This paper presents the results of three pilot units comprising one UASB reactor followed by two anaerobic filters (AFs) operating in parallel (upflow and downflow modes). The UASB reactor had a volume of 416 litres, being operated at hydraulic retention times (HRT) of 6 and 4 hours. The anaerobic filters had total volumes of 102 litres (32 litres of packing material), being operated at HRT varying from 24 to 1.5 hours (upflow velocities varying from 0.06 to 1.44 m/h). These different operational conditions characterised seven phases of research. Both reactors were fed with domestic sewage pumped directly from the main interceptor of Belo Horizonte city – Brazil.

After almost one year of continuous monitoring, the UASB/AF system produced very good results in terms of COD and BOD removal, and also very low solids concentration in the final effluent. The average results of COD and BOD removal varied from 85 to 95%, sufficient to maintain the COD concentration in the final effluent in the range of 60 to 90 mg/l and the BOD values systematically below 40 mg/l. The overall averages of SS in the final effluent were kept below 25 mg/l.

The UASB/AF system could become a very promising alternative for the treatment of domestic sewage in developing countries, since the system can be designed at very short hydraulic retention times (6 hours for the UASB reactor and 3 to 4 hours for the AF), resulting in a very compact and low cost treatment unit. Besides, there is no energy consumption and the labour costs are minimum.

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