Dispersion Modelling

The odour impact assessment on the local community (sensitive receptor) is carried out according to the national standard represented by the Lombardy Region Guidelines (“Guidelines for the characterization and authorization of gaseous emissions into the atmosphere from an odour-impact activities” issued with the Regional Government Resolution of 15 February 2012 – No. IX / 3018) where the use of mathematical dispersion modelling is recommended in Annex 1.

The approach establishes exposure criteria in terms of ground level odour concentration, set at 1, 3 and 5 ouE/m3 at the 98th percentile of hourly peak odour concentrations over a year, i.e. the maximum odour concentration should not be exceeded for more than 2% of the hours in a year.

Modelling atmospheric dispersion of odours allows us to estimate the extent of the area affected by both existing and future emissions scenarios. The calculations fulfil many purposes in engineering, risk assessment and regulatory authorisation of industrial plants (Italian AIA – Integrated Environmental Authorisation (IEA), Italian VIA – Environmental Impact Assessment (EIA), etc.).

First of all, the studies provide the assessment of the emission scenario, i.e. the measurement or estimated forecast of all emissions inside the plant or induced by the plant, such as point sources (e.g., gas stacks), active area sources (e.g., biofilters), passive area sources (e.g., landfill surfaces and wastewater treatment tanks), diffuse volume and fugitive sources (e.g., buildings, passive ventilation apertures, valve and flange leakage). For each identified odour source, the Odour Emission Rate (OER) is estimated, expressed in odour units per second (ouE/s) with its frequency. Furthermore, the geometrical (e.g., height, diameter, orientation) and physical data (e.g., air flow/speed and temperature) also have to be specified.

The spatial domain of the simulation should be chosen so as to include all the emission sources to be studied, and all the receptors that are believed to be impacted by the emitted odours and their geographical coordinates must be indicated.

The acquisition of meteorological and topographical data is of crucial importance for atmospheric dispersion modelling purposes. In general, requested data include wind speed, wind direction and information on the atmospheric stability conditions, identification of sensitive receptors impacted by the emitted odours, and their geographical coordinates, as well as the orography of the terrain.

The choice of the most adequate model for a given application should be evaluated case by case based on several factors. The models that are most commonly used are Gaussian models (e.g., AERMOD) and CALPUFF or more advanced models such as Lagrangian particle models, if the vertical emissive profile is important.

The results of the odour dispersion simulation are represented in the maps of ground level concentration contours at the 98th percentile of the hourly peak odour concentrations over a year (Regional Government Resolution of 15 February 2012 – No. IX / 3018), reporting the isopleths set at 1, 3 and 5 ouE/m3 or representations of the odour plume for predetermined scenarios (worst case) or tables with the values of odour concentration at the receptors.