Methane (CH4) is a potent greenhouse gas often emitted in low concentrations from waste sector activities. Biological oxidation techniques have the potential to offer effective methods for the remediation of such emissions. In this paper, methods of improving the CH4 oxidation performance of a horizontal flow biofilm reactor (HFBR) technology, operated at low temperatures, are investigated.Three pilot scale HFBRs were operated over three studies (Study 1,2 & 3) lasting 310 days in total. The reactors were loaded with 13.2 g CH4/m(3)/h during each study and operated at an average temperature of 10 degrees C.In Study 1, nutrients were added to the biofilm via a liquid nutrient feed (LNF) comprising water and nutrient mineral salts. Average removals were 4.2, 3.1 and 2.3 g CH4/m(3)/h for HFBRs 1, 2 and 3 respectively.In Study 2 silicone oil was added to the LNF of all three HFBRs. Average removals increased, when compared to Study 1, by 31%, 79% and 78% for HFBRs 1, 2 and 3 respectively.In Study 3 a non ionic surfactant (Brij 35) was added to the LNF and silicone oil liquid phase of HFBRs 1 and 2. The operating conditions of HFBR 3 were not changed and it was used as a control. A concentration of 1.0 g Brij 35/L proved most effective in improving reactor performance; with removal rates increasing by 105% and 171% for HFBRs 1 and 2 respectively when compared to Study 1.These results indicate the potential of liquid phase optimisation for improving mass transfer rates and removal performances in biological reactors treating CH4. (C) 2014 Elsevier B.V. All rights reserved.