November 19, 2015

The Tricky Business of Composting

Amy Mulvihill

Amy Mulvihill

Freelance Journalist


littlegunpowderfarm3Along with a hybrid in the driveway and solar panels on the roof, an earthy mound of compost decaying in the backyard has long been a signifier of an eco-conscious lifestyle—and with good reason. It is a cheap, easy, and natural way to divert organic waste from a landfill where it would otherwise almost certainly fester and release greenhouse gases, including uber-potent methane and nitrous oxide. In addition, composting results in a soil amendment that can be used to stabilize soils: runoff is reduced, moisture is retained, and crop yields are increased, all of which are ever more important as global population surpasses 7 billion. It is an elegant closed loop.

So why don’t we do more of it? According to a 2014 report on U.S. composting practices produced by the nonprofit Institute for Local Self-Reliance, “almost half the materials Americans discard—food scraps, yard trimming, and soiled paper—is compostable.” And yet only 58 percent of yard trimmings and a measly 4.8 percent of food scraps are recovered every year. What is preventing more widespread adoption?

littlegunpowderfarm4Brenda Platt, the co-director of the Institute for Local Self-Reliance and co-author of the 2014 report, notes that one barrier is a lack of infrastructure. But her dream scenario doesn’t necessarily revolve around the mega-composting facilities that cities such as San Francisco and Seattle use to process organic matter collected as part of their much-heralded mandatory composting programs.

“We advocate for a diverse composting infrastructure, and diverse composting programs,” she says. “Seattle and San Francisco are such remarkable programs. They’re just dealing with tens of thousands of tons of organic material and composting it, but there’s not as much of the community-based or backyard composting happening.”

In an ideal world, Platt would love to see municipalities fund training programs for backyard and schoolyard composting so there could be a tiered network of composting sites starting with the hyper-local (i.e. backyard or schoolyard) and then moving outward to community/neighborhood gardens, urban farms, municipal sites, and then, finally, regional facilities.

Such a system, she says, would offer flexibility, something very much necessary with composting, which can encompass raw materials ranging from food scraps to treated sewage sludge and quantities that fluctuate depending on everything from weather to lifestyle trends. “I think composting in general is a very flexible and forgiving process,” Platt says, “And there’s no one way to do it. There’s such a wide spectrum of scales, technologies, and feedstocks, and even final products.”

Steve Savage, an agriculture technology consultant with a BS in biology from Stanford University and a PhD in plant pathology from the University of California Davis, is, like Platt, dissatisfied with many of the current municipal composting models, though for different reasons.

littlegunpowderfarm1In January 2013, he authored a blogpost titled “The Shocking Carbon Footprint of Compost” that stirred up no small amount of debate and outrage. Mostly, the post extrapolates from the results of one 2001 study that looked at emission from composting cattle manure. The manure was composted in one passive pile (fancy science talk for an untouched mound) and one turned windrow pile (essentially, a long, narrow pile of biomaterial; “turned” simply means the material is stirred occasionally). Because a windrow is a tried-and-true composting format, and “turning” is generally understood to aid the composting process by allowing for better air circulation, the expectation is that the turned windrow would have produced fewer emissions. But the study concluded two curious things. One: the passive pile emitted fewer greenhouse gases. And two: the greenhouse gas emissions from the turned windrow pile were not insignificant—with total GHG emissions expressed as CO2–C equivalent of 240.2 and 401.4 kg C Mg−1 for passive and active aeration, respectively.

Provocatively, Savage writes, “Most people think of composting as a very ‘green’ thing to do, but few realize that composting actually generates a significant amount of the potent greenhouse gases (GHG), methane and nitrous oxide.”

Naturally, this riled the composting faithful, with many questioning Savage’s methods, logic, rigor, and ethics. How, critics wondered, could Savage malign every method of composting based on one study with very specific parameters? (For what it’s worth, other studies do contradict the 2001 study Savage cites, and the EPA declares methane and nitrous oxide emissions from composting negligible.) But at least one more recent study (also involving dairy cattle manure) came to a conclusion similar to the 2001 study, namely that “turned piles emitted about 20 percent more GHG than static piles due to greater emission of CH4 and N2O.”

In any event, Savage insists his goal was not to invalidate composting (which he practices with worm bins at his Southern California home) but to highlight the need for other methods and practices that can complement or even surpass composting, especially in large-scale applications.

littlegunpowderfarm2“Composting is certainly better than sending organic wastes to a landfill, but there are even better options,” he wrote in an email this summer.

In Savage’s mind, chief among these options is anaerobic digestion (AD), a process in which decomposition occurs in airless conditions, and which has long been used by livestock farmers to process manure.

One of the main byproducts of anaerobic digestion is methane, which, if captured, can be used as a fuel to power the digester with the excess potentially sold back into the power grid. Its potential for both waste diversion and electricity generation has many, including Savage, excited.

“I visited the wastewater treatment plant for my community and they have an anaerobic digester that generates enough electricity from methane so that the plant supplies almost all of its own power,” enthused Savage in the email. “Whenever I send waste down the disposal, I know it’s probably the ‘greenest’ option I have.”

Platt also sees the benefits of anaerobic digestion, though she is not as gung-ho as Savage. She notes that anaerobic digesters usually require a big startup investment. Furthermore, they can be complicated to operate, making them impractical for household waste. But, she adds that, for some materials—like manure, wastewater, and even food scraps—it’s an appropriate method of waste diversion. Perhaps its best application, she says, is in using it in conjunction with composting.

“Composting and AD are very compatible,” she says. “If you do have yard waste or a bio-based plastic cup that’s going into an AD facility, it won’t digest, but it will come out as digestate and then that can feed into a compost facility.”

The system Platt describes is almost identical to the system in place in Edmonton, Canada, which has made headlines for a municipal composting program that diverts more than 50 percent of household waste from landfill through recycling and composting. The system’s crown jewel is the Edmonton Composting Facility, which holds the largest co-composter by volume and capacity in North America.

According to Allan Yee, senior engineer of organics processing for the city’s waste management services, Edmonton collects mixed residential solid waste (“food, soiled paper, grass, leaves, etc.”) and, after sorting, sends it to the composting facility. Meanwhile, the city also collects the biosolids that have been processed by an anaerobic digester at the city’s wastewater treatment plant. These biosolids eventually end up mixed with the organic solid waste or separately with wood chips for composting. Along the way, the anaerobic digestion process generates a biogas of 50 to 60 percent methane, which is used for burning in boilers, and earns the city money from the sale of carbon savings credits. Plus, there is, of course, the compost, which is sold as Second Nature Horticultural Compost all over the Edmonton metro area, returning the organic material from whence it came.

Such are the possibilities with the backing of government or industry, but what about the little guy?

About 10 miles north of the Baltimore Beltway, the suburban sprawl of Baltimore County gives way to a landscape of rolling fields divided between the grand estates of gentleman’s farms and homesteads of family farmers.

Connor Horne and Christy Ottinger joined the ranks of the latter in 2013 when they founded Little Gunpowder Farm. The 12-acre farm uses “organic or better standards” to produce vegetables and eggs, which are then sold at area farmers’ markets, through a CSA, and directly to local farm-to-table restaurants. Having earned degrees in environmental science and policy at The College of William and Mary in Williamsburg, Virginia; apprenticed at the Center for Environmental Farming Systems in Goldboro, North Carolina; and worked at a Baltimore urban farm, Horne and Ottinger come to farming with considerable academic and practical experience. Even so, Horne admits compost is a fickle mistress.

“The thing about doing compost is that it’s different every time because you have different ingredients,” he muses while waiting out a summer storm in his barn in June.

At the urban farm in Baltimore, Horne oversaw the compost program for which he created a passively aerated windrow by mixing kitchen scraps dropped off by community members with cardboard, wood chips, and straw. The pile—approximately 6 feet tall by 10 feet long by 6 feet wide—was built upon perforated PVC pipes to allow air to penetrate the mound. Horne has decided to keep it even simpler on his own farm.

“Here we just do a standard windrow pile that we flip every now and then. It’s pretty basic,” he says, as he leads the way across the drive and up a small hill to where three large mounds are heaped. The first, with what looks like an arrow embedded about halfway up one side, is the farm’s compost pile. “This is compost that’s probably close to done. It’s been there for close to a month now, month and a half,” Horne says, retracting the long, thin rod—a dipstick that he’s been using in lieu of a real thermometer—from the pile. (It is recommended that a compost pile’s internal temperature reach between 135 and 160 degrees Farenheit in order to kill off harmful pathogens.) Sure enough, when pulled from the pile, the end of the dipstick is warm to the touch, but not scalding. “Which is good,” Horne notes, “It should be far past the stage when it’s burning hot in there.”

The second pile is kitchen scraps mixed with straw and the third is a deposit of wood chips, just delivered by an arborist with excess. These are the raw materials of Little Gunpowder Farm’s future compost pile, but right now they are standing sentinel over Horne’s newest experiment: two worm bins outlined by cinderblocks and filled with yet more kitchen scraps, plant materials from harvested crops, and dirt, earthworms wriggling contentedly in the crevasses. The compost produced will be used to help start seedlings over the winter.

After spending years obsessing over compost mixtures and methods—“there are some pretty complicated Excel spreadsheets I’ve made,” he cracks—Horne seems to have arrived at both a balanced equation and outlook.

“The easiest rule of thumb is just to add the same volume of the two streams, the nitrogen-rich and the carbon-rich,” he advises. “And that works because most of the carbon-rich stuff, like leaves and sticks, is less dense when you pile it up than the heavy wet [nitrogen-rich] stuff.” Thus, even though you start with equal inputs by volume, the decaying process will result in compost with a desirable 30:1 or 20:1 carbon to nitrogen ratio.

If the ratio is slightly off or the piles don’t get turned as regularly as recommended, Horne isn’t worried. This is composting in the real world. “You can get really, really complicated with compost and it can be really overwhelming if you start reading about the really sort of esoteric recipes and designs and systems for doing it,” Horne says. “But any organic material will turn into compost eventually. It varies in quality and richness and all that, but as long as you get the basic ingredients right, you’re going to make something that’s good for your soil. You don’t really need to stress about it.”

Image of Amy Mulvihill by David Colwell. Images of Little Gunpowder Farm by Amy Mulvihill.

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This work by Amy Mulvihill and the Johns Hopkins Center for a Livable Future is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
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