DESIGN OF A HORIZONTAL AIRFLOW BIOFILTER

Transcription

1 Paper No DESIGN OF A HORIZONTAL AIRFLOW BIOFILTER D.D. Mann Assistant Professor, Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB R3T 5V6 E.M. Garlinski M.Sc. Student, Department of Biosystems Engineering, University of Manitoba, Winnipeg, MB R3T 5V6 Written for presentation at the AIC 2002 Meeting CSAE/SCGR Program Saskatoon, Saskatchewan July 14-17, 2002 Abstract Simple, uncovered vertical flow biofilters have been shown to be effective at removing odour from the air exhausted from livestock barns. Unfortunately, uncovered biofilters are plagued by problems such as rodent infestations and surface weed growth. It is anticipated that enclosing the biofilter will prevent both rodent entry and growth of weeds. To minimize the cost of such a structure, the surface area of the biofilter media must be minimized. To accommodate a constant volume of biofilter media, the depth of the bed will increase substantially. Research has shown that airflow resistance of biofilter media consisting of woodchips and mature compost is significantly lower with horizontal airflow. This paper will discuss the design of an enclosed horizontal flow biofilter that will be fitted to a commercial swine facility in southern Manitoba. Attempts will be made to optimize the performance of the biofilter by incorporating an automatic watering system and a dust filter. The effectiveness of the biofilter will be assessed by measuring inlet and outlet concentrations of odour and hydrogen sulfide. Papers presented before CSAE/SCGR meetings are considered the property of the Society. In general, the Society reserves the right of first publication of such papers, in complete form; however, CSAE/SCGR has no objections to publication, in condensed form, with credit to the Society and the author, in other publications prior to use in Society publications. Permission to publish a paper in full may be requested from the CSAE/SCGR Secretary, PO Box 316, Mansonville, QC J0E 1X0. Tel/FAX The Society is not responsible for statements or opinions advanced in papers or discussions at its meetings.

2 Introduction As the swine industry continues to grow in Manitoba and elsewhere, odour continues to be a major issue, particularly with rural neighbours. Current practices to reduce odour include the sub-surface injection of liquid manure and the covering of earthen manure storages with straw or synthetic covers. Another significant source of odour is the barn itself. Biofiltration is a technique that can be used to treat odourous air from livestock barns. Research efforts at the University of Minnesota have identified shallow, open biofilters consisting of wood chips and compost as a viable alternative (Nicolai and Janni 1997; 1998; 1999; 2001a; 2001b). In 1999, research at the University of Manitoba confirmed that shallow, open biofilters remained operational without the need for insulation or supplemental heat at ambient temperatures of - 20LC (Mann et al. 2002). Despite the positive research results, there are several disadvantages associated with this type of biofilter. Problems such as rodent infestations and surface weed growth can be alleviated with good management. A more significant disadvantage is the large surface area required by this type of biofilter. Increasing the depth of biofilter media increases the airflow resistance to be overcome by the ventilation fans. The solution is to use larger, more expensive ventilation fans to force the air through the biofilter media. Sadaka et al. (2002) made an important discovery related to biofiltration. They reported that airflow resistance in the horizontal direction was approximately 60% of the airflow resistance in the vertical direction. This means, that for the same depth of biofilter media (where depth refers to the dimension in the direction of airflow), a smaller ventilation fan could be used. This result indicates that surface area can be reduced with an associated lower cost for ventilation fans than could be achieved by vertical airflow. The purpose of this paper is to discuss the design of a horizontal airflow biofilter being constructed at a commercial grower-finisher swine operation near Niverville, MB. Biofilter Design Initially, the design of the horizontal-flow biofilter was conceptualized as follows: a large rectangular box filled with media would have air entering on one side and leaving out the other side (Fig. 1). Unless the width of the biofilter (i.e., the depth of the media) was kept small, the pressure drop to be overcome by the fans would have become excessive. Consequently, the biofilter was designed with an internal plenum down the middle of the structure. Air was introduced to the central plenum and exited through the media on either side of the plenum (Fig.2). For the same total depth of media, the pressure drop to be overcome by the fan would be reduced by a factor of approximately three. The biofilter will be constructed as a post-frame structure (Fig. 3). The structure will be situated alongside the grower-finisher barn in such a way that the internal plenum is parallel to the length of the barn. The walls of the internal plenum and the exterior side walls of the biofilter will be covered with perforated metal sheets to allow air movement while holding the media. No floor 1

3 Fig. 1. Artist s conceptualization of the enclosed, horizontal airflow biofilter. Odorous air enters on the left and exits from the right end after passing through the biofilter media. Fig. 3 Schematic (end view) showing construction details of the horizontal airflow biofilter. Ventilation air will enter the center plenum and exit through the biofilter media to either the right or the left. 2

4 will be constructed to facilitate subsequent removal of the media (when its useful life has expired) with a tractor or skid-steer loader. Fig. 2 Overhead view showing movement of air into the centre plenum and through the biofilter media located on either side of the centre plenum. A problem associated with horizontal airflow through any material is short-circuiting in the head space above the material (i.e., it is easier for the air to travel above the material rather than through the material). For this biofilter, the problem has been overcome with the use of a pressurized head space. An inflatable bladder will be placed between the top surface of the media and the lower surface of the roof. The inlet opening to the inflatable bladder will be attached to the internal plenum. Air from the plenum will pressurize the bladder, creating a tight seal between the bladder and the top surface of the media. As the media settles over time, the bladder will continue to expand to maintain the seal. It has been determined that biofilter media should have high porosity to yield low airflow resistance, the ability to retain large quantities of water (but easily release water for biological processes), and a large diverse biological population (Devinny et al. 1999). This leaves a number of products available for use as a biofilter media. A mixture of wood chips and mature compost will be used in this project based on the positive results from previous researchers with this type of biofilter media (Nicolai and Janni 1997; 1998; 1999; 2001a; 2001b). In this mixture, the compost provides a start-up biological community for the biofilter and, through the life of the biofilter, a source of nutrients (Devinny et al. 1999). The wood chips are more porous than compost, lowering the airflow resistance for a given thickness of media (Sadaka et al. 2002). In addition, a mixture of wood chips and compost has good water retention properties and an adequate surface area to promote the formation of biofilm (Devinny et al. 1999). For this design, an 80:20 mixture of wood chips and compost was selected. It is assumed that the media will compact over time, increasing the resistance to airflow. For this reason, the airflow resistance characteristics for a 60:40 mixture of wood chips and compost (Sadaka et al. 2002) were used in the calculations to size the biofilter. The porosity of the media is assumed to be 60% based on the results presented by Sadaka et al. (2002). The biofilter will be built to handle an airflow of 39 m 3 /s with a true residence time of 5 s. Based on these requirements, the biofilter volume is calculated to be 325 m 3 using the equation below. 3

5 V = τ Q θ (1) Where: V = biofilter bed volume (m 3 ), g = true residence time (s), Q = airflow rate (m 3 /s), and O = porosity of biofilter media. For practical purposes, the height of the biofilter (h) should not exceed 3.05 m (10 ft). Based on airflow resistance characteristics determined previously (Sadaka et al. 2002), a total media depth (d) of 3.05 m (10 ft) divided equally on either side of the internal plenum was considered acceptable. The biofilter length (l) was calculated to be 34.9 m. A length of 36.6 m (120 ft) was chosen as a practical length for construction purposes. With half of the airflow entering each half of the biofilter, the surface loading (SL) was calculated to be 0.17 m 3 s -1 m -2. The resulting pressure drop was calculated to be 102 Pa using the coefficients for Shedd s equation provided by Sadaka et al. (2002) shown below. This pressure drop will be accommodated by installing larger ventilation fans in the walls of the grower-finisher barn. P d SL = ln( SL) (2) Ventilation air being exhausted from livestock barns contains large concentrations of suspended particles (i.e., dust). Although the biofilter media will successfully filter such particles from the air, it is undesirable to allow the biofilter media to remove the particles because the pore spaces will become clogged and the pressure drop will increase. There is no easy way to clean the accumulated dust from the biofilter media. It is preferable, therefore, to use a separate dust filtration system. An undergraduate student will evaluate a number of designs to determine the one most suited to this application. Instrumentation of Biofilter The biofilter will be instrumented for both moisture content and temperature. Moisture content will be measured using Aquaflex sensors (Streat Instuments, Christchurch, New Zealand). The Aquaflex sensor measures average moisture content in a cylindrical volume of 6 L over a 3-m length using a technique known as Time Domain Transmission (TDT). By packing fine material (i.e., ground wood chips, sand, or compost) around the Aquaflex sensor, it is suitable for measuring moisture content within a biofilter medium consisting of wood aggregate. The Aquaflex sensors will be situated as shown in Fig. 4. Temperature within the biofilter bed will be measured primarily with the use of thermocouples, although the Aquaflex sensors also measure temperature. Thermocouples will be dispersed throughout one half of the biofilter in a regular pattern (Fig. 5) with a more intense grid located 4

6 in the region where the Aquaflex sensors will be located (Fig. 6). The thermocouples and Aquaflex sensors will be connected to a data acquisition system so that a permanent record of the temperature and moisture profiles within the biofilter bed will be available. Irrigation of the biofilter media will be controlled based on the moisture content output from the Aquaflex sensors. Due to the symmetry of the biofilter, it will be assumed that both halves of the biofilter have similar moisture content and temperature characteristics. Samples of biofilter media will be removed periodically and moisture content determined by drying at 103LC for 24 h (ASAE Standard S358.2 DEC98 Moisture Measurement - Forages) m 0.31 m 1.07 m 0.15 m 0.46 m 0.76 m 1.07 m Fig. 4 End view of one half of the biofilter showing the placement of the Aquaflex sensors. Air is moving from left to right m 0.38 m 0.15 m 0.92 m 0.84 m 0.84 m Fig. 5 End view of one half of biofilter showing the placement of thermocouples. Fig. 6 End view of one half of biofilter showing the placement of thermocouples in the intense grid. 5

7 Proposed Experimental Procedure Once the biofilter has been constructed and filled with media, exhaust air from the growerfinisher barn will be vented through the biofilter. After allowing approximately 1 month for a natural population of microorganisms to develop, hydrogen sulfide and odour samples will be taken bi-monthly for 12 months. Inlet samples will be taken from a sampling port located in the duct leading to the biofilter. Outlet samples will be collected at the biofilter outlet using a sampling hood. Hydrogen sulfide will be measured with the use of a Jerome Meter (Jerome 631-X Hydrogen Sulfide Analyzer, Arizona Instrument Corporation, Phoenix, AZ) which has a detecting limit of 1 ppb. Odour concentration will be measured with the use of a dynamic dilution olfactometer (AC SCENT International Olfactometer, St. Croix Sensory, Stillwater, MN). Odour samples collected in 10 L Tedlar bags will be presented to 6 human panelists within 24 h of collection. The procedure used to screen panelists was detailed by Mann et al. (2002). Concluding Thoughts To our knowledge, no one has ever constructed and tested a horizontal airflow biofilter. Based on previous research conducted at the University of Manitoba (Sadaka et al. 2002), we feel that horizontal airflow may be appropriate for biofiltration of air exhausted from livestock barns because it can reduce the costs associated with ventilation fans. The concept of a pressurized head space to reduce short-circuiting of the air above the media seemed to work in the previous research. It is anticipated that construction will be completed and the biofilter will be operational by September Time will tell whether the design is a success. Acknowledgments The authors thank the Manitoba Livestock Manure Management Initiative and Chemawawin Cree Nation for funding this project and Matt McDonald and James Allen for their technical assistance. 6

8 References Mann, D.D., J.C. DeBruyn and Q. Zhang Design and evaluation of an open biofilter for treatment of odour from swine barns during sub-zero ambient temperatures. Canadian Biosystems Engineering 44: Nicolai, R.E. and K.A. Janni Development of a low cost biofilter for swine production facilities. ASAE Paper No St. Joseph, MI: ASAE. Nicolai, R.E. and K.A. Janni Comparison of biofilter residence time. ASAE Paper No St. Joseph, MI: ASAE. Nicolai, R.E. and K.A. Janni Effect of biofilter retention time on emissions from dairy, swine and poultry buildings. ASAE Paper No St. Joseph, MI: ASAE. Nicolai, R E and K. A. Janni. 2001a. Biofilter media mixture ratio of wood chips and compost treating swine odors. Water Science and Technology 44: Nicolai, R. E. and K. A. Janni. 2001b. Determining pressure drop through compost-wood chips biofilter media. ASAE Paper No St. Joseph, MI: ASAE. Sadaka, S., C.R. Magura and D.D. Mann Vertical and horizontal airflow characteristics of wood/compost mixtures. Accepted for publication in Trans. ASAE. 7