Co‐substrate composition is critical for enrichment of functional key species and for process efficiency during biogas production from cattle manure

Cattle manure has a low energy content and high fibre and water content, limiting its value for biogas production. Co‐digestion with a more energy‐dense material can improve the output, but the co‐substrate composition that gives the best results in terms of degree of degradation, gas production and digestate quality has not yet been identified. This study examined the effects of carbohydrate, protein and fat as co‐substrates for biogas production from cattle manure. Laboratory‐scale semi‐continuous mesophilic reactors were operated with manure in mono‐digestion or in co‐digestion with egg albumin, rapeseed oil, potato starch or a mixture of these, and chemical and microbiological parameters were analysed. The results showed increased gas yield for all co‐digestion reactors, but only the reactor supplemented with rapeseed oil showed synergistic effects on methane yield. The reactor receiving potato starch indicated improved fibre degradation, suggesting a priming effect by the easily accessible carbon. Both these reactors showed increased species richness and enrichment of key microbial species, such as fat‐degrading Syntrophomonadaceae and families known to include cellulolytic bacteria. The addition of albumin promoted enrichment of known ammonia‐tolerant syntrophic acetate‐ and potential propionate‐degrading bacteria, but still caused slight process inhibition and less efficient overall degradation of organic matter in general, and of cellulose in particular.