Utilising biohydrogen to increase methane production, energy yields and process efficiency via two stage anaerobic digestion of grass

• Continuous, two stage anaerobic digestion of grass was performed successfully.
• Two stage AD was compared with single stage AD in parallel.
• 13% more energy was obtained from comparable substrate utilisation with two stage.
• Two stage AD increased process stability and required shorter retention times.

Real time measurement of gas production and composition were used to examine the benefits of two stage anaerobic digestion (AD) over a single stage AD, using pelletized grass as a feedstock. Controlled, parallel digestion experiments were performed in order to directly compare a two stage digestion system producing hydrogen and methane, with a single stage system producing just methane. The results indicated that as well as producing additional energy in the form of hydrogen, two stage digestion also resulted in significant increases to methane production, overall energy yields, and digester stability (as indicated by bicarbonate alkalinity and volatile fatty acid removal). Two stage AD resulted in an increase in energy yields from 10.36 MJ kg−1 VS to 11.74 MJ kg−1 VS, an increase of 13.4%. Using a two stage system also permitted a much shorter hydraulic retention time of 12 days whilst maintaining process stability.