Bioenergy waste residues as alternatives to conventional inorganic fertilisers for sustainable food production in sub-Saharan Africa

Soils provide, support and regulate fundamental processes in the environment, including nutrient cycling, plant growth, and
have a strong influence on ensuring purity of the atmosphere, as well as water supply and quality. Through the delivery of these ecosystem services, vital global biodiversity and, ultimately, the sustenance of the human population is maintained. However, exploitation of soils through intensive agricultural practices such as the over application of phosphorus and nitrogen fertilisers, has resulted in their degradation and, as a result, a diminishment of soil fertility, threatening future global food security.
Phosphorus is a vital, non-renewable element required for crop growth, upon which agriculture is now almost entirely dependent to maintain current levels of food production. The extraction and processing of phosphorus, is also extremely environmentally damaging, and originates from a non-renewable source for which demand is rapidly increasing with no alternative available in the volume required. The production of nitrogen fertiliser is also a highly energy intensive and unstainable process, is tied strongly to the price and availability of fossil fuels. As the global population is expected to reach 9 billion by 2050, humanity faces an urgent need to balance an ever increasing demand for energy and natural resources, with the sustainable management of ecosystems and the vital services that they provide.
A huge amount of waste is generated daily in African countries, much of which is dumped or landfilled. Zoomlion, a major waste management company in West Africa, have reported that there are high levels of organic content of waste generated in most African cities. In Ghana, for example, about 66% of the total waste generated is organic. African countries can transform their energy landscape through waste-to-energy, but there needs to be political and institutional commitment. It is encouraging to note that in recent times a lot of governments in Africa are gradually embracing the Green Growth development pathway, with some having already mainstreamed Green Economy actions in their national development plans. These steps signal great prospects for waste-to-energy development in Africa because Green Growth developmental actions foster economic, social and environmental development. The knock on from this is what to do with the residual waste streams from waste-to-energy processes including anaerobic digestate, organic residues from bioethanol production, ash from burning wood and soots from charcoal.
In discussing this with scientists and policy makers from sub-Saharan Africa, it is clear that soils are vulnerable to degradation through environmental conditions (extremes of wetting and drying, leaching, erosion, loss of organic matter) and anthropogenic impacts (fertiliser and pesticide application, land use, deforestation, dumping of wastes and pollution). However, the maintenance of soil health and fertility is crucial to continued sustainable production of food, welfare of families and communities and local and national economies. A shift in emphasis to more sustainable farming systems, utilising recycled and recovered nutrient products, incorporating OM to improve soil structure and biodiversity, and improved crop-soil management for nutrient acquisition are needed. By using these bioenergy residues, soils will become more resilient to environmental impacts and will offer a sustainable alternative to expensive and finite made N and mined P fertilisers.

This research will contribute to a paradigm-shift to change in the way in which bioenergy 'wastes' are perceived, from an expensive problem, to a truly sustainable substitute to traditional fertiliser products, with holistic environmental and economic value in the UK and sub-Saharan Africa. Through providing a market for waste-streams from bioenergy generation, this will remove the cost burden, which is presently a significant disincentive to the generation of renewable biomass energy schemes, promoting further bioenergy developments, while promoting business opportunities. Direct benefit therefore, will extend from the commercial bio-energy operators by way of reduced waste disposal costs, waste management companies who can maximise the value of the materials by processing, through to the grower who can use lower cost and more sustainable products. Beyond immediate commercial beneficiaries, these benefits will transfer through the supply chain to the wider public, as food and power consumers, who will access more cost effective, sustainable and secure food products, as well as lower impact energy supply. Longer term, nutrient recovery will ensure that food production remains economically viable in the face of depreciating supplies of raw phosphate, and higher cost nitrogen, whilst a more viable and extensive network of low-carbon bio-energy production facilities will provide improved energy security. Both improved food and energy security will allow the UK to meet its obligations for carbon reduction under the Kyoto and Paris Protocols.
Looking globally, the use of rock phosphate reserves is increasingly geopolitically sensitive, with the mineral deposits under the control of a handful of countries such as China, the US and Morocco. China has recently imposed a 135% export tariff to ensure domestic supply. In addition, the import of P rock from Morocco is sensitive, as it currently occupies the Western Sahara, controlling its P reserves. Hence, trading with these regions is highly condemned by the United Nations. Further to these politically unstable sources, the USA is estimated to have less than 30 years of high quality rock phosphate reserves remaining. Hence, for countries such as the UK with no natural rock phosphate mineral reserves, political, legal and economic challenges related to primary P use are likely to become increasingly important issues as global supply declines. Countries, will therefore be increasingly dependent upon recycling of P to ensure economic competitiveness of agriculture.
With respect to nitrogen, the production of N fertilisers relies heavily on adequate supplies of methane from which hydrogen gas can be generated; hence it is tied strongly to the price and availability of fossil fuels. Reduced dependence on the Haber Process as a result of N formation from anaerobic digestion of waste materials will significantly reduce the a country's dependence on foreign gas imports for fertiliser production or the import of expensive inorganic fertiliser. This change in perception of waste-derived fertilisers therefore presents a benefit to any user of fertilisers who is currently reliant on traditionally sourced products.
Although waste recycling and reuse has come a long way in recent years in developed countries, the perception of wastes as ingredients to be blended and reused is not yet established or even considered acceptable in developing countries. The use of bioenergy waste in food production is an application which is potentially highly sensitive to the end-user, and as such, success in this area will do much to promote movement and change in perceptions. This project will promote and support the discussion for future waste re-use, sustainable agriculture as well as waste policy and legislation in sub-Saharan Africa.