Bacteria are categorized by the way that they obtain oxygen. In wastewater treatment, there are three types of bacteria used to treat the waste that comes into the treatment plant: aerobic, anaerobic and facultative.
Aerobic bacteria are used in most new treatment plants in an aerated environment. This means that there is dissolved oxygen available for the respiration of the bacteria.
They use the free oxygen in the water to degrade the pollutants in the incoming wastewater into energy they can use for growth and reproduction. In most cases, the oxygen has to be added mechanically to the wastewater through the use of aerators in the aerated section of the treatment plant. With a normal influent load of pollutants, the dissolved oxygen content in the aerated section of most plants should be kept between 3 and 5 MG/L.
Anaerobic bacteria are normally used in an anaerobic digester to reduce the volume of sludge to be disposed of and to produce methane gas. This process is completed in anaerobic conditions, without any dissolved oxygen in the water. The anaerobic bacteria normally get the oxygen needed for their respiration from their food source. This process is also called fermentation.
As mentioned above, during the anaerobic digestion process, methane gas is produced by the anaerobic bacteria. This gas, if properly cleaned and collected, may be used as an alternative energy source. You can read more about anaerobic digestion in our previous post.
Another use of anaerobic bacteria is in the biological removal of phosphorus. During this process, a part of the aeration section of the treatment plant may be made into an anaerobic zone to facilitate the growth of phosphorus accumulating organisms, which in turn lowers the amount of phosphorus in the effluent. You can find more information about this process in our post entitled, Biological Phosphorous Removal.
Lastly, facultative bacteria are able to change their mode of respiration from aerobic to anaerobic and back again. These bacteria are able to adapt to either condition, although they prefer the aerobic condition.
These three types of bacteria are grouped only by their method of respiration. There are many species of bacteria in a wastewater treatment plant. In a recent study, over 300 species were identified in an aeration basin. However, they can all be classified as aerobic, anaerobic or facultative.