A critical review on liquid-gas mass transfer models for estimating gaseous emissions from passive liquid surfaces in wastewater treatment plants.

Emission models are useful tools for the study and management of atmospheric emissions from passive liquid surfaces in wastewater treatment plants (WWTPs), which are potential sources of odour nuisance and other environmental impacts. In this work, different theoretical and empirical models for the gas-side (k) and liquid-side (k) mass transfer coefficients in passive surfaces in WWTPs were critically reviewed and evaluated against experimental data. Wind forcing and the development of the wind-wave field, especially the occurrence of microscale wave breaking, were identified as the most important physical factors affecting mass transfer in these situations. Two approaches performed well in describing the available data for k for water vapour. One is an empirical correlation whilst the other consists of theoretical models based on the description of the inner part of the turbulent boundary layer over a smooth flat plate. We also fit to the experimental data set a new, alternate equation for k, whose performance was comparable to existing ones. However, these three approaches do not agree with each other in the whole range of Schmidt numbers typical for compounds found in emissions from WWTPs. As to k, no model was able to satisfactorily explain the behaviour and the scatter observed in the whole experimental data set. Excluding two suspected biased sources, the WATER9 (US EPA, 1994. Air Emission Models for Waste and Wastewater. North Carolina, USA. EPA-453/R-94-080A) approach produced the best results among the most commonly used k models, although still with considerably high relative errors. For this same sub-set, we propose a new, alternate approach for estimating k, which resulted in improved performance, particularly for longer fetches. Two main gaps were found in the literature, the understanding of the evolution of the mass transfer boundary layer over liquid surfaces, and the behaviour of k for larger fetches, especially in the range from 40 to 60 m.