Blowing in the wind - odour emission control in Italy

By Lindsay Gale12 April 2011

Sampling the air on the La Spezia site to determine the source of offending odours

Sampling the air on the La Spezia site to determine the source of offending odours

From 2005, General Smontaggi has been working on the reclamation of the land formerly occupied by an oil refinery at the Italian port of La Spezia, which it previously demolished, with the goal of returning the extensive site to residential, commercial and industrial use. The project requires the cleansing of 700,000 cubic metres (24.7 million cubic feet) of land, involving excavation and material handling, landfarming, soilwashing and the creation of suitable disposal plants/external recovery.

From the beginning of the excavation activity, problems associated with the emissions from odourous substances appeared and therefore it became evident that on-site treatment was required in order to achieve the reclamation objectives. Odour emissions generally raise the fear of possible health risks, and generate requests for explanations and reassurances from nearby residents that must be satisfied quickly in order to avoid the growth of a climate of fear and mistrust. As a result, in the initial phase of the project the aim was to evaluate the possible danger of the emissions through an accurate chemical analysis. Air samples were taken from the site and then concentrated to reach a high degree of sensitivity. The analyses were looking for dangerous substances, such as benzene and aromatic polycyclic hydrocarbons, which fortunately were not present. The odours were recognised as not being dangerous, but irrespective of this, it was necessary to limit these emissions as much as possible. To this end a monitoring system was established in order to verify, in real time, the "smelliness" of the site and its local area.

Chemical characterisation and olfaction site metric

A further step was made towards a more accurate chemical characterisation and an olfaction metric of the site contamination. In order to eliminate the odours, therefore, the technique adopted consisted of sampling site air in bags of nalophan, an inert material that has no odour, filled to act as a lung by an air pump. The first problem to resolve was the difficulty in identifying the substances responsible for the odour, because human perception of odour is often more sensitive than classical analysis tools. With the application of advanced techniques it was possible to identify some of the components responsible for the smell in the site surroundings. These substances, even although present in low concentrations, were clearly perceptible because they possess an extremely low level of olfaction perception. In practice, Smontaggi was confronted with a group of different odours, from garlic to naphtha, with different intensity and persistence. The contribution made by the weather of the area, which tended to transport masses of contaminated air to nearby residents and provoked their protests, could not be ignored. A continuous monitoring system was therefore installed so as to be able to indicate in advance the excess concentration of odour contamination affecting nearby residences in order to permit immediate intervention on the part of company personnel.

Monitoring the emissions

The approach adopted was based upon the "System Olfaction Artificial" (SOA), better known as "electronic nose". Two of these systems have been installed near the boundary of the reclamation area in positions corresponding to normal wind directions. The SOA are systems made up of a group of sensors of different kinds that supply a different electrical response according to the chemical composition of the analysed air. This generates a characteristic olfaction imprint that is different for each kind of odour emission. These imprints are recorded in the system, which permits the SOA to recognise the odour in exactly the same as the brain calculates the olfaction perception - it compares them with those perceived in the past and identifies them on the basis of their similarity.

For this reason the electronic nose had to be 'trained' in advance to recognise the odours that it must identify. At La Spezia, initially they were calibrated with emissions produced during excavation. In order to be able to transform the electrical measurements to quantify the odours, it was necessary to present the same gas sample to a panel of evaluators. The concentration of odours, expressed in olfattometric units per m3, was estimated according to European standard UNI EN 13725, which turns a subjective evaluation of the odour into an objective measurement. In practice, this panel was no different from a panel of tasters that has the job of evaluating the quality of a beverage or a meal. After having calibrated the reply of the SOA directly into a concentration of odours, the problem became that of finding the relation between the odour measured by the noses at the boundary of the worksite and odours perceived at the sensitive sites, the nearby residential properties.

Chemical analysis

Since the sensitivity technique of the olfactionmeter was applicable to odour emissions, but not to less intense odours present on site, Smontaggi searched for the origins of the odour emissions - those substances not necessarily in themselves odorous but present in the emission in a sufficiently elevated amount as to be detected even at a distance from the emission point.

Odour dispersion model

With the information obtained as above, a mathematical model of the odour dispersion was developed that was able to forecast the concentration of odours on the basis of the weather conditions and on the electronic replies of the two SOA at the nearest properties to the site. The predictive capacity of the model was controlled and validated with chemical and sensitivity measurements, and the limit value at the boundary of the plant measured by the SOA that corresponds to an acceptable odour level. In this way the two "electronic noses" perform continuous boundary monitoring of the air during land reclamation operations and transmit by wireless odour measurements to the central computer. In the event that levels are greater than the predefined maximum, the system sends a signal to the personnel carrying out the land reclamation to allow them to carry out the appropriate remedial action.

Conclusions

All the monitored measurements, such as meteorological conditions, are recorded by the system - it is therefore possible to access site data to remotely monitor the current or historic position. This allows supervising personnel to monitor the state of the land reclamation and to collate reports of odour events with the activities on the site. This minimises those critical situations that can cause slowdowns and difficulties in land reclamation activities.

The technique employed at La Spezia is completely innovative, Such a continuous monitoring by "electronic noses" placed at the edge of a worksite had not been previously used at any land reclamation sites. The validity of this approach has been demonstrated by the experience General Smontaggi gained during the project.

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