Biological treatment is one of the most efficient methods for removal of the organic pollutants.

Biological treatment is the decaying of the organic materials found as suspended or dissolved in wastewater by the bacteria and the transformation to the fixed inorganic compounds that flee to the atmosphere which remains in the liquid with biological flocs able to deposit. Biological treatment systems can be classified in different ways. These systems which are classified as Aerobic and Anaerobic according to oxygen situation in the environment can also be classified as Suspended and Biofilm processes based on the conditions of the microorganisms used in the system. Bacterial performance of the system can be affected by the operation problems such as increase in capacity, variable air temperature, growth of undesirable species such as filamentous, equipment malfunctions, changes in pH, salinity, high amounts of grease, chlorinated compounds which can create toxic impact on bacteria or inability to control the entrance of the petrochemical heavy organics to the treatment plant.