Organic waste : The (un)hidden benefits to soil

The first ISWA report analysed municipal solid waste (MSW) production and organic waste from a global perspective, focusing specifically on the current status of organic waste recycling. Organic waste is between 44% and 46% (by mass) of the MSW fraction, comprising mostly food waste when it comes to cities. Globally, it was estimated that about 935 million tonnes of organic waste are generated annually in towns and cities, but unfortunately only a fraction of this is currently recycled through composting and anaerobic digestion and returned to soil; this leads to the creation of odours and release of the powerful greenhouse gas methane into the atmosphere, and attracts vermin.

The link between recycling societies’ organic wastes and the soil from which these wastes have been generated is not as clear-cut as it should be. If the global quantity of organic waste was recycled solely into compost, annual pro- duction would result in about 309 million tonnes of compost. This amount – assuming an application rate of 10 tonnes per hectare (t/ha) – could be used to restore fertility to about 31 million hectares of arable agricultural soil, representing 2.4% of the world’s land area being cultivated according to the Food and Agriculture Organization of the United Nations.

It is important to note that the benefits described in the ISWA project may only be realised if the compost or digestate is not contaminated. Quality compost and digestate should only be derived from clean organic feedstock which has been kept and collected separately from other wastes. Physical and chemical contaminants, such as plastics, glass, metals, heavy metals and organic substances, might pollute soil and have the potential to accumulate over time following repeated application of contaminated compost; this is not sustainable, and nor is it desirable. The challenge for waste planners and waste managers is therefore to source and collect clean, contaminant-free organic wastes, then recycle them into quality compost and digestate. EU member states are currently engaged in this, since separate collection of biowaste will become mandatory by 2023.

Soil is a complex mixture of minerals, organic matter, air and water. It can take many thousands of years to form but can be destroyed very quickly, sometimes within decades. Soil is not only the source of almost all of the world’s food, but is also an important store of carbon and provider of ecosystem services. The poor condition of many soils around the world is a cause for significant concern and the loss of soil organic matter is cited as one of the main reasons why a great deal of agricultural land is becoming progressively less productive; this has the potential to undermine humans’ ability to grow and harvest food crops sustainably.

Globally, around 30% of the world’s soil is moderately to highly degraded due to erosion, nutrient depletion, acidification, salinisation, compaction and chemical pollution, with an estimated 10 million hectares of agricultural land being lost through soil erosion every year.

The third ISWA report overviews the status and threats to soils in five different countries located in different regions of the world (Australia, Brazil, Chile, Italy and the United Kingdom). All five countries were found to suffer from varying degrees of soil erosion and loss of soil organic matter. In all five, agricultural productivity was identified as being adversely affected by soil degradation and erosion; loss of organic matter could potentially be reduced through the application of high-quality compost to soil.

The second ISWA report summarises the benefits of applying quality compost and digestate to soil. Compost and anaerobic digestate differ in their chemical composition, which is due to the different types of feedstocks from which they have been derived (in particular, whether or not they contain lignin) and the biological treatment processes themselves (i.e. whether the process was aerobic and/or anaerobic). The combination of these two factors means that both compost and digestate differ in the levels of humic substances (stable organic carbon) and the type and availability of plant nutrients they contain.

Anaerobic digestate can be best classified as an organic fertiliser, as it contains plant nutrients that are present in a form readily available for crop uptake; in the short term, it also increases soil microbial activity in comparison to inorganic fertilisers or untreated controls. However, the long-term benefits to soil of anaerobic digestate are less clear-cut than those of compost, and it is thought that it has a negligible effect on soil organic matter in the long term.

Compost, on the other hand, can be classified as an organic soil improver. Generally, it has lower plant nutrient levels than anaerobic digestate, but it has been shown to increase soil organic matter levels, thereby helping to improve soil structure and function. Repeated compost application has been shown to increase soil aggregate stability and soil pore structure, reduce compaction and increase water-holding capacity.

The fourth ISWA report concentrates on modelling the benefits of applying quality compost to soil; it quantifies an economic value derived from a carbon sequestration, nutrient and financial point of view.

Organic matter in compost is transformed through soil microbes into more stable forms of carbon that are partially ‘locked up’ in soil and taken out of the atmosphere; applying compost to soil is therefore a means of sequestering carbon and thus helps to mitigate climate change. The studies summarised in this ISWA investigation suggest that over a period of between 4 and 12 years, in the region of 11 to 45 percent of the organic carbon applied to soil as compost remained as soil organic carbon. The potential is not finite, as an equilibrium will be reached in about 20 years when the rate of soil organic carbon (SOC) formation is equal to its degradation.

The modelling outlined in the report suggests that over a 20-year period, the levels of SOC stocks could be increased (in absolute terms) by between 0.40 and 0.55 percent (mass/mass) depending upon the density of the receiving soil, with significant implications in the restoration of soils low in organic matter.

If all organic waste produced globally in cities and towns (935 million tonnes) were collected separately and converted into quality compost, about 34 million tonnes of carbon dioxide equivalents could potentially be sequestered in soil annually. This is approximately 2 percent of the annual two to five gigatonnes of CO2 equivalent that global soils are technically able to sequester.

Pricing the carbon in compost using World Bank figures indicates that compost can be valued in carbon sequestration terms at between €3.50 and €8.10 per tonne (fresh mass).

Estimates were also made of the total macronutrient content of compost using published figures for two types of compost derived from either green waste only or a mixture of green and food waste (biowaste) feedstocks. The total nutrient value of compost falls in the region of €17 to €20 per tonne on a fresh mass basis.

Taken together, the total carbon and nutrient value of compost is estimated to lie in the region of €21.20 to 28.20 per tonne (fresh mass). With an estimated global potential value of €6.6 to €8.8 billion a year in product alone, the composting sector needs to be recognised for the important role it has to play in sequestering carbon and improving degraded soils.

In conclusion, ISWA’s investigation clearly documents how waste management and the recycling of organic waste can contribute to the improvement of soils. As just under a billion tonnes of organic municipal solid waste are thought to be generated annually, this is a valuable resource, containing both carbon and plant nutrients. Recycling it into com- post and digestate can help improve soil and slow its degradation, and thus goes some way towards meeting the United Nations Sustainable Development Goals 12 and 15.

The manifesto “SOS Soil”, launched at the 2019 ISWA Annual Conference, reinforces the request for a global commitment to preserve and to protect soils, starting with the adoption of legislative measures and actions by the countries in the European Union.

Marco Ricci-Jürgensen, Jane Gilbert and Aditi Ramola are members of the ISWA Working Group on Biological Treatment of Waste