Bioaerosol Monitoring and Dispersion from Composting Sites
Environmental consultancy ADAS outlines its research into tackling the dispersal of gases from composting sites without causing a stink.
Having its own commercial composting centre at St Ives in Cambridgeshire, ADAS is well aware that the composting of organic waste from industrial and domestic sources is an essential component in the modern waste management mix and key to meeting increasingly stringent recycling targets.
The potential harmful effects of bioaerosol emissions from composting operations are a concern for the Environment Agency, local authority environmental health departments and planners, health and safety practitioners and the public at large.
With this in mind a project to analyse the levels of bioaerosol emissions that occur on commercially operated sites was developed by Patrick Pierrepont, business manager at Envar Ltd – part of the ADAS group of companies.
While protocols for the measurement of bioaerosols are circulated by the Compost Association, ADAS developed the techniques and equipment to be used throughout the research.
The objective of this research study was to measure bioaerosol and odour levels from a range of different types of commercial composting operations.
In particular, the data was analysed to obtain a greater understanding of the rate of decline of bioaerosol concentrations over distance from these facilities and the variation in odour from material at different stages in the process.
The study also sought to establish if the bioaerosol source can be predicted via the use of dispersion modelling and to identify opportunities and management practices to help reduce emissions of bioaerosols and odours from composting facilities.
During the course of the study, three different types composting facility were examined including continuous vertical flow, windrow composting and fixed batch tunnels. The research study has afforded a wealth of data for the determination of the effect of different commercial composting activities on the generation and release of emissions. The results also clearly illustrate the problems in attempting to identify, quantify and model the generation and distribution of these emissions in a complex, variable and open-air environment, such as a composting facility.
Bioaerosol sampling and enumeration was undertaken for multiple types of mould including actinomycetes, Aspergillus fumigatus, and the total bacteria count.
When sampling, all samplers were operated concurrently. A weather station was also used to log meteorological conditions (wind speed and direction, air temperature and relative humidity) during all sampling periods.
Sampling was undertaken when weather conditions were dry and when air temperatures were not less than 5ºC. In addition, sampling was generally undertaken on cloudy and breezy days to create as near as practically possible a neutral atmospheric stability dispersion profile.
This considerably reduced the potential number of sampling opportunities, which was a major operational difficulty in this study.
Research was also conducted over a wide range of composting activities, including shredding, turning, loading, unloading and screening.
It also took place while different materials were being composted, including municipal, agricultural and kitchen waste.
Odour sampling was also undertaken at these composting sites, though sample collection was mostly on static windrows, during the various stages of the composting process.
The majority of samples were collected using the ventilated hood method, although a small number of samples were collected as point sources directly from the windrow. Most importantly, odour analysis was undertaken to the European Standard for olfactometric analysis (BS EN 13725).
In total, 146 datasets where collected from the three sites, which were converted from the laboratory reported colony forming unit (cfu) plate counts to concentration data (cfu/m3), and mass data (mg/m3) for the dispersion modelling.
Although the up and downwind pattern of bioaerosol concentrations varied considerably, approaching half of all the concentration results showed the expected pattern or trends towards the expected pattern.
And, despite selecting composting sites with no identifiable third party sources of bioaerosols, 17% of the data was identified as having an upwind contamination.
Results from the Atmospheric Dispersion Modelling System exercise demonstrate that the downwind deposition of bioaerosols from composting activities can in part be successfully modelled.
However, the confidence in such a mathematical prediction must be regarded as low due to bioaerosols being generated and dispersed in such a wide variety of ways, as mentioned above.
This puts into question the usefulness of using this type of model to predict deposition of bioaerosols.
The research also highlighted that no systematic changes in micro-organism particle size distribution with distance from the composting activity source occurred.
In addition, 91% of all samples taken across the three sites were below 1,000 cfu/m3 at 125 metres downwind from the boundary.
Although 125 metres downwind of the facility boundary was the greatest sampling distance in this study, it would appear likely that at a position 200 metres downwind, upwind concentrations would more or less be achieved in the majority of cases.
The study has shown that the containment of the entire composting process, including the treatment of process air, is the most effective way of controlling bioaerosols (and odours).
However, this level of containment is very expensive and not always a practical option, therefore containment on this prerequisite should be based on risks to nearest sensitive receptors.
Based on the limited odour measurements reported, it was evident that well managed green waste based undisturbed windrows generally had odour emission rates of around 10-40 ouE/s/m2.
Higher, short-term emissions are likely to occur during and after turning, while undisturbed, maturing, well made compost can be expected to be less odorous.
Where feedstocks contained a high proportion of putrescible materials, it seemed that the composting process is likely to be much more odorous, mainly as a consequence of the feedstock materials being stored in unmanaged “heaps”, resulting in anaerobic conditions.
In this case, it is believed that odour mitigation measures should be focused on improved feedstock management and conditioning.
Given that almost all composting sites operate with part of the process outside, there is always potential for sites to generate emissions and so the containment and control of bioaerosols should be based on the proximity of the nearest sensitive receptors.
Envar and the waste industry as a whole have a genuine concern to operate with minimal impact and this particular project has significantly enhanced the knowledge we have about emissions.
This study has provided a much greater understanding of the factors that influence emissions of bioaerosols from composting processes and the dispersion characteristics that are observed around commercial sites.
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