Turbidity Creep After a Spring Rain at a Small Filtration Plant
Your very small surface-water plant treats a spring-fed source through coagulation, flocculation, sedimentation, and a single mixed-media filter, rated at 50 gpm. It serves 90 connections. Your state requires filtered water turbidity of 0.3 NTU or less in at least 95% of monthly measurements, with a maximum of 1 NTU. After two days of heavy rain, raw turbidity jumped from 2 to 18 NTU and raw color rose. Your finished-water turbidity readings today are 0.28, 0.31, 0.35, 0.42, and 0.55 NTU on grab samples every few hours. Your coagulant feed is unchanged, filter run is at 40 hours (typical is 48), and head loss is rising faster than normal.
1. Are you currently in compliance, and what is the risk in the next 24 hours if you change nothing?
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Compliance check: The rule is a percentage-based limit (0.3 NTU in ≥95% of measurements) plus an absolute cap (1 NTU). Today's five readings: 0.28, 0.31, 0.35, 0.42, 0.55. Four of five are above 0.3, so today's sample set is already failing the 95% criterion (only 20% at or below 0.3). You are also drifting toward the 1 NTU cap. So even though no single reading is over 1 NTU, you are not in compliance for the day.
Risk in 24 hours: With raw turbidity at 18 NTU and rising head loss, the filter is loading up. If the coagulant dose is not adjusted for the higher particle load, you will get filter breakthrough - turbidity climbing past 1 NTU, which is a clear violation and a pathogen-risk signal. The rising head loss at 40 hours suggests the filter is near the end of its useful run and may need backwashing before 48 hours.
2. What are your immediate operational adjustments, in order, and what do you monitor to confirm they work?
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Order of operations:
- Verify the coagulant dose is matched to the new raw quality. Raw turbidity tripled and color rose - the dose that worked at 2 NTU is almost certainly too low now. Run a jar test (or at minimum a bench settle test) to find the right dose, then adjust. Do not guess-and-crank blindly; a jar test takes 20 minutes and prevents over- or under-dosing.
- Check and tune pH for coagulation - color removal especially is pH-sensitive, and the rain may have shifted raw pH.
- Backwash the filter now rather than waiting for 48 hours, since head loss is rising fast. A loaded filter will break through.
- Slow the filtration rate if you can - lower flow reduces particle loading per unit area and buys turbidity margin.
- Increase monitoring frequency - turbidity every hour, not every few hours, until stable.
Confirmation: Finished turbidity back under 0.3 NTU consistently, head loss rising at a normal rate, and floc visibly forming in the settling basin. If turbidity keeps climbing after backwash and dose correction, take the filter out of service or reduce output rather than send out-of-spec water.
3. If the problem persists and you cannot get turbidity under control, what regulatory and public-health steps apply?
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Persistent turbidity above limits is a treatment-technique violation and a pathogen-risk indicator (turbidity shelters microbes from disinfection).
Steps:
- Notify the primacy agency promptly of the exceedance and your corrective actions.
- Consider a boil-water notice if turbidity is high enough that disinfection may be compromised or if you cannot maintain the required limits - coordinate with the agency on the threshold.
- Document raw and finished turbidity, dose changes, backwash times, and flow rates - this is your defense and your improvement record.
- Investigate root cause for the next storm: is your coagulant dose procedure written down? Do you have jar-test capability? A written rain-event response plan (trigger raw turbidity → test dose → adjust → increase monitoring) is the durable fix for a plant this small.