A Citrus Peel Waste Biorefinery for Ethanol and Methane Production

This paper deals with the development of a citrus peel waste (CPW) biorefinery that employs low environmental impact technologies for production of ethanol and methane. Three major yeasts were compared for ethanol production in batch fermentations using CPW pretreated through acid hydrolysis and a combination of acid and enzyme hydrolysis. The most efficient conditions for production of CPW-based hydrolyzates included processing at 116 °C for 10 min. Pichia kudriavzevii KVMP10 achieved the highest ethanol production that reached 30.7 g L^-1 in fermentations conducted at elevated temperatures (42 °C). A zero-waste biorefinery was introduced by using solid biorefinery residues in repeated batch anaerobic digestion fermentations achieving methane formation of 342 mL g_VS^-1 (volatile solids). Methane production applying untreated and dried CPW reached a similar level (339-356 mL g_VS^-1) to the use of the side stream, demonstrating that the developed bioprocess constitutes an advanced alternative to energy intensive methods for biofuel production.