Efficient bioconversion of deinked paper sludge into valuable biobased succinic acid by engineered Escherichia coli

With the ongoing rise in petroleum prices, there has been growing interest in the microbial production of succinic acid from industrial wastes. Deinked paper sludge (DPS) is a significant waste, rich in cellulose and hemicellulose, generated in paper recycling plants. The disposal of DPS raises environmental concerns, and potential health hazards. This study explored an alternative and integrated platform, combining effective pre-treatment and enzymatic hydrolysis of DPS, and efficient fermentation process, for biobased succinic acid production by Escherichia coli KJ122. Initially, 10% (w/v) DPS was pre-treated with 1 N NaOH for 16 h followed by autoclaving at 120 °C for 20 min to remove inhibitors. The cellulase loading of 60 PCU (Protein Centered Unit)/g NaOH-pre-treated DPS was found to be optimal to release the highest fermentable sugars to 84.6 ± 1.8% after hydrolysis. For separated hydrolysis and fermentation (SHF), succinic acid at 27.80 ± 0.37 g/L with a yield of 0.68 ± 0.20 g/g and productivity of 0.50 ± 0.00 g/L/h was achieved. However, DPS loading at 100 g/L NaOH-pre-treated DPS in SHF posed challenges due to high viscosity of the broth, requiring increased agitation and energy consumption, and prolonged operating time, particularly for large-scale operations. To address these issues, a fed-batch semi-SHF process was employed, significantly improving succinic acid yield (0.97 ± 0.01 g/g; 88.1% theoretical maximum) and productivity (0.55 ± 0.02 g/L/h). According to the mass balance analysis, producing 1 kg of succinic acid required 2.5 kg NaOH-pre-treated DPS. This work provides an alternative and effective method for converting DPS into high-value bio-succinic acid, offering potential applications in the paper industry and contributing to bio-circular economy and zero-waste principles.