The basic objective of the activated sludge secondary treatment process is to remove organic matter (carbon), and this is done in a large tank full of sewage, mixed with oxygen and bacteria.  The bacteria use the sewage for food, and the oxygen or air for respiration.  If you are interested in learning more, an understanding of the bacteriological chemistry involved requires research into heterotrophic and autotrophic bacteria, which are subdivided into aerobic (require free oxygen), anaerobic (do not require free oxygen), and facultative (use free oxygen if available, but can live without), each of which transform organic matter into compounds (mostly CO2 and H2O) plus energy in their own way. Anaerobic bacteria are predominant in the anaerobic sludge digestion process. The objective of the anaerobic digestion process is to stabilize the sludge by transforming organic volatile compounds into inert ones and methane (mostly CO2, H2O and CH4 (methane)).  

No bacteria are added in the process... they are inherent and ubiquitous to wastewater. No other additives or chemicals are added to the treatment processes before release to the river. The majority of bacteria are of the genera Alcaligenes, Flavobacterium, Bacillus, and Pseudomonas. 

Other organisms involved in secondary treatment include Fungi, Molds, and Algae.  Many of these are unwanted as they are filamentous and inhibit settling, an important component of secondary treatment.  Another important group is the higher order organisms (multi-cellular, motile), called protozoa (eg. amoeba, stalked cilia, flagellates, rotifers), and less important, nematodes.  These feed on the other groups and are an indicator of a mature secondary process. 

The wastewater treatment operator is like a micro-zookeeper who's job is to promote an environment where the correct mix of these bugs accomplish carbon removal + good settling + bio-gas production (for digestion), and to apply corrections if any group is dieing off or tries to "take over" the zoo.

Ron Shulyer, PE:  "What's Buggin' You Today?"             
          http://www.rtweng.com/media/newsletter/winter93.pdf

Ron Shulyer, PE:  "What's Buggin' You Part II?"
          http://www.rtweng.com/media/newsletter/spring93.pdf