Enhanced biohydrogen production from tofu residue by acid/base pretreatment and sewage sludge addition

Abstract

Anaerobic dark fermentation is considered a promising technology for clean energy production and waste reduction. In the present work, tofu residue and sewage sludge were utilized as substrates for fermentative hydrogen production. To increase the biodegradability, tofu residue was pretreated for 30 min in the presence of HCl and NaOH at various concentrations (0, 0.5, 1.0, and 2.0%), and then fermented by a thermophilic (60 °C) mixed culture. The solubility (SCOD/TCOD ratio) of the tofu residue increased from 4 to 30–40% after pretreatment, and the increased soluble constituents were mainly protein rather than carbohydrate compounds. However, in spite of a slight increase of carbohydrate solubility, the H 2 production performance was significantly enhanced by pretreatment, owing to the degradability of thermophilic cultures used on insoluble tofu residues. The limited H 2 yield of 0.30 mol H 2/mol hexose added achieved in the raw tofu residue was increased 1.6- to 4-fold with the highest H 2 yield of 1.25 mol H 2/mol hexose added at 1.0% HCl concentration. Carbohydrate degradation and the H 2 production rate also increased from 39 to 50–65% and 27 to 50–120 mL H 2/L/h, respectively. The role of pretreatment was not only to increase the biodegradability but also to suppress the activity of indigenous non H 2-producers such as lactic acid bacteria and propionic acid bacteria. When sewage sludge was added to acid pretreated (1.0% HCl) tofu residue as a co-substrate, the H 2 yield and H 2 production rate increased to 1.48 mol H 2/mol hexose added and 161 mL H 2/L/h, respectively, which was attributed to the abundant minerals, vitamins, and metals contained in sewage sludge.