UPcycling, returning products to a useful life over and over again, is a concept that McDonough and Braungart see as key in their conception. REcycling is not enough. Recycling is down-cycling and ultimately leads to the dump... the process is simply delayed. It still conforms to the paradigm of cradle to grave that the authors seek to change.
One half of the cradle to cradle conception is bio-metabolism. This one is easy enough to understand, and seems most immediately do-able. It would seem that nature can filter and re-use biological waste if it is properly returned and devoid of excessive poisons. Even here, the use of wetlands and plant life to filter sewage and manufacturing bio-waste is proposed and examples given.
I had to chuckle though. Contaminated biological waste is known to cause genetic damage to wildlife. Might it not do the same to plant life if used as wetlands and swales to cleanse water? Perhaps plants are not as important as animals... although I occasional rail in a kidding way that when a cow is milked, the cow remains unharmed, but when soy milk is used... well, millions of soy beans gave their lives for that.
But we are animals and consume other life to sustain ourselves. We don't photosynthesize.