Understanding biofilter performance and determining emission concentrations under operational conditions

As early as the 1990s, concerns were growing regarding the risks associated with the emission of bioaerosols. In the absence of dose response relationships in the community, the Environment Agency used a precautionary approach in the regulation of these sites in relation to the location and operation of composting plants, to minimise any potential health impacts from these sites and in particular open windrow sites. This together with an increased target for recovery of biodegradable waste and diversion from land fill resulted in a rise in the number of enclosed composting facilities operating in the UK. Biofilters have been used as an abatement technology at biowaste plants primarily for odour removal, but also for the control of bioaerosols, for many years and over that time very little has changed in terms of the fundamental biofilter design criteria. Research over the past 20 years has led to a better understanding of the principles of operation of biofilters in relation to odour. However, there are still many gaps in the knowledge which need to be addressed if biofilters are to be designed to control all emissions and to perform efficiently. This research project was commissioned by Sniffer on behalf of the UK Regulators together with industry representation. The project was carried out by a team led by the University of Leeds, in partnership with Odournet UK Ltd. The overall objective was to determine the extent to which abatement methods incorporating either open or enclosed biofilters reduce both bioaerosols and odour emissions from enclosed biowaste treatment operations. The research also attempted to answer the following questions: 1. What current technologies (e.g. combination of biofilter and scrubber) are being used throughout the UK biowaste industry to treat emissions? 2. What emission concentrations are being achieved by current technologies under operational conditions, and what rates of reductions relative to untreated emissions do these represent? 3. What design configurations and operating conditions (e.g. empty bed residence time, media type, moisture content, etc.) are required to ensure that maximum reduction rates are achieved, taking into consideration the different processes? 4. What is the degree of aerobicity/anaerobicity in existing, enclosed, biowaste treatment processes? 5. How significant of an impact does the degree of aerobicity/anaerobicity have on the levels and types of bioaerosol and odour emitted? 6. What impact does the ratio of aerobic to anaerobic activity at a site have on the site’s overall environmental performance? 7. Which technology (or technologies) might be put forward as candidates for Best Available Techniques (BAT) for bioaerosol and odour abatement? 8. What final bioaerosol and odour concentrations are achievable by the candidate BAT(s) for each of the available, enclosed, biowaste treatment operations? This research involved an initial review of the available literature pertaining to the performance of biofilters in their application to treat emissions from biowaste sites. This was followed by evaluation of the performance of biofilter systems at eight biowaste treatment sites in the UK, which was conducted over a period of one year. The findings of the research are primarily based on the data obtained during this phase of the study. The sites were chosen to ensure that as large a range of different abatement system arrangements and process parameters as possible were captured. The study acknowledged the current position in the UK were the majority of abatement systems consist of open biofilters, although enclosed systems are increasingly applied at new, large scale biowaste facilities. The key variables that were considered were; • whether the biofilter was open or enclosed, • whether the abatement system included a scrubber or not, • the type of biofilter media being used (e.g. woodchips, brash and granular peat), • the biowaste type, and the treatment process being used. The sites selected included two Eco Deco bio-drying systems, three in-vessel composting (IVC) tunnel sites, two enclosed windrow sites and one rotating drum system. Sampling and analysis was undertaken for odour (using olfactometry), hydrogen sulphide, ammonia, Volatile Organic Compounds (VOCs) and the bioaerosols, Aspergillus fumigatus, total bacteria and gram negative bacteria. The techniques applied were standardised and delivered using accredited procedures where available. It is important to note that all of the biofilters sampled as part of this study were observed to be in good condition and were well monitored. The media condition and particle size were generally good and the operational parameters of each system were within the ranges identified through review of relevant literature. As a result, the data obtained in terms of emissions and performance is likely to be representative of abatement systems that can be considered to be well designed, operated and maintained (Best Available). The results of this study do not therefore, allow conclusions to be drawn regarding the performance of abatement systems that are badly designed and operated or poorly maintained. Further sampling would need to be undertaken to determine the impact on the emission of odour and bioaerosols and removals that can be achieved by such systems.