Effects of solid-liquid separation on recovering residual methane and nitrogen from digested dairy cow manure

The feasibility of optimizing methane and nitrogen recovery of samples obtained from farm biogas digester (35 °C) and post-storage tank (where digested material is stored for 9-12 months) was studied by separating the materials into different fractions using 2, 1, 0.5 and 0.25 mm sieves. Mass-balances revealed that digested material mainly consists of <0.25 mm (60-69%) and >2 mm (18-27%) fractions, while fractions between 2 and 0.25 mm made the rest. Incubation of solid fractions >0.25 mm of digester material at 35 °C resulted in specific methane yields of 0.060-0.085 m³ kg^-1 volatile solids (VS) during initial 30-50 d and 0.16-0.18 m³ kg^-1 VS at the end of 340 d incubation. Similarly, fractions >0.25 mm of post-storage tank material produced 0.055-0.092 m³ kg^-1 VS and 0.13-0.16 m³ kg^-1 VS of methane after 30-50 d and after 250 d, respectively. Methane yields for fractions <0.25 mm of post-storage tank was 0.03 m³ kg^-1 VS after 30-50 d and 0.05 m³ kg^-1 VS after 250 d compared to 0.20 m³ kg^-1 VS and 0.41 m³ kg^-1 VS, respectively for the same fraction of digester material. Separation of digested cow manure into solids and liquid fractions to recover methane may be feasible only for post-storage tank material and not for digester material. Nitrogen management would not be feasible with neither material as total nitrogen and ammonium-nitrogen concentrations were equally distributed among the segregated fractions.