Loss of Chloramine Residual in a Storage-Tank-Dominated Zone
A 14,000-customer distribution system chloraminates at the treatment plant. To stay under a TTHM limit, the operator cut the chlorine dose, pushing the applied Cl₂:NH₃-N weight ratio from about 4.5:1 to roughly 3.2:1. Six months later, residual in the northwest pressure zone falls from 1.8 to 0.2 mg/L, HPC counts rise, and coliform samples show a few non-total-coliform positives at dead-end sites. The zone is fed through a 2.0 MG ground storage tank; metered through-flow averages 0.6 MGD but the tank is typically held near 85–95% full by an altitude valve. An extended-period water-quality model puts water age at the worst dead end at about 6 days in summer. Free ammonia at the tank outlet is 0.35 mg/L as N.
1. Diagnose the likely cause of the residual loss and list the specific field and lab data that would confirm or refute it before you change operations.
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Diagnosis: nitrification in the distribution system, driven by excess free ammonia plus long water age.
The pieces line up:
- Lowering the Cl₂:NH₃-N ratio to ~3.2:1 leaves unreacted free ammonia. A healthy monochloramine system runs roughly 4.5–5:1 by weight, which is why the ratio change is the trigger event.
- Free NH₃-N of 0.35 mg/L at the tank outlet is well above the ~0.1 mg/L threshold at which nitrifying bacteria grow readily.
- Water age: the tank alone holds 2.0 MG against 0.6 MGD of through-flow - over 3 days of theoretical detention before distribution, and ~6 days at the worst dead end. Warm summer temperatures accelerate nitrifier growth.
- The residual decay pattern (gradual, worse at dead ends, seasonal) and rising HPC are classic; a simple chlorine-demand problem would typically show a sharper, more uniform drop.
Confirming data: free NH₃-N and monochloramine (not just total chlorine) at multiple points along the trunk and at dead ends; nitrite and nitrate across the zone (nitrite rising plus ammonia falling is near-conclusive); heterotrophic plate counts trend; temperature log; chlorine demand curve (hold a sample 24 h) to separate demand from biological decay; system-wide survey of which pressure zones are affected. If nitrite is climbing while free ammonia falls, stop investigating and act.
2. Lay out the operational response, with the trade-off you are accepting for each action.
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Short term (weeks):
- Restore the Cl₂:NH₃-N ratio to ~4.5–5:1 by trimming ammonia feed, not by adding more chlorine, so free ammonia drops below 0.1 mg/L. Trade-off: minimal TTHM impact because monochloramine chemistry is largely unchanged.
- Unidirectional flushing of dead ends and the northwest extremities until free ammonia and nitrite clear. Trade-off: water loss, public complaints, temporary low-pressure perception.
- Raise the residual leaving the tank toward 2.0–2.2 mg/L total chlorine (not above 3 mg/L, which drives nitrite issues and taste complaints). Trade-off: more chloramine taste/odor calls.
Medium term (1–3 months):
- Operate the tank through a real cycle - draw down to roughly 40–50% and refill, rather than parking at 85–95%. Cutting average age in the tank from >3 days to under 1.5 days attacks the root cause. Trade-off: reduced emergency storage and more valve cycling; confirm fire-flow/storage criteria before adopting.
- Tank mixing or a baffled inlet if the tank stratifies - often cheaper than extra turnover.
- A seasonal free-chlorine burn (2–4 weeks, residual 1.5–2.0 mg/L free) to reset nitrifier populations, then return to chloramine. Trade-off: DBP spike during the burn; check TTHM formation potential and the LRAA compliance period before scheduling.
What not to do: add ammonia to “stabilize” the residual. Extra free ammonia is the food source and will make the problem worse.
3. The utility wants to keep the tank at high level for fire storage and argues the water age is unavoidable. How would you test that claim and what would you accept as a defensible operating plan?
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Test the claim with the tank's own operating record, not with an assertion.
- Pull 12 months of SCADA level and flow data and compute actual daily fill/drain volume and turnover time. “Held at 85–95%” often still produces 20–40% daily swing; if so, the age problem is largely the altitude valve setting and sequencing, which is a control change, not a capital project.
- Run the water-quality model with the observed level time series (not an average) to identify the true worst-case age, and check whether high-level holding is genuinely required by the fire-flow analysis or is simply a habit.
- Confirm whether the tank has separate inlet/outlet or a common pipe, and whether thermal stratification is isolating a stagnant layer.
Defensible operating plan:
- Set a normal operating band with a minimum level chosen from the fire-flow/storage model and a maximum that forces a defined daily turnover (target average age under 2 days at the tank).
- Keep the high-level top-up as a reserve mode, not the default, with automatic cycling.
- Add continuous or grab monitoring for free ammonia, nitrite, and monochloramine at the tank outlet and at two dead ends, with a written trigger (e.g., free NH₃-N > 0.1 mg/L or nitrite > 0.05 mg/L) that initiates flushing and ratio correction.
- If SCADA review shows the tank cannot cycle without violating fire storage, then document that and fund tank mixing or a second inlet - that is the point at which “unavoidable” becomes a real conclusion.