Some Earthlings, not content to flush everything down the drain, already take their waste into their own hands.

Composting toilets can be purchased from more than a dozen manufacturers or resellers, with names like Water Conservation Systems Inc., AlasCan of Minnesota, Inc., Nature-Loo and Rota-Loo. A few municipalities and parks have tried them out. And a handful of eco-friendly subdivisions suggest or require their installation. But these are generally small-scale problem-solvers -- where sewage treatment systems aren't available -- or conscientious decisions.

Critics charge that not enough is known about the potential dangers to groundwater and to the supply of human food when processed human waste is used on a large scale as compost or fertilizer.

Other complaints are in the wind.

"Local governments are increasingly trying to ban or regulate further [use as fertilizer] because neighbors to application sites are complaining about odors and health effects," Harrison said.

In what might not surprise many readers, human waste, it turns out, stinks.

The pieces and parts of MELISSA are being built in various research labs in Europe and Canada. Construction of the project began in 1995, and now early versions of two of the three fermentation chambers are up and running in Barcelona. The complete pilot plant is expected to be fully operational, with all five chambers and all three rats, by 2005.

The first chamber will liquefy waste using thermophillic anaerobic fermentatives. The second will degrade fatty acids, carbon dioxide, sulphers and other unwanted substances, employing small critters called thiocapsa roseopersicina and rhodobacter capsulatus. And nitrifying bacteria do the finish work in chamber 3.

"MELISSA is a bit like a lake," Lasseur said. "At the bottom is sludge (raw waste) which undergoes anaerobic (without oxygen) fermentation in darkness. Higher up there's light but no oxygen. At the surface, there's carbon dioxide, oxygen and light. This is where higher plants can thrive."

Meanwhile, the European researchers say versions of MELISSA might prove attractive to the casino industry, contractors of large office buildings, and others who generally build spaces where indoor air quality is an issue.

Down the road, an eventual design of a sophisticated Mars toilet depends on the ultimate culinary expectations of future Mars diners.

"The size of MELISSA will ultimately depend on the number of astronauts and the quality of the food you want to grow," Lasseur said. "You would need about 10 square meters (108 square feet) growing area per person if you want to live on wheat, far less if you’re happy with algae."

One is then compelled to wonder how a diet of algae might affect MELISSA's loop.