Caracterização dos compostos traços influentes no aproveitamento energético do biogás gerado em reator UASB no tratamento de esgoto doméstico

Abstract

Given the perspective of biogas use as energy resource, there has been an increasing interest about the biogas’s specific chemical composition and the possible implications regarding its use. This paper investigates the trace compounds which influence the energy recovery from biogas, among them hydrogen sulfide, siloxanes and halogenated compounds. In literature, the papers carried out on siloxanes in biogas come from studies in sludge digesters, and not the biogas produced in sewage treatment itself, hence the importance of this research. The potential use of biogas and validation of a model for estimating COD mass balance in a UASB reactor are also investigated. Biogas and sewage samples were collected from the wastewater treatment plant of Piaçú/ES, composed of pre-treatment, anaerobic treatment (UASB) and aerobic treatment (submerged aerated biological filters).The following analysis were performed, all tests according to the Standard Methods for the Examination of Water and Wastewater (APHA, 2005): total and filtered COD, sulfate, sulfide and solids in affluent and effluent sewage; solids and precipitated sulfide in sludge. Samples of biogas produced in the up flow anaerobic sludge blanket (UASB) reactor were collected in 10 L Nalophan® bags and pre-concentrated in CarbopackTM sorbent tubes. Concentrations of hydrogen sulfide, halogenated compounds and siloxanes, as well as the main gaseous components (methane and others) were analyzed by gas chromatography coupled to thermal conductivity detector (GCTCD) or mass spectrometry (GC-MS).The amount of methane in the captured biogas was 19.7 Nm3 day-1 (0.093 Nm3 /m3 sewage / 0.24 Nm3 /kgDQOrem) and its combustion would liberate a power of 8.2 kW as heat, capable of being converted in a power of 2.7 kW of electricity. Methane recovery and biogas production were properly estimated using the mathematical model proposed by Lobato, Chernicharo and Souza (2012).In general, siloxanes and halogenated compounds were found in varieties and concentrations lower than those typically reported in the literature. The concentration of hydrogen sulfide (2078 ppm) was considerably high, which may represent a limitation to the energy use of biogas.