Environmental Engineering Reference
In-Depth Information
Ingredients with relatively high nitrogen content include
• Green plant material (fresh or wilted) such as crop residues, hay, grass clippings, and weeds
• Manure of poultry and herbivorous animals such as horses, cows, and llamas
• Fruit and vegetable trimmings.
The most efficient composting occurs by seeking to obtain an initial C:N mix of 25-30 to 1 by
dry chemical weight. Grass clippings have an average ratio of 10-19 to 1 and dry autumn leaves
from 55-100 to 1. Mixing equal parts by volume approximates the ideal range.
Poultry manure provides much nitrogen but with a ratio to carbon that is imbalanced. If
composted alone, this results in excessive N-loss in the form of ammonia—and some odor. Horse
manure provides a good mix of both, although in modern stables, so much bedding may be used as
to make the mix too carbonaceous.
For home-scale composting, mixing the materials as they are added increases the rate of
decomposition, but it can be easier to place the materials in alternating layers, approximately 15 cm
thick, to help estimate the quantities. Keeping carbon and nitrogen sources separated in the pile can
slow down the process, but decomposition will still occur.
Some people put special materials and activators into their compost. A light dusting of agricultural
lime (not on animal manure layers) can curb excessive acidity, especially with food waste. Seaweed
meal provides a ready source of trace elements. Finely pulverized rock (rock flour or rock dust) can
also provide minerals, whereas clay and leached rock dust are poor in trace minerals.
Some materials are best left to a high-rate thermophilic composting system, as they decompose
slower, attract vermin and require higher temperatures to kill pathogens than backyard composting
provides. These materials include meat, dairy products, eggs, restaurant grease, cooking oil, manure
and bedding of nonherbivores, and residuals from the treatment of wastewater and drinking water.
Meat and dairy products can be recycled using bokashi, a fermentation method, which uses bokashi
bran (Wikipedia 2009), wheat bran inoculated with effective microorganisms (EMs).
There are two major approaches to composting: active and passive. These terms are somewhat of
a misnomer because both active and passive composts can attain high heating, which increases the
rate of biochemical processes. But the terms active and passive are appropriate descriptions for the
nature of human intervention used.
An active compost heap, steaming on a cold winter morning, is kept warm by the exothermic
action of the bacteria as they decompose the organic matter.
Active (hot) composting is composting at close to ideal conditions, allowing aerobic bacteria to
thrive. Aerobic bacteria break down material faster and produce less odor and fewer pathogens and
destructive greenhouse gases than anaerobic bacteria. Commercial-grade composting operations
actively control the composting conditions such as the C:N ratio. For backyard composters, the charts
of carbon and nitrogen ratios in various ingredients and the calculations required to get the ideal
mixture can be intimidating, so many rules of thumb exist for approximating it.
Pasteurization is a misnomer in composting, as no compost will become truly sterilized by high
temperatures alone. Rather, in very hot compost where the temperature exceeds 55°C for several
days, the ability of organisms to survive is greatly compromised. Nevertheless, there are many
organisms in nature that can survive extreme temperatures, including the group of pathogenic
Clostridium, and so no compost is completely safe. To achieve the elevated temperatures, the
compost bin must be kept warm, insulated and damp.
Aerated composting is an efficient form of composting from the chemical point of view as
it produces ultimately only energy in the form of waste heat and CO
2
and H
2
O. With aerated
composting, fresh air (i.e., oxygen) is introduced throughout the mix of materials using any
appropriate mechanism. The air stimulates the microorganisms that are already in the mix, and
their byproduct is heat. In a properly operated compost system, pile temperatures are sufficient
to stabilize the raw material, and the oxygen-rich conditions within the core of the pile eliminate
