41 Hours Out of Turbidity Compliance, and an Overdue Calibration

Compliance calculation:

• Readings at or below 0.3 NTU: 720 - 41 = 679
• Percentage: 679 ÷ 720 = 94.31%
• (Or: 41 ÷ 720 = 5.69% exceedance)

Allowance: 5% of 720 = 36 readings. Up to 36 exceedances would still leave 684 ÷ 720 = 95.0% and pass.

Result: 41 exceedances is 5 above the allowance. The month fails the 95% requirement and is a treatment technique violation.

Two things that do not change the answer:

• The 1.0 NTU absolute limit was not breached, so there is no separate exceedance on that standard this month. It does not offset the 95% failure.
• The two clusters have different causes (a procedural failure on filter restart, a chemical feed failure), but the rule counts readings, not causes. Root-cause analysis is required after the count, not instead of it.

No. It is not defensible, and it is the single most dangerous decision in the scenario.

Why each argument fails:

• "The instrument was probably wrong." A lapsed calibration is a QA deficiency to be documented and corrected - it is not a license to discard valid data retroactively. You may not know the direction of the error, and the grab sample actually reads lower than the online unit, which would only matter if you were trying to prove more excursions occurred, not fewer.
• The grab sample is not a substitute. It is one point in time at one location, not flow-proportional and not at the combined effluent sample point. Turbidity can differ between a filter's individual effluent and the combined header. A single grab reading cannot override 41 hourly records.
• The exceedances have identified physical causes. A filter returned to service without filter-to-waste and a coagulant feed upset are known, documented mechanisms. That is corroboration, not instrument error.

What to do instead:

1. Calibrate now with an approved primary standard, log the as-found and as-left values, and note the QA gap in the monthly report.
2. Report the 41 readings as recorded, with a written explanation of the two events and the calibration lapse.
3. If the instrument is later proven out of tolerance by more than the allowed limit, take that to the primacy agency and let them decide on data qualification. The operator does not get to decide.
4. Falsifying or selectively omitting monitoring data is a separate, serious regulatory matter and typically a criminal exposure for the person signing the report.

Regulatory follow-up:

• Report the violation to the primacy agency on the required monthly report and by the state's reporting deadline; confirm the exact deadline with the state rather than assuming one.
• A treatment technique violation generally requires Tier 2 public notice within 30 days unless the state directs otherwise - confirm the tier and timing with the primacy agency.
• Retain all raw data, calibration records, and the corrective action documentation. Assume they will be reviewed.

Operational fixes, tied to the two root causes:

1. Filter restart procedure. Rewrite the SOP so filter-to-waste is mandatory and cannot be bypassed, and add a written check-off before a filter is placed in line.
2. Turbidity alarms. Set a high-turbidity alarm at 0.15-0.20 NTU (well below the 0.3 limit) so you find problems before they become exceedances, with an automatic notification to the on-call operator.
3. Coagulant feed reliability. Add a low-dose alarm, a day-tank level interlock, and a calibration drawdown check per shift.
4. Calibration program. Move the turbidimeter to a tracked schedule with a secondary sign-off; a 45-day lapse should be impossible, not routine.
5. Trend, don't tabulate. Plot hourly CFT turbidity so a 26-hour plateau is visually obvious, not buried in a spreadsheet of numbers.