URBAN DRAINAGE IN THE 21ST-CENTURY - ASSESSMENT OF NEW TECHNOLOGY ON THE BASIS OF GLOBAL MATERIAL FLOWS

The process of innovation has been described as a function of the forc es of technology ''push'' and, in the present study, environmental ''p ull''. In order to assess what form of urban drainage system might be desirable by the middle of the next Century, speculation on the possib le evolution of the associated technology and the forms of the ''ultim ate'' standards of ''environmental quality'' is necessary. Interwoven with these speculations is also the necessity to think through the imp lications of the scenarios for possible climate change and changes in the social fabric of cities in the future. The paper takes a first ste p in the direction of such speculation. It is noted that operational d efinitions of a ''sustainable'' city, or of ''environmental quality'' (beyond sustainability), are lacking. Cities, like organisms, are asso ciated with flows of material and energy. Within the broad context of the global cycles of certain principal materials, and in the absence o f a good knowledge of the forces of environmental ''pull'', the ways i n which an urban drainage system of the future might introduce minimal distortion of these ''natural'' material cycles are explored. Specifi cally, 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 assoc iated with the activities of a city. Much of the analysis points towar ds the desirability of returning the non-aqueous output fluxes of the urban drainage system to the land, as opposed to the aquatic environme nt. This is hardly surprising given the history of social developments (in moving from a rural to urban society). The challenge is to combin e the more specific insights from this study of a hypothetical ideal w ith the obvious practical constraints of existing infrastructures of s ewer networks and wastewater treatment facilities.