Article pubs.acs.org/est
Testing a Biofilter Cover Design to Mitigate Dairy Effluent Pond Methane Emissions Chris Pratt,* Julie Deslippe, and Kevin R. Tate Landcare Research - Manaaki Whenua, Riddet Road, Palmerston North, New Zealand ABSTRACT: Biofiltration, whereby CH4 is oxidized by methanotrophic bacteria, is a potentially effective strategy for mitigating CH4 emissions from anaerobic dairy effluent lagoons/ponds, which typically produce insufficient biogas for energy recovery. This study reports on the effectiveness of a biofilter cover design at oxidizing CH4 produced by dairy effluent ponds. Three substrates, a volcanic pumice soil, a garden-waste compost, and a mixture of the two, were tested as media for the biofilters. All substrates were suspended as 5 cm covers overlying simulated dairy effluent ponds. Methane fluxes supplied to the filters were commensurate with emission rates from typical dairy effluent ponds. All substrates oxidized more than 95% of the CH4 influx (13.9 g CH4 m−3 h−1) after two months and continued to display high oxidation rates for the remaining one month of the trial. The volcanic soil biofilters exhibited the highest oxidation rates (99% removal). When the influx CH4 dose was doubled for a month, CH4 removal rates remained >90% for all substrates (maximum = 98%, for the volcanic soil), suggesting that biofilters have a high capacity to respond to increases in CH4 loads. Nitrous oxide emissions from the biofilters were negligible (maximum = 19.9 mg N2O m−3 h−1) compared with CH4 oxidation rates, particularly from the volcanic soil that had a much lower microbial-N (75 mg kg−1) content than the compostbased filters (>240 mg kg−1). The high and sustained CH4 oxidation rates observed in this laboratory study indicate that a biofilter cover design is a potentially efficient method to mitigate CH4 emissions from dairy effluent ponds. The design should now be tested under field conditions.
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The filter sustained high CH4 oxidation rates (up to 16 g CH4 m−3 h−1) over 16 months without maintenance. However, the filter design was cumbersome and consisted of a gas recovery system on the pond’s surface, gas transfer pipes, and an enclosed biofilter unit on the pond’s edge that itself required a fan to provide air to the methanotrophs. Scaled up to whole pond size, this design would be so expensive as to eliminate any cost or performance advantage over a thermal combustion system. However, the structure and cost effectiveness of soil biofilter technology may be improved by designing a shallow biofilter that covers an effluent pond, either by floatation or suspension. This design eliminates the need for a gas recovery and transfer system and for a fan or pump because diffusion of air through the relatively thin layer of soil would provide sufficient oxygen for the methanotrophs. A functional cover biofilter for dairy effluent pond emissions would present a novel and cost-effective solution to farmers to reduce their GHG emissions. Hence, we used a series of labscale biofilter experiments to test the effectiveness of a biofilter cover design for treating CH4 emissions from dairy effluent ponds. Our goal was to establish whether this novel design has
INTRODUCTION Lagoons or ponds are used on dairy farms around the world to store effluent washdown from milking sheds, as well as runoff from feed lots.1−3 These ponds are a significant source of methane (CH4), a potent greenhouse gas (GHG) with a global warming potential at least 21 times greater than that of CO2.4 For farms with large herd sizes, the CH4 produced from effluent ponds may be harnessed as an energy source.5 However, the reality is that most dairy farms are too small to make this option economically viable. With no incentive to mitigate GHG emissions, CH4 generated from effluent ponds is left to escape to the atmosphere. Yet, with the imminent introduction of emissions trading schemes (ETS) in many countries, farmers will soon need practical and cost-effective options to mitigate their GHG emissions. Biofiltration offers a potentially sound approach to attenuate dairy effluent pond CH4 emissions, which are typically too low for viable energy recovery. Methane biofilters harbor methanotrophs that oxidize CH4 and convert it to water vapor and CO2 (a much weaker GHG than CH4). Methane biofilters have been well studied for their application to reduce landfill CH4 emissions (e.g., see refs 6 and 7), yet very little information has been published regarding their potential to mitigate agricultural emissions.8 In a previous study, we documented the successful performance of a soil biofilter treating CH4 emissions from a section of a dairy effluent pond.9 © 2012 American Chemical Society
Received: Revised: Accepted: Published: 526
August 8, 2012 November 15, 2012 November 16, 2012 November 30, 2012 dx.doi.org/10.1021/es303225h | Environ. Sci. Technol. 2013, 47, 526−532
Environmental Science & Technology
Article
filters was doubled to assess how the filters would perform on effluent ponds with higher than average CH4 emission rates. This trial was conducted for four weeks. Gas samples from the outlet vents were collected three times per week on average, while samples from the air space between the filter substrate layer and the water were collected twice a week in order to assess if sufficient oxygen was delivered to biofilters. Gas samples were collected manually using 10 mL syringes. Gas Analyses. Methane, CO2, and N2O concentrations in gas samples were measured by gas chromatography (Varian CP-3800) using a flame ionization detector (FID), a thermal conductivity detector (TCD), and an electron capture detector (ECD), respectively. Oxygen concentrations in the gas samples were measured by a hand-held probe (Apogee, Model 201). Biofilter Substrate Sampling and Analyses. Biofilter substrates were sampled for physicochemical analysis on four occasions: after one month, two months, three months, and at the completion of the experiment after the high CH4 inlet dose trial. Within each biofilter, three replicate 100 mL substrate samples were randomly selected using a 3 × 5 square grid. Samples were replaced with equivalent volumes of fresh substrate, and all sampled locations were exempt from future sampling. We measured the physicochemical parameters that have been shown to have the greatest impacts on biological CH4 oxidation.14 At the beginning of the experiment, three replicate aliquots of each of the original substrates were analyzed for pH, available (Olsen)-P, total C, total N, ammonium (NH4+)-N, nitrate (NO3−)-N, microbial biomass N and C, water holding capacity, and moisture content. The same analyses were carried out on samples collected after three months of the experiment and after the four week double inlet CH4 dose trial. We also collected three replicate substrate samples at monthly intervals during the initial experiment (i.e., after one and two months); these were analyzed for NH4+-N, NO3−-N, available-P, and microbial biomass N and C, as we expected the greatest fluctuation in these variables during the experiment. Moisture content was determined by oven-drying the samples for 24 h at 105 °C. Water holding capacity was determined on an aliquot of each sample by saturating substrates with water in a 1:2 ratio overnight followed by air drying for 3 h before measuring the moisture content. pH was measured in a 1:2.5 water suspension.15 Total C and N were measured by combustion in a FF-2000 CNS analyzer (LECO Corporation, St. Joseph, MI, USA). Olsen-P was determined by extraction with bicarbonate (0.5 M sodium bicarbonate, pH 8.5, 1:20 substrate/extractant, 30 min shaking), and phosphorus concentrations were measured in the extracts by the ascorbic acid/ammonium molybdate/antimony potassium tartrate colorimetric method on a Lachat FIA 8000. Nitrate-N and NH4+N were extracted using 2 M KCl (1:10 substrate/extractant, 1 h shaking) and measured colorimetricaly on a Lachat QuickChem FIA 8000. Unless otherwise stated, all data are expressed per oven-dry (105 °C) weight. The particle density, dry and wet bulk densities, and porosity of the substrates were calculated following the techniques described by Gradwell.16
the potential to lead to a workable technology to mitigate dairy effluent CH4 emissions. Efforts to field test the design are thus dependent on the outcome of this research.
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MATERIALS AND METHODS Substrate Collection and Preparation. Two substrates were used as biofilter media in this study. The first was a volcanic pumice soil collected from a landfill cover in the central North Island of New Zealand. Soils from this region are classified as pumice (Andisols) derived from volcanic activity that occurred approximately 2000 years ago.10 The second substrate was a 6 month old compost from a green-wasteprocessing site in the lower North Island of New Zealand. The volcanic soil exhibited high CH4 oxidation rates in previous column experiments,11,12 while the compost showed high CH4 uptake rates in preliminary batch tests. The substrates were sieved to
