Compost pile in a home garden.
Technical Summary

Composting

Project Drawdown defines composting as: the conversion of biodegradable waste to a useful soil amendment, while avoiding emissions from landfills. This solution replaces the disposal of biodegradable waste in landfills.

Organic wastes account for 46% of global solid waste and contribute, on average, 469 tons of carbon dioxide-equivalent greenhouse gases per million metric tons of organic solid waste due to anaerobic breakdown resulting in methane emissions. Composting is a flexible, scalable approach that reduces those emissions by more than 50%. Drawdown has defined the composting solution based on urban organic waste that is largely being managed today via landfills and, in some regions, open dumping. The mitigation impact reported in Drawdown for this solution does not consider any potential carbon biosequestration benefits from the use of compost as a soil amendment, nor any potential savings from reducing demand for nitrogen fertilizers.

Methodology

To arrive at the mitigation and financial results for composting, a forecast for the total global urban organic waste production from 2014-2050 was first calculated. Next, the current composting rate of existing municipal solid waste was determined, the future plausible adoption of composting was forecast to 2050, and the emissions mitigated were calculated in comparison to a Reference Scenario that keeps adoption of composting fixed at the current percentage of global organic waste.

Total Addressable Market[1]

The total addressable market for organic waste is defined based on the global urban organic waste production from 2014-2050. The global market was calculated using a composite of forecasts including a linear interpolation of World Bank data from 2010-2025, an extrapolation to extend those projections to 2050, and a per capita extrapolation[2] using IPCC data. It is expected that by 2050, global organic waste will be around 1979 million metric tons. Current adoption[3] of composting in the year 2014 was estimated to be 13% of urban organic waste.

Adoption Scenarios[4]

Due to the lack of reliable future projections of the growth of composting, adoption estimates were made based on present urban composting rates in the US and Europe – approximately 38% and 57%, respectively – with negligible rates in developing and least-developed countries.

Impacts of increased adoption of composting from 2020-2050 were generated based on two growth scenarios, which were assessed in comparison to the Reference Scenario mentioned above.

  • Scenario 1: For this scenario, it is assumed that composting will increase from present rates to 38% in middle- and low-income countries, and to 57% in high-income (OECD) countries by 2050. Globally, compost production increases from 106 million metric tons per year in 2015 to 758 million metric tons per year in 2050.
  • Scenario 2: Here, it is assumed that composting scale and costs are optimized, the market value of compost increases, and source-separated collection is subsidized.[5] In this scenario, the amount of compost produced in 2050 is 956 million metric tons.

Emissions Model

Emissions data for each scenario was calculated using variables selected from sources which measured the comparative emissions of composting and/or landfilling. The variables were normalized to total direct emissions per million metric tons of organic waste, as a variety of sources reported complete emissions and some segmented emissions data by collection, transportation, processing, and other indirect measures.[6]

Financial Model

Financial results were reached by comparing the costs of creating and operating compost facilities to creating and operating sanitary landfills for an equivalent volume of organic waste. Collection and transportation costs were assumed to be the same for each approach. The cost of establishing new compost facilities over the period in question is calculated to be US$110 billion, which is US$61 billion less than the cost of establishing new landfills.[7]  However, operating compost facilities costs more than operating landfills, even considering the revenue generated from the sale of finished compost. The data used for modeling both composting and landfilling was generated based on a mix of values, including worldwide averages reported by World Bank’s ‘What a Waste’ and UNEP.

Integration[8]

For integration, Drawdown first considered the reduction of organic waste from reduced food waste and the increase in organic waste from an assumed compostable percentage of the increased adoption of bioplastic, to refactor the market for composting for each year.[9]  Modeled compost adoption was then compared to the new organic waste market and adjusted if needed.[10]

Results

Drawdown found a potential of 2.14 gigatons of carbon dioxide-equivalent greenhouse gas reductions in the Scenario 1 over 2020-2050, corresponding to a 48% adoption of composting, with a net implementation savings of $61 billion but a net operational cost of US$58 billion. The first cost of landfilling is almost double the cost of composting facilities; however, the operational cost of composting is 10% more than the cost of landfilling due to the maintenance incurred for aeration, moisture, and temperature control of compost piles. For the Scenario 2, the emissions avoided amount to 3.1 gigatons with a 60% adoption.

Discussion

The impact of composting is bounded by the amount of organic waste created. There are some indicators that as regions develop, the per capita amount of waste increases then plateaus. All scenarios modeled are highly dependent on a significant increase in adoption in the Asia (sans Japan) region. Whereas less than 3% waste to landfills has already been achieved in several European countries, Drawdown has chosen more conservative growth scenarios for both China and Asia (sans Japan) to reflect the uncertainty of growth of composting, considering current investments in waste-to-energy facilities in the region.[11]  The financial case for adoption of composting is distorted by local regulations and compost demand.[12] Where regulations, space, and logistics create cost barriers to landfill expansion, there is already a self-evident business case for composting. In conclusion, this analysis suggests that composting can grow to offset a significant portion of landfilling, while reducing climate emissions. This is limited by the education required for composters and consumers, as well as the realization of a financial advantage that compost may have over landfilling.[13]

 

[1] For more on the Total Addressable Market for the Materials Sector, click the Sector Summary: Materials link below.

[2] Using UN 2015 median urban population forecast.

[3] Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.

[4] For more on Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Materials Sector-specific scenarios, click the Sector Summary: Materials link.

[5] Perhaps through increased regulation and cost burden on landfill practices.

[6] Sources include the EPA WARM model, the Composting Council of Canada, and meta-analyses such as those by Zaman, et al.   

[7] All monetary values presented in US2014$.

[8] For more on Project Drawdown’s Materials Sector integration model, click the Sector Summary: Materials link below.

[9] Reduced food waste already incorporates considerations of plant-rich diet adoption.

[10] E.g., adjusting adoption down if composting adoption exceeds in total or a reasonable percentage of organic waste.

[11] If China is slow to adopt composting, then the overall global mitigation impact of composting will likely need to be discounted further.

[12] Whereas the model shows a potentially uninspiring business case for adopting composting due to the increased operating cost of a compost facility over a managing a landfill, it would take only an increase in market price of finished compost (driven by demand of more climate-friendly agricultural practices) and/or a decrease in the operating costs through innovation and process design to make a compelling financial argument in favor of composting over landfill.

[13] Particularly in areas with informal and unregulated dumps.