Hopefully, you're not just throwing your used coffee grounds in the garbage ... are you? Not only are they compostable, but they can also be used in robot hands, biofuel engines for cars, warm sports clothing, and as printer ink. Now, it turns out that they have yet another use - a scientist from The City College of New York has discovered that they're good at soaking up stinky sewer gas.

Specifically, the grounds have been found to act as an effective filter for hydrogen sulfide (H2S) gas. Not only does H2S have a rotten-egg reek, but it can also be dangerous. The intense odor can overwhelm peoples' sense of smell, to the point that that they become desensitized to it - people working around the gas, such as sewer workers, have died from overexposure to H2S as a result.

City College professor of chemistry and chemical engineering Dr. Teresa Bandosz realized that coffee grounds could be made into a particularly H2S-hungry type of activated carbon filter, due to their caffeine content. Caffeine in turn contains nitrogen, and nitrogen increases carbon's ability to capture airborne sulfur. Existing carbon-based sulfur filters require the addition of chemicals such as ammonia, melamine, or urea to bring their nitrogen content up to scratch. Used coffee grounds, by contrast, are an abundant resource that don't require the expense of a nitrogen boost.

The grounds can't just be used straight out of the Mr. Coffee filter, however.

Dr. Bandosz and her team started by preparing a slurry of coffee grounds, water and a "chemical activator," zinc chloride. They dried the mixture, then proceeded to bake it at 800ºC (1,472ºF). The resulting carbon particles were found to contain a multitude of tiny nitrogen-lined holes, which are apparently perfect for trapping hydrogen sulfide molecules wafting through the material.

It is hoped that the research could result in a commercially-available eco-friendly H2S filter.

A paper on the project was recently published in the Journal of Hazardous Materials.