Influência da temperatura e do tempo de permanência sobre o desempenho de reatores UASB tratando esgoto sanitário.

Abstract

In this study, the influence of temperature and retention time on the performance of anaerobic reactors treating domestic sewage was investigated. The experimental set up consisted of four anaerobic sludge blanket reactors (UASB), with a volume of 25 L each, which were operated with constant temperatures of 12, 15, 20 and 25°C called R12, R15, R20 and R25, respectively. The goal of the experimental investigation was not only the determination of the performance of the reactors in terms of removal of organic material, but also the characteristics of the generated sludge in the reactors and expelled from them. The influent of the reactors was raw sewage of the city of Campina Grande-PB and the reactors were fed with constant flow. The experiment was conducted in three stages operating in which for each of the defined temperatures a range of retention times (Rh) were applied, starting with a long Rh and reducing it for every stage. R12 and R15 Rh reactor operated at 12, 8 and 6 hours in Phase 1, 2 and 3 respectively, while R20 and R25 were operated with Rh 12, 6 and 4 hours in Phase 1, 2 and 3 respectively. The system operated stably under all imposed conditions and no accumulation of volatile acids was observed, nor consumption of alkalinity, or drop in pH. Under the conditions prevailing during the investigation, the organic material removal efficiency was affected more by temperature than by liquid retention time. Considering the average of the whole period of investigation, the R25 had a mean removal efficiency of 78% while the R12 removal was 62%.Methane production was affected by temperature and organic load. The highest production of methane was obtained for a temperature of 25°C and was 4, 9 and 12 gCH4.d-1 for Phase1, 2 and 3, respectively. The biomass characterization indicated that the percentage of volatile solids in the reactor did not vary to a great degree with the temperature or retention times remaining around 63%. The sludge from the reactors showed a specific methanogenic activity (SMA) ranging between 0.10 and 0.34 gDQO/gSSV.d-1. The sludge operating temperatures of the reactors 25 and 20°C showed better sludge settleability than the reactor operating at 15 to 12°C. The sludge expelled from reactors showed SMA values similar to that of sludge kept in the reactor. However, there was a clear difference in settleability of sludge in the reactors and sludge expelled from these. It is concluded that there was selective expulsion, so that the sedimentation of sludge remaining in the reactors improved. The results show that the temperature has an influence on the process of anaerobic digestion. However, this influence is mitigated by the use of reactors that have an efficient sludge retention mechanism, enabling thus the application of anaerobic systems in temperate regions.