effects of anaerobic digestion and separation of pig ...

EFFECTS OF ANAEROBIC DIGESTION AND SEPARATION OF PIG SLURRY ON ODOR EMISSION M. N. Hansen, P. Kai, H. B. Møller ABSTRACT. Storage and land application of livestock manure causes considerable odor nuisance to the surrounding neighborhood. Anaerobic digestion and separation of slurry change composition and physical properties of slurry and may therefore lessen the odor pollution during storage and land application. An experiment was set up to study the effects of anaerobic digestion and separation of slurry on the emission of odor. Odor concentration above treated and untreated slurry was compared during storage and following land application. Concentrations of odorous gasses were measured using GC/MS analysis and odor concentrations were determined using dynamic dilution olfactometry. Slurry concentrations of malodorous volatile fatty acids were reduced by between 79% and 97% by anaerobic digestion, while concentrations of malodorous phenolic and indolic odor components above the slurry were reduced by both anaerobic digestion and subsequent separation. Odor concentration in air sampled above slurry stores was slightly reduced by anaerobic digestion; however, odor concentration was found to be higher above stores of anaerobically digested slurry following mixing of the slurry prior to land application. Odor concentration in air sampled above land applied slurry was reduced by 17% by anaerobic digestion and by 50% by combined anaerobic digestion and separation. Keywords. Odor abatement, Manures, Slurry, Odorants, Volatile fatty acids, Anaerobic digestion, Slurry separation.

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ivestock production causes environmental problems, such as odor nuisance, which damages the reputation of livestock producers both regionally and nationally, and may cause a reduction in the value of neighboring properties (Hansen and Petersen, 2003). In addition, some odorants may present health hazards (Donham et al., 1982; Schiffman, 1998). Odor is caused by a large number of chemical components produced during animal growth and present in livestock manure (Spoelstra, 1980; Tanaka, 1988; O’Neill and Phillips, 1992; Schiffman et al., 2001). Many of these odorants have been identified in livestock manure by Hobbs et al. (1995) and the major odor components in pig slurry has been found by Hobbs et al. (1998) to belong to sulfides, volatile fatty acids (VFA), and phenolic and indolic groups. The odor components are released from the slurry surface. A large surface area of slurry and low diffusion resistance therefore facilitate emission of these gases. High emission of odor components consequently takes place from uncovered stores of slurry and during handling and land application of slurry. The emission of odors from manure storages depends on the concentration of odorants in the manure (Misselbrook et al., 1997; Hobbs et al., 2001) and on the presence of a slurry cover system (Clanton et al., 1999; Hörnig et al., 1999; Nahm, 2003). These factors vary according to type of slurry,

Article was submitted for review in June 2005; approved for publication by the Structures & Environment Division of ASABE in October 2005. The authors are Martin N. Hansen, Scientist, Peter Kai, PhD Student, and Henrik B. Møller, Scientist, Department of Agricultural Engineering, Danish Institute of Agricultural Sciences, Denmark. Corresponding author: Martin N. Hansen, Danish Institute of Agricultural Sciences, Department of Agricultural Engineering, Research Centre Bygholm, Schüttesvej 17, 8700 Horsens, Denmark; phone: +45 89991900; fax: +45 89993100; e-mail: [email protected].

but may also depend on how the slurry has been treated prior to storage. Likewise, emission of odor components after slurry application may be influenced by how fast the slurry percolates (infiltrates) into the soil after land application. As the infiltration rate depends on the dry matter content of slurry (Sommer and Olesen, 1991; Sommer et al., 2004), there has been considerable interest in slurry treatment technologies, such as anaerobic digestion and separation of slurry, that change the composition and the physical properties of slurry (Pain et al., 1990a; Møller et al., 2002). Slurry separation is a technology that separates slurry into a minor solid fraction containing the majority of the dry matter and phosphorus content of the slurry, and a larger liquid fraction containing most of the ammonium-nitrogen and potassium content (Møller et al., 2002). Separation of slurry is in Denmark mainly performed to reduce the risk of phosphorus accumulation on agricultural land in areas of dense livestock production, as it allows export of slurry phosphorus at a lower cost due to the volume reduction (Møller et al., 2000). Moreover, reduced content of dry matter in the liquid fraction is likely to enhance infiltration, thereby reducing the potential for odor nuisance following slurry application. Anaerobic digestion of slurry is performed to produce methane (biogas), which can be used for production of heat and electricity. Anaerobic digestion is a biological process which reduces the dry matter content of the slurry and ensures degradation of some important organic odorants such as VFA (Pain et al., 1990a), thereby reducing the odor releasing potential of slurry during subsequent storage and land application. Besides, the reduction in the dry matter content following anaerobic digestion may enhance infiltration of applied slurry, and therefore reduce the potential for odor nuisance following slurry application. As both anaerobic digestion and separation of slurry have potential to reduce the emission of odor, the odor reduction

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E 2006 American Society of Agricultural and Biological Engineers ISSN 0883−8542

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effects of both anaerobic digestion, and anaerobic digestion combined with slurry separation were studied during subsequent storage and slurry application.

MATERIALS AND METHODS SLURRY TREATMENT Unstored pig slurry taken from a pig finisher house was either anaerobically digested (AD), anaerobically digested and separated (AD-sep), or left untreated (Untreat). The anaerobic digestion was performed on a commercial farm biogas facility (Dansk Biogas, Hegndal, Denmark) in a 600-m3 digester at 48_C with a hydraulic retention time of 15 days. Before digestion, 3% by weight mixed organic industrial waste was added to the slurry to increase biogas production (co-digestion). Separation was subsequently performed using a decanting centrifuge (Alfa Laval NX 309). Thirty m3 of each slurry type were stored for five months in three experimental slurry tanks (diameter = 4.5 m; height = 3.0 m) under identical outdoor storage conditions (average temperature = 2.7_C). During storage all types of slurry were covered by an artificial crust consisting of 0.15-m lightweight-expanded clay aggregates (Leca, Randers, Denmark) to reduce ammonia emission. SLURRY COMPOSITION AND ODOR CONCENTRATION ABOVE SLURRY STORES Concentrations of nutrients and odor components in the slurry were determined before and after the storage period by analyzing three well-stirred samples (each consisting of four representative subsamples collected during establishment and mixing of the slurry stores) per treatment. Each sample was analyzed for slurry composition using standard techniques (DS/EN ISO 11732). Dry matter (DM) was determined after drying samples at 100_C for 24 h, total N was determined by the Dumas procedure (LECO CN 2000, St. Joesph, Mich.), total ammoniacal N (TAN) was measured with an Autoanalyzer 3 (Bran+Luebbe), and pH was measured with a standard electrode (Porotrode W.O.C). Concentrations of VFA in the slurry were determined by gas chromatographic analyses (HP 6850 series). Prior to analysis, well-mixed samples of 1.0 g each were centrifuged at 12,000 rpm for 10 min. The centrifuged samples were then filtered and had 0.3-M oxalic acids added before gas chromatographic analyses were performed. Identifications of odorous components in the headspace air collected above 1.0 L of the different slurry types at 15°C were obtained by the following procedure. Volatile odorants in 10 L of headspace air were absorbed onto SOlid Phase Micro-Extraction (SPME) fibers. The SPME fibers (50/30 µm DVB/Carboxen/PDMS; Supelco, USA) were placed in a PTFE in-liner (universal septum injector; Omnifit, England), inserted between the headspace and an air pump that sucked air with a sampling rate of 0.001 m3 min−1. The amounts of molecules of the different odorants collected by SPME were then thermally desorbed and identified by gas chromatography/mass spectrometry (GC/MS) with a non-polar capillary column (60 m × 0.32 mm × 1.0 µm VF-1MS FactorFour; Varian CP 3800, USA). Quantification of the odorous components was achieved by relative estimations of the areas of peaks obtained for the different components.

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Odor concentration in air sampled above the surface of the different slurry types were additionally determined by dynamic dilution olfactometric analyses. Care was taken that air sampling took place as simultaneously as possible to minimize changes in climatic conditions. Prior to air sampling, the slurry tanks were covered by a plastic sheet (0.15-mm polyethylene) to ensure equal climatic conditions during sampling. The 1.1-m headspace between the surface of the slurry and the plastic sheet was mixed by an internal oscillating ventilator (Air King 18-in., model 9018, California, USA) to simulate wind speed and to ensure mixing of the headspace air. After 20 min of covering, 30 L of headspace air was sucked into 30-L Tedlar bags by means of vacuum boxes. All sampling of air was replicated twice. Concentrations of odor in the air samples were estimated by dynamic dilution olfactometric analysis within 24 h in accordance with the European Standard Draft prEN 13725 (CEN, 1999). Using the same procedure, sampling was repeated immediately after the surface crust was broken by a thorough mixing of the different slurry types by means of a tractor-driven slurry propeller. ODOR CONCENTRATION FOLLOWING SLURRY APPLICATION Emission of odor after land application of the differently treated slurries was quantified by means of a static flux chamber technique. The chamber, made of hardened PVC (height = 0.60 m, length = 2.40 m, width = 1.30 m), was equipped with two oscillating internal ventilators (Desk Fan, FT6, Zhongshan, China) fixed to the top of the chamber to allow for simulation of internal wind speed and for mixing of internal air. The external surface of the chamber was covered by aluminum foil to restrict unequal solar heating of the chamber during sampling. Each slurry type was applied using the same band application technique (distance between bands = 0.30 m) to separate 2.0- × 4.0-m plots. Slurry was applied in the morning at a rate equal to 30 t ha-1. The slurry-treated surface was covered by the static flux chamber immediately after slurry application, and 20 min after covering, 30 L of air was sucked from the chamber into 30-L Tedlar bags using vacuum boxes. The odor concentration in the Tedlar bags was determined by dynamic dilution olfactometry within 24 h (CEN, 1999). All air samplings and odor analyses were replicated twice. Quantification of the odor emission from the slurry-treated surfaces was repeated 4 h after slurry application using the same procedure. Temperatures of soil and slurry/soil mixture inside the chamber were continuously recorded during sampling by thermo-elements attached to an Eltek data-logger (Squirrel, series 1000, Cambridgeshire, UK). STATISTICAL ANALYSIS Statistical analyses of the manure composition were performed using generalized linear model (GLM) procedure (SAS Institute, 1988). Assumption of equal variance of different groups was tested by Bartlett’s test prior to analysis. Where significant differences were found within groups, Duncan’s multiple range test was used to test for significant differences in means. Statistical analyses of climatic conditions were performed using paired t-tests. For all the statistics, a significance level of α = 0.05 was applied.

APPLIED ENGINEERING IN AGRICULTURE

DISCUSSION

The dry matter content of the slurry was significantly reduced by anaerobic digestion and even more by succeeding separation (table 1). Concentrations of malodorous VFA in the slurry (2-methyl propanoic acid, butanoic acid, 3-methyl butanoic acid, and pentanoic acid) were significantly reduced by between 79% and 97% by anaerobic digestion; while only the concentration of 3-methyl butanoic acid was found to be further reduced by subsequent separation of slurry (table 1). Concentrations of the malodorous VFA in the three differently treated slurry types were not significantly influenced by storage (data not shown). Concentrations of phenolic and indolic odor components in the headspace air were found to be reduced by anaerobic digestion (AD), and further by succeeding separation (AD-sep). Due to unreplicated sampling, no statistical analyses were performed.

Concentrations of VFA in the slurry were found to be significantly reduced by anaerobic digestion. Similar results have been reported by Pain et al. (1990a) and Powers et al. (1999). The lower concentrations of VFA in anaerobically digested slurry is caused by the fact that VFA are easily degradable and serve as the main carbon source for

ODOR FOLLOWING SLURRY APPLICATION Following slurry application, odor concentration was highest in headspace air sampled above the untreated slurry type, both immediately and 260 min after application (table 2). Odor concentration, especially from the untreated slurry type, was found to be increased 260 min after application, probably caused by higher temperature of soil and slurry-soil mixture (table 2).

250 200 150 100 50 0 Untreat

AD

b

25000 20000 15000 10000 5000 0 Untreat

AD

Odor components in headspace air Phenol 4-Methyl phenol (p-cresol) 4-Ethyl phenol Indole 3-Methyl indole (skatole) [a] [b] [c]

AD−Sep

Type of slurry Figure 1. Concentration of odor in odor units (OU m-3) in air sampled above stores of untreated (Untreat), anaerobically digested (AD), and anaerobically digested and separated slurry (AD-sep). Sampling of air was performed before (a) and immediately after (b) an artificial crust on top of the slurry store was broken by a thorough mixing of the slurry stores. Bars indicate maximum and minimum values of observations (two replicates only). Note difference in scales of y-axis.

Table 1. Composition and average concentrations of volatile fatty acids in the slurry types investigated and concentration of malodors phenolic and indolic components in headspace air above them during storage. Units Untreat[a][b] AD Slurry component Dry matter pH Total N NH+4-N 2-Methyl propanoic acid (iso-butanoic acid) Butanoic acid 3-Methyl butanoic acid (iso-valeric acid) Pentanoic acid (valeric acid)

AD−Sep

Type of slurry

Odor, OU m −3air

ODOR DURING STORAGE Odor concentration was found to be highest in headspace air sampled above the untreated slurry type, when the slurry was covered by an artificial crust (fig. 1a). However, when the crust was broken by a thorough mixing of the slurry, odor concentration in headspace air was increased by a factor of 10 to 100 (fig. 1b) and was found to be highest in air sampled above the anaerobically treated slurries, despite the fact that these types had a lower concentration of VFA and a lower emission of phenolic and indolic components (table 1).

a Odor, OU m −3 air

RESULTS

%

AD-sep

g L-1 slurry g L-1 slurry mg L-1 slurry mg L-1 slurry mg L-1 slurry mg L-1 slurry

3.3 (0.14) a 7.2 (0.03) a 4.3 (0.1) a 3.1 (0.1) a 435 (0.9) a 883 (5.0) a 596 (1.2) a 226 (1.2) a

2.8 (0.09) b 8.1 (0.02) b 5.2 (0.2) b 3.7 (0.1) b 91 (0.8) b 22 (0.3) b 115 (0.9) b n.d.[c]

2.2 (0.03) c 8.2 (0.07) c 4.8 (0.1) a 3.6 (0.2) b 91 (0.4) b 36 (0.2) b 106 (0.2) c 15 (0.3) b

% of untreat % of untreat % of untreat % of untreat % of untreat

100 100 100 n.d. 100

42.0 24.6 24.0 n.d. 68.3

18.5 18.4 25.3 n.d. n.d.

Values in parentheses are standard errors of the mean. Values in the same row followed by same letter are not significantly different. Not detected.

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Table 2. Temperature of slurry, soil, and slurry-soil mixture and odor concentration in air sampled over land receiving untreated slurry (Untreat), anaerobically digested (AD), and anaerobically digested and separated slurry (AD-sep).[a] Temperature, °C Odor Concentration, OU m-3 Minutes after Application Slurry Slurry/soil Soil Untreat AD AD-sep 20 260

7.9

10.9 (0.4) a 15.5 (0.1) b

12.2 (0.07) a 15.7 (0.07) b

300 [300−300] 1000 [1000−1000]

250 [200−300] 450 [400−500]

150 [100−200] 150 [100−200]

[a] Values in parentheses are standard deviations, and values in square brackets are minimum and maximum values obtained (only two replicates). Values in the same column followed by the same letter are not significantly different.

methanogenic microorganisms during anaerobic digestion of slurry (Brock and Madigan, 1988). VFA is an important group of odorants, which have often been used for quantification of odor strength in slurry (Barth and Polowski, 1974; Ndegwa et al., 2003). The use of short-chain VFA concentrations for quantification of odors has been questioned by Zhu et al. (1999), who found that the degree of offensiveness of odors from pig manure was better represented by the long-chain and branching VFA (C4–C9) than by the short and straight chains; and by Hobbs et al. (2001), who found a weak correlation between the concentration of a short-chain VFA, acetic acid, and olfactory response. Accordingly, only concentrations of long-chain and branching VFA (malodorous VFA) were taken into account in our study. Concentrations of malodorous VFA were not found to be reduced by slurry separation, which indicates that the malodorous VFA are not related to the solid fraction of the slurry. Similar results were obtained by Zhu et al. (2001) and Ndegwa et al. (2002), who found that although solid–liquid separation caused a small reduction in VFA concentration in the liquid fraction (12%), this technique did not significantly reduce the odor nuisance from stores of separated slurry. Lower concentration of malodorous VFA in the anaerobically digested slurry types was not found to be correlated with lower odor emission during storage (table 1, fig. 1b). This indicates that other groups of odor components have significant influence on the odor emission from slurry. These groups of odorous compounds may include phenols (Powers et al., 1999) and sulfur-containing components such as hydrogen sulfide, produced during storage (Clanton and Schmidt, 2000), which have been identified as major groups of odorous compounds in slurry (Hobbs et al., 1998, 2001). A possible source of these odor components could be the organic industrial waste that was added to the slurry before the anaerobic treatment. Addition of organic industrial waste before anaerobic digestion (co-digestion) is a common procedure for boosting methane production and for recycling organic waste (Wulf et al., 2002; Murto et al., 2004); however, the addition of malodorous substances such as slaughterhouse waste and fish offal, which in Denmark are commonly used organic substrates, may subsequently increase the odor emission from stores of anaerobically treated slurry. However, increased concentrations of odorous phenolic and indolic components were not observed in headspace air above the undisturbed anaerobically digested slurry types (table 1). Lower methane production in slurry stores that have been previously anaerobically digested (Clemens and Ahlgrim, 2001) could be a possible explanation for the reduced odor emission prior to mixing of these slurry types. Methane production produces bubbles that rise to the slurry surface, which may facilitate the transport of odor components in the slurry toward the surface. Emission of odor components from the anaerobically digested slurry may

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therefore be restricted by lower methane production potential. Mixing of slurry stores was found to cause a significant increase in odor emission. Mixing is performed prior to slurry application in order to facilitate pumping and to ensure homogeneity of nutrient concentration of the slurry applied. Mixing temporarily breaks down the surface crusts, and odor components in and below the crust layer are therefore released to the ambient air. The lower odor emission from the untreated slurry type following mixing indicates that this slurry type had a lower concentration of non-VFA odor components before mixing, which may be explained by greater methane-bubble-facilitated transport of odor components. Following slurry application, the concentration of odor in air sampled above land-applied slurries was reduced by 17% by anaerobic digestion and by 50% by combined anaerobic digestion and separation. The same order was observed 260 min after slurry application, but the values were higher (table 2). The higher values observed after 260 min were not expected, but are probably explained by the increase in temperature of the slurry-soil mixture. Reduced odor emission from anaerobically digested slurry has also been observed by Pain et al. (1990a), who found that odor emission during the first 6 h following application was reduced by 70% to 80% following anaerobic digestion. Dry matter concentration was found to be reduced by anaerobic digestion, which is in agreement with results obtained by Pain et al. (1990a). The concentration of dry matter was found to be further decreased by slurry separation, which is in agreement with results reported in the literature (Møller et al., 2000, 2002). Lower concentration of dry matter in treated slurry facilitates the infiltration of the slurry following land application. Slurries pretreated by anaerobic digestion and separation, therefore, have a lower tendency to stay on the soil surface after application and will wet a relatively smaller area of soil surface, leading to reduced odor emission. Lower odor emission from separated slurry has been observed by Pain et al. (1990b), who found that separation of slurry through a 1.25-mm screen prior to an aerobic treatment reduced odor emission following land application by 26%. They similarly attributed the lower odor emission to a more rapid infiltration into the soil of the thinner slurry type.

CONCLUSIONS Slurry concentrations of malodorous VFA were significantly reduced by between 79% and 97% by anaerobic digestion, while emissions of malodorous phenolic and indolic odor components were found to be lower from stores of both anaerobically digested and combined anaerobically and separated slurry. Quantified by dynamic dilution olfactometry, odor concentration in headspace air sampled above


undisturbed stored slurry was found to be reduced by anaerobic digestion, while odor concentration was found to be higher in headspace air sampled above stores of anaerobically digested slurry following mixing of slurry store. Odor concentration in air sampled above land applied slurry was reduced by 17% by anaerobic digestion and by 50% by combined anaerobic digestion and separation. ACKNOWLEDGEMENTS The authors thank Merete Maahn, Inge Marie Gregersen, and Peter Ravn for valuable technical assistance. This study was part of the project ’Documentation of the Environmental Effects of Anaerobic Digestion and Separation of Slurry,’ funded by the Danish Ministry of Food, Agriculture and Fisheries, The Danish Directorate for Food, Fisheries and Agri business, and the National Committee for Pig Production.

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