ABSTRACT
Aeration was initially employed as a possible means of reducing odours in wastewater holding tanks. At about the same time it was noted that during soil percolation if the flow of wastewater was stopped intermittently to allow aeration of the soil, an improved treatment efficiency and loading capacity was obtained. Later, it was realized that purification was due to biochemical oxidation and a major step forward was the development of a system which incorporated a continuous inflow of wastewater and continuous aeration. Such a system was the forerunner of the activated sludge process, while the development of soil percolation systems led to the process known as trickling or percolating filtration. These two processes, or modifications thereof, are by far the most frequently used in the UK for the treatment of domestic wastewater at municipal works today. Moreover, in the vast majority of applications they are secondary treatment processes, designed for the oxidation of soluble or dispersed colloidal materials in the wastewater. The raw wastewater, however, passes through several stages before entering the secondary treatment process. In the next section these preliminary and primary stages are described briefly, for the sake of completeness, although they are not biological processes.
