Milestones in the development of the activated-sludge process over the past eighty years (abridged) - Discussion

The biological filter was introduced during the latter part of the last cen tury but, in 1913, a radical new process was developed which resulted in a four-fold reduction in the size of sewage-treatment plants. This new method was called the 'activated-sludge process', and whilst biological filters a re still in use - especially for small works activated sludge is now the do minant secondary treatment process and is the 'engine' around which modern treatment systems are constructed. This paper (a) reviews the milestones in the evolution of the activated-slu dge process from 1913 to 1954, centering on reliable removal of BOD and sus pended solids, (b) highlights the improvements in design capabilities (1955 -74), with the emphasis on the attainment of consistent nitrification, and (c) discusses the removal of nitrogen and phosphorus (1975-97), with brief reference to computer modelling and the control of sludge properties.