MOLECULAR-WEIGHT DISTRIBUTION OF HYDROLYSIS PRODUCTS DURING THE BIODEGRADATION OF MODEL MACROMOLECULES IN SUSPENDED AND BIOFILM CULTURES .2. DEXTRAN AND DEXTRIN

To improve wastewater treatment models, it is important to consider th at wastewater is composed of a variety of complex molecules, many mole cules having large molecular weights. Previous experiments have shown that hydrolytic enzymes are cell-associated and that hydrolytic fragme nts accumulate in bulk solution during the degradation of a model poly saccharide (dextran) in pure culture. These results indicate that inco mpletely hydrolyzed macromolecules are released into solution prior to their complete degradation. The authors wanted to determine whether t he release of incompletely degraded molecules was specific to dextran degradation by pure cultures or whether it could be generalized to mix ed culture systems and the degradation of other polysaccharides. To ac complish this, both pure and mixed (wastewater) cultures were used to examine the degradation of dextran and another macromolecular polysacc haride, dextrin, in batch suspended culture, continuous suspended cult ure and fixed-film reactor systems. Membrane ultrafiltration was used to monitor the molecular weight distribution of polysaccharides in sol ution during degradation. In all reactor configurations, and for all s ubstrates and inocula investigated, small-molecular-weight (< 1000 amu ) oligosaccharides accumulated in solution during polysaccharide degra dation. These results, in conjunction with results of enzyme studies, support a generalized model for macromolecular degradation by cells th at features cell-bound hydrolysis of polysaccharides and the subsequen t release of hydrolytic fragments back into bulk solution. This hydrol ysis and release is repeated until fragments are small enough(< 1000 a mu) to be assimilated by cells. Essential features of this model are t hat polysaccharide diffusivity changes during its degradation and that different enzymes, with different methods of operation and different kinetic characteristics, may be used in successive hydrolytic cleavage s. These features are particularly important to consider in evaluating macromolecule degradation by aggregates and biofilms and in understan ding overall uptake kinetics in bioreactors. (C) 1997 Elsevier Science Ltd.