NOVELTY - Simultaneous treatment of a methane containing gas and liquid effluent containing ammonium or nitrate and other toxic pollutants, comprises: biofiltration of effluent by percolation through a filter medium comprising an organic material supporting growth of microorganisms degrading pollutants to transform ammonium nitrate; and simultaneously directing gas through filter medium under aerobic condition. The nitrates contained in effluent or product from ammonium, serve as nutrient for growth of methanotrophic microorganisms to convert methane to biomass and carbon dioxide, in filter medium.    USE - The process is useful for simultaneous treatment of a methane containing gas and liquid effluent containing ammonium or nitrate and other toxic pollutants, where the methane-containing gas is landfill gas and the liquid effluent is a landfill leachate (claimed), preferably technical landfill leachate.    ADVANTAGE - The process: utilizes nitrates contained in the effluent or product from the ammonium as nutrient for growth of methanotrophic microorganisms, and the exothermic reaction thus producing a quantity of heat to maintain the temperature of at least a portion of the organic medium within an operational range for the conversion of methane and the degradation of pollutants (all claimed), even during winter periods; the methanotrophic microorganisms can survive in the toxic conditions of the leachate and their growth and activity allows to reduce nitrates in the leachates; and utilizes biofiltration system, which eliminates the need for two specific processing lines for methane and effluent, thus reduces implementation costs and simplifies system design.    DETAILED DESCRIPTION - Simultaneous treatment of a methane containing gas and a liquid effluent containing ammonium or nitrate and other toxic pollutants, comprises: biofiltration of the effluent by percolation through a filter medium comprising an organic material, at least a portion of the organic material supporting the growth of microorganisms degrading the pollutants to transform the ammonium nitrate, and degrading the other pollutants; and simultaneously directing the gas through the filter medium under aerobic condition. The nitrates contained in the effluent or product from the ammonium, serve as nutrient for growth of methanotrophic microorganisms in the filter medium. The methanotrophic microorganisms are adapted to convert the methane to biomass and carbon dioxide via an exothermic reaction, thus producing heat to maintain the temperature of at least a portion of the filter medium within an operating range for the conversion of methane and the degradation of the pollutants. An INDEPENDENT CLAIM is also included for a system for simultaneous treatment of the methane containing gas and liquid effluent containing ammonium or nitrate and other toxic pollutants, comprising percolation biofilter on organic support with fixed biofilm adapted to perform biofiltration of the effluent, the organic carrier is adapted to support the growth of microorganisms adapted to convert the ammonium nitrate and degrading other pollutants, the organic support is also adapted to support the growth of methanotrophic microorganisms adapted to convert the biomass into methane and carbon dioxide via an exothermic reaction, ventilation adapted to introduce the gas based on the organic support and push up through the filter medium under aerobic conditions; capture at base of the biofilter to recover and evacuate the treated effluent; and organic support surface adapted for discharging the treated gas into the atmosphere, where the nitrate content in the effluent or product from ammonium serve as nutrients for growth methanotrophic microorganisms, the exothermic reaction thus producing a quantity of heat to maintain the temperature of at least a portion of the organic medium within an operational range for the conversion of methane and the degradation of pollutants.