The use of an upflow fixed bed reactor for treatment of a primary settled domestic sewage

Abstract

This study presents and demonstrates results obtained from an half full-scale upflow fixed bed reactor (UFBR) treating a primary settled domestic sewage. This study used expanded clay with an effective size of 2.7 mm containing hematite and magnetite as a granular medium. The content of TSS in the effluent treated was always between 10 and 20 mg l −1 for bed depths ranging from 2 to 3 m and filtration rates of 3–6 m 3m −2h −1. The profiles taken all along the reactor show that the activity of the biomass is constant over the whole height of the reactor. Moreover, an air/water volume ratio of 2:1 is amply sufficient to satisfy the oxygen demand of the biomass. The average removal efficiency based on the soluble COD remains virtually unchanged as a function of the filtration rate at about 70% of the influent. For a final BOD 5 of 30 mg l −1, loadings of 4.5–8 kg BOD 5m −3 can be applied. This corresponds to filtration rates of 3–6 m 3m −2h −1. The removal efficiencies for BOD 5 are then about 80%. After optimization of the backwashing conditions, the consumption of backwash water is about 5% of the volume of filtered water. Sludge measurements carried out during our experiments indicate an excess sludge production of 1 kg kg −1 BOD 5 eliminated. The nature of these sludges is very similar to the biological sludges produced in the high rate activated sludge process. This study has made it possible to establish design parameters of an UFBR and to develop technology for applications. These results are applied to two wastewater treatment plants which began to operate in 1984: these plants serve population equivalents of 40,000 and 11,000.