UV Transmittance Drop Threatens Cryptosporidium Credit
A 6 MGD conventional plant filters to a UV building with four reactors; three are in service because the fourth reactor's ballast is on backorder. Each reactor is validated by third-party bioassay to deliver a reduction equivalent dose of 40 mJ/cm2 at 2.0 MGD when UV transmittance at 254 nm (1 cm) is 88 percent or higher. The state grants 3-log Cryptosporidium credit at 22 mJ/cm2. The validation report table for 2.0 MGD reads: 88 percent UVT gives 40 mJ/cm2; 80 percent gives 31; 75 percent gives 25; 72 percent gives 21; 70 percent gives 19. Over three weeks, UVT fell from 90 to 74 percent after a reservoir drawdown raised filtered-water TOC from 2.0 to 3.4 mg/L. Filtered turbidity remains 0.06 NTU, lamp intensity checks are within 5 percent, sleeves were cleaned 10 days ago, and the UVT analyzer was calibrated four days ago. Peak plant demand is 6.0 MGD.
1. Is the plant still meeting the 22 mJ/cm2 requirement? Show how you establish the dose and what you would document.
Read the worked answer
At 74 percent UVT, interpolate between the validated points: 75 percent gives 25 and 72 percent gives 21, so 74 percent is one-third of the way down, giving roughly 25 minus 1.3, or about 23.7 mJ/cm2 per reactor at 2.0 MGD. That is nominally above the 22 mJ/cm2 required.
Treat it as not reliably compliant. The margin is only about 1.7 mJ/cm2, which is inside the combined uncertainty of the UVT analyzer, the interpolation, lamp aging, and sleeve fouling. Within a few tenths of a percent of UVT the plant is out of credit.
How to establish the dose properly: use the reactor's validated dose-monitoring algorithm exactly as written, with continuous flow, UV intensity sensor readings, and UVT as inputs, rather than a hand interpolation. Cross-check the online UVT analyzer against a grab sample on a calibrated bench spectrophotometer, since a biased analyzer silently changes the reported dose. Verify sleeve fouling and lamp output are inside the validation assumptions.
Document: the calculated dose trend, the validation table, the UVT calibration records, the lamp and sleeve maintenance dates, and the decision logic for any flow reduction. This record is what demonstrates compliance if the state asks.
2. What are your options to restore margin, and what must you not do?
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First, calculate what flow reduction would buy. Dose approximately scales inversely with flow at constant UVT, so to reach 30 mJ/cm2 from about 24 at 2.0 MGD, each reactor would run at 2.0 x 24/30, or 1.6 MGD. With three reactors that is 4.8 MGD against a 6.0 MGD peak, a real shortfall. Concessions would be needed: draw down storage, shift demand, or curtail large users.
Better option: fix the cause upstream. The TOC rose from 2.0 to 3.4 mg/L after the drawdown, and TOC is what dropped UVT. Enhanced coagulation (lower coagulation pH, higher coagulant or a polyaluminum chloride, optimized jar testing for TOC removal) can recover several points of UVT and also lowers DBP precursors, so it pays twice.
Other moves: expedite the fourth reactor ballast to add capacity and redundancy, clean sleeves early, and confirm no lamp is degraded.
What not to do: do not turn up lamp power, increase reactor flow, or operate outside the validated envelope without revalidation. Operating beyond validated conditions voids the credit no matter what the dose calculation says. Also do not assume the UVT decline is a sensor artifact without bench confirmation, and do not blend around the UV reactors, since UV is the Crypto barrier and chlorine does not substitute for it.
3. If UVT keeps falling and the calculated dose drops below 22 mJ/cm2, what is your compliance and communication response?
Read the worked answer
Below 22 mJ/cm2 the plant has lost part of its Cryptosporidium treatment credit, which is a treatment technique failure, not a monitoring hiccup. Response sequence:
- Immediately reduce flow per reactor to whatever restores the required dose with margin, even if that means operating below peak demand.
- Notify the primacy agency the same day, describe the calculated doses and the cause, and follow their direction on public notification. Because Crypto is the organism of concern and chlorine is ineffective against it, the state may require a notice; do not decide that alone.
- Increase monitoring: hourly UVT and dose logging, daily bench UVT verification, daily lamp and sleeve checks, and grab samples for TOC and turbidity to confirm the coagulation response.
- Accelerate the fixes: coagulation optimization, ballast delivery, and any alternative barrier the state accepts.
- Document everything contemporaneously: dose calculations, notification times, the individuals contacted, and the corrective action timeline. For the longer term, add a UVT alarm and automatic flow setback so that the plant responds to transmittance loss before credit is lost, and review the source-water monitoring that would have given earlier warning of the reservoir drawdown.