A system and method are disclosed for processing grease trap waste to produce brown grease oil. Heat to promote waste grease stratification is provided by combustion of brown grease itself. A portion of the brown grease, which would normally be transported to an end user, is retained in a separate holding vessel at the grease processing site. The brown grease within the holding vessel is thermally conditioned to adjust viscosity so that it may be drawn through one or more pumps to increase pressure for atomization in a boiler. The heat of the brown grease combustion is applied to grease trap waste in the containment vessel by heating tubes immersed in the grease trap waste in which a heat transfer medium circulates and gives up its heat to the grease trap waste to thereby promote stratification of the grease trap waste and production of the brown grease oil.