Why is the wind tunnel system recommended rather than an isolation chamber system for the determination of odour emission rate from area sources?
Odours are emitted from wastewater processing units through volatilization of odorous compounds at the liquid surface. Firstly, odour volatilization occurs by diffusion at the liquid and air interface. The process happens when odorous compound concentrations at the water surface are much higher than ambient concentrations. The odorous compounds volatilize, or diffuse into the air, in an attempt to reach equilibrium between aqueous and air phases. Secondly, convective mass transfer takes place in the air phase. Convection occurs when air flows over the water surface, sweeping odorous compounds in the air phase near the water surface. The convective mass transfer rate relates directly to airflow velocity over the water surface. Together, diffusion and convection determine rates of volatilization of odorous compounds and odour emission rates from a water surface to the air.
The isolation chamber system was developed by the USEPA in 1983. The mixing characteristics of the chemicals and the carrier gas are the critical design parameters (Gholson et. Al. 1991). Several uncertainties have been reported:
- It was reported by the original authors that the complete mixing only occurred at a zone of 2 - 9.5 cm above the air and water interface (Gholson. et. al. 1989). This stratification is dependent on the temperature of the carrier gas, surface temperature and ambient air temperature. The variations in the stratification layer thickness under different sampling conditions could significantly affect the repeatability and reproducibility of the testing results.
- The selection of the sweep air (carrier gas) rate has been found not to be fully satisfactory (Reihart et. al. 1992). By increasing the sweep air rate it was found that the chemical concentration inside the isolation chamber did not alter (Hwang, 1985).
- The measured emission rates largely depend on the configuration of the enclosure and operating procedures (Reihart, et. al. 1992). It has also been shown that the isolation chamber could not easily be used for aerated liquid surfaces (Gholson, et. al. 1989).
All these factors may limit the application of the isolation chamber in the determination of emission rates. Figure 9-1 illustrates the US EPA isolation chamber.
The design of static isolation chambers is based on the two-film model that is frequently used to explain the experimental phenomenon of the volatilisation of organic compounds from water in the laboratory. For gas phase controlled VOC emissions, the volatilisation process will be influenced by wind induced gas phase turbulence. Static techniques do not make provision for simulating wind turbulence. A USEPA evaluation study for the isolation chamber did not consider gas phase controlled processes (Gholson, et. al. 1989).
Figure 9-1 Schematic of US EPA isolation chamber
In 1993, a portable wind tunnel system was developed by the Centre for Wastewater Treatment at the University of New South Wales to measure odour and VOC emission rates from area sources at wastewater and industrial wastewater treatment plants, cattle feedlots, mushroom composting, piggeries etc. This system operates with activated carbon filtered air supplied at a controlled rate by a blower. A set of diffusers, and a perforated baffle control the aerodynamics of the wind tunnel. This is intended to create an environment where the boundary layer is well developed. Convective mass transfer takes place above the confined surface. The aerodynamics of the wind tunnel and a mathematical model of the mass transfer process inside the wind tunnel have been reported and represented a significant advance in wind tunnel technology.
It is almost impossible for natural ground level wind conditions to be duplicated inside a small wind tunnel. Therefore, the portable wind tunnel is designed to create an environment where the boundary layer is well developed and convective mass transfer occurs. The aerodynamic performance of the wind tunnel system can be repeated in the sampling process.
The DynaTunnel portable wind tunnel system is illustrated in Figure 9-2. The geometric size of system was designed taking into account transport and operational factors.
Figure 9-2 DynaTunnel portable wind tunnel
In summary, the isolation chamber is not designed to take into account convective mass transfer caused by air movement. The aerodynamics of the isolation chamber do not guarantee the repeatability and reproducibility of the emission rates measured. The isolation chamber should not be used to determine the emission rates from an area surface for the use of dispersion model. The portable wind tunnel system simulates wind movement in the atmosphere and is considered to be a more appropriate sampling technique in the determination of odour and VOC emissions from an area surface.
Gholson, A. R., Albritton, J. R. Jayanty, R. K. M., Knoll, J. E. and Midgett, M. R. (1991). Evaluation of an enclosure method for measuring emissions of volatile organic compounds from quiescent liquid surfaces. Envirn. Sci. Technol., 25:3, 519-524.
Gholson, A. R. Albritton, J. R. Jayanty, R. K. M. 1989, Evaluation of the flux chamber method for measuring volatile organic emissions from surface impoundments, EPA/600/3-89/008, U.S. Environmental Protection Agency, Research Triangle Park, NC. PB89-148589.
Reinhart, D. R., Copper, D. C., Walker, B. L. (1992). Flux chamber design and operation for the measurement of municipal solid waste landfill gas emission rates. J. Air Waste Manage. Assoc., 42, 1067-1070.
Hwang, S.T., (1985). "Model prediction of volatile emissions: A comparison of several models for predicting emissions for hazardous waste treatment facilities" Environmental Progress, 4:2, 141-144.