There is no reason why it wouldn't work, but it would have to a rather large wastewater treatment plant to produce enough Methane for a viable project.
the EPA has already used the GTL F-T technology to transform coal mine methane to synthetic diesel.
click this link http://www.fischer-tropsch.org/DOE/DOE_reports/6202j/epa_6202j.pdf
it's hard to say.
most of those small scale plants are already using the methane from the digester's to run local co-generation plants to reduce their commercial power consumption.
it would depend completely on the size of the plant and what, if anything, they are already doing with the methane.