Filter Run Shortened by a Coagulant Change
A conventional 6 MGD surface-water plant treats a reservoir source with turbidity that swings from 4 to 30 NTU after storms. The plant uses alum coagulation, flocculation, sedimentation, and four dual-media (anthracite/sand) filters. Historically filter runs lasted 36–48 hours at a loading of about 4 gpm/ft². Last week the plant switched to a new alum supplier. Since then, finished turbidity has crept from 0.08 NTU to 0.22 NTU, filter runs have dropped to 14–18 hours, and the filters show mudball formation and short terminal headloss runs. Jar tests on the new alum show the optimum dose is 18 mg/L versus the 12 mg/L the plant is feeding. The sedimentation basin effluent turbidity is 3.5 NTU, up from 1.2 NTU. The state requires combined filter effluent ≤0.3 NTU and individual filter effluent ≤1.0 NTU with no more than 5% of monthly measurements over 0.5 NTU.
1. Trace how the coagulant change propagates through the treatment train to the filter problems. What is the first corrective action?
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Propagation:
- Underdosing. Jar tests show optimum dose is 18 mg/L but the plant feeds 12 mg/L - a 33% shortfall. The new alum may differ in strength or alkalinity demand, so the same feed setting gives less effective coagulation.
- Poor floc. Insufficient coagulant produces small, weak, poorly settled floc. Sedimentation effluent rises from 1.2 to 3.5 NTU - that extra solids load hits the filters.
- Filter loading. More solids per gallon means filters load faster; runs fall from 36–48 h to 14–18 h. Weak floc penetrates the anthracite and forms mudballs and surface matting, causing short terminal-headloss runs.
- Turbidity breakthrough. Finished turbidity rises to 0.22 NTU - still under the 0.3 NTU combined limit but trending toward it, and individual filter effluent could exceed 1.0 NTU during a storm.
First action: correct the coagulant dose to the jar-test optimum (18 mg/L) and verify with a fresh jar test and streaming-current or turbidity titration. Then re-check sedimentation effluent turbidity before touching the filters. Do not increase backwash frequency as the primary fix - that treats the symptom.
2. The plant is at 0.22 NTU finished and a storm is forecast to raise source turbidity to 30 NTU. What operational and reporting steps do you take?
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Operational steps:
- Raise coagulant dose and confirm optimum with jar tests at the higher raw turbidity; adjust flocculator energy and possibly polymer as a filter aid.
- Increase filter surveillance: check individual filter effluent turbidimeters every 2 hours and watch for the 0.5 NTU trigger that counts toward the 5% monthly limit.
- Be ready to take a filter out of service for backwash if individual effluent approaches 1.0 NTU; never let a filter run past breakthrough to save water.
- Verify backwash is effective (air scour, bed expansion, mudball breakup) since mudballs indicate the backwash is not cleaning the bed.
- Check finished-water alkalinity and pH - higher alum dose consumes alkalinity and can drop pH, hurting coagulation.
Reporting:
- Document the coagulant change, jar-test results, dose correction, and turbidity trend in the operating record.
- If any individual filter exceeds 1.0 NTU, that is a treatment-technique violation requiring notification to the primacy agency; follow the state's reporting timeline.
- Track the 5% over-0.5-NTU rule monthly so you know your margin before the month closes.