During daytime in a sunny day, the soil is heated and convective flows in the air layers located near the soil are generated. If the gas emission takes place under these conditions, the referred convective air flows dilute the contaminating gas and elevate it several meters above the soil, thus avoiding the odour perception at near populated areas.
Likewise, during windy conditions at the plant, the turbulence existing near the soil generates a strong mixture of the gas with fresh air and therefore the odour perception is limited to the plant area and its very adjacent areas.
Based on the previous information it is important to note that in the absence of the atmospheric conditions explained before (convective air movements that dilute the contaminant taking it several meters above the soil; and winds that mix the emitted gas with fresh air), and even if significant horizontal air movements occur, the dilution and elevation of the emitted gas does not occur and the unpleasant odour can be sensed by people living up to several kilometers far from the wastewater plant.
The atmospheric conditions that favour odour perception are those existing during clear and calm nights (without clouds and wind respectively). These conditions may be extended beyond sunrise if the sky is covered by clouds (the soil is not heated) and the absence of breezes is maintained.
In the atmospheric conditions of clear sky by night, absence of breeze and of fog, the surface cools by radiation through the atmosphere, which is semitransparent to electromagnetic radiation of long wave longitude. The surface, colder than air, cools it by radiation transmission from air to surface, and to a lesser degree by turbulent and molecular conduction. Cooling is more marked in the lower layers of the atmosphere (first tens of meters) and keeps reducing with height above the soil.
Atmosphere stratification process
Under these conditions, in the atmosphere near the surface we have the so-called “thermal inversion”, since on the contrary to what happens during the day when air temperature decreases with altitude, in the described conditions, air temperature increases with altitude in the mentioned lower layers. As air density diminishes with temperature, the atmosphere becomes stabilized as the lower layers are the coldest and therefore the densest ones. Under these conditions the atmosphere can be represented as an ideal vertical succession of “strata” or horizontal “layers” of air of increasing density and decreasing temperature the nearer they are to the surface. This phenomenon is referred to as “stratified” atmosphere.
Temperature and density distribution in height for a stratification night.
The stratified atmosphere presents two very interesting characteristics in relation with the odour propagation from a wastewater plant. The referred characteristics are the following:
a- The particle movement is mainly horizontal, the mixture in the vertical direction is reduced and therefore the contaminant dilution in the air is also reduced
b- Near the soil the so called “downslope winds” or “density currents” are generated. These currents are produced due to the combination of the atmosphere stratification and the presence of soil hillsides or slopes (complex topography). “Downslope winds” present their maximum speed near the soil (on the contrary of the winds generated by a pressure difference or “synoptic winds” whose speed increases with height above the surface), being able to transport the contaminated particles several kilometres downslope keeping them near the soil.
Downslope wind
In the stratified atmosphere (clear and clam nights) the colder air (denser) flows downhill, in the form of a density current (see figure below). This physical phenomenon has also been called “downslope wind” because its origin lies in the presence of a stratified atmosphere on a sloping surface and not on pressure gradients (usual cause of wind).
Density current in a stratified atmosphere.
The mentioned density currents are responsible for moving the cold air downslope, along the hillside to the lower areas of the surface.
Odour propagation and downslope winds
As “downslope winds” flow near the soil and have a reduced height compared with the length of their route (around 0.6% of their route), if they go through an odour emitting plant they become an excellent mean of odour transport due to the fact that they limit the contaminant dilution to the height of the downslope wind, which in 1000m of slope it is only 6m. The following figure presents a scheme where it is shown the effective odour propagation process that these “downslope winds” are capable of establishing.
Odour propagation from a wastewater plant through a downslope wind during a calm and clear night
As a consequence of the previously presented it is concluded that when there is a wastewater plant from which there is an odour propagation process that affects near populated areas that occurs during calm and clear nights or during cloudy and calm days, the referred process is, with a high probability, explained by the aspects mentioned previously. If this is confirmed by measuring the relevant parameters of the phenomena at the subject place, then the odour propagation problem can be solved through the application of the SIS technology.
