The Effects of Post-Treatments and Temperature on Recovering the Methane Potential of >2 MM Solid Fraction of Digested Cow Manure

Abstract

The effects of thermal and chemical treatments, mechanical maceration and freezing and thawing on recovering the remaining methane potential of the >2 mm solid fraction of digested cow manure – which accounted for 30% of the original potential of digested cow manure - were studied in laboratory batch assays at 5-20°C and at 35-55°C to evaluate the treatment effects both under long-term (340 d) storage of solids and during active digestion (30 d), respectively. The effects of different treatments on the methane production of the solids varied with incubation temperatures and time. However, in all cases, methane productions at 15°C and lower were slow and low for both untreated and treated solids even after long-term incubation. At 35 and 55°C more methane was recovered from untreated solids producing up to 61-82 ml g−1 volatile solids (VS)added in 30 d and 179-215 ml g−1 VSadded in 340 d. Only chemical treatment with or without thermal treatment enhanced the methane yields while some treatments even decreased the yields. An increase in temperature to 35°C of the assays incubated for 6 months at ≤ 20°C initiated more significant methane production. In conclusion, the methane potential of the digested solids in a farm-scale biogas system can be recovered by active digestion at 35 or 55°C and can be improved to a smaller extent through chemical treatment of separated solids fraction, while methane recovery at lower temperatures and with some of the treatments studied would not be effective.