Sewage stinks. But did you know that bacteria can make it smell better?

Certain forms of bacteria feasting on sewage give off hydrogen sulfide, a really malodorous chemical, and one of several that give sewage its "pleasing" smell. In general this problem has been treated with powerful chemicals like bleach--not at all good for the environment.

Marc Deshusses, of University of California Riverside, argues that we could use other bacteria instead. His technique is catching on. There's already a sewage treatment plant in California that's using bacteria that degrade hydrogen sulfide, using the oxygen in air, to get rid of the stink.

The payoff isn't just environmental. Using "biofilters," as this kind of system is called, means plant operators can decrease the amount of dangerous chemicals onsite. Biofiltering is also cheaper than chemically treating the problem with bleach or lye.

So, some bacteria break down sewage. They give off hydrogen sulfide. Other bacteria break down hydrogen sulfide. They give off hydrogen sulfate--better known as sulfuric acid-- almost odorless at the fairly low concentrations produced.