Whether sustainability can, or should, be defined in a practical operational sense, it is clear that the emergence of such a notion has prompted what seems to be a profound re-thinking of whether our society, economic system, and technology are as we would wish them to be. Sustainable development, clean technology, life-cycle analysis, pollution prevention, and so on, are expressions of a willingness to leave no stone unturned, as it were, in the search for what would be appropriate. With respect to the design and operation of a city's wastewater infrastructure, in particular, this search is characterised by a seeming explosion in the possible combinations of appropriate technologies, gross uncertainty about how novel technologies - only now emerging - might perform in the very long term, and a continuing absence of specific criteria of sustainability for determining the grounds on which any candidate technology might be preferred over another. The paper introduces a simple computational procedure for generating and screening candidate combinations of unit-process technologies for an urban wastewater infrastructure. This is based on the use of Monte Carlo simulation, with the identification of those specific technologies (and combinations thereof) that appear to have the greatest probability of being selected for use under different, possibly evolving, criteria of sustainability. Application of the procedure is illustrated with respect to just a part of this infrastructure, i.e., the wastewater treatment plant.