Analysis of methods for estimating the emission rate of odorous compounds from passive liquid surfaces

Abstract

The determination of the emission rate of odorous compounds from passive liquid surfaces is critical for the study and management of the environmental impacts associated with odour emissions from wastewater treatment plants (WWTPs). This research analysed two methods widely employed for estimating the emission rate of odorous compounds from such surfaces, namely, predictive emission models and direct sampling with a flux hood. Several theoretical and empirical models for the gas-side (kG) and liquid-side (kL) mass transfer coefficients in passive surfaces in WWTPs were evaluated against experimental data. The analysis of the compiled data set led to the development of an alternate approach for estimating kL which resulted in improved performance, particularly for longer fetches. Since the friction velocity (u*) is a critical input variable for the modelling of emissions, this work also evaluated different parametrisations of u* against available wind friction and wave data measured at wind-wave tanks with liquid surfaces of the same scale as WWTP units. For the first time, the most frequently used u* parametrisation was verified against representative data for WWTPs. Furthermore, new, alternative correlations were derived and combined in an approach that described the u* data set more accurately and in more detail, incorporating the size of the tanks together with the wind speed in the parametrisation of u*. A sensitivity analysis was conducted, in order to understand how different emission models are affected by the use of different u* parametrisations. The mass transfer of compounds inside the US EPA flux hood (one of the enclosure devices most commonly employed for the direct measurement of emissions) was studied by means of experiments that assessed kG and kL in the microenvironment created by the flux hood and the effects of concentration build-up in the hood`s headspace. The mass transfer of gas phase-dominated compounds inside the US EPA flux hood was found equivalent to conditions of very low wind speeds. A procedure was presented to scale the emission rates of these compounds to conditions of higher winds, by combining the application of emission models and the flux hood measurements.