AbstractThe influence of variations in carbon source concentration, cell inocula, pH, presence of other substrates, and other organisms on the biodegradation of 2‐chlorophenol (2‐CP) was studied for Alcaligenes sp. isolated from natural sources. Assays of biodegradation were performed in batch and continuous‐flow fluidized‐bed aerobic reactors. Evaluation of biodegradation was performed by determining total phenols, chemical oxygen demand (COD), and 2‐CP by ultraviolet (UV) spectrophotometry. Measurement of microbial growth was carried out by the plate count method. Bioassays of acute toxicity were performed to evaluate detoxification by using Daphnia magna. Results obtained show that under batch conditions with initial inocula of 106 cells/mL the strain grew exponentially with 100, 200, and 300 mg/L of 2‐CP within 48 hr. A lag period was observed with low cell density inocula (105 cells/mL). The strain showed marked delay in the biodegradation of 2‐CP at pH 5. Removal of target substrate from mixtures containing other carbon sources demonstrated the possibility of concurrent growth. Mineralization of 2‐CP was assessed by gas chromatography carried out at the end of the batch assays and at the exit of the continuous‐flow reactor. The presence of other organisms (bacteria, rotifers, ciliate, and algae) that developed in the fluidized‐bed reactor did not affect the efficacy of the biodegradation of 2‐CP. The removal of 2‐CP in the two assayed systems was over 97% in all cases. Toxicity was not detected at the exit of the continuous reactor. © 2001 John Wiley & Sons, Inc. Environ Toxicol 16: 306–313, 2001