Pulsed fed-batch strategy towards intensified process for lactic acid production using recycled paper sludge

Abstract

A process has previously been optimized for production of lactic acid from recycled paper sludge (RPS), the ultimate solid waste obtained in the wastewater treatment plant of a local paper recycling mill. In this work, both separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) processes were improved under batch mode in bench-scale bioreactor. The use of bioreactor was advantageous, providing higher conversion rates, together with increased yield. By running SSF in bioreactor, 73.2 g L−1 of lactic acid has been produced by Lactobacillus rhamnosus ATCC 7469 from 179 g L−1 RPS, corresponding to 76 % of maximum theoretical yield. Further process intensification, with improved kinetics and final product concentration, was achieved by applying a pulsed fed-batch strategy. With six pulsed additions of RPS (40 g at each 5 h), 108.2 g L−1 of lactic acid was produced (after 120 h, corresponding to 62 % yield). This achievement contributes to make more realistic the large-scale upgrading of RPS, a waste raw material exhibiting negative cost.