465 (14) Fellner, J.; Döberl, G.; Laner, D.; Brunner, P.H. (2011) Landfill Simulation Reactors A Method to Predict the behaviour of Field-Scale

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1 465 (14) Fellner, J.; Döberl, G.; Laner, D.; Brunner, P.H. (2011) Landfill Simulation Reactors A Method to Predict the behaviour of Field-Scale Landfills?, In: Proceedings 4th International Workshop Hydro-physico-mechanics of landfills (HPM4), University of Cantabria, April, Santander, Spain, p. 1-7.

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3 LANDFILL SIMULATION REACTORS A METHOD TO PREDICT THE BEHAVIOUR OF FIELD-SCALE LANDFILLS? J. FELLNER 1, G. DÖBERL 2, D. LANER 1 AND P. H. BRUNNER 1 1 Institute for Water Quality, Resource and Waste Management, Vienna University of Technology, Vienna, Austria. johann.fellner@tuwien.ac.at 2 Umweltbundesamt Federal Environment Agency of Austria, Department for Contaminated Sites, Spittelauer Lände 5, A-1090 Vienna, Austria SUMMARY: Investigations into landfill simulation reactors and a full scale landfills are used for predicting emissions from municipal solid waste landfills. In the present study the water flow and solute transport through a landfill simulation reactor and a full scale landfill (using the same waste) has been investigated. The results indicate that, due to preferential flow pathways the water flow in field-scale landfills is less uniform than in laboratory experiments. Based on tracer experiments, it can be discerned that in laboratory-scale experiments around 40% of pore water participates in solute transport, whereas this fraction amounts to less than 0.2% in the investigated full-scale landfill. Main consequences of the difference in water flow and thus solute transport are: (a) emissions from full-scale landfills decrease faster than predicted by laboratory experiments, and (b) the stock of materials remaining in the landfill body, and thus the long-term emission potential, is likely to be underestimated by laboratory landfill simulations. 1. INTRODUCTION Methods for predicting the future behaviour of landfills are based either on the observation of landfills, on laboratory experiments, on mathematical models for describing biochemical, chemical, and physical processes, and on the observation of natural analogs such as peat bogs. All methods show different advantages and disadvantages (Döberl, 2004), and have been used for various purposes and time scales. The methodology probably most often used in the past for predicting emissions from landfills is based on laboratory experiments. They include waste analysis, batch tests (waste mass investigated in the range of some hundred grams, e.g. leaching tests to determine the potentially mobile fraction), and the operation of so called landfill simulation reactors (LSR) with an experimental volume ranging from 0.1 to 10 m³. While batch tests are mainly performed to determine the potentially mobile substance fraction in wastes, LSR are largely used to assess the future emission behaviour of landfills. Using the fact that the rate of water flow accelerates or slows down the rate of biochemical, chemical, and Fourth International Workshop Hydro-Physico-Mechanics of Landfills Santander, Spain; April 2011

4 physical processes within landfills, landfill simulation reactors are usually operated at high water flow rates. According to Beaven and Knox (1999) determining the amount of water (given in liters) passing through a given waste mass (given in kg dry matter of waste), commonly known as liquid to solid (L/S)-ratio, allows relating the LSR results to full-scale-landfills. However, the transformation of LSR results to full scale landfills implies a similar water distribution within landfills and LSR, because the kinetics of metabolic reactions largely depend on the water content (Klink and Ham, 1982) and its distribution within the waste (Bendz, 1998). The present paper addresses the crucial question; whether the basic assumption of a uniform distribution of water flow within landfilled MSW is valid for different scales. 2. MATERIALS & METHODS The study presented is based on investigations of a full scale landfill (see Figure 1, volume ~ 80,000 m³) and two landfill simulation reactors LSR with a volume of less than 0.10 m³ (see Figure 2). The waste built in the LSR was excavated from the full scale landfill, hence alike waste used in both investigations. silt 0,9 m gravel 0,2 m humus 0,1 m gravel 1,3 m humus 0,1 m compost 0,7 m compost 0,7 m gravel 1,3 m recirculation compartment I MSW12 m compartment II MSW 12 m compartment III MSW 14 m silt 1,7 m foil pipes silt 1,7 m foil pipes foil pipe Figure 1: Cross-section of the full scale landfill investigated (Huber et al., 2004) landfill gas Waste V~ 0,15 m³ recirculation Figure 2: Landfill Simulation Reactor (after Döberl et al., 2005)

5 The hydraulic conditions in both systems (landfill and landfill simulation reactor) were investigated by tracer experiments. For the tests at the landfill, 800 g of the dye uranine was applied in an aqueous solution of 100 liters by a single pulse. For the injection of the tracer, the landfill cover was partly removed by digging trenches over a total area of 30 m². Two days before the tracer was applied, 200 liters of water were poured into the trenches to pre-wet the waste. The day after tracer application, again 200 liters of water were applied. After complete infiltration, the trenches were filled again with the material excavated, thus restoring initial hydraulic permeability of the cover layer. The concentration of the dye in the was determined by photometry, whereby prior to analysis, samples were filtrated (0.45 μm) and the ph value was adjusted to 8.5. The detection limit for uranine in the was about 0.05 mg/l. In the landfill simulation reactors, 150 g NaCl dissolved in 500 ml water were applied as a tracer. Since a linear correlation between the concentration of chloride and the electric conductivity of the was observed in experiments, the tracer breakthrough was determined by measuring the electric conductivity of the discharged. In order to minimize interferences with Cl contained in the waste material, tracer tests were performed after the waste in the reactors has been flushed with water. The tracer experiments resulted in a break-through-curve (tracer concentration versus time or water discharged, respectively). This curve was analyzed applying the transfer-function-model as described in Jury and Roth (1990) in order to calculate the median of the tracer travel time and the volume fraction participating in solute transport. For a narrow-pulse input of a tracer through the surface of an experimental volume at time zero, the solute travel time probability density function g(t) can be expressed by the equations given in White et al. (1986). g(t ) ln t e 2 t (1) Where and are fitting parameters and t is the time variable. From these variables, the solute transport volume st and the solute travel time distribution t are calculated (White et al., 1986): st q t L (2) where t exp (3) The solute transport volume st is defined as the volume fraction of the wetted pore space active in the advective transport of solute. 3. RESULTS & DISCUSSION 3.1. Full Scale Landfill At the landfill the tracer (uranine) was detected in the already 30 hours after injection

6 (deposition height ~ 12 m). 54 hours later, the main peak was observed. A second, smaller peak appeared after two months. Three months after the tracer application the uranine concentration in the dropped below the detection limit of 0.05 mg/l. Until this time, 50% of the tracer mass applied has been recovered in the Uranine [mg/l] Tracer application 22 nd of March tracer conc. [mg/l] discharge [mm/d] discharge [mm/d] time [d] Figure 3. Tracer concentration (uranine) and discharge rate during the experiment at the full scale landfill (Fellner et al., 2009) The discharge rate during the tracer experiment a varied between 0.1 and 5 mm/d, whereby the occurrence of tracer peaks corresponds with high discharge rates, indicating the rapid flow of infiltrating rainwater through preferential pathways towards the bottom of the landfill. Applying the transfer function model to the observed tracer breakthrough curve (normalized tracer mass flux converted to a steady state water flow density q of 0.58 mm/d) results in the following values of the fitting parameters: = 0.75 and = 2.4. For a detail description how these values were derived see Fellner et al. (2009). The values of the fitting parameters and denote a median of the solute travel time t of 11 days and a solute transport volume st of m³/m³ at the full scale landfill Landfill Simulation Reactors 500 ml of NaCl solution were sprinkled over the surface of the LSR (1200 cm²) with a flow rate of 25 ml/s. After the tracer has been applied, water was added with the same flow rate for about 1,800 seconds. The breakthrough curves observed during the experiment are given in Figure 4. The main tracer peak was detected after 230 seconds at both reactors. In total between

7 70 and 80 % of the tracer mass applied were recovered in the of the reactors LSR 1 LSR Cl [g/l] ,000 1,200 1,400 1,600 1,800 2,000 Time [s] Figure 4. Tracer concentration (chloride) during the experiments at the landfill simulation reactors When the transfer function model (Jury and Roth, 1990) is applied to the normalized tracer breakthrough curves (which are obtained from the breakthrough curves in Figure 4) the solute transport volume st amounts to m³/m³ and the solute travel time t is about 500 seconds. This means that the pore volume participating in solute transport expressed by st is distinctly higher than in the field experiment with the full scale landfill. Considering the volumetric water content of the waste, which was about 0.35 m³/m³ in the LSR more than one third of the pore water is exchanged by convective transport. For comparison, in experiments with 3.5 m³ of MSW a similar range for the volume fraction participating in solute transport (around 25% of the wetted pore space was active in advective solute transport) was found (Rosqvist and Bendz, 1999). The results at the full scale landfill however, indicate that less than 0.2 % of the pore water (0.40 m³/m³) was taking part in the transport of the tracer. Baumann and Schneider (1998), who investigated the water flow through a full sized landfill (average deposition height 6 m), obtained comparable results. They noticed that 1/8 to 3/8 of the water input reaches the collection system at the landfill bottom with negligible delay. Fellner (2004), who evaluated the observations of Baumann and Schneider, calculated a solute transport volume st of m³/m³, which implies that only around 1% of pore water participates in tracer transport. For a better visualization of the hydraulic conditions in both systems (landfill and LSR), tracer flow for theoretically ideal homogeneous water distribution is illustrated in Figure 5. It is obvious that conditions in LSR are much closer to uniform water flow, but far away from the conditions in landfills.

8 Normalized tracer mass flow (sum) [-] Field scale Landfill Simulations Reactors Ideal homogeneous water flow Increasing heterogeneity Bed-volumes [-] Figure 5 Tracer mass flow (normalized) vs. bed volumes for the experiments at the full scale landfill and the landfill simulations reactors 4. CONCLUSIONS The results of the investigation clearly indicate a significant difference in the heterogeneity of water flow in landfills and landfill simulation reactors. As the presence of water plays a key role in the metabolism of landfills (Straub and Lynch, 1982; Ehrig, 1983), the transformation of results obtained from landfill simulation reactor experiments to full scale landfill is highly questionable. In order to evaluate the significance of LSR results, the consequences of diverse water distribution on the emission behavior of waste deposits need to be investigated in detailed. First results addressing this research question have been presented by Fellner et al. (2009). They indicate that on the one hand emissions from full scale landfills decrease faster than predicted by laboratory experiments, and on the other hand the stock of materials remaining in the landfill body, and thus the long term emission potential, is likely to be underestimated by laboratory landfill simulations. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support of the Austrian Ministry of Agriculture, Forestry, Environment and Water Management.

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