Activated sludge communities demonstrate oxygen consumption per unit dry weight ratios two to three orders of magnitude lower than those observed for pure cultures under non-limiting conditions. Flocs appear as irregular particles with filaments protruding from the surface, and range in size from 20 - 200 micrometers in diameter. Achromobacter , Bacillus , Flavobacterium , and Pseudomonas have been tentatively proposed as ubiquitous genera significant in the sludge process. Many early studies also focused on the bacterium Zoogloea ramigera , which had been identified as the most important floc-forming species in all activated sludges. Subsequent studies indicated that members of the genera Escherichia , Alcaligenes , Bacillus , and Pseudomonas readily form flocs in pure cultures, and that Zoogloea may constitute a major, minor, or insignificant floc component in a given sludge.
Aeration is either passive or by surface aerators.
Can achieve aerobic BOD reduction and anaerobic sludge stabilization in a single basin. Used by small communities and industries. A very cost effective treatment system. Towers packed with solid media to support biofilm formation. Typical microorganisms are Zoogloea, Pseudomonas, Alcaligenes, Flavobacterium, Bacillus, Corynebacterium, Streptomyces, Nocardia, fungi and protozoa Rotating biological contactors .