BACKGROUND This paper aimed to provide a critical appraisal on maximizing sludge generation and energy conservation in high-rate activated sludge (AS) configurations. The role of gravity settling and the positive attributes of high rate membrane bioreactors were emphasized. The appraisal covered data reported in the literature on 40 different experiments testing high-rate AS configurations for sludge generation and energy conservation. RESULTS In systems with gravity settling, effluent chemical oxygen demand (COD) was higher than 125 mg L-1 in 60% of the experiments, with similarly high effluent soluble COD values. In high-rate MBR systems, permeate COD fluctuated around 12-18 mgCOD L-1; in gravity systems, COD loss in the effluent was higher than 200 mg L-1 in 17 runs (65%) and 300 mg L-1 in 12 runs (46%). Corresponding observed yields were only 9-43%. Membrane systems consistently yielded much higher observed yields of 50-60%. CONCLUSION For energy recovery, AS systems require a major transition, reducing the sludge age to 1.0-2.0 d; only high-rate systems are capable of maximizing energy conservation. This argument does not apply to the contact stabilization process, which is inherently unsuitable for energy recovery. A transition is also needed to replace gravity settling by membrane systems for effective control of particulate and soluble COD components. (c) 2018 Society of Chemical Industry