Disclosed is a system and method for treating wastewater. The system includes a bioreactor which defines a basin for receiving wastewater to be treated; a membrane module in fluid communication with the bioreactor; and a dissolution tank. The tank includes a pressure vessel that contains a portion of the wastewater to be treated and provides a regulated, pressurized gas head space above the wastewater. The tank also includes at least one liquid spray nozzle that permits passage of the wastewater into the gas head space of the pressure vessel and an outlet for discharging the wastewater having a desired gas concentration from the pressure vessel. The system also includes a pumping mechanism for supplying the wastewater to the spray nozzle of the tank such that fluid droplets are formed in the gas head space and the gas contained within the pressurized head space is dissolved into the wastewater.