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Composting sites generate odour nuisances along the site boundaries. The setting up of a continuous odour monitoring system based on eNose technology means operators can get a better understanding of the way unpleasant odour incidents occur and do something about their process to reduce emissions.
A continuous odour monitoring system, fitted with five eNoses, a weather station and odour dispersion modelling software was set up at one of the biggest composting sites in Europe. Although several sources were identified from the beginning as being likely to create odour nuisances, monitoring the odours enabled the identification of the zero to three week fermentation source as being responsible for 87% of the nuisance.
The operator’s efforts reduced odour nuisances generated by more than 40% over a six month period, and by nearly 75% after two years. The nuisance level in inhabited areas was, to start with, relatively low, but was greatly reduced by the operator’s work, using measurements made by the system in real time, and by continuous improvement of the process.
Introduction
Composting is an organic waste reuse process. It produces a soil improver rich in humus after an aerobic fermentation phase, followed by a maturation phase, which stabilises the product.
The scope for the application of composting has broadened with the development of composting techniques and the issue of collective management of household waste. This field involves all types of organic waste including green waste, household bio-waste, the sludge from wastewater treatment plants, agri-food industry waste or livestock waste.
Composting generates odours. Where composting sites are close to inhabited areas, there is a risk that the odours generated by the composting process will be a nuisance to the neighbourhood.
This situation can rapidly become a problem in the community. In France, a ministerial order stipulates that the legal limit for exposure to odours, within a 3km radius around a composting site, is a maximum of 5 uo/m3 for 98% of the time. So, if it is assumed that 2% of the time production incidents are likely to generate a significant and uncontrollable nuisance, the odour level in the area around a composting site must be low.
his 5 uo/m3 odour limit is a new restriction for site managers. It means production has to be organised so as not to generate a significant nuisance and is likely to prevent production increases, or worse a shut down of activities for non-compliance with this rule.
This article describes the feedback from the installation of an odour monitoring system at a sludge and green waste composting site located in the north of France.
Measuring odours
Measuring specific molecules to evaluate odour has two major disadvantages. Firstly, by not including all the molecules, the risk of an error in evaluating the odour is great. Furthermore, if some odour molecules are excluded from the analysis, the effects of inhibition and synergy between the molecules cannot be fully explained. Secondly, analysing many molecules has a material and time cost. It is then necessary to have a detector for each molecule, or a facility comprising a separator and analyser.
It therefore seems necessary to correctly evaluate odour nuisances, to quantify odours with an aggregate measurement, giving an odour concentration.
Study site
The composting site has a surface area of just over eight hectares. It is located in a rural area in the north of France and processes 110,000 tonnes of wastewater treatment plant sludge and green waste every year. It is bordered to the north by a main road and to the east by a navigation canal, from where some of the waste treated arrives by barge. The main point affected by the odours from the site is the village of Moeuvres (1km from site).
The green waste and sludge composting processes are separate on the site. The sludge received is stored until it is mixed with the co-products.
Fermentation is divided into two stages: a first three week stage where the ventilated piles (positive aeration) are in the open air and a second three week stage where the piles are ventilated (positive aeration) but covered by a metal structure.
Next comes the maturation period, in the open air, of four to eight weeks. Green waste is treated in two separate areas on the site, where the process for both is the same. The green waste is gathered into piles and turned regularly. The piles of green waste are not ventilated. The run-off water is collected in five leachate collection tanks and treated by an evaporator.
The size of the site and the large quantity of waste treated makes it impossible to enclose its activities. The process therefore takes place entirely in the open air, so the odours cannot be channelled or treated.
Ever since the site was established, the residents and elected representatives from neighbouring communities have raised the issue of odour nuisance. In an effort to be completely transparent, the plant’s operators wanted to install continuous odour measurement tools, to precisely evaluate the odour nuisances generated by the site and to be able to measure any improvements made to the process.
Continuous odour monitoring system
A new system has been developed comprised of one or more e-noses, a weather station and software for atmospheric dispersion modelling and viewing the measurement results.
The e-noses can be deployed strategically around the site, close to the odour sources, so as to characterise (identify and quantify) the odour emissions. Once calibrated, using data taken from the field and analysed by dynamic dilution olfactometry in accordance with European standard EN 13725, the e-noses quantify the odours given off by the site in units of odour per cubic metre of odourless air (o.u./m3).
The meteorological data measured by the weather tower and the odour concentration data measured by the e-noses allow the system to model the atmospheric dispersion of the odours. In addition, it displays the odour plume in real time (colour coded according to concentration) by superimposing it on the aerial map of the site.
Results and discussion
The continuous odour monitoring system has been in operation since July 2007. The data shown is from the system’s first year of operation.
Continuous odour monitoring was carried out over one full year. The continuous odour monitoring system was operational for more than 95% of the time.
The results obtained can be used for two types of analysis: an instant analysis, to identify the odour level in the environment, and a long term analysis, to identify problematic sources and the average odour level to which the site’s neighbours are subjected.
The Southern Green Waste source refers to the green waste composting area to the south of the site, positioned between the sieve, the area in which the compost has been fermenting for three to six weeks, and the anaerobic digestion unit. The Canal Green Waste source refers to the green waste composting area located next to the main road and the canal next to the site.
The biggest source in terms of odour emissions is the area in which the compost has been fermenting for zero to three weeks. This source represents 87% of all the odours given off by the site. By comparison, the tanks and green waste composting area next to the canal are only responsible for 2% of all the site’s odour emissions.
This result is surprising. Although there was no doubt that the fermentation areas generated odours, it was also believed that the leachate tanks would be a significant source of nuisance. So, with a relative proportion of 87% of odour emissions, the zero to three week fermentation source was responsible for nearly all the potential nuisance.
This situation is explained by the combination of two parameters. Firstly, the strong positive ventilation of the piles results in an emission rate that is much higher in the fermentation areas, and therefore produces greater odour emissions. Secondly, the odour level is much higher at the start of fermentation. The combination of these two parameters creates a situation where a source, which represents less than 15% of the site’s total surface area, is responsible for the majority of the odours given off.
Assessing the odour impact
The odour impact is the graphical representation, using isopleths, of the nuisance caused by the site in its environment, over the study period. The average concentration corresponds to the average of all of the odour impacts calculated during the study period.
Apart from the areas very close to the site (radius of 150 metres), the average level of exposure to the odour is below the sensory threshold (1 o.u./m3).
This result means that the operators are not always responsible for odour nuisances in the inhabited areas.
It is important to note that the isoconcentration curves do not follow the directions of the prevailing winds, as the odours are more easily dispersed by strong winds. Episodes of odour perception are therefore caused by light winds from the south or north.
Another type of result, the exceeding of the thresholds, is used for assessing the amount of time in which exposure to the odour exceeds a certain value.
Two values are used, 1 o.u./m3 (odour sensory threshold) and 5 o.u./m3 (threshold at which the odour can be recognised or discerned). The 1 o.u./m3 threshold corresponds to the odour sensory threshold, the odour level where only one person in two will detect the odour from the site. At this level of exposure, the odour is not recognisable.
The villages neighbouring the site are exposed to an odour level greater than 1 o.u./m3 less than 2.5% of the time, which is fewer than 10 days over the whole of the monitoring period. The 5 o.u./m3 threshold corresponds to the threshold at which the odour becomes recognisable. Although not very intense for most people, at this threshold it is possible to link the odour detected to the activities on the composting site.
Resident exposure is negligible. Those most exposed are motorists driving on the main road next to the site. This is an additional parameter to exceeding the threshold, because it is the percentage of the time during which the odour level will be reached. For example, the 98th percentile determines the odour level that will be exceeded for 2% of the time (147 hours during the course of the year). It is therefore a matter of determining an odour level at which 147 hours in the year will show values above this odour level, while the remaining 8,584 hours in the year will show lower values.
This representation gives an indication of the frequency of the neighbourhood’s exposure to the highest concentrations over the course of the year. It is this type of result that is used in French law pertaining to odours generated by rendering activities.
The 98th percentile is the one used by the order relating to composting operations. This order, which is the strictest in terms of odours in France, stipulates that the odour level must not be above 5 o.u./m3 more than 2% of the time in inhabited areas.
The diagram below shows the 98th percentiles for the first six months of the study and then the next six months. The efforts made by the operators in the first six months had a direct impact on the nuisance level generated in the following six months. In terms of the 98th percentile, nuisance was reduced by more than 40% over this period.
Conclusion
The results from monitoring the composting site’s odour emissions show that the odour outbreaks were short term in the neighbouring villages. Although there can be no doubt that the composting site sometimes causes a nuisance, it is minor and non-recurrent. The continuous odour monitoring system made it possible to identify the main source of odour nuisances, in this case, the zero to three week fermentation area. This source, which is responsible for 87% of odour emissions, is now the top priority for improvement, in order to minimise odour outbreaks outside the site.
Furthermore, the system used at this particular site means the operator can manage its odour nuisances proactively, by identifying the causes of odour outbreaks, which means it can deal with them immediately.
The process of improving the composting site has been constant since the system was first used. The continuous odour monitoring system provides a response and analysis tool that is essential for this approach.
Author Andrew Cowell is the Director of Business Development for Odotech Inc in Montreal, Canada. Mr Cowell works with companies across a number of industries to educate, assess and ultimately deal with issues pertaining to odour emissions. Odotech Inc is the world leader in measurement and monitoring of environmental odours. With corporate headquarters in Montreal, Odotech now has offices in the USA, France, Chile and most recently in the UK.
OdoWatch is an odour monitoring and measurement system. It is comprised of one or more electronic noses (eNoses) deployed on a site, close to odour sources and linked to a weather station. The system software integrates the eNose data with AERMOD and computes the atmospheric dispersion plume of odours right to your desktop computer, tablet or smart phone.
For more information visit www.odotech.com or subscribe to our blog at http://blog.odotech.com .
If you have any questions please contact Andrew at [email protected]
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Published: 10th Jun 2011 in AWE International
