Greenhouse Gas (GHG) Emissions from Mechanically Ventilated Deep Pit Swine Gestation Operation

Emission of greenhouse gases (GHGs) from mechanically ventilated deep pit manure storage was monitored in a swine gestation operation. Air samples were collected from pit exhaust fans at different times of the year (fall, summer, and spring) using a vacuum chamber and Tedlar bags. GHGs concentrations were measured with a greenhouse gas chromatograph (GC) within 24 hours of collection. Air flow rates from exhaust fans were measured using a 160 mm bi-directional Gill propeller anemometer and the ventilation rate was determined as the summation of air flow rates from all fans. The average methane (CH4) concentration was 88±61 ppm and CH4 concentration differences were statistically significant among sampling dates and seasons. The carbon dioxide (CO2) concentration followed the same trend as CH4. The average CO2 concentration was 1105±1063 ppm. Nitrous oxide (N2O) concentrations ranged from 0.02 to 0.66 ppm. Methane emissions varied between 115.94 to 572.18 g d-1 AU-1 and higher methane emission was observed during summer (480.28 g d-1 AU-1). The average carbon dioxide emissions varied from 5.35 to 15.83 kg d-1 AU-1, whereas average N2O emissions varied from 0.06 to 7.30 g d -1AU-1. Significant variation of GHG concentrations and emissions were observed among fall, summer and spring seasons. *Corresponding author: Shafiqur Rahman, Department of Agricultural and Biosystems Engineering, North Dakota State University, NDSU Dept. 7620, PO Box 6050, Fargo, ND 58108-6050, USA, Tel: +1 701 231 8351; Fax: +1 701 231 1008; E-mail: s.rahman@ndsu.edu Received December 06, 2011; Accepted January 07, 2012; Published January 09, 2012 Citation: Rahman S, Lin D, Zhu J (2012) Greenhouse Gas (GHG) Emissions from Mechanically Ventilated Deep Pit Swine Gestation Operation. J Civil Environment Engg 2:104. doi:10.4172/2165-784X.1000104 Copyright: © 2012 Rahman S, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
The demand for animal products are expected to grow [1] and animal production will have a large impact on the world's natural resources and contributes significantly to environmental problems, such as pollution, climate change and loss of biodiversity [2]. Livestock production operation and manure storage generate greenhouse gases (GHGs) such as carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) [3,[4][5][6] and contribute to climate change by emission of these gases. Generation of GHGs from animal production facilities are from enteric fermentation, housing confinement, manure storage, manure treatment and land application of manure [7,8] and GHG emissions from animal production can vary with animal species, different diets, feed conversion mechanisms, manure management practices, and environmental conditions [7][8][9][10][11][12].
Swine production plays an important economic role in key hog producing states in the United States [13]. The growth of swine production in the United States, as well as in North Dakota, is expected to continue. The main environmental concerns with animal production facilities are soil, water, and air pollution (i.e. odor, ammonia, hydrogen sulfide, and greenhouse gases) [14,15]. However, little is known about the relative contributions to GHG emissions from mechanically ventilated deep pit manure storage barns under temperate climatic conditions. Due to confined and intensive swine production in a concentrated area, there are many outdoor and indoor (i.e., deep pit and shallow pit) manure storage systems. Deep pit manure storage systems are commonly used for swine operation for long-term storage of manure. Manure in deep pit storages undergoes anaerobic decomposition and generates pollutant gases including ammonia, hydrogen sulfide, and GHGs [16]. Production of N 2 O during storage can occur due to incomplete nitrification-denitrification of nitrogen contained in the wastes. Anaerobic decomposition of organic matter in manure generates methane. The amount of methane produced during decomposition is influenced by ambient temperature and manure management practices.
Many researchers have identified temperature as an important factor for CH 4 emissions from manure storage facilities. Low temperatures can suppress microbial activity and metabolism and therefore production of CH 4 [17]. High temperatures may expedite decomposition of organic matter in manure and increase CH 4 production. At the same time, high ambient temperatures require high ventilation rates and are correlated to high CH 4 emissions [18]. Despite the rather contrasting differences in operational practices, data on GHG emission rates under temperate climatic production conditions are inadequate. In order to address environmental concerns and to adapt a management practice, it is important to monitor GHG emissions under different climatic conditions and manure management practices. Therefore, the purpose of this study is to quantify GHG emissions from mechanically ventilated deep pit exhaust fans in swine gestation operation in temperate climatic conditions.

Description of facilities and management practices
This study was conducted at a commercial swine gestation operation in North Dakota, USA ( Figure 1). The total capacity of this facility is 5000 animals. The facility has two gestation-barns (g-barn) and each g-barn (165 m×24 m) has 2100 gestation-stalls with deep manure pits for collection. The deep pit size is 165 m×24 m and the maximum operating depth is 3 m. The two g-barns are identical in size, Typically, the producer empties the deep pit storage system twice peryear (fall and spring).

Air sample collection
Because of the large number of exhaust fans, only a limited number of samples (16 to 18) were collected during each sampling event for GHG analysis. Two ambient air samples were collected at the upwind site of the barn during each sampling event to obtain background concentrations. For sampling consistency, samples were collected in duplicate from the same pit fans and at the same time of a day (10 am-12 noon) each time. All air samples were collected in Tedlar bags using a vacuum chamber (SKC Inc., 863 Valley View Rd., Eighty Four, PA 15330) from the exhaust side of the fans for biosecurity reasons. Samples were collected from inside of an exhaust fan as shown in the Figure 1 to minimize dilution of sample with ambient air.

Measurement
Within 24 hours of collection, air samples were analyzed for CH 4 , CO 2 , and N 2 O using a greenhouse gas chromatograph (GC) (Model No. 8610C, SRI Instruments, 20720 Earl St., Torrance, CA 90502) ( Figure 2) equipped with a flame ionization detector (FID) and an electron captured detector (ECD). An air sample from the Tedlar bag was injected into a 1mL sample loop using the inbuilt vacuum pump interface ( Figure 2) and the event program. Before injecting any sample into the sampling loop, the FID detector temperature was One of the pit exhaust fans sampling location.  raised to 300°C and the ECD detector temperature was raised to 350°C. The system was operated on a nitrogen carrier at 20 PSI for the ECD, while hydrogen and air were supplied to the FID/methanizer using a built-in air compressor at 20 PSI. In this system, the ECD detector detects N 2 O, while the FID/methanizer detector detects both CH 4 and CO 2 . Gas chromatographs were recorded and analyzed with the Peak Simple Chromatography Data System Software (Version 3.72, SRI Instruments, 20720Earl St., Torrance, CA 90502). Before and after sample analysis, calibration gases were used to ensure that the GC was functioning properly. Blank samples were also run between samples using the same procedure to check any contamination from previous analysis. To generate calibration equations, three points calibration were conducted for CH 4 (20, 100, and 1000 ppmv), CO 2 (100, 1000, and 2500 ppmv), and N 2 O (0, 1, and 10 ppmv) gases. Calibration equations and R 2 value of equations are listed in Table 1.
The average air velocity rates (ms -1 ) from all running pit exhaust fans were measured continuously using a 160 mm bi-directional Gill propeller anemometer (Model 27106RS, RM Young Company, 2801 Aero Park Dr., Traverse City, MI 49686) ( Figure 3) as also used by other researchers [18]. The average air velocity across the radius of an exhaust fan was measured in at least 10-15 locations (Figure 4). A single propeller was installed on the exhaust side of a fan and the output signal of the anemometer (0 to 1 VDC) was recorded with a CR10X data logger (Campbell Scientific, Inc., 815 West 1800 N., Logan, UT 84321). The air flow rate (m 3 s -1 ) of each running fan was calculated from the measured average air velocity and the fan cross-sectional area. The total ventilation rate from each gestation barn was determined as the summation of the air flow rates of all fans.

Data analysis
Data were pooled and pair wise means were compared among sampling dates and seasons. Both concentration and emissions were analyzed at P<0.05 to quantify the seasonal effect. The significance of the differences in concentration and emissions were examined according to Duncan's multiple range tests [20].

Results and Discussion
Methane, carbon dioxide, and nitrous oxide concentrations from deep pit exhaust fans    manure storage beneath the g-barn was 88±61 ppm, and the CH 4 concentration differences were statistically significant among sampling dates (Table 2). During spring, the CH 4 concentration was significantly higher than in the fall (September-October) and summer (June-August) ( Table 3). Elevated CH 4 concentrations during the spring were likely due to the amount of manure stored in an anaerobic condition for an extended time (six-nine months) in the deep pit and when the ventilation rate was also low. Lower ventilation rates resulted in a greater concentration of CH 4 . The longer the manure is stored in a deep pit under anaerobic conditions, the more methane will be produced. Also, a crust was observed on the manure surface, which might have also contributed to high CH 4 production as also reported by others [21]. In the month of July, a higher CH 4 concentration was observed ( Figure 5), which was likely due to elevated anaerobic digestion of organic matter in the manure from higher ambient temperature. Methane generation and emissions are mainly depends on anaerobic digestion of organic matter in the manure [22,23] the duration of manure storage [23,24] and the manure removal frequency. In a deep pit manure storage system, it is common that bubbles rise from the liquid manure, which can carry methane and increase the concentration noticeably. The measured CH 4 concentration was higher than reported by Zhang et al. [19] but close to that reported by Lague [25]. This difference could be due to the manure storage system. The manure storage system in this study was deep pit storage under the g-barn, whereas in the Zhang et al. [19] study, it was an outdoor manure storage system and a shallow pit was used in the Lague [25] study.
Similarly, CO 2 concentrations are shown in Figure 6. The CO 2 concentration followed the same trend as CH 4 and the average CO 2 concentration was 1105±1063 ppm. During spring and fall, the CO 2 concentration was higher when the ventilation rate was low (Table 3, Figure 6). As the ventilation rate increased, the CO 2 concentration became lower. Carbon dioxide generation is mostly from animal respiration and from anaerobic digestion of organic matter in manure [26]. The variation of concentration is due to combination of management practices, animal activities, and ambient temperature during the study period. The methane and CO 2 concentrations followed a parallel trend (Figure 7) as also reported by others [27]. The measured CO 2 concentrations were within the range reported by Zhang et al. [19] and Lague [25].
The variation of N 2 O concentrations are presented in Figure 8. Due to the malfunctioning of the ECD detector, no N 2 O concentrations were measured during the 6/28 and 7/12 sampling events ( Figure  8). The measured N 2 O concentrations from the deep pit fans ranged from 0.02 to 0.66 ppm ( Table 2). The highest N 2 O concentration was observed during the summer and fall. This was likely due to the surface crust on the manure in storage, where N 2 O production took place in the interface between manure and surface crust. A similar conclusion was also drawn by others [28]. Although a higher N 2 O concentration was observed during summer and fall, the N 2 O concentrations from the deep pit fans were found to be less than 1.0 ppm, averaging 0.35 ppm which was close to the background level (0.3-0.4 ppm). This means that under cold climatic conditions like North Dakota, manure storages would likely have lower N 2 O emissions than in warmer climatic conditions.

Emissions of methane, carbon dioxide, and nitrous oxide from deep pit manure storage exhaust fans
Methane emissions varied from 115.94 to 572.18 g d -1 AU -1 ( Figure  9).These values are comparable to those reported by Costa and Guarino [3] and Zhang et al. [19]. However due to manure storage differences, the variation was the same but magnitude was different. The higher methane emissions were observed during summer (Table 4) when ambient temperatures and the ventilation rates were high (Figure 9). Statistically significant differences were noticed between summer and fall and also between summer and spring methane emissions, but not between fall and spring emissions (Table 4). During June and July, Carbon dioxide emissions followed a trend similar to the CH 4 emissions ( Figure 10). As mentioned before, CO 2 generation is mostly from animal respiration and from anaerobic digestion of organic matter in manure. Additionally, a significant amount of CO 2 is also produced at the manure-air interface by the aerobic microbial degradation process [21,23]. As a result, higher CO 2 emissions were observed during the summer than in the fall and spring (Table 4), when anaerobic degradation of manure at high temperature resulted in higher CO 2 production, as well as aerobic decomposition at the manure-air interface [19,24]. However, no statistically significant differences were noticed between summer and fall emissions, but there was a clear difference between summer and spring emissions (Table 4). Higher ventilation rates contributed to higher CO 2 emissions during warm months. The average CO 2 emissions varied from 5.35 to 15.83 kg d -1 AU -1 . Results obtained in this study compare well with those of other studies [3,7,19].
The N 2 O emissions trend is shown in Figure 11. As stated before, due to an ECD detector malfunction, no N 2 O concentrations were measured during the 6/28 and 7/12 sampling times. As a result, no N 2 O emissions were calculated for those days. Average N 2 O emissions varied from 0.06 to 7.30 g d -1 AU -1 . Like other GHGs, significantly higher N 2 O emissions were observed during the summer and fall as accelerated microbial decomposition of organic matter in deep pit manure might occur and amplified the CH 4 production. In general, when the CH 4 concentrations were low, the emissions rates were high due to the high ventilation rates.