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Agricultural Biofilters – Reflections on a Decade of Research

Prepared by Danny Mann, Ph.D., P.Eng.

Department of Biosystems Engineering University of Manitoba

Winnipeg, MB R3T 5V6 Telephone: 204-474-7149

E-mail: Danny_Mann@umanitoba.ca

March 19, 2010

Introduction

Over the past decade, I have had the opportunity to research the use of biofiltration for treatment of odour from hog barns. A total of 8 papers have been published in refereed scientific journals (see listing at conclusion of paper). In addition to the knowledge contained in those papers, other information has been reported in project reports and student theses. The purpose of this document is to pull together my observations and conclusions related to the use of biofiltration for the treatment of hog odours. It should be recognized that some of the observations included in this document lack sufficient scientific evidence to be considered scientific truths.

The Beginnings

I cannot take credit for the idea of researching agricultural biofilters in Manitoba. The idea originated from an employee of Manitoba Hydro. This individual had a near-fatal accident on a northern Manitoba lake that was littered with floating trees caused by a previous hydroelectric project. As a consequence of his near-fatal accident, the employee decided to investigate potential uses for the dead trees floating on the lake. He approached the Department of Biosystems Engineering with the idea of using woodchips generated from the floating trees as a media source for biofilters to treat the odour from hog barns in southern Manitoba. After some preliminary investigation, I learned that agricultural biofilters were being researched at the University of Minnesota at the time. Manitoba Hydro provided funding beginning in 1999 to begin research on agricultural biofilters in Manitoba. The original assumption was that this could be a win-win-win situation. Manitoba Hydro would benefit by helping to clean up an environmental problem caused by an earlier hydroelectric project. The northern community would benefit from the economic development associated with “harvesting” and chipping the floating trees. Finally, the hog industry in southern Manitoba would benefit from an odour-control technology. Thus, the research of agricultural biofilters in Manitoba began.

Investigation of Open–bed Biofilters

With no previous knowledge of biofilters, a logical starting point was to copy the open-bed biofilter design being researched at the University of Minnesota at the time. There

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had already been some evidence to suggest that a biofilter could successfully treat livestock odours with relatively short residence times (in the order of a few seconds). The open-bed design was extremely simple. Essentially, a wooden air plenum was constructed on the ground adjacent to the barn wherever ventilation fans were present. The plenum connected the barn’s ventilation fans to the biofilter. The biofilter was nothing more than a mixture of woodchips and compost piled on top of the plenum. Exhaust air from the barn moved vertically through the biofilter media before being released to the atmosphere (Fig. 1).

Figure 1. Photograph of an open-bed biofilter constructed on a farm near Landmark, MB.

Four biofilters were constructed on a farm near Landmark, MB (Mann et al. 2002). Although there were a couple of variations in plenum design compared to the biofilters being researched at the University of Minnesota, the main objective of my research was to determine how the open-bed (uncovered) biofilter would behave during the winter months. From this perspective, the project was a success (Fig. 2). Despite measured ambient temperatures below -20°C, temperatures within the biofilter bed were maintained at approximately 16°C without the use of either insulation or supplemental heat. Odour concentration reductions ranged from 56 to 94%. Despite some promising results, a number of practical problems were observed. The open-bed biofilters were susceptible to infestation by both rodents and weeds. Rodents were attracted to the warm environment. Their burrows caused short-circuiting of the air which likely contributed to non-uniform and inadequate residence times. Weed growth on the surface of the biofilter also contributed to non-uniform residence times because of clumping of the biofilter media in the presence of roots. Control of these infestations would be an on-going maintenance challenge for the operation of open-bed biofilters.

Perhaps the greatest disappointment of this research project was that data could not be collected for a complete 12-month period as had been intended. Just a couple of months into the data collection period, one of the ventilation fan controllers malfunctioned. According to an electrician employed to fix the malfunction, there was no evidence to suggest that the problem was due to the presence of the biofilter. Unfortunately, the producer was concerned that the biofilters would cause problems and requested that the biofilters be disconnected. From this point, it became clear that the promotion of biofilters for control of livestock odours would face an uphill battle.

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Figure 2. Open-bed biofilter covered with snow following a snowstorm. Cracks in the snow show where air was escaping. Within a couple of days, the top surface was free of snow.

In the remaining months of the project, another experiment was completed at the University of Manitoba’s Glenlea Research Station. Small biofilters were constructed using various types of bulking agents (i.e., woodchips, hemp hurds, and wheat straw) mixed with composts originating from various sources (i.e., leaves, grocery waste, and poultry manure) (Fig. 3). Biofilters were designed for large residence times of 35 and 70 s to ensure full treatment of the odour. Mixtures containing wheat straw dried out too quickly causing inadequate treatment due to short-circuiting; wheat straw is not an appropriate bulking agent (this conclusion was later supported by results from an undergraduate thesis project (Hayward 2002)). Passage of odourous air (originally at an average concentration of 958 OU) through the biofilter chambers reduced the odour concentrations to 22 to 51 OU. By comparison, ambient farmyard air (initial odour concentration of 34 OU) had odour concentrations ranging from 26 to 51 OU after passing through the biofilter chambers. This research demonstrated that there seems to be a lower limit beyond which it is impossible to achieve lower odour concentrations with a biofilter consisting of mature compost and either woodchips or hemp hurds (DeBruyn et al. 2001).

Figure 3. Small biofilter units tested at the University of Manitoba Glenlea Research Station.

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Horizontal-airflow Biofilters

After I finished the project on open-bed biofilters, I began to think about innovative biofilter designs (Fig. 4) based on the belief that producers might be more inclined to spend money on biofiltration if they were getting something more than a pile of woodchips situated next to their million-dollar barns. I began to consider how best to enclose a biofilter within a structure.

Figure 4. Initial concept of a horizontal-airflow biofilter.

The first step was to determine whether air should be moved vertically or horizontally through the biofilter media. I was aware of research that demonstrated that there was less resistance to airflow through grain bulks in the horizontal direction than in the vertical direction. With the assistance of a couple of summer students, two biofilter chambers were constructed on a trailer (Fig. 5). One chamber allowed vertical airflow through the media while the other allowed horizontal airflow through the media. Experiments were conducted to determine the pressure drop experienced in each of these directions. As had been previously demonstrated for grain, resistance to airflow was less in the horizontal direction. The results indicated that resistance to airflow in the horizontal direction was approximately 0.65 times the resistance to airflow in the vertical direction (Sadaka et al. 2002). This result stimulated interest in developing a “horizontal-airflow biofilter.”

Figure 5. Horizontal-airflow and vertical-airflow biofilter chambers constructed for the purpose of determining airflow resistance characteristics for biofilter media in the horizontal and vertical directions.

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Creating horizontal airflow through a bulk of media consisting of woodchips is challenging because the bulk settles over time. When it settles, the tendency is for the air to channel over the top of the bulk without being treated. Two designs were developed with the intention of preventing such channeling. The first design consisted of an inflatable bladder situated in the headspace on top of the biofilter bulk (Fig. 6). In theory, the inflatable bladder would simply expand to occupy the headspace when settling occurs thereby preventing channeling over the top of the biofilter. To test this design, a biofilter was constructed at the University of Manitoba’s Glenlea Research Station (Garlinski and Mann 2005) (Fig. 7). Our research did not find any evidence of channeling despite significant settling over two summers of operation. Although this design might be considered a success for research purposes, it possesses practical challenges that would it difficult to implement in a commercial setting. For an inflatable bladder to work, there must be a roof structure. It is difficult to load woodchips into a structure with a roof. It is difficult to install the bladder in the small space between the top surface of the biofilter media and the bottom surface of the biofilter ceiling. The inflatable bladder is also subject to rips and tears which could be problematic from a maintenance perspective.

Figure 6. Schematic illustrating the variables associated with a pressurized-headspace horizontal-airflow biofilter.

Where:

Ph = pressure drop per unit thickness in the horizontal direction (Pa/m)

Pv = pressure drop per unit thickness in the vertical direction (Pa/m)

Ph = headspace pressure (Pa)

Po = ambient pressure (Pa)

Pp = plenum pressure (Pa)

t = thickness of biofilter media (m)

h = height of biofilter media above biofilter wall without direct access to pressurized plenum (m) 

Pp  Po 

Ph 

h 

              t 

 

Solid Material 

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Figure 7. Experimental horizontal-airflow biofilter constructed with a pressurized headspace.

The second design for a horizontal-airflow biofilter consisted of a non-pressurized headspace (Mann et al. 2008). The roof was eliminated from this design to facilitate filling of the biofilter chamber. Baffles were installed from the top of the biofilter to direct the movement of the air through the biofilter bed (Fig. 8). In theory, as long as the horizontal thickness of the biofilter is less than the vertical depth of the baffle, there will be less resistance in the horizontal direction (based on research results reported in Sadaka et al. 2002) and the air will travel horizontally through the biofilter media. If some air moves upward, it must still pass through the media and will be treated at least partially. At some point, settling would be so substantial that the chambers would need to be “topped up.” To test this design, seven biofilters were constructed at a farm near Niverville (Fig. 9). During the data collection period, exit velocity from the side of the biofilter was greater than the exit velocity from the top of the biofilter in 44% of cases. In 49% of cases, there were no significant differences. In the remaining 7% of cases, exit velocity was actually greater from the top than from the sides. Overall, the non-pressurized headspace biofilter did not behave as expected because there was significant movement of air through the top. Excessive settling and inadequate irrigation may be possible contributing factors.

Figure 8. Schematic of the horizontal-airflow biofilter with a non-pressurized headspace illustrating the desired movement of air through the biofilter media (black arrow) and the movement of air that will occur as the media settles (hollow arrows).

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Figure 9. Experimental horizontal-airflow biofilters constructed with a non-pressurized headspace.

Overall, the horizontal-airflow biofilter with a non-pressurized headspace seems to offer more promise than the design with the pressurized headspace. Filling the biofilter chambers was easily achieved with either a front-end loader or a potato conveyor. There were no hassles associated with installation and maintenance of an inflatable bladder. Finally, even though more air was exiting through the top of the biofilter than had been theorized, the air exiting through the top was still being treated.

Zeolite as a Biofilter Media Additive

At the time when the biofilter units at Niverville were being constructed, I became aware of research being done with zeolite as an agent for odour removal. A decision was made to add zeolite to half of the biofilter chambers to investigate its potential to improve the effectiveness of the biofilter. Biofilter effectiveness was evaluated based on the percentage removal of hydrogen sulfide. With the exception of the first sampling date, chambers with zeolite always resulted in greater reduction in hydrogen sulfide concentration than those chambers without zeolite. The zeolite used in this research was in the form of crushed rock. Greater effectiveness could likely be achieved if smaller-sized particles were used as a biofilter additive. Before recommending the use of zeolite as a biofilter additive, however, one would need to investigate the economics carefully. Although the zeolite was actually donated for the research project, the freight charge to haul a load of “rock” across the prairies was substantial.

Biofilter Wall Loads

Several of the experimental biofilter structures experienced bulging of the walls that seemed to increase with time. The bulging also seemed to be related to the settling of media that was occurring with time. A doctoral student was recruited to study the lateral pressures induced by biofilter media on the walls of biofilter structures. Due to a lack of information on the physical properties of woodchips, the first step was to measure several key physical properties (Ima and Mann 2007). There has been much research on lateral pressures within grain-holding structures, but grains and oilseeds are typically dry when in storage. Biofilter media must be maintained at high moisture contents for the biofiltration process to occur. Furthermore, the media is subject to wetting and drying

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cycles due to periodic irrigation of the biofilter media. Ima (2008) attempted to develop mathematical relationships to predict the lateral pressures induced by biofilter media under these varying conditions. Experiments confirmed that lateral pressures increased as the moisture content of the woodchips increased (Ima and Mann 2008) and that lateral pressures increased after each subsequent drying-wetting cycle (Ima 2008). The work completed by Ima (2008) represents a first approximation of the data needed during the design process to predict the lateral pressures induced by the wetting-drying cycles typical of an agricultural biofilter. Additional work is needed to develop better predictions.

Challenges with Retrofitting

Although it may not have been emphasized in publications arising from my research, an on-going challenge was the process of retrofitting a biofilter to an existing barn. Barns were not designed with the thought of attaching a biofilter. Specifically, ventilation systems were not designed to deal with the additional load of a biofilter. From a practical perspective, this meant the addition of booster fans to ensure sufficient airflow through the biofilter without changing the required ventilation rates. This posed another challenge to the operation of the biofilter because ventilation rates are not constant from hour to hour or from day to day. Even with perfect synchronization between the barn’s ventilation fans and the biofilter’s booster fans, biofilter performance would suffer because the load on the biofilter would not be uniform.

Another challenge with retrofitting is that the biofilter was perceived as an expensive add-on that provided little value to the producer. Problems that occurred with the barn’s ventilation system were automatically blamed on the presence of the biofilter. In one case, the owner’s insurance company threatened to discontinue fire insurance if the biofilter was physically connected to the barn. To satisfy the insurance company, the experimental biofilters had to be physically separated from the barn (Fig. 10). Although the biofilter inlets were aligned with the barn’s ventilation fans, we were certainly not able to capture and treat all of the odourous air. For obvious reasons, this further reduced the perceived value to the producer.

Figure 10. Photograph showing the separation between the barn and the experimental biofilter units.

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Based on my experiences, I am cautious about retrofit scenarios. I believe a retrofit solution could be made to work if the owner is fully committed to the biofilter and is willing to allow the biofilter’s air movement system to be integrated with the barn’s ventilation system (likely with modifications to the barn’s ventilation system). Without such integration, it may be more appropriate to include biofilters in the design of new barns rather than as retrofit solutions.

Locating a Biofilter Inside the Barn

Based on my concerns related to retrofitting, the idea of situating a biofilter inside a barn was evaluated. The intention was to reduce odourous emissions from the barn by improving the quality of the air inside the barn. A biofilter was designed and constructed that consisted of eight removable (and replaceable) cartridges. The free-standing biofilter provided a ground clearance of approximately 1 m to allow pigs to freely move around beneath the biofilter (Fig. 11). Air was drawn into the biofilter from the bottom and discharged out the side near the top of the biofilter. Because the biofilter was independent of the barn’s ventilation system, it could be operated continuously at constant rates which is desirable for the operation of a biofilter. A biofilter was placed inside of a barn in the spring of 2008. Despite some challenges getting the prototype inside the barn, everything was progressing until we attached the irrigation system to wet the biofilter media. A soaker hose had been placed on the top surface of the woodchips to allow water to be added slowly with sufficient time for the woodchips to soak up the added water. Within a couple of minutes, however, water was running through the biofilter media and “raining” onto the pigs standing below the biofilter. In my opinion, it was not acceptable i) to have water “raining” down onto the pigs or ii) to be wasting water (the water was flowing through the biofilter so rapidly that the moisture content of the woodchips could not be increased substantially). After observing this situation, I was forced to conclude that the biofilter had a fundamental design flaw. An attempt was made to collect data without irrigating the biofilter media; poor results were obtained because the media was too dry to allow the biofiltration process to occur. These results were not published, however, they were reported in a final project report submitted to MLMMI (Final Report for MLMMI 06-01-03).

 

Figure 11. Photograph of the free-standing biofilter structure situated inside a barn.

 

 

 

 

 

 

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I am still intrigued by the concept of placing a biofilter inside a barn. Improving the quality of the air in the barn will improve the working conditions for the barn workers, improve the living conditions for the pigs, and reduce the odour emissions from the barn. To solve the irrigation problem, I believe a biofilter with a horizontal-airflow configuration should be designed. This would allow the biofilter to be constructed with a solid bottom. Irrigation water that flowed rapidly through the media would pool in the bottom and slowly get sucked back up into the woodchips by capillary action. Further design and evaluation would be useful.

Other Applications of Biofilters – Integrating a Barn, Biofilter, and Greenhouse

In the past decade, I have been involved in one other project involving biofilters. Another colleague of mine had done work with an innovative greenhouse design. His research had shown that the innovative solar-energy greenhouse was able to store almost enough energy from the sun to keep the greenhouse temperature above freezing throughout winter nights. It was hypothesized that the warm air being ventilated from hog barns throughout the winter might contain sufficient energy to provide the necessary supplemental heat to a solar-energy greenhouse to enable plant growth throughout the winter months. Simply ducting the odourous air into a greenhouse would not be feasible, however, treating the air with a biofilter prior to release into the greenhouse was seen as a potential solution. Thus, an integrated barn-biofilter-greenhouse system was designed and evaluated (Mahmood et al. 2009) (Fig. 12). The prototype system displayed potential as a means of “harvesting” the waste ventilation air from livestock barns. In addition to harvesting the heat from the air, the contaminants from the airstream can be converted into water and carbon dioxide as by-products of the biofiltration process. Elevated levels of carbon dioxide inside the greenhouse are advantageous to plant growth.

Figure 12. Schematic of the barn-biofilter-greenhouse system.

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Student Theses Completed on the Topic of Biofiltration

2000 M.Sc. DeBruyn, J. Biofiltration for odour control from swine housing in Manitoba. 2000 B.Sc. Martin, H. Evaluation of media infiltration using a bench-scale model of

biofiltration. 2002 B.Sc. Hayward, R. Study of airflow resistance characteristics of straw/compost

biofilter media 2004 M.Sc. Garlinski, E. Airflow characteristics of a horizontal airflow biofilter. 2006 B.Sc. Wilson, N. Evaluation of a non-pressurized headspace horizontal airflow

biofilter. 2008 M.Sc. Mahmood, K. An integrated barn-biofilter-greenhouse system. 2008 Ph.D. Ima, S. Prediction of woodchip-induced lateral pressures on biofilter

walls.

Papers published by D.D. Mann on the Topic of Biofiltration

DeBruyn, J.C., D.D. Mann and Q. Zhang. 2001. Comparison of the odour levels of biofiltered air and ambient farmyard air. Canadian Biosystems Engineering 43:6.7-6.11.

Mann, D.D., J.C. DeBruyn and Q. Zhang. 2002. Design and evaluation of an open biofilter for treatment of odour from swine barns during sub-zero ambient temperatures. Canadian Biosystems Engineering 44:6.21-6.26 (Technical Note).

Sadaka, S., C.R. Magura and D.D. Mann. 2002. Vertical and horizontal airflow characteristics of wood/compost mixtures. Applied Engineering in Agriculture. 18(6): 735-741.

Garlinski, E.M. and D.D. Mann. 2005. Evaluation of airflow through a horizontal-airflow biofilter with a pressurized headspace. Canadian Biosystems Engineering 47:6.29-6.34.

Ima, C.S. and D.D. Mann. 2007. Physical properties of woodchip:compost mixtures used as biofilter media. Agricultural Engineering International: The CIGR Ejournal. Vol. IX.

Mann, D.D., K.N. Wilson and C.S. Ima. 2008. Evaluation of airflow through a horizontal-airflow biofilter with a non-pressurized headspace. Canadian Biosystems Engineering 50:6.1-6.5.

Ima, C.S. and D.D. Mann. 2008. Wall pressures caused by wet woodchips in a model biofilter bin. Agricultural Engineering International: The CIGR Ejournal. Vol. X.

Mahmood, K., D.D. Mann, Q. Zhang and A. Brule-Babel. 2009. A perpetual harvest greenhouse system: integrating barn, biofilter and greenhouse. Agricultural Engineering International: The CIGR EJournal. Vol. XI.  

 

 

 

 

 

 

 

 

 

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