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.