text: The activated sludge process is an example for a more high-tech, energy intensive or "mechanized" process that is relatively expensive compared to some other wastewater treatment systems. It can provide a very high level of treatment.
  Activated sludge plants are wholly dependent on an electrical supply to power the aerators to transfer settled solids back to the aeration tank inlet, and in many cases to pump waste sludge and  final effluent. In some works untreated sewage is lifted by pumps to the head-works to provide sufficient fall through the works to enable a satisfactory discharge head for the final effluent.  Alternative technologies such as trickling filter treatment requires much less power and can operate on gravity alone.