Modelagem matemática da formação e emissão do gás sulfídrico no tratamento de esgotos domésticos
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Data
2011-08-26
Autores
Sá, Leandro Melo de
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Universidade Federal do Espírito Santo
Resumo
Emission models of gases from wastewater treatment plants (WWTP) are based on a mass balance for each compound in the liquid phase of the treatment facility. This mass balance includes mechanisms of compound removal from liquid phase (volatilization, stripping, biodegradation, chemical oxidation), which result from its interaction with the reaction medium through physical, chemical and biological processes, typical of each treatment process. Emission of odorous gases from sewage treatment comprises a large number of compounds, where hydrogen sulfide (H 2 S) is the main responsible by olfactory nuisance, due to its considerable emission rate and low recognition threshold (0.5 ppb). This compound is capable of causing adverse human health effects and lead to death in high concentrations. Aerobic treatment systems to domestic sewage can oxidize odorous compounds such as H 2 S and convert them into non-odorous compounds. Anaerobic treatment of wastewater containing sulfate may lead to formation of H 2 S, which can volatilize into the atmosphere and to cause environmental impact. Formation of H 2 S is a process in which sulfate acts as electron acceptor during chemical oxidation of organic compounds in a reaction mediated by sulphate-reducing bacteria (SRB), which reduce sulfate in H 2 S. This work aims (1) to verify the relevance of H 2S removal mechanism by chemical oxidation in a submerged aerated biofilter using four models for H 2 S oxidation, (2) to investigate the effect of wind speed on volatilization of H 2 S from quiescent surfaces in a wind tunnel and compare predictions of three volatilization models, (3) to estimate the formation of H 2 S in the liquid phase from an anaerobic reactor (UASB) and compare experimental H 2 S emission rates with predictions of three emission models which consider formation and volatilization of H 2 S in their mass balance. The main mechanisms of H 2 S removal in the biofilter were biodegradation and chemical oxidation. Biodegradation was the main mechanism of H 2 S removal when chemical oxidation was calculated by three of the four oxidation models investigated. Stripping and volatilization presented minimal contributions to H 2 S removal from biofilter. Friction velocity showed weak influence on the overall mass transfer coefficient of H 2 S determined with the use of wind tunnel, pointing that the volatilization of H 2 S can be considered nondependent of wind speed when a friction velocity is less than 0.3 m s -1 . At high wind speeds, H 2 S volatilization can show dependence on friction velocity, as suggested by volatilization models. WATER9 model showed better agreement with experimental results, although it overestimated the overall mass transfer coefficientof H 2 S by a factor of 4.0. Other models have overestimated the overall mass transfer coefficient of H 2S. The formation of H 2 S at the UASB reactor presented an average of 412.5 µg s -1 , equivalent to specific average formation of 15.6 µg m -3 s -1 , and was shown to be mainly due to consumption of acetate (66%), followed by consumption of hydrogen (25%) and propionate (9%). TOXCHEM+ and WATER9 models exhibited a better ability to estimate overall H 2 S emission rate from UASB reactor. Emission models investigated overestimated H2 S emission rate from the settling compartment of UASB reactor. However, prediction of the models resulted within the range of 95% confidence interval of overall average emission rate from reactor, in six out of twelve experiments.
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Hydrogen sulphide , emission models , volatilization , H2S formation , sulphate reduction , chemical oxidation , formação de H2S , redução de sulfato , oxidação química , Sulfeto de hidrogênio , modelos de emissão , volatilização