Biohydrogen production from lactic acid: Use of food waste as substrate and evaluation of pretreated sludge and native microbial community as inoculum

Nowadays, lactic acid (HLa) has caught the eye as a precursor of H-2 production from substrates that harbor a significant amount of lactic acid bacteria (LAB), such as Lactobacillus and Lactococcus. Since the native microbial community of food waste (FW) is dominated by LAB, the production of H-2 during dark fermentation might be inhibited by the displacement of H-2-producing bacteria by LAB. Therefore, lactate-driven dark fermentation in two steps is an alternative to overcome the overproliferation of LAB. In this process, FW is converted into HLa, which can subsequently be transformed into H-2 by HLa-consuming bacteria during a cross-feeding mechanism with H-2-producing bacteria. In the present study, FW was evaluated for HLa production at different organic loading rates (OLR) (12.5, 25.0, 37.5, and 50.0 gVS/L/d) in a semi-continuous reactor, and the HLa obtained was used for the H-2 production. Results showed that OLR significantly influences HLa production during the self-fermentation of FW (p < 0.05). The highest ORL of 50.0 gVS/L/d and an HRT of 1.25 days generated the net maximum HLa production of 10.8 g/L. The HLa production was companied by the presence of acetate, propionate, and succinate, which accounted for only 20 % of the total organic acids produced. Then, effluent with HLa concentrations of 5.0, 10.0, 15.0, 20.0, and 25.0 g/L was evaluated for H-2 production in batch tests using thermally pretreated sludge and the native microbial community of the effluent as inoculum. Even though different metabolic pathways for H-2 production can be noted during the fermentation, which differs from one inoculum to another, the highest value of H-2 production of 596.3 mL/L-reactor was obtained with the native microbial community at the highest concentration of HLa (25.0 g/L). During the fermentation process for H-2 production, HLa and propionate were mainly transformed into butyrate and acetate. The present study demonstrated the feasibility of obtaining H-2 from a complex substrate, such as effluent from a previous fermentation, using different inocula.