Integration of energy production and nutrient management in farming
Charles J Banks
School of Civil Engineering and the Environment, University of Southampton, SouthamptonSO17 1BJ, UK email
The paper looks at how the process of anaerobic digestion (AD) can be integrated into the farming system to help provide both better control of nutrients generated from livestock and other organic waste sources on the farm, and a means by which nutrients can be safely imported onto the farm as part of a wider cycle of food production and consumption - waste management and recycling, involving both urban and rural communities. The measures that are required to ensure biosecurity on farms when a circular approach to nutrient management is adopted are considered along with potential benefits the process can bring in enhancing biosecurity of farm-generated wastes.
Farms offer a large potential for generation of renewable energy both from agricultural wastes and by the production of dedicated biomass grown mainly or specifically for the purpose of energy production i.e. energy crops. The paper looks at examples of how energy and energy crop farming has spread in Europe and the economic drivers and legislative factors that have encouraged this diversification in agricultural activity. There is currently considerable debate as to the sustainability, both environmental and financial, of energy farming with dedicated mono-crops. Attention is now moving to the development of integrated farming systems in which energy crop biomass may be part of a crop rotational cycle, possibly involving intercropping and undercropping, with further use of land normally left fallow during certain periods. There is also some potential for the use of specific crop species in both energy production and nutrient enhancement, e.g. through the cultivation of legumes for biomass production with return of digestates to land for nitrogen enrichment and soil preservation. There are however limitations on the use of digestate in closed cycle agricultural systems. Conflict between nutrient demands and the times at which organic fertiliser materials can be applied may cause operational difficulties and lead to potential nutrient imbalances that need to be corrected by supplementation with inorganic fertilisers.
AD can assist in providing a way in which better nutrient management can be achieved while also providing a source of renewable energy. The economic feasibility of energy schemes needs careful consideration, however, particularly when dealing with farm wastes which may have only a small potential energy yield per unit of volume, as is the case for example with cattle slurry. At the same time governments and policy makers should consider other benefits from on-farm AD, which may include the prevention of nutrient pollution of rivers and aquifers; improvements in biodiversity through the return to more traditional but scientifically-managed farming techniques; preservation of soil structure and organic content. Finally AD of farm and other wastes may have a role to play in reduction of fugitive greenhouse gas emissions from the agricultural sector. At present however too little is known about the overall losses and gains associated with a reduction in methane emissions versus the potential for nitrous oxide emissions from application of digestates to farm land, and more research is needed in this area.