The main characteristic of the activated sludge is the occurrence of microorganisms, which take up solved food over their body surface or a cell mouth thereby contributing to wastewater cleaning. The bio conenoses of the activated sludge gives information over the condition of the activated sludge and the cleaning achievement.

Bacteria
Bacteria are simple, colorless, one celled plants that use soluble food and are capable of self reproduction without sunlight. As decomposers they fill an indispensable ecological role of decaying organic matter in stabilizing organic wastes in treatment plants. They are responsible for activated sludge growing in domestic wastewater treatment. A wide variety of bacteria can be found in a sludge flake.

Beside bacteria, a number of species of protozoa such as flagellate-, ciliated- and amoebae protozoa have been identified in activated sludge. Protozoa are single-celled organisms that can consume food such as bacteria and particulate matter. 
These organisms are partly on the activated sludge flakes. Other protozoa move lively on or between the activated sludge flakes. The nematodes or rotifers are ranked among the multi-cellular organisms.

Nematode

Rotifer

Ciliate

The sludge abstracted after the wastewater treatment process contains in the unloaded stabilized condition:

Carbon       (50-70 %), 
Hydrogen    (6,5-7,3 %), 
Oxygen       (21-24 %), 
Nitrogen      (15-18 %), 
Phosphorus  (1-1,5 %) and 
Sulphur       (0-2,4 %). 

 

Water is the main component of sludge. Its amount depend on the sludge sort (primary-, secondary or tertiary sludge) and the way of stabilization (aerobic, anaerobic). Raw sludge has a water content of 93 % to 99 %. Therefore a dewatering (up to approx. 35 % dry substance content) or drying (to over 85 % dry substance content) can be necessary for a further utilization.
The second main component is the dry substance, which is made up of organic and inorganic substances.