Life cycle assessment of biogas digestate processing technologies

Driven by a high increase of large scale biogas plants based on bio waste, agricultural by-products and waste from food industry, there is a rapid structural development of the agricultural holdings in Germany. Particularly in regions with intensive livestock husbandry, this leads to an overprovision of nutrients. New technologies have been introduced during the last years to treat biogas digestate for optimal transport and application conditions. An environmental Life Cycle Assessment (LCA) was carried out in order to compare the environmental impacts and the energy efficiency of seven treatment options of biogas digestate. The treatment options include one conventional digestate management option (storage and application of untreated manure on agricultural land), one stabilization process (composting), three mechanical drying options (belt dryer, drum dryer and solar dryer), one option using thermal vaporization (concentration) and finally one physical–chemical treatment (combination of separation, ultra-filtration, reverse osmosis and ionic exchanger). Primary energy demand (PED), global warming potential (GWP) and acidification potential (AP) were analysed and presented per kg of digestate on the input side of the system as functional unit (fu). Based on the default parameter setting, four scenarios have been defined to analyse the influence of different feedstock, different kinds of energy supply, different emission reductions techniques and different logistic chains on the LCA results. In the overall comparison, solar drying, composting and physical–chemical treatment were identified to be the most suitable options to reduce the use of resources and environmental impacts compared to the conventional digestate management. Belt drying turned out to be the handling process with the highest PED demand, GWP and AP among the compared options. Total PED varies from −0.09 MJ/fu (i.e. savings) in the composting option up to 1.3 MJ/fu in the belt drying option. The GWP was in a range between 0.06 CO2 eq./fu for solar drying to 0.1 kg CO2 eq./fu for belt drying. The amount of AP ranged from 2.7 kg SO2 g eq./fu in composting to 7.1 g SO2 eq./fu in belt drying. The results indicate that the environmental impact depends largely on nitrogen related emissions from digestate treatment, storage and field application. Another important aspect is the amount and kind of fuel used for heat supply (biogas, natural gas) and the procedure chosen for the allocation among heat and power.

► Environmental impact of seven digestate treatment options.
► The best digestate management options are composting and solar drying.
► Nitrogen related emissions from digestate management are of major importance.
► Emission reduction technologies show large potential for improvement.
► Waste heat from biogas CHP improves environmental performance of drying technologies.