Chlorine Residual Present, Coliforms Still High

Where the chlorine is going With 14 mg/L of ammonia as N in the effluent, essentially all applied chlorine reacts to form monochloramine first, then some dichloramine. That is why the total residual is 2.6 mg/L while free residual is only 0.05 mg/L - the chlorine is combined, not free.

Why coliforms survive Monochloramine is a much weaker oxidant than hypochlorous acid. For fecal coliform the CT product needed with chloramines is one to two orders of magnitude higher than with free chlorine, and chloramines are notably poor against viruses and Giardia. In a 35-minute basin the CT achievable at 2.6 mg/L combined is around 91 mg·min/L, and much of that time is not at plug flow. Meanwhile any solids, biofilm, or clumps in the sample shield organisms, and a high TSS effluent gives the bacteria somewhere to hide.

Chlorine dose on a mass basis 4.5 × 4.0 × 8.34 ≈ 150 lb/day of chlorine.

Could you just raise the dose? Breakpoint chlorination needs roughly 7.6-8.3 lb of chlorine per lb of ammonia-N (about 10:1 on a Cl2 to NH3-N mass basis): 14 × 7.6 ≈ 106 mg/L, or 106 × 4.0 × 8.34 ≈ 3,540 lb/day of chlorine - more than 20 times the current dose, and after the breakpoint is reached almost all of that is free residual that must then be fully dechlorinated to meet the 0.1 mg/L outfall limit. That is not a feasible operating answer: the chemical cost, the sulfur dioxide or bisulfite demand, the DBP formation (trihalomethanes, especially with any organics in the effluent), and the gas-handling risk all rule it out. Increasing dose is not the solution.

Breakpoint chlorination - rejected on dose (roughly 106 mg/L), residual destruction cost, and DBP formation, as shown above.

Nitrify the secondary process - the real fix Effluent ammonia at 14 mg/L means the plant is not nitrifying, and until it does, combined chlorine is the only chemistry available. Nitrification requires SRT of roughly 8-12 days at cold temperatures, DO of at least 2 mg/L in the last pass, and about 7.1 lb of alkalinity as CaCO3 per lb of ammonia-N oxidized. With influent ammonia around 30-35 mg/L, that is roughly 230 mg/L of alkalinity consumed - the plant may need supplemental alkalinity or may simply not have the aeration capacity. Data needed: influent and effluent ammonia, influent alkalinity, winter water temperature, current SRT, and aeration capacity versus demand.

UV - attractive, but verify the prerequisites UV needs low TSS (well below 30, and preferably below 10-15), low BOD, and good UV transmittance (65%+ is a common design floor). At 18 mg/L TSS the effluent may still be too turbid; check UVT on a filtered and unfiltered sample first, and confirm whether the permit requires virus and protozoa credits that only a validated UV system can deliver.

Short-term: optimize what you have

1. Dechlorinate properly - this is mandatory under the 0.1 mg/L limit anyway. Set the sulfur dioxide (or bisulfite) dose with a residual analyzer and a target of 0.02-0.05 mg/L total residual, and dechlorinate after the required contact point (compound loop). Indiscriminate dechlorination upstream of the contact time defeats disinfection.
2. Squeeze more out of the contact basin: verify the actual minimum detention at peak flow, confirm the sample point is downstream of the required CT, and look for short-circuiting.
3. Improve effluent quality: lower TSS to reduce shielding, keep the filters or clarifiers at their best, and avoid recycling solids-laden streams into the chlorine contact basin.
4. Measure and record free and total chlorine at several points daily, plus daily fecal coliform until the geometric mean recovers.

Recommendation Short-term, optimize contact and dechlorination and push the process toward nitrification as far as the aeration and SRT allow; if that fails, move to UV. Capital plan: either a nitrification upgrade (aeration and SRT) or UV with filtration, chosen from the UVT, TSS, and permit-disinfection-credit data.

Check the arithmetic first Volume = 125,000 gal. At 4.0 MGD: 125,000 / 4,000,000 × 1,440 = 45 min. At a peak factor of 1.3 (5.2 MGD): 125,000 / 5,200,000 × 1,440 ≈ 34.6 min. So the 35-minute figure is the peak-flow number and the basin roughly meets it on paper - but only on paper.

Check the physical reality

• Level and dead volume: confirm the basin is operated at full depth; a drawn-down basin loses detention time directly.
• Baffling: a single-pass basin without baffles develops density currents - dense chlorinated flow runs along the bottom, warm or buoyant flow skims across the top, and the real detention time can be a fraction of the theoretical value. Look for inlet/outlet baffles and tie-across walls.
• Tracer test: a slug of fluoride, lithium, or a conductivity pulse with a curve of time versus concentration will give the actual modal and 10th-percentile contact times. This is the only way to know the short-circuiting, and it is worth the day of effort before spending capital money.
• Sample point: confirm the coliform sample and the chlorine residual sample are taken after the full contact length and before dechlorination - sampling a dead zone reads a residual that no organism ever saw.

At peak flow

• If the tracer shows poor plug flow, raise the operating level (more volume) and add or repair baffles before considering more chlorine.
• Verify that the basin is not receiving a surge with a high TSS slug from upstream - solids protect the organisms.
• If the peak flow reliably exceeds 35 minutes of detention, evaluate flow equalization, or move to a disinfectant that is not CT-limited by contact time (UV) or add a second basin in series.
• Never compensate for lost contact time by dosing more chlorine: you will blow the 0.1 mg/L total residual limit and create DBP problems while making no measurable disinfection gain.