Published online 4 January 2008
Published in J Environ Qual 37:146-153 (2008)
DOI: 10.2134/jeq2006.0191
© 2008 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
Spectroscopic and Wet Chemical Characterization of Solid Waste Organic Matter of Different Age in Landfill Sites, Southern Germany
Rupert Bäumlera,b,* and
Ingrid Kögel-Knabnera
a Technische Universität München, Lehrstuhl für Bodenkunde am Wissenschaftszentrum für Ernährung, Landnutzung und Umwelt, Department für Ökologie, Am Hochanger 2, D-85350 Freising-Weihenstephan, Germany
b present address: Institut für Geographie, Friedrich-Alexander Universität Erlangen-Nürnberg, Kochstr. 4/4, 91054 Erlangen, Germany
* Corresponding author (baeumler{at}geographie.uni-erlangen.de).
Received for publication May 12, 2006.
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ABSTRACT
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Landfill sites are potential sources of hazardous emissions by degradation and transformation processes of waste organic matter. Its chemical composition and microbial degradability are key factors for risk management, after-care, and estimation of potential emissions. The aim of the study is to provide information about composition and extent of transformation of waste organic matter in four landfill sites in Bavaria, Southern Germany by means of 13C NMR spectroscopy, acid-hydrolyzable carbohydrates, chloroform-methanol extractable lipids, acid-hydrolyzable proteins, and lignin compounds after CuO oxidation. Ten samples of about 20 to 25 yr, 15 to 20 yr, and 5 to 10 yr of deposition each were taken at 2 m depth intervals by grab drilling till 10-m depth. Increasing temperatures from about 15°C at 2-m depth to >40°C at 10-m depth are found at some of the sites, representing optimum conditions for mesophile methane bacteria. Moisture contents of 160 to 310 g kg–1 (oven dry), however, provide limiting conditions for anaerobic biodecay. Spectroscopic and chemical variables generally indicate a low extent of biodegradation and transformation at all sites despite a considerable heterogeneity of the samples. Independent of the time and depth of deposition more than 50% of the carbohydrate fraction of the waste organic matter provide a high potential for methane emissions and on-site energy production. There was no significant accumulation of long-chain organic and aromatic compounds, and of lignin degradation products even after more than 25 yr of rotting indicating higher extent of decomposition or stabilization of the waste organic matter. Installation of seepage water cleaning and recirculation systems are recommended to increase suboptimal moisture contents with respect to microbial methanogenesis, energy production, and long-term stabilization of municipal solid waste.
Abbreviations: MSW, municipal solid waste • WOM, waste organic matter • NMR, nuclear magnetic resonance • Sec., emplacement section • TOC, total organic carbon • Ntot, total nitrogen
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INTRODUCTION
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WASTE management is one of the big issues in fast-growing urban environments of the 21st century. In common, municipal solid waste (MSW) is either incinerated or disposed at urban or regional landfill sites. Interstate MSW trading networks developed. Waste was globally merchandised, and economically seen waste reduction was more or less counterproductive. Landfill sites, however, are potential sources of hazardous gaseous and leachate emissions caused by multiple degradation and transformation processes of the waste organic matter (El-Fadel et al., 1997). Environmental impact may last for centuries, and after-care actions may burden future generations (Tojo et al., 2005). Landfills are regarded as one of the most important sources of methane which is thought to be one of the most effective greenhouse gases. In addition harmful polycyclic aromatic hydrocarbons are mobilized and may contaminate ground and drinking water. Chemical composition and microbial degradability of the waste organic matter are therefore key factors for risk management, after-care actions, and the calculation of potential emissions. Within the last decades, however, environmental awareness increased and waste management policy changed toward sustainability including an overall reduction of waste, resource recycling, and composting (Katers and Walczak, 2005). Since 1993 the German legislation claims special waste treatment procedures which are separate collection of organic waste, paper, and various recyclable materials before deposition to reduce or avoid hazardous emissions (Siedlungsabfall, 1993). Before 1993, however, untreated solid waste of unknown composition was deposited having high amounts of organic matter. In general, more than 70% of untreated MSW is of organic origin (Zeng et al., 2005). In the federal state of Bavaria in southern Germany about 4000 of such landfill sites exist with an unknown risk potential. Complex biochemical and physicochemical reactions are running in their landfill bodies causing emissions of volatile and liquid by-products. Those emissions demand time-consuming and cost-intensive aftercare actions, which are difficult to be calculated in most cases to avoid or minimize environmental impact (Stief, 1996; Calvo et al., 2005). Little is known about those chemical reactions and decomposition processes of untreated waste, and estimation of future potentials by measuring recent emissions is limited as it strongly depends on the abiotic conditions in the landfill bodies (Kuchlik et al., 1996) and on a detailed knowledge of the chemical composition of the residual waste organic matter in (old) landfill sites.
The aim of the study is, therefore, to provide information about composition and stability of the organic material and the extent of decomposition of solid waste organic matter of known age in different municipal landfill sites in southern Germany by selective chemical and spectroscopic methods. In this context time-dependent carbohydrate decomposition is considered to be one of the best qualified stabilization criteria of solid waste in landfill sites (Kelly et al., 2006). The results may help to estimate future emission potentials and adverse environmental impacts, and to reduce costs of essential after-care actions.
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Material and Methods
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Study Sites
The studied landfill sites are located in Bavaria in southern Germany. Four sites were selected for comparison which are all are top sealed. The first site named Im Dienstfeld is located near Aurach in the administrative district of Ansbach. 47,000 Mg of untreated solid waste were deposited each year between 1979 and 1998 within a total area of 12.3 ha in sections of different age, section I from 1979–84, section II from 1984–88, section IIIa from 1988–1992, and IIIb from 1992–1999. The age sequence may help to identify processes and extent of transformation with increasing time of rotting since deposition. The dumped material consists of 57% municipal waste including biowaste, 31% industrial waste, 8% bulk waste, and 4% others including sewage sludge. Methane emissions are used for energy production since 1992. A seepage water recirculation system was installed in 2001 after sampling to increase the moisture content in the landfill body to enhance microbial methanogenesis and energy production, and to reduce the costs of leachate treatment.
The landfill sites Pyras and Georgensgmünd in the administrative district of Roth were sampled for comparison. Pyras is separated into an older site without base sealing operated from 1977 to 1988 (sections I, II, and III), and a newer section IV which was filled between 1987 and 1993. In the new section the composition of the dumped waste is similar to the site near Aurach. The composition of the delivered waste could not be reconstructed further at the older site. Methane emissions are used for energy supply of a local brewery. 3.71 106 J power were generated in 1999. Power generation significantly dropped to such comparably low values after top sealing of the landfill body. Georgensgmünd was operated from 1979 to 2001. The two oldest sections I and II are without base sealing and were filled between 1979 and 1983. Section III was filled from 1984–1992, and the youngest section IV until 2001. The dumped material consists of 69% municipal waste including biowaste, 19% industrial waste, 9% bulk waste, and 3% construction waste.
The landfill Erbenschwang (administrative district Weilheim-Schongau) was additionally included in the study. At Erbenschwang leachate re-infiltration experiments were performed in one of the emplacement sections since 1996 to increase the water content in the landfill body to enhance methanogenesis. The site was filled from 1989 to 1996. A methane combustion system was installed for energy production. After top sealing of the landfill, microbial activity decreased drastically, and methane production was too low for energy production. Four sites were sampled 5 yr after beginning of the experiments: two sites outside of the infiltration area and two sites at a distance of 2.5 and 7.5 m from the main infiltration core.
Sampling and Sample Preparation
Samples were taken from each section in duplicate at regular depth intervals of 2 m by grab drilling of 1 m in diameter till 10-m depth. After sampling, the material was stepwise prepared for analysis. Glassware, stones, and metals were removed on site by hand as far as possible, and their portion was estimated. Thereafter, a barrel of about 200 L was filled with each sample. The material was shredded to <1 to 2 cm for homogenization, and divided by piling and quatering to aliquots of about 7 kg. One aliquot was again split into four subsamples. One of the subsamples was further ground by a cutting mill in two steps (5 mm and 1 mm; SM 2000; Retsch, Haan, Germany), and by a centrifugal mill to <0.5 mm (Pulverisette 14, Fritsch, Idar-Oberstein, Germany) for chemical analysis. The application of the complex multilevel sample preparation was successfully tested with residual waste after mechanical-biological pretreatment to get representative samples (Pichler, 1999). Pichler (1999) found standard deviations of 4.2% for total organic carbon (TOC), 3.3% for total N, and 2.7% for loss of ignition by analyzing 16 subsamples out of one main sample which was split before shredding and grinding.
At the site Im Dienstfeld all samples were analyzed separately to identify trends with depth and time since deposition. At the three other sites the samples at 0 to 2 m, 4 to 6 m, and 8 to 10 m were analyzed.
Analytical Methods
Waste organic matter is chemically composed of different groups of organic compounds, such as carbohydrates, proteins, lipids, or lignin components that are as well present in natural ecosystems. In addition variable amounts of components typical for civilized societies, such as plastics are present. Numerous methods exist to characterize different groups of organic compounds in soils and sediments (Kögel-Knabner, 1995) that could be successfully adjusted on waste organic matter (Pichler, 1999; Pichler & Kögel-Knabner, 2000), and which were applied in the present study. Temperatures were measured on-site during sampling. pH was measured with a glass electrode in the supernatant of a 10:1 (w/w) water/waste suspension. Total organic and inorganic carbon and total nitrogen were determined with an elemental analyzer based on gas chromatography (Vario EL, ELEMENTAR, Hanau, Germany). Ignition losses were determined after heating the samples at 550°C to constant weight for 3 h after DIN 18128 (2002). Hydrolyzable proteins were extracted by 6 mol L–1 HCl and analyzed colorimetrically at 550 nm by determining the 
-amino-group of amino acids with ninhydrine (Spectronic 501 UV/VIS spectrophotometer, Bausch & Lomb, Rochester, NY; Stevenson and Cheng, 1970). The lipid fraction was extracted by chloroform-methanol and determined gravimetrically after dehydration with anhydrous Na2SO4 (Bligh and Dyer, 1959). Lignin contents are representative of the sum of methoxyphenols obtained after alkaline CuO oxidation (Kögel, 1986; Kögel-Knabner, 1995). The oxidation products were analyzed by gas chromatography using Fisons GC 8000 and FID. Cellulose and non-cellulosic carbohydrates were solubilized by acid hydrolysis and measured colorimetrically at 635 nm after reaction with 3-methyl-2-benzothiazolinonehydrazone (Spectronic 501 UV/VIS photometer, Bausch & Lomb, Rochester, NY) as described in detail by Kögel-Knabner (1995). All analyses were performed in duplicate of the dry weight material and are referred to total organic carbon. Nuclear magnetic resonance spectroscopy was performed on freeze-dried material after removal of paramagnetic compounds by 5M HF treatment. Solid-state 13C NMR spectra were obtained by a DSX 200 spectrometer (Bruker, Rheinstetten, Germany) at 50.32 MHz with a commercial Bruker double air bearing probe and 7 mm o.d. rotors. Chemical shifts were calibrated with glycine and reported relative to tetramethylsilane (= 0 ppm). The spectra were obtained at a spinning speed of 6.8 KHz, and up to 10,000 scans were accumulated at pulse delays between 8 and 15 s. The tentative peak assignment to chemical shift ranges for municipal solid waste is given in Pichler et al. (2000). The range between 220 and 160 ppm chemical shift is assigned to carboxyl/carbonyl groups, aldehydes, ketones, amides, and esters; 160 to 140 ppm are representative of aromatic C-O-R- and C-N-R groups and C-1 compounds in polystyrene; 140 to 100 ppm are representative of aromatic C-H groups and non-C-1 carbon in polystyrene; 110 to 90 ppm are representative of C-1 in carbohydrates and C-2 and C-6 in syringyl units of lignin; 90 to 60 ppm are representative of C-2 and C-6 in carbohydrates and side chains in lignin; 60 to 50 ppm are methoxyl groups and C in amino acids; 50 to 40 ppm are C and Cβ in polyvinyl chloride and in polystyrene, and Cβ in polypropylene; 40 to 25 ppm are methylene groups in lipids, proteins, polyethylene, polyisoprene, and polyamide; 25 to 0 ppm are methyl groups, e.g., of lipids, peptides, and polypropylene. Fresh municipal solid waste materials are dominated by signals in the O-alkyl-region (110–50 ppm). During the process of rotting and decomposition, the relative intensity in the O-alkyl-region should decrease by a preferential microbial decay of carbohydrates, while all other chemical shift ranges should increase to a different extent (Pichler et al., 2000; Fuentes et al., 2006; Kelly et al., 2006).
Side effects and chemical reactions in landfill bodies may lead to an under- or overestimation of the different organic compounds with regard to the applied methods. As far as we know there are no specific studies on that problem with special emphasis on interferences with the waste matrix. Lipids might be additionally extracted from plastics compared to their extraction from soil organic matter. This holds especially for polyvinylchloride (PVC). Pichler (1999) found 8.1% lipids extracted from PVC by the chloroform-methanol method. However, as the amount of PVC is around 4% on average in MSW, the influence can be neglected in total. Recovery experiments with MSW gave 99 ± 13% retrieval with glucose, 85 ± 8% with cellulose, 96 ± 6% with proteins (glycine), and between 88 ± % (p-hydroxybenzaldehyde) and 98 ± 10% (ferula-acid) with lignin degradation products (Pichler, 1999).
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Results and Discussion
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Temperatures increase in general with depth at the study sites (Fig. 1
). Temperatures of <20°C within the first 2 m of the waste body were influenced by the climatic conditions at the landfill sites independent of the age of the waste and of the time of deposition. Differences increased with increasing depth and age of the waste. The younger the waste, the higher are the temperatures. In the younger sections of the sites Im Dienstfeld and Georgensgmünd temperatures rise up to 50°C providing optimum conditions for thermophile bacteria. Comparable low temperatures (<30°C) in the older sections and at the two other sites Pyras and Erbenschwang in general might either be caused by adverse abiotic or toxic conditions, or by a decline of the microbial activity, as easily biodegradable fractions like polysaccharides might already be removed by mineralization (Inbar et al., 1990; Barberis and Nappi, 1996). A minor tendency of increased temperatures was observed at Erbenschwang after re-infiltration of seepage water (Fig. 1).
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Fig. 1. Mean temperature values (°C) and standard deviations in four landfill bodies during sampling at each sampling depth of the filling sections at all sites.
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The moisture content is one of the key factors for microbial decay of organic waste in landfill bodies. Optimum values are 500 to 600 g kg–1 waste for methanogenesis (Zavala and Funamizu, 2005; Kelly et al., 2006). The found values varied between 60 and 420 g kg–1 independent of age and depth of the waste at all sites (Fig. 2
). Moisture content still seemed to be controlled by structure and bedding of the waste at the time of deposition causing preferential flow (Huber et al., 2004; Katers and Walczak, 2005; Xie et al., 2006). It is especially indicated at Erbenschwang. Re-infiltration caused slightly higher moisture contents at three sampling depths. Moreover, mean values of 160 to 310 g kg–1 point toward permanent limiting conditions with regard to biodegradation and stabilization of the waste organic matter (Zavala and Funamizu, 2005).
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Fig. 2. Moisture content (g kg–1) and standard deviations of the samples in four landfill bodies at each sampling depth during sampling at all sites.
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With the exception of one sample consisting mainly of compressed wood waste (pH = 5.9) pH values varied between 7.1 and 8.4 typical of methanogenic processes (not shown in Table 1
).
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Table 1. Mean values and standard deviation of selected analytical data in the different filling sections and in the re-infiltrated and not infiltrated area of the four landfill sites.
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Ignition loss, total organic carbon, and total nitrogen (Ntot) provide the quantitative frame of the characterization and turnover processes of organic matter in soils, sediments, or municipal waste. The mean values of TOC varied between 117 and 264 g kg–1 dry weight. Mean inorganic carbon was <17 g kg–1 most probably caused by minor additions of construction waste and bones (Table 1). Inorganic carbon should increase during mineralization in landfill bodies. However, this could not be confirmed in the present study with respect to the age and depth of the samples. Ignition losses varied between 201 and 536 g kg–1 (Table 1). Amounts of more than 600 g kg–1 indicate high incipient amounts of organic compounds rich in carbon (plastics and lipids), or a relative accumulation of these compounds during biodegradation. Loss of ignition should characterize the amount of organic matter, and should decrease during decomposition. However, no significant differences were found with regard to sampling depth and age of the waste organic matter at all sites. This is in accordance with findings of Davidsson et al. (2007) whereupon waste samples of different origin and pre-treatment had minor variations in the chemical composition despite significant methane production during digestion experiments. Their results may in turn confirm our findings of increasing temperatures with sampling depth despite nonsignificant variations in the chemical composition. Moreover, strong variations of the TOC values up to 130 g kg–1 within each emplacement section indicate the heterogeneity of the parent landfill material. In comparison, 400 to 690 g kg–1 ignition losses were found in residual waste (Pichler, 1999).
Mean total nitrogen values varied between 4.5 and 15.4 g kg–1 (Table 1). It is related to TOC (r = 0.58) and ignition loss (r = 0.55; Spearman's rank order correlation; n = 30; P < 0.05). Highest values were found in the youngest emplacement sections independent of site and sample depth. The lower values in older sections may indicate advanced biodegradation, but also changes in the consumer's behavior at the end of the 1980s by waste separation and disposing of waste rich in carbon (plastics). However, this is in contrast to the findings of Davidsson et al. (2007) of minor variations in the chemical composition of waste samples independent of the origin, pre-sorting, and pre-treatment. Mean C/N values varied between 19 and 41 similar to immature composts (Table 1; Inbar et al., 1990; Barberis and Nappi, 1996).
The mean amount of lipids of 50 to 162 g kg–1 TOC dry weight (Table 1) were partly lower compared with 120 to 130 g kg–1 found in mechanical-biologically pretreated residual waste (Pichler, 1999). During biodegradation the amount of lipids may increase by a relative accumulation of long-chain compounds which might hardly be attacked by microorganisms. However, no significant changes occurred with time and depth at the study sites. This may point to a low extent of transformation and stabilization of the organic waste material. However, degradation trends might be leveled by microbial re-synthesis and the unknown structure of the lipids. The same holds for lignin and lignin degradation products varying between 99 and 220 g kg–1 (Table 1) without significant trends except for Pyras. At Pyras, the amount of lignin degradation products increases nonsignificantly from the youngest (IV) to the oldest section (I). However, interpretation is difficult as the older sections II-IV could not exactly be differentiated any more with regard to the age of the waste, original composition, and the time of emplacement by missing records of the operator. No significant differences or trends could be observed after re-infiltration of seepage at Erbenschwang for both groups of organic compounds. That can be explained by the comparably short period of 5 yr between the beginning of re-infiltration and the sampling by the problems coming along with re-wetting and preferential flow (Huber et al., 2004; Xie et al., 2006).
The analysis of proteins gives similar results. At one of the sites (Im Dienstfeld) the amount of proteins tends to increase with the age of the waste, while it is the opposite trend at Pyras (Table 1). The two other sites fit into the patchy figure. Five yr of re-infiltration of seepage water had no significant influence or clear trend on the amount of proteins.
A dominance of carbohydrates of up to >600 g kg–1 of the total organic fraction was found in the samples independent of the degree of rottenness (Table 1). No significant reduction could be observed, however, in the older sections with increasing age of the waste at the sites Im Dienstfeld, Pyras, and Georgensgmünd. Non-cellulosic carbohydrates, however, tend to be slightly enriched in these three landfill sites. Mean ratios of cellulose and non-cellulosic carbohydrates >1 point to a dominance of paper waste even in the youngest sections, that were deposited after legally forced waste separation and recycling (data not shown in Table 1). At Erbenschwang the results indicate enforced decay of carbohydrates after re-infiltration of seepage supporting the presumption of limiting moisture contents in the examined landfill bodies with regard to microbial decay and long-term transformation and stabilization of the waste organic matter.
Solid-state 13C NMR spectroscopy was used as a complementary technique to support the results of the chemolytic analysis of the waste organic matter in the different landfill sites. During biodecay of waste a decrease of the relative intensity of the O-alkyl C region (110 to 50 ppm chemical shift range; mainly carbohydrates) with a concurrent increase of the relative intensity of alkyl C (50 to –10 ppm chemical shift range; long-chain aliphatics, i.e., plastics and lipids, proteins) and of aryl C (160 to 110 ppm; sp2 C in aliphatic and aromatic compounds) is expected.
The results (Fig. 3
and 4
; Table 1) again show the dominance of carbohydrates of more than 50% of the total organic fraction in most of the samples independent of the time of deposition. Moreover, a distinct signal around 29 ppm indicates methylene groups of long-chain aliphatic compounds (lipids, proteins, plastics). Their portion was 17% of TOC on average. Signals of around 129 ppm comprise sp2 C in aliphatic and aromatic structures of 
15% on average. There were no significant changes with sampling depth despite the temperature variations (Fig. 1). And again the results point to an enhanced turnover of carbohydrates after re-infiltration of seepage at Erbenschwang compared to the non-infiltrated area (Table 1). In addition the effect of re-infiltration seems to decrease with distance from the main infiltration core.
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Fig. 3. Solid state CPMAS 13C NMR spectra of the municipal solid waste in the different filling sections of the site Georgensgmünd at sampling depth of 0 to 2 m, 4 to 6 m, and 8 to 10 m.
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Fig. 4. Solid state CPMAS 13C NMR spectra of the municipal solid waste in the re-infiltrated and not infiltrated area of the site Erbenschwang at sampling depth of 0 to 2 m, 4 to 6 m, and 8 to 10 m.
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In general both the results of NMR spectroscopy and of chemolytic analysis indicate a low degree of microbial transformation and mineralization of the waste organic matter even after more than 25 yr of deposition.
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Conclusions
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Municipal solid waste of different age of deposition from four landfill sites in southern Germany was studied with regard to the composition of the organic waste material to provide information about potential emissions and future site management. Analytical values indicate that the overall extent of biodegradation and transformation is low in all samples. It is indicated by the fact that no significant changes with time of deposition were found at all sites. The same holds for the sampling depth. A total average moisture content of 240 g kg–1 found in the landfill bodies after top sealing might be the most important factor of limitation with regard to oxidation and long-term stabilization of the organic matter (Zavala and Funamizu, 2005; Kelly et al., 2006). This is indicated by the results of re-infiltration experiments of seepage water at one of the sites, which generally show a higher degree of decomposition. Moreover, one of the problems in landfill bodies is a complete moistening of the total waste material. Heterogeneity of the waste and mechanically induced layers during deposition of the waste material will induce preferential water flows and accumulation of water above horizontally oriented plastic pieces, while large volumes of the landfill body will stay dry (Huber et al., 2004; Xie et al., 2006). Huber et al. (2004) showed that most of the waste material revealed unchanged even after more than 15 yr since deposition by inadequate wetting, and that leachate analysis will therefore underestimate long-term emission potentials. In addition, Xie et al. (2006) found that the hydraulic conductivity decreases with time in MSW landfill bodies.
More than 500 g kg–1 of waste organic matter (WOM) were present in the carbohydrate fraction, the most suited compounds for microbial decay, still providing a high potential for methane emissions and energy production at the studied sites and in landfill sites of similar conditions in general despite more than 25 yr of rotting. Carbohydrates, especially cellulose, are the most important source of energy for microorganisms and are preferably decayed as they are easily accessible in most environments (Dignac et al., 2005). Installation of seepage water cleaning and recirculation systems can be recommended to increase the moisture content with respect to microbial methanogenesis and the use of landfill sites for energy production over decades. Removal of the legally regulated top sealing might be an alternative to increase the moisture content and to enhance microbial decay. However, it should be again considered that heterogeneity within the landfill body may cause preferential flow preventing large areas of the waste from re-wetting (Huber et al., 2004; Doeberl et al., 2005; Xie et al., 2006).
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ACKNOWLEDGMENTS
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We gratefully acknowledge the financial support by the Bavarian Ministry of Development and Ecology, Munich (StMLU). We are indebted to the district administrations of Ansbach, Roth, and Weilheim-Schongau, and to the WEIMA Company, Ilsfeld, for their manifold support. We thank our colleague Dr. Heike Knicker for her manifold support with the NMR spectroscopy.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
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