AbstractThe fate of atrazine in biologically active laboratory‐scale granular activated carbon (GAC) reactors was studied through analysis of atrazine and its breakdown products in the influent and effluent water of the GAC reactor and sand reactor. The biological activity in the GAC bed, influent, and effluent was monitored by measuring the heterotrophic plate count (HPC) bacteria. The HPC reached equilibrium concentrations of 1.37 to 1.67 × 106colony‐forming units per milliliter (cfu/ml) after two to three weeks in the GAC effluents, and 7.65 × 103cfu/ml after four to six weeks in the effluent of the sand reactor. A solution of natural ground water spiked with 200 μg/L atrazine was continuously applied to the biologically active reactors at a flow rate of 5 ml/min for 70 d. Composite weekly samples of the effluent were extracted and analyzed for atrazine, deethylatrazine, and deisopropylatrazine using GC. Hydroxyatrazine was analyzed by liquid chromatography. Deethylatrazine was consistently detected in the effluent of the sand reactor only, and hydroxyatrazine was detected in the effluent of one of the GAC reactors, before complete development of bacterial activity. The percentage of atrazine disappearance was 98.10 to 98.67% in the GAC reactors and 42.10% in the sand reactor. The results suggest the possibility of complete mineralization of atrazine at 200 μg/L in biologically active GAC, with significantly low exportation of toxic breakdown products to the effluen