anaerobic digestion Technology: general issues 12 biogas 11 History of engineering & technology 8 biomass 8 biorefinery 7 biochar 6 sustainability 6 biodiesel 5 circular economy 5 fermentation 5 gasification 5 life cycle assessment 5 lignocellulosic biomass 5 pretreatment 5 torrefaction 5 wastewater 5 wastewater treatment 5 Research & information: general 4 adsorption 4 combustion 4 manure 4 microalgae 4 optimization 4 renewable energy 4 syngas 4 transesterification 4 biochemical methane potential 3 bioenergy 3 biogas production 3