Association of coliform, bacteria with wastewater particles: impact of operational parameters of the activated sludge process

The fraction of particles with associated coliform bacteria (PAC) in the ac tivated sludge process was evaluated using a 16S rRNA oligonucleotide probe specific to the family Enterobacteriaceae. The PAC was found to decline ex ponentially with increasing mean cell residence times (MCRTs). The factors influencing the formation of PAC, identified with simplified mass balance r elationships, are the concentration of particles, the concentration of disp ersed (non-particle associated) coliform bacteria, and the MCRT. The concen tration of dispersed coliform bacteria was found to decline with increasing MCRTs, The rate of decline was greater than the typical half-life attribut ed to endogenous decay, suggesting that other factors (e.g., predation by p rotozoa) influence the concentration of dispersed coliform. bacteria, and s ubsequently the formation of PAC. Given that the association of targeted or ganisms with particles adversely impacts the performance of a disinfection system, studies targeted at the fate of organisms other than coliform bacte ria in the activated sludge process are of paramount importance in assessin g the health risks of post-disinfected effluents. (C) 2001 Elsevier Science Ltd. All rights reserved.