CALVET, S.1, HUNT, J. 2, MISSELBROOK, T.2
1 Universitat Politècnica de València, Institute of Animal Science and Technique. Camino de Vera s.n. 46022 Valencia, Spain
2 Rothamsted Research, North Wyke, Okehampton, Devon EX20 2SB, UK;
ABSTRACT: The aim of this study was to quantify the effect of low frequency aeration of pig slurry on gas emissions and to establish the underlying mechanisms. A batch experiment was designed with 6 tanks with 1m3 of pig slurry each. Three of these tanks were subjected to aeration (2 minutes every 6 hours, airflow 10 m3 h-1), whereas the other three tanks remained as a control. Emissions of NH3, CH4, CO2 and N2O were measured. Aeration increased NH3 emissions by 20% with respect to the controls (8.48 vs. 7.07 g day-1 per m3 of slurry, P<0.05). A higher pH was found in the aerated tanks at the end of this phase (7.7 vs. 7.0 in the aerated and control tanks, respectively, P<0.05). CH4 emissions were 40% lower in the aerated tanks (2.04 vs. 3.39 g day-1 per m3 of slurry, P<0.05). These differences in NH3 and CH4 emissions remained after the aeration phase had finished. No effect was detected for CO2, and no relevant N2O emissions were detected during the experiment. Our results demonstrate that low frequency aeration of stored pig slurry increases slurry pH and NH3 emissions.
Keywords: Slurry, Swine, Storage, Aeration, Mixing, pH, GHG, NH3
INTRODUCTION: Pig slurries emit considerable amounts of ammonia (NH3) and greenhouse gases (GHG) to the atmosphere, mainly as methane (CH4). Slurry treatment techniques may contribute to reduce these emissions (Amon et al., 2006), in accordance with the environmental regulations. Aeration of slurries has been widely studied in the literature as a technique to reduce nitrogen and organic matter loads, with potential benefits associated to reductions of odour and CH4 emissions. However, increased nitrous oxide (N2O) emissions arise as a relevant side effect. Recent research suggests that low frequency aeration of slurries may reduce NH3
emissions because mixing induces changes in the physical-chemical characteristics of the slurry surface, without increasing N2O emissions (Dai and Blanes-Vidal, 2013; Van Dooren et al., 2016).
A pH gradient is described at the surface of pig slurries, with higher pH at the layer in contact with the air. Mixing slurries breaks this gradient. Aeration contributes to the aerobic degradation of organic matter and could reduce CH emissions. However, aeration increases pH as a
with 6 tanks with 1 m3 of pig slurry each, which was obtained from a fattening unit of commercial farm (total solids 25.4 g/kg, total nitrogen 4.0 g/kg).
After an initial phase of 7 days when none of the tanks were aerated (phase 1), an experimental phase of 4 weeks (phase 2) subjected three of the tanks to aeration (2 minutes every 6 hours, airflow 10 m3 h-1 per m3 of slurry), whereas the other three tanks remained as a control. A final phase of 9 days was established (phase 3) with no aeration at any tank, to compare permanent effects of aeration. Slurry samples were taken at the start of each experiment phase and were analysed for total solids, volatile solids, total nitrogen and ammonium nitrogen. pH was measured three times per week using a portable meter with pH probe HI 9025, Hanna Instruments, Leighton Buzzard, UK). Measurements were conducted throughout the experimental period at 1 cm and 10 cm depth. Temperature of ambient air and of each slurry tank was continuously monitored using a data logger (Grant Data Acquisition Series 2040, UK).
Gas concentrations were measured using Los Gatos analysers (Model 911-0016 for NH3 and 915-0011 for CH4 and CO2, Los Gatos Research, California). A sampling protocol was devised to measure emissions in the moments of aeration. As a validation of NH3 concentration measurements, these were also measured by means of acid absorption flasks twice per week.
Finally, spot measurements of N2O and H2S concentrations were also conducted by means of gas chromatography (Clarus 500, Perkin Elmer, Buckinghamshire, UK) and colorimetric detection tubes (Draeger Safety), respectively.
Emissions were integrated on a daily basis and a two-way analysis of variance was conducted to determine the effects of treatment, phase and their interaction. During the moments of aeration, the immediate effect of aeration on slurry emissions was analysed in qualitative terms.
2. RESULTS AND DISCUSSION: The evolution of temperature of ambient air and tanks is shown in Figure 1. Temperatures ranged mostly between 14ºC and 18ºC. The effect of aeration on slurry temperature was of minor importance (less than 0.5ºC variation).
Figure 1. Evolution of ambient and tank temperature (left) and pH (right) during the experiment. Aeration and control tanks are shown separately.
No statistical differences were found in pH during phase 1, where no aeration was conducted in any tank. During phase 2 pH measured at 10 cm depth raised steadily for the first days of aeration and reached a constant level of about 0.7 pH units above the pH of slurries in control tanks (Figure 1). These differences were statistically significant and continued in phase 3, where no aeration was conducted. Slurry surface pH was also measured, but the divergence between surface and bulk measures was lower than the difference between aeration and control tanks.
Emission factors and air quality
The aerobic degradation of volatile fatty acids (VFA) is very likely the cause for this pH increase (Burton, 1992).
The evolution of daily emissions of NH3 and CH4 are shown in Figure 2. During the first phase, where no aeration was conducted, no statistical differences were obtained in emissions from any gas. However, during the second phase aeration increased NH3 emissions and decreased CH4
emissions. The increase in NH3 emissions was attributed to the increase of slurry pH previously described. The formation of crust in control tanks, compared with aeration tanks (in which crust was not formed) may additionally explain the higher NH3 emissions of NH3. A slight effect of pH gradient reduction was also detected at the beginning of phase 2, according to previous research (Blanes-Vidal et al., 2012). During the short instants of aeration (2 minutes every 6 hours), most CH4 was emitted to the atmosphere in the aerated tanks, but this quantity was still lower than the CH4 emitted from control tanks.
Figure 2. Evolution of NH3 (left) and CH4 (right) emissions during the experiment. In the aerated tanks, CH4 emissions excluding the instants of aeration are also plotted (Aeration).
No effects of aeration were detected on CO2 emissions, which remained constant, and on N2O emissions, which could not be detected in both control and aerated slurry. On the contrary, spot emissions of H2S were detected during the aeration moments. Mixing was considered a driving force of gas emission dynamics. Insoluble gases such as CH4 and CO2 generated in the slurry are retained in the slurry body as small bubbles and then released in relatively high amounts during the aeration moments. On the contrary, NH3 did not show this behaviour, and the emission dynamics was more related pH changes both in the short term (pH gradient reduction due to mixing, leading to lower emissions) and in the short term (pH increase due to VFA removal, leading to higher emissions).
Acknowledgements. North Wyke Farm and technical support staff. Mr. R. Knox, Tor Pigs, Devon, UK for providing slurry. Spanish Ministry of Education, Culture and Sports, in the framework of the State Programme to Promote Talent and Employability in R+D+I, Sub-program on Mobility of the Plan on Scientific and Technical Research and on Innovation 2013-2016, and Spanish Ministry of Economy, Industry and Competitiveness (Project AGL2014-56653-C3-2-R). Rothamsted Research is supported by the UK Biotechnology and Biological Sciences Research Council.
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Amon, B., Kryvoruchko, V., Amon, T., Zechmeister-Boltenstern, S. 2006. Methane, nitrous oxide and ammonia emissions during storage and after application of dairy cattle slurry and influence of slurry treatment. Agric. Ecosyst. Environ. 112, 153–162
Blanes-Vidal, V., Guardia, M., Dai, X. R., Nadimi, E. S. 2012. Emissions of NH3, CO2 and H2S during swine wastewater management: Characterization of transient emissions after air-liquid interface disturbances. Atmos. Environ. 54, 408–418.
Burton, C. H. 1992. A review of the strategies in the aerobic treatment of pig slurry: Purpose, theory and method. J Agric. Eng. Res, 53, 249-272.
Dai, X.R., Blanes-Vidal, V., 2013. Emissions of ammonia, carbon dioxide, and hydrogen sulfide from swine wastewater during and after acidification treatment: Effect of pH, mixing and aeration. J. Environ. Manage. 115, 147-154.
Van Dooren, H.J., Bokma, S., Zonderland, J., 2016. Preliminary ammonia emission measurements from Aeromix system for slurry mixing. CIGR-AgEng Conference 2016, Aarhus, Denmark
Emission factors and air quality
GAS EMISSIONS FROM DEEP LITTER SYSTEMS FOR DAIRY CATTLE IN CONTRASTED FEEDING