Modelling atmospheric emissions from wastewater treatment plants: Implications of land-to-water roughness change

Atmospheric emissions from passive liquid surfaces, such as wastewater treatment plants (WWTP), are common sources of impacts to the environment and to the health of communities, due to odours, greenhouse gases and other air pollutants. Emission models have been broadly employed for assessing these emissions, with the wind friction velocity (u_∗) being a key variable. The usual practice in the context of WWTP is to parametrise u_∗ based on reference wind speeds measured over the land, without considering the internal boundary layer (IBL) development due to the change in aerodynamic roughness as the wind blows from the land to the liquid surface, nor the stability of the wind flow. The potential consequences of these conceptual inconsistencies are major knowledge gaps in emission modelling. Addressing these, a customised computation was implemented to couple the wind friction parametrisation with the evolution of the IBL downwind of the land-to-water roughness change. A sensitivity analysis with different emission models, considering ranges of fetch, wind speed and surface roughness encompassing typical conditions in WWTP, showed that not incorporating the roughness change leads to systematic overestimation of u_∗ and the overall mass transfer coefficient K_L for two compounds analysed (liquid phase and gas phase-controlled volatilisation). A modelling approach was devised, comprising the u_∗ parametrisation that incorporate the roughness change combined with the Prata-Brutsaert emission model and alternative calculation of the gas-side mass transfer coefficient k_G from local IBL variables. Evaluation against experimental data and physical considerations support the adoption of this approach for modelling the volatilisation of compounds from passive liquid surfaces in WWTP. A simplified equation to approximate u_∗ after a change in roughness is presented, which can be used for quick emission modelling of liquid phase-controlled compounds. Furthermore, a preliminary exploration demonstrated that the effects of atmospheric stability on the response of u_∗ to the land-to-water roughness change can be substantial under certain conditions.