The co-production of biodiesel and methane gas from grease trap waste (GTW) was evaluated and compared against theoretical predictions of methane production from sole anaerobic digestion of GTW. The GTW was first processed into two separate phases comprised of fats, oil, and grease (FOG) and high strength wastewater (GTW wastewater). The GTW wastewater was then anaerobically digested in bio-char packed up-flow column reactors to produce methane gas and a low-strength wastewater effluent while the FOG phase was set aside for conversion into biodiesel. Anaerobic digestion efficiencies that yielded chemical oxygen demand (COD) reductions up to 95% and methane headspace concentrations between 60 and 80% were achieved along with FOG to biodiesel conversion efficiencies of 90%. Methane production yields (m(3) per kg COD reduced) achieved theoretical maximums with near total depletion of the volatile organic acids. High resolution images of biochar samples confirmed extensive coverage with thick biofilm communities. Microbial analysis revealed broad spectrum populations of anaerobic bacteria that ferment organic substrates to produce acetate, ethanol, and hydrogen as major end products as well as archaeal populations that produce methane gas. Energy calculations validated the co-production of biodiesel and methane gas from GTW as a competitive option relative to its co-digestion with sewage sludge. (C) 2013 Elsevier Ltd. All rights reserved.