Black liquor as an alternative alkaline agent in thermo-alkaline pre-treatment to enhance volatile fatty acid production from excessive sludge

Abstract

The inherent alkalinity and organic constituents of black liquor from chemical pulping enables chemical saving while serving as a sustainable alkaline substitute for thermal-alkaline pre-treatment of excessive sludge. The feasibility of using black liquor from pulping as an alkaline substitute to enhance volatile fatty acids production through anaerobic digestion of thermo-alkaline pretreated excessive sludge was evaluated in the present study. The results of batch methanogenic fermentation demonstrated that over 60 % of bio-available organic remain in the liquid phase which contributed to the kinetics acceleration of bioconversion. Besides, the addition of black liquor suppressed the methanogenic activity therefore conserved more volatile fatty acids as final production (the production of volatile fatty acids reached 300 mg per gram of volatile solids, in equivalence of chemical oxygen demand) and 160 mg acetic acid per gram of volatile solid. Black liquor presents the capability to inhibit the bio-activity of acetate-consuming methanogenesis. The electron buffer capacity of Lignin-carbohydrate complexes in black liquor might be responsible to the selective inhibition of acetate-utilizing methanogens, thereby reducing the metabolic consumption of non-utilized volatile fatty acids. This work establishes black liquor assisted thermo-alkaline pretreatment as a feasible strategy for magnitude volatile fatty acids production from excessive sludge.