Biomethane production from maize and liquid cow manure – Effect of share of maize, post-methanation potential and digestate characteristics

This study investigates the co-digestion of liquid cow manure and maize with different share of maize in continuously stirred tank reactors (CSTRs). The objective was to determine the methane yield of reactor and the post-methanation potential of the digestate from different reactor trials. The highest specific methane yield (259 Nl CH4/kg volatile solids (VSs)) was obtained when the share of maize in the feedstock was 40% (VS) and the second highest specific methane yield was when the proportion of maize was 60% (VS) (234 Nl CH4/kg VS). The post-methanation potential of the digestate was determined in batch assays. The minimum value (maize 40%, 75 ± 1 Nl CH4/kg VSfeed) occurred when the methane yield in the reactor was at a maximum and when the methane yield was at a minimum (maize 67%, 153 ± 46 Nl CH4/kg VS) the potential at a maximum (140 Nl CH4/kg VSfeed). Plant nutrients and trace elements were determined in the feedstock and the digestate. As maize contained fewer nutrients than manure the concentration of the nutrients and trace elements in the digestate decreased when the share of maize in the feedstock increased. Thus it seems that even though the CSTR co-digesting maize and manure can be operated with high organic loading rate (OLR) and short hydraulic retention time (HRT) a significant part of the methane yields of the feedstock may be lost, if the post-methane potential is not considered. Based on the yield results of the reactor trials a biogas plant (270 kW) could produce energy (methane) almost 2400 MW h, when the reactor operates at OLR 2.5kg VS/m3/d and feedstock consists of 60% VS of maize and 40% VS of liquid cow manure.