Cogenerating plants, require an accurate maintenance. Part of this maintenance regards the lubricating oil change. This oil has to be changed since it becomes quickly polluted from acids as Hydrogen Sulfide and from Hydrogen Chloride. This acids becomes from the binding of water contained in air and Sulfuric Acid (H2S) and Chlorine (Cl), contained in methane obtained respectively from the fermentation of animal wastes or from rubbish dumps. This acids, once dispersed in lubricating oil, intake the inner engine parts, creating serious corrosion damages. Due to what written, lubricating oils must be changed frequently to maintain as low as possible the acid concentration. A customer of us, utilizer of a cogenerating system supplied by methane obtained by animal waste fermentation, had the problem of a very high intake percentage of H2S and asked us for a system to lower this percentage.