Estudo quantitativo e qualitativo de escuma acumulada em reatores UASB tratando esgotos domésticos

Abstract

The aim of this work was to characterize the scum accumulated in three demonstration scale UASB reactors fed on domestic wastewater. Two identical reactors (R1 and R2) were operated under different hydraulic retention times (HRT) - 9.0, 7.0, 5.0 and 9.0 hours -, representing four distinct operational phases. The only difference between the reactors R1 and R2 is that the latter was preceded by a forced screening unit. Differently from R1 and R2, the scum layer of R3 was monitored under the same operational condition for a long time and did not involve the complete scum removal in the settlig tank. Scum accumulation in all reactors was periodically assessed in each operational phase, and its characterization involved analyses of BOD, COD, solids, as well as microscopic observations and biodegradability tests. The results obtained for R1 and R2 reactors showed that the accumulation of scum, BOD, COD, solids and biodegradability measured, in general, were similar in both reactors. However, from the first to the third operational phase, althogh the relation VTS/TS decreased, the scum accumulation, solids and COD values increased. The differences between the early stages of phase 1 and the end of phase 3 may be caused by the decrease in the HRT and the increase in the upflow velocity. For the R3 reactor, it was observed low scum accumulation and this was probably due to the external factors such as excess sludge descharge and heavy rainfall. The biodegradability tests showed that the scum accumulated in the settling tank of all reators were better degraded under anaerobic than aerobic conditions, especially for the long time incubation experiment. The scum accumulated in the three-phase separator was different fromthat formed in the settling tanks, being accumulated in lower rates and with lower relation VTS/TS ratios. Microscopic analysis showed the presence of several bacterial morphotypes including filaments and rods with internal sulfur granules in the scum from the settling tank. Cyanobacteria-like cells and eukaryotic photosynthetic organisms were also observed, however they appeared to be more frequent in the R3 reactor.