Biofiltration is not a one-size-fits-all technology. In order to properly design the biological odour control process, the foul air source needs to be accurately characterized. The optimal biological odour control configuration will depend strongly on the compounds contributing to odour. Considering the application of biological odour control to wastewater treatment plants specifically, this paper first describes the most common odorous compounds and how each can be biologically degraded.

   Several case studies demonstrate the importance of selecting the proper biological technology based on the foul air source. This paper is intended as a Manual of Best Practices for environmental professionals interested in applying the latest developments in advanced biological odour control techniques.

   Competing interests: The author has declared that no competing interests exist.

   Academic editor:  Carloz N. Díaz

   Content quality: This paper has been peer-reviewed by at least two reviewers. See scientific committee here

   Citation: D. Webb, C. Olesen. 2021. Navigating the complex landscape of biological odour control solutions for Waste Water applications, 9th IWA Odour& VOC/Air Emission Conference, Bilbao, Spain, Olores.org.

   Copyright:  2021 Olores.org. Open Content  Creative Commons license. It is allowed to download, reuse, reprint, modify, distribute, and / or copy articles in olores.org website, as long as the original authors and source are cited. No permission is required from the authors or the publishers.

   ISBN: 978-84-09-37032-0

   Keyword: emissions; biofilters; biotricklers; abatement; technology selection.

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Abstract

  Biofiltration is not a one-size-fits-all technology. In order to properly design the biological odour control process, the foul air source needs to be accurately characterized. The optimal biological odour control configuration will depend strongly on the compounds contributing to odour. Considering the application of biological odour control to wastewater treatment plants specifically, this paper first describes the most common odorous compounds and how each can be biologically degraded. Guidelines based on the strengths and weaknesses of various biofilter configurations are presented for the application of biofiltration to different wastewater treatment plant foul air sources. Several case studies demonstrate the importance of selecting the proper biological technology based on the foul air source. This paper is intended as a Manual of Best Practices for environmental professionals interested in applying the latest developments in advanced biological odour control techniques. These approaches safeguard economics while ensuring that the highest rates of treatment efficiency can be achieved in a reliable and sustainable manner.

 1. Introduction

   In the most general terms, biological odour control consists of a system which allows contaminants to absorb from a gaseous phase to an aqueous phase and where subsequent oxidation of the contaminants occurs. The microorganisms which are responsible for the conversion can either be immobilized on a matrix (referred to as the biofiltration media) or they can be suspended in solution. The former process is known as a fixed-film bioreactor whereas the latter process is known as a suspended growth bioreactor.

   Biological odour and VOC control has been in use for several decades. The earliest biofilters were constructed in the 1950’s, but it is in the 1980’s and 1990’s that they became common in Europe and were starting to be used in North America (Devinny, Deshusses, & Webster, 1999). More recent developments have included transitioning away from organic media to engineered or synthetic medias and the development of different system configurations.

   Given the many different configurations and media types to choose from, choosing to treat a foul air source biologically requires properly selecting the most appropriate process. This paper considers what some typical foul air characteristics are from various areas within a wastewater treatment plant, as well as the suitability of various biological odour control configurations.

 2. Wastewater treatment plant odours and biodegradation

   The primary wastewater treatment plant (WWTP) compounds contributing to odour typically consist of a mixture of H₂S as well as organic sulphur compounds (OSCs). The probable biodegradation pathway for these reduced sulphur compounds is shown in Figure 1. As can be seen, both dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) are converted to methyl mercaptan (MM). MM in turn is oxidized to H₂S, which is oxidized to sulphuric acid. The relative removal rates of these sulphide compounds by biofiltration are H₂S > MM > DMDS > DMS. Part of the reason for the lower removal rates of organic sulphur species (especially DMS) is that their removal is inhibited by the accumulation of sulphuric acid (which is produced from H₂S bio-oxidation), with an observable decrease below a pH of 6 and nearly complete inhibition below a pH of 4.

   Biological odour control systems that favour organic sulphur compound removal will operate at a neutral pH and require sufficient contact time to treat these slower degrading compounds. While H₂S and MM are found throughout the entire WWTP, DMS and DMDS are primarily found in biosolids processing operations (McGinley & McGinley, 2018). 

 Figure 1: Reduced sulphur oxidation pathway (Zhang, 2007), (Smith & Kelly, 1988)

   Also common at WWTPs are fatty acids, aldehydes and ketones and amines. These compounds are typically secondary odour contributors and are not usually the target of odour control efforts (McGinley & McGinley, 2018). When considering the design of biological odour control, the presence of these compounds is important as they are very hydrophilic and readily biodegradable. These compounds can cause a biofilm to grow very quickly, to the point where the biofilter media becomes plugged and excessive pressure drop prevents proper operation. The presence of these volatile organic compounds (VOCs) at concentrations of 1 ppm or higher usually require mitigating design aspects.

 3. Biological Odour Control Configurations

   The following descriptions of biological odour control configurations has been prepared from a combination of literature sources such as (Talaiekhozani, Bagheri, Goli, & Khoozani, 2016), (Kennes, Rene, & Veiga, 2009) and (Devinny, Deshusses, & Webster, 1999) as well as from BIOREM’s own field experience with having designed and delivered biological odour control systems worldwide since 2000. Biofilters and biotrickling filters are focused on as these are most commonly used at WWTPs. Other biological odour control systems such as bioscrubbers (suspended growth bioreactors) and membrane bioreactors are not discussed.

3.1 Biofiltration

   A biofilter uses microorganisms that grow in a biofilm fixed to the surface of biofilter media. The air is typically conditioned in a humidifier prior to the biofilter, and the biofilter media is intermittently irrigated with fresh water to maintain optimal moisture levels. Biofilter medias commonly used are summarized in Table 1.

 Table 1: Commonly used biofilter media types

   Biofilters that are designed for WWTP applications typically feature an up-flow configuration. This is to prevent early acidification of the media bed from sulphuric acid released by microbial conversion of H₂S. The area of the bed with the highest acid production rate will be the bottom, and any generated acid will travel by gravity back down through the bed away from the neutral bed further up. Maintaining a neutral pH in a biofilter bed is desirable as microorganisms which grow at a neutral pH are able to remove VOC’s as well as OSCs whereas a low pH favours acidiphiles such as thiobacillus which primarily remove H₂S only. Engineered coated biofilter media often
includes a pH buffer which helps to keep biofilm at a neutral pH even as sulphuric acid is produced.

3.2. Biotrickling Filtration

A biotrickling filter is similar to a biofilter in that a biofilm is fixed to the surface of the biotrickling filter media. The difference is that the liquid phase in a biofilter is stationary, while the liquid phase in a biotrickling filter is mobile and down over the biofilm by gravity. This helps to remove excess biomass, allows greater external control of temperature, pH and nutrients and the removal of accumulated metabolites. Biotrickling filter medias commonly used are summarized in Table 2.

 Table 2: Commonly used biotrickling filter media types

   In the context of WWTP applications, biotrickling filters are primarily used for the removal of H₂S, low pH recirculation water favours the removal of H₂S and the growth of acidiphiles such as Thiobacillus. There are commercial designs and offerings for intermittent once-through irrigation schemes which aim to develop a neutral operating pH at the top of the bed (targeting VOC and OSC removal) and an acidic pH at the bottom of the bed (targeting H₂S removal). To date no scientific literature has been published which shows that such an operation is possible for high rates of OSC removal. There are concerns that a typical biotrickling filter design does not have the EBRTs required for OSC removal and that diurnal variations in H₂S loading can negatively impact the growth of beneficial microbial colonies for the removal of more recalcitrant compounds.

4. Guidelines for biological treatment process

   As has been discussed, the process design depends strongly on the gas composition, which itself varies depending on the specific source. The following recommendations are a starting point for technology selection. Foul air properties have been estimated from a combination of information provided in (WEF, 2004) and (McGinley & McGinley, 2018) as well as from BIOREM’s field experience. Specific EBRT’s have not been recommended, as this will depend on the media type.

Table 3: Biological odour control process by WWTP source

5. System Case Studies

   BIOREM designs biotrickling filters, biofilters and dry scrubbers as stand-alone systems as well as multi-stage systems. The following performance data was obtained by sampling the foul air with Tedlar bags and a vacuum chamber. The odour analysis was performed by St. Croix Sensory Inc. following ASTM E679. The reduced sulphur analysis was performed by Mayfly Odor Laboratory by GC-FPD.

Table 4: WWTP odour control case studies

   As can be seen from the data presented in Table 4, the first case (Oviedo) is of a dual-stage biofilter drawing air from a headworks related tank. H₂S and methyl mercaptan are each present at elevated concentrations, while the DMS and DMDS concentrations are very low. This is typical for liquid stream processes found throughout a WWTP. The dual stage biofilter removes all compounds to below detection threshold, and the measured odour removal of the system is >99.6%. High odour removal rates from liquid stream processes are often possible with relatively low EBRT’s since a large fraction of the odour is due to the easily removed H 2 S.

   The second case (Rocky River) is of a stand-alone biotrickling filter drawing air from a dewatered sludge storage bunker. As expected, the biotrickling filter achieved  99.8% removal of H₂S, however the organic sulphur removal is poor with only 64.9% removal of the combined OSCs and only 30% removal of DMS. A two-stage system with engineered biofilter media would have better suited this foul air source.

   The third case (Little Patuxent) is of a stand-alone biofilter drawing air from the exhaust of a sludge dryer. The specific sampling event took place shortly after the dryer had started (based on hourly temperature data not included in this paper) so the odorous compound concentrations were quite low. Even at these low concentrations, the biofilter achieved 90.0% odour removal, 91.5% removal of OSCs and 81.2% removal of DMS.

   These examples clearly show the importance of characterizing the foul air, and how optimal process design depends strongly on the foul air source.

 6. References

   Devinny, J., Deshusses, M., & Webster, T. (1999). Chapter 1: Introduction. In Biofiltration for Air Pollution Control (pp. 13-15). Florida: CRC Press LLC.

   Kennes, C., Rene, E., & Veiga, M. (2009). Bioprocesses for air pollution control. Journal of Chemical Technology & Biotechnology, 84(10), 1419-1436.

   McGinley, C., & McGinley, M. (2018). Chapter 6: Odor and Air Emissions Management. In WEF, WEF Manual of Practice 8: Design of Water Resource Recovery Facilities (pp. 225-362). Alexandria, VA: WEF Press.

   Smith, N., & Kelly, D. (1988). Mechanism of Oxidation of Dimethyl Disulphide by Thiobacillus thioparus Strain E6. Journal of General Microbiology, 134, 3031-3039.

   Talaiekhozani, A., Bagheri, M., Goli, A., & Khoozani, M. (2016). An overview of principles of odor production, emission, and control methods in wastewater collection
and treatment systems. Journal of Environmental Management, 170, 186-206.

   WEF. (2004). Manual of Practice 25: Control of Odors and Emissions from Wastewater Treatment Plants. Alexandria, VA: Water Environment Federation.

   Zhang, Y. (2007). Biofiltration of dimethyl sulphide in the presence of methanol. PhD Thesis.