Compost

"Of the humble things that might save this imperiled planet, compost is near the top of the list." - Bill McKibben, environmentalist, educator, bestselling author, and cofounder of 350.org

Decomposition is the process which creates compost from waste food products and other organic material, including foliage and animal/human waste. The process turns the waste in compost which can then be used as a fertiliser for growing veg/plants etc. It also produces heat and gas as by-products which can be harvested and used. 

The Pain Mound is a large pile of woody biomass, aka mulch. Invented by French farmer Jean Pain in the 1970s, it is made of woodchips and sawdust, surrounded by a ring of hay bales for structure and insulation. As the Pain Mound decomposes, heat is produced and harnessed using a hydronic loop. The Pain Mound will produce heat for up to 18 months, after which time the remains (nutrient rich, earthy humus) can be used to build soil.

Decomposition is an exothermic process - it generates heat

In the process of composting, microorganisms break down organic matter and produce carbon dioxide, water (leechate), heat, and humus - the relatively stable organic end product (compost). Under optimal conditions, composting proceeds through three phases:

1) the mesophilic, or moderate-temperature phase, which lasts for a couple of days,

2) the thermophilic, or high-temperature phase, which can last from a few days to several months, and finally,

3) a several-month cooling and maturation phase.

Bacteria are the smallest living organisms and the most numerous in compost; they make up 80 to 90% of the billions of microorganisms typically found in a gram of compost. Bacteria are responsible for most of the decomposition and heat generation in compost. They are the most nutritionally diverse group of compost organisms, using a broad range of enzymes to chemically break down a variety of organic materials.

Many microbes need oxygen, just as animals need oxygen. During the early stages of composting, the oxygen-loving microbes predominate. The microbes combine the oxygen with the carbon from the decaying matter. In that way the microbes produce energy in the form of heat. This makes the compost pile warm.

aerobic composting:
organic materials + oxygen + water = carbon dioxide + water + energy

If the oxygen in the pile is not replenished by stirring or aeration, the microbes that do not need oxygen tend to take over. These are the anaerobic bacteria.They do not produce heat. They do produce a good deal of ammonia, that gives off a tell-tale smell. The ammonia is a waste product; it comes from the microbes as they seek to discard the unneeded nitrogen in their small bodies. Another gas that can get produced in this anaerobic state is hydrogen sulfide, which smells like rotten eggs.

anaerobic composting:
organic materials + water = carbon dioxide + methane + hydrogen sulfide + energy

http://www.dailydump.org/decomposition