A bio-refinery concept for production of bio-methane and bio-ethanol from nitric acid pre-treated corncob and recovery of a high value fuel from a waste stream

The present paper reports the bio-refinery concept on exploiting the pre-treated non-detoxified corncob hydrolysate for bio-gas production and simultaneously utilizing the delignified cellulosic residue for bio-ethanol and black liquor for extraction of lignin. Based on the optimized conditions, corncob was pre-treated with HNO3 at 121 °C for 15 min at a solid concentration of 20% (w/v). Maximum sugar concentration achieved was 110.12 mg mL−1. The resultant corncob hydrolysate for bio-gas production was investigated in three anaerobic hybrid rectors. The reactors were operated at an OLR of 1.000, 1.428 and 2.500 g COD L−1d−1 and HRT of 10, 7 and 4 d. Maximum bio-gas production rate was in R2 (18.7 Ld−1), followed by R1 (18.41 Ld−1) and R3 (12.06 Ld−1), respectively at 7 d HRT. The COD removal efficiency was 81.0, 83.3 and 79.0% in R1, R2 and R3 reactors, respectively. Simultaneous saccharification and fermentation studies were carried out at three different enzyme loadings (9, 15 & 21 FPU g−1 substrate) by Kluveromyces marxianus at 42 °C for 96 h. 1% NaOH pre-treated cellulosic residue yielded maximum ethanol concentration (33.14 mg mL−1) with 15 FPU g−1 substrate enzyme load and at 87 h. The extracted lignin has a calorific value of 21.05 MJ kg−1.