According to this method, a filter media to fix organisms on it is immersed and aerated. The purification principle is the same as a sprinkling filtration process and a rotating biological contactor. Like the other methods, the removal ratio in general reduces if a pollution load is high, but a proper pollution load makes purification ability high. In addition, since large protozoans appear and the amount of attached organisms are much, the process is tolerative to load fluctuation, and a bulking is hard to occur.
Excess sludge is relatively less generated.
The equipment cost is relatively more expensive than an activated sludge method. The operation cost is the power one for an aeration, and expensive due to its continuos operation. The operation and maintenance,however, is easy.