One of the most difficult problems influencing wastewater treatment in the developed world is the treatment and disposal of sewage sludge. Sewage sludge is often disposed of either spreading it over land used for agriculture or by making a cake that may be deposited in a landfill or burned. Water needs to be taken out in order to make a sludge cake. The manner in which water is bonded to the flocs or sludge particles limits the amount of water that may be removed by pressure-based dewatering technologies. In order to drive water under an electrical gradient from the positive (anode) electrode to the negative (cathode) electrode, a DC voltage must be applied across the sludge. The typical metallic anode dissolves quickly in the acidic environment produced by the electrolysis of water, which is one of the reasons this technology has not been used in practice. Currently, membrane separation, ion exchange, electrodialysis, and adsorption are the chemical and physical treatment techniques utilized to dispose of wastewater. But these treatment techniques always result in some issues, such excessive expense, and secondary pollution, hence these technologies are only used under specific circumstances. On the other hand, biological treatments—particularly the activated sludge process—are recommended due to their superior economic performance and ability to prevent secondary pollution. Accordingly, this chapter will discuss the different biological treatment technologies of sewage/wastes, also called secondary treatment methods, in a very comprehensive way.