August
2014
Controlling Compost
Biochemistry for Heat
production
A White Paper
Toby Gouker, Ph.D.
© 2014 Triea Systems, LLC
Compost Biochemistry for Heat Production
“A good compost pile should get hot enough to poach an egg, but not so hot it would cook a lobster.” –
Anonymous
Introduction
Composting of manure is nature’s way of recycling the energy in materials that has otherwise passed its
useful life. Composting involves air, microorganisms, fuel, and water to oxidize and break down, or
biodegrade, complex materials into simpler elements and molecules.
The conversion of litter and manure to rich humus during composting involves several types of bacteria
and fungi. Fungi initiate the composting process by breaking down cellulose and other complex carbon
containing molecules in the waste. Fungus populations increase rapidly as composting progresses. The
temperature inside the pile may rise to 150-160F, inactivating harmful disease organisms. After several
months, the temperature decreases, the fungi disappear, and millions of bacteria continue gradual
breakdown of the organic materials into rich, dark, crumbly humus.
The fungi and bacteria feed on the organic material and, through respiration, generate energy that they
use for movement, growth, reproduction or stored energy. The byproduct of microorganism respiration
is excess heat. As microbes consume carbon and respire oxygen, they produce the gas carbon dioxide.
The reaction that changes carbon to carbon dioxide is ‘exothermic’, which means energy is released in
the form of heat during the reaction.
Air
An aerobic composting process consumes large amounts of oxygen. During the first days of composting,
easily degradable components are greatest during the early stages and then decrease as the process
continues. If the supply of oxygen is limited, the composting process slows and the process becomes
anaerobic (without oxygen). A minimum oxygen concentration of 5% within the pore spaces of the
composting material is recommended for a well-managed compost pile.
In addition to providing oxygen, aeration removes heat, water vapor and other gases trapped within the
composting materials. In fact, the required rate of aeration for heat removal can be 10 times greater
than for supplying oxygen. Temperature frequently determines how much and how often aeration is
required.
Microorganisms
As a matter of convenience, biologists have classified the temperature ranges in which certain
microorganisms are most active and given these ranges names. The two ranges f importance are known
as Mesophilic and Thermophilic. Mesophilic bacteria thrive from 70° to 90° F, but just survive at
temperatures above and below (40° to 70° F, and 90° to 110° F). In many composting piles, these midrange bacteria do most of the work. However, if conditions are right, they produce enough heat to
activate the thermophilic, or heat loving bacteria. Thermophilic bacteria work fast. Their optimum
temperature range is from 104° to 160° F. For completeness it is worth mentioning that some bacteria
© 2014 Triea Systems, LLC — www.trieasystems.com
Compost Biochemistry for Heat Production
can even decompose organic material at temperatures below freezing. These are called psychrophilic
bacteria, and while they work best at around 55°, they can continue to work down to as low as 0° F.
Fuel
When living organisms that use oxygen feed upon organic matter, they develop cell protoplasm from
the carbon, nitrogen, phosphorus, and other required nutrients. Carbon serves as a source of fuel
energy for organisms and is burned up and respired as carbon dioxide (CO2). Since carbon serves both as
a source of energy and as an element in the cell protoplasm, much more carbon than nitrogen is
needed. Generally, organisms respire about two-thirds of the carbon they consume as CO2, while the
other third is combined with nitrogen in the living cells.
Biological activity diminishes if the compost mix contains too much carbon in relation to nitrogen.
Several cycles of organisms are required to burn excess carbon. When organisms die, their stored
nitrogen and carbon become available to other organisms. These new organisms form new cells which
again need nitrogen to burn carbon and produce CO2. Thus, the amount of carbon is reduced and the
fixed amount of nitrogen in the compost is recycled. Phosphorus, potash, and various micronutrients are
also essential for biological growth. These are normally present in more than adequate amounts in
compostable materials.
Water
Like all other life, compost organisms need water to survive and thrive. While life-giving, water also
provides some microorganisms the ability to move, using a film of water to slip and slide to another
section of the pile. If adequately aerated, compost with moisture content between 30% and 100% will
be aerobic. For efficient composting, however, high moisture content must be avoided because water
displaces air from the interstices between the particles causing anaerobic conditions. Conversely, low
moisture content must also be avoided because it deprives organisms of water needed for their
metabolism, and inhibits their activity. Ideally, compost piles should contain 40 - 60% moisture. It should
feel as moist as a wrung out sponge. Dry manure can be watered as the pile is built, then water can be
recycled as needed to control moisture levels to an optimum range.
Control of Compost Biochemistry
Air, microorganisms, fuel and water are the components that generally dictate how much, and for how
long, heat can be generated from composting manure or litter. The engineers and scientists at Triea
Systems have researched many different configurations for the optimum generation of heat from
composting manure and litter. From open windrows, both static and aerated, to covered and
containerized systems, Triea has studied the biochemistry of composting to generate metrics that will
optimize conditions for a wide variety of configurations. When an optimized compost operation is
coupled with Triea’s Biomass- HRS™, an on-farm, sustainable heat source can be created to meet the
majority of a farmer’s needs for heat in their operation, while substantially reducing energy costs.
© 2014 Triea Systems, LLC — www.trieasystems.com