NOVELTY - The process fluid is mixed with oxidant in the reactor.    USE - To purify biogas, especially to remove sulphur. To remove hydrogen sulfide by oxidation with the assistance of microorganisms. To purify biogas for conversion into electricity in fuel cells (the foregoing are claimed applications).    ADVANTAGE - Prior art method and plant are simplified. Oxidant, especially air, is not added to the gas ahead of processing. As a result there is no reduction in the quality of the treated gas, as a result of (nitrogen) dilution. It is added instead, to the liquid, and a high oxygen concentration is achievable in solution. There is no reduction in calorific value of the gas produced and no potentially-explosive mixture formation. Because there is no oxygen content in the gas produced, it is suitable for direct use in fuel cells. The reactor is operated continuously. It is not switched between oxidation- and oxygen-enrichment states in alternation. This simplifies the plant and reduces its cost. The plant is particularly compact. Existing plants (digesters) exploiting biogas are readily converted for purification, with few problems. Ambient air is fully suitable as the oxidant.    DETAILED DESCRIPTION - Gaseous oxidant, e.g. air or oxygen, is employed. It is dosed to the process fluid before wetting the microorganisms. For dosing of oxidant, a predetermined quantity of fluid is introduced into the reactor. This is achieved by controlling a liquid column between minimum and maximum levels. On falling below a minimal level, a flow connection between a section of the reactor for oxidant supply and a section for oxidation of the gas component, is interrupted. The connection is a perforated plate with a slide capable of closing its openings. It is connected to the level controllers (18) and pump (11) associated with the liquid column. A bubble column is employed to enrich the process fluid with oxygen. Gas leaving the irrigated bed reactor (3) passes through a droplet separator. Oxygen supply is controlled as a function of oxygen content in the liquid, which is heated, preferably to 20degreesC to 45degreesC. The reactor is at least partially heated by a jacket (30). An instrument measures concentration of salts formed as a result of separating the gas. A lock releases process fluid to remove process fluid containing these products. The process fluid supply control- and locking systems are further detailed.    DESCRIPTION OF DRAWING(S) - A schematic plant diagram is shown, of a first implementation.    irrigated bed reactor (3)    pump (11)    level controllers (18)    heating jacket (30)