When San Francisco, one of the greenest cities in America, offered its residents free compost, many were excited to take it. After all, purchasing enough compost for even a small 10 x 10-foot garden can cost over $50, and generating one's own compost in high enough quantities for such a garden takes a long time.

Few of the gardeners who lined up to receive the free compost at events like last September's Big Blue Bucket Eco-Fair suspected that the 20 tons of free bags labeled "organic biosolids compost" actually contained sewage sludge from nine California counties. On Thursday, March 4, angry San Franciscans returned the toxic sludge to the city, dumping it at Mayor Gavin Newsom's office in protest.

Sewage sludge is the end product of the treatment process for any human waste, hospital waste, industrial waste and — in San Francisco — stormwater that goes down the drain. The end goal is treated water (called effluent), which San Francisco dumps into the Pacific Ocean and San Francisco Bay. But the impurities and toxins removed from the water do not go away. With the water removed, the remaining byproduct is a highly concentrated toxic sludge containing anything that went down the drain but did not break down during the treatment process. That usually includes a number of heavy metals, polycyclic aromatic hydrocarbons, pharmaceuticals, steroids, flame-retardants, bacteria (including antibiotic-resistant bacteria), fungi, parasites and viruses.