(JP10502863-W)      Treating a liquid containing organic material and/or microorganisms comprises a surface, over which the liquid flows, carrying a semiconductor film (3) which is irradiated such that a photo chemical reaction is produced. A voltage (5) is applied to the semiconductor material to ensure that electrons, liberated by the reaction, are transported away from the semiconductor. Also claimed are: (I) The use of a thermally or aerogel formed semiconductor film for use in the above; and (II) The manufacture of an aerogel catalyst for use in the above comprising heating a semiconductor sol in a fluid phase to 200-300 C at a rate of 1 C/min. and 200-500 psi. The temperature is maintained for 20-40 minutes and the aerogel is then allowed to cool.    USE -   The process can be used in the decomposing of organic solutions or removal of microorganisms from water, e.g. purification of waste containing organic residues or compounds and removal of bacterial pathogens from water supplies.    ADVANTAGE -   The process does not exhibit the environmental problems experienced with traditional methods of disposal (e.g. incineration), is cost effective with respect to e.g. oxidation and does not require the addition of oxidation agents as in photo-oxidation. The semiconductor layer has enhanced oxidation properties, particularly when thermally or aerogel formed. The photochemical reaction efficiency is improved by the voltage applied between the semiconductor film (3) and counter electrode (4) which causes holes to migrate to the film surface (3) to oxidise the organic compounds. Electrons are transported to the counter electrode and ideally to a cathode where reduction of oxygen can take place.    (US5932111-A)      Treating a liquid containing organic material and/or microorganisms comprises a surface, over which the liquid flows, carrying a semiconductor film (3) which is irradiated such that a photo chemical reaction is produced. A voltage (5) is applied to the semiconductor material to ensure that electrons, liberated by the reaction, are transported away from the semiconductor. Also claimed are: (I) The use of a thermally or aerogel formed semiconductor film for use in the above; and (II) The manufacture of an aerogel catalyst for use in the above comprising heating a semiconductor sol in a fluid phase to 200-300 C at a rate of 1 C/min. and 200-500 psi. The temperature is maintained for 20-40 minutes and the aerogel is then allowed to cool.    USE -   The process can be used in the decomposing of organic solutions or removal of microorganisms from water, e.g. purification of waste containing organic residues or compounds and removal of bacterial pathogens from water supplies.    ADVANTAGE -   The process does not exhibit the environmental problems experienced with traditional methods of disposal (e.g. incineration), is cost effective with respect to e.g. oxidation and does not require the addition of oxidation agents as in photo-oxidation. The semiconductor layer has enhanced oxidation properties, particularly when thermally or aerogel formed. The photochemical reaction efficiency is improved by the voltage applied between the semiconductor film (3) and counter electrode (4) which causes holes to migrate to the film surface (3) to oxidise the organic compounds. Electrons are transported to the counter electrode and ideally to a cathode where reduction of oxygen can take place.