Continuous-flow chemostats were operated at different mean-cell-residence-times (theta c) and influent concentrations of a xenobiotic (2,4-D) and biogenic substrates. Steady state chemostat biomasses' performances in 2,4-D degradation were analyzed with a mathematical model to determine the quantities of degrader the biomasses contained. The qualification for microbial cells to become degraders is a high grade of degradation kinetics. This qualification uniformly applies to all biomasses. The quantities of degraders vary inversely with the chemostats' theta c. Biogenic substrates increase degrader yield such that a high biogenic and a high xenobiotic influent optimize degrader mass output. Economics evaluation performed around the optima finds the influent containing 5-25% 2,4-D carbon (TOC) in approximately 900 mg/l biogenic TOC, and the theta c of 2-5 d, are suitable operating conditions for a degrader producing bioreactor that may serve as a selector of biomass for bioaugmentation purposes. (C) 2012 Elsevier Ltd. All rights reserved.