Comparison of Physical and Operational Characteristics of Flux Chambers for VOCs Measurement from Porous Media

There is an increasing emphasis on the impact of volatile gaseous emissions from area sources such as anaerobic treatment ponds, biosolids treatment processes, feedlot pads, compost windrows, and municipal wastewater sites, due to the concerns related to malodors, greenhouse gases, micropollutants, or bioaerosols. The estimation of emission rates are critical to support the assessment of environmental and social impacts such as nuisance and human health as well as climate change. Emission sampling is an essential stage in determining emission rates, where sampling methods seek to simulate the real emission scenarios. In a previous review (Liu et al. 2022), the benefits and disadvantages of a series of sampling methods (flux chamber, wind tunnel, static chamber, headspace methods) were compiled. The review showed there was lack of the understanding of the role of sampling methods, creating difficulties for industries or users in the analysis and interpretation of the emission data. Flux chambers are a commonly used method, due to the presence of standards, ease of operation and perception as a method that simulates real-world emissions. During method development, flux chambers were extensively tested to assess the impact of operational factors such as flushing rate and placement depth, however, experimental implications of different designs of flux chambers were limited. Based on the finding by Hudson and Ayoko (2008), 19 called flux chamber cylindrical devices were found out of 76 dynamic devices, while only 5 of 19 shared the same design in terms of dimension and operating system (Gholson et al. 1991, Sarwar et al. 2005). Such differences in design, limits the comparison of data and the establishment of an emission model for evaluating emission from area sources such as biosolids storage and application sites. In this research, four different flux chambers were used to measure the volatile emission rate from porous media using typical operating conditions (Table1). The comparison of flux chambers took into account inlet gas distribution system, chamber materials and variations in hood dimensions. The sweep gas flow rate in the chamber was varied from 1 L/min to 5 L/min. The results were interpreted using the relationship between gas velocity, turbulence intensity and the physical design of the device. Chamber II was set up based on the U.S. EPA flux chamber design (Kienbusch 1986) and tested as a benchmark. The emission rate measured using chamber III, which was duplicated from chamber II, showed 20%-60% variations, indicating the necessity of calibrations for each flux chamber before use. Compared to chamber II, variations of sweep air flow rate in chamber IV showed less effect on changes in the emission rate. However, the overall emission rate from chamber IV was higher, e.g. 103% higher at 1L/min flow rate. This difference was attributed to altered sweep gas jet direction which affects the circulation of sweep gas over the area surface and generated turbulence. To verify the hypothesis that sweep gas direction can affect the emission rate, axially vertical jets of sweep gas were used in chamber IV resulting in a higher emission rate compared to chamber II, while the increased number of jets further increased the measured emission rate (chamber I). The study findings emphasised the importance using a standardised flux chamber configuration, while linking the inlet gas distribution setups to recirculation motion in the chamber as well as the effects on emission rates from porous media. This is in agreement with prior studies conducted using CFD on liquid surfaces (Andreao et al. 2019). The outcome of this study will contribute to the more consistent use of flux chamber within which consistent, reproducible conditions could be established.