Coliform Excursions in a Contact Basin Sitting on the Monochloramine Hump
An 8 ML/d tertiary plant discharges to a cold-water river. The permit sets fecal coliform at 200 per 100 mL monthly geometric mean and total residual chlorine at 0.02 mg/L maximum, so sodium hypochlorite is fed ahead of a serpentine contact basin and dechlorinated with sodium bisulfite. The basin holds 95 m³; peak flow is 0.12 m³/s. Effluent ammonia is 2.5 mg/L as N and nitrite 0.4 mg/L as N. At a dose of 4.0 mg/L, the DPD test gives total residual 2.2 mg/L and free residual 0.05 mg/L. Fecal coliform has averaged 500 per 100 mL for three weeks. The filter upstream is running well, turbidity 0.9 NTU, and a tracer test last year gave a t10 of 5 minutes at peak flow.
1. What do the residual numbers tell you about where on the breakpoint curve this plant is operating, and why does that explain the coliform result?
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Locate the operating point. The dose-to-ammonia ratio is 4.0 mg/L Cl2 ÷ 2.5 mg/L NH3-N = 1.6:1 by weight. Monochloramine formation peaks near 5:1, and breakpoint is reached around 7.6:1. At 1.6:1 you are on the rising limb, well below breakpoint: essentially all the chlorine is being consumed to form combined chlorine, and the 0.05 mg/L free residual confirms it - there is effectively no free chlorine in the water.
Why that fails. Monochloramine is a much weaker oxidant than hypochlorous acid. For a 3-log fecal coliform inactivation, free chlorine needs a Ct on the order of a few mg·min/L, while combined chlorine needs hundreds to thousands. Your actual contact is thin:
- Hydraulic retention at peak flow = 95 m³ ÷ 0.12 m³/s ≈ 790 s ≈ 13 min theoretical.
- The tracer test gives t10 = 5 min, and disinfection credit should be based on t10, not theoretical detention.
- Ct = 5 min × 2.2 mg/L ≈ 11 mg·min/L of combined chlorine, which is nowhere near enough.
Two secondary points. Nitrite exerts a fast, near-stoichiometric chlorine demand and will strip chlorine ahead of the ammonia reaction, so 0.4 mg/L NO2-N is quietly eating dose. And because the contact basin is serpentine with a t10 of only 5 minutes against a 13-minute theoretical, there is meaningful short-circuiting to fix.
The plant is not under-dosing in a simple sense - it is dosing into the wrong chemistry.
2. Compare the realistic options for meeting 200/100 mL, including what each costs you in chemicals, byproducts, and compliance risk.
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Option A - Nitrify upstream so the ammonia disappears. If effluent ammonia drops below roughly 0.5 mg/L, the same chlorine dose produces free chlorine and the Ct math changes by orders of magnitude. Lowest operating cost, fixes the root cause, and reduces chlorine demand overall. Downside: lead time (weeks to months in cold weather) and it depends on the secondary process being able to nitrify, which is not guaranteed here.
Option B - Go to breakpoint. Breakpoint dose is roughly 7.6 × 2.5 ≈ 19 mg/L, plus demand from nitrite and organics - call it 22–25 mg/L. That means a five- to six-fold increase in hypochlorite, a correspondingly large bisulfite demand (roughly 1.3–1.5 mg bisulfite per mg chlorine; verify with your supplier's figures), sharply higher THM/HAA formation on an effluent with organics, and a fragile total-residual compliance position: holding 0.02 mg/L while swinging 20+ mg/L of dose requires tight, fast dechlorination control. One feed upset becomes a fish kill and a permit violation.
Option C - UV. No residual, no byproducts, no dechlorination. Requires good upstream quality; 0.9 NTU is generally workable but UV transmittance must be measured, not assumed. High capital, moderate maintenance (lamp/sleeve fouling, quarterly validation), and it removes your chlorine residual as a coliform barrier downstream.
Option D - Improve contact time only. Retrofitting baffles to raise t10 might get you from 5 to 9–10 minutes. Useful, cheap, and worth doing regardless - but it multiplies a Ct that is already two orders of magnitude short. It cannot close the gap alone.
Practical sequencing: tracer-test and re-baffle now, tighten the dose to the actual ammonia load with feed-forward control, push nitrification upstream, and plan UV as the capital path if the ammonia cannot be reliably driven down.
3. While the fix is being engineered, what changes to monitoring and control would you make, and what would you tell the regulator?
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Monitoring and control changes:
- Split the DPD test every time. Report free and total residual separately, not just total. A total residual that is nearly all combined tells you instantly that you are on the wrong side of breakpoint.
- Get an ammonia signal on the chlorine feed. Grab samples three times a day at minimum until an online ammonia analyzer is in service; feed-forward from ammonia load instead of a fixed dose.
- Sample at the worst moment. Coliform grabs taken at the morning low-flow period flatter the plant. Sample at peak flow, when t10 is shortest, and composite the geometric mean honestly.
- Instrument the dechlorination. An ORP or residual controller on the bisulfite feed, with the analyzer on the outfall, so a dose change does not roll through as a residual exceedance.
- Daily check of the contact basin for short-circuiting, baffle damage, or sludge accumulation that steals volume.
What to tell the regulator: report the exceedances as required by the permit, on time, with the cause (ammonia loading putting the plant below breakpoint), the corrective actions underway, and a schedule. Do not wait for the monthly average to be formally computed - the credibility you gain from a prompt, technically literate notification is worth more than the few days you might buy. Also confirm whether the 0.02 mg/L residual limit has been approached during the higher dosing, because that is a second, separate violation waiting to happen.