Application of FT-IR for assessment of the biological stability of landfilled municipal solid waste (MSW) during in situ aeration

Abstract

Disposal of untreated municipal solid waste leads to gaseous emissions as well as liquid degradation products. In situ aeration is an emerging means for remediation of abandoned landfills. It aims at an accelerated mineralization and stabilization of waste organic matter and thus reduces significantly the emission potential of the site. In order to prove the success of the technique, evaluation of the biological stability of the aerated material has been suggested. Fourier Transform Infrared Spectroscopy (FT-IR) provides comprehensive information on the chemical composition of solid waste samples. Different stages of organic matter degradation are reflected by changes in the infrared spectral pattern. In the present study the feasibility of applying FT-IR for assessment of the stability of waste material derived from abandoned landfills and for in situ aeration process control was investigated. Waste samples derived in the course of pilot-scale and lab-scale aeration experiments were characterized by FT-IR (4000–400 cm⁻¹, KBr-technique, transmission mode) and a set of conventional parameters describing biological stability. The occurrence of distinct indicator bands was correlated with chemical and biological waste properties using 206 solid waste samples. Visual spectra interpretation was found to be appropriate in proving a reduced emission potential of initially rather reactive waste (respiration activity over 4 days (RA₄) > 7 mg O₂ g⁻¹ DM) during aeration. Furthermore, cluster analysis was applied successfully to differentiate between original and aerated waste samples, even for rather stable material (RA₄ < 7 mg O₂ g⁻¹ DM), when visual spectra interpretation was limited.

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Introduction

Disposal of municipal solid waste (MSW) generates both gaseous emissions (mainly CH₄) and liquid degradation products (NH₄-N, organic carbon) due to the predominantly anaerobic decomposition of waste organic matter. Minimization of emissions from MSW-landfills currently represents an important issue in European environmental policy. Thus the European Directive on the Landfill of Waste sets targets to reduce the quantities of biodegradable MSW going to landfill.¹ By the year 2016 the quantity must be reduced to 35% by weight of the total amount of biodegradable MSW produced in 1995. The current approach applied in achieving this goal is pretreatment of MSW, either mechanical–biological or thermal, before final disposal.

For old landfills, where untreated MSW has been disposed of, aerobic in situ stabilization of the waste is an emerging means of site remediation, aiming at sustainably reduced emissions.² The technology is based on the supplementation of ambient air under low excess pressure into the landfill body and simultaneous collection and treatment of the exhaust gas. Adjustment of aerobic conditions leads to accelerated mineralization and stabilization of waste organic matter, thus reducing the remaining emission potential. A shift of gaseous C-emissions from methane to carbon dioxide occurs and the leachate load of ammonium and organic compounds (i.e. indicated by the chemical oxygen demand, COD) declines to environmentally sound levels within several years, compared to at least 30 to hundreds of years, estimated for leachate of landfills where untreated MSW has been disposed of. Currently, control of the degradation process is accomplished by monitoring landfill gas composition, temperature in and settlement of the landfill body. Furthermore, leachate properties such as the concentrations of NH₄-N, biological oxygen demand BOD₅, COD or AOX are determined. When in situ aeration is applied for remediation of abandoned sites usually no leachate is available for analysis. Furthermore, the decomposition process in the landfill may be inhibited due to, for example, toxic conditions or extreme dryness, thus determination of landfill temperature or settlement might be misinterpreted. Therefore, monitoring of the biological stability of the aerated waste, which is supposed to correlate with the remaining emission potential, is suggested. In this paper biological stability refers to a stage of decomposition when biological activity and turnover rates have reached a low level.

For assessment of stability of MSW several chemical, physical and biological parameters are currently employed. Sum parameters, such as total organic carbon (TOC) or loss on ignition (LOI), however, do not necessarily characterize waste stability, due to the presence of inert plastics etc. Laboratory leachate (prepared according to EN 12457) provides more information on mobile nitrogen and carbon fractions: low concentration levels of N and C (the Austrian landfill ordinance for example stipulates acceptance criteria at landfills for non hazardous waste of 300 mg kg⁻¹ DM for NH₄–N and 500 mg kg⁻¹ DM for TOC) indicate, in general, low reactivity.^3,4 Nevertheless, it has to be taken into account that mineral components are capable of influencing the extraction behavior. Aerobic and anaerobic biological tests reflect the bio-available organic fractions of waste material (i.e. respiration activity, gas generation by incubation test, gas generation by fermentation test, dynamic respiration index). Although the latter provide more accurate information on the emission potential of the waste material, biological tests might suffer from falsification due to unfavorable sample properties, such as toxic components or extreme dryness, inhibiting microbial activity. Furthermore, the realization is laborious and inter-laboratory comparison has shown lower reproducibility than usually obtained for chemical parameters.⁵

The application of Fourier Transform Infrared (FT-IR) spectroscopy for the characterization of MSW stability provides several advantages. The method sheds light on the solid sample as a whole whilst avoiding chemical extraction. Infrared spectra, resulting from characteristic molecular vibrations, reflect the molecular composition of complex waste material in a general way. Different stages of organic matter degradation are indicated by changes in the infrared spectral pattern. Provision of comprehensive information on degradation, the rapid accomplishment and being less error prone than biological tests, thus, make IR-spectroscopy a promising cost effective and reliable alternative for the determination of the stability of aerated waste material. To date infrared spectroscopy has been widely used to investigate complex environmental samples, such as soil, sewage sludge, compost and mechanically–biologically pretreated MSW. Haberhauer and Gerzabek characterized differences in soil organic matter due to various agricultural activities, land management systems and soil amendments.⁶ FT-IR in combination with multivariate data analysis has been successfully applied for the prediction of chemical and physical soil properties closely related to the bulk properties of the soil, such as organic matter content, cation exchange capacity, water and clay content from MIR-spectra.^7–9 A comparison of NIR and MIR calibration models for quantification of soil carbon of a highly diverse set of soils was carried out by McCarty et al.¹⁰ FT-IR spectroscopy has been used to monitor composting processes, to determine compost maturity and to monitor mechanical–biological treatment of MSW.^11–16 Identification of unknown landfill material was supported by comparison of IR spectra with a spectra library of pure substances.¹⁷

The present study was conducted to investigate whether FT-IR is appropriate for use in assessing biological stability of MSW derived from abandoned landfills in general and in particular for monitoring the success of in situ aeration.

Materials and methods

Experimental setup

Municipal solid waste material from two abandoned landfills (located in Austria (M) and Germany (K)) was used to conduct aeration experiments, both at lab-scale and in situ. Both sites showed an anaerobic status of the landfill body and have been identified as constituting a relevant risk of pollution, in particular to the groundwater. Table 1 presents general features of the sites.

Table 1 General features of the two abandoned landfills

Landfill	Volume/m^3	Average thickness/m	Age of the material/a	Type of waste	Landfill gas composition	Base seal	Landfill cover
M	∼200 000	8–10	10–20	MSW (∼60% m/m) demolition waste (∼40% m/m)	CH4: 60% (v/v)	Deficient mineral layer	0.2–0.5 m topsoil
					CO2: 40% (v/v)
K	∼220 000	8–10	10–20	MSW, bulky waste, demolition waste (variable portions)	CH4: 60% (v/v)	None	Provisional
					CO2: 30% (v/v)
					N2: 10% (v/v)

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In situ aeration experiment

Via six air injection wells (depth: 8 m) a portion of approximately 20 000 m³ of the Austrian landfill was aerated over 2.5 years with an average of 600 m³ air h⁻¹ and a pressure of 10 000 Pa. By means of adjusted negative pressure landfill gas was extracted via two further gas wells in order to prevent diffuse waste gas migrations to the atmosphere. The composition of the exhaust gas (O₂, CH₄, CO₂) was recorded on-line. Solid waste material was characterized prior to in situ aeration (45 composite samples), as well as after 6 (n = 22), 18 (n = 23), 24 (n = 24) and 30 months (n = 29) of aeration, respectively. After the periods specified the following air/solid ratios were achieved: 0.14 (6 months), 0.43 (18 months), 0.57 (24 months) and 0.66 m³ kg⁻¹ DM (30 months), respectively. Sampling was performed employing grab drilling (0.6 m diameter) applying a regular raster of a horizontal distance of ∼30 m. Approx. 20 increments (one shovel) each were taken from 3 vertical ranges (∼0–3, 3–6, 6–8 m) resulting in 3 composite samples per profile. Composite samples were homogenized by screening through a sieve with a mesh size of 20 mm. Laboratory samples were prepared by fractional shoveling. Solid samples were characterized by LOI, TC, TOC, total content of nitrogen (N_t), respiration activity over 4 days (RA₄) and gas generation sum within 21 days (GS₂₁). Leachates prepared according to EN 12457,³ were characterized by pH, electrical conductivity (eC), COD, BOD₅, contents of NH₄-N, NO₃-N, Cl⁻, PO₄-P and SO₄-S. The biological tests RA₄ and GS₂₁ were performed according to the Austrian Standards “Stability parameters describing the biological reactivity of mechanically-biologically pretreated residual wastes”.^18,19 The remaining parameters were determined following the analytical protocols cited in the Austrian Standard “Contaminated Sites: Risk assessment concerning the pollution of groundwater which is to be safeguarded”.²⁰

Lab-scale aeration experiments

Mixed samples of the solid waste material derived from the abandoned landfills were prepared (screened through a sieve—20 mm mesh size). The homogenized material was filled into 20 l landfill simulation reactors (LSR) and incubated at a constant temperature of 35 °C. The columns made of plexiglass were equipped with suctions for irrigation (on the top) and aeration (at the bottom) as well as for exhaust gas and leachate collection. The waste was either aerated with ambient air or stored anaerobically. The operation parameters of the individual test modes are presented in Table 2. At several times in the course of the test runs the content of the particular LSR was retained, thoroughly mixed and the resulting solid waste samples were characterized by assessing the same properties termed under “in situ aeration experiment”. The quality of the collected leachate was characterized for pH, eC, COD, NH₄-N and NO₃-N. Furthermore, the composition of the generated landfill gas (O₂, CH₄ and CO₂) was recorded online.

Table 2 Operation parameters of lab-scale experiments

Run	Test mode	Description of test mode	Aeration rate/l h^−1 kg^−1 DM	Leaching rate/l d^−1 kg^−1 DM
a Runs A and B were carried out using material from landfill M, run C with material from landfill K. b The aeration rate was adjusted to the O2 concentration in the exhaust gas.
A^a	—	Anaerobic storing	—	0.0034
	+	Low aeration rate	0.034	0.0034
	++	High aeration rate	0.068	0.0034
	+9−	Low aeration rate (9 months)/anaerobic storing	0.034/—	0.0034
	++9−	High aeration rate (9 months)/anaerobic storing	0.068/—	0.0034
B^a	+	Aeration, low leaching rate	0.081	0.0017
	+w	Aeration, high leaching rate	0.081	0.0069
	—	Anaerobic storing, low leaching rate	—	0.0017
	−w	Anaerobic storing, high leaching rate	—	0.0069
C^a	+	Aeration	0.037–0.002^b	0.0031

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FT-IR spectroscopic investigations of solid samples

Laboratory samples (screened through 20 mm) derived from lab-scale as well as from the in situ aeration experiment were air dried, ground in an agate mill and sieved <0.63 mm. Two milligrams of the prepared material were mixed thoroughly with 198 mg (FT-IR grade) KBr and pressed to a pellet. However, since the required particle size for a sample size of 2 mg (according to prEN 15002) was calculated to be <0.2 mm, each sample was measured at least 3 times in order to prove repeatability and thus representativeness.²¹ The pellet was measured using a mid-infrared (4000–400 cm⁻¹) FT-IR-spectrometer (Equinox 55, Bruker, Karlsruhe, Germany) in the transmission mode. Detection was performed using a DLATGS detector. Resolution was set to 4 cm⁻¹, 32 scans were recorded, averaged for each spectrum and corrected against ambient air as background.

Data analysis

Data analysis was performed using STATISTICA 6.0 (Statsoft), OPUS 4.0 (Bruker) and UNSCRAMBLER 9.1 (Camo).

Results and discussion

General characterization of mid infrared spectra of material derived from abandoned landfills

Fig. 1 illustrates how the spectra of waste material encountered at abandoned landfills vary considerably from similar to fresh MSW (landfill K) to showing more soil like properties (landfill M). Spectra of waste from landfill M show a predominance of bands caused by inorganic compounds, such as those related to carbonate (875 cm⁻¹ (C–O out of plane vibration), 1430 cm⁻¹ (C–O stretch vibration) and 2520 cm⁻¹), silica and clay minerals (1030 cm⁻¹ (Si–O–Si, Si–O stretch vibrations)) or phosphate (500–600 cm⁻¹). These spectral characteristics reflect the basic chemical and biological properties (Table 3) of the waste material of landfill M. Compared to further complex environmental samples such as compost, sewage sludge, mechanical–biological treated (MBT) waste material^12,15,17 and organic soil layers,²² previously subjected to FT-IR analysis, the total organic carbon content of the material from the two abandoned landfills was within a low range (14–120 g kg⁻¹ DM). Furthermore, comparably moderate values for biological reactivity were observed. High values were found for TIC (Table 3) which can be explained by large amounts of demolition and bulky waste (approx. 1/3 by mass) deposited.

Fig. 1 Representative spectra of waste material derived from the landfills M and K, fresh MSW and soil.

Table 3 Chemical and biological properties of the waste samples derived from landfill M before in situ aeration

	Solid samples					Laboratory leachates
	TIC/g kg^−1 DM^f	TOC/g kg^−1 DM	Nt/g kg^−1 DM	RA4/mg O2 g^−1 DM	GS21/Nl^g kg^−1 DM	eC/μS cm^−1	pH	NH4-N/mg kg^−1 DM	COD/mg O2 kg^−1 DM	BOD5/mg O2 kg^−1 DM
a Number of composite samples analyzed. b Standard deviation. c Median. d Minimum value. e Maximum value. f Dry matter. g Norm litre.
n ^a	45	45	45	43	10	45	45	45	45	45
Mean	40.6	80.1	5.1	3.6	6.2	1660	8.5	900	6211	2739
SD^b	5.8	23.0	1.4	1.9	3.7	534	0.3	384	3249	1747
MED^c	40.0	79.0	5.0	3.6	6.0	1660	8.5	835	6620	2470
MIN^d	29.8	14.0	1.4	0.8	0.1	524	8.1	244	860	350
MAX^e	53.0	120.0	8.0	11.8	13.5	3130	9.1	1756	16 560	7620

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In contrast to the materials mentioned above, bands at wavenumbers of 2925, 2850 and 1630 cm⁻¹ were the only bands assigned to functional groups of organic molecules, occurring in the spectra of all composite samples drawn from the investigated landfills to a remarkable degree. The bands at 2925 and 2850 cm⁻¹ are assigned to C–H stretching vibrations of aliphatic methylene groups constituting the skeleton of organic molecules.²³ The latter unfailingly occur in spectra of MSW samples. Intensities have been reported to decrease to a constant level during all biochemical decomposition processes.^15,17 The band at ∼1630 cm⁻¹ is attributed to C=C stretching vibrations of alkenes and aromatic rings and to C=O stretching vibrations of carboxylates and amides.^21,22,24 The intensity of the band has been reported to show a divergent evolution in the course of decomposition processes due to organic matter degradation and synthesis of biomass.¹⁰ Furthermore, the accumulation of aromatic compounds in the course of progressing decomposition has been reported to be reflected by the relative increase of the band compared to aliphatic methylene bands.¹² However, OH-bending vibrations from water adsorbed to functional groups of the organic matter at 1635 cm⁻¹ can overlap vibrations from these groups. Bands of protein-originated functional groups at 1570–1540 cm⁻¹ (Amide II) and ∼1320 cm⁻¹ (aromatic primary and secondary amines) frequently reported in FT-IR spectra of fresh MSW, sewage sludge and organic soil layers^12,15,20 occurred at a visible level in the spectra of the material derived from landfill K and a few particularly reactive samples of landfill M. Bands at 1260–1240 cm⁻¹ (C–O stretching vibrations of carboxylic acids, C–N stretching vibration of amides²¹ developed at the most as a slight shoulder in the spectra of the investigated waste material. Further bands, such as those at ∼1730 cm⁻¹ (C=O vibrations of aldehydes, ketones and carboxylic acids) indicating the presence of early decomposition products and thus occurring in rather fresh MSW, as well as at ∼1510 cm⁻¹ (aromatic C=C vibrations) stemming from waste components containing lignin^22,25,26 were rather weak. Table 4 gives an overview about relevant bands, associated functional groups and their occurrence in fresh MSW and waste material from abandoned landfills.

Table 4 Indicator bands changing in the course of MSW degradation^a

Wavenumber/cm^−1	Vibration	Functional group or compound	Waste from abandoned landfills Occurrence	Fresh MSW Occurrence
a Bold letters: bands disappearing in the course of successful aeration, bold italics: bands diminishing in the course of successful aeration.
2925	C–H	Aliphatic methylene	+	+
2850	C–H	Aliphatic methylene	+	+
1740–1720	C[double bond, length half m-dash]O	Aldehyde, ketone, carboxylic acids, esters	—	±
1630	C[double bond, length half m-dash]O	Amide I, carboxylates	+	+
	C[double bond, length half m-dash]C	Aromatic ring modes, alkenes
1570–1540	N–H	Amide II	±	+
1515–1505	Aromatic	Lignin		±
1320	C–N	Aromatic primary and secondary amines	±	+
1260–1240	C–O	Carboxylic acids	±	+
	C–N	Amide III

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Correlation of indicator bands with conventional waste properties

With respect to the variation observed within the spectra recorded from composite samples derived from the two investigated landfills during aeration experiments, the occurrence of two bands (at 1260–1240 and at ∼1540 cm⁻¹) was correlated with further chemical and biological properties. To enable comparison of the properties of waste material registering differences in the manifestation of the predominantly weak band at 1260–1240 cm⁻¹ 169, composite samples were subjected to descriptive statistics. The two groups were of similar size (presence of the band: n = 85; absence: n = 84). Whereas the values of TOC, RA₄, GS₂₁ and eC were almost normally distributed within the particular group, the laboratory leachate parameters NH₄-N, SO₄-S, PO₄-P, BOD₅ and COD showed an accumulation of values in the lower range in both groups. The values of the parameters presented in Fig. 2 proved to be significantly different (p < 0.001) between the two groups on applying the t-test (TOC, RA₄, eC) and Kolmogorov–Smirnov test (BOD₅, COD, NH₄–N), respectively. The remaining parameters did not differ between the two groups.

Fig. 2 Mean (for properties approx. normally distributed) and median (for properties not normally distributed) values, resp., 25–75%…interquartile range, Min–Max…Range, yes…presence of the band at 1260–1240 cm⁻¹ (n = 85), no…absence of the band at 1260–1240 cm⁻¹ (n = 84).

The properties of samples differing in the occurrence of the band at ∼1540 cm⁻¹ are given in Table 5. Although the mean values of COD, BOD₅ and NH₄-N were proven to be significantly different (t-test, p < 0.01) there was a substantial overlap between the two groups. However, for RA₄ the minimum value observed within the sample where the band occurred (n = 8) was 7.5 mg O₂ g⁻¹ DM. Waste samples (n = 155) without the band showed a maximum value for RA₄ of 8.8 mg O₂ g⁻¹ DM. Austrian and German regulations, i.e., set targets for RA₄ of 7 mg O₂ kg⁻¹ DM for acceptance at landfills for non-hazardous waste.⁴ Thus screening FT-IR spectra for occurrence of this band might be applied as a means of roughly discriminating between the ability of waste material to comply with this criteria, respectively. However, it should be kept in mind that respiration activity determined for MBT-material has a different significance regarding the gaseous emission potential than for anaerobically stored MSW of old landfills.²⁷

Table 5 Comparison of stability-relevant properties of waste samples with and without the band at 1540 cm⁻¹

Parameter	Band at ∼1540 cm^−1	n	Mean	SD^a	MED^b	MIN^c	MAX^d
a Standard deviation. b Median. c Minimum value. d Maximum value.
RA4/mg O2 g^−1 DM	Presence	8	8.9	1.46	8.4	7.5	11.8
	Absence	155	2.7	1.38	2.6	0.2	8.8
COD/mg O2 kg^−1 DM	Presence	8	16 920	10 942	16 560	5460	34 610
	Absence	155	5871	4576	4890	150	31 590
BOD5/mg O2 kg^−1 DM	Presence	8	4356	2311	4050	1740	7620
	Absence	155	1439	1342	1000	30	5610
NH4-N/mg kg^−1 DM	Presence	8	1360	482	1335	697	1996
	Absence	155	798	429	772	5	3032

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Changes of FT-IR spectra caused by aeration of waste material

Lab-scale experiments showed that bands manifested at 1570–1540 cm⁻¹ and ∼1320 cm⁻¹ in the spectra of the original waste sample (as observed for the samples derived from landfill K) had disappeared after aeration of the material. Material derived from landfill M in large part did not exhibit these bands. Therefore, general determination of enhanced stabilization due to aeration by visual inspection of IR-spectra is limited.

However, application of cluster analysis allowed discrimination between “original” waste samples and “stabilized” ones, although IR-spectra were not distinguishable visibly (1260–1240, ∼1540 and 1320 cm⁻¹, see above). To establish a model, samples drawn at landfill M prior to in situ aeration (only composite samples derived from the part of the landfill where in situ aeration was conducted, n = 18, indicated by II) and waste samples collected in the course of the lab-scale aeration experiments A and B (n = 17, see Table 6) were used as “original” and “stabilized” reference samples, respectively. Three spectral windows (3000–2800, 1790–1530 and 1350–1200 cm⁻¹), including the indicator bands mentioned above, were selected for the calculation. Vector normalized 2nd derivative spectra, Ward’s algorithm and Euclidean distance led to the result reported in Fig. 3. This dendrogram illustrates how samples stored anaerobically or drawn at an early stage of low rate aeration (0.034 l air h⁻¹ kg⁻¹ DM) were divided from those obtained after further aeration within the group of “stabilized” samples. The first subgroup of waste samples—having received a load of less than 0.1 m³ air kg⁻¹ DM—is indicated by III, the second (having received a load of more than 0.1 m³ air kg⁻¹ DM) is indicated by IV. Fig. 4 and Fig. 5 show a comparison of chemical and biological properties of the two main groups (“original” and “stabilized”) distinguished (left hand side). Differences in the considered parameters were deemed significant at the p < 0.001 level (t-test). Further samples drawn in the course of the in situ aeration experiment (n = 101) were tested using the model. Sixty five samples were assigned to the group of “original” samples, 34 to the group of “stabilized” ones. Descriptive statistics of these test sets are presented in Fig. 4 and Fig. 5 (right hand side). Differences between the two groups proved to be significant (BOD₅: p < 0.01, remaining parameters: p < 0.001, t-test). The parameters TOC, RA₄, eC, NH₄–N and COD showed a similar level and distribution in the group of “original” waste samples, both in the reference and the test set. The mean value of BOD₅, however, was significantly lower in the test set classified as “original” (1310 mg O₂ kg⁻¹ DM) than in the group of “original” reference samples (3100 mg O₂ kg⁻¹ DM). Since all waste samples in the test set were obtained after the beginning of the in situ aeration experiment, this finding indicates that the content of bio-available compounds in the laboratory leachate was significantly reduced at an early stage of the in situ aeration before actual stabilization (confirmed i.e. by RA₄) had occurred. The observed differences between the mean values (Fig. 4 and 5) of “stabilized” samples of the reference set and “stabilized” samples of the test set can be explained by the heterogeneity of the two sets. This stresses the importance of obtaining reference samples representative of the material to be tested. However, with regard to this specific investigation the latter was not achieved, since in the course of the lab-scale experiments, forming the reference set, a higher degree of stabilization was reached than in the course of the in situ aeration experiment over a period of 2.5 years (test set).

Fig. 3 Cluster analysis of “original” waste samples (II) and more (IV) or less (III) “stabilized” ones obtained in the course of the lab-scale aeration experiments, Spectral region…3000–2800, 1790–1530, 1350–1200 cm⁻¹, vector normalization, 2nd derivative spectra, Ward’s algorithm.

Fig. 4 Comparison of the properties (TOC, RA₄, eC) of the two groups of reference waste samples (left hand side: “original”, n = 18, “stabilized”, n = 17) distinguished applying cluster analysis and waste samples derived from the in situ aeration experiment classified by the model (right hand side: “original”, n = 65, “stabilized”, n = 36), ^astandard error, ^bstandard deviation.

Fig. 5 Comparison of the properties (NH₄-N, COD, BOD₅) of the two groups of reference waste samples (left hand side: “original”, n = 18, “stabilized”, n = 17) distinguished applying cluster analysis and waste samples derived from the in situ aeration experiment classified by the model (right hand side: “original”, n = 65, “stabilized”, n = 36), ^astandard error, ^bstandard deviation.

Table 6 Description of samples drawn from lab-scale experiments and their emission behavior serving as stabilized reference samples

Sample	Air/solid /m^3 kg^−1 DM	Liquid/solid /l kg^−1 DM	Discharge via gas^b ∑CO2–C, CH4–C/g kg^−1 DM	Discharge via leachate^b		Leachate concentration^b
				COD/mg O2 kg^−1 DM	NH4-N/mg kg^−1 DM	COD/mg O2 l^−1	NH4-N/mg l^−1
a The first part of the sample name refers to the amount of aeration (III ≤ 0.1 m^3 kg^−1 DM, IV > 0.1 m^3 kg^−1 DM), the middle part to the test mode of the particular run and the number at the final position indicates the duration (months) of aeration of anaerobic storing. b At the time of sampling.
III_A–9^a	0.00	1.00	2.28	1992	763	843	282.0
III_A–17	0.00	2.03	2.76	2352	1082	189	229.0
III_A+2	0.05	0.20	3.72	293	29	336	2.8
III_A+4	0.10	0.35	5.58	312	29	300	<0.1
IV_A+7	0.17	0.63	7.70	248	29	436	<0.1
IV_A+9	0.22	0.84	9.22	294	29	114	0.3
IV_A+17	0.87	1.59	14.61	523	69	129	0.2
IV_A+9–8	0.22	1.79	9.51	459	60	193	31.6
IV_A++2	0.11	0.20	4.13	109	15	339	0.8
IV_A++4	0.19	0.36	5.69	171	15	361	<0.1
IV_A++7	0.32	0.60	9.50	181	15	139	<0.1
IV_A++9	0.46	0.79	11.62	231	15	138	1.9
IV_A++9–8	0.46	1.65	11.90	328	37	156	36.9
IV_B+12	0.64	0.74	15.21	516	102	127	<0.1
IV_B+w12	0.64	2.67	19.01	997	165	27	<0.1
III_B–12	0.00	0.74	2.69	1123	531	338	554.0
III_B–w12	0.00	2.59	3.82	2098	1130	9	165.0

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Assessment of the biological stability of MSW derived from abandoned landfills using FT-IR

This investigation showed that the biological stability of MSW material derived from abandoned landfills is reflected by the mid infrared spectral pattern which is an indicator of different stages of organic matter degradation. Several IR-spectral features have been proposed to date as characterizing sufficient stability of MSW to guarantee sustainably low emissions after final disposal. Smidt and Schwanninger proposed the absence of bands at ∼1560, 1320 and 1260–1240 cm⁻¹ as an indication of successfully stabilized MBT-material.¹⁷ The present investigation showed that abandoned sites comprised of waste material, although meeting the above IR-spectral criteria to a large degree, have not necessarily reached a low-emission state. Compared to average concentrations of organic compounds and ammonium reported in the leachate of MSW-landfills with a duration of 11–20 years of deposition (COD = 1830 mg O₂ l⁻¹, NH₄–N = 555 mg l⁻¹,²⁸) concentrations in the leachate of landfill M prior to in situ aeration were rather high (COD ∼2500 mg O₂ l⁻¹, NH₄-N ∼ 1000 mg l⁻¹). A ratio of BOD₅/COD in the leachate of ∼0.5 as well as a landfill gas composition of 60% (v/v) CH₄ and 40% (v/v) CO₂ furthermore indicate that landfill M was still in a phase of stable methane production associated with considerable emissions. Moreover, the quality of leachate from landfill M did not fulfill the requirements for direct discharge into the environment, as e.g. set by the Austrian regulation for the leachate of landfills.²⁹ Several parameters exceeded the threshold values (NH₄-N = 10 mg l⁻¹, COD = 50 mg O₂ l⁻¹, AOX = 0.5 mg l⁻¹). Concentrations of ammonium did not even meet the criteria (200 mg l⁻¹) for discharge into public sewerage. However, these findings are in accordance with the characterization of the waste material by means of biological and conventional chemical parameters. Mean values of RA₄ (3.6 mg O₂ g⁻¹ DM) and GS₂₁ (6.2 Nl kg⁻¹ DM) of landfill M prior to in situ aeration fall considerably below target values (RA₄ < 7 mg O₂ g⁻¹ DM, GS₂₁ < 20 Nl kg⁻¹ DM) for materials suited to disposal in landfills for non hazardous waste.⁴ This furthermore indicates that these criteria, established for MBT-material, are not necessarily appropriate in demonstrating the low-emission state of old landfills. Indeed, for practical reasons less severe stability requirements can be applied for MBT-material than for MSW from abandoned landfills (deposition at landfills complying state-of-the-art, equipped with proper leachate collection facilities). Furthermore, anaerobic storing of MSW leads to a different significance of chemical and biological parameters. As an example, the present study emphasised how the fraction of total gas generation potential covered by GS₂₁ is far lower when determined for MSW from abandoned sites (GS₂₁/GS₅₈₀ = 0.34–0.54) than for MBT-material (>0.8).^27,30 The threshold value set for TOC is 50 g kg⁻¹ DM.⁴ However, Fig. 4 clearly demonstrates that this value is exceeded on average even in the group of “stabilized” samples (no longer causing significant emissions) obtained during the lab-scale experiments. In particular at abandoned landfills, where usually no source-separated waste has been deposited, a considerable fraction of TOC might result from inert plastics which, however, do not contribute to the medium-term emission potential.

Besides analysis of the occurrence of distinct indicator bands, constant heights of aliphatic methylene bands over time have been proposed by several authors to prove stabilization during biological treatment processes (MBT, composting).^14,31 In the course of in situ aeration a decrease of the intensities of aliphatic methylene bands (at 2925 and 2850 cm⁻¹) was also observed. However, proving the success of site remediation by repeated sampling would seem to be rather inappropriate, since sampling of (abandoned) landfills is much more complex and expensive than sampling at waste treatment plants.

Conclusions

The aim of this investigation was to perform a basic interpretation of the mid IR-spectroscopic pattern of MSW samples derived from abandoned landfills, with particular regard for their biological stability and the related emission potential. FT-IR spectra of material from old landfills was found to vary considerably (according to the composition of the input material and the duration of disposal) over a wide range, from similar to fresh MSW to soil-like.

In the case of particularly reactive waste material, visual spectra interpretation (similar to suggestions for process control of mechanical–biological waste treatment) was found to be appropriate for use in monitoring the success of in situ aeration (occurrence of bands at 1260–1240, ∼1540 and 1320 cm⁻¹). However, the indicators proposed for sufficiently stabilized MBT-material can not be transferred directly to demonstrate a low-emission state of samples derived from old landfills.

For rather stable material, which does not exhibit the relevant indicator bands, this approach is limited. However, multivariate data analysis was successfully applied to discriminate between such rather stable material obtained prior to and after in situ aeration. In order to utilize the comprehensive information of the IR-spectra, lab-scale simulation experiments could be performed using samples with a well known emission behaviour to establish models for subsequent use in testing waste samples taken during in situ aeration.

The tentative correlation between IR-spectral features and biological/chemical properties and the corresponding emission potential of waste from abandoned landfills elaborated in this study is based on the analysis of material derived from two sites. In order to develop generally valid spectral criteria to be used for process control of in situ aeration and assessment of the emission potential of abandoned landfills an extended analysis of waste samples from further sites is necessary. Furthermore, additional efforts should be undertaken on appropriate sampling plans for proper assessment of the emission potential of a site based on FT-IR analysis of solid waste samples, in particular consideration of the spatial heterogeneity of abandoned landfills.

Acknowledgements

This research was funded by the Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management (BMLFUW). The authors thank all staff members of the Institute of Waste Management and of the NUA, Niederösterreichische Umweltschutzanstalt GmbH contributing to the present study as well as Marco Ritzkowski (Technical University Hamburg-Harburg, Institute of Waste Management) for providing the samples of the German landfill.

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