Influência do tempo de detenção hidráulica sobre a autoinoculação na partida de um reator UASB tratando esgoto sanitário.

Abstract

This report presents the results of an experimental investigation on the start-up period of the upflow anaerobic sludge blanket (UASB) reactor used for sewage treatment in the city of Campina Grande, Brazil. During the investigation no inoculum was used, so that the start-up was carried out under the most difficult conditions that could be found in practice. Under these conditions the period to obtain a stable and good performance of the reactor was established as a function of the hydraulic retention time. Important factors that influence the start-up are the time to generate the required anaerobic sludge mass and the risk of souring of the reactor contents (prevalence of acid fermentation over methanogenic fermentation). The experimental work was carried out in two four-months stages. In each stage three identical pilot scale UASB reactors were simultaneously operated, using raw sewage from the city as influent. Different but constant flows were fed to the reactors to establish values of the hydraulic retention time varying between 2 and 14 hours. It was observed that the equilibrium between acid and methanogenic fermentation was maintained throughout the investigation for all hydraulic retention times: the concentration of volatile fatty acids always remained low and the pH around the neutral point. When the UASB reactor was operated at a retention time of 2 hours, the reactor showed an irregular behaviour pattern, mixing short periods of good performance with long periods of poor COD removal efficiency. For a retention time of 2 hours the start-up was not reached up to 18 weeks of operation. A 13 weeks period was necessary to the startup without inoculum be established, considering retention times ranging from 4 to 14 hours.