BACKGROUND: Using a suspended or immobilized microbial community obtained through chemostat selection from agricultural soils formerly treated with triazinic herbicides, the atrazine biodegradation kinetics in continuous aerobic reaction systems was studied. RESULTS: When microbial cells were continuously cultivated on atrazine as the sole nitrogen and carbon source, atrazine removal efficiencies eta(ATZ) near to 100% were reached, although accumulation of metabolic byproducts was detected. The fluidized-bed biofilm reactor allowed atrazine removal rates R-V,R-ATZ higher than that permitted by suspended cell cultures with similar removal efficiencies. In this system, the highest volumetric removal rate was obtained (R-V,R-ATZ -12.2 mg L-1 h(-1)), with herbicide removal efficiencies eta(ATZ) near 100% and reduced accumulation of byproducts. CONCLUSIONS: With the operational conditions probed in continuous suspended-cell culture, increasing the C: N ratio in the inflowing medium resulted in higher cell growth yields but not in better atrazine removal rates. Kinetic results showed that for similar working conditions higher R-V,R-ATZ values and reduced amounts of degradation byproducts of recalcitrant organic compounds could be expected with multi-stage biofilm reactors. (C) 2009 Society of Chemical Industry