Published online 3 April 2006
Published in J Environ Qual 35:830-836 (2006)
DOI: 10.2134/jeq2005.0239
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
Methane Production during Storage of Anaerobically Digested Municipal Organic Waste
Trine Lund Hansena,
Svend G. Sommerb,
Søren Gabriela and
Thomas H. Christensena,*
a Institute of Environment & Resources, Technical University of Denmark, Building 113, DK-2800 Lyngby, Denmark
b Danish Institute of Agricultural Sciences, Foulum, Blichers Allé, Postbox 50, DK-8830 Tjele, Denmark
* Corresponding author (thc{at}er.dtu.dk)
Received for publication June 14, 2005.
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ABSTRACT
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Anaerobic digestion of source-separated municipal organic waste is considered feasible in Denmark. The limited hydraulic retention in the biogas reactor (typically 15 d) does not allow full degradation of the organic waste. Storage of anaerobically digested municipal organic waste can therefore be a source of methane (CH4) emission that may contribute significantly to the potential global warming impact from the waste treatment system. This study provides a model for quantifying the CH4 production from stored co-digested municipal organic waste and estimates the production under typical Danish climatic conditions, thus quantifying the potential global warming impact from storage of the digested municipal organic waste before its use on agricultural land. Laboratory batch tests on CH4 production as well as temperature measurements in eight full-scale storage tanks provided data for developing a model estimating the CH4 production in storage tanks containing digested municipal organic waste. The temperatures measured in separate storage tanks on farms receiving digested slurry were linearly correlated with air temperature. In storage tanks receiving slurry directly from biogas reactors, significantly higher temperatures were measured due to the high temperatures of the effluent from the reactor. Storage tanks on Danish farms are typically emptied in April and have a constant inflow of digested material. During the warmest months the content of digested material is therefore low, which limits the yearly CH4 production from storage.
Abbreviations: EDOM, enzyme degradable organic matter • Nm3, normal cubic meter (one cubic meter at 0°C and 1 atmosphere) • VS, volatile solids (the loss from oxidation of a dried sample at 550°C, % of dry matter)
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INTRODUCTION
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DURING ANAEROBIC DIGESTION of municipal organic waste in biogas plants, 75 to 85% of the organic matter is typically degraded (Davidsson et al., 2006), leaving 15 to 25% that may be degraded in the storage tank where the digested waste is often kept for months before application to agricultural land. According to Danish regulations, organic waste and manure can be spread on land only twice a year to enhance nutrient uptake and limit loss to surface water and ground water. Since CH4 has a global warming potential 23 times that of CO2, CH4 emissions from storage tanks for anaerobically digested organic waste may considerably affect the global warming impact from the waste management system. These emissions have not previously been investigated and the significance of CH4 emissions from digested organic waste is unknown. Therefore, quantification of CH4 emissions from storage tanks is essential for environmental assessment of treatment of municipal organic waste in biogas plants.
The CH4 production from storage of digested waste is governed by degradability of the material, temperature, and retention time in the storage tank. Whether the produced CH4 is emitted to the atmosphere depends on such factors as the physical features of the storage tank and whether the tank is covered and has gas collection. The Intergovernmental Panel on Climate Change (IPCC) has not developed guidelines for estimating methane emissions from storage of digested organic waste. However, according to the IPCC Good Practice Guidelines (Intergovernmental Panel on Climate Change, 2001), reliable estimates of CH4 emissions from storage of manure should consider the amount of volatile solids (VS), time of storage, CH4 potential of the material (VS basis), and a CH4 conversion factor depending on the manure management system and climate region. Hence, these factors should also be included when determining the potential CH4 emissions from storage of anaerobically digested municipal organic waste.
Methane emissions from storage of anaerobically digested municipal organic waste could not be measured directly, since co-digestion with either manure or sewage sludge is the most common anaerobic treatment technology used for municipal organic waste in Denmark. Accordingly, investigations of emissions from storage tanks have typically concentrated on digested manure and not on municipal organic waste.
The objective of this study was to model possible CH4 production in storage tanks. This modeling was based on measurements of temperature and filling degree in full-scale storage tanks as well as the results of laboratory batch tests, where the methane production of digested municipal organic waste was determined at seven different temperatures (5–55°C).
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MATERIALS AND METHODS
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Methane Production Tests
The CH4 production of anaerobically digested municipal organic waste was determined by batch tests on the effluent from pilot-scale biogas reactors treating only source-sorted municipal organic waste collected in the city of Aalborg. The stirred pilot-scale reactors of 35 L were thermophilic (55°C) and continuously fed with waste diluted to 5% dry matter. The hydraulic retention time was 15 d, which is a normal hydraulic retention time for Danish biogas plants. The reactors were run for 2 to 3 mo and the effluent for the batch tests was sampled during periods with stable gas production. The pilot-scale biogas experiments are further described in Davidsson et al. (2006). Table 1 shows the chemical composition of the digested waste with respect to dry matter, VS, ash, fat, fibers, nitrogen, phosphorus, potassium, carbon, hydrogen, sulfur, chloride, and calorific value. Furthermore, the enzyme degradable organic matter (EDOM) was determined by treating the waste samples with acid and heat followed by washing and treatment with three types of enzyme to determine the enzyme degradable fraction. The analytical procedures are described in detail in Hansen et al. (2006b).
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Table 1. Composition of effluent from anaerobic pilot-scale digestion of source-sorted municipal organic waste (collected and pre-treated in the Danish city of Aalborg) used in the methane production batch tests.
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The batch tests for determining the CH4 production of the anaerobically digested waste were performed in triplicate in 2-L glass bottles according to the procedure described in Hansen et al. (2004). The tests covered a broad spectrum of seven different temperatures representing possible conditions in storage tanks in Denmark: 5, 10, 15, 22, 28, 35, and 55°C. No inoculum was used, since the tests should determine the actual CH4 production in a storage tank for anaerobically digested organic waste. For each temperature, parallel tests were performed where cellulose was added to the effluent samples (triplicates) to test whether slow or missing CH4 production was caused by lack of degradable organic matter or microbial inactivity due to low temperature. The batch tests lasted 125 d.
The results from the batch tests were given as the accumulated CH4 production over time (Nm3 CH4/g VS, where Nm3 is one cubic meter at 0°C and 1 atmosphere) per gram of VS in the digested material.
Temperature and Filling Degree of Storage Tanks
To estimate the CH4 production from stored anaerobically digested municipal organic waste, the temperature and amount of stored waste must be known. At Danish biogas plants municipal organic waste usually constitutes less than 5% of the material digested. Manure, sewage sludge, and other organic wastes constitute the main part. Therefore, it was assumed that there were no differences in temperature and filling degree between storage tanks containing only anaerobically digested manure and those containing anaerobically digested manure mixed with municipal organic waste. The temperature and depth of digested manure was measured with 3-wk intervals throughout 1 yr in eight full-scale storage tanks containing anaerobically digested manure.
Three of the eight tanks were separate tanks, tank volumes of 2800 to 4000 m3, located on farms receiving anaerobically digested manure by truck from the biogas plants (Blåskærvej, Lintrup, and Kolstrupvej). The tanks all had a floating surface cover of straw, but no gas collection.
Five of the eight storage tanks were located at biogas plants and directly connected to the main reactor (Linkogas, Filskov, and Århus 1–3). These tanks were all covered and the gas was collected together with gas from the biogas reactor. The volumes of the tanks were 1000 to 3000 m3.
The temperature was measured with thermocouple sensors (Type T, data collection unit Type 1033-130K; Eltek, Haslingfield, UK) mounted on an 8-m pole at 0.1, 1.1, 2.1, 3.1, and 4.1 m from the tip inserted in the storage tank. Calibration of the sensors showed variations of 0.5%. The vertical, horizontal, and 45° temperature profiles were determined; in tanks with low slurry content the horizontal and 45° angle temperature profiles could not be determined. The depth of the slurry was also measured to determine the volume of digested material in the tanks at each measurement.
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RESULTS AND DISCUSSION
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Methane Production Tests
Methane production from the digested waste was slow in batch tests at temperatures lower than 28°C. At 5°C, the CH4 production was almost undetectable, both for the digested waste and the cellulose sample (Fig. 1), indicating that the microorganisms in the reactors were not capable of degradation at this low temperature. At 10, 15, and 22°C, the CH4 production was low and the accumulated CH4 production over time was linear (see Fig. 1). At 10 and 15°C, the CH4 production from the digested waste and the cellulose was of the same order of magnitude, whereas at 22°C the cellulose produced 10 times as much CH4 per kg VS as did the digested waste. This indicates that low degradability of the organic matter in the digested waste limited the degradation process at 22°C. At 28 and 35°C, the CH4 production was substantial in the early phases, but decreased after about 70 and 50 d, respectively, indicating depletion of all easily degradable matter in the digested material. Furthermore, within 50 d at 35°C, the accumulated CH4 production from the cellulose samples reached the theoretical maximal CH4 potential determined by the Buswell formula (Symons and Buswell, 1933). At 55°C, the CH4 production rate decreased after 10 d, but continued for another 60 d. The cellulose samples reached the theoretical CH4 potential within 15 d at this temperature. The microorganisms present in the effluent were already adapted to 55°C and therefore fast and effectively degraded easily degradable matter at this temperature. The degradation of slowly degradable organic matter (in this case biogas plant effluent) was, as expected, slower.
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Fig. 1. Accumulated methane production over time (days) for anaerobically digested organic municipal waste and cellulose. The methane production was measured in batch tests (triplicates) at seven temperatures: 5, 10, 15, 22, 28, 35, and 55°C. Nm3, cubic meter at 0°C and 1 atmosphere pressure; VS, volatile solids in the organic waste.
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The average CH4 production for the effluent material obtained in the batch tests at 55°C was 225 Nm3 CH4/Mg VS (see Fig. 1). In Hansen et al. (2006a) the measured average CH4 potential for municipal organic waste at 55°C was found to be 460 Nm3 CH4/Mg VS with the average EDOM value being about 90% of VS (equaling 510 Nm3 CH4/Mg degradable VS). Using the EDOM value in Table 1 as a measure of degradability, the maximal obtainable CH4 production for the digested waste is 285 Nm3 CH4/Mg VS. Thus, around 80% of the degradable VS in the digested waste, as determined by the VS and EDOM of the material, was degraded in the batch tests at 55°C.
The Methane Production Model
The results from the batch tests showed an approximately linear CH4 production rate over time in the beginning of the tests for the digested waste samples. As the easily degradable organic matter is degraded, the CH4 production decreases until finally no degradable VS is present. The CH4 production rate (Nm3 CH4/Mg VS h) was determined from the linear part of the CH4 production curves for the investigated temperatures (5–55°C) and is, therefore, valid only within the period where substrate for CH4 production is abundant. For the temperature range 5 to 35°C an exponential correlation between temperature and CH4 production rate was found (see Fig. 2 and Eq. [1]). The rate for 55°C was not included, since this temperature is normally not relevant for storage tanks under Danish conditions:
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where ECH4 is the production rate for methane (Nm3 CH4/Mg VS h) and t is the temperature of the digested waste within the interval 5 to 35°C.
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Fig. 2. Estimated correlation between temperature and methane production rate for anaerobically digested organic municipal waste based on laboratory batch tests. Nm3, cubic meter at 0°C and 1 atmosphere pressure; VS, volatile solids in the organic waste.
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The results of the batch tests showed a clear linear relation between the log-transformed estimated CH4 production and the inverse temperature in Kelvin (R2 = 0.98) as was also proven for measured CH4 emissions by Husted (1994), Sommer et al. (2000), and Khan et al. (1997).
At the lowest temperatures (5 and 10°C) the production of CH4 within the first 24 h was relatively high compared to the overall production. When the effluent entered the batch reactors it still had the temperature of the pilot-scale reactors (55°C) and, therefore, a comparably higher microbial activity. This activity decreased as the material cooled. However, this initial production was included in the linear determination of the CH4 production rate.
Figure 3 shows the calculated cumulative CH4 production from storage of 1 Mg of VS over 1 yr at different temperatures, assuming constant CH4 production as determined by Eq. [1]. The horizontal line represents the obtainable CH4 potential (VS basis) assuming a maximal CH4 potential of 510 Nm3 CH4/Mg degradable VS (Hansen et al., 2006a) and a maximal degradability of 56% of the VS (EDOM value in Table 1). With storage of digested waste over 1 yr at temperatures at 22°C or lower, the degradable organic matter is unlikely to be the limiting factor for the CH4 production. At these temperatures, the CH4 production can, therefore, be assumed to be linear over time, which is a basic assumption for the model estimates. At 28°C the degradable organic matter will be depleted within approximately 7 mo and at 35°C within 4 mo. With storage at these temperatures the linear CH4 production cannot be assumed to be valid beyond these time limits.
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Fig. 3. Linear estimation of methane production from stored anaerobically digested organic municipal waste (1 Mg VS) over time. The horizontal curve represents the maximal obtainable methane production from the waste. Nm3, cubic meter at 0°C and 1 atmosphere pressure; VS, volatile solids in the organic waste.
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When using the batch test results for estimation of CH4 production in full-scale storage tanks, it should be considered whether the microorganisms in the full-scale tanks, which are well adapted to the low temperatures, would be more productive than those in the batch tests adapted to 55°C. It was, however, assumed that the length of the tests (125 d) allowed adaptation of the microorganisms to the lower temperature and this is, therefore, not considered a crucial uncertainty.
Temperature and Filling Degree
The horizontal, vertical, and 45° temperature profiles in the storage tanks showed little variation in space and no clear patterns. Wind and temperature gradients in the stored slurry may enhance natural convection, thereby contributing to homogeneous temperatures in the storage tanks (Olesen and Sommer, 1993). Therefore, the average temperature from each occasion of measuring temperature in the stored slurry was used (Fig. 4) and related to the average monthly Danish air temperature based on 30 yr of monitoring (www.dmi.dk, verified 31 Jan. 2006). Figure 4a shows that the temperature of the separate storage tanks for digested manure was generally slightly higher than the air temperature. This indicates that the temperature in farm tanks may be slightly elevated due to microbial activity in the digested material, but that the temperature is mainly affected by average air temperature. A linear regression of the temperature of the material in the tanks as a function of the average monthly Danish air temperature (Fig. 5) showed the following correlation:
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where tsl is the slurry temperature in the tank (°C) and tair is the air temperature (°C, monthly Danish average, www.dmi.dk).
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Fig. 4. Temperature in storage tanks measured over 1 yr in Denmark. Figure 4a shows measured temperatures in storage tanks for anaerobically digested manure in separate storage tanks, while Fig. 4b shows measured temperatures in storage tanks for anaerobically digested manure directly connected to the biogas reactor.
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Fig. 5. Correlation between temperature measured in separate storage tanks for anaerobically digested manure and the Danish average monthly temperature.
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The measured temperatures in storage tanks directly connected to biogas reactors are shown in Fig. 4b. These were significantly higher than the average monthly temperature and mainly depended on the temperature of the effluent from the reactor. The temperature variations in these tanks were larger than in the separate storage tanks due to intermittent inflow of heated effluent from the biogas reactors.
The digested material is applied to agricultural fields in April and the tanks are then emptied. From April the storage tanks are gradually filled until the following spring. This pattern reflects the Danish regulations enforcing farmers to apply slurry in spring at the beginning of the growth season. The addition of slurry to the storage tanks generally varies only slightly, since the biogas plant treats the same amount of manure throughout the year.
Estimation of Methane Production in Storage Tanks
The CH4 production from storage tanks for anaerobically digested organic waste was modeled from Eq. [1] and assumptions based on the field measurements: (i) the temperature of the digested material in the separate storage tanks follows the average air temperature with respect to Eq. [2], (ii) the tanks are emptied in April, and (iii) the tanks have a constant monthly inflow (see Table 2). Over 1 yr the CH4 production from the digested material was estimated to be 0.08 Nm3 CH4/average Mg digested waste. Assuming 1.3% dry matter and 60.3% VS (see Table 1), this CH4 production equals 10 Nm3 CH4/Mg VS in the digested waste or 2 Nm3 CH4/Mg VS in the waste entering the biogas reactor [assuming 80% degradation as measured in the pilot-scale reactors in Davidsson et al. (2006)]. This equals 0.4% of the CH4 potential of the waste going into the biogas reactor (460 Nm3 CH4/Mg VS). The estimated CH4 production represents the maximal potential (worst case) emissions, assuming no cover of the storage tank and no oxidation of CH4 in the tank. In Denmark, slurry in open tanks must be covered by a layer of straw, floating granules of leca pebbles, or similar to reduce ammonia emission. It has been shown that these porous covers may significantly reduce CH4 emissions (Petersen et al., 2005; Sommer et al., 2000).
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Table 2. Estimated methane production in a 1200-m3 storage tank for anaerobically digested organic waste. The tank is emptied in April, while the inflow of digested material is distributed over the year.
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Emptying the tanks in spring is an important factor for the magnitude of the CH4 production. Figure 6 shows the lowest filling degree of the storage tanks during the warmest months, where the potential CH4 production rate is the highest. Application of the digested material in the autumn (September) would increase the estimated yearly CH4 production for the average digested material by up to 70% to 0.13 Nm3 CH4/Mg of digested waste. These data are shown in Table 3.
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Table 3. Estimated methane production in a 1200-m3 storage tank for anaerobically digested organic waste. The tank is emptied in September, while the inflow of digested material is distributed over the year.
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The importance of the relatively low Danish monthly average temperature is shown in Table 4. The CH4 production for a storage tank similar to that in Tables 2 and 3 is estimated assuming the highest monthly average air temperature measured in Denmark since 1874 (www.dmi.dk). The temperature difference is approximately 4°C compared to the average temperatures in Table 2 resulting in a 60% increase in the yearly CH4 production. Assuming elevated temperatures in January, February, and December only (average temperatures for the rest of the year) increased the CH4 production by around 10% compared to production at average temperature.
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Table 4. Estimated methane production in a 1200-m3 storage tank for anaerobically digested organic waste assuming a monthly average temperature corresponding to the highest measured monthly averages in Denmark since 1874 (www.dmi.dk). The tank is emptied in April, while the inflow of digested material is distributed over the year.
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The CH4 produced from the digested organic waste could be collected and could contribute to the gas production of the main reactor. This type of gas collection is already in place at many biogas plants, especially for storage tanks directly connected to the biogas reactor. The amount of gas collected in storage tanks may be a significant part of the gas production at the plant. Typically, the retention time in these tanks is 1 to 2 mo. Assuming 30°C in the storage tank, Eq. [1] estimates a CH4 production of 68 Nm3 CH4/Mg VS in the digested waste or 13.6 Nm3 CH4/Mg VS in the waste entering the biogas reactor. This equals approximately 3% of the CH4 potential of the waste delivered to the biogas plant (460 Nm3 CH4/Mg VS).
If the degradation in the biogas reactor is less than the assumed 80%, due to decreased retention time, overload, or operational problems, CH4 production in the storage tanks may be significantly higher than estimated above.
Environmental Assessment
Environmental assessment of biogas treatment of municipal organic waste, including electricity produced from the gas and utilization of the effluent on agricultural fields, will often show savings in the impact category "global warming" due to substitution of energy based on fossil fuels and substitution of commercial fertilizers. Kirkeby et al. (2006) evaluated the environmental impacts from different treatment methods of the organic waste from the municipality of Århus (Denmark). Based on their data and assumptions, the savings in global warming from biogas treatment of 1 Mg of municipal organic waste could be estimated to be 261 kg CO2 equivalents. They did not include the CH4 emissions from storage of the digested waste. Estimated according to the proposed model, these emissions would potentially constitute around 8 kg CO2 equivalents/Mg waste (based on the production of 10 Nm3 CH4/Mg VS in the digested waste, 80% degradation in the biogas reactor as well as 80% VS, and 30% dry matter content in the waste entering the biogas reactor). If emitted to the atmosphere, the potential savings in global warming, resulting from the biogas treatment, will be reduced by 3%.
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CONCLUSIONS
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This paper presents a model for estimating the CH4 production from stored digested municipal organic waste. The model takes into account typical patterns of managing the storage tanks and the average temperatures during the months where the digested waste is actually stored.
For a storage tank under typical Danish conditions, the model estimated the CH4 production to be 0.4% of the CH4 potential of the waste delivered to the biogas plant. Whether the produced CH4 is emitted to the atmosphere depends on the physical features of the storage tank (e.g., cover and gas collection).
For storage tanks directly connected to biogas reactors the temperature was higher and the estimated potential CH4 production corresponded to 3% of the CH4 potential of the organic waste treated in the biogas plant. The gas produced in these tanks is typically collected and, therefore, contributes to the overall gas production of the biogas plant. Storage in these tanks before transfer to the separate storage tanks may, therefore, decrease the content of degradable VS in the digested waste and thereby limit the methane production and potential emissions from later storage in the separate tanks on the farms.
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REFERENCES
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- Hansen, T.L., H. Spliid, J.C. Jansen, Å. Davidsson, and T.H. Christensen. 2006b. Composition of source-sorted municipal organic waste collected in Danish cities. Waste Manage. (in press).
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- Olesen, J.E., and S.G. Sommer. 1993. Modelling effects of wind speed and surface cover on ammonia volatilization from stored pig slurry. Atmos. Environ. 27A:2567–2574.
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ABSTRACT
INTRODUCTION
