1. Stabilized organic material usually gives off obnoxious odors.
2. When doing a 30-minute activated sludge settleability test, a 1000-mL imhoff cone should be used..
3. Dissolved air floatation thickeners that treat primary sludge should be equipped with bottom sludge scrappers and sludge removal equipment.
4. Primary sludge is generally easier to treat than excess biological sludge in a dissolved air floatation thickener.
5. Visual observations by the operator play an important role in day-to-day operation of gravity thickeners.