The impact of biogas production on the circularity of nitrogen flows around a dairy farm

Keywords: recycling nitrogen flows, circular economy, biogas production Dairy farms require a significant amount of nitrogen to enter the production system via cattle fodder, which in intensive farming can be traced back to... [ view full abstract ]

Keywords: recycling nitrogen flows, circular economy, biogas production

Dairy farms require a significant amount of nitrogen to enter the production system via cattle fodder, which in intensive farming can be traced back to artificial fertilizers. As a by-product of dairy farms, cattle manure contains undigested nitrogen that allows the farmers to reuse it for their grassland and reduce net imported artificial nitrogen. The manure is traditionally applied directly on the land, however nowadays, when anaerobic digestion is adopted, the farmers can gain extra income from biogas production and still have nitrogen remaining in the digestate to use as a fertilizer.  To increase the biogas yield, the farmers can also decide to digest their manure with other co-feeds. As a consequence of using co-feeds for biogas digesters, the nitrogen concentration in the digestate is largely increased and this allows the recycled nitrogen flow to exceed the boundary of the dairy. Nevertheless, it is noticed that the recycled nitrogen flow is never a closed loop because of the continuous stream of nitrogen that leaves the system via the dairy products. Therefore it is interesting to figure out to which extent different options of biogas production and nutrient recycling can help farmers to reduce their dependency on artificial nitrogen.

This study is a material flow analysis of nitrogen around intensive dairy farms combined with biogas production. Based on a literature review, we map different recycling fates of nitrogen in this combined system and then use material flow accounting to assess how different system configurations impact the potential for nitrogen cycling.

Nitrogen recovery can be achieved via four potential recycling loops:
1) direct application of manure to the grassland of the dairy farm
2) mono-digestion; the digestate is applied directly to grassland of the dairy farm;
3) co-digestion; the digestate is applied directly on both the dairy farm and the farm which provides co-feed for the production;
4) co-digestion; nitrogen in the digestate is extracted to produce artificial fertilizers and then comes back to the dairy farm and to the farm which provides co-feed for the production.

These models were examined with different co-feeds which vary on nitrogen content and biogas yield. The results provide an overview of the different extents of the circularity of recycling nitrogen in the combined system of biogas production and related agriculture production. In a broader perspective, the results also point out that the quantitative characteristics of a material flow play a critical role in the feasibility of its recycling pathways.