Assessment of a New Waste Biodegradation Model for MODUELO

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Assessment of a new waste biodegradation model for MODUELO

A. Lobo, J. Muñoz & I. Tejero Environmental Engineering Group, University of Cantabria, Spain

Abstract

Several research groups are working on the development of dynamic simulation tools for the design, management and monitoring of municipal solid waste landfills. One of these tools is the MODUELO software, which the Environmental Engineering Group of the University of Cantabria has been developing since 1998. Based on meteorological data, waste generation and landfill layout data, the program estimates the leachate flows and the gas generated in time. The three dimensional model of the landfill is composed of layers formed by square horizontal section cells. The hydrological and biodegradation phenomena are simulated by means of simplified models that have been specifically created or adapted from existing ones. In the recently developed second version, MODUELO 2, both the hydrologic and the waste biodegradation modules have been improved. This paper presents the developed new waste biodegradation model and a first assessment of its simulation results. The changes from the original model affect both the waste characterization parameters and the definition of the biodegradation processes. Waste biodegradability is defined by means of three factors: fbio (biodegradable fraction of the organic matter), fdr (non biodegradable fraction that can be dragged along in the hydrolysis) and fah (accessibility to the waste for the microorganisms). The waste biodegradation process has been divided into three stages, hydrolysis, acetogenesis and gasification, which give place to seven kinetics modelled through first order laws. This new model has been initially checked by simulating the biodegradation process of municipal solid waste in a theoretical closed cell. The simulated emissions show a good agreement with the general tendencies observed by other authors in several facilities, which constitutes a first step for the general verification of the modified version of the program. Keywords: landfill, biodegradation, organic pollution, leachate, modelling, municipal solid waste, dynamic simulation.

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1 Introduction

Different types of protection systems are installed in sanitary landfills to prevent impacts and, in some cases, harness the gases generated during the waste degradation process: leachate collection systems, gas collection nets, etc. The design of these and other measures has traditionally been based on previous experiences, mathematical formulae developed from very simplified landfill models, and, less commonly, on “in situ” tests. Similar to other fields of environmental engineering, modelling can constitute a valuable complement to these techniques. Simulation programs can be used to optimize the design, facilitating the analysis of alternatives and the monitoring of the projected measures. However, modelization has not yet become as widespread in municipal waste landfills as it is in other fields such as wastewater treatment. Although several programs and models simulating particular aspects of the landfill have been published, special difficulties appear in this case, such as the heterogeneity of the studied medium and the amount of local factors that condition the landfill’s response to different actions. These are the reasons why a program of generalized use in different landfill facilities which integrate the several phenomena to consider has not been established to date. The development of “integrated” landfill simulation programs started at the end of the last century. Since then several research groups have been working on this subject, such as the waste management groups of the Universities of Southampton (Great Britain), Braunschweig (Germany), Central Florida (United States) and Cantabria (Spain). These researchers develop models to simulate the waste mass hydrologic, degradation and settlement processes from different approaches. 2 MODUELO

2.1 MODUELO 1 Among these “integrated” models is MODUELO, which the Environmental Engineering Group (GIA) of the University of Cantabria has been developing since 1998. MODUELO is a tool designed to help in the environmental evaluation of landfills, quantifying the pollution and settlement caused by its biodegradation. The first version, MODUELO 1, simulates rain infiltration, water movement through the waste, its decomposition by microorganisms, and the transport of organic material dissolved in liquid, based on meteorological, waste production and fill configuration data. A 3-dimensional model of the landfill is created, which is formed of layers composed of square cells. Each cell is filled with waste or its corresponding material during the simulation, reproducing the landfill’s history. As a result, the daily leachate flow, its organic pollution and the production and composition of the landfill gas is obtained. The program incorporates an independent waste management simulation module, which allows the study of the landfill’s life cycle under different

Waste Management and the Environment II 421 hypotheses, such as policies of urban waste management (for example, the effects of the reduction of organic material dumped), as well as the form of filling the available surface. The models implemented initially in the program, presented in detail in Lobo et al. [1, 2] were selected after an analysis of work published on mathematical modeling of each topic (hydrological surface balance, groundwater flow, anaerobic decomposition processes) in order to generate, using this information, an overall tool of direct use. Some expressions were directly incorporated, others were simplified and still others were created according to modeling needs. For this, an object-oriented programming language (C++) was employed with which an interface was created allowing the user to model the morphology of the landfill, input data characterizing it (refuse production, topography, meteorology), analyze the results numerically and graphically, as well as export these results to data bases for later use and consultation. The three main sections of the program —hydrology, biodegradation and settlement— have been improved since the original version.

2.2 The second version

MODUELO 2 is an improved version of the program that keeps the main hypothesis of the hydrological model: the existence of saturated flow within the waste layers that are covered by a significant thickness of low permeability material. Its hydrological module incorporates some changes in the mathematical algorithms and two new phenomena: the variation of hydraulic conductivity of the waste as it is buried and the influence of surface runoff in the leachate generation. This new hydrological model was tested and compared to its previous version and HELP (Schroeder et al. [3]) in Lobo et al. [4]. The results presented in that paper showed that MODUELO 2 adjusts the field data available in the Meruelo I landfill (located in Cantabria, Spain) more adequately than its previous version and HELP. The long-term tendencies resulting from the new model also seemed to be more appropriate, leading to the conclusion that MODUELO 2 constituted the best base for the biodegradation simulation. With respect to the biodegradation module, during the applications of MODUELO 1, some difficulties were detected to adequately simulate the nitrogen pollution and the changes in the landfill gas composition. Trying to solve these difficulties a new biodegradation model has been created. In this paper the proposed new biodegradation model and its simulation results in a synthetic case, as a first assessment, are presented. 3 The new biodegradation model

3.1 Introduction

The recently developed biodegradation model incorporates changes in the waste biodegradability mathematical characterization and in the processes definition.

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The modelization of the waste biodegradability in the previous model included two factors to relate the organic matter with its “hydrolizable” and “biodegradable” fractions. As the hydrolysis is a biodegradation process, these two factors have been substituted by one, that distinguishes the “biodegradable” and the “non biodegradable” fraction. The possible appearance of non biodegradable matter dissolved in the leachate has been modelled by means of another parameter, described in the next section. The biological break down of the residue was simplified in MODUELO 1 in two stages: “hydrolysis” and “gasification”. According to this model the dissolved nitrogen remains as organic nitrogen until its degradation in the methanogenic stage. This leads to simulated concentrations of ammonia nitrogen lower than those observed. The ammonification process actually takes place much before methane generation. The stage that delays the appearance of ammonia in the leachate is the hydrolysis, which has been assumed in the new model. On the other hand, as in the original model all the sequential gas generation processes were grouped in one unique step (“gasification”), the simulated gas composition was only sensitive to the changes in the biodegradable waste “formula”. Neither temporal variations nor mass loss as gas previous to the methanization could be simulated, and this led to greater methane generation than that observed. The new model tries to solve these problems by introducing gas generation (CO2 and H2) in the previous stages (hydrolysis and acetogenesis).

3.2 Modelling waste biodegradability

According to its first version, the biodegradation model only considers the organic matter of the waste. It is divided into two fractions: the biodegradable and the non biodegradable fraction. A part of the former is readily hydrolyzable (MSrb) and another slowly hydrolyzable (MSsb). Both are characterized by their chemical formula (CcrbHhrbOorbNnrbSsrb y CcsbHhsbOosbNnsbSssb). The biodegradable fraction of the waste can be estimated through the biochemical methane potential (BMP) experimentally observed /theoretical BMP ratios corresponding to each component (paper, cardboard, food waste, yard waste, etc.) reaching the landfill. These ratios, defined for each waste component, have been called “fbio” in MODUELO. The potential appearance of non biodegradable organic compounds in the leachate (non biodegradable COD) is modelled by means of the “dragging” factor, “fdr”, that gives the amount of this kind of substances that are dissolved as the ready and slow hydrolysis are produced. Finally a parameter describing the accessibility of the matter to the microorganisms, “fah”, is maintained. “fah” establishes which part of the total biodegradable matter can actually be hydrolyzed in the specific conditions of the landfill studied. It has to be considered as a site specific calibration parameter that can vary from 0 (microorganisms can not access any fraction of the waste

Waste Management and the Environment II 423 and the landfill would remain “mummified”) to 1 (the ideal situation in which all the degradable matter can be broken down).

3.3 Biodegradation processes

The new model is based on the substrate balance reported for similar-to- digesting-sludge anaerobic ecosystems by Zhender et al. [5]. The two stages initially stated have been redefined and a new intermediate reaction has been introduced. Table 1 presents the seven chemical reactions and kinetics of these three stages, which are described in the following paragraphs.

Hy droly si s. The hydrolysis groups all of the phenomena (by biological activity or chemical or physical “dragging”) that give way to dissolving the particulated material that initially makes up the solid waste. Depending on the kind of matter affected by the process, two reactions, ready and slow hydrolysis, are distinguished. Both directly generate “intermediate compounds”, acetic acid, carbon dioxide and hydrogen, as well as ammonium and hydrogen sulphide coming from the breaking down of the organic nitrogen and sulphur. Most of the “intermediate compounds” are carboxylic acids generated from the carbohydrates, but their “global chemical formula” will depend on the ratio of carbohydrates/proteins/lipids of the waste and the predominant metabolic paths. On the other hand the non biodegradable matter dragged does not give way to specific subproducts (reactions 3 and 4 in table 1), as proposed for every compound in MODUELO 1.

Acetogenesis. It accounts for the breaking down of the intermediate compounds into acetate, carbon dioxide and hydrogen.

Gasification. It is the final decomposition process that converts organic matter into gas. Two independent reactions are proposed to represent the two biological pathways of methane generation: acetate break down (by the acetophilic methanogens) and CO2 reduction with H2 (hydrogenophilic methanogens). As these reactions simplify the real metabolic pathways, their stequiometric factors can vary from one landfill to another. For this reason they have been introduced as model parameters whose values can be changed by the user. If no specific data are available the values obtained from the diagram of Zhender et al. [5] are proposed: [fAC = 0.2; fCHO = 0.76; f’AC = 0.68]. The seven reactions representing the three decomposition stages are simulated as first order kinetics with respect to the substrate. The “activation time” parameters (tact), described in Lobo et al. [2], have also been included. These parameters account for a possible initial lag in the hydrolysis (“tact,rea” and “tact,slo”) and methanization (“tact, met”).

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2004 WIT Press, www.witpress.com, ISBN1-85312-738-8 Table 1: Waste biodegradation reactions as defined in MODUELO 2. a s t e

N REACTION RATE M a n

1 ... [oHeCcfSsNnOoHhCc .22( ].). Ocff a rbrbrbrbrb → CHOrbCHO + rb CHO rb CHO ++−++ CHOrbAC g e kh . SM m rbAC .. 3COOHCHcf 1( AC −−+ rbCHO .). COcff 2 + rea rb e n t

. +++ SHsNHnHa a

rb 2 rb 3 rb 2 n d

2 t sbsbsbsbsb → CHOsbCHO + sb... CHO [oHeCcfSsNnOoHhCc sb .22( CHO ++−++ ].). Ocff CHOsbAC h e

sbAC .. 3COOHCHcf 1( AC −−+ sbCHO .). COcff 2 + khslo. SMsb n v i r

. +++ SHsNHnHa o

sb 2 sb 3 sb 2 n m

3 rnbrnbrnbrnbrnb → SsNnOoHhCcSsNnOoHhCc nbnbnbnbnb dissolved)( fdr.khrea. SMrb e n t

4 I snbsnbsnbsnbsnb → SsNnOoHhCcSsNnOoHhCc nbnbnbnbnb dissolved)( fdr.khslo. SMsb I 5 f ' OHC []−+ ).'2( − . OHOCf → AC .. COOHCHC −+ .).'1( COCf + CHO CHO CHO AC CHO CHO 2 2 CHO 3 AC CHO 2 k .CHO − ).'1.(4 −+ .2 OHCf A + AC CHO CHO CHO .H 2 2 6 COOHCH → + COCH 3 24 kAC.AC 7 4 →+ + 2 OHCHHCO 22 24 kH2.H2

Note: b = ..2 cfe bCHO if > + + + .2.3)..(2 sncffh bbbCHOACb ; = bb − − − .2.3..2 sncfhe bbbAC if ≤ + + + .2.3)..(2 sncffh bbbCHOACb .

b .(2 AC +− −− .2.3). sncffh bbbCHO a = if > + + + .2.3)..(2 sncffh bbbCHOACb ; ab = 0 if ≤ + + + .2.3)..(2 sncffh bbbCHOACb b 2

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The model is solved for each simulation day in every cell making up the landfill. It is assumed that the reactions in each cell do not influence the adjacent cells. The interchange of dissolved substances between cells and towards the leachate collection system is simulated in the hydrologic module, supposing every substance is homogeneously distributed in the water of each cell. The gases movement model has not been incorporated yet. The gas release calculation is simplified through an interfacial balance model that assumes an atmospheric constant pressure within the waste’s pores.

100000 1200 SMrb 90000 1100 SMsb 1000 80000 CCHO AC 900 70000 NH3 800

60000 700 50000 600

40000 500

Solid mass (kg) (kg) mass Solid 400 30000 300 20000 200 Dissolved (mol/1000) substances 10000 100 0 0 1 501 1001 1501 2001 2501 3001 3501 Time (d) Figure 1: Quantities of substances dissolved in the water of the closed cell throughout time resulting from the simulation with MODUELO 2 [SMrb: rapidly biodegradable solid matter; SMsb: slowly biodegradable solid matter; CCHO: intermediate compounds carbon].

4 Simulation of the closed cell

As a first verification of the new model the degradation of a municipal waste in an ideal closed cell (without entries or outlets of water or residues) with a surface area of 14x14 m2 and 1.9 m height during 30 years was simulated. The composition (Lobo et al. [1]), moisture content (48%) and specific weight (0.9 ton/m3) of the waste were chosen similar to that of the waste buried in the Meruelo I landfill (Cantabria, Spain). The calibration criteria were based on the following observations made in different landfill facilities or laboratory tests by a number of authors compiled in Lobo [6]:

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.- Landfill gas and leachate pollution – time curves published by Tchobanoglous et al. [7], obtained from reports of several authors. .- Average landfill gas production in the first few years between 10 and 50 L gas/kgSM/year and total gas generation between 100 and 190 Lgas/kgSM. .- Degradation rates for the different stages in the range of those reported. After 30 years of emissions, they should have significantly dropped, as is habitual in real landfills. .- The CH4 generated via hydrogen after 30 years should be approximately 30% of the total CH4, according to the proportions in which matter is distributed as reported by Zhender et al. [5]. The simulation results are presented for the first four thousand days of degradation in figures 1 and 2. The calibration values of the parameters were: -1 -1 -1 -1 0.01 d for krb, 0.0005 d for ksb and kA, 0.001 d for kAC, 50 d for kH2, 0 d for tact,rea and 365 d for ttact,slo. In figure 1 a rapid appearance of the intermediate compounds (represented by CCHO) as a consequence of the hydrolysis is shown. From these substances acetate is generated. It reaches a maximum accumulation from which the methanogenesis rate becomes greater than the acetogenesis rate, making the acetate disappear. The amount of ammonia nitrogen present in the water is always increasing. The slower the hydrolysis becomes the slower the amount of ammonia nitrogen increases, as it does not suffer any later transformation.

0,6 releasedCO2 80% releasedH2 releasedCH4 0,5 %CO2 70% %H2

%CH4 ) 60% 0,4 50%

mol/1000 0,3 ( 40% as mixture g as g

30% e in 0,2 g

20% Released 0,1 Percenta 10%

0,0 0% 1 501 1001 1501 2001 2501 3001 3501 Time (d) Figure 2: Gas emissions throughout time resulting from the simulation of the closed cell with MODUELO 2.

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Analyzing the results, it has to be considered that the amounts of pollutants accumulated in the water are greater than those that would appear in a real case, in which the dissolved substances would be dragged by the water circulation. The peak of CO2 generation that can be observed in figure 2 coincides with the maximum release of organic compounds in the leachate. In the graphs compiled in Tchobanoglous et al. [7] this peak agrees in time with the COD peak concentration, but in this case the accumulation effect of the pollutants has to be considered, as it can alter the results. The methane, carbon dioxide and hydrogen percentages throughout the time curves in figure 2 can be compared to the reference curves (Tchobanoglous et al. [7]). The three of them develop as expected. CO2 reaches a peak around 85%, later descending to values near 50%, where it remains until the end of degradation. CH4 gradually increases until around 50%, a percentage that stays in time as CO2. The H2 appears in proportions under 20% (in this case it reaches 10% as a maximum), according to the values reported in the literature (Young [8]). The initial “adjustment-to-anaerobic conditions” phase in which oxygen is consumed generating carbon dioxide is not included in MODUELO. The curves shown correspond to the acid and methane fermentation phases, which can be delayed in the model, when the initial aerobic periods become significant, by means of the “activation time” parameters. The maximum gas generation occurs during the first few years and are within the mentioned ranges: 22 Lgas/kgSM, of which 7 L are CH4, are emitted during the first year and 20 Lgas/kgSM, of which 10 L are CH4 during the second one, for example. The release of landfill gas after thirty years has been 170 L/kgSM, of which 84 L are CH4 (49%), of which 69% is produced by methanogenesis from the acetate, the rest being transformed H2.

5 Conclusions

In this paper an improved waste biodegradation model for the landfill dynamic simulation program MODUELO is presented. The new model characterizes the biodegradability of the readily and slowly biodegradable waste by means of three main parameters, fbio (biodegradable fraction of the organic matter), fdr (non biodegradable fraction that can be dragged in the hydrolysis) and fah (accessibility to the waste for the microorganisms). Three biodegradation processes have been considered—hydrolysis, acetogenesis and methanogenesis—which give place to seven kinetics that are simulated as first order reactions with respect to the main substrate. The calibration and simulation results of an ideal closed cell of waste demonstrate that the new model solves the difficulties detected in the previous model. The appearance of the carbonaceous organic matter as well as the ammonia nitrogen dissolved in the leachate is in accordance with the observations reported in the literature. Furthermore, the simulated changes in the

428 Waste Management and the Environment II landfill gas composition, once the transition to anaerobic conditions stage is finished, coincide with what was expected from the general references. Nevertheless, to verify its usefulness in real landfill monitoring and design, the effect on the simulated emissions of the water passing through the waste remains to be analyzed. With this aim, the results of a first application of MODUELO 2 (with both the hydrologic and biodegradation modules modified) to a real landfill will be presented in a forthcoming paper.

References

[1] Lobo, A.; Herrero, J.; Montero, O.; Fantelli, M. & Tejero, I., Modeling for Environmental Assessment of Municipal Solid Waste Landfills (Part 1: Hydrology). Waste Management and Research, 20(2), pp. 198 – 210, 2002. [2] Lobo, A.; Herrero, J.; Montero, O.; Fantelli, M. & Tejero, I., Modeling for Environmental Assessment of Municipal Solid Waste Landfills (Part 2: Biodegradation). Waste Management and Research, 20(6), pp. 514 – 528, 2002. [3] Schroeder, P. R.; Dozier, Tamsen S.; Zappi, P. A.; McEnroe, B. M,; Sjostrom, J. W. & Peyton, R. L., The hydrologic evaluation of landfill performance (HELP) model. Engineering documentation for version 3, EPA/600/r-94/168b, Environmental Protection Agency, Cincinnati, OH, United States, 1994. [4] Lobo, A.; Muñoz, J.; Sánchez, M. M. & Tejero, I., Comparative analysis of three hydrological landfill models through a practical application (MODUELO 2, HELP and MODUELO 1), Proceedings of the Ninth International Landfill Symposium, Sardinia 2003. CISA, Cagliari, Italy, 2003. [5] Zhender, A. J. B.; Ingvorsen, K. & Marti T., Microbiology of methane bacteria. Anaerobic digestion: second international, Elsevier Biomedical Press, Amsterdam, pp. 45 – 66, 1982. [6] Lobo, A., Desarrollo de MODUELO 2: herramienta para la evaluación de la contaminación producida en vertederos de residuos sólidos urbanos, Doctoral Thesis, Dep. Sciences and Techniques of Water and the Environment, University of Cantabria, Spain, 2003. [7] Tchobanoglous, G.; Theisen, H. & Vigil, S., Integrated solid waste management. Engineering principles and management issues. McGraw Hill, Inc., United States, 1993. [8] Young, A., Mathematical modelling of the methanogenic ecosystem. Senior, E., Microbiology of landfill sites. Lewis Publisher, United States, 1995.