Anaerobic Biogranulation as Microbial Response to Substrate Adequacy

Abstract

Microbial granulation was examined in four lab-scale upflow anaerobic sludge blanket (UASB) reactors under varied microbial load indices (MLIs). The dimensionless MLI is defined as the ratio of the sludge loading rate to the specific methanogenic activity, and represents the relative substrate sufficiency subjected to by the sludge. A synthetic feed consisting of glucose, peptone, and meat extract was used as substrate and anaerobic digested sludge as seed. The four reactors were designed with the same diameters yet different heights, aiming at achieving a different biological selection pressure (solely MLI) yet a similar physical selection pressure in the reactors. Granulation under the high MLIs of 0.8 and 0.6 developed well within three and four months of operation, respectively, whereas that under the low MLIs of 0.3 and 0.2 did not even after six months. The granulation process was characterized by three phases of acclimation, granulation, and maturation. The MLI provides a quantitative approach toward stepwise sludge loading rate increases during start-up, with the appropriate values of 0.6–0.8 for fast and well-proceeded granulation.