IMPACT OF THE REACTOR HYDRODYNAMICS AND ORGANIC LOADING ON THE SIZE AND ACTIVITY OF ANAEROBIC GRANULES

Wastewater treatment processes based on the upflow anaerobic sludge be d design are strongly dependent on the aggregation of biomass into mac roscopic granules (1-3 mm) which settle well. The reactor hydrodynamic s is of importance in the granulation process. The effect of the liqui d upflow velocity v(UP) associated with the operating time on the mean granule size and on the hydrogen, formate, acetate, propionate and gl ucose specific activities was studied, at various specific loading rat es, in upflow sludge bed and filter reactors of 13 1 fed with sugar wa stewater. 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, beca use an immediate spatial gradient of the sludge particle sizes was ind uced. The v(UP) had a significant positive effect on mean granule size . A specific loading rate increase from 0.5 to 1.5 g chemical oxygen d emand per gram of volatile suspended solids per day raised proportiona lly the biomass growth rate, but had no positive effect on the granule development in size. Moreover, the v(UP) had little effect on the spe cific 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 sever al mechanisms related to fluidization: improved penetration of substra tes into biofilm; insignificance of liquid shear relative to the shear of gas; reduction of particle friction and attrition with the bed voi dage. Acidogenic (glucotrophic) activity decreased when v(UP) increase d yielding minimum values at intermediate V-UP, between 2 and 5 m h(-1 ). Postacidogenic activities (propionate, acetate, formate, H-2) were positively influenced by v(UP) to a slight extent. Glucose activity gr adient within the granule bed was highly and inversely correlated to t he 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 popul ations within the granule spatial structure.