The breakdown of dissolved TCE in a two-step bioremediation system is described. In the first reactor, the organism Methylosinus trichosporium OB3b is grown; in the second reactor, consisting of three 17-L column reactors in series, the cells degrade TCE. A special design allowed both for the addition of air (u(G,s) = 0.01-0.04 mm s(-1)) in the conversion reactor to prevent oxygen limitation while minimizing stripping of TCE, and for the use of methane as exogenous electron donor. In two-step systems presented thus far, only formate was used (excess, 20 mM). We found formate additions could be reduced by 75% (15 degrees C), whereas small amounts of methane (0.02-0.04 mol CH4/g cells) could replace formate and led to equally optimal results. Example calculations show that up to 90% reduction in operating cost of chemicals can be obtained by using methane instead of formate. A model was developed to describe each of the conditions studied: excess formate and optimal methane addition, suboptimal formate addition and suboptimal methane addition. Using parameters obtained from independent batch experiments, the model gives a very good description of the overall TCE conversion in the two-step system. The system presented is flexible (oxygen/methane addition) and can easily be scaled up for field application. The model provides a tool for the design of an effective and low-cost treatment system based on methane addition in the conversion reactor.