The process of innovation has been described as a function of the forces of technology ‘push’ and, in the present study, environmental ‘pull’. In order to assess what form of urban drainage system might be desirable by the middle of the next Century, speculation on the possible evolution of the associated technology, U1d the forms of the ‘ultimate’ standards of ‘environmental quality’ is necessary. Interwoven with these speculations is also the necessity to think through the implications of the scenarios for possible climate change and changes in the social fabric of cities in the future. The paper takes a first step in the direction of such speculation. It is noted that operational definitions of a ‘sustainable’ city, or of ‘environmental quality’ (beyond sustainability), are lacking. Cities, like organisms, are associated with !lows of material and energy. Within the broad context of the global cycles of certain principal materials, and in the absence of a good knowledge of the forces of environmental ‘pull’, the ways in which an urban drainage system of the future might introduce minimal distortion of these ‘natural’ material cycles are explored. Specifically, the cycles of C-, N-, P- and S-bearing materials, together with those of heavy metals, synthetic organic chemicals and pathogens, are examined. These represent the principal categories of pollution associated with the activities of a city. Much of the analysis points towards the desirability of returning the non-aqueous output fluxes of the urban drainage system to the land, as opposed to the aquatic environment This is hardly surprising given the history of social developments (in moving from a rural to urban society). The challenge is to combine the more specific insights from this study of a hypothetical ideal with the obvious practical constraints of existing infrastructures of sewer networks and wastewater treatment facilities.

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