The performance of an anaerobic mesophilic packed bed reactor, with a mixture of GAC and tezontle, followed by an aerobic suspended growth system was studied for the treatment of organic chemical wastewater with a high COD concentration (22–29 g/L). The testing of the anaerobic–aerobic system was conducted in an experimental set-up for almost 2.5 years. Different operational conditions were evaluated. The anaerobic reactor showed performance stability and COD removals higher than 80% were obtained with loads up to 16.6 kg.m−3.d−1. The acclimation of the aerobic biomass to the substrate in the anaerobic effluent was very quick and COD removals higher than 94% were obtained even at high organic loads. The combined anaerobic–aerobic system allowed total COD removals higher than 99.5% and the accomplishment of the discharge requirements of 200 mgCOD/L when the anaerobic reactor was operated with loads of 8–11 kg.m−3.d−1 and the aerobic reactor with 0.33 kg.kg−1.d−1, being the total HRT of 4.4. The average TKN removal in the anaerobic–aerobic system was 97%, the average for the anaerobic reactor being 52% and that one for the aerobic system being 94%.

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