Bioenergy recovery from two-stage mesophilic-thermophilic anaerobic digestion of cheese whey

Abstract

Among renewable energy sources, hydrogen and methane are gaseous fuels that have a higher energy density than petroleum-derived gasoline and diesel. In recent years, there has been increasing interest in converting existing anaerobic digestion systems to a two-stage process that produces hydrogen in the first stage and then methane in the second stage and is characterized by increased recovery of biofuels and bioenergy. In this study, an assessment of the energy recovery from two-stage mesophilic-thermophilic anaerobic digestion of cheese whey (CW) was carried out. After appropriate dilution, CW was fed at three COD levels of 6.8, 9.2 and 13.8 g O 2 /L to a mesophilic acidogenic reactor with a hydraulic retention time (HRT) of 10 h. The acidogenic reactor effluent was then fed to three thermophilic methanogenic reactors, which operated at different HRTs: 72, 48, and 24 h. Polyurethane foam was used to immobilize the anaerobic acidogenic and methanogenic sludge. The heating value (HV) was determined according to the experimental method using a bomb calorimeter for modes with the highest methane content and with the highest hydrogen content, as well as a calculation method based on the content of combustible gases in biohythane. The obtained deviations of the calculated from the experimentally determined HV of biohythane (4.84–7.60%) may be due to the fact that the HV of biohythane is probably not equal to the sum of the HV of pure hydrogen and methane. While an increase in the hydrogen content in biohythane leads to a decrease in the deviation of the calculated HV of biohythane from the experimental one. The maximum energy production rate was 53.2 kJ/(L d) at a COD level of 13.8 g O 2 /L, HRT in an acidogenic reactor of 10 h, and HRT in a methanogenic reactor of 48 h. The maximum energy yield of 14.42 kJ/g COD was obtained at a COD level of 9.2 g O 2 /L, HRT in an acidogenic reactor of 10 h and HRT in a methanogenic reactor of 72 h.