Organic matter removal in combined anaerobic–aerobic fixed-film bioreactors

A combination of two fixed-film bioreactors (FFB) with arranged media, the first anaerobic and the second aerobic, connected in series with recirculation was fed continuously for 133 days with wastewater from a poultry slaughterhouse. Oxidation of the organic carbon compounds and nitrification were carried out in the aerobic FFB and methanogenesis and denitrification were performed in the anaerobic FFB. The average organic loading rate was 0.39 kg COD/m^3 d and 92% removal efficiencies of organic matter were achieved. COD-removal occurred mainly in the anaerobic FFB, increasing when the recirculation ratio rose from 1 to 6 due to the increase in the anoxic denitrification. The influence of the C/N ratio of the raw wastewater over the proportion in which the COD-removal was carried out by oxidation in the aerobic FFB, methanogenesis or denitrification in the anaerobic FFB was studied. When the volume of the aerobic FFB became smaller than that of the anaerobic one the fraction of organic matter removed in the anaerobic FFB increased, but also the ratio between the respective volumetric rates (rCODan/rCODae) increased. High recirculation and low C/NO-N ratio in the anaerobic FFB feed favoured the denitrification to the detriment of the methanogenic process. Regarding nitrogen removal for nitrogen applied loads around 0.064 kg TKN/m3 d the removal efficiency was of 95%, which decreased to 84% for 0.14 kg TKN/m^3 d. The stability of the nitrification process was the controlling factor of the nitrogen removal. High ammonia concentration caused by high recirculation ratio, specially when the aerobic FFB volume was smaller, caused nitrification inhibition which destabilised the system.