Residual biochemical methane potential (BMP) of concentrated digestate from full-scale biogas plants

In Denmark 50% of animal manure must be used for biogas production by 2020. Danish manure is generally managed as slurry with a dry matter content of 1–10%, and this process limits its volumetric biogas yield due to diluted feedstock. This leads to a demand for co-substrates in order to concentrate feedstock to make biogas production financially viable, since only digesting animal manure is not economically feasible. However, most of the potential co-substrates from the Danish food industry are currently being utilised, leading to a demand for new co-substrates such as concentrated animal slurry. This study investigates the potential benefits of recirculating concentrated digestate to biogas digesters by determining the biochemical methane potential (BMP) of the digestate. The BMP was 156–240 CH4 NL kg−1 VS, which is similar to the BMP of untreated cattle slurry. The gravimetric BMP (BMP per kg wet waste) however, estimated as 15–49 CH4 NL kg−1, was much higher from the digestate than from untreated animal slurry. The results from this study indicate that recirculating concentrated digestate to biogas digesters may be beneficial by increasing biogas yield capacity and process stability