Climate Change Corner
Composting as a Strategy to Combat Global Warming (Part 2)
Reduction of carbon emission through composting
Composting of food waste indirectly reduces the emission of GHG in two ways. On one hand it avoids the GHG emission from the landfills and on the other hand land application of compost increases the carbon sequestration in agricultural soil preventing the emission of GHG. In Hong Kong, complete diversion of food waste from landfill theoretically could reduce about 515,308 to 1,141,158 tons of CO2 equivalent (CO2E), which is ~1.27 to 2.74% of local GHG emission, and will promote the Region’s mission to reduce carbon emission.
The IPCC framework for estimating greenhouse gas fluxes is based on a 100-year time frame and will only consider compost carbon as ‘sequestered’ if it remains locked in the soil for at least 100 years. Compost application to soils results in retaining 45% of carbon applied over a 20-year period, 35% over a 50-year period, and 10% over a 100-year period. For a 10 t/ha application of compost results in the GHG reduction of 5.05, 3.53, and 1.01 t CO2E over 20, 50 and 100 years, respectively, indicating that composting can sequestrate carbon in the soil pool. The 'CENTURY' model predicts that new equilibrium for soil organic carbon will not be reached before 200 and 300 years for annual compost application rates of 10 and 15 t ha-1, respectively, for northern European conditions; therefore the soil carbon sequestration through land application of compost is likely to be longer than 20 years. On the basis of carbon sequestration data, it is clear that compost application in soils promotes carbon sequestration that reduces the overall emission of CO2 from the organic wastes indirectly.
Application of composting in Hong Kong
Back to the situation in Hong Kong, the three existing strategic landfills in Hong Kong are expected to be full in another 4-8 years’ time. Therefore, landfilling of food waste in Hong Kong exerts a great demand on the landfill space. Finding new landfill space is a crucial problem currently; therefore diverting the food wastes to composting treatment offers great advantages for Hong Kong. Of the food waste generated in Hong Kong in 2011, every day 1,056 tonnes were produced in the Commercial & Industrial (C&I) Sector and 2,528 tonnes/day were generated from the domestic sector. Therefore, it is more feasible under the current situation to utilize the food wastes from the C&I sector in order to achieve better quality composts. However, almost 70% of the food waste is generated in the domestic sector that requires appropriate waste separation programme, and the heterogeneity of the organic fraction of the MSW could influence the quality of the compost product. Therefore, the key to solve the huge amount of food waste generated in Hong Kong is not the treatment technologies; instead policies like waste charging and mandatory waste separation should be implemented to encourage waste separation in the territory.
To conclude, composting facilitates recycling of organic matter and nutrients in the ecosystem, offers an alternative and potential treatment method for the diversion of food wastes from landfills; and most importantly reduces ~1.27 to 2.74% of local GHGs emission that promotes the Region’s mission to reduce carbon emission and global warming. Albeit, the large land requirement for composting makes it unrealistic to adopt composting as the sole treatment technology for food waste in Hong Kong. Instead to combine composting with anaerobic digestion would provide a good viable hybrid technology to handle the food waste generated in Hong Kong with the end products as biogas and compost fertilizer, which will, on the hand, exhibit a lower capability in reducing GHG emission than composting alone.
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This article is contributed by Professor Jonathan WC Wong, MH, Sino-Forest Applied Research Centre for Pearl River Delta Environment, Hong Kong Baptist University, with the co-ordination of the Environmental Division.
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