Secondary (Biological) Treatment There are two principal types of biological sewage treatment: (i) The percolating filter (also referred to as a trickling or biological filter). (ii) Activated sludge treatment. Both types of treatment utilize two vessels, a reactor containing the microorganisms which oxidize the BOD, and a secondary sedimentation tank, which resembles the circular radial flow primary sedimentation tank, in which the microorganisms are separated from the final effluent. Percolating Filter These units consist of circular or rectangular beds of broken rock, gravel, clinker or slag with a typical size in the range of 50-l00 mm. The beds are between 1.5 and
2.0 m deep and of very variable diameter or size depending on the population to be served.
The proportion of voids (empty spaces) in the assembled bed is normally in the range 45 to
55% (Figure 4). The settled sewage trickles through interstices of the medium which constitutes a very large surface area on which a microbial film can develop. It is in this gelatinous film containing bacteria, fungi, protozoa and on the upper surface algae that the oxidation of the BOD in the settled sewage takes place.