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 degrees 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 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 degrees C resulted in specific methane yields of 0.060-0.085 m(3) kg(-1) volatile solids (VS) during initial 30-50 d and 0.16-0.18 m(3) 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 m(3) kg(-1) VS and 0.13-0.16 m(3) kg(-1) VS of methane after 30-50 d and after 250 d, respectively. Methane yields for fractions