On October 8, 2014

The Outside Story: Compost provides hot action

By Madeline Bodin

At the University of New Hampshire’s organic dairy research farm near Durham, N.H., even the heat for the wash water is organic and locally-sourced.

The heat comes from the farm’s composting facility, a building that looks like an eight-bay garage but actually contains cutting-edge composting technology, as well as a whole lot of rotting stuff.

Of course, compost heat doesn’t require sophisticated technology or the attention to detail that doctoral students provide to farm chores. However, managing heat generation is tricky. Even academics and professional composters can’t always get everything in the right balance for perfect decomposition.

Two kinds of decomposition provide most of the action in compost piles. When oxygen is limited (think the fruit skins in the bottom of your smelly garbage pail), anaerobic microorganisms are likely to thrive. They’re not very efficient, they don’t create a lot of heat, and they create noxious gases as a byproduct. Still, despite the smell, anaerobic decomposition has benefits. Throw organic material into an anaerobic digester–a huge, high-tech plastic bag–and you get methane, a fuel that can be used much like natural gas.

What most composters try to do is create a high-oxygen environment where aerobic microbes can shine. These oxygen-loving microorganisms work a lot faster and create a lot of heat in the process. They are responsible for the steam rising from your compost pile on a chilly autumn morning, and the warm water at the UNH research farm. When you see a farmer turning a compost pile, it’s to infuse the whole thing with oxygen and stimulate these little guys.

The specific species of microbes in a compost pile vary, depending on factors such as ambient temperature and what’s on the menu. Many of them are literally as common as dirt. Deborah Neher, chair of the University of Vermont’s department of plant and soil science, looked at the DNA of the microbes in compost piles and discovered that many of them were organisms that also live in decomposing leaf litter on the forest floor.

Neher pointed out the sanitary benefits of hot compost. The heat not only breaks down the material in the pile down, but also kills the pathogens you don’t want there anyway. Most organisms can’t survive a compost pile’s peak temperatures, which can reach about 170 degrees. Initial pathogens that may be lurking in the compost are killed, making a good, hot compost pile self-purifying.

Of course, there are risks with such high temperatures. At the UNH farm, the students are not only constantly monitoring the balance of all the components of the decomposition process to keep the process moving along, they are also on the lookout for signs that the pile may be getting too hot.

“Compost pile fires are more common than people would like to believe,” explained Matt Smith, a doctoral student at UNH’s natural resources department. “These fires start when new, dry organic feedstock is put too close to old, wet compost that is already hot. This easily can happen in towering compost piles that receive little tending,” he said.

That very same decomposition process sometimes ignites fires in hay bales, causing barn fires. Wet hay in the center of the bale supports decomposition and gets hotter and hotter until it ignites the dry hay on the outside of the bale.

Getting the heat just right means keeping many things in proportion, but the main factors are oxygen, water, and the carbon/nitrogen balance, said Smith. Carbon is the microbes’ fuel and nitrogen is their building material. When there is not enough carbon, things cool down. Smith aims for a balance of 25 parts carbon to one part nitrogen.

Research on the compost heat system at the UNH organic dairy farm is moving beyond its earliest stage. According to Smith, the farm has applied for grants to expand the use of compost heat on the farm. If all goes well, in the future, compost will heat not only the farm’s water (which currently gets a boost from other energy sources) but also a greenhouse and wood chips that are used for the cows’ bedding.

Everything comes together in that eight-bay building. Cow manure and bedding get recycled in a sanitary way. Nutrients are returned to the hayfields when the compost is spread, and hot water keeps the dairy equipment sparkling. Who knew that life on the farm could be such hot stuff?

Madeline Bodin is a writer living in Andover, Vt. The illustration was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine.

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