Solids Washout and Hydraulic Overload

1Symptom

During high flows, the clarifier blanket rises quickly and solids wash over the weirs. Effluent TSS spikes. After the event, MLSS is lower than before and nitrification may suffer.

2Check first

1. Note the peak hour flow and the surface overflow rate (SOR) per clarifier.
2. Check the RAS rate and whether it was increased too far (overloading the clarifier hydraulically).
3. Measure blankets hourly during the event.
4. Check MLSS before and after.

3Possible causes

• Peak flow beyond clarifier capacity for the current SVI.
• High MLSS going into the storm.
• Poor flow split.
• Excessive RAS that adds to clarifier loading without lowering the blanket.

4Next investigation

• Model the event with the state point tool: storm flow, SVI and clarifiers in service.
• Review the wet weather operating plan.

5Corrective actions

1. Before storms: lower MLSS moderately where SRT allows, have all clarifiers in service, and confirm RAS pumps.
2. Step feed or move solids to the head of the basin (if the design allows) to reduce the solids reaching clarifiers.
3. Set RAS by state point: too little thickens poorly, too much adds loading.
4. After: rebuild SRT with reduced wasting and check ammonia.