By GREG GERRITT/ecoRI News contributor
Composting is the breaking down of organic materials such as food scrap, leaves and manure by bacteria in the presence of oxygen, and transforming it into what is essentially the organic and high-nutrient portion of topsoil. Composting recycles the nutrients and organic matter in what we have thrown away so that plants can reabsorb the nutrients and use them to grow.
In a forest, the leaves fall to the ground each year, but they never fill the forest because below the surface of the leaf litter bacteria are turning the leaves back into soil.
The breakdown of organic matter takes place in several steps over the course of time, each step in the breakdown being done by a different mix of bacteria. The speed at which organic matter breaks down and turns to compost varies according to conditions.
Reducing the size of the organic matter to be composted provides more surface area for the bacteria to work and speeds things up. Other factors effecting the speed of composting include the oxygen flow to the compost pile, which can be controlled by either an aeration system or by turning the pile, moisture and temperature. By regulation, and sound composting practice, commercial compost piles have to heat up through bacterial action to more than 160 degrees Fahrenheit for several days or they are not acceptable for agricultural purposes.
Properly managed composting operations can turn food scrap and leaves into finished compost in about eight weeks.
Another approach that is considered to be within the realm of composting is to use worms to process the organic matter. Vermiculture turns food scrap into worm castings that can be directly returned to the soil, working exactly like compost. Mostly practiced at home, there are large-scale vermiculture operations selling compost. Worms can compost all vegetable matter, but there are limits to what they can eat, so to fully capture food scraps, vermiculture must be supplemented with another approach.
It should be noted that there are many compostable items, such as expired dairy products and meats, that can be successfully composted, but require a higher temperature than is usually created in a home compost pile. Therefore home composting on its own will never meet the need for complete organics composting, even for the most ambitious home composters.
Therefore, a complete system must include facilities handling large quantities of compostables collected from the community and businesses and able to compost the hard to compost items.
Anaerobic digestion is the bacterial breakdown of organic matter with no oxygen present, as opposed to the composting process, which is the breakdown of organic matter in the presence of oxygen. The bacteria in the anaerobic process create different gases than the bacteria that work in the presence of oxygen, with methane being the leading end product.
Burying food scrap in a landfill causes it to breakdown without oxygen, creating the smells no one wants to experience and much methane that no one wants in the atmosphere. The Central Landfill collects between 50 percent and 75 percent of the methane produced by food buried in the landfill, with a piped collection system, and it is burned in a turbine to create electricity. The rest eventually escapes into the atmosphere.
If one is actively pursuing anaerobic digestion in a closed system — essentially some kind of vat or chamber with pressure controls to keep out oxygen and keep in the gases — it is possible to capture all of the methane and other gases. This captured gas is then burned in turbines to produce electricity. Another alternative is to clean up the gas and then put it into the commercial gas pipeline system, as methane is the basic component of natural gas. Natural gas is simply the product of anaerobic digestion millions of years ago that remained trapped in the ground or under the sea.