What primaries remove
Typical primary clarifiers remove about 50 to 70% of TSS and 25 to 40% of BOD. White River's design removals are 60% TSS and 32% BOD. White River's design primary effluent is BOD 150 mg/L and TSS 96 mg/L from influent BOD 220 and TSS 240.
Key numbers
- Detention time: often 1.5 to 2.5 hours. White River: four 65 ft clarifiers with 893,554 gal total volume; at 8.0 MGD, detention is about 2.7 hours.
- SOR: design values are often 800 to 1,200 gpd/ft² at average flow. White River's 13,273 ft² of primary area gives about 603 gpd/ft² at 8.0 MGD.
- Sludge concentration: anywhere from about 1 to 6% solids depending on pumping strategy and whether the primaries thicken. White River pumps about 1.2% on timers.
Effect on activated sludge
- Less BOD and TSS to aeration means less oxygen and less sludge.
- Grease removal reduces foam formers.
- Too much removal can short denitrification of carbon in BNR plants.
- Septic primary sludge (held too long) sends sulfide and odors downstream.
Good habits
- Pump primary sludge often enough to prevent septic conditions, but not so much that it is thin.
- Keep scum removal working.
- Check sludge blanket and solids concentration daily.