In accordance with at least one aspect of this disclosure, a system for dissolving gases into a liquid without side-stream pumping includes a pressure vessel defining a liquid inlet and a liquid outlet, a gas inlet device disposed within an internal chamber of the pressure vessel, a gas source in selective fluid communication with the gas inlet device and the internal chamber of the pressure vessel through a gas control valve and configured to provide a gas pressure, a liquid inlet pipe in selective fluid communication with the liquid inlet of the pressure vessel through a liquid inlet valve, and an outlet pipe in selective fluid communication with the liquid outlet through a liquid outlet valve for discharging the liquid from the internal chamber of the pressure vessel. The gas pressure both facilitates the dissolving of the gas in the liquid and forces the liquid out of the pressure vessel when the liquid is exposed to the gas pressure.