This pilot-scale study aimed at improving the design and operation of biofilters to achieve simultaneous significant odour and bioaerosol reductions from waste air streams emitted from enclosed waste management facilities, using a materials recovery facility (MRF) as the source of contaminated air. The specific objectives included (i) to determine the key biofilter design and operating parameters required for a high level of odour and bioaerosol removal and to refine operational ranges and firmly define boundary conditions between normal and abnormal biofilter operations; (ii) to evaluate and characterise the concentrations of odour and bioaerosols that are being emitted as a result of waste management operations within the MRF; (iii) to evaluate the potential for biofilters to control bioaerosol emissions, and the potential for net emission of bioaerosols from biofilters both in terms of the overall concentration, and also the individual species; (iv) to determine the impact of gas residence time, media moisture content and media depth on simultaneous reduction of bioaerosols and odour in the process air; (v) to assess the impact of different biofilter media types (woodchips [old and new], peat and wheat straw) in terms of bioaerosol and odour emissions and removal; and (vi) to evaluate the possibility of improving a single biofilter for the removal of both bioaerosols and odour.
A pilot-scale biofiltration system was constructed for this study and comprised of four vertical up-flow plastic reactors filled with wood chips as the initial biofilter media and connected to a common plenum. Each reactor had a media volume of 181.5 L located above an air-space (for air distribution) separated by a metal mesh which supported the media. A six-stage Andersen sampler was used to measure the concentrations of four groups of bioaerosols (Aspergillus fumigatus, total fungi, total mesophilic bacteria and Gram negative bacteria) in the airstream before and after passing through the biofilters and these were expressed as cfu m-3. Air for odour analysis was collected into air-tight Nalophan bags which were sent off to Concept Life Science odour testing laboratory for analysis within 30 hours of sampling. Olfactometry analysis was carried out on the odour samples in accordance with BS EN 13725 to determine the odour concentration of the samples in European odour units (OUE m-3). The performance of the pilot biofilters was evaluated on the basis of removal efficiency (%) for odour and bioaerosols.
The data showed that the concentration of bioaerosols in the process air (as indicated by the inlet air samples) varied from visit to visit in the range of 103 – 105 cfu m-3. The concentration of odour in the process air also varied between visits typically ranging from 94 to 489 OUE m-3. This was thought to be due to the complex interactions between the specific process operating conditions, the types of waste being processed and the configuration of the air ventilation system installed on the site. Overall, this study shows that biofilters designed and operated for odour degradation can also achieve significant bioaerosol reductions in the process air of waste treatment facilities, provided that the inlet concentration is high - which is the case for most enclosed waste treatment facilities. The biofilters achieved average removal efficiencies of 70% (35 to 97%) for A. fumigatus, 71% (35 to 94%) for total fungi, 68% (47 to 86%) for total mesophilic bacteria and 50% (-4 to 85%) for Gram negative bacteria, while odour reduction efficiency was in the range of 34 – 76%. Thus, biofilters can be effective for the control of potentially pathogenic species in the emissions from such treatment facilities. The performance of the biofilters was highly variable at low inlet concentration with some cases showing an increase in outlet concentrations, suggesting that biofilters had the potential to be net emitters of bioaerosols. Bioaerosol particle size distribution varied between the inlet and outlet air, with the outlet having a predominantly greater proportion of smaller size particles (3.3 µm) that represent a greater human health risk as they can penetrate the respiratory system more deeply, and even to the lung alveoli where gaseous exchange occurs. However, the outlet concentrations were low, and further reduction would be achieved by the combined effect of wind dilution and dispersal as well as exposure to environmental stress from temperature, desiccation and oxygen in full scale biofilter applications.
It appears that variations in gas residence time may not impact on bioaersosol removals; thus, gas residence time may not be critical for bioaerosol control. However, longer empty bed residence time (EBRT) delivered significant (p < 0.05) reductions in odour compared to shorter EBRT, implying that the longer EBRT accommodates the time required for both odorous contaminants diffusion transfer from the gas phase into the biofilm, and their subsequent biodegradation within the biofilm layer on the media materials, as established in literature. There also appears to be no media moisture content dependent differences (p > 0.05) in the bioaerosols reductions reported in this study. On the other hand, although not statistically significant (p > 0.05), differences did exist in odour performance between the two groups, with the higher moisture content (40 – 70%) consistently showing better removals (odour RE range of 44 – 63%) than media moisture content in the range of 10 – 40%.
Furthermore, the two media depths (0.50 m and 0.25 m) investigated in this study showed potential capacity to control bioaerosols emissions from the process air of the MRF, and possibly other waste treatment facilities. Both depths achieved significant (p < 0.05) reductions of the inlet concentrations of bioaerosols as measured at the outlet. Although there were no statistically significant differences between the performances of both media depths, the 0.5 m media depth showed improved control of fungi than bacteria while the 0.25 m media depth had better removals of bacteria than fungi. This observation with the higher media depth has been thought to be a function of the large surface available for particles impaction; airflow rates and larger particles of fungi.
From the data, there were variations in the performance of the different media types assessed. Peat consistently delivered the highest simultaneous reduction of odour and bioaerosols; however, this was a much more expensive option. The performance of the wheat straw was the poorest both in terms of bioaerosols and odour reductions. Woodchips appeared to be the preferred choice particularly because they are relatively cost effective and offered satisfactory odour and bioaerosol removals (though not as high as peat). Nonetheless, the data indicated that the performance of woodchips may improve over time especially as the one year old woodchips indicated better removals of odour than the new woodchips which were freshly acquired for this study.
Overall, this study suggests that the ideal biofilter to simultaneously control bioaerosols and odour would be a woodchips-based reactor operated with a minimum media depth of 0.50 m and an EBRT of 16 s maintained at a moisture content level of between 40 and 70%, all of which lie within operational ranges reported in literature.
