It is a biological process for treating sewage and industrial wastewaters.
Over the years, microbes have been used to treat sewage (waste water) by the process of activated sludge formation as it helps recycle the water in nature. It is a measure for secondary treatment of sewage water. In this process sludge is used which contain bacteria and algae.
While this process is fairly efficient for the removal of BOD, COD and nutrients it seems to be expensive.