Primary settlement is the first stage of the sewage treatment process and occurs within the primary settlement tank, which may be either a totally separate unit, or a compartment within a larger structure. In either case the heavier solids will sink to the bottom of the tank and the lighter material will form a scum at the surface. Both are retained, allowing the liquid to pass forward to the next stage of treatment by displacement.
Biological oxidation is the term used to describe the process in which naturally occurring micro-organisms, which colonize the filter bed, utilize the organic material as a food source, to reduce the Biochemical Oxygen Demand (effectively a measure of the ‘polluting strength’ ) prior to discharge.
In order to achieve this the micro-organisms themselves require an adequate supply of oxygen, which, in the case of the ‘SAFE’ plant, is supplied by the venturi/ pump combination that both entrains air and distributes the liquor through the filter.
Final settlement is provided by a humus tank, also known as a secondary settlement tank, or clarifier. The purpose of this is to settle out biomass that has died, and fallen away from the filter media, when it has become too thick for oxygen to reach the innermost cells. This biomass is continually being replaced by new growth, quickly achieving a state of equilibrium.
Depending on the application, secondary sludge may be automatically returned to the primary settlement tank, to maintain efficiency and reduce de-sludging requirements.