Composting and Climate Change

Agriculture is a significant contributor to climate change with producing  at least 14% of global greenhouse. Large quantities of methane are emitted as a result of activities such as : ploughing, which  releases carbon dioxide from the soil,  and  livestock. Farming is a significant user of fossil fuels and, of course, fertilizers
However, while much  attention has been focused on the problems caused by farting cows  farming can help reduce climate change by  carbon sequestration

Making compost can contribute to the efforts to reduce the rate of global warming as spreading compost and using a non-dig method can boost the soils carbon storage ability. In agriculture animal husbandry is a significant  contributor  to global warming but research has shown that if is spread on  grazing lands it could capture a significant amount of greenhouse gas emissions with carbon storage utilising  humus formation. The beneficial  effect  would continue into the future  resulting in improvement of the overall greenhouse gas balance.

  The Intergovernmental Panel on Climate Change and the European Commission has recognised as Carbon sequestration in soil one of the measures through which greenhouse gas emissions can be mitigated. Composting originated in agriculture land and it has now been recognised that there is a role  it could play in carbon sequestration of  agriculture land with some large industrial  farms using large-scale composting techniques to deal with  animal waste.

Home Composting

By aerobic  composting at home, on the allotment, in businesses and at school everyone can  play a part in  reducing the quantity of methane released into the atmosphere.

One of the environmental  reasons for composting at home and on the allotment is to avoid sending organic waste to landfill where it will decompose to produce methane, a major greenhouse gas. which forms under the  anaerobic conditions found inside a landfill, Methane is  twenty-six  times more potent than carbon dioxide The methane can  be collected and burnt  to generate electricity but often it is just lost to the atmosphere.

Anaerobic composting is commonly used to treat human effluent and livestock waste on farms and from houses in hotter countries as well as food and other organic waste as an alternative to landfill.  Anaerobic digestion (AD) involves the fermentation of the waste under anaerobic micro-organisms this methane and other biogases produced are used for power heating or power generation or heating and inorganic solids. The  remaining bio-solids can be used as a  fertilizer.

Aerobic Composting

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 composting.

 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.

 Most 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.


Sources and further reading