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Dairy manure-based biogas, an emerging source of renewable energy, is a result of a recycling process which often leads to the thought that manure production is the beginning of this biogas supply chain by energy producers. However, dairy manure is only a byproduct of an agricultural system whose main activities are crops and milk production. And thus, researching a combined biogas and agricultural production system provides a more comprehensive and systematic view of opportunities and impacts resulting from biogas production. Nitrogen is a critical input for any agricultural production system. Crops need nitrogen for their growth. Dairy farmers make decisions on nitrogen inputs for their cows by choosing co-feeds which result in different levels of milk production. The undigested nitrogen is often recycled by returning manure or biogas digestate to the fields. Choices related to milk production influence the amount of manure and nitrogen concentration of the farm (Nennich et al., 2005) which eventually affects the production of biogas as well as its nitrogen-related environmental problems. On the other hand, alternatives strategies for biogas production which may result in higher biogas yields, such as the use of certain co-substrates and mixing ratios, might also make the nitrogen contained in the digestate heavily exceed the need of the dairy farm. For these reasons, this paper researches the mutual interaction between biogas production and its agricultural production via the nitrogen flows within the combined system. In order to do that, the research first quantitatively modelled the nitrogen flows within the combined system which actually happen in practice. The combined system boundary includes the dairy farm itself where the choice of milk and biogas production are made; the co-feed farm and the co-substrate farm (Figure 1). We used the indicator of nitrogen recycling potential which is the ratio between the amount of nitrogen required by the agriculture system and the nitrogen contained in the manure or digestate produced by the dairy farm. We also varied animal density of the farm, options for milk and biogas production as well as crop characteristics such as biogas yield and harvest yield to investigate the influence of these factors on the nitrogen recycling potential of the system. At the end, we implemented an optimization on top of the material accounting model to figure out which commonly used crops would bring the best nitrogen recycling potential in certain combinations of biogas and milk production. The model observed that the nitrogen recycling advantage of one production combination in comparison with others might switch based on levels of a farm’s animal density; and these switching points are unforeseen by simple calculations. Therefore, to make wise decisions of nitrogen use and recycling for dairy manure-based biogas system, a quantitative material flow model is beneficial. Furthermore, we examined the nitrogen recycling potential from a perspective of the dairy farm itself and of the whole agriculture production system. The results showed that high biogas production or high milk production does not always couple with high nitrogen recycling potential, which leaves us a discussion on the trade-offs among these production options. |
