SIS technology is based on the application of the devices called Selective Inverted Sinks (SIS).
SIS are fans of low specific number, vertical shaft and horizontal propeller rotating plane, confined inside a cylindrical structure with a conical shaped inlet.
There are SIS devices of different sizes and models (see Products).
SIS operation principles
In a stratified atmosphere the SIS selectively drain the denser air layers (this means that the layers closer to the soil are drained before the layers located farther from the soil), that is where, in a wastewater plant, the larger hydrogen sulphide concentrations are located.
The SIS perform the referred extraction through its lower section (inlet) expelling the fluid through its upper section (outlet).
The selective drainage of the SIS (whose model, number and location inside the wastewater plant and neighbouring areas needs to be customized for each case) must be done in the neighbouring of the odour sources. The SIS will drain, as mentioned, the denser layers inside which it is produced (in the plant) the contamination with the hydrogen sulphide gas. Hydrogen sulphide gas has a specific weight of 1.1895 in relation to air in the 15° - 20°C temperature interval.
The contaminated denser layers drained by the SIS are impelled upwards in the form of a vertical conical jet that mixes the contaminated air expelled by the SIS with the fluid captured by the jet which comes from the layers that the jet goes through.
The referred discharge of the fluid in higher layers is possible due to the strong mixture that occurs inside the jet between the denser layers (hydrogen sulphide gas and fresh air) selectively captured by the SIS inlet and the fluid (fresh air) contained in the layers that the jet goes through.
As the mixture contained in the up going jet is denser than the layers the jet goes through, the density of the mixture existing inside the jet decreases as it goes up. Also as the jet transforms its kinetic energy (which is maximum at SIS outlet) in potential energy, the mixture will finally stop its way up and will remain at the layer whose density matches the mean density of the mixture contained in the jet.
The described process assures that, due to atmosphere stratification, the particles vertically impelled by the SIS can not return back to the lower layers due to the existence of the layers located at intermediate height which the jet went through in its way up. This “block” to the descent of the mixture is explained by the larger density of the intermediate layers in comparison to the density of the mixture discharged by the jets at their maximum height. Therefore the mixture will float on the intermediate layers and will propagate horizontally throughout the night.
By regulating the kinetic energy that the SIS introduce to the denser layers extracted from near the soil, it can be regulated the height over the soil at which the mixture will disperse horizontally in the stratified atmosphere.
The jet impelled by the SIS, in most of the usual applications, stops its way up at a height of between 60m and 120m from soil.
In the following figure this aspects are presented for a SIS device, using a colour scale for expressing (qualitatively) the level of contamination in the air (brown means larger density than red).
The SIS technology acts in two ways:
A- Dilution of the contaminant
The SIS technology acts, as already explained, moving large amounts of air, solving in an easy and cost effective way the dilution of the contaminant. The dilution is made by mixing the H2S emitted at the wastewater plant with large amounts of fresh air, thus reducing in hundreds of times the contaminant concentration at the initial stage of the up going jet. The level of dilution is a design parameter for the SIS installation at the considered wastewater plant.
The mixture of contaminant and fresh air drained by the SIS and impelled vertically upwards, in the form of a jet that, captures in its way up, atmospheric air (free of contaminant) therefore increasing the initial dilution of the contaminant in the mixture.
B- Discharge of the diluted mixture (in comparison to the emission conditions) to the atmosphere at a height from which it is not possible its return to the level of the populated area.
In order to set the height of the mixture discharge into the atmosphere it must be considered the desired level of dilution of the mixture (the dilution level increases with the height of the discharge) as well and the depth of the intermediate layers above which the mixture discharge is desired. The referred intermediate layers will act by blocking the propagation of the diluted contaminant though molecular and turbulent diffusion towards the location of the populated area whose protection is desired.
By increasing the final height of the jet, the dilution will be increased as well as the blocking effect to the return of the diluted contaminant to the populated area. The limit to the final height of the jet is given by economic considerations in regards to the investment in the SIS devices required and the operative cost of the system (machines required power).
The following figure shows the effect of SIS technology acting in an odour emission plant and controlling the odour propagation to the near populated areas.
According to what presented previously, a possible way for eliminating or significantly reducing the odour detection at the populated areas located near wastewater plants, during stratification conditions (clear and calm nights or cloudy and calm days) is acting at the plant by introducing, through the application of the SIS technology, the processes of dilution and elevation of the contaminant. In this way a significant change in regards to odour propagation will be achieved, similar to the one observed after comparing the referred odour propagation during sunny and windy days with the same propagation under stratification conditions.
