Response to recent article on composting and greenhouse gas
27/09/2006

Jane Gilbert, Chief Executive of the Composting Association commented:

"We were interested to read the points raised in the article concerning composting, which miss the bigger picture and fail to acknowledge research findings published in the scientific press, including the Environment Agency's own research.

"We do feel that these comments were unnecessarily negative and could have had a harmful effect on the numbers of people composting at home.  The average home composting bin or centralised facility produces comparatively little methane and the benefits of diverting green waste away from landfill and the carbon stored by compost in the soil more than make up for this.

"Composting is widely accepted as playing an important role in helping to reduce this country's reliance on landfill and contributing to a reduction in greenhouse gases.  It would be a shame if scare stories like this had a negative impact on the phenomenal commitment the British people have made to composting their organic waste.  There is a great deal of scientific research to back up the benefits of composting."

Here are the facts:

Compost as a carbon sink

Composting has a major role to play in storing carbon in the soil.  The upcoming Thematic Strategy on Soil Protection will acknowledge that ‘compost is the best exogenous input for humus formation".  Similarly, estimates by the European Commission suggest that compost from municipal biodegradable waste has the potential to store 1.4 Mt CO2-equivalent per year in the EU.  Over a 100-year period, the use of compost as a soil amendment would store 54 kg CO2-equivalent per tonne of compost used, or some 22 CO2-equivalent per tonne of putrescible waste prior to composting.

Home composting

Research carried out by the Environment Agency did not identify significant quantities:  "The emissions from the composting systems are, as expected, low and difficult to monitor.  It appears that methane levels are very low and that whilst methane has been monitored within the pile in other studies of home composting systems it is effectively oxidised in the surface layers of the composting system."

The Environment Agency has stated: "We believe that home composting activities will produce some methane and other emissions, such as ammonia.  However, it is important not to view home composting in isolation - all waste management processes have some impact on the environment.

The impacts of an individual home composting site will obviously be minimal, and therefore our study seeks to compare the impacts for managing the same amount of waste in other ways using a life cycle approach.  This approach takes into account the combined impacts of all the waste composted at home and any associated benefits, for example those relating to savings in transport of waste.

Overall, home composting will have a lower impact on global climate change than taking waste to a landfill site and it may help the UK meeting its targets to divert biodegradable municipal waste under the Landfill Directive."

Composting process

If a generally aerobic environment is maintained within the windrow, methane production is not detected.  In fact, in the experiment carried out by AEA Technology Environment (2000) on behalf of the Environment Agency on a windrow composting process of green waste, anaerobic activity, indicated by methane evolution, was not detected at any time. Furthermore, CH4 production was only occasionally found in the experiment carried out by Chroni et al. (2006) on a composting process of food wastes and garden trimmings.

However, Cabanas and Stentiford (2006) demonstrated that during the maturation phase of composting, the methane emissions are not a major concern.  They showed that methane is never detected within 0.20 meters of the surface of an open composting windrow, as result of the oxidation of methane by methanotrophic bacteria.  As shown by Jackel et al. (2005), during composting process, associated with the metabolic activity of thermophilic microorganisms which produce methane, thermophilic microorganisms which oxidize the CH4 are also present.  The optimum range for these microorganisms (45-55 ºC) correspond to the optimum temperatures for the methane oxidation and the temperatures measured in the upper layer (0-40 cm) of the compost heaps were found to be also in these range. Jackel et al. (2005) found that 46-98% of the produced CH4 is consumed by methanotrophic bacteria before is released to atmosphere.  Therefore, ‘these organisms are effectively capable to reduce the potential methane emissions from compost'.

Methane emissions in composting are generally associated with composting of animal manures.  In a study carried out on a composting process of a mixture of animal manure and barks, Morand et al. (2005) detected concentrations of methane ranging from 5 to 300 ppm and extensive aeration of manure  However these were measured in different zones within the heaps and the zone with strong methane concentrations were associated with anaerobic part.  ‘Even if it is produced, methane is released unequally depending on the zone and on the presence of microbial population that consume it as it progresses through the heap' (Morand et al., 2005).

References

Wheeler P. A. & Parfitt J. P., 2002. Life cycle assessment of home composting. Proceedings of Waste 2002 Conference, Stratford. 

Jackel U, Thummes K, Kampfer P., 2004. Thermophilic methane production and oxidation in compost. FEMS Microb Ecol., 52 (2): 175-184

R&D Technical Report P355, 2000. Monitoring the Production of Compost from Wastes on a Continuous Basis. Produced for the Environment Agency by AEA Technology Environment

Chroni C., Kyriacou A., Oikonomou I, Georgaki I., Manios T., Kotsou M., Lasaridi K, 2006. Microbial community succession during composting of biowaste as evaluated by plating techniques and denaturing gradient gel electrophoresis analysis. Proceedings of the International Conference ORBIT 2006 Biological Waste management

 Morand P., Peres G., Robin P., Yulipriyanto H. and Baron S., 2005. Gaseous emissions from Composting Bark/Manure Mixtures. Compost Science & Utilization, Vol. 13, 1, 14-26

Cabanas-Vargas D.D., Stentiford E.I., 2006. Oxygen and CO2 profiles and methane formation during the maturation phase of composting. Compost Science & Utilization, Vol. 14, 2, 86-89

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