TECHNOLOGY FOCUS - BIOLOGY - Preferred Micro-organisms: Mixed or pure cultures of e.g., Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli etc. (11 types listed) and/or types or strains of bacteria isolated from the effluent treatment plants of siloxane producers and/or combinations of these with other types and strains.    TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: A mixture of biogas and oxidising agent (6) is formed by adding gaseous (6), especially oxygen and/or air, to the biogas and then mixed with process liquid (9) in the reactor and optionally transferred to a collecting tank (8). A stoichiometric amount of oxidiser (6) is used, and the amount of (6) fed in is regulated in the tank (8) and/or reactor (4), according to the content of (6) in the liquid (9) and/or biogas (2) in a feed-in or take-off device (3, 7). Mixing is achieved with a distributor (11) in the tank (8), especially a stirrer, and the liquid (9), biogas (2) and oxidiser (6) and/or their mixture are heated to a temperature in the range 20-45 (preferably 28) degrees C, preferably in the tank (8). Fresh process liquid (9) is fed into the circulation from a supply tank (12), preferably via the collecting tank (8) and optionally regulated by level control, and used liquid (9) is removed through a receiver tank (13). The liquid (9) is taken upwards from the tank (8) to the top of the reactor interior (21) by a pump (15) and optionally sprayed evenly into the reactor (4) through a distributor (18), especially a sprinkler head; further distribution is achieved by means of distributor tray(s) (19). The micro-organisms (10) are supported, preferably on a column packing (22) such as swelling clay, plastic packing (e.g. polyurethane foam), Raschig rings etc. and the process liquid (9) may be inoculated, especially at the start of the process, with the same micro-organisms as those on the packing (22). The process may involve counter-current operation, preferably with the biogas flowing upwards and the process liquid downwards, or co-current operation with gas and liquid circulating in the same direction. Preferred Apparatus: Feeder and take-off devices (3, 7) are distributed over the entire diameter of the reactor (4), preferably in the same plane and/or at regular intervals. A device for enriching the biogas with air or oxygen is mounted outside the reactor and the final concentration of oxygen in the purified biogas (2) is 0.1-2 (preferably 0.5-1) %. The circulation system is provided with meter(s), especially for measuring oxidising agent (6), salt concentration, pH etc. The reactor contains distributor tray(s) (19), preferably in the form of perforated plates at regular intervals inside the reactor. The collecting tank (8) (optionally with a stirrer) is used for feeding in and taking off process liquid (9), metabolic products from the micro-organisms, etc.; this tank is fitted with a heater, preferably a heating mantle (16). A mist eliminator (23) is mounted downstream from the reactor (4). The system (1) shows an efficiency of 80-90%. Preferred Process Liquid: Plant fertiliser, especially liquid fertiliser containing nitrogen, phosphorus and potassium in the ratio 3:1:3 and trace nutrients such as 0.01% manganese, 0.002-0.005% zinc and/or copper and possibly taces of boron and/or iron. This is used in a dilution of (1:50)-(1:200), preferably 1:100, based on the original concentration, and shows a pH value of 5-7.5, preferably 6-6.5. The liquid (9) also contains a solvent with a polarity between those of water and siloxanes, especially a surfactant in amounts of 0.1-5 (preferably 0.5-1) % based on (9).