Alkaline and fungal pretreatments for improving methane potential of Napier grass

Alkaline and biological pretreatments were compared for enhancing the biological methane potential of Napier grass. The earlier reported biotreatments for Napier grass did not use the edible white-rot fungus Pleurotus sajor-caju, as in the present work. Dry Napier grass was ground to different particles sizes (20–30 mm, N1-L; ≤0.6 mm, N1-S). The N1-L grass was treated with alkali and designated as the alkali treated grass N2. The samples N1-S, N1-L and N2 were used separately as substrates for growing the fungus for 14 days at room temperature (30 ± 2 °C) in a solid-state biotreatment. Alkali treatment delignified the grass 2.1- to 10.7-fold better than the fungus. Fungal treatment resulted in 3.8- to 8.3-fold loss in glucan compared to alkali treatment. Maximum xylan loss occurred in the N1-S fine-ground grass after fungal growth. The fungus-grown grass samples (N1-FL, N1-FS, N2-F), the untreated ground samples (N1-L, N1-S) and the alkali treated sample (N2) were anaerobically digested to determine the biological methane potential. The fungus-grown grass samples had a maximum daily methane production in the range of 44–50 cm3 g VS−1, significantly higher than the samples not treated with the fungus. The alkali treated grass gave a significantly higher cumulative methane yield than the untreated grass and the biological methane potential was ∼71–77% of the theoretical methane potential. The proportion of methane in the total gas produced from the treated grass was in range of 74–83% by volume whereas it was 57–68% for the untreated grass.