Methane and hydrogen sulfide emissions in UASB reactors treating domestic wastewater

The release of CH₄ and H₂S in UASB reactors was evaluated with the aim to quantify the emissions from the liquid surfaces (three-phase separator and settler compartment) and also from the reactor's discharge hydraulic structures. The studies were carried out in two pilot- (360 L) and one demo-scale (14 m³) UASB reactors treating domestic wastewater. As expected, the release rates were much higher across the gas/liquid interfaces of the three-phase separators (5.4–9.7 kg CH₄ m⁻² d⁻¹ and 23.0–35.8 g S m⁻² d⁻¹) as compared with the quiescent settler surfaces (11.0–17.8 g CH₄ m⁻² d⁻¹ and 0.21 to 0.37 g S m⁻² d⁻¹). The decrease of dissolved methane and dissolved hydrogen sulfide was very large in the discharging hydraulic structures very close to the reactor (>60 and >80%, respectively), largely due to the loss to the atmosphere, indicating that the concentration of these compounds will probably fall to values close to zero in the near downstream structures. The emission factors due to the release of dissolved methane in the discharge structure amounted to around 0.040 g CH₄ g COD_infl⁻¹ and 0.060 g CH₄ g COD_rem⁻¹, representing around 60% of the methane collected in the three-phase separator.