
A 6.2 MGD distribution system buys finished water and applies chloramines at the entry point with a chlorine-to-ammonia-nitrogen weight ratio of 4.5 to 1. Over three weeks the picture has shifted. Total chlorine at the entry point holds at 2.6 mg/L, but at a dead-end sampling station 4.8 miles out and 60 ft higher in elevation, total chlorine has fallen from 1.4 to 0.3 mg/L. Free ammonia at that station is 0.42 mg/L (historically 0.05), nitrite is 0.18 mg/L, and the heterotrophic plate count is 210 CFU/mL. A customer on that dead-end street reports a chemical, swimming-pool taste.
System pH has drifted from 8.1 to 7.6 since the utility converted from lime to caustic soda for corrosion control. Two distribution storage tanks, 0.5 MG and 1.0 MG, have about 3.5 days of turnover. The dead-end hydrant was last flushed in March. Water temperature is 24 C. Coliform results for the month are not yet complete.
1. What is happening in this zone, and which data points support the diagnosis? What else could explain the residual loss, and how do you rule it out?
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This is nitrification. Chloramine decays and releases free ammonia, and nitrifying bacteria oxidize that ammonia to nitrite and then nitrate. The signature is unmistakable in the data set:
- Total chlorine falls from 1.4 to 0.3 mg/L at the dead end while the entry point holds at 2.6 mg/L, so the loss is in the distribution system, not at the plant.
- Free ammonia rises from 0.05 to 0.42 mg/L. That is the substrate being released by chloramine decay.
- Nitrite at 0.18 mg/L is the intermediate product of ammonia oxidation and is close to diagnostic.
- HPC at 210 CFU/mL shows general regrowth, and the customer's chemical taste fits chloramine decay and the aldehydes and organic by-products that go with it.
Contributing conditions: pH 7.6 slows nothing and speeds chloramine decay, especially after the caustic switch; 24 C water; 3.5 days of tank turnover; a dead end that has not been flushed since March.
Rule out alternatives. Heavy chlorine demand from tuberculated cast iron or a new main would drop the residual but would not raise free ammonia or produce nitrite. An ammonia feed error or a drifting analyzer would also leave excess free ammonia and mimic the biology, so verify the chlorine-to-ammonia ratio and calibrate the entry-point analyzers before concluding the problem is bacterial.
2. What are your first actions over the next 72 hours, and what do you tell the customer?
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Stop the growth and remove the substrate. Flush the dead end and the low-velocity mains until residual recovers and free ammonia and nitrite drop, then work systematically through the zone rather than flushing only the one hydrant. Restore pH to the 8.0 to 8.3 range with caustic, because lower pH accelerates chloramine decay and you have drifted to 7.6. Cycle the tanks on a fill-and-draw basis to cut detention; 3.5 days of turnover is a reactor, not a reservoir. Sample free ammonia, total chlorine, nitrite, and HPC at six to eight points to map where the nitrification front actually is.
Consider a temporary free-chlorine burn in the affected zone, coordinated in advance with the primacy agency: shut off or reduce ammonia feed, carry free chlorine through the area long enough to knock the biofilm back, then re-establish chloramines with fresh ammonia. Plan the transition carefully, because the changeover can produce a strong taste spike and a temporary rise in chloramine formation.
Check coliform samples; if any are positive, follow the total coliform rule and state assessment requirements rather than improvising. Tell the customer what happened, that flushing and treatment changes are underway, and that you will follow up on results. Do not issue a boil notice without a basis, and do not let the taste complaint drive an unnecessary advisory.