A combined cellulosic and starchy ethanol and biomethane production with stillage recycle and respective cost analysis.

To minimize waste generation in ethanol production from starchy and cellulosic feedstocks, the feasibility of stillage recycle to fermentation process was studied. For sugarcane bagasse (SCB) and palm empty fruit bunch (EFB), optimal hot-compressed water (HCW) pretreatment and enzyme hydrolysis (10% wt) gave the highest total reducing sugar (TRS) yields of 64.2% and 67.3%, respectively. Ethanol fermentation of SCB, EFB and fresh cassava by Saccharomyces cerevisiae TISTR5606 gave the highest yields of 0.31 g g−1, 0.40 g g−1, and 0.31 g g−1 TRS, respectively. For recycling of SCB and EFB stillage, a slight decline of ethanol yield was found while ethanol yield of cassava significantly increased from 60.8% to 89.9%. The ethanol yields from the 2nd recycle of cassava was still 10% higher than no recycle. Addition of 10%–20% v/v black liquor from lignocellulosic HCW pretreatment into anaerobic digestion system noticeably enhanced the chemical oxygen demand removal and methane production. Analysis of variable operating cost showed that stillage recycles for 20% for fresh cassava and 10% for SCB is cost-effective process for ethanol production. • Cassava stillage recycle significantly increased ethanol yield from 60.8% to 89.9%. • The ethanol yields from the 2nd recycle of cassava was still 10% higher than control. • Adding 10–50% of black liquor from SCB enhanced COD removal and CH 4 yield. • Adding 10–20%v/v black liquor from EFB enhanced COD removal and CH 4 yield. [ABSTRACT FROM AUTHOR]