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Representative sampling of the organic fraction of fresh municipal solid waste
Biomass 11 (1986) 91-98
Representative Sampling of the Organic Fraction of Fresh Municipal Solid Waste G. H. Beckers, E.-J. Nyns and H. P. Naveau Unit of Bioengineering, University of Louvain, Place Croix du Sud, 1 bte 9, B- 1348 Louvain-la-Neuve, Belgium (Received 3 July 1986; accepted 22 August 1986) ABSTRACT A method has been developed for sampling the highly heterogeneous organic fraction of fresh municipal solid waste, especially for chemical oxygen demand (COD) determination. The method consists of progressive crushing, taking samples which are a few orders of magnitude greater than the size of an average single particle of that sample. A t each step, the crushing and homogenization system is adapted to the sample size and the characteristics of the material. The method developed permits the sampling of representative quantities as small as the 0 3 g necessary for COD measurements. Based on 24 replicate results, the method yields" a confidence level of plus or minus 7"5% at P = 0"95. The accuracy of the proposed sampling method in the measurement of total solids, volatile solids, ash and carbonate content has also been investigated. Key words: Municipal solid waste, sampling, chemical analysis, chemical oxygen demand (COD).
INTRODUCTION Municipal solid waste (MSW) is steadily increasing in importance as its potential value as a fuel is realized. For instance, there are m o r e and m o r e studies and projects dealing with the application of biomethanation to this substrate. Yet, because of the extremely heterogeneous nature of MSW, one of the first problems that arises is chemical characterization prior to design of any treatment process. In the literature, municipal solid wastes are usually characterized by subdivision into different classes (paper, plastics, glass, etc.). This classification does not always meet the needs of the engineer or the researcher 91 Biomass 0144-4565/86/S03.50- © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
92
G. H. Beckers, E.-J. Nyns, H. P. Naveau
who has to know exactly what is being treated. In particular there is a need for accurate quantitative measurement of the organic fraction of MSW. When MSW is considered in its totality, the measured parameters are generally its total solids (TS) and/or volatile solids (VS) content. These parameters are determined by heating the material to temperatures around 105 and 450 to 600°C respectively. This method of determination of the TS content has the disadvantage of not discriminating between the relative fractions of organic and inorganic material and the VS content is ascribed to the organic fraction. At the same time this method does not include products, such as alcohols and low molecular weight volatile organic acids, that have already volatilized during the heating step at 105°C. In addition, this parameter does not take into account the variable oxidation state of the organic material. A more useful method for determination of the organic material is provided by the chemical oxygen demand (COD). This parameter takes into account almost all organic compounds and their oxidation state. Determinations of COD, however, require very small samples. In this paper, a procedure adapted for sampling of the organic fraction of fresh MSW is described. The method is based on progressive crushing and sampling steps, adopting two specific strategies. First, at all stages, the size of samples taken are a few orders of magnitude greater than the size of an average single particle of that sample. Secondly, at each stage the crushing and homogenization system is adapted to the sample size and the characteristics of the material. The accuracy of the proposed method has also been investigated for determining other parameters (TS, VS, etc.).
MATERIALS AND METHODS Substrate
The material used originated from the municipal solid waste treatment plant of the 'Intercommunale de Salubrit6 Publique Hennuybre' located in Mons, Belgium. No special precaution was taken in collecting a representative sample from the treatment plant of the so-called organic fraction, obtained by crushing MSW by an industrial hammer-mill and sieving at 50 nun. This material still presents a high degree of heterogeneity: containing paper, glass, organic material, etc. It was stored at - 20°C until use.
Sampling of municipal solid waste
93
Analytical methods Chemical oxygen demands (COD) were determined by the sulphuricacid-dichromate method I modified according to Leithe. 2 A more concentrated sulphuric acid solution (57% v vol-l in water rather than 50% v vol- 1), allows the reflux heating time to be reduced from 2 h to 10 mJn. The determination of TS, VS, ash and carbonate content was based on gravimetric measurements after heating at different temperatures to constant weight. The TS or total solids value corresponds to the weight after heating at 105°C overnight. The VS or volatile solids value is defined as the loss of weight between 105 and 450°C when heated overnight and is assumed to represent the organic matter. The residue after 3 h at 1000°C defines the ash content. The conventional 'CaCO 3' content value is calculated from the loss of weight between 450 and 1000°C (loss assumed to be CO2 from CaCO~), which is multiplied by 100/44, this being the ratio between the molecular weights of CaCO 3 and CO2. Triplicate measurements were made on each of three sub-samples from each of three samples, giving 27 values.
Sampling method A schematic representation of the sampling method is given in Fig. 1. A quantity of about 40 kg of MSW was discharged onto a platform and mixed for 15 rain with a shovel. No leachate was observed at this stage. Three samples of about 1500 g each (labelled A, B and C) were taken from three different places in the heap. Each of these samples was sieved with a 5 mm sieve. The fraction (about half the sample) which passed through the sieve contained most of the glass particles and other small hard pieces that would wear the mill. This was saved to be mixed with the remaining fraction, which did not pass through the sieve, after its subsequent treatment. The mainly organic fraction was then submitted to a manual sorting to discard the little pieces of hard material (essentially metallic) that would block or damage the mill rapidly. In the interpretation of the results, this material is assumed to have an impact only on the TS and ash contents. The discarded fraction represented 1.2% of the total solids. In order to shred the remaining larger pieces of the organic fraction, crushing was necessary. However, it proved impossible to crush this wet fraction, even in a frozen form, with a laboratory hammer mill due to its
94
G. H. Beckers, E.-J. Nyns, H. P. Naveau J HOMOC~NIZATION WITH A SHOVEL
Sam[~, A
Same procedure
I SIEVING AT 5mm
ll
]
J
'
I MAHUAL SORT,NG I ,
..,,,.
~
ROUGH CRUSHING ON HOUSEHOLD I CHOPPING KNIFE
I
t
J REMIXING AND SAMPLIH~ BY SUCCESSIVE FRACTIONS
[ S u b - sa
pie Aa
[
J
~ Sub-sampl i Ab .~ Sub-~amPle Ac Same p r o c e d u r e
I °`LUT'°H I t
J
FINE CRUSHING BY PLANETARY BALL GRINDER
J COD, TS, VS, "" MEASUREMENTS (triplicate/)
Fig. 1.
J J
Schematic representation of the sampling method.
high fibre content. Therefore, it was chopped by passing it several times through a domestic food processor ('Moulinette S', Moulinex, Gosselies, Belgium). This device has the advantage that both the knife and the jar may be replaced at low cost when damaged. This chopping was carried out with frozen material, but later experiments showed that this is not necessary. The chopped material was thoroughly mixed with the fraction that had passed through the 5 mm sieve. At this stage, three sub-samples of about 100 g (labelled a, b and c) were taken from each of the first samples. To do this, the material was spread evenly over the bottom of a rectangular container and divided into four equal parts by tracing diagonal lines. Two opposing triangles were eliminated and the remaining two were spread once again over the container. This process was repeated several times until the material remaining was of the size desired for sampling. Each of the 100 g sub-samples were diluted with 50 ml water and treated for 10 min in a 'Pulverisette 5' (Fritsch, Idar-Oberstein, West Germany) planetary ball grinder with chrome-nickel stainless steel jars (300 ml in volume) and one metallic ball of 40 mm in diameter per jar.
Sampling of municipal solid waste
95
The result of this fine crushing was a relatively homogeneous semi-solid sludge. Samples of this (about 0.3 g for C O D and 4 - 7 g for TS) were used for triplicate measurements of each parameter.
RESULTS A N D DISCUSSION Chemical oxygen demand (COD) The C O D values obtained are shown in Table 1 which also gives the mean value for all measurements and the corresponding coefficient of variation. The latter parameter corresponds to the ratio of sample standard deviation cr to mean value 2, expressed as a percentage? A n initial inspection was made by looking for aberrant values. A confidence interval can be calculated from 2 + at where t is the Student value at a probability of P = 0.95 and for a number N - 1 of values. All values that are outside this confidence interval can be considered as aberrant. TABLE 1 Chemical Oxygen Demand (COD)
Chemical oxygen demand (COD) (g CO1) kg 1)
Triplicates
Sample A
Sample B
Sample C
Sub-samples Aa Ab Ac
Sub-samples Ba Bb Bc
Sub-samples Ca Cb Cc
249 263 261
264 259 255
260 264 279
258 281 253
Mean (of 27 (24) replicates) Coeff. of variation (%) Mean (of triplicates) Coeff. of variation (%) Mean (of 3 (2) sub-samples) Coeff. of variation (%) Mean (of 3 samples) Coeff. of variation (%)
247 273 271
257 253 262
319 328 326
267 265 279
265 259 277
324 1-46
270 2"80
267 3-43
270(263)" 7"95( 3"58) 258 2.94
268 3'74 263 1.92
264 5"66
259 1'74
264 5.49
257 1'75
260 1.24
287(269) 11-2(0"88)
270(263) 5.50(1-67)
~'Numbers in brackets are results obtained after removal of aberrant values. In these cases, the mean values are calculated according to the number of replicates taken into consideration.
96
G. H. Beckers, E.-J. Nyns, H. P. Naveau
In the case of the COD values, this led to the elimination of three results out of 27. They corresponded to the triplicate values of the same sub-sample. Probably, the particular sampling step at this point was influenced by an unexplained accidental error. Interest was then focused on the 24 remaining results which showed a coefficient of variation of 3.6% giving a confidence interval of + 7.5%. This means that every new measurement made on the same substrate would have a 95% probability of being within this interval around the mean value. Taking into account the extreme heterogeneity of the original MSW substrate, we concluded that the precision of the COD measurement procedure, including sampling, was satisfactory. An attempt was made to determine which of the different sampling steps, in particular, led to the variability of the results. The second half of Table 1 gives coefficients of variation obtained among the replicates within each sub-sample, among the sub-samples within each sample and among the samples. For the non-aberrant data, the variabilities between the samples and between the sub-samples are low with a coefficient of variation below 2%. A statistical treatment, using the analysis of variance method applied to one-way classification for samples within samples, 4 shows no significant differences between the samples and between the sub-samples. A greater variability exists between the different replicates within a sub-sample. Although the method is quite satisfactory, this is the step eventually which needs to be improved if more accuracy is desired.
Other parameters Table 2 gives the results obtained, using the same sampling procedure, for other parameters. For the total solids content (TS) no aberrant values could be detected and the confidence interval was + 4%. As far as the volatile solids content (VS) was concerned it was necessary to eliminate six out of 27 samples. These data do not belong to any one sub-sample in particular, as was the case for the COD values. On the contrary, they are spread over different samples and sub-samples. Therefore, these cases probably do not arise from accidental sampling errors. The reason therefore must probably be found in the VS measurement technique itself. More work is being done to determine both the optimal heating time and the appropriate temperature for the measurement of municipal wastes. The calculations based on the remaining 21 non-aberrant values show that it should be possible to obtain a high precision on this parameter since the confidence interval is + 4.5%. As far as the COD/VS ratio is concerned, the results are of course dependent upon the variability of the two parameters involved.
97
Sampling of municipal solid waste TABLE 2
Summary of Results for COD, TS, VS, Ashes, 'CaCO3' and COD/VS Ratio of the Organic Fraction of MSW
COD TS VS COD/VS Ashes 'CaCOf
Number of aberrant values (out of 27)
Mean value a (g kg- /)
Coefficient of variation a (%)
Confidence interval ~
3 0 6 3 1 0
263 650 221 1.20 ~ 383 27
3'6 2'2 2-1 4.2 4-0 17.0
_+7"5 _+4.4 _+4-5 _+8.') _+8.2 _+34"8
Based on non-aberrant values only. bDimensionless. (For determination of aberrant values, see text.)
However, the variability of the ratio p r o v e d to be greater than that of each of the individual parameters concerned. This leads to the conclusion that the C O D and VS contents of M S W do not vary in the same way. As far as ash content is concerned, the observation of only one aberrant value and the narrow confidence interval of _+ 8-2% imply a relatively high precision. T h e results for the 'CaCO 3' content, on the other hand, show a very large variability with a confidence interval of + 34"8%. T h e r e are three reasons for this. First, in comparison with the other parameters, the ' C a C O 3' content values are very small. T h e same absolute error leads therefore to a greater relative error on this parameter. Secondly, the ' C a C O 3' content is calculated from the difference between two m e a s u r e m e n t s , namely the ash and VS contents, and is therefore influenced directly by two possible errors. Thirdly, this parameter includes the VS measurement, which was shown to present some imprecisions. Of course, i m p r o v e m e n t in the m e a s u r e m e n t of VS would improve the accuracy of estimation of the 'CaCO3' content.
CONCLUSIONS T h e sampling m e t h o d reported here, based on progressive crushing and sampling steps in such a way that particle size and sample size decrease simultaneously, results in a satisfactory precision for the C O D (chemical oxygen demand) content (+7-5%), for the TS (total solids) content ( _+4.5%) and for the ash content ( _+8-2%) of MSW.
98
G. H. Beckers, E.-J. Nyns, H. P. Naveau
Determination of the VS (volatile solids) content and the related COD/VS ratio proved to be less precise. To improve accuracy, further research is needed on the optimal heating time and temperature applied to the organic fraction of MSW. The 'CaCO 3' determination proved to be unsatisfactory when carried out using the proposed sampling technique.
ACKNOWLEDGEMENTS This research was supported by grant no. RNW-114-B from the Commission of the European Communities.
REFERENCES 1. Taras, M.-J., Greenberg, A. E., Hoak, R. D. & Rand, M. C. (1971). In: Standard methods for the examination of water and wastewater, 13th ed., American Public Health Association, Washington,DC, USA, pp. 286-8. 2. Leithe, W. (1970). Ein beschleunigtes Verfahren zur Bestimmung des chemischen Sauerstoffbedarfs in W~issern mit Kaliumpyrochromat, Osterreichische Abwasser Rundschau, 1fi, 25-8. 3. Lacroix, Y. (1962). Analyse chimique. Interpretation des R~sultats par le Calcul Statistique, Masson, Paris, France, 66 pp. 4. Snedecor, G. W. & Cochran, W. G. (1967). Statistical Methods, 6th ed., Iowa State University Press, Iowa, USA, 593 pp.
Representative Sampling of the Organic Fraction of Fresh Municipal Solid Waste G. H. Beckers, E.-J. Nyns and H. P. Naveau Unit of Bioengineering, University of Louvain, Place Croix du Sud, 1 bte 9, B- 1348 Louvain-la-Neuve, Belgium (Received 3 July 1986; accepted 22 August 1986) ABSTRACT A method has been developed for sampling the highly heterogeneous organic fraction of fresh municipal solid waste, especially for chemical oxygen demand (COD) determination. The method consists of progressive crushing, taking samples which are a few orders of magnitude greater than the size of an average single particle of that sample. A t each step, the crushing and homogenization system is adapted to the sample size and the characteristics of the material. The method developed permits the sampling of representative quantities as small as the 0 3 g necessary for COD measurements. Based on 24 replicate results, the method yields" a confidence level of plus or minus 7"5% at P = 0"95. The accuracy of the proposed sampling method in the measurement of total solids, volatile solids, ash and carbonate content has also been investigated. Key words: Municipal solid waste, sampling, chemical analysis, chemical oxygen demand (COD).
INTRODUCTION Municipal solid waste (MSW) is steadily increasing in importance as its potential value as a fuel is realized. For instance, there are m o r e and m o r e studies and projects dealing with the application of biomethanation to this substrate. Yet, because of the extremely heterogeneous nature of MSW, one of the first problems that arises is chemical characterization prior to design of any treatment process. In the literature, municipal solid wastes are usually characterized by subdivision into different classes (paper, plastics, glass, etc.). This classification does not always meet the needs of the engineer or the researcher 91 Biomass 0144-4565/86/S03.50- © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
92
G. H. Beckers, E.-J. Nyns, H. P. Naveau
who has to know exactly what is being treated. In particular there is a need for accurate quantitative measurement of the organic fraction of MSW. When MSW is considered in its totality, the measured parameters are generally its total solids (TS) and/or volatile solids (VS) content. These parameters are determined by heating the material to temperatures around 105 and 450 to 600°C respectively. This method of determination of the TS content has the disadvantage of not discriminating between the relative fractions of organic and inorganic material and the VS content is ascribed to the organic fraction. At the same time this method does not include products, such as alcohols and low molecular weight volatile organic acids, that have already volatilized during the heating step at 105°C. In addition, this parameter does not take into account the variable oxidation state of the organic material. A more useful method for determination of the organic material is provided by the chemical oxygen demand (COD). This parameter takes into account almost all organic compounds and their oxidation state. Determinations of COD, however, require very small samples. In this paper, a procedure adapted for sampling of the organic fraction of fresh MSW is described. The method is based on progressive crushing and sampling steps, adopting two specific strategies. First, at all stages, the size of samples taken are a few orders of magnitude greater than the size of an average single particle of that sample. Secondly, at each stage the crushing and homogenization system is adapted to the sample size and the characteristics of the material. The accuracy of the proposed method has also been investigated for determining other parameters (TS, VS, etc.).
MATERIALS AND METHODS Substrate
The material used originated from the municipal solid waste treatment plant of the 'Intercommunale de Salubrit6 Publique Hennuybre' located in Mons, Belgium. No special precaution was taken in collecting a representative sample from the treatment plant of the so-called organic fraction, obtained by crushing MSW by an industrial hammer-mill and sieving at 50 nun. This material still presents a high degree of heterogeneity: containing paper, glass, organic material, etc. It was stored at - 20°C until use.
Sampling of municipal solid waste
93
Analytical methods Chemical oxygen demands (COD) were determined by the sulphuricacid-dichromate method I modified according to Leithe. 2 A more concentrated sulphuric acid solution (57% v vol-l in water rather than 50% v vol- 1), allows the reflux heating time to be reduced from 2 h to 10 mJn. The determination of TS, VS, ash and carbonate content was based on gravimetric measurements after heating at different temperatures to constant weight. The TS or total solids value corresponds to the weight after heating at 105°C overnight. The VS or volatile solids value is defined as the loss of weight between 105 and 450°C when heated overnight and is assumed to represent the organic matter. The residue after 3 h at 1000°C defines the ash content. The conventional 'CaCO 3' content value is calculated from the loss of weight between 450 and 1000°C (loss assumed to be CO2 from CaCO~), which is multiplied by 100/44, this being the ratio between the molecular weights of CaCO 3 and CO2. Triplicate measurements were made on each of three sub-samples from each of three samples, giving 27 values.
Sampling method A schematic representation of the sampling method is given in Fig. 1. A quantity of about 40 kg of MSW was discharged onto a platform and mixed for 15 rain with a shovel. No leachate was observed at this stage. Three samples of about 1500 g each (labelled A, B and C) were taken from three different places in the heap. Each of these samples was sieved with a 5 mm sieve. The fraction (about half the sample) which passed through the sieve contained most of the glass particles and other small hard pieces that would wear the mill. This was saved to be mixed with the remaining fraction, which did not pass through the sieve, after its subsequent treatment. The mainly organic fraction was then submitted to a manual sorting to discard the little pieces of hard material (essentially metallic) that would block or damage the mill rapidly. In the interpretation of the results, this material is assumed to have an impact only on the TS and ash contents. The discarded fraction represented 1.2% of the total solids. In order to shred the remaining larger pieces of the organic fraction, crushing was necessary. However, it proved impossible to crush this wet fraction, even in a frozen form, with a laboratory hammer mill due to its
94
G. H. Beckers, E.-J. Nyns, H. P. Naveau J HOMOC~NIZATION WITH A SHOVEL
Sam[~, A
Same procedure
I SIEVING AT 5mm
ll
]
J
'
I MAHUAL SORT,NG I ,
..,,,.
~
ROUGH CRUSHING ON HOUSEHOLD I CHOPPING KNIFE
I
t
J REMIXING AND SAMPLIH~ BY SUCCESSIVE FRACTIONS
[ S u b - sa
pie Aa
[
J
~ Sub-sampl i Ab .~ Sub-~amPle Ac Same p r o c e d u r e
I °`LUT'°H I t
J
FINE CRUSHING BY PLANETARY BALL GRINDER
J COD, TS, VS, "" MEASUREMENTS (triplicate/)
Fig. 1.
J J
Schematic representation of the sampling method.
high fibre content. Therefore, it was chopped by passing it several times through a domestic food processor ('Moulinette S', Moulinex, Gosselies, Belgium). This device has the advantage that both the knife and the jar may be replaced at low cost when damaged. This chopping was carried out with frozen material, but later experiments showed that this is not necessary. The chopped material was thoroughly mixed with the fraction that had passed through the 5 mm sieve. At this stage, three sub-samples of about 100 g (labelled a, b and c) were taken from each of the first samples. To do this, the material was spread evenly over the bottom of a rectangular container and divided into four equal parts by tracing diagonal lines. Two opposing triangles were eliminated and the remaining two were spread once again over the container. This process was repeated several times until the material remaining was of the size desired for sampling. Each of the 100 g sub-samples were diluted with 50 ml water and treated for 10 min in a 'Pulverisette 5' (Fritsch, Idar-Oberstein, West Germany) planetary ball grinder with chrome-nickel stainless steel jars (300 ml in volume) and one metallic ball of 40 mm in diameter per jar.
Sampling of municipal solid waste
95
The result of this fine crushing was a relatively homogeneous semi-solid sludge. Samples of this (about 0.3 g for C O D and 4 - 7 g for TS) were used for triplicate measurements of each parameter.
RESULTS A N D DISCUSSION Chemical oxygen demand (COD) The C O D values obtained are shown in Table 1 which also gives the mean value for all measurements and the corresponding coefficient of variation. The latter parameter corresponds to the ratio of sample standard deviation cr to mean value 2, expressed as a percentage? A n initial inspection was made by looking for aberrant values. A confidence interval can be calculated from 2 + at where t is the Student value at a probability of P = 0.95 and for a number N - 1 of values. All values that are outside this confidence interval can be considered as aberrant. TABLE 1 Chemical Oxygen Demand (COD)
Chemical oxygen demand (COD) (g CO1) kg 1)
Triplicates
Sample A
Sample B
Sample C
Sub-samples Aa Ab Ac
Sub-samples Ba Bb Bc
Sub-samples Ca Cb Cc
249 263 261
264 259 255
260 264 279
258 281 253
Mean (of 27 (24) replicates) Coeff. of variation (%) Mean (of triplicates) Coeff. of variation (%) Mean (of 3 (2) sub-samples) Coeff. of variation (%) Mean (of 3 samples) Coeff. of variation (%)
247 273 271
257 253 262
319 328 326
267 265 279
265 259 277
324 1-46
270 2"80
267 3-43
270(263)" 7"95( 3"58) 258 2.94
268 3'74 263 1.92
264 5"66
259 1'74
264 5.49
257 1'75
260 1.24
287(269) 11-2(0"88)
270(263) 5.50(1-67)
~'Numbers in brackets are results obtained after removal of aberrant values. In these cases, the mean values are calculated according to the number of replicates taken into consideration.
96
G. H. Beckers, E.-J. Nyns, H. P. Naveau
In the case of the COD values, this led to the elimination of three results out of 27. They corresponded to the triplicate values of the same sub-sample. Probably, the particular sampling step at this point was influenced by an unexplained accidental error. Interest was then focused on the 24 remaining results which showed a coefficient of variation of 3.6% giving a confidence interval of + 7.5%. This means that every new measurement made on the same substrate would have a 95% probability of being within this interval around the mean value. Taking into account the extreme heterogeneity of the original MSW substrate, we concluded that the precision of the COD measurement procedure, including sampling, was satisfactory. An attempt was made to determine which of the different sampling steps, in particular, led to the variability of the results. The second half of Table 1 gives coefficients of variation obtained among the replicates within each sub-sample, among the sub-samples within each sample and among the samples. For the non-aberrant data, the variabilities between the samples and between the sub-samples are low with a coefficient of variation below 2%. A statistical treatment, using the analysis of variance method applied to one-way classification for samples within samples, 4 shows no significant differences between the samples and between the sub-samples. A greater variability exists between the different replicates within a sub-sample. Although the method is quite satisfactory, this is the step eventually which needs to be improved if more accuracy is desired.
Other parameters Table 2 gives the results obtained, using the same sampling procedure, for other parameters. For the total solids content (TS) no aberrant values could be detected and the confidence interval was + 4%. As far as the volatile solids content (VS) was concerned it was necessary to eliminate six out of 27 samples. These data do not belong to any one sub-sample in particular, as was the case for the COD values. On the contrary, they are spread over different samples and sub-samples. Therefore, these cases probably do not arise from accidental sampling errors. The reason therefore must probably be found in the VS measurement technique itself. More work is being done to determine both the optimal heating time and the appropriate temperature for the measurement of municipal wastes. The calculations based on the remaining 21 non-aberrant values show that it should be possible to obtain a high precision on this parameter since the confidence interval is + 4.5%. As far as the COD/VS ratio is concerned, the results are of course dependent upon the variability of the two parameters involved.
97
Sampling of municipal solid waste TABLE 2
Summary of Results for COD, TS, VS, Ashes, 'CaCO3' and COD/VS Ratio of the Organic Fraction of MSW
COD TS VS COD/VS Ashes 'CaCOf
Number of aberrant values (out of 27)
Mean value a (g kg- /)
Coefficient of variation a (%)
Confidence interval ~
3 0 6 3 1 0
263 650 221 1.20 ~ 383 27
3'6 2'2 2-1 4.2 4-0 17.0
_+7"5 _+4.4 _+4-5 _+8.') _+8.2 _+34"8
Based on non-aberrant values only. bDimensionless. (For determination of aberrant values, see text.)
However, the variability of the ratio p r o v e d to be greater than that of each of the individual parameters concerned. This leads to the conclusion that the C O D and VS contents of M S W do not vary in the same way. As far as ash content is concerned, the observation of only one aberrant value and the narrow confidence interval of _+ 8-2% imply a relatively high precision. T h e results for the 'CaCO 3' content, on the other hand, show a very large variability with a confidence interval of + 34"8%. T h e r e are three reasons for this. First, in comparison with the other parameters, the ' C a C O 3' content values are very small. T h e same absolute error leads therefore to a greater relative error on this parameter. Secondly, the ' C a C O 3' content is calculated from the difference between two m e a s u r e m e n t s , namely the ash and VS contents, and is therefore influenced directly by two possible errors. Thirdly, this parameter includes the VS measurement, which was shown to present some imprecisions. Of course, i m p r o v e m e n t in the m e a s u r e m e n t of VS would improve the accuracy of estimation of the 'CaCO3' content.
CONCLUSIONS T h e sampling m e t h o d reported here, based on progressive crushing and sampling steps in such a way that particle size and sample size decrease simultaneously, results in a satisfactory precision for the C O D (chemical oxygen demand) content (+7-5%), for the TS (total solids) content ( _+4.5%) and for the ash content ( _+8-2%) of MSW.
98
G. H. Beckers, E.-J. Nyns, H. P. Naveau
Determination of the VS (volatile solids) content and the related COD/VS ratio proved to be less precise. To improve accuracy, further research is needed on the optimal heating time and temperature applied to the organic fraction of MSW. The 'CaCO 3' determination proved to be unsatisfactory when carried out using the proposed sampling technique.
ACKNOWLEDGEMENTS This research was supported by grant no. RNW-114-B from the Commission of the European Communities.
REFERENCES 1. Taras, M.-J., Greenberg, A. E., Hoak, R. D. & Rand, M. C. (1971). In: Standard methods for the examination of water and wastewater, 13th ed., American Public Health Association, Washington,DC, USA, pp. 286-8. 2. Leithe, W. (1970). Ein beschleunigtes Verfahren zur Bestimmung des chemischen Sauerstoffbedarfs in W~issern mit Kaliumpyrochromat, Osterreichische Abwasser Rundschau, 1fi, 25-8. 3. Lacroix, Y. (1962). Analyse chimique. Interpretation des R~sultats par le Calcul Statistique, Masson, Paris, France, 66 pp. 4. Snedecor, G. W. & Cochran, W. G. (1967). Statistical Methods, 6th ed., Iowa State University Press, Iowa, USA, 593 pp.