In addition to the benefits that that have long been associated with aerobic composting e.g. the recycling of plant nutrients, improvement to soil structure, and the recycling of minerals the
gaseous emissions are composed mainly of water (H2O) and carbon dioxide. Aerobic composting does not produce methane as the microbes involved in aerobic composting utilise some of the carbon in the organic waste for energy and lock
the remaining carbon as in the soil as humus.(carbon sequestration).
Home composting has a role to play. It reduces the carbon footprint associated with the collection and transport of the organic waste by the
council by eliminating the road-miles involved in collecting and taking it to the landfill tip or processing plant. Home composting also saves the road-miles involved in transporting commercially produced compost to retailers and homes. It
is to be welcomed that landfilling of domestic organic waste is increasingly being replaced by commercial composting, but this still involves the road-miles in collection. In addition, methane emission from home composting is less than from commercial
While each individual home and backyard composter can only play a small role the total land that could be composted would be significant if the majority of gardens, lawns and parks were brought into use.
Many County and City Councils have recognized that there is a climate emergency and may be actively promoting home composting offering subsidized compost bins, advice and training through Master Composter and similar schemes. It might be interesting
to ask whether they are adopting systems of composting and carbon sequestration in their parks and grounds using the compost made from the green waste they collect from households. Many city parks also have large grass areas.
people now live in cities or suburban areas and only have small gardens but even these are likely to have either a lawn or if not, there will be a managed area of grass around the apartment block or houses as well as round local
commercial buildings and offices. In country towns and villages, the garden and grassed area will be larger .
Environmentally lawns have not had a good press. They can reduce biodiversity,
encourage the use of with synthetic which can pollute the environment and kill unintended beneficial species. The mowing, fertilization and general high level of maintenance necessary can result in a net emission of carbon
dioxide and nitrous oxide both contributors to global warming. This could be countered bytheir carbon sequestration capability especially if managed environmentally
But just because we are trying
to compost aerobically does not mean that all the heap will be aerobic all the time. If the feedstock has a high moisture content Compost Moisture combined with a high
temperature or is not adequately aerated anaerobic zones will be formed in the “aerobic” compost heap leading to the production of methane and nitrous oxide. It is recognised that aerating the feedstock (organic material)
during composting speeds the decomposition process. It also helps the environment as the greater amount of oxygen available to the composting micro-organisms the less methane, and the more carbon dioxide, is produced during decomposition. The use
of composting techniques which maximise aerobic conditions in the home compost heap include the design and set up of the bin to allow good access of air, ensuring there are air spaces in the material, which may require the addition of a bulking agent,
in addition to the ensuring correct mix of greens and browns. Traditionally regular aeration (turning) has been recommended as well as careful control of the moisture content so that water does not fill the air spaces in the material. A lid
or cover to the bin or heap is a key factor in controlling the moisture level to prevent the compost becoming saturated during heavy rain.
The need avoid the formation of anaerobic zones within the composting material might
be thought to favour the use of hot composting techniques by the environmentally aware composter as the organic waste is aerated regularly when it is most active during the first week or two and careful control of the moisture content.
However, the situation is complicated as not all the greenhouse gases produced in anaerobic areas are emitted into the atmosphere some of the methane is converted to carbon dioxide by aerobic
methane-oxidising bacteria (Methanotroph) on reaching the surface layers of the heap. Turning or mixing of the heap to aerate all of the content to encourage hot composting and remove anaerobic areas within the bin will disrupt the activity of these
methanotrophic aerobes as the surface layers in which they live and work are turned to the centre of the pile allowing any small amounts of methane from any anarobic areas, not broken up by the mixing, to be emitted to the
atmosphere until the Methanotrophs are re-established at the heap.
The evidence would appear to show that part of the price of killing pathogens and producing compost quickly by weekly mixing is that there
will be slightly higher emission factors when compared with cold composting techniques with no mixing. It has been reported that hot composting within the temperature range of 50-60°C produces the highest methane emissions.
At temperatures of over 65°C methane emissions are reduced. Monitoring the temperature so that temperatures within the 50-60°C range should not be prolonged past the time necessary to kill weeds and potential pathogens is necessary to keep the methane
emissions as low as possible,
Often when using cold composting at home where the organic material is added in small amounts composting will take up to 18 months to a year. The above suggests that there
might be an environmental case for not aerating the bin during the active stages of composting even if this may allow small pockets of anaerobic activity to develop. Even careful aeration using a commercial aerator that reaches the lower
depths of the bin to encourage continuation of aerobic decomposition will result in disturbance of the surface layers. But the bin will need aerating so a better procedure might be to set up the composting bin to provide greater aeration. For example,
the bin can be mounted on a wooden pallet or a layer of wood chips to provide for the entry of air at the base. Airflow within the lower part of the bin can be increased by the addition of a layer of twigs and small sticks or a layer of criss-crossed
sticks and/or a ventilation tube can be introduced to the bin.
Home composting may produce some greenhouses gases but less than other processes and the compost produced
will when converted to humus will provide a valuable carbon sink.