Process performance of biogas digesters incorporating pre-separated manure

The yield from anaerobic digestion of liquid manure is often too low to make biogas production economically viable. This study examined the potential for partly substituting the liquid manure with solids from solid–liquid separation. The proportion of manure solids was gradually increased to 60%, which resulted in a higher gas production per digester volume than could be achieved by liquid manure alone. During the period with 60% substitution, the yield in the high-solids digester was almost twice that in the digester based on liquid manure alone. The yield in the high-solids digester was slightly above 200 L CH4 kg− 1 VS for the most part, while the yield in a reference digester containing only liquid pig manure was mostly above 300 L CH4 kg− 1 VS. Thus, the high ratio of solid matter reduced gas yield as a proportion of VS under thermophilic conditions, indicating that the high-solids process was inhibited by the high amounts of solid manure. This was most likely caused by NH3 inhibition, as the NH4–N level increased to more than 5 g L− 1. However, post-digestion in a mesophilic digester almost fully compensated for the lower conversion of VS in the high-solids digester.