Kinetics and fundamental mechanisms of starch removal by activated sludge: hydrolysis of starch to maltose and maltotriose is the rate-determining step

Abstract

The mechanisms of hydrolysis and uptake of starch, a polymer in sewage, by activated sludge (AS) were investigated, taking into consideration the mechanisms of the maltose transport system, responsible for the uptake of hydrolysis products of starch, in Gram-negative bacteria. AS was acclimated to dextrin, and the molecular weight distributions of the substrates during dextrin removal are analyzed using a GPC method. The removal rates of substrates such as dextrin, various maltodextrins (maltotriose, etc.), maltose and glucose by AS, and the oxygen uptake rates (OURs) of AS during the removal of these substrates were compared. Low-molecular-weight saccharides such as glucose, maltose and maltotriose were undetectable in the substrates during dextrin removal. The removal rates of maltose and maltotriose were higher than those of dextrin and glucose, and the OURs of AS during the removal of maltose and maltotriose were higher than those for other substrates. Therefore, hydrolysis of starch to maltose (dimer) and maltotriose (trimer), and not to glucose (monomer), appears to be the rate-determining step during the removal of starch, although maltodextrins with molecular weights up to 1152 (maltoheptaose) appear to penetrate the bacterial outer membrane.