Assessing the sustainability of small-scale anaerobic digestion systems with the introduction of the emergy efficiency index (EEI) and adjusted yield ratio (AYR).

•Emergy analyses of digesters for human waste in Haiti and dairy manure in the US.•New emergy indices (EEI and AYR) were created for comparisons of analogous products.•Food and waste emergy analyses were conducted to determine waste emergy values.•Inputs from wastes accounted for more than 99% of the emergy in digesters.•When emergy waste values were zeroed, apparent sustainability improved.