The mathematics of purely mechanical ecological reconstruction would have been stupendous. The largest single water purifying or recycling unit ever developed processed five hundred cubic meters of water per standard minute. Had one million of these units been employed and had they been required to purify every drop of water on the planet, and had no drop ever been processed more than once, the process would have taken more than 50,000 years.
Beyond that, the resource drain on the Empire would have been astronomical. For a self-contained unit to be effective, an incorporated fusion plant was required, with all contaminants processed either reduced to basic elements or elemental hydrogen.
While nature's natural processes, given time, are also effective with the most critical areas of land and water pollution, neither nature nor the puny mechanical aids of man could have been totally effective in reclaiming Old Earth, not in the time scale in which reclamation was actually accomplished.
Dawn for a Distant Earth - chapter:LXXII