Biodegradation of model macromolecules (proteins and polysaccharides) in wastewaters

Abstract

Macromolecules such as proteins and polysaccharides can constitute a significant portion of dissolved organic carbon (DOC) in wastewater, but limited information is available on how these compounds are degraded in biological wastewater treatment systems. Bacteria cannot assimilate intact macromolecules but must first hydrolyze them to monomers or small oligomers. To better understand the mechanism of macromolecule degradation in wastewater treatment systems this study investigates two important questions of macromolecule metabolism. First, does hydrolysis occur in close proximity to the cell or are the hydrolytic enzymes released into bulk solution, and second, if hydrolysis is cell-associated, are hydrolyzed fragments directly assimilated into the cell or are they released back into solution? Fluorescent model substrate analogs were used to determine the location of leucine aminopeptidase and a-glucosidase activity in wastewater inoculated biofilm and suspended cultures and in trickling filter effluent. In biofilm and suspended cultures at least 93% of hydrolytic activity was cell-associated. In trickling filter effluent hydrolysis rates were at least five times higher in contact with cells and sloughed biofilm pieces than in cell-free solution. To determine whether hydrolytic fragments were directly assimilated or released into solution molecular size distributions in bulk solution were monitored using membrane ultrafiltration techniques during the degradation of the model protein, bovine serum albumin, and the model polysaccharides, dextran and dextrin, in batch and continuous suspended cultures, and in fixed-film reactor systems. Measurable amounts of macromolecule hydrolytic fragments accumulated in all reactor configurations, substrates and inocula tested. Relatively larger concentrations of hydrolytic fragments accumulated in pure culture than in wastewater culture inoculated reactors. These results support a generalized model for macromolecule degradation by bacteria that features cellbound hydrolysis of protein and polysaccharides and the subsequent release of hydrolytic fragments back into bulk solution. This hydrolysis and release is repeated until fragments are small enough ( < 1000 amu) to be assimilated by cells. A separate, but related, part of this study adapted pre-column orthophthaldialdehyde derivatization, reverse phase high performance liquid chromatographic separation and fluorometric detection to measure free and combined amino acids in unconcentrated wastewaters and to assess their treatability in wastewater treatment systems.