A novel horizontal flow biofilm reactor (HFBR), recently developed and optimised as a wastewater treatment technology, has been adapted and tested for its efficacy in treating hydrogen sulphide (H2S) and methane (CH4) gas. 6 pilot scale HFBR reactors were designed and commissioned, 3 each treating CH4 (HFBR 1, 2 and 3) and H2S (HFBR 4, 5 and 6) respectively. The reactors were operated at 10oC, typical of ambient wastewater and air temperatures in Ireland and were simultaneously dosed with air containing the gas in question and with a synthetic wastewater. The methane reactors were operated over 3 phases (Phases 1 – 3) which lasted 215 days in total. During each phase the air mixture flow rate (AFR) and top plan surface area (TPSA) loading rate to the 3 CH4 reactors was 1.2 m3/m3 reactor/hr and 0.6 m3/m2 TPSA/hr respectively. The average CH4 loading rate was 8.6 g CH4/m3/hr (4.3 g CH4/m2 TPSA/hr). Despite the low operating temperature, CH4 removal efficiencies (RE) of up to 88.3% were observed at an empty bed retention time (EBRT) of 50 minutes.Triplicate reactors treating an air mixture containing H2S, were loaded at an AFR of 15 m3/m3 reactor/hr (7.5 m3/m2 TPSA/hr) with an average H2S loading rate of 3.34 g H2S/m3/hr (1.67 g H2S/m2 TPSA/hr). After 35 days of operation, the RE reached 100% for all 3 reactors at an EBRT of 4 minutes.In each of the reactors, profile samples of biofilm, air and liquid were taken periodically from various regions of the HFBR. These allowed detailed description of removal processes and optimisation of the reactors by detailing changes in air, liquid and biofilm composition as air moved through the reactor.The results to date indicate that the HFBR has excellent potential to biologically treat odorous and greenhouse gases in an effective manner, lowering emissions of odours, toxins and environmentally hazardous gases.