Abstract
UASB reactors are a common technology for wastewater treatment. However, certain
disadvantages must be considered. One of the disadvantages relates to the presence of dissolved gases,
hydrogen sulfide and methane, in the effluent, which can potentially be released into the atmosphere. This can cause malodours and contribute to the greenhouse effect. In this perspective, this work investigated alternative techniques to minimize these disadvantages: air stripping inside the settling compartment; and a dissipation chamber immediately after the reactor outlet. Results achieved with the air stripping technique showed low removal efficiencies for methane, around 30%, and in the range of 40 to 60% for hydrogen sulfide. On the other hand, the removal efficiencies obtained with the dissipation chamber technique were much higher, consistently reaching 60% or more for both gases, plus a relatively lower exhaust flow. For the best operational condition tested, median removal efficiencies of 73 and 97% were observed for dissolved methane and dissolved sulfide, respectively.
Keywords: Dissipation chamber; Stripping; Dissolved methane; Hydrogen sulphide; Gaseous emissions;
UASB reactor.
INTRODUCTION
When anaerobic UASB-type reactors are employed for the treatment of domestic sewage, the
generation of gaseous by-products, notably methane and hydrogen sulfide, is unavoidable. The former is a greenhouse gas and the latter causes bad odours and corrosion. In addition, methane losses mean less
energy potential to be exploited. In this sense, a higher dissolved methane concentration in the reactor
effluent leads to a decrease in the CH4 recovery efficiency, which stands for the percentage of the
total CH4 produced (biogas + losses with the effluent) that is actually recovered with the biogas, inside
the three-phase separator. This parameter is another important issue of concern when CH4 is intended to
be used as energy source (Giménez, 2012).
Some alternatives to reduce the concentrations of dissolved methane from the effluent of anaerobic
reactors have been tested, such as micro-aeration (Hartley and Lant, 2006) and membranes (Cookney
et al., 2010); however, none of them proved to be truly feasible and effective. In a recent study using
membranes to remove dissolved gas, Luo (2014) obtained high removal efficiencies for methane,
around 86%; however, it is still an expensive technique. Other researchers are focusing on improving
the post-treatment of the anaerobic effluent to promote a controlled biological oxidation of dissolved
CH4, for example, using a closed-type down-flow hanging sponge (DHS) reactor (Hatamoto et al.,
2010; Matsuura et al., 2010; Hatamoto et al., 2011).
In relation to dissolved sulfide, various techniques have been applied to attempt its removal from
anaerobic reactor effluents, notably from UASB reactors. Some studies have been carried out in this
direction, including: aeration of the effluent, with removal efficiency of approximately 86% (Khan et
al., 2011); electrochemical technique, with removal efficiency of approximately 82% (Dutta et al., 2010);
micro aeration technique, with removal efficiency of around 16% (Krayzelova et al., 2014) and biochemical
technique (photosynthetic bacteria), with removal efficiencies ranging between 81 and 95% (Kobayashi, 1983).
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