Upgrading a wastewater treatment plant may be necessary to meet the existing effluent quality and/or to meet the stricter future effluent quality requirements and/or to increase its capacity because of population growth or sewerage expansion to serve more areas. Inability to meet the existing effluent quality may result from lack of proper plant operation and maintenance, inadequate plant design capacity and increased hydraulic or organic loading rate caused by a change in wastewater flow or characteristics.

As the space available at a WWTP is limited, processes are required which can accommodate the need for increased treatment capacity and/or improvement of effluent quality without requiring much more space.

In general, no standard solutions for the upgrading of wastewater treatment systems are available and the transfer of a certain measure from one case to another is limited. In fact, the optimum answer in each individual case is largely site dependent. Accordingly, there is a broad spectrum of measures that might be employed. In most cases it is not a singular action to be taken, but the upgrading will incorporate a set of different interlocking components both at the drainage system and the wastewater treatment plant.