The idea behind all biological methods of wastewater treatment is to introduce contact with bacteria (cells), which feed on the organic materials in the wastewater, thereby reducing its BOD content. In other words, the purpose of biological treatment is BOD reduction.

Typically, wastewater enters the treatment plant with a BOD higher than 200 mg/L, butprimary settling has already reduced it to about 150 mg/L by the time it enters the biological component of the system. It needs to exit with a BOD content no higher than about 20-30 mg/L, so that after dilution in the nearby receiving water body (river, lake), the BOD is less than 2-3 mg/L. Thus, the biological treatment needs to accomplish a 6-fold decrease in BOD.

Principle:

Simple bacteria (cells) eat the organic material present in the wastewater. Through their metabolism, the organic material is transformed into cellular mass, which is no longer in solution but can be precipitated at the bottom of a settling tank or retained as slime on solid surfaces or vegetation in the system. The water exiting the system is then much clearer than it entered it.

A key factor is the operation of any biological system is an adequate supply of oxygen. Indeed, cells need not only organic material as food but also oxygen to breathe, just like humans. Without an adequate supply of oxygen, the biological degradation of the waste is slowed down, thereby requiring a longer residency time of the water in the system. For a given flowrate of water to be treated, this translates into a system with a larger volume and thus taking more space.

Advantages:

Like all biological systems, operation takes place at ambient temperature. There is no need to heat or cool the water, which saves on energy consumption. Because wastewater treatment operations take much space, they are located outdoor, and this implies that the system must be able to operate at seasonally varying temperatures. Cells come in a mix of many types, and accommodation to a temperature change is simply accomplished by self adaptation of the cell population.

Similarly, a change in composition of the organic material (due to people’s changing activities) leads to a spontaneous change in cell population, with the types best suited to digest the new material growing in larger numbers than other cell types.

http://engineering.dartmouth.edu/~cushman/courses/engs37/BioTreatmentTypes.pdf