The Green Plate: When the Sludge Hits the Fan

ColumnWhat, exactly, is in your city’s free “organic” compost?

Have you ever wondered what happens to the food scraps and yard waste that your city picks up as compost? Where does it go? What becomes of all those avocado peels and geranium stalks?

As more cities go green, they’re making it easy for businesses and residences to compost by picking up such waste and hauling it away. This great city service keeps food scraps out of landfills where they can generate methane, a potent greenhouse gas, and also conserves space in our landfills.

Some cities even give away the resulting compost free to home gardeners who want to amend their soil. While smart in theory, before you accept free compost from your city, you’ll want to find out what else is in the mix. Even the greatest green practices sometimes have a flip side.

The city of San Francisco stepped in a big steaming pile of, well, over its free “organic” compost giveaways. One problem was the use of the term “organic” to describe a substance that is most likely organic, but not in the way usually applied to food. The issue is heating up again after an article in the Washington Post was published last week about a facility on the East Coast that will soon produce a similar type of compost.

Few home gardeners are aware that the free compost given away by cities is often a mixture of waste water (including human waste and everything we put down the drains in our homes and businesses), and yard and food waste. Hardly the organic melange you want to spread around your yard.

All over the country, waste water treatment plants utilize methane digesters to convert copious amounts of waste into electricity. Most of these plants have excess capacity, so green city programs have added food waste to the mix of human, household, and industrial overflow that is put into anaerobic digesters. The digesters then use bacteria to break down the waste and release methane as a byproduct. The plants then capture the methane and use it as a renewable source of energy to power their own operations. The leftover sludge is repackaged and sold or given away as compost.

The problem with this practice is that we don’t really know if this compost is a safe medium in which to grow crops.

The Environmental Protection Agency (EPA), endorses the practice, yet its own surveys have found a variety of heavy metals, semi-volatile organics, polycyclic hydrocarbons, pharmaceuticals, steroids, hormones, and flame retardants.

What we don’t know (and neither does the EPA), is whether the concentrations of these substances are dangerous, whether they are taken up by plants and transferred to humans, how toxic they are, or how quickly they break down.

Not all cities that give away free compost are necessarily giving away sewage sludge. I checked in with the program in Berkeley, CA. The information on their website states that the compost given away to Berkeley residents is a safe, organic-certified product. It’s made from green waste in an aerobic process that includes temperature and moisture control as well as daily turning. The carbon to nitrogen ratio and C02 levels are monitored to create a high quality microbial compost.

Until we know more about the substances present in treated sludge, and the possible effects on our health, beware of municipalities bearing free compost. Or at least find out what’s in your city’s compost before you dig in.

Image: sk8geek via Flickr

This is the latest installment in Vanessa Barrington’s weekly column, The Green Plate, on the environmental, social, and political issues related to what and how we eat.






Vanessa Barrington

Vanessa Barrington is a San Francisco based writer and communications consultant specializing in environmental, social, and political issues in the food system.