Impact of the reactor hydrodynamics and organic loading on the size and activity of anaerobic granules

Abstract

Wastewater treatment processes based on the upflow anaerobic sludge bed design are strongly dependent on the aggregation of biomass into macroscopic granules (1–3 mm) which settle well. The reactor hydrodynamics is of importance in the granulation process. The effect of the liquid upflow velocity υ UP associated with the operating time on the mean granule size and on the hydrogen, formate, acetate, propionate and glucose specific activities was studied, at various specific loading rates, in upflow sludge bed and filter reactors of 13 l fed with sugar wastewater. Reactors which were operated at 0.9 m h −1 behaved as fixed beds while those run at 2.2, 4.4 and 6.6 m h −1 were fluidized, because an immediate spatial gradient of the sludge particle sizes was induced. The υ UP had a significant positive effect on mean granule size. A specific loading rate increase from 0.5 to 1.5 g chemical oxygen demand per gram of volatile suspended solids per day raised proportionally the biomass growth rate, but had no positive effect on the granule development in size. Moreover, the υ UP had little effect on the specific wash-out rate of the smaller particles. Henceforth the resulting final size of granules is essentially a function of the hydrodynamic regime. Major impact on granule net steady size is attributed to several mechanisms related to fluidization: improved penetration of substrates into biofilm; insignificance of liquid shear relative to the shear of gas; reduction of particle friction and attrition with the bed voidage. Acidogenic (glucotrophic) activity decreased with υ UP increased yielding minimum values at intermediate υ UP, between 2 and 5 m h −1. Postacidogenic activities (propionate, acetate, formate, H 2) were positively influenced by υ UP to a slight extent. Glucose activity gradient within the granule bed was highly and inversely correlated to the granule size, while for acetate activity gradients, the correlation was direct, although less strong. These observations are discussed in detail with regard to an ordered distribution of the consortium populations within the granule spatial structure.