Integrated anaerobic–aerobic fixed-film reactor for slaughterhouse wastewater treatment

An integrated anaerobic–aerobic fixed-film pilot-scale reactor with arranged media was fed during 166 days with slaughterhouse wastewater. Operation temperature was 25 °C and the anaerobic–aerobic volume ratio was decreased from 4:1 to 3:2 and finally to 2:3. Overall organic matter removal efficiencies of 93% were achieved for an average organic loading rate of 0.77 kg COD/m3 d, and nitrogen removal efficiencies of 67% were achieved for nitrogen loading rates of 0.084 kg N/m3 d. The high internal recirculation associated to the air-lift effect linked to the aeration of a part of the reactor section caused high mixing between the anaerobic and aerobic zones, so that most organic matter was removed aerobically. The nitrification process achieved an efficiency of 91% for nitrogen loads of 0.15 kg N/m3 d when the anaerobic–aerobic volume ratio was 2:3 and was limited by dissolved oxygen concentration below 3 mg/l. The influence of the heterotrophic biomass growing in the outer biofilm was checked. Denitrification only implied the 12–34% of the total nitrogen removal and was limited by dissolved oxygen concentration in the anaerobic zone above 0.5 mg/l caused by the mixing regime. Most removed nitrogen was employed in synthesis of heterotrophic bacteria.