Technical Report

Exploring the viability of small scale anaerobic digesters in livestock farming

December 2015

Executive Summary

Worldwide GHG emissions from livestock supply chains are estimated to produce 7.1 gigatonnes of carbon dioxide CO2 equivalent (CO2e) per annum. This represents 14.5% of all human-induced emissions. Of the total, storage and handling of manure represents 10 per cent (FAO, 2013a). On-farm anaerobic digestion (AD) of manures has significant potential to capture methane as a renewable energy source and, as a consequence, to reduce net global GHG emissions.

Animal manure, however, is a massively underexploited biomass resource but presents many challenges in any attempt to harness its full potential. This is attributable in part to the low energy density of the material, and also arises because agriculture worldwide is comprised of relatively small units of production. If the benefits are to be realised, strategies need to be developed for on-farm AD whatever the size of the farm. In many countries, subsidies are used as an inducement to encourage such actions on account of its high capital cost.

The use of methane from agricultural biomass not only removes a direct source of GHG emissions, but also displaces the use of fossil fuels in terms of fertiliser and energy production, thus further reducing net GHG emissions. When livestock manures are used, there are other environmental benefits including better nutrient management which should be taken into account. These include:

• Improved air quality from the replacement of fossil fuels, wood and peat
• Biofertiliser availability in the form of digestate
• Resource efficiency (recycling of nutrients)
• Reduced odours
• At least 90% reduction of pathogens harmful to animal, human and plant life
• Reduction in weed seeds

Overall this creates a circular economy based on zero waste of resources

The purpose of this report

• To assist farmers who are considering the adoption of AD either to improve the overall productivity of the livestock enterprise or for farm diversification. It aims to provide the farmer with an overview of the types and designs of anaerobic digesters that are available and the factors which can affect both the capital and operating costs.
• To provide policy makers with an illustration of the capital and operating costs for farm-based anaerobic digestion that will allow assessment of the effectiveness of legislation and its impact on the adoption of AD technology.

The methodology

Four scenarios for a dairy farm of 100 milking cows are considered in order to demonstrate the extent to which energy prices, incentives and capital grants can influence the cost of GHG reduction through the use of AD for slurry management. All costs used in the examples are for illustration only and can be replaced with those appropriate for individual countries and farms. It is assumed throughout that cost calculations are based on best practice in plant design and management including digestate application. The outcomes can be used by policy makers and regulators for guidance in making decisions that will maximise the potential contribution from AD towards meeting internal and external targets for the reduction of greenhouse gas (GHG) emissions.

Concluding remarks and the way ahead

AD is a multi-purpose process. It reduces the GHG emissions from the storage of the livestock manure. The recovered biogas replaces oil, kerosene or wood as fuels and in doing so reduces the release of particulates and toxins into the atmosphere. As a consequence, their detrimental effects on human health are reduced, while the reduction or elimination of pathogens during the process can lead to improved human health as well as animal health and productivity. These small scale plants wherever they are located can usually be integrated into a wholly sustainable farming system for the reduction of pollution to land, air and water.

Extensive investigations at the international level all reach the same conclusion that AD is the most effective, indeed recommended technology for the removal of methane emissions from storage of livestock manure. There is a cautionary note that the AD process could potentially increase the amount of ammonia which can be released from the digestate into the atmosphere. This, however, is a matter for best practice management and therefore not an insuperable problem. It can be resolved in the main by gas tight storage of the digestate, as well as by the timing and method of application of the digestate to land as a biofertiliser. The cost calculations assume best practice in the plant design and management.

There is a different attitude and psychology to the use of AD when it is used as a basic farm process to enhance the productivity and monetary value of slurry. Dairy farmers generally need a simple, efficient and minimal cost system for dealing with slurry, and preferably one that reduces net cost to the farm. The investigations show that a small AD system can meet this need. For the farmer, the avoided costs and a predictable level of expenditure on energy, for example, are as important as any additional income from outside sources. This factor must not be overlooked in any evaluation of these small plants. Nevertheless, there is still the underlying dependence on the relationship between capital cost, energy prices and incentives, quite apart from any extra costs incurred to meet national regulations.

Conclusions to this effect have been reached by a number of studies made over the last three decades. There has been considerable progress in understanding process management and plant design since many of these studies were undertaken. It is, however, an oversimplification to assert that high levels of incentives are needed to offset the high capital costs of such plants.

The driving force behind the incentive systems adopted by national governments is to encourage renewable energy production as for example in Europe to replace the use of fossil fuels and so remove the sources of the GHG emissions. These incentives have and still are fulfilling that purpose. However, they fail to recognise the full environmental benefit which AD can offer. They have not been favourable therefore to the development of anaerobic digestion at a small scale (or indeed any scale) and especially manure based plants simply because these have been geared to electricity production which involves further complexity and investment in generation capacity. To tackle the issues of agricultural GHG emissions, the approach used shows a lead as to how AD, as the acknowledged best available technology for the reduction of GHGs from manure, can also be achieved without the combined heat and power (CHP) option where it fulfils the needs for heating or cooling within the buildings of the farmstead as a whole rather than the dairy in isolation.

Widespread adoption can drive the cost of small scale AD plants down through innovation, development and production and, where incentives are present, can allow support so that such plants can be fitted into the existing farming system, rather than having to alter the farming system in order to accommodate the digester and the incentive scheme.

Given the stimulus of a favourable combination of capital cost, energy price and incentive at the outset, there is a win for the policy maker and a win felt in the pocket of the farmer.