Particulate matter expelled from tunnel-ventilated animal feeding operations (AFOs) is known to transport malodorous compounds. As a mitigation strategy, vegetative environmental buffers (VEBs) are often installed surrounding AFOs in order to capture particulates and induce lofting and dispersion. Many farmers are or are interested in implementing VEBs, yet research supporting their efficacy remains sparse.
Currently, point measurements, often combined with models, are the primary means by which emission rates from AFOs and VEB performance has been investigated. The existing techniques lack spatial resolution and fail to assign the observed particulate reduction to capture, lofting, or dispersion.
In recent years, lidar has emerged as a suitable partner to point measurements in agricultural research. Lidar is regarded for its ability to capture entire plume extents in near real time. Here, a technique is presented for estimating the capture efficiency of a VEB using lidar. An experiment was conducted in which dust was released upwind of a VEB at a known rate, and the emission rate downwind of the VEB was estimated using an elastic scanning lidar.
Instantaneous lidar scans showed periodic lofting well above the VEB, but when scans were averaged over several hours, the plumes appeared Gaussian. The experiment revealed a capture efficiency ranging from 21-74%, depending on the time of day. The methodology presented herein addresses deficiencies in the existing techniques discussed above, and the results presented add to the lacking body of research documenting VEB capture efficiency.
EXPERIMENTAL SITE & EQUIPMENT
The study took place at the broiler house at the University of Delaware Carvel Research and Education Center (38.64,-75.47), between June 24 and 26, 2013. The multi-row VEB surrounding the facility was established in the Spring of 2003. In this study, we focused on the northeast section of the VEB and conducted experimental runs only when the wind was out of the southwest, perpendicular to the VEB.
The northeast VEB section was planted 19 m from the exhaust fans in sequential rows parallel to the end wall of the broiler house with 12 bald cypresses (Taxodium distichum), 13 green giant arborvitaes (Thuja plicata x tandishii), and 14 white pines (Pinus strobus). Each tree was 8to 10m tall.
The University of Iowa’s elastic scanning lidar (Figure 2) utilizes a laser, telescope, photo detector, and computer to measure the backscatter of light from suspended particulates. The lidar operates by emitting a pulse of infrared laser light (wavelength λ = 1.064 μm) into the atmosphere.
Particulates interact with the pulse and scatter a fraction of the light back to the telescope. The scattering is elastic, so no energy is lost by the photons, and the detected light is at the same wavelength as the emitted light.
DATA ANALYSIS METHODS
n this section, the technique is presented for inverting lidar, wind, and particulate size data into capture efficiency. The analysis presented below was performed for all six runs. Each variable needed to calculate capture efficiency was time-averaged over the run duration, yielding six total estimates. In the methodology described hereafter, over bars () indicate time-averaging over the run duration.
Particulate Size Distribution:
Particulate size distributions (PSDs) were required to obtain mass extinction efficiency, a parameter needed to invert extinction coefficients to mass concentration. PSD methods are described in Wang-Li et al. (2013) and Buser (2004) ; a brief summary of the method is provided here. A Beckman Coulter LS230 laser diffraction system (Beckman Coulter Inc., Miami, FL) with software version 3.29 was used to perform the particle size analyses on the filter and wash samples.
Lidar Signal Inversion Uncertainty:
The uncertainty in the transformation of the raw lidar signal into extinction coefficients propagates to the capture efficiency estimation. Klett’s Lidar inversion algorithm (Equation (3), rewritten below for reference) contains 4 major sources of uncertainty which propagate to the extinction coefficient alpha R.
Mass Extinction Efficiency Uncertainty:
The uncertainty in the PSD measurements correspond to uncertainty in MEE values and propagate to capture efficiency estimates. The uncertainty in MEE is represented by the standard deviation of the MEE values estimated at all sampler locations.
RESULTS & DISCUSSION
Observed VEB Capture Efficiencies:
The capture efficiencies ranged between 21 and 74% amongst the six runs. The performance of the VEB varied based on time of day. The VEB captured a larger fraction of particulates during the night and a smaller fraction during the day.
Five of the six runs were performed during the day, one at night. Since sampling times were non-uniformly distributed amongst a 24-hour period, the average diurnal capture efficiency was not determined.
The methodology presented here is a reliable technique for estimating emission rates under complex flow regimes. The method was applied to determine the capture efficiency of a VEB, as documentation of VEB efficacy is currently lacking.
The results of this study indicate that a VEB can effectively capture between 21 and 74% of PM transported through it, depending on atmospheric conditions. Higher capture efficiency is observed at night, during stable atmospheric conditions with low TKE. However, the same conditions may discourage lofting and consequently result in more odor nuisance to downwind neighbors. Conditions associated with low capture efficiency (daytime, unstable, and high TKE) may encourage lofting and dispersion.
Capture efficiencies exhibited a slight relationship with the particulate release distance from the VEB. Due to the limited number of runs performed and the varying atmospheric conditions associated with each run, only two pairs of runs were available with similar atmospheric conditions and different release distances. Under both situations, a further release distance yielded greater capture efficiencies.
The results of this experiment show that a VEB is an effective mitigation strategy for capturing particulate matter which often transports malodorous compounds. Even during its worst performance, the VEB captured 21 % of particulates, and at its best, it captured 74 %. We hope these results will provide farmers with some assurance that the technology many of them are interested in implementing (or have already implemented) is in fact effective.
Source: University of Iowa
Author: William Brandon Willis