Systems and methods for upgrading conventional activated sludge plants

A conventional activated sludge wastewater treatment plant is upgraded by adding one or more of a membrane filtration unit, a membrane aerated biofilm reactor (MABR) unit, and a screen. The membrane filtration unit is added between a process tank and the secondary clarifier. The membrane filtration unit extracts treated effluent at a rate up to 25% of the influent flow rate. The plant is not converted into a membrane bioreactor (MBR) since the MLSS concentration is still less than 4,000 mg/L. The membrane-aerated biofilm reactor (MABR) unit is added to a process tank of the plant and provides attached growth biological treatment. The screen extracts solids from water flowing to the process tanks. When optionally used in combination, the added units and processes increase the capacity of the primary separation, biological processing and secondary separation functions of the plant. The influent flow rate may be increased.