Last year at the Seattle Garden Show, I met the people at the Kings County Wastewater Division. They were handing out samples of GroCo, which is a composted biosolids product.
Yes, that means sewer sludge. But stick with me here. They compost this stuff with sawdust and produce a product that is much, much lower in metals than the mandatory minimums that are imposed on any compost product. And of course they are pathogen-free.
The potential for using worms to digest biosolids and create an even more nutrient-rich product is enormous. Sewage treatment plants all around the country are upgrading to new, clean technology that uses nothing but heat, beneficial microbes, and ultraviolet light to clean the stuff. Already the water is safe to use in irrigation. Feeding the solids to worms and selling or giving away the finished product is the next step. Right now, many sewer treatment plants pay to haul the stuff away, even if it's got the EPA's highest rating for cleanliness and safety.
Now, of course, we need to monitor what's in this stuff and do a better job of preventing people (and industry) from depositing materials in the waste stream that they shouldn't. (Paint and solvents, for instance, should be properly disposed of, not poured down the drain, so they don't end up at the sewer treatment plant to begin with.)
Kings County has been very responsible about this stuff, responding to concerns that locally-produced compost may contain the herbicide Clopyralid by testing GroCo and finding it completely free of the chemical.
Check it out.
Kings County GroCo