Turbidity Excursion After a Source Water Change
A surface water plant treating a reservoir source switches to a river source after a storm raises reservoir turbidity to 60 NTU. The river runs 12 NTU but carries higher organic content (UV254 doubled). The plant uses coagulation with alum at a fixed dose of 28 mg/L, flocculation, sedimentation, and dual-media filtration. Within six hours of the switch, filtered water turbidity climbs from 0.08 to 0.42 NTU, filter run times shorten from 36 to 14 hours, and chlorine demand in the clearwell rises by 40 percent. The combined filter effluent (CFE) turbidity limit is 0.3 NTU, and the utility must report any exceedance. The lead operator is off shift; a Class II operator is on duty.
1. Explain what is happening in the treatment train and identify the immediate operational decisions.
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The symptoms point to coagulation failure under a changed raw water, not a filter defect:
- Higher organic content (UV254 doubled) consumes coagulant and shifts the optimum pH and dose. The fixed 28 mg/L alum dose is now insufficient to destabilize particles and NOM.
- Poorly coagulated floc carries through sedimentation, loads the filters, and causes early breakthrough - hence 0.42 NTU CFE and shortened runs.
- The 40 percent rise in chlorine demand reflects residual NOM not removed, which also raises disinfection byproduct risk.
Immediate decisions:
- Notify the lead operator and the responsible official - a CFE exceedance of 0.3 NTU is a reportable event; do not wait for the shift change to document it.
- Run a jar test on the new raw water to find the correct coagulant dose and pH; do not guess with a large step change.
- Increase coagulant dose incrementally based on jar results and adjust pH if needed.
- Reduce filtration rate or take a filter out of service to protect the CFE while coagulation is corrected.
- Increase monitoring frequency on raw, settled, and filtered turbidity.
- Confirm disinfection CT is still met as demand rises.
2. The Class II operator wants to double the alum dose immediately to fix it. Evaluate that action.
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Doubling the dose blindly is risky and not the right first move.
- Overdosing alum can reverse the charge and re-stabilize particles, making turbidity worse, and can drive pH down, which may push the plant outside its coagulation optimum.
- It also raises sulfate and sludge production and can shorten filter runs further if floc is poor.
- Without a jar test, the operator cannot know whether the problem is dose, pH, mixing energy, or polymer.
Appropriate action: run a rapid jar test across a dose range (e.g., 28, 35, 45, 55 mg/L) and pH adjustment, then apply the verified optimum. In the interim, protect the CFE by reducing filter loading and increasing surveillance. If jar testing equipment or reagents are unavailable, the operator should escalate rather than make an unverified large change.
This is also a supervision issue: the Class II operator should have clear standing authority and a written protocol for source changes so decisions are not made by guesswork.
3. Draft the reporting and documentation steps the utility must take.
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- Record the event: raw and filtered turbidity readings with timestamps, the source switch time, dose changes, filter run data, and jar test results.
- Determine reportability: a CFE turbidity exceedance of 0.3 NTU is a treatment technique violation in most jurisdictions; confirm the state primacy agency's specific reporting timeline (often within 24 hours for the initial notice, with written follow-up).
- Notify the primacy agency per the required timeline and method, documenting who was contacted and when.
- Assess public health impact: confirm disinfectant residual and CT were maintained; if not, consult on a boil-water notice.
- Public notification if required by the violation type and tier.
- After-action review: update the source-change SOP, add jar testing as a required step, and retrain operators on authority and escalation.
All records should be retained per the utility's retention schedule.