L-Lactic Acid Production from Rice Straw via In Situ Product Removal with Recycling for Reduced Water Footprint

Abstract

Water consumption and wastewater generation are crucial for sustainable lignocellulosic biorefineries. In this work, a closed-loop fermentation with a zero liquid discharge (ZLD) downstream approach has been developed for l-lactic acid (LA) production from acid-pretreated rice straw. Fermentation of rice straw sugars was conducted using in situ product removal (ISPR) with Lactobacillus casei and Indion-860S, a weakly basic anion-exchange resin. The process involves recycling the fermentation spent broth and the regenerated resin for successive fermentation cycles, resulting in gradual xylose enrichment. The study was conducted over 10 cycles and resulted in an average yield of 0.77 ± 0.09 g LA/g consumed glucose. The average glucose consumption was 81.7%. The inlet xylose concentration of the hydrolysate increased 3.2 times from 5.2 to 16.7 g/L by Cycle 10, without any additional unit operation. Furthermore, a ZLD downstream strategy was proposed to convert the eluted sodium lactate into purified free LA, with an integrated mass and energy balance for estimating the variable operating costs of the proposed scheme. Three variable operating cost (VOC) scenarios were evaluated. Scenario 1 (current study, USD 2.164/kg LA), Scenario 2 (ISPR with Amberlite IRA-67, USD 1.520/kg LA), and Scenario 3 (optimized ISPR with Indion-860S resin and spent-broth recycling, USD 1.260/kg LA). Scenario 3 projected the lowest estimated VOC of LA, demonstrating the potential of the recycling strategy for economic and environmental sustainability of lactic acid production from rice straw.