The solids in this inflow will be almost entirely organic. The non-commutible solids removed will have been removed by screening. The BOD of this inflow will be many thousands. In these two views of a settling tank, you can see the gantries running along the top, supporting baffles that dip below the surface. Deep end of primary settling tank Similar baffles draw the sludge to the deep end of the tank where it is eventually pumped out.
Primary effluent is the liquid phase from the primary settling tanks.
There is, however, still much colloidal and soluble material to be removed. The BOD may be several hundred. The primary effluent is led into one of the Activated Sludge Tanks for secondary treatment. The effluent is kept in the Activated Sludge Tank for several hours whilst air is bubbled through it. The oxygen in the air, together with the microflora, lowers the Biological Oxygen Demand to around eight.