Storm Upset at the Conventional Plant

What changed. Two things happened at once. Particle load went up roughly twentyfold (2 → 45 NTU), so the coagulant demand rose. And alkalinity collapsed (180 → 55 mg/L), so the same 28 mg/L alum dose now consumes a much larger share of the available buffer - alum consumes roughly 0.45–0.5 mg/L of alkalinity as CaCO3 per mg/L of alum. The settled pH almost certainly dropped below the optimum range for alum (typically around 6.0–6.5), where aluminum stays soluble, floc is light and pin-flocculent, and it carries into the filters.

The evidence fits: settled turbidity 4.5 NTU against a 2 NTU goal means the filters are removing particles the clarifier should have taken out. Run time collapsing from 60 to 18 hours is a filter doing clarifier work. The rising clearwell chlorine demand is the same soluble organics and aluminum passing through.

First three actions. (1) Pull a settled-water pH and check filtered aluminum - confirm whether this is a pH/coagulation problem rather than a hydraulic one. (2) Throw out the old jar test and run a fresh matrix on today's raw water: alum versus alum plus polymer, across a pH range. (3) Walk the filters and verify the instruments - check for media loss, bed upset after backwash, and confirm the turbidimeters are calibrated; a 0.35 versus 0.15 gap is the kind of thing that is sometimes real and sometimes a dirty lens.

Combined filter effluent. The limit is ≤0.3 NTU in at least 95% of monthly measurements, and never above 1.0 NTU. At 0.15 NTU you are compliant with room to spare.

Individual filters. The individual filter effluent is tracked separately. The trigger for a Tier 2 public notice is an individual filter at or above 1.0 NTU in two consecutive 15-minute samples, or a filter that fails to return to ≤0.5 NTU within four hours after backwashing or being returned to service. Filter 3 at 0.35 NTU is not a violation - but it is more than double the combined value, which means that filter is carrying a disproportionate share of the load and is trending toward the trigger. This is a warning, and the correct response is to act now rather than to record it and move on.

What to verify. Confirm the 0.35 reading with a second instrument or a calibration check before you act on it, and confirm the sample point is truly the filter effluent and not a mixed sample. Then establish whether the reading rises through the run (breakthrough) or is high from the start of the run (media problem or inadequate backwash).

Reporting. Any Tier 2 trigger must be reported to the primacy agency and public-noticed within the required timeframe; do not wait for the quarterly report to mention it.

First 24 hours.

1. Cut loading. Reduce plant flow to the extent storage and demand allow; a lower hydraulic loading rate buys settling time and extends filter runs.
2. Fix the chemistry. Adjust the alum dose from the fresh jar test and add a polymer as a filter aid; add caustic or lime to hold settled pH in the optimum range. Do not simply raise alum - at 55 mg/L alkalinity that pushes pH further down and can make things worse.
3. Protect the filters. Put filter 3 on filter-to-waste until it stabilizes, backwash on turbidity or headloss rather than on a fixed clock, and confirm backwash rates are not washing media to the troughs.
4. Protect the clearwell. Increasing chlorine to hold residual will raise DBP formation on exactly the high-TOC water you have right now - watch the residual, but document the dose change and sample DBPs on schedule.
5. Document everything. Log the readings, the dose changes, the jar test results, and the times.

To the state. Report the filter 3 individual filter effluent exceedance if it reaches the 1.0 NTU trigger (with the two consecutive 15-minute readings and the corrective actions), and include the turbidity and filter-to-waste data in the monthly report. If you take a filter out of service, note it. Nothing here is a Tier 1 situation, but the record has to show that you detected the trend and responded to it.