This paper presents the results of pilot system comprised of one UASB reactor followed by one trickling filter (TF). The UASB reactor had a volume of 416 litres, being operated at an average hydraulic retention time (HRT) of 4 hours. The trickling filter had a useful volume of 60 litres, being operated at hydraulic and organic loading rates varying from 3.4 to 30.6 m3/m2.d and 0.3 to 3.9 kgBOD/m3.d, respectively. These different operational conditions characterised eight research phases. Both reactors were fed with domestic sewage pumped directly from the main interceptor of Belo Horizonte City, Brazil.
After almost 16 months of continuous monitoring, the UASB/TF 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 74 to 88% and from 80 to 94%, respectively, sufficient to maintain the COD concentration in the final effluent in the range of 60 to 120 mg/L and the BOD values systematically below 60 mg/L. The overall averages of SS in the final effluent were kept below 30 mg/L.
The UASB/TF system is a very promising alternative for the treatment of domestic sewage in Brazil and other developing countries, since the system can be designed with very short hydraulic retention times, resulting in a very compact and low cost treatment unit. Besides, the energy consumption and the labour costs are minimal.