NASH liquid ring pumps draw the digester gas off the top of the digester and recirculate it to improve mixing and rate of reaction. The pump's operating liquid cools the digester gas by absorbing the heat of compression, thus providing an almost isothermal mode of operation. This has a positive effect on the service life of downstream valves, gas burners and motors, and it reduces maintenance costs. The cooling function of the operating liquid simultaneously acts as a flame arrester and reduces expenditures for explosion protection. The contaminated raw gas is also cleaned in the water ring as though in a gas scrubber and is partially dried. The gas is thus oxygenated and returned to the wastewater treatment process.