ABSTRACT
The biological treatment systems discussed to this point (ponds, trickling filters, and rotating biological contactors) have been around for years. The trickling filter, for example, has been used successfully since the late 1800s. The problem with ponds, trickling filters, and rotating biological contactors is that they are temperature sensitive and remove less biochemical oxygen demand (BOD); also, trickling filters, for example, cost more to build than the activated sludge systems that were later developed.
