Anaerobic digestion of pig manure fibres from commercial pig slurry separation units

• Biochemical methane potential of manure fibres are depended on separation method.
• Methane production rate of manure fibre is lower than that of pig slurry.
• Re-circulating separated manure fibres is beneficial for biogas yield.
• CH4 production from manure fibres and lignocellulose content does not correlate.
• CH4 production from manure fibres and lignocellulose composition does not correlate.

The composition of manure fibres (MF) from 17 commercially separated pig slurries and seven raw pig slurries were characterised in terms of dry matter (DM), volatile solids (VS), protein, hemicellulose, cellulose and lignin. The average lignocellulose concentration in manure fibres and pig slurries was 790 and 370 g kg−1 [VS] respectively. Biochemical methane potential was ascertained after 60 days, revealing a trend in biochemical methane potential between the different separation technologies used: pig slurry > shaking filter and screw press combined ≈ decanter centrifuge > flocculation, belt and screw press combined ≈ screw press. The maximum methane yield of manure fibres from decanter centrifuges and the combined shaking filter and screw press was approximately 330 l [CH4] kg−1 [VS] at standard temperature and pressure (STP), while manure fibres from a screw press and a combination of belt press and screw press on average produced approximately 220 l [CH4] kg−1 [VS]. Initial methane production can be described using a first-order kinetic model. The average rate constant for manure fibres was 0.030 d−1 and for pig slurry 0.071 d−1, showing that pig slurry is digested much faster than manure fibres.