Primary Sedimentation Tanks

The primary sedimentation tanks are designed to reduce the velocity of the wastewater flow for organic solids (called raw sludge) to settle. Scrapers present in the tank move continuously along the floor of the tank to deposit the raw sludge in hoppers for removal. The scum which floats to the surface is directed by water jets to the sludge sump. The raw sludge is then pumped to the sludge digesters for biological treatment. Approximately 60% of suspended solids and 35% of BOD removal efficiency can be achieved at this stage.

Design of Primary Sedimentation Tanks:

Circular Sedimentation Tank

Rectangular Sedimentation Tank


Primary Settling followed                                                           Primary Settling with waste   by secondary treatment:                                                             activated-sludge return:

Item                                    Range             Typical                    Range                Typical

Detention time(hr) 1.5-2.5 2 1.5-2.5 2
Average Overflow rate(mt3/mt2 . d) 32-48 40 24-32 28
Peak hourly Overflow rate(mt3/mt2 . d) 80-120 100 49-99 59
Weir loading (mt3/mt.d) 125-500 250 125-500 250