Abstract

Granular sludge, a self-aggregating spherical biofilm, possesses better stability compared to flocculent sludge under extreme conditions. This study compared the ability of anaerobic granular sludge (AnGS) and flocculent waste activated sludge (WAS) to convert food waste (FW) into highly-valuable optically active lactic acid (LA), a central and versatile intermediate platform molecule. Different loadings (30–60 g volatile suspended solids (VSS)/L) were applied and LA was accumulated to 52.7 g chemical oxygen demand (COD)/L on day 7 with optical activity (OA) of 77% using granular consortia under the highest FW loading, exhibiting a 48% increase in the yield over flocculent WAS on day 6. An initially delayed utilization of carbohydrates and subsequently enhanced glycolysis occurred with AnGS, which was in line with its high accumulation of LA. Furthermore, the AnGS consortia were dominated by LA bacteria (81%), including Bacillus, Lactobacillus, and Bifidobacterium, much higher than the WAS consortia (48%). The AnGS was more tolerant to LA inhibition, preventing cytoplasmic pH decrease (intracellular H+ in AnGS was 42% lower than in WAS) from the dissociation of carboxylic acid inside the cells. This might be due to its particular multi-layered aggregate extracellular polymeric substance structure and the enhancement of H+-transporting ATPase (increased by 120% in AnGS). This study provides a practical way to overcome carboxylic acid inhibition and increase the recovery of value-added platform molecules from organic wastes.

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