A Citrus Peel Waste Biorefinery for Ethanol and Methane Production

Abstract

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.

Keywords: bioethanol; biomethane; biorefinery; biorefinery residues; citrus peel waste.

Figure 1

Process flow sheet of the zero-waste biorefinery used for citrus peel waste (CPW) valorization.

Figure 2

Εthanol titre achieved in Pichia kudriavzevii KVMP10, Saccharomyces cerevisiae, and Kluyveromyces marxianus fermentations of hydrolyzates obtained through dilute acid treatment of CPW.

Figure 3

Cumulative methane production using 6 g_VS L⁻¹ of untreated CPW (), dried citrus peel waste (DCPW) (), and solid biorefinery residues (BR) (). A control fermentation was conducted without the addition of CPW (), while all experiments were conducted at 37 °C. Dashed lines represent the time where substrate refeed was applied.

Figure 4

Concentration of volatile fatty acids, (A) formate, (B) propionate, (C) acetate, (D) butyrate, and (E) valerate, formed during anaerobic digestion of untreated CPW (), DCPW (), solid BR (), and in the control experiment ().

Figure 5

Yield of methane (A) as mL of product per g of initial volatile solids (VS) and (B) as mL of product per g of raw material (rm) generated from anaerobic digestion of CPW, DCPW, and BR following completion of each batch.

Figure 6

Cumulative methane production (A) and methane content per g of initial VS (B) using 3 g L⁻¹ (), 6 g L⁻¹ (), 12 g L⁻¹ (), and 24 g L⁻¹ () initial volatile solids of CPW. A control fermentation was performed without the addition of CPW (), while all experiments were conducted at 37 °C.