A gas mixing reactor (GMR) is a special reactor in which gas is used to mix the reaction medium. In water and wastewater treatment, GMR reactors are used to improve the exchange of substances between the microorganisms and the substances to be treated. These reactors are often used in anaerobic treatment processes where the gas (e.g. methane or carbon dioxide) is produced as a by-product of the biological processes and is used to mix the reactor without using mechanical agitators.