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Hydrolysis rates, methane production and nitrogen solubilisation of grey waste components during anaerobic degradation
Bioresource Technology 96 (2005) 501–508
Hydrolysis rates, methane production and nitrogen solubilisation of grey waste components during anaerobic degradation J.P.Y. Jokela a, V.A. Vavilin b, J.A. Rintala a
a,*
Department of Biological and Environmental Science, University of Jyv€askyl€a, P.O. Box 35, FIN-40351 Jyv€askyl€a, Finland b Water Problems Institute, Russian Academy of Sciences, Gubkina str. 3, 119991 Moscow, Russia Received 15 May 2002; accepted 23 March 2004 Available online 6 August 2004
Abstract Municipal grey waste (i.e. the remaining fraction in municipal waste management systems in which putrescibles (biowaste) and other recyclables (paper, metals, glass) are source-segregated) was manually sorted into six main fractions on the basis of composition and also separated by sieving (100 mm mesh size) into two fractions, oversized and undersized, respectively. In practice, in waste management plant the oversized fraction is (or will be) used to produce refuse-derived fuel and the undersized landfilled after biological stabilisation. The methane yields and nitrogen solubilisation of the grey waste and the different fractions (all studied samples were first milled to 5 mm particle samples) were determined in a 237-day methane production batch assay and in a water elution test, respectively. The grey waste was found to contained remnants of putrescibles and also a high amount of other biodegradable waste, including packaging, cartons and cardboard, newsprint, textiles and diapers. These waste fractions comprised 41%-w/w of the grey waste and produced 40–210 m3 methane (total solids (TS))1 and less than 0.01 g NH4 -N kg TS1 added except diapers which produced 9.8 g NH4 -N kg TS1 added in the batch assays. In the case of the two sieved fractions and on mass bases, most of the methane originated from the oversized fraction, whereas most of the NH4 -N was solublised from the undersized fraction. The first-order kinetic model described rather well the degradation of each grey waste fraction and component, showing the different components to be in the range 0.021–0.058 d1 , which was around one-sixth of the values reported for the source-segregated putrescible fraction of MSW. 2004 Published by Elsevier Ltd. Keywords: Anaerobic degradation; Components; Grey waste; Hydrolysis rate; Landfill; Municipal solid waste; Methane; Nitrogen; Solubilisation; Source-segregation
1. Introduction The source-segregation of putrescibles and other recyclables from municipal solid waste (MSW) has been practised in many countries since the beginning of the 1990s, mainly as a result of the enactment of legislation to promote sustainable development and minimise the environmental effects of the landfill disposal of MSW. A putrescible fraction of MSW (PFMSW) around 25% of the total amount of MSW produced by households can be separated by source-segregation. Given that the PFMSW fraction is the main source of methane and nitrogen
* Corresponding author. Tel.: +358-14-2602-1211; fax: +358-142602-2321. E-mail address: jrintala@jyu.fi (J.A. Rintala).
0960-8524/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.biortech.2004.03.009
emissions from the landfill disposal of MSW, a high reduction in environmental emissions can be obtained by the source-segregation of putrescibles and separate stabilisation by e.g. anaerobic digestion (e.g. Pavan et al., 2000). The remaining proportion of MSW, so called ‘‘grey waste’’, is either landfilled or incinerated. Recent studies have shown that if landfilled, this residual grey waste continues to have a high methane (Scherer et al., 2000) and nitrogen emission potential (Jokela et al., 2002). Generally, the composition of MSW is dependant on consumer habits in the region, the waste management legislation in force, and the extent to which source-segregation is practised as well as the composition of the materials that end up as waste. Many studies have been done on the composition of unsorted MSW (reviewed by e.g. Barlaz et al., 1990; CEC, 1992) and some studies, in which the methane production rate has been included,
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have been done on the individual components of MSW (e.g. Eleazer et al., 1997). Regardless of variations in its composition, some of the main components of MSW can be readily identified. Grey waste may, for example, contain a high proportion of cellulose-containing waste components, like packaging, cardboard and newsprint, which have the potential to yield high amounts of methane (Clarkson and Xiao, 2000) if disposed of as landfill. In some countries, various mechanical–biological pretreatments have been or are to be introduced to decrease the amount and greenhouse gas production potential present in biodegradable MSW ending up in landfills. Mechanical treatment typically includes shredding, metal removal and sieving of the material. The sieved oversized fraction is generally used to produce refusederived fuel. Similarly, the undersized fraction is biologically stabilised before landfilling (e.g. Leikam et al., 1997). Along with aerobic treatment, anaerobic digestion has been proposed and applied as one option for the biological stabilisation of grey waste or the undersized fraction (Mata-Alvarez et al., 2000). Anaerobic degradation of complex organic matter has been described as a multistep process of series and parallel reactions in which several key groups of bacteria take part (Batstone et al., 2002). With respect to the rate of MSW degradation, numerous studies have been produced on the kinetics of the anaerobic degradation of different waste materials (reviewed by e.g. Sanders, 2001) such as putrescible kitchen waste (e.g. Veeken and Hamelers, 1999) or wastes from the food-processing industry (Salminen, 2001). On the other hand, before anaerobic digestion can be applied to grey waste more knowledge about the kinetics of degradation of its various components needs to be obtained. Such knowledge would help in selecting the appropriate method of pretreatment and in optimising the anaerobic digestion of grey waste. The objective of this study was to characterise grey waste by defining its composition using a sorting procedure and by sieving as well as to test the potential for methane production and nitrogen solubilisation of different waste components by means of an anaerobic incubation test. In addition, the degradation kinetics of the sorted components was determined and the main sources of methane and soluble nitrogen were traced. By these means, the pollution potential of grey waste and the rate and extent of anaerobic degradation of the sorted components were determined.
PFMSW (from estates with more than six households), metals, paper, cardboard, and glass were source-segregated by households, whereas the residual grey waste was mechanically processed at the waste management plant. The unprocessed grey waste was delivered in a 1.0 tonne bale wrapped in plastic film. The bale contained 2.5 m3 of waste. The bale was stored for about one month at ambient temperature. For this study, 0.5 m3 of the waste was shredded to a maximum size of 200 mm of which 0.2 m3 was sieved using a 100 mm mesh size. This was done to simulate the process carried out in the Tarastenj€arvi waste treatment plant and was as follows: magnetically removed waste was shredded to a maximum size of 200 mm and sieved (mesh size 100 mm; the undersized contained e.g. glass, gravel, ceramics, and some PFMSW), and the oversized was further shredded to a maximum size of 50 mm. The sieved fractions are henceforth referred as the undersized and the oversized (Table 1). Besides the grey waste samples, virgin computer printer paper (Canon Office A4, 80 g m2 ) was used in the present studies. All the samples were milled to a maximum particle size of 5 mm in a laboratory with a hammer mill (Retsch) before the anaerobic incubation test. 2.2. Sorting procedure About 1.5 m3 of the grey waste was manually sorted into six categories (Table 1) that would, according to previous sorting studies (Huovinen, 1994), represent the composition of the grey waste. The category of packaging included all packaging materials that consisted of either card alone or of card and layers of other materials such as plastic and/or aluminium. Packaging materials consisting solely of plastic and other plastic wastes were sorted into separate category. The category of cardboard consisted mainly of brown cardboard boxes and some other cardboard items, such as packaging. The category of textiles included textiles that were made of natural materials, like wool and cotton, as well as of synthetic materials, such as polyester. A considerable amount of newsprint as well as diapers were found in the grey waste sample, and these components were thus sorted into their own categories. The category of plastics was not studied for its pollution potential as plastics are considered biologically recalcitrant and thus do not contribute to the gaseous emissions. 2.3. Elution test
2. Methods 2.1. Waste Grey waste was obtained from Tarastenj€ arvi waste treatment plant in the Tampere region (Finland). The
The amounts of elutable soluble chemical oxygen demand (SCOD) and nitrogen in the different waste samples were tested in the elution test (CEN/TC 292/96, 1996). A portion (100 g) of the total solids (TS) of the sample was first added to deionised water in Duran glass vessels to obtain a liquid/solids (L/S) ratio of 2. The
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Table 1 The composition of the residual grey waste after source-segregation as received at the waste management plant and characteristics of the waste components and the sieved fractions as well as the office paper Grey waste component
(%-w/w)
TS (%)
VS (%)
VS/TS
Elutable SCOD (COD% of TS)
TKN (% of TS)
Elutable NH4 -N (% of TKN)
Packaging Cardboard Textiles Newsprints Diapers Plastics Oversizeda Undersizedc Grey waste Sorted fractions Miscellaneous unsorted Office paper
4.4 10 12 11 3.6 15 68b 32b 100 56 44
84 87 91 86 38 n.d. 85 78 84 n.d. n.d.
72 67 84 59 29 n.d. 70 54 58 n.d. n.d.
0.85 0.77 0.92 0.68 0.78 n.d. 0.82 0.69 0.69 n.d. n.d.
0.02 0.02 0.01 0.01 0.09 n.d. 0.01 0.02 0.02 n.d. n.d.
0.03 0.04 0.06 0.06 1.5 n.d. 0.25 0.32 0.32 n.d. n.d.
23 100 19 0 7.2 n.d. 4 16 29 n.d. n.d.
n.d.
97
72
0.74
0.02
0.02
n.d.
n.d.: Not determined. a Oversized from a 100 mm mesh size sieve. b Data supplied by the waste management company. c Undersized from a 100 mm mesh size sieve.
samples were stirred in a rotary shaker for 8 h, after which the liquid samples (50 ml) were collected. Deionised water was then added to yield a L/S ratio of 8 (overall L/S ratio of 10). The samples were further stirred in a rotary shaker for 16 h and the liquid samples were collected. All the liquid samples were filtered through glass fibre filters (Schleicher & Schuell). The procedures were performed at 22 ± 2 C. The concentrations after the elution test were expressed as the total cumulative amount of released SCOD or nitrogen per kg TS obtained from the elutions with L/S ratios 2 and 8. 2.4. Biochemical methane potential (BMP) test The BMP assays were carried out in three replicates with the different waste samples at 35 C in 2 l glass vessels for 237 days. The waste samples were added to the vessels to obtain the ratio 1.5 g-volatile solids (VS)/ g VSinoculum using 0.5 l of mesophilically digested municipal sewage sludge (TS ¼ 3.4%, VS ¼ 1.8%) from Nen€ ainniemi (Jyv€ askyl€ a, Finland) as the inoculum. A nutrient solution was added to ensure the availability of micronutrients and trace metals (Lepist€ o and Rintala, 1997), and 5.5 g l1 sodium bicarbonate was added as buffer. The final liquid volume of 1.6 l in each test vessel was obtained by the addition of deionised water. The inoculum was assayed separately so as to be able to subtract its methane production from those of the samples. The gas phases of the vessels were flushed with N2 /CO2 (80%/20%) and the vessels were sealed with butyl rubber stoppers. The gas produced was led through Viton tubing to aluminium gas sampling bags (Tecobag PETP/AL/PE-12/12/75 of 10 l, Tesseraux Spezialverpacknungen).
2.5. Analyses The volume of biogas produced was measured by a displacement method. Methane concentration was analysed with a Perkin Elmer Autosystem XL gas chromatograph, equipped with a flame-ionisation detector and fitted with a PE Alumina column (30 m · 0.53 mm). The carbon dioxide and oxygen content of the biogas were determined with an IR analyser (Geotechnical Instruments). TS and VS were measured according to Standard Methods (American Public Health Association, 1998). COD (dichromate) was determined according to Finnish Standards (SFS standard 5504, 1988). pH was measured with a 774 pH meter (Metrohm). Total Kjeldahl nitrogen (TKN) analyses were performed according to the Tecator application procedure (Perstorp Analytical/Tecator AB, 1995) with the same Kjeltec after digestion with a 2006 digestion. Ammonium nitrogen (NH4 -N) was determined by using a Kjeltec system 1002 distilling unit (Perstorp Analytical/Tecator AB) in accordance with Standard Methods (American Public Health Association, 1998). 2.6. Calculations The cumulative methane yield of each waste material was calculated by dividing its cumulative methane production by the amount of waste added to each vessel. The rate of degradation of the materials was assumed to follow a first-order rate of degradation, and thus the following formula was used to describe the methane production of each category of material: Y ¼ Yl ð1 exp½k tÞ;
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where Y represents the cumulative methane yield at time t, Yl represents the ultimate methane yield (m3 tTS1 added ) of the sample and k is the first-order rate constant. The parameters Yl and k were estimated by using non-linear regression fit to the yield data of a triplicate set with SPSS for Windows 10.1.3 software.
3. Results 3.1. Waste sorting and elutable nitrogen and organic matter in the sorted fractions The composition and the characteristics of the waste fractions in the source-segregated grey waste as received at the waste management plant are presented in Table 1. Fifty-six percent w/w of the grey waste was sorted into six waste categories (packaging, cardboard, textiles, newsprint, diapers and plastics), leaving 44% of miscellaneous waste that did not fall into the six categories. The miscellaneous residual undefined waste contained small unclassified items e.g. mixtures of putrescibles, plastics and metals, and that would probably end up in the undersized fraction. The data indicated that the main components of the grey waste per wet weight were plastics, textiles, newsprints and cardboard, which accounted for 15%, 12%, 11% and 10% w/w of the grey waste, respectively, whereas packaging and diapers represented 4.4% and 3.6% w/w of the grey waste (Table 1). The high content of inorganic materials in the undersized fraction was indicated by its low VS/TS ratio. Generally, the waste components had a VS/TS ratio of between 0.77 and 0.92, except for newsprint, which had a ratio of 0.68. The low fraction of SCOD in the TS (Table 1) obtained in the elution test indicated that the organic matter was mostly present in the sorted
grey waste components as solids. The amount of elutable SCOD was below 0.02%-TS, except in the diapers, where it was 0.09%-TS. The office paper consisted almost solely of insoluble organic matter, as indicated by a high TS and VS/TS ratio and low elutable SCOD. Total nitrogen content was low in most of the sorted components (between 0.03% and 0.06% of TS) with the exception of the diapers, which had a clearly higher TKN content (1.5% of TS). Also, the elutable NH4 -N fraction of the TKN was low in most of the sorted components (below 23%), whereas in the cardboard all of the TKN was eluted as NH4 -N. The undersized fraction had a somewhat higher TKN content than the oversized fraction. Only 4% of TKN was elutable from the oversized fraction, whereas the values were clearly higher in the undersized fraction (16%) and in the grey waste (29%). 3.2. Batch assays The specific methane yields from the triplicate vessels of each waste component after 237 days, and from those prepared in a previous study (Jokela et al., 2002) after 110 days with the putrescibles incubated at 35 C, together with the estimates of the ultimate methane yield (Yl ) and the first-order rate constant are presented in Table 2. Methane production began within 3 days in all samples. Most of the methane was produced within 43 and 66 days (data not shown). Of the grey waste components, the highest methane yield during the 237 days of the assay was from textiles (average 210 m3 tTS1 ) (Table 2). The packaging, diapers and cardboard yielded between 140 and 168 m3 tTS1 , whereas the newsprint yielded considerably less methane (40 m3 tTS1 ). The methane yields of the undersized and oversized fractions were similar but higher than that of the grey waste (101
Table 2 The specific methane yields of the grey waste components during the 237 days of the biochemical methane potential test (BMP) at 35 C with added methanogenic inoculum and the estimated values of the ultimate (Yl ) and the first-order kinetic constant (k) (standard deviations in parenthesis) Grey waste component
Specific methane yield 3
(m t Packaging Cardboard Textiles Newsprint Diapers Oversizeda Undersizedb Grey waste Office paper Putresciblesc a
118 146 192 34 60 127 118 85 243 67
waste1 added )
(81) (25) (99) (6.1) (5.1) (26) (5.4) (2.4) (18) (6.9)
Oversized from a 100 mm mesh size. Undersized from a 100 mm mesh size. c From a similar batch assay (Jokela et al., 2002). b
(m
tTS1 added )
(m
tVS1 added )
140 168 210 40 158 150 151 101 252 410
(96) (29) (109) (7.1) (13) (31) (6.9) (2.8) (18) (42)
165 217 228 58 204 182 219 147 340 527
(112) (38) (118) (10) (18) (38) (10) (4.1) (24) (54)
3
3
Yl (m3 tTS1 added )
k (d1 )
150 (100) 183 (32) 238 (121) 43.6 (7.0) 177 (14) 166 (35) 170 (7.3) 112 (3.8) 277 (21) 426 (42)
0.058 0.046 0.021 0.056 0.025 0.031 0.026 0.031 0.036 0.107
(0.01) (0.009) (0.001) (0.008) (0.003) (0.003) (0.002) (0.001) (0.006) (0.04)
J.P.Y. Jokela et al. / Bioresource Technology 96 (2005) 501–508
m3 tTS1 ). The grey waste also yielded significantly less methane than did the PFMSW in our previous assay (Jokela et al., 2002; Table 2) or the office paper in the present assay (mean yield 252 m3 tTS1 ). The value of Yl and the first-order kinetic rate constant are presented in Table 2. Regardless of the differences in the specific methane yields, the differences between those yields and the values of Yl seem to be fairly small. Also, the high coefficients of determination (above 0.88 with the methane yields of all the individual vessels, values not shown) indicated that the first-order kinetic model showed a good fit with the methane yield data. The grey waste and different components produced methane at a significantly lower rate (k-value between 0.021 and 0.058 d1 ) than the PFMSW (Table 2). The highest rate of production was from the packaging and newsprint, whereas the textiles and diapers produced the lowest rate (k-values of 0.021 and 0.025 d1 respectively). Also, the undersized fraction produced methane at a somewhat lower rate than the oversized fraction. The office paper showed approximately the same rate as the grey waste. The degradation of solids (VS and TS) and the VS/TS ratio as well as the specific NH4 -N solubilisation of each of the grey waste components and fractions after 237 days of anaerobic incubation are presented in Table 3. A high reduction in VS (more than 40%) was obtained for all the grey waste components except the newsprint. A higher VS reduction was obtained with the oversized fraction of the grey waste (51%) compared to the undersized fraction and the grey waste (both 35%). In most of the samples the VS/TS ratio was between 0.43 and 0.47, except in the textile and office paper samples, which had ratios of 0.37 and 0.31, respectively. Significantly more (approximately 10-fold) NH4 -N was solublised from the undersized compared to oversized fraction of the grey waste during the 237 days of anaerobic incubation. The grey waste sample yielded
505
approximately one-third of the amount produced by the undersized fraction. The diaper was the only grey waste component that produced a significant amount of soluble nitrogen (9.8 g NH4 -N kg TS1 added ).
4. Discussion 4.1. The composition of grey waste e The present sorting procedure showed that, along with the remnants of putrescibles, a high amount of other biodegradable waste components was present in the grey waste, including packaging cartons (4.4%) and cardboard (10%), newsprint (11%), textiles (12%) and diapers (3.6%). Together these comprised 41% of the grey waste. Previously, in a similar sorting procedure with grey waste the proportion of the same components was 40%, made up of 16% of various packaging materials (multilayer cartons, cardboard and paper), 5% newsprint, magazines and office paper, 8% textiles as well as 9% diapers (Huovinen, 1994). In the same study plastics contributed 11% of the grey waste, and putrescibles 26%. Putrescibles were omitted in our study, but the miscellaneous unsorted fraction probably contained residuals of putrescibles, as in packaging, which is often contaminated with putrescibles. On the basis of the data presented in this study and the waste component data obtained from the waste management authority in the region of origin, the total proportion of the biodegradable waste component was estimated to be around 70% w/w of the grey waste, which was made up of 60% w/w non-putrescible and 40% putrescible components. For unsorted MSW the proportion of non-putrescible biodegradable waste has been between 20% and 42% (reviewed by CEC, 1992; reviewed by Barlaz et al., 1990). On the other hand, in a previous study the grey waste fraction contributed 51%-w/w of total MSW (Huovinen,
Table 3 Reduction of solids (TS and VS), VS/TS and specific NH4 -N solubilisation of the grey waste and its different components (inoculum subtracted) as well as NH4 -N/TKN ratio of samples (inoculum included) after the 237 days of the biochemical methane potential test (BMP) at 35 C with added methanogenic inoculum (standard deviations in parenthesis) Grey waste component
TSred: (% of waste
Packaging Cardboard Textiles Newsprint Diapers Oversizeda Undersizedb Grey waste Office paper
52 50 70 29 40 49 31 33 73
a b
added )
(7.6) (4.1) (3.1) (1.3) (1.2) (4.1) (1.1) (1.1) (0.6)
Oversized from a 100 mm mesh size sieve. Undersized from a 100 mm mesh size sieve.
VSred: (% of wasteadded )
VS/TS (of waste)
52 54 79 27 43 51 35 35 88
0.46 0.43 0.37 0.47 0.46 0.45 0.45 0.45 0.31
(4.7) (12.8) (2.9) (1.0) (2.3) (3.5) (0.4) (2.0) (2.7)
(0.03) (0.09) (0.2) (0.01) (0.02) (0.01) (0.02) (0.01) (0.01)
NH4 -N/TKN (of waste + inoculum) 0.53 0.52 0.51 0.49 0.69 0.9 0.59 0.58 0.56
Specific NH4 -N solubilisation (gN kg TS1 added ) <0.01 <0.01 <0.01 <0.01 9.8 (0.44) 0.32 (0.05) 3.5 (0.25) 1.0 (0.003) <0.01
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1994), whereas in our study the grey waste accounted for approximately 75% of the household MSW collected, according to the waste management company. It is assumed that a significant proportion of the non-putrescible biodegradable components would end up in the oversized fraction after mechanical treatment, whereas the putrescibles in the grey waste would end up in the undersized fraction.
may be incorporated in e.g. woody biomass that remains fairly intact even during long-term anaerobic incubation. The great differences in NH4 -N yield between the oversized and undersized fraction suggest that the presently applied separation of fractions by sieving is an effective method of reducing the nitrogen load from the waste management if the undersized fraction is properly treated and end-disposed.
4.2. Nitrogen solubilisation from the grey waste fractions
4.3. Extent of methane yield from the grey waste components
A high amount of nitrogen (Tables 3 and 4) was solublised in the form of NH4 -N from the undersized fraction of the grey waste during methanation, whereas only 16% of TKN from the undersized fraction was eluted at the start (Table 1), indicating that almost all of the NH4 -N was produced during the 237-day exhaustive BMP test. In contrast, even more NH4 -N was eluted from the oversized fractions of the grey waste during the elution test than during methanation of the sample, indicating that almost all of the NH4 -N in the oversized fraction was present as soluble NH4 -N contaminant. The high level of NH4 -N production from the undersized fraction probably resulted from the remnants of protein-containing putrescibles, as the solublised NH4 N could not be traced to any of the sorted components (Table 4). Previously, up to 7.9 g NH4 -N kg TS1 was eluted from source-segregated putrescibles after a 51day aerobic lysimeter experiment (Jokela et al., 2002), which accounted for approximately 100% of the TKN, whereas in the present study only 1.0 g NH4 -N kg TS1 was eluted from the grey waste after the 237-day anaerobic incubation experiment. Regardless of the high proportion of solublised nitrogen (Table 3), the total contribution of diapers to the amount of TKN and the NH4 -N solubilisation was minor, 7.6% and 3.4% respectively (Table 4). A small proportion of TKN, namely one-third and one-tenth of the grey waste and the oversized fraction, was solublised. This indicates the presence of a nitrogen fraction in the waste that is highly resistant to biological solubilisation. These fractions
Most of the residual grey waste components (packaging, cardboard, textiles and diapers) seem to yield high amounts of methane in the presence of a proper methanogenic inoculum. Previously, in a similar BMP study, methane yields of between 318 and 343 m3 tVS1 were obtained for packaging materials, including milk cartons, wax paper and food board (Owens and Chynoweth, 1993). The evidently higher yields compared to the present study may be due to the 3.3-times larger maximum particle size used in the present study or the higher content of plastics in form of protective film on cartons and cardboard in the waste in the present study. This may have impeded the hydrolysis of solids by decreasing the access of hydrolytic enzymes to the cellulose-containing waste components, as previously discussed by Owens and Chynoweth (1993). This was not the case with the newsprint, which yielded around half of the methane in the present study compared to the study by Eleazer et al. (1997) (74 m3 tTS1 ), whereas the methane yield from office paper was similar in both studies. It is noteworthy that the maximum particle size of the piece of paper in that study was 2 · 5 cm as against 0.5 · 0.5 cm in the present study. Thus, a high concentration of inorganic clay filler and lignin in the paper probably impeded the complete destruction of VS in the present study. The high concentration of inorganic clay filler can be seen in the somewhat lower VS content in the newsprint and in the office paper if compared to the content in the packaging. Therefore,
Table 4 Contribution of the grey waste components to the amount of TS, VS, TKN and to methane yield as well as NH4 -N solubilisation of the grey waste calculated on the basis of the present sorting test and after the 237 days of the biochemical methane potential test (BMP) Waste component
TS (%)
VS (%)
TKN (%)
CH4 yield (%)
NH4 -N solubilisation (%)
Packaging Cardboard Textiles Newsprint Diapers Oversizeda Undersizedb
4.4 10 13 11 1.6 69 30
5.5 12 17 11 1.8 82 29
0.41 1.3 2.4 2.1 7.6 54 30
6.1 17 27 4.4 2.5 102 44
n.d. n.d. n.d. n.d. 3.4 28 104
n.d.: Not determined. a Oversized from a 100 mm mesh size sieve. b Undersized from a 100 mm mesh size sieve.
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the characteristics of the waste component, e.g. the method of paper and carton production used, effects the rate and extent of methanation and destruction of solids. On the other hand, compared to newsprint and packaging the anaerobic incubation of office paper and cardboard resulted in a high rate and extent of methanation as well as a high level of VS destruction. Previously, an 80% reduction in cellulose was obtained for newsprint after 300 days anaerobic incubation (Clarkson and Xiao, 2000). Alkali pre-treatment further increased the degradation by 10%. In the same study, a methane yield around 60% lower was obtained with 0.6 · 5 cm particles of newsprint and office paper compared to paper samples ground into powder. Nevertheless, the high cellulose-containing components of the grey waste will eventually degrade and yield high amounts of methane. Methanation may thus be applied as a biological method of treatment before landfill disposal of the grey waste. The non-putrescible biodegradable fraction contributed almost half of the TS and VS of the grey waste, and together these waste components accounted for 57% of the methane yield of the grey waste (Table 4). The highest contribution was from textiles, which produced 13%-TS, 17%-VS and 27% of the total methane yield. In addition, the discarded miscellaneous unsorted fraction may also have yielded some methane. The various results obtained from different tests and analyses can be seen as discrepancies in the contributions of the undersized and oversized fractions (sum more than 100%). In comparison to the undersized fraction, the oversized fraction evidently made a higher contribution to the amount of solids (both TS and VS) as well as to the methane yield. 4.4. Kinetics of solid hydrolysis for grey waste components The estimated curve with the values of Yl plotted against the specific methane yield showed that the firstorder kinetic model described rather well the degradation of each grey waste fraction. The high standard deviation of Yl and low standard deviation of k indicates that Yl was dependant on the sample in the individual vessel, whereas the k-value was independent of the sample. Thus, the degradable matter in the individual waste components degraded at the same rate, whereas the variation between the samples of the individual waste components in the concentration of degradable organic matter (as VS) resulted in a high deviation in Yl . This is especially noticeable in the Yl of textiles and packaging. In a previous study (Owens and Chynoweth, 1993), slightly higher constant was obtained for the waste components of newsprint (between 0.069 and 0.084 d1 ) and for the component of corrugated unbleached cardboard (0.058 d1 ) to those of the present study (newsprint 0.056 d1 and cardboard 0.046 d1 ),
507
whereas the constants for packaging (between 0.099 and 0.119 d1 ) and especially for office paper (0.136 d1 ) were clearly higher than ours (packaging 0.058 d1 and office paper 0.036 d1 ). This might result from differences between the studies in terms of the types and amounts of plastic film (packaging) used or amount of clay filler (office paper) in the waste components, which in our study might have decreased the transfer of mass from the waste particles. Also, the kinetic rate constants obtained for the grey waste and for the sieved fractions were somewhat lower (0.031 d1 ) than that obtained previously (0.075 d1 ) for unsorted MSW (Owens and Chynoweth, 1993). The range of the constant for putrescible components has been reported to be between 0.10 and 0.35 d1 (Veeken and Hamelers, 1999). A similar constant to that obtained in our present study (between 0.038 and 0.048 d1 ) has been reported for putrescibles studied in 5 m3 reactors with recirculation (ten Brummeler et al., 1991), but as previously noted by Veeken et al. (2000), the conditions were not optimal for the hydrolysis of solids. Nevertheless, the present study indicates that the first-order rate constant for the grey waste components was in the range of 0.021 and 0.058 d1 , whereas for the putrescibles from our previous study (Jokela et al., 2002) the first-order rate constant was 0.11 d1 .
5. Conclusions The grey waste produced after the source-segregation contained a high amount of biodegradable waste components, including packaging, cartons and cardboard, newsprint and also textiles and diapers, which together comprised 41%-w/w of the grey waste. The total proportion of the biodegradable waste components was around 70%-w/w of the grey waste. Under anaerobic conditions and in the presence of methanogenic microorganisms, these components yielded 40–210 m3 methane (tTS1 ) and during complete methanation <0.1 g NH4 -N kg TS1 added were produced except from diapers, which produced 9.8 g NH4 -N kg TS1 added . The grey waste yielded 101 m3 methane tTS1 . On the mass bases, the oversized fraction produced more methane than the undersized fraction. The NH4 -N was solublised mainly from the undersized fraction obtained after sieving the grey waste through a 100 mm mesh, and thus the nitrogen pollution stream from waste management is potentially controllable by reducing the particle size and sieving of grey waste. The curve estimated from the values of Yl and the data on specific methane yields showed that the first-order kinetic model gave rather a good description of the degradation of each of the grey waste fractions. The first-order rate constant of the grey waste components was in the range of 0.021 and 0.058 d1 ,
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which is around one-sixth of the values reported previously for source-segregated putrescibles.
Acknowledgements This study was sponsored by the Finnish Graduate School in Environmental Science and Technology (EnSTe). We wish to thank the personnel at the Tarastenj€ arvi landfill for providing the waste samples as well as associate professor Pedro Aphalo for valuable expertise in the statistical analysis.
References American Public Health Association, American Water Works Association & Water Environment Federation, 1998. In: Clesceri, L.S., Greenberg A.E., Eaton A.D. (Eds.), Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association. Washington DC, USA. Batstone, D.J., Keller, Angelidaki, I., Kalyuzhnyi, S.V., Pavlostathis, S.V., Rozzi, A., Sanders, W.T.M., Siegrist, H., Vavilin, V.A. Anaerobic Digestion Model No. 1. Scientific and Technical Report No. 13. IWA Task Group for Mathematical Modelling of Anaerobic Digestion Processes. IWA Publishing, London. Barlaz, M.A., Ham, R.K., Schaefer, D.M., 1990. Methane production from municipal refuse: a review of enhancement techniques and microbial dynamics. Crit. Rev. Env. Control 19, 557–584. CEN/TC 292/96, 1996. Draft prEN 12457. Characterisation of wasteleaching–compliance test for leaching of granular waste materials. Determination of leaching of constituents from granular waste materials and sludges. European Committee for Standardisation. CEC (Commission of the European Communities), 1992. Landfill gas. From environment to energy. In: Gendebien A., Pauwels, M., Constant, M., Ledrut-Damanet, M.-J., Nyns, E.-J., Willumsen, H.C., Butson, J., Fabry, R., Ferrero, G.-L. (Eds.), CEC. Clarkson, W.W., Xiao, W., 2000. Bench-scale anaerobic bioconversion of newsprint and office paper. Wat. Sci. Tech. 41, 93–100. Eleazer, W.E., Odle, W.S., Wang, Y.-S., Barlaz, M.A., 1997. Biodegradability of municipal solid waste components in laboratoryscale landfills. Environ. Sci. Technol. 13, 911–917. Huovinen, H., 1994. Waste collection experiment with four bins. Technical Research Centre of Finland (VTT) (in Finnish).
Jokela, J.P.Y., Kettunen, R.H., Rintala, J.A., 2002. Methane and leachate pollutant emission potential from various fractions of municipal solid waste (MSW): effects of source segregation and aerobic treatment. Waste Manage. Res. 20, 424–433. Leikam, K., Heyer, K.-U., Stegmann, R., 1997. Mechanical–biological pretreatment of residual municipal solid waste and the landfill behaviour of pretreated waste. In: Proceedings Sardinia 97, Sixth International Landfill Symposium, 13–17 October 1997, S. Margherita di Pula, Cagliari, Italy, vol. 1, pp. 267–278. Lepist€ o, S., Rintala, J., 1997. Start-up and operation of thermophilic upflow anaerobic sludge blanket reactor treating vegetable processing wastewaters. J. Chem. Technol. Biotechnol. 68, 331–339. Mata-Alvarez, J., Mace, S., Llabres, P., 2000. Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Biores. Technol. 74, 3–16. Owens, J.M., Chynoweth, D.P., 1993. Biochemical methane potential of municipal solid waste components. Wat. Sci. Tech. 27, 1–14. Pavan, P., Battistoni, P., Cecchi, F., Mata-Alvarez, J., 2000. Twophase anaerobic digestion of source sorted OFMSW (organic fraction of municipal solid waste): performance and kinetic study. Wat. Sci. Tech. 41, 111–118. Perstorp Analytical/Tecator AB, 1995. The determination of nitrogen according to kjeldahl using block digestion and steam distillation. Tecator application note. Salminen, E., 2001. Anaerobic digestion of solid poultry slaughterhouse by-products and wastes. Ph.D. thesis, University of Jyv€askyl€a, Finland. Sanders, W.T.M., 2001. Anaerobic hydrolysis during digestion of complex substrates. Ph.D. thesis, Wageningen University, The Netherlands. Scherer, P.A., Vollmer, G.R., Fakhain, T., Martensen, S., 2000. Development of a methanogenic process to degrade exhaustively the organic fraction of municipal ‘‘grey waste’’ under thermophilic and hyperthermophilic conditions. Wat. Sci. Tech. 41, 83–91. SFS standard 5504, 1988. Determination of chemical oxygen demand (CODcr ) in water with the closed tube method, oxidation with dichromate. Finnish Standard Association, Helsinki, Finland (in Finnish). ten Brummeler, E., Horbach, H.C.J.M., Koster, I.W., 1991. Dry anaerobic batch digestion of the organic fraction of municipal solid waste. J. Chem. Tech. Biotechnol. 50, 191–209. Veeken, A., Hamelers, B., 1999. Effect of temperature on hydrolysis rate of selected biowaste components. Biores. Technol. 69, 249– 254. Veeken, A., Kalyuzhnyi, S., Scharff, H., Hamelers, B., 2000. Effect of pH and VFA on hydrolysis of organic solid waste. J. Environ. Eng. 126, 1076–1080.
Hydrolysis rates, methane production and nitrogen solubilisation of grey waste components during anaerobic degradation J.P.Y. Jokela a, V.A. Vavilin b, J.A. Rintala a
a,*
Department of Biological and Environmental Science, University of Jyv€askyl€a, P.O. Box 35, FIN-40351 Jyv€askyl€a, Finland b Water Problems Institute, Russian Academy of Sciences, Gubkina str. 3, 119991 Moscow, Russia Received 15 May 2002; accepted 23 March 2004 Available online 6 August 2004
Abstract Municipal grey waste (i.e. the remaining fraction in municipal waste management systems in which putrescibles (biowaste) and other recyclables (paper, metals, glass) are source-segregated) was manually sorted into six main fractions on the basis of composition and also separated by sieving (100 mm mesh size) into two fractions, oversized and undersized, respectively. In practice, in waste management plant the oversized fraction is (or will be) used to produce refuse-derived fuel and the undersized landfilled after biological stabilisation. The methane yields and nitrogen solubilisation of the grey waste and the different fractions (all studied samples were first milled to 5 mm particle samples) were determined in a 237-day methane production batch assay and in a water elution test, respectively. The grey waste was found to contained remnants of putrescibles and also a high amount of other biodegradable waste, including packaging, cartons and cardboard, newsprint, textiles and diapers. These waste fractions comprised 41%-w/w of the grey waste and produced 40–210 m3 methane (total solids (TS))1 and less than 0.01 g NH4 -N kg TS1 added except diapers which produced 9.8 g NH4 -N kg TS1 added in the batch assays. In the case of the two sieved fractions and on mass bases, most of the methane originated from the oversized fraction, whereas most of the NH4 -N was solublised from the undersized fraction. The first-order kinetic model described rather well the degradation of each grey waste fraction and component, showing the different components to be in the range 0.021–0.058 d1 , which was around one-sixth of the values reported for the source-segregated putrescible fraction of MSW. 2004 Published by Elsevier Ltd. Keywords: Anaerobic degradation; Components; Grey waste; Hydrolysis rate; Landfill; Municipal solid waste; Methane; Nitrogen; Solubilisation; Source-segregation
1. Introduction The source-segregation of putrescibles and other recyclables from municipal solid waste (MSW) has been practised in many countries since the beginning of the 1990s, mainly as a result of the enactment of legislation to promote sustainable development and minimise the environmental effects of the landfill disposal of MSW. A putrescible fraction of MSW (PFMSW) around 25% of the total amount of MSW produced by households can be separated by source-segregation. Given that the PFMSW fraction is the main source of methane and nitrogen
* Corresponding author. Tel.: +358-14-2602-1211; fax: +358-142602-2321. E-mail address: jrintala@jyu.fi (J.A. Rintala).
0960-8524/$ - see front matter 2004 Published by Elsevier Ltd. doi:10.1016/j.biortech.2004.03.009
emissions from the landfill disposal of MSW, a high reduction in environmental emissions can be obtained by the source-segregation of putrescibles and separate stabilisation by e.g. anaerobic digestion (e.g. Pavan et al., 2000). The remaining proportion of MSW, so called ‘‘grey waste’’, is either landfilled or incinerated. Recent studies have shown that if landfilled, this residual grey waste continues to have a high methane (Scherer et al., 2000) and nitrogen emission potential (Jokela et al., 2002). Generally, the composition of MSW is dependant on consumer habits in the region, the waste management legislation in force, and the extent to which source-segregation is practised as well as the composition of the materials that end up as waste. Many studies have been done on the composition of unsorted MSW (reviewed by e.g. Barlaz et al., 1990; CEC, 1992) and some studies, in which the methane production rate has been included,
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have been done on the individual components of MSW (e.g. Eleazer et al., 1997). Regardless of variations in its composition, some of the main components of MSW can be readily identified. Grey waste may, for example, contain a high proportion of cellulose-containing waste components, like packaging, cardboard and newsprint, which have the potential to yield high amounts of methane (Clarkson and Xiao, 2000) if disposed of as landfill. In some countries, various mechanical–biological pretreatments have been or are to be introduced to decrease the amount and greenhouse gas production potential present in biodegradable MSW ending up in landfills. Mechanical treatment typically includes shredding, metal removal and sieving of the material. The sieved oversized fraction is generally used to produce refusederived fuel. Similarly, the undersized fraction is biologically stabilised before landfilling (e.g. Leikam et al., 1997). Along with aerobic treatment, anaerobic digestion has been proposed and applied as one option for the biological stabilisation of grey waste or the undersized fraction (Mata-Alvarez et al., 2000). Anaerobic degradation of complex organic matter has been described as a multistep process of series and parallel reactions in which several key groups of bacteria take part (Batstone et al., 2002). With respect to the rate of MSW degradation, numerous studies have been produced on the kinetics of the anaerobic degradation of different waste materials (reviewed by e.g. Sanders, 2001) such as putrescible kitchen waste (e.g. Veeken and Hamelers, 1999) or wastes from the food-processing industry (Salminen, 2001). On the other hand, before anaerobic digestion can be applied to grey waste more knowledge about the kinetics of degradation of its various components needs to be obtained. Such knowledge would help in selecting the appropriate method of pretreatment and in optimising the anaerobic digestion of grey waste. The objective of this study was to characterise grey waste by defining its composition using a sorting procedure and by sieving as well as to test the potential for methane production and nitrogen solubilisation of different waste components by means of an anaerobic incubation test. In addition, the degradation kinetics of the sorted components was determined and the main sources of methane and soluble nitrogen were traced. By these means, the pollution potential of grey waste and the rate and extent of anaerobic degradation of the sorted components were determined.
PFMSW (from estates with more than six households), metals, paper, cardboard, and glass were source-segregated by households, whereas the residual grey waste was mechanically processed at the waste management plant. The unprocessed grey waste was delivered in a 1.0 tonne bale wrapped in plastic film. The bale contained 2.5 m3 of waste. The bale was stored for about one month at ambient temperature. For this study, 0.5 m3 of the waste was shredded to a maximum size of 200 mm of which 0.2 m3 was sieved using a 100 mm mesh size. This was done to simulate the process carried out in the Tarastenj€arvi waste treatment plant and was as follows: magnetically removed waste was shredded to a maximum size of 200 mm and sieved (mesh size 100 mm; the undersized contained e.g. glass, gravel, ceramics, and some PFMSW), and the oversized was further shredded to a maximum size of 50 mm. The sieved fractions are henceforth referred as the undersized and the oversized (Table 1). Besides the grey waste samples, virgin computer printer paper (Canon Office A4, 80 g m2 ) was used in the present studies. All the samples were milled to a maximum particle size of 5 mm in a laboratory with a hammer mill (Retsch) before the anaerobic incubation test. 2.2. Sorting procedure About 1.5 m3 of the grey waste was manually sorted into six categories (Table 1) that would, according to previous sorting studies (Huovinen, 1994), represent the composition of the grey waste. The category of packaging included all packaging materials that consisted of either card alone or of card and layers of other materials such as plastic and/or aluminium. Packaging materials consisting solely of plastic and other plastic wastes were sorted into separate category. The category of cardboard consisted mainly of brown cardboard boxes and some other cardboard items, such as packaging. The category of textiles included textiles that were made of natural materials, like wool and cotton, as well as of synthetic materials, such as polyester. A considerable amount of newsprint as well as diapers were found in the grey waste sample, and these components were thus sorted into their own categories. The category of plastics was not studied for its pollution potential as plastics are considered biologically recalcitrant and thus do not contribute to the gaseous emissions. 2.3. Elution test
2. Methods 2.1. Waste Grey waste was obtained from Tarastenj€ arvi waste treatment plant in the Tampere region (Finland). The
The amounts of elutable soluble chemical oxygen demand (SCOD) and nitrogen in the different waste samples were tested in the elution test (CEN/TC 292/96, 1996). A portion (100 g) of the total solids (TS) of the sample was first added to deionised water in Duran glass vessels to obtain a liquid/solids (L/S) ratio of 2. The
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503
Table 1 The composition of the residual grey waste after source-segregation as received at the waste management plant and characteristics of the waste components and the sieved fractions as well as the office paper Grey waste component
(%-w/w)
TS (%)
VS (%)
VS/TS
Elutable SCOD (COD% of TS)
TKN (% of TS)
Elutable NH4 -N (% of TKN)
Packaging Cardboard Textiles Newsprints Diapers Plastics Oversizeda Undersizedc Grey waste Sorted fractions Miscellaneous unsorted Office paper
4.4 10 12 11 3.6 15 68b 32b 100 56 44
84 87 91 86 38 n.d. 85 78 84 n.d. n.d.
72 67 84 59 29 n.d. 70 54 58 n.d. n.d.
0.85 0.77 0.92 0.68 0.78 n.d. 0.82 0.69 0.69 n.d. n.d.
0.02 0.02 0.01 0.01 0.09 n.d. 0.01 0.02 0.02 n.d. n.d.
0.03 0.04 0.06 0.06 1.5 n.d. 0.25 0.32 0.32 n.d. n.d.
23 100 19 0 7.2 n.d. 4 16 29 n.d. n.d.
n.d.
97
72
0.74
0.02
0.02
n.d.
n.d.: Not determined. a Oversized from a 100 mm mesh size sieve. b Data supplied by the waste management company. c Undersized from a 100 mm mesh size sieve.
samples were stirred in a rotary shaker for 8 h, after which the liquid samples (50 ml) were collected. Deionised water was then added to yield a L/S ratio of 8 (overall L/S ratio of 10). The samples were further stirred in a rotary shaker for 16 h and the liquid samples were collected. All the liquid samples were filtered through glass fibre filters (Schleicher & Schuell). The procedures were performed at 22 ± 2 C. The concentrations after the elution test were expressed as the total cumulative amount of released SCOD or nitrogen per kg TS obtained from the elutions with L/S ratios 2 and 8. 2.4. Biochemical methane potential (BMP) test The BMP assays were carried out in three replicates with the different waste samples at 35 C in 2 l glass vessels for 237 days. The waste samples were added to the vessels to obtain the ratio 1.5 g-volatile solids (VS)/ g VSinoculum using 0.5 l of mesophilically digested municipal sewage sludge (TS ¼ 3.4%, VS ¼ 1.8%) from Nen€ ainniemi (Jyv€ askyl€ a, Finland) as the inoculum. A nutrient solution was added to ensure the availability of micronutrients and trace metals (Lepist€ o and Rintala, 1997), and 5.5 g l1 sodium bicarbonate was added as buffer. The final liquid volume of 1.6 l in each test vessel was obtained by the addition of deionised water. The inoculum was assayed separately so as to be able to subtract its methane production from those of the samples. The gas phases of the vessels were flushed with N2 /CO2 (80%/20%) and the vessels were sealed with butyl rubber stoppers. The gas produced was led through Viton tubing to aluminium gas sampling bags (Tecobag PETP/AL/PE-12/12/75 of 10 l, Tesseraux Spezialverpacknungen).
2.5. Analyses The volume of biogas produced was measured by a displacement method. Methane concentration was analysed with a Perkin Elmer Autosystem XL gas chromatograph, equipped with a flame-ionisation detector and fitted with a PE Alumina column (30 m · 0.53 mm). The carbon dioxide and oxygen content of the biogas were determined with an IR analyser (Geotechnical Instruments). TS and VS were measured according to Standard Methods (American Public Health Association, 1998). COD (dichromate) was determined according to Finnish Standards (SFS standard 5504, 1988). pH was measured with a 774 pH meter (Metrohm). Total Kjeldahl nitrogen (TKN) analyses were performed according to the Tecator application procedure (Perstorp Analytical/Tecator AB, 1995) with the same Kjeltec after digestion with a 2006 digestion. Ammonium nitrogen (NH4 -N) was determined by using a Kjeltec system 1002 distilling unit (Perstorp Analytical/Tecator AB) in accordance with Standard Methods (American Public Health Association, 1998). 2.6. Calculations The cumulative methane yield of each waste material was calculated by dividing its cumulative methane production by the amount of waste added to each vessel. The rate of degradation of the materials was assumed to follow a first-order rate of degradation, and thus the following formula was used to describe the methane production of each category of material: Y ¼ Yl ð1 exp½k tÞ;
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where Y represents the cumulative methane yield at time t, Yl represents the ultimate methane yield (m3 tTS1 added ) of the sample and k is the first-order rate constant. The parameters Yl and k were estimated by using non-linear regression fit to the yield data of a triplicate set with SPSS for Windows 10.1.3 software.
3. Results 3.1. Waste sorting and elutable nitrogen and organic matter in the sorted fractions The composition and the characteristics of the waste fractions in the source-segregated grey waste as received at the waste management plant are presented in Table 1. Fifty-six percent w/w of the grey waste was sorted into six waste categories (packaging, cardboard, textiles, newsprint, diapers and plastics), leaving 44% of miscellaneous waste that did not fall into the six categories. The miscellaneous residual undefined waste contained small unclassified items e.g. mixtures of putrescibles, plastics and metals, and that would probably end up in the undersized fraction. The data indicated that the main components of the grey waste per wet weight were plastics, textiles, newsprints and cardboard, which accounted for 15%, 12%, 11% and 10% w/w of the grey waste, respectively, whereas packaging and diapers represented 4.4% and 3.6% w/w of the grey waste (Table 1). The high content of inorganic materials in the undersized fraction was indicated by its low VS/TS ratio. Generally, the waste components had a VS/TS ratio of between 0.77 and 0.92, except for newsprint, which had a ratio of 0.68. The low fraction of SCOD in the TS (Table 1) obtained in the elution test indicated that the organic matter was mostly present in the sorted
grey waste components as solids. The amount of elutable SCOD was below 0.02%-TS, except in the diapers, where it was 0.09%-TS. The office paper consisted almost solely of insoluble organic matter, as indicated by a high TS and VS/TS ratio and low elutable SCOD. Total nitrogen content was low in most of the sorted components (between 0.03% and 0.06% of TS) with the exception of the diapers, which had a clearly higher TKN content (1.5% of TS). Also, the elutable NH4 -N fraction of the TKN was low in most of the sorted components (below 23%), whereas in the cardboard all of the TKN was eluted as NH4 -N. The undersized fraction had a somewhat higher TKN content than the oversized fraction. Only 4% of TKN was elutable from the oversized fraction, whereas the values were clearly higher in the undersized fraction (16%) and in the grey waste (29%). 3.2. Batch assays The specific methane yields from the triplicate vessels of each waste component after 237 days, and from those prepared in a previous study (Jokela et al., 2002) after 110 days with the putrescibles incubated at 35 C, together with the estimates of the ultimate methane yield (Yl ) and the first-order rate constant are presented in Table 2. Methane production began within 3 days in all samples. Most of the methane was produced within 43 and 66 days (data not shown). Of the grey waste components, the highest methane yield during the 237 days of the assay was from textiles (average 210 m3 tTS1 ) (Table 2). The packaging, diapers and cardboard yielded between 140 and 168 m3 tTS1 , whereas the newsprint yielded considerably less methane (40 m3 tTS1 ). The methane yields of the undersized and oversized fractions were similar but higher than that of the grey waste (101
Table 2 The specific methane yields of the grey waste components during the 237 days of the biochemical methane potential test (BMP) at 35 C with added methanogenic inoculum and the estimated values of the ultimate (Yl ) and the first-order kinetic constant (k) (standard deviations in parenthesis) Grey waste component
Specific methane yield 3
(m t Packaging Cardboard Textiles Newsprint Diapers Oversizeda Undersizedb Grey waste Office paper Putresciblesc a
118 146 192 34 60 127 118 85 243 67
waste1 added )
(81) (25) (99) (6.1) (5.1) (26) (5.4) (2.4) (18) (6.9)
Oversized from a 100 mm mesh size. Undersized from a 100 mm mesh size. c From a similar batch assay (Jokela et al., 2002). b
(m
tTS1 added )
(m
tVS1 added )
140 168 210 40 158 150 151 101 252 410
(96) (29) (109) (7.1) (13) (31) (6.9) (2.8) (18) (42)
165 217 228 58 204 182 219 147 340 527
(112) (38) (118) (10) (18) (38) (10) (4.1) (24) (54)
3
3
Yl (m3 tTS1 added )
k (d1 )
150 (100) 183 (32) 238 (121) 43.6 (7.0) 177 (14) 166 (35) 170 (7.3) 112 (3.8) 277 (21) 426 (42)
0.058 0.046 0.021 0.056 0.025 0.031 0.026 0.031 0.036 0.107
(0.01) (0.009) (0.001) (0.008) (0.003) (0.003) (0.002) (0.001) (0.006) (0.04)
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m3 tTS1 ). The grey waste also yielded significantly less methane than did the PFMSW in our previous assay (Jokela et al., 2002; Table 2) or the office paper in the present assay (mean yield 252 m3 tTS1 ). The value of Yl and the first-order kinetic rate constant are presented in Table 2. Regardless of the differences in the specific methane yields, the differences between those yields and the values of Yl seem to be fairly small. Also, the high coefficients of determination (above 0.88 with the methane yields of all the individual vessels, values not shown) indicated that the first-order kinetic model showed a good fit with the methane yield data. The grey waste and different components produced methane at a significantly lower rate (k-value between 0.021 and 0.058 d1 ) than the PFMSW (Table 2). The highest rate of production was from the packaging and newsprint, whereas the textiles and diapers produced the lowest rate (k-values of 0.021 and 0.025 d1 respectively). Also, the undersized fraction produced methane at a somewhat lower rate than the oversized fraction. The office paper showed approximately the same rate as the grey waste. The degradation of solids (VS and TS) and the VS/TS ratio as well as the specific NH4 -N solubilisation of each of the grey waste components and fractions after 237 days of anaerobic incubation are presented in Table 3. A high reduction in VS (more than 40%) was obtained for all the grey waste components except the newsprint. A higher VS reduction was obtained with the oversized fraction of the grey waste (51%) compared to the undersized fraction and the grey waste (both 35%). In most of the samples the VS/TS ratio was between 0.43 and 0.47, except in the textile and office paper samples, which had ratios of 0.37 and 0.31, respectively. Significantly more (approximately 10-fold) NH4 -N was solublised from the undersized compared to oversized fraction of the grey waste during the 237 days of anaerobic incubation. The grey waste sample yielded
505
approximately one-third of the amount produced by the undersized fraction. The diaper was the only grey waste component that produced a significant amount of soluble nitrogen (9.8 g NH4 -N kg TS1 added ).
4. Discussion 4.1. The composition of grey waste e The present sorting procedure showed that, along with the remnants of putrescibles, a high amount of other biodegradable waste components was present in the grey waste, including packaging cartons (4.4%) and cardboard (10%), newsprint (11%), textiles (12%) and diapers (3.6%). Together these comprised 41% of the grey waste. Previously, in a similar sorting procedure with grey waste the proportion of the same components was 40%, made up of 16% of various packaging materials (multilayer cartons, cardboard and paper), 5% newsprint, magazines and office paper, 8% textiles as well as 9% diapers (Huovinen, 1994). In the same study plastics contributed 11% of the grey waste, and putrescibles 26%. Putrescibles were omitted in our study, but the miscellaneous unsorted fraction probably contained residuals of putrescibles, as in packaging, which is often contaminated with putrescibles. On the basis of the data presented in this study and the waste component data obtained from the waste management authority in the region of origin, the total proportion of the biodegradable waste component was estimated to be around 70% w/w of the grey waste, which was made up of 60% w/w non-putrescible and 40% putrescible components. For unsorted MSW the proportion of non-putrescible biodegradable waste has been between 20% and 42% (reviewed by CEC, 1992; reviewed by Barlaz et al., 1990). On the other hand, in a previous study the grey waste fraction contributed 51%-w/w of total MSW (Huovinen,
Table 3 Reduction of solids (TS and VS), VS/TS and specific NH4 -N solubilisation of the grey waste and its different components (inoculum subtracted) as well as NH4 -N/TKN ratio of samples (inoculum included) after the 237 days of the biochemical methane potential test (BMP) at 35 C with added methanogenic inoculum (standard deviations in parenthesis) Grey waste component
TSred: (% of waste
Packaging Cardboard Textiles Newsprint Diapers Oversizeda Undersizedb Grey waste Office paper
52 50 70 29 40 49 31 33 73
a b
added )
(7.6) (4.1) (3.1) (1.3) (1.2) (4.1) (1.1) (1.1) (0.6)
Oversized from a 100 mm mesh size sieve. Undersized from a 100 mm mesh size sieve.
VSred: (% of wasteadded )
VS/TS (of waste)
52 54 79 27 43 51 35 35 88
0.46 0.43 0.37 0.47 0.46 0.45 0.45 0.45 0.31
(4.7) (12.8) (2.9) (1.0) (2.3) (3.5) (0.4) (2.0) (2.7)
(0.03) (0.09) (0.2) (0.01) (0.02) (0.01) (0.02) (0.01) (0.01)
NH4 -N/TKN (of waste + inoculum) 0.53 0.52 0.51 0.49 0.69 0.9 0.59 0.58 0.56
Specific NH4 -N solubilisation (gN kg TS1 added ) <0.01 <0.01 <0.01 <0.01 9.8 (0.44) 0.32 (0.05) 3.5 (0.25) 1.0 (0.003) <0.01
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1994), whereas in our study the grey waste accounted for approximately 75% of the household MSW collected, according to the waste management company. It is assumed that a significant proportion of the non-putrescible biodegradable components would end up in the oversized fraction after mechanical treatment, whereas the putrescibles in the grey waste would end up in the undersized fraction.
may be incorporated in e.g. woody biomass that remains fairly intact even during long-term anaerobic incubation. The great differences in NH4 -N yield between the oversized and undersized fraction suggest that the presently applied separation of fractions by sieving is an effective method of reducing the nitrogen load from the waste management if the undersized fraction is properly treated and end-disposed.
4.2. Nitrogen solubilisation from the grey waste fractions
4.3. Extent of methane yield from the grey waste components
A high amount of nitrogen (Tables 3 and 4) was solublised in the form of NH4 -N from the undersized fraction of the grey waste during methanation, whereas only 16% of TKN from the undersized fraction was eluted at the start (Table 1), indicating that almost all of the NH4 -N was produced during the 237-day exhaustive BMP test. In contrast, even more NH4 -N was eluted from the oversized fractions of the grey waste during the elution test than during methanation of the sample, indicating that almost all of the NH4 -N in the oversized fraction was present as soluble NH4 -N contaminant. The high level of NH4 -N production from the undersized fraction probably resulted from the remnants of protein-containing putrescibles, as the solublised NH4 N could not be traced to any of the sorted components (Table 4). Previously, up to 7.9 g NH4 -N kg TS1 was eluted from source-segregated putrescibles after a 51day aerobic lysimeter experiment (Jokela et al., 2002), which accounted for approximately 100% of the TKN, whereas in the present study only 1.0 g NH4 -N kg TS1 was eluted from the grey waste after the 237-day anaerobic incubation experiment. Regardless of the high proportion of solublised nitrogen (Table 3), the total contribution of diapers to the amount of TKN and the NH4 -N solubilisation was minor, 7.6% and 3.4% respectively (Table 4). A small proportion of TKN, namely one-third and one-tenth of the grey waste and the oversized fraction, was solublised. This indicates the presence of a nitrogen fraction in the waste that is highly resistant to biological solubilisation. These fractions
Most of the residual grey waste components (packaging, cardboard, textiles and diapers) seem to yield high amounts of methane in the presence of a proper methanogenic inoculum. Previously, in a similar BMP study, methane yields of between 318 and 343 m3 tVS1 were obtained for packaging materials, including milk cartons, wax paper and food board (Owens and Chynoweth, 1993). The evidently higher yields compared to the present study may be due to the 3.3-times larger maximum particle size used in the present study or the higher content of plastics in form of protective film on cartons and cardboard in the waste in the present study. This may have impeded the hydrolysis of solids by decreasing the access of hydrolytic enzymes to the cellulose-containing waste components, as previously discussed by Owens and Chynoweth (1993). This was not the case with the newsprint, which yielded around half of the methane in the present study compared to the study by Eleazer et al. (1997) (74 m3 tTS1 ), whereas the methane yield from office paper was similar in both studies. It is noteworthy that the maximum particle size of the piece of paper in that study was 2 · 5 cm as against 0.5 · 0.5 cm in the present study. Thus, a high concentration of inorganic clay filler and lignin in the paper probably impeded the complete destruction of VS in the present study. The high concentration of inorganic clay filler can be seen in the somewhat lower VS content in the newsprint and in the office paper if compared to the content in the packaging. Therefore,
Table 4 Contribution of the grey waste components to the amount of TS, VS, TKN and to methane yield as well as NH4 -N solubilisation of the grey waste calculated on the basis of the present sorting test and after the 237 days of the biochemical methane potential test (BMP) Waste component
TS (%)
VS (%)
TKN (%)
CH4 yield (%)
NH4 -N solubilisation (%)
Packaging Cardboard Textiles Newsprint Diapers Oversizeda Undersizedb
4.4 10 13 11 1.6 69 30
5.5 12 17 11 1.8 82 29
0.41 1.3 2.4 2.1 7.6 54 30
6.1 17 27 4.4 2.5 102 44
n.d. n.d. n.d. n.d. 3.4 28 104
n.d.: Not determined. a Oversized from a 100 mm mesh size sieve. b Undersized from a 100 mm mesh size sieve.
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the characteristics of the waste component, e.g. the method of paper and carton production used, effects the rate and extent of methanation and destruction of solids. On the other hand, compared to newsprint and packaging the anaerobic incubation of office paper and cardboard resulted in a high rate and extent of methanation as well as a high level of VS destruction. Previously, an 80% reduction in cellulose was obtained for newsprint after 300 days anaerobic incubation (Clarkson and Xiao, 2000). Alkali pre-treatment further increased the degradation by 10%. In the same study, a methane yield around 60% lower was obtained with 0.6 · 5 cm particles of newsprint and office paper compared to paper samples ground into powder. Nevertheless, the high cellulose-containing components of the grey waste will eventually degrade and yield high amounts of methane. Methanation may thus be applied as a biological method of treatment before landfill disposal of the grey waste. The non-putrescible biodegradable fraction contributed almost half of the TS and VS of the grey waste, and together these waste components accounted for 57% of the methane yield of the grey waste (Table 4). The highest contribution was from textiles, which produced 13%-TS, 17%-VS and 27% of the total methane yield. In addition, the discarded miscellaneous unsorted fraction may also have yielded some methane. The various results obtained from different tests and analyses can be seen as discrepancies in the contributions of the undersized and oversized fractions (sum more than 100%). In comparison to the undersized fraction, the oversized fraction evidently made a higher contribution to the amount of solids (both TS and VS) as well as to the methane yield. 4.4. Kinetics of solid hydrolysis for grey waste components The estimated curve with the values of Yl plotted against the specific methane yield showed that the firstorder kinetic model described rather well the degradation of each grey waste fraction. The high standard deviation of Yl and low standard deviation of k indicates that Yl was dependant on the sample in the individual vessel, whereas the k-value was independent of the sample. Thus, the degradable matter in the individual waste components degraded at the same rate, whereas the variation between the samples of the individual waste components in the concentration of degradable organic matter (as VS) resulted in a high deviation in Yl . This is especially noticeable in the Yl of textiles and packaging. In a previous study (Owens and Chynoweth, 1993), slightly higher constant was obtained for the waste components of newsprint (between 0.069 and 0.084 d1 ) and for the component of corrugated unbleached cardboard (0.058 d1 ) to those of the present study (newsprint 0.056 d1 and cardboard 0.046 d1 ),
507
whereas the constants for packaging (between 0.099 and 0.119 d1 ) and especially for office paper (0.136 d1 ) were clearly higher than ours (packaging 0.058 d1 and office paper 0.036 d1 ). This might result from differences between the studies in terms of the types and amounts of plastic film (packaging) used or amount of clay filler (office paper) in the waste components, which in our study might have decreased the transfer of mass from the waste particles. Also, the kinetic rate constants obtained for the grey waste and for the sieved fractions were somewhat lower (0.031 d1 ) than that obtained previously (0.075 d1 ) for unsorted MSW (Owens and Chynoweth, 1993). The range of the constant for putrescible components has been reported to be between 0.10 and 0.35 d1 (Veeken and Hamelers, 1999). A similar constant to that obtained in our present study (between 0.038 and 0.048 d1 ) has been reported for putrescibles studied in 5 m3 reactors with recirculation (ten Brummeler et al., 1991), but as previously noted by Veeken et al. (2000), the conditions were not optimal for the hydrolysis of solids. Nevertheless, the present study indicates that the first-order rate constant for the grey waste components was in the range of 0.021 and 0.058 d1 , whereas for the putrescibles from our previous study (Jokela et al., 2002) the first-order rate constant was 0.11 d1 .
5. Conclusions The grey waste produced after the source-segregation contained a high amount of biodegradable waste components, including packaging, cartons and cardboard, newsprint and also textiles and diapers, which together comprised 41%-w/w of the grey waste. The total proportion of the biodegradable waste components was around 70%-w/w of the grey waste. Under anaerobic conditions and in the presence of methanogenic microorganisms, these components yielded 40–210 m3 methane (tTS1 ) and during complete methanation <0.1 g NH4 -N kg TS1 added were produced except from diapers, which produced 9.8 g NH4 -N kg TS1 added . The grey waste yielded 101 m3 methane tTS1 . On the mass bases, the oversized fraction produced more methane than the undersized fraction. The NH4 -N was solublised mainly from the undersized fraction obtained after sieving the grey waste through a 100 mm mesh, and thus the nitrogen pollution stream from waste management is potentially controllable by reducing the particle size and sieving of grey waste. The curve estimated from the values of Yl and the data on specific methane yields showed that the first-order kinetic model gave rather a good description of the degradation of each of the grey waste fractions. The first-order rate constant of the grey waste components was in the range of 0.021 and 0.058 d1 ,
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which is around one-sixth of the values reported previously for source-segregated putrescibles.
Acknowledgements This study was sponsored by the Finnish Graduate School in Environmental Science and Technology (EnSTe). We wish to thank the personnel at the Tarastenj€ arvi landfill for providing the waste samples as well as associate professor Pedro Aphalo for valuable expertise in the statistical analysis.
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