A system and method for treating wastewater includes a treatment container for receiving and treating wastewater and a mixing/aeration system disposed in the treatment container. Wastewater is pumped under pressure from the treatment container through a solids/liquid separation system, preferably comprising multiple parallel membrane bioreactors, to separate the solids and liquids in the wastewater. Liquid retentate from the solids/liquid separation system is cycled back to the treatment container via the mixing/aeration system. The closed-loop system and method for solids/liquid separation and aeration/mixing provides for reduced equipment requirements and energy usage during operation.