The world is becoming ever more populous and urbanized. Cities are inherently unmitigated environmental "bads"; with no extenuating circumstances; like bulls in china shops. Man's burden on the environment --- woe, that is --- will continue to be piled upon woe. So runs the popular mind-set.
Yet things do not have to be this way, no matter how hard it may today be to 
conceive of cities as forces for good in the environment. Far from the burden of infrastructures having to compensate for the ills of cities, the two should "act" deliberately to contribute positively to enhancement of the environment about them. That is our grand challenge for Engineering; and this is how we might begin to think of responding to the challenge.
In introducing their concept of the "urban ecological footprint" --- massive, of course, for cities such as Paris, New York, and so on --- William Rees and Mathis Wackernagel invite us to conceive of the city as a "large animal grazing in its pasture". We imagine that animal as a bull. The "bull" of intense social and economic activity in the city is to be shod, we suggest, with the "padded athletic trainers" of re-engineered infrastructures and imbued with a technological deftness and intelligence sufficient for restoring the business of running the environmental "china shop" in which it charges about --- indeed, profitably expanding the shop's operations.
The city, continuing the large grazing animal analogy, takes in its daily grass and daily water, together with life-sustaining breath; and we, for readily 
understandable but increasingly unsustainable reasons, have engineered the return of the residuals of this metabolism to the air, water, and land environments surrounding the city. In the Global North, a good deal of the city's daily water is used to remove the residuals of its daily grass --- as wastewater --- so that citizens can lead healthy and productive lives. And much technological effort has been invested in treating that wastewater, not always to the good of the air, missing an opportunity to benefit the land, while not being a wholly unmitigated good for the water environment. In short, wastewater treatment in the Global North can end up shunting nitrogen into the atmosphere, to avoid fertilizing the aquatic environment, while we labor awfully energetically with the Haber-Bosch process to pull that nitrogen out of the atmosphere to produce industrial fertilizer.
How, then, can the built infrastructure be re-engineered to restore the natural capital and ecosystem services of the nature that occupied the land before the city arrived there; how can they be re-engineered to enable the city to act as a force for good, deliberately to compensate positively for the ills of the rest of Man's interventions in Nature?
And how can cities of the Global South avoid adopting the same technological 
trajectory? Can they, in other words, leapfrog the Global North by missing out the entire human-waste-into-the-water-cycle phase, thereby ending up one step ahead, as it were?
How, more profoundly, can the engineering of city infrastructure be deployed expressly so that those at the bottom of the pyramid of dignified human development may be brought to such a level where they care to engage in such a debate, over such a grand challenge for the next century --- of cities as forces for good --- beyond their desperate needs of survival for just today and tomorrow?