Determination of biological stability in compost: A comparison of methodologies

ARTICLE IN PRESS

Soil Biology & Biochemistry 39 (2007) 1284–1293 www.elsevier.com/locate/soilbio

Determination of biological stability in compost: A comparison of methodologies Claudio Baffia, Maria Teresa Dell’Abateb, Antonio Nassisic, Sandro Silvaa, Anna Benedettib, Pier Luigi Genevinid,e, Fabrizio Adanid,e, a

Universita` Cattolica S. Cuore, Istituto di Chimica Agraria e Ambientale, via Emilia Parmense, 84, 29100 Piacenza, Italy b CRA - Istituto Sperimentale per la Nutrizione delle Piante, via della Navicella 2/4, 00184 Roma, Italy c ARPA Emilia-Romagna, sezione di Piacenza, Eccellenza Suolo, via XXI Aprile 48, 29100 Piacenza, Italy d Universita` degli Studi di Milano, Dipartimento di Produzione Vegetale, sezione FCA, via Celoria, 2, 20133 Milano, Italy e Centro Ricerche Nazionale Biomasse, Via S.Lucia, 20, Napoli, Italy Available online 26 December 2006

Abstract Biological [dynamic respiration index (DRI); chemical humification indexes: humification index (HI), degree of humification (DH) and humification rate (HR); and thermoanalytical (thermostability index, R1 and a labile fraction)] indexes were used to assess compost stability of 15 end products. By use of these three techniques independent assessment of compost stability was made possible. Evidence of unstable materials was found where labile, easily biodegradable, and non-humified organic fractions were present. The DRI was used as a reference index for biological stability, and no significant correlation was observed between DRI and the humification indices (HI, DH, HR) and the thermogravimetry index (R1). On the other hand, significant correlation was observed for DRI vs. non-humified carbon (NHC), which was determined using the chemical method and the labile fraction determined by thermogravimetry, as well as for NHC vs. ‘‘labile fraction’’. These fractions represent labile, easily biodegradable, and non-humified organic matter. Significant correlations were also observed between the three above mentioned measurements and TEC, suggesting that this fraction is mainly formed of the easily degradable organic fraction. These results suggest that the integrated use of biological, chemical, and thermoanalytical methods could represent a useful tool in differentiating stabilized composts from non-stabilized ones, and it could provide more reliable information for both managerial and sanitary health aspects involved in good agricultural practice. r 2006 Elsevier Ltd. All rights reserved. Keywords: Biological stability; Compost; Dynamic respirometric index; Thermal stability indices; Humification parameters

1. Introduction New strategies in municipal solid waste (MSW) management, i.e., separate collection of the organic fraction (EU Directive 1999/31/EC) and the need to reduce the biodegradable-MSW fraction allocated in landfills (EU Directive 2003/33/EC), have favored the development of composting as a useful biotechnology in transforming organic wastes into suitable agricultural products (Senesi and Brunetti, 1996). Corresponding author. Universita` degli Studi di Milano, Dipartimento di Produzione Vegetale, sezione FCA, via Celoria n. 2, 20133 Milano, Italy. Tel.: +39 2 5031 6546; fax: +39 2 5031 6521. E-mail address: [email protected] (F. Adani).

0038-0717/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2006.12.004

Compost quality is an important issue since compost is thought to be beneficial for both agriculture and the environment. What is meant by compost quality is a plant friendly, soil friendly, environmentally friendly, and a socially responsible product. The first three of these four points concerning ‘‘friendliness’’ are, respectively, related to the absence of both phytotoxicity and seed weeds, to the content of non-desired elements (e.g., Na), and to the presence of both organic and inorganic contaminants. The use of compost should be socially responsible in terms of being used safely in both full field and pot container applications (e.g., products should be pathogen free so as not to adversely affect both plants and humans). Problems connected with compost transportation and conservation (e.g., odor production, self-heating and self-combustion,

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biogas production, and pathogen re-growth) should be minimized. As many of these aspects are directly (e.g., odors, self-heating, etc.) or indirectly (e.g., phytotoxicity, seed weeds, etc.) related to microbial activity in the compost, the importance of measuring the biological stability of a compost is an important scientific and technical argument over which there is some dispute (AAVV, 1993). Among previously proposed methods (Inbar et al., 1990; Barberis and Nappi, 1996), respirometric tests are recognized as being well tested methods of measuring biological stability (e.g., ASTM, 1996; The U.S. Composting Council, 1997) as they are a direct measure of microbial activity (Adani et al., 2004). On the other hand, both chemical and physical methods have also been proposed as they are less expensive (chemical approach) or less time consuming (thermoanalytical approach) (Dell’Abate et al., 1998; Tittarelli et al., 2002). The chemical approach is based on the extraction and fractionation of humic-like and non-humic (NH) organic C. It uses humification parameters, some of which, e.g., humification index (HI), degree of humification (DH) and humification rate (HR) are based on the humified and more stabilized organic fraction. These measurements have proved to be particularly suitable for describing and assessing compost stability during maturation (Dell’Abate et al., 1998; Hsu and Lo, 1999; Tittarelli et al., 2002; Mondini et al., 2003). In the thermoanalytical approach, thermogravimetry and differential scanning calorimetry (DSC) are the most frequently used techniques as they are simple, fast, reproducible, and can provide information about thermal stability of organic matter without sample pre-treatment (Blanco and Almendros, 1994; Dell’Abate et al., 2000). In spite of using the most sophisticated techniques, several authors (Inbar et al., 1990; Barberis and Nappi, 1996; Chen et al., 1996; Ita¨vaara et al., 2002) agree that by itself no technique or method can be successfully used to evaluate compost stability because the differences between raw materials used to produce compost are so great that these may affect biological stability itself (Mondini et al., 2003). This point is controversial, and the authors of this paper also disagree among themselves about whether or not more than one method is required. Support for the respirometry approach as a satisfactory measure of biological stability is provided by its routine success in assessing compost quality in many countries (e.g., Italy, Germany, Austria, United Kingdom, United States), and methods are now well codified at the international level (e.g., ASTM, 1996; The U.S. Composting Council, 1997). On the other hand, the opinion is also acceptable that an integrated use of different measurements and/or indices describing the properties of the materials used for composting processes gives a more complete picture of the nature and composition of compost. It can provide more information and so the compost can be used more appropriately. The aim of this paper was to evaluate the biological stability of 15

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composts using biological, chemical, and thermoanalytical techniques. 2. Materials and methods 2.1. Compost description and preparation Fifteen composts of different origins and compositions were investigated (Table 1). In particular, #1–9 were either composts obtained from the organic fraction of the municipal solid waste collected separately (MSWOM), or lignocellulose wastes (LW). In the same way as the previous nine composts, compost #10 came from a mixture, but was amended with chicken manure at the end of the process. Composts from #11 to #15 came from mixtures of MSWOM, LW, and newspaper (NP). Compost samples could be differentiated by origin of raw material, as stated above, but no other information (e.g., compost age, plant typology, etc.) was available due to the nature of the service. This activity was composed of certified external laboratories that carried out dynamic respiration index (DRI) determinations in order to certify the biological stability of compost received from producers. The DRI (Adani et al., 2004) was used to determine biological stability of samples collected from private and public Italian composting plants. The DRI analyses were performed at the Dipartimento of Produzione Vegetale— DiProVe-Universita` degli Studi di Milano, Italy (Lab 1). The initial wet weight of compost samples ranged from 20 to 30 kg. Samples were stored at 4 1C and processed within 1–4 d after reception. A homogeneous sub-sample of 1 to 3 kg (depending on the particle size) was taken from each bulk sample according to the method of The U.S. Composting Council (1997). Sub-samples were oven-dried at 105 1C in order to determine moisture and total solid contents (TS). Following this, a mill was used to grind the residual sub-sample to a size of less than 1 mm (Cyclotec 1093 sample mill, Foss Tecator, Sweden). Ash and volatile solid (VS) contents were then determined according to the methods reported by the The U.S. Composting Council (1997). The main characteristics of compost samples are listed in Table 1. Part of each dried sample was then sent to the Istituto di Chimica Agraria ed Ambientale of the Universita` Cattolica Sacro Cuore, Piacenza, Italy and ARPA Emilia-Romagna, Eccellenza Suolo, Piacenza, Italy (Lab 2) for C fractionation and determination of humification indices. A second part of the dried sample was sent to CRA-Istituto Sperimentale per la Nutrizione delle Piante, Roma, Italy (Lab 3) for thermo-gravimetric analysis and DSC. 2.2. DRI determination The DRI was determined according to the method of Adani et al. (2001). This method was proposed for use with different types of composts obtained from wastes such as MSW and derived products, yard waste, source separated

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Table 1 Mean values of volatile solids, moisture and dynamic despiration index (DRI) in investigated samples Sample

Type

Volatile solids (g kg1 dm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Organic green Organic green Organic green Organic green Organic green Organic green Organic green Organic green Organic green Compost/chicken-dung Waste/green/organic Green/paper/organic Green/paper/organic Green/paper/organic Green/paper/organic

75.5 72.7 78.9 70.1 78.4 71.3 33.7 67.0 71.1 71.7 74.9 83.3 76.9 77.3 74.1

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

1.09 1.72 0.68 2.66 0.66 1.01 0.99 0.23 1.42 0.89 1.76 0.19 1.00 0.14 0.65

Moisture (g kg1 w w1)

DRIa (mg O2 kg VS1 h1)

58.0 58.8 41.8 48.4 38.2 40.6 21.2 27.9 40.6 36.8 30.9 57.3 67.7 33.1 20.8

3350 5150 3360 2400 1300 2300 230 940 960 5000 110 4100 1150 1100 810

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

170cb 257a 190c 160d 90e 130d 44g 60ef 66ef 300a 25g 250b 106ef 80ef 88f

a

DRI ¼ average value of 12 instantaneous respiration indexes (DRIi) taken during 24 h characterized by the most intense biological activity on a basis of 4 d observation and calculated by using the following equation: DRI ¼

X24 y¼0

ðDRIi Þ=12.

b Values of the same column followed by the same letter are not significantly different at Po 0.05 using the Student–Newmann–Keuls multiple range test.

organic waste, and other types of organic wastes with toxic concentrations that do not inhibit microorganisms (ASTM, 1996). The oxygen (O2) uptake rate was determined first, by measuring the difference in O2 concentration (ml l1) between the inlet and the outlet of an air flowing through the biomass. Then secondly, a determination was made of VSs in the biomass (kg), the flow rate (1 h1), and the time (h) during which oxygen consumption was measured. The instantaneous DRI (DRIi) (mg O2 kg VS1 h1) was calculated as DRIi ¼ Q  y  DO2  Vg1  31:98  VS1  y1 ,

(1)

1

where Q (l h ) is the airflow, y the acquisition time (2 h), DO2(ml l1) the difference in O2 concentration in the inlet and outlet air flows of the reactor, Vg (l mol1) the volume occupied by one mole of gas at inlet air temperature, 31.98 g mol1 is the molecular weight of O2, and VS (kg) represents the total VSs present at the time of measurement (starting VS). The degree of biological stability was expressed by DRI, which was calculated using the method of Adani et al. (2004). In brief, on the basis of a 4 d test-period during which DRI was continuously recorded, the DRIi representing the 24 h period (12 DRIi) characterized by the most intense biological activity (higher DRIi) was used to define the DRI index. This was mathematically expressed by applying the following equation (Adani et al., 2006):

DRI ¼

24 X ðDRIi Þ=12. y¼0

(2)

2.3. Organic carbon, total nitrogen analysis, and humification measurements In Lab 2, aliquots of dried samples ground to o 0.5 mm were analyzed for total C (TC) and total N (TN) using an NA 2000 elemental analyzer (Fisons, CE, Instrument, Milan, Italy). Total organic C (TOC) was obtained by subtracting total inorganic C (TIC) from TC. TIC was determined according to the ISO TC 190 10693 Method (1995), suitable for the determination of total soil carbonates. The TOC fractionation was set up as reported by Ciavatta et al. (1990). In particular, solid samples were extracted at 65 1C for 24 h using 0.1 mol l1 NaOH plus 0.1 mol l1 Na4P2O7 solution (1:50, solid:liquid ratio). The samples were then centrifuged at 5000  g and the supernatants were filtered through a 0.20-mm Millipore filter (Millipore, Billerica, MA) (total extracted C, TEC). The humic-like acid (HA) fraction was separated from the fulvic-like acid (FA) and the NH fractions by precipitation after acidification of the alkaline solution (supernatant) to pHo 2. Chromatography on a column of polyvinylpyrrolidone (PVP, Aldrich, Germany) was used to separate the NH from the FA. The FA was then combined with the HA to obtain total humified fraction (HA+FA). The organic C content (TEC and (HA+FA)C) and TN of each fraction were determined by dry combustion using the NA 2000 elemental analyzer, after neutralizing and freeze drying the extracts at 40 1C for 2 d using a freeze dryer (Edwards mini fast 680, Edwards, Norfolk, England). To optimize the analytical conditions, TC and TN were determined simultaneously and methionina (Carlo Erba, Milan) was used as the standard for calibration. A certified compost

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sample (Comp ARPAV – Ce.Lo. 2002) was employed to evaluate accuracy and was always present in each series of measurements together with an analytical blank. The nonhumified carbon (NHC) was determined as the difference between TEC and (HA+FA)C. Humification indexes HI, DH, and HR were determined according to the following formulas of Sequi et al. (1986) and Ciavatta et al. (1990): HI ¼ NHC=ðHA þ FAÞC, DH% ¼ ½ðHA þ FAÞC  100=TEC, HR% ¼ ½ðHA þ FAÞC  100=TOC: 2.4. Thermal analysis DSC and thermogravimetry (TG) were carried out in Lab 3 simultaneously by using a Netzsch STA 409 Simultaneous Analyzer (Netzsch-Geratebau GmbH, Selb, Germany) equipped with a TG/DSC sample carrier supporting a type S thermocouple (PtRh10-Pt). Under a static oxidizing (air) atmosphere, samples were heated from room temperature to 900 1C at a heating rate of 10 1C min1. About 11–12 mg of sample was analyzed in an aluminum crucible and the same amount of calcinated kaolinite was used as the inert sample reference for DSC. The thermobalance was calibrated for buoyancy effect to obtain a quantitative estimate of weight changes. Heat production in the heat-flux DSC was calibrated under the same conditions using a sapphire standard and subtracting a baseline obtained by an additional run for the empty crucibles. The Netzsch software SW/cp/31.01 was used for data processing. Three replicated thermal scans were carried out on whole samples. Two of these replicates involved samples ground in a micro-mill (Culatti IKA, Janke & Kunker, Germany) with a 1.00-mm sieve followed by gentle manual grinding in an agate mortar preparation procedure (Dell’Abate et al., 1998, 2000; Dell’Abate and Tittarelli, 2002). The third thermal scan was performed on aliquots of the samples used for the determination of humification, so that the possible influence of the grinding method, and consequently sample particle sizes on thermal profiles could be verified. During DSC measurements, the temperature difference between sample and reference material was recorded as a direct measure of the difference in the heat-flow rates. Deflections from the baseline occur according to the endothermal or exothermal nature of reactions or transformations, producing peaks at characteristic temperatures. The sample weight change—gain or loss (expressed as %)—in TG is measured during the thermal program (heating). The first derivative of the TG trace (DTG) represents the weight loss rate (expressed as % min1): calculation of DTG onset and peak temperatures mean that subsequent decomposition steps can be distinguished.

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Results are expressed as the total weight loss of the sample attributed to organic matter decomposition in the temperature range 180–600 1C (Exo tot); the sample weight loss associated with the first exotherm in the temperature range 180–410 1C (Exo 1), which reflects the cellulosic component; and the sample weight loss occurring in the temperature range 130–180 1C (labile fraction), which has an endothermic effect. The sample weight loss occurring in the temperature range 410–600 1C (Exo 2), associated with a second exothermic peak, reflects more complex and condensed organic molecules, such as lignin and humified compounds. The ratio between the weight losses associated with the second and the first exothermic reactions (R1 ¼ Exo 2/Exo 1) is a thermostability index representing the relative amount of the thermally stable organic matter fraction compared to the less stable one, regardless of moisture level or ash content in either sample (Dell’Abate et al., 1998, 2000). 2.5. Statistical analysis All results were expressed on a dry weight (DW) basis and represent the mean of three replicates. Data underwent univariate ANOVA and sample means were compared using the Student–Newmann–Keuls test (Snedecor and Cochran, 1980). A multivariate analysis with the use of Pearson coefficients was carried out on all measured parameters by using the SPSS Version 11.5.1 statistical package (SPSS Inc., 2002). 3. Results 3.1. Biological and chemical stabilities of composts Table 1 presents data relevant to biological stability obtained in determinations of DRI. Assuming 1000 mg O2 as kg VS1 h1 to be a maximum threshold value for biological stability (medium stability) (Adani et al., 2004), the stability values of the majority of products investigated were not satisfactory because they are above the threshold. Tables 2 and 3 present the values detected of parameters used for assessing chemical stability. TOC shows homogeneous values ranging from 30.6% to 40.7%, except for sample 7 which shows the lowest value (16.3%). TEC, representing on average 58% of TOC, exhibits equal percentages (50%) of (HA+FA)C and NHC. Higher values for TEC and (HA+FA)C and lower ones for NHC are observed in green-organic composts than in those with paper added. Assuming the HI threshold value for chemical stability to be 0.5 (Ciavatta et al., 1990; Ciavatta and Govi, 1993), all samples, with the exception of samples 1, 7, and 11 exhibited far higher values of HI than this threshold, as shown in Table 3. Similarly, except for 1, 7 and 11 for DH, and for sample 7 for HR, values of DH and HR are low for all samples which confirms the poor chemical stability, as assessed by determination of humification indexes.

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Table 2 Mean values of TOC, TN, TOC/TN, TEC and C(HA+FA) in compost samples Sample

TOCa (g kg1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

369 364 388 325 360 365 163 310 306 335 359 407 376 360 360

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

4cf 3.4c 1.8b 1d 4.8c 3c 8.2f 3.5e 4.1e 6.3d 3c 1.8a 25bc 7c 17c

TNb (g kg1) 16.6 12.8 23.7 15.2 18.6 24.7 13.9 17.3 17.6 23 24.8 12.9 12.8 11 14.6

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

TOC/TNc

0.6bc 0.6ef 0.5a 0.4cd 0.3b 0.7a 0.7de 1b 0.5b 0.7a 0.3a 1.7ef 1.8ef 0.2f 1.5de

22.2 28.5 16.4 21.3 19.4 14.8 11.7 18.0 17.4 14.6 14.5 32.0 30.0 32.8 24.9

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

TECd (g kg1)

0.52bc 1.18a 0.33de 0.62bc 0.25cd 0.37de 0.04e 0.81cd 0.22cd 0.58de 0.20de 4.23a 5.84a 1.07a 2.95b

196 251 240 202 196 227 135 192 198 234 201 223 178 158 163

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

8.56de 4.01a 9.12ab 3.70cd 9.10de 6.20b 1.80g 8.500de 9.0de 6.55b 3.40d 7.06c 8.90ef 8.55f 9.00f

CHAþFA e (g kg1) 119 113 131 95.9 98.2 118 81.4 94.2 98.8 101 121 99.4 74.4 71.0 78.2

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

3.70bc 4.00c 3.40a 4.10d 3.41d 3.85bc 3.35e 0.75d 4.05d 4.15d 3.41b 1.15d 2.20fg 0.90g 1.45ef

a

TOC: total organic carbon. TN: total nitrogen. c TOC/TN: total organic carbon/total nitrogen. d TEC: total extractable carbon. e CHAþFA : humified carbon (humic+fulvic acid C contents). f Values in the same column followed by the same letter are not significantly different at Po 0.05 using the Student–Newmann–Keuls multiple range test. b

Table 3 Mean values of NHC, HI, DH and HR in compost samples Sample

NHCa (g kg1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

77.6 138 108 106 98.0 109 53.8 97.5 99.6 133 79.8 123 104 87.0 84.8

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

4.86ee 7.29a 8.78b 6.73b 8.19bcd 2.35b 1.64f 8.12bcd 3.70bcd 9.39a 6.01e 7.62a 11.1bc 7.65cde 8.40de

HIb 0.65 1.22 0.83 1.11 1.00 0.92 0.66 1.03 1.01 1.33 0.66 1.24 1.40 1.23 1.09

DHc (%) 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

0.02f 0.11abc 0.08ef 0.11bcd 0.09cde 0.01de 0.05f 0.09cde 0.08cde 0.14ab 0.06f 0.09abc 0.19a 0.10abc 0.10cd

60.5 45.2 54.8 47.6 50.1 52.2 60.2 49.2 49.8 43.1 60.3 44.7 41.9 45.0 48.1

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

0.76a 2.16def 2.20b 2.60cde 2.27cd 0.27bc 1.70a 2.02cd 1.94cd 2.64ef 2.44a 1.69def 3.34f 1.87def 2.31cde

HRd (%) 32.5 30.7 33.9 29.5 27.3 32.4 50.1 30.5 32.2 30.1 33.7 24.5 19.8 19.8 21.7

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

0.86bc 1.16bc 0.98b 1.17c 1.07d 0.94bc 3.49a 0.43bc 1.52bc 1.61c 1.18b 0.39e 0.81f 0.60f 1.18f

a

NHC: non-humified carbon. HI: humification index ¼ NHC/(HA+FA)C. c DH: degree of humification ¼ [(HA+FA)C  100)/TEC]. d HR: humification rate ¼ [(HA+FA)C  100)/TOC]. e Values in the same column followed by the same letter are not significantly different at Po 0.05 using the Student–Newmann–Keuls multiple range test. b

Nevertheless, humification indexes, i.e., HI, HR and DH (Table 3) were higher for compost made from mixtures of MSWOM+LC (#1–10) than for compost based on only MSWOM+LC+NP (#11–15). These results probably reflected the origin of the compost, i.e., the higher content of easily degradable organic matter (more MSWOM) in the first group of composts.

The values for TN (Table 2) confirm the previous observations as there were higher values for the majority of MSWOM+LW+NP composts compared to samples containing NP.TOC/TN ratio values were lower for MSWOM+LW+NP mixtures, indicating a greater degree of organic matter stabilization. Furthermore, higher values of TOC/TN ratio were observed

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for the mixtures of MSWOM+LW and NP organic materials. 3.2. Thermoanalytical data Examination of thermal stability revealed that there were different regions of DSC traces according to different temperature ranges (Fig. 1a,b): the first, with Tp180 1C, shows endothermic peaks resulting from dehydration reactions; the second, with temperatures between 180 and 600 1C, is the thermally active temperature range for organic material decomposition and oxidation; the third, at T4600 1C, indicates oxidation of refractory C as well as the decomposition of both mineral and biogenic salts, such as carbonates. Pectin, hemicellulose, cellulose, and micro-

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bial cell walls seem to be progressively involved in oxidation during the first exothermic peak (Sharma, 1995; Kaloustian et al., 1997). On the other hand, more stable components such as lignin oxidize in the second range, as Lopez-Capel et al. (2005) confirm in data obtained for alpha cellulose and sugarcane lignin. TG traces recorded the weight losses involved in all these reactions. The examination of TG and DTG traces in the range up to 180 1C (Fig. 1c) showed that most composts had two distinct weight losses in the ‘‘dehydration’’ reaction range: the first up to about 130 1C, and the second between 130 and 180 1C. In this second range more tightly bound water is released (McBrierty et al., 1996) together with more volatile components released from lateral chains and low molecular weight compounds.

Fig. 1. Differential scanning calorimetry (DSC), thermogravimetry (TG), and derivative thermogravimetry (DTG) traces of compost samples obtained in oxidising conditions for air: (a) sample 2, (b) sample 7; and (c) expanded graph of TG and DTG traces of sample 10, with indication of the weight loss registered in the temperature range 130–180 1C (labile fraction).

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The corresponding weight losses mentioned above ranged from 0.28% to 1.44%, and are shown as the labile fraction in Table 4. Values of total weight loss in the temperature range 180–600 1C (Exo tot) caused by organic matter decomposition ranged from about 52% to 68%. Exo tot values were slightly higher for the MSWOM+LW+NP composts than for green organic composts (mean values 64% vs. 58%). Exo 1 mean values (i.e., weight losses due to the less stable organic fraction, detected in the temperature range 180–410 1C) were slightly, but not significantly, lower in the MSWOM+LC composts, except for sample 7, which showed the lowest values of both Exo tot (28%) and Exo 1 (17%). The combination of these two results seems to indicate organic matter is slightly more transformed in the first compost group than in the second. The majority of samples showed low values for the thermostability index R1, which seems to indicate incomplete transformation of the organic matter. Thermograms of the samples (Fig. 1) show the lowest (0.31) and the highest (0.64) R1 values (samples 2 and 7, respectively). DSC traces (Fig. 1) revealed that sample 2 had a more intense first exothermic peak than sample 7, confirming that the latter is more mature than compost 2.

biological stability is determined by the extent to which readily biodegradable organic matter has decomposed. It identifies the actual point reached in the decomposition process and represents a gradation of values on a recognized scale. This enables comparisons of the process of decomposition. Therefore, indices or parameters directly related to the measurement of the content (or the absence) of easily degradable organic matter in compost can be used to assess biological stability. A high content of an easily degradable fraction means low biological stability; the opposite also applies (Adani et al., 2004). The DRI has been successfully used to measure biological stability (e.g., Adani et al., 2006) because it is able to match the biological stability definition, and it is able to measure microbial oxygen uptake when easily biodegradable organic matter is degraded (Adani et al., 2004). Today DRI is an EU-suggested method (EU, 2001), has been recognized as an official method in Italy (UNI, 2006), and acknowledgment is ongoing at the European level (EU-CEN/TC 343, 2005). Therefore, the DRI can be assumed to be a reference index that can be used to compare and to relate to other indices. 4.2. Considerations of chemical and thermoanalytical data for assessing stabilities of composts

4. Discussion 4.1. Considerations of biological stability based on ease of biodegradation of substrates A current interpretation of biological stability, proposed by Lasaridi and Stentiford (1996), would indicate that the

Both chemical (HI, DH, and RH indexes) and thermoanalytical (R1 index) methods have been used to describe the evolution of the organic matter during composting (Dell’Abate et al., 1998, 2000; Dell’Abate and Tittarelli, 2002; Mondini et al., 2003). In studies of compost samples

Table 4 Mean values of main thermogravimetric weight losses attributed to organic matter decomposition, temperature ranges in which they occurred and R1 values Sample

Labile fractiona

Exo 1b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

–f 0.73 1.20 0.60 0.96 1.44 0.28 0.62 0.66 1.17 0.71 0.98 0.28 0.41 0.32

44.7 51.6 42.3 34.3 41.4 37.4 17.4 38.8 40.1 39.7 42.5 45.0 41.9 43.7 44.2

a

7 7 7 7 7 7 7 7 7 7 7 7 7 7

0.04cdg 0.39ab 0.21cde 0.03bc 0.07a 0.08e 0.08cde 0.12cde 0.12ab 0.27cd 0.04bc 0.07e 0.11de 0.13de

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

Exo 2c(% of total sample) 2.93b 1.01a 1.18bc 2.27d 0.97bc 1.82cd 1.35e 1.77bcd 2.63bc 0.35bc 2.10bc 5.18b 2.02bc 2.07bc 2.91b

18.6 15.8 24.0 17.4 19.3 23.5 11.2 18.6 19.3 23.7 18.2 20.7 16.8 16.9 20.0

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

0.40bcd 0.29d 0.49a 1.05d 1.44bc 1.04a 1.35e 1.74bcd 1.22bc 0.34a 0.84b 1.09b 1.18d 1.41d 1.60bc

Exo totd 63.3 68.4 66.4 51.7 62.5 60.9 28.5 57.3 59.4 63.4 62.6 65.7 62.5 64.3 64.2

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

1.72abc 2.59a 1.02ab 3.30d 1.53abc 2.29abc 2.69e 3.32c 3.85bc 0.14abc 2.11abc 4.75abc 1.74abc 5.82abc 4.19abc

R1e (dimensionless) 0.42 0.31 0.57 0.51 0.47 0.63 0.64 0.48 0.48 0.60 0.43 0.47 0.40 0.39 0.45

7 7 7 7 7 7 7 7 7 7 7 7 7 7 7

0.03de 0.01f 0.02b 0.01c 0.03cd 0.04a 0.03a 0.03cd 0.02cd 0.01ab 0.03de 0.07cd 0.04de 0.02e 0.03cde

Labile fraction ¼ sample weight loss in the temperature range 130–180 1C. Exo 1 ¼ sample weight loss (associated with the first exotherm) in the temperature range 180–410 1C. c Exo 2 ¼ sample weight loss (associated with the first exotherm) in the temperature range 410–600 1C. d Exo tot ¼ sample weight loss in the temperature range 180–600 1C. e R1 (thermostability index) ¼ Exo 2/Exo 1. f Value not available. g Values in the same column followed by the same letter are not significantly different at Po 0.05 using the Student–Newmann–Keuls multiple range test. b

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taken at different stages of transformation, these methods have proved to be good assessors of the chemical and thermal stability of the organic matter. Nevertheless, as these indices do not reflect directly the content of easily degradable OM in composts, and as they are also affected by the composition of the input material (e.g., R1), they do not substantially describe biological stability correctly (Dell’Abate et al., 1998, 2000; Dell’Abate and Tittarelli, 2002). The above idea is confirmed by the absence of a significant correlation between DRI, the humification parameters HI, DH, HR, and the thermostability index R1. This could be due to the differing natures of these indices since DRI is related to the easily biodegradable organic fraction, HI, DH, and HR to non-humified vs. humified material, and R1 to more thermally recalcitrant organic fractions (Exo 2) vs. the medium thermally labile fraction (Exo 1) (Table 5). Significant correlation was observed between DRI, NHC, and the labile fraction (Table 6). In particular, DRI was well correlated with both NHC (r ¼ 0:789; Po0.01) and the labile fraction (r ¼ 0:651; Po0.05), while NHC was correlated with the labile fraction (r ¼ 0:631; Po0.05). The NHC represents the organic C fraction soluble in both alkali and acid, but under acid conditions it is not retained by the PVP resin during the purification of the FA fraction. NHC is composed of carbohydrate-like and protein-like molecules (Lowe, 1975). The labile fraction indicates thermal labile organic molecule whose chemical nature should be interpreted, once again, as simple carbohydrates, simple lipids, and amino acids (Dell’Abate et al., 1998, 2000). Therefore, since the NHC and the labile fraction are components of the easily degradable fraction, they could provide a suitable set of parameters with which to measure biological stability.

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As a result of this, both chemical and thermo-analytical methods can be used to measure biological stability, but then results need to be used correctly. This means that instead of the humified (humification indexes) and more recalcitrant (R1 index) fractions, indices must consider the easily degradable fraction (NHC and the labile fraction). Significant correlation was also observed between the above three indices and TEC (Table 6). TEC contains both humified [(HA+FA)C] and NHC. This contrasts with DRI, NHC, and the labile fraction, as these represent a measure of the content of easily degradable fractions. Nevertheless, methods used to extract humified material from compost (soil-derived methods) are not completely selective for humified material since co-extraction of a

Table 6 Multivariate analysis: significant contrasts observed for NHC, DRI and labile thermal fraction with the use of Pearson correlation coefficients Fraction

Contrasts

NHC

TEC 0.839** TOC 0.566* HI 0.682** DH 0.697** DRI 0.789** Labile 0.631*

DRI

TEC 0.790** NHC 0.789** Labile 0.651*

Labile

TN 0.721** TEC 0.888** C HAþFA 0.756** NHC 0.631* DRI 0.651*

** Statistically significant for P o 0.01.*Statistically significant for P o 0.05.

Table 5 Correlation matrix and Pearson coefficients (two-tail correlation) TOC

TN

TOC/ TN

TEC

CHAþFA

HI

NHC

DH

HR

DRI

Labile

Exo1

Exo tot

0.132 1

0.531** 0.753* 1

0.573** 0.572** 0.158 1

0.331 0.696* 0.403 0.758* 1

0.373 0.286 0.480 0.186 0.490 1

0.566* 0.261 0.104 0.839* 0.280 0.682* 1

0.380 0.263 0.457 0.206 0.473 0.994* 0.697* 1

0.718* 0.326 0.750* 0.029 0.394 0.731* 0.371 0.735* 1

0.425 0.118 0.143 0.790* 0.448 0.350 0.789* 0.341 0.061 1

0.416 0.721* 0.341 0.888* 0.756* 0.070 0.631* 0.034 0.133 0.651* 1

0.877* 0.036 0.587** 0.426 0.292 0.233 0.383 0.233 0.658* 0.372 0.218 1

0.937* 0.155 0.475 0.535** 0.336 0.315 0.506 0.323 0.688* 0.404 0.392 0.960* 1

0.463 0.527** 0.734* 0.129 0.122 0.109 0.089 0.083 0.547** 0.044 0.437 0.725* 0.536** 1

R1 TOC TN TOC/TN TEC C HAþFA HI NHC DH HR DRI Labile Exo1 Exo tot R1

1

Statistically significant for P o 0.01.*Statistically significant for P o 0.05.

**

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great deal of non-humified material occurs. This, above all, takes place for compost that has not been well stabilized, as was the case in this study (Adani et al., 1995; Genevini et al., 2002). Co-extracted materials are consistent with fatty acids, carbohydrates, and proteinaceous material, all of which represent molecules rapidly degraded during composting (Genevini et al., 2002). Therefore, for the composts studied, rather than humified material, it is assumed that TEC is primarily formed of easily degradable material, represented by the sum of NHC and of the nonhumified fraction co-extracted during the isolation of the humic fraction. Therefore, the correlations found for TEC vs. DRI, NHC, and the labile fraction reflect the main nature of TEC for unstable composts; i.e., the easily degradable fraction. 5. Conclusions Composting can be a reliable technology producing organic amendments for profitable use in agriculture. Consequently, compost quality, i.e., biological stability, must be assessed accurately. In using the definition of biological stability, the ability of a single measurement to describe the content of the easily degradable organic fraction must be used as a starting point in the development of a descriptive index. This study shows that the uses of biological (DRI), chemical (NHC), and thermochemical (labile fraction) indices measures the biological stability of composts because all three indices are able to measure the easily degradable organic fraction. As this is the first time that NHC and labile fraction have been investigated in this way, more in-depth research is required before these can become routine laboratory tests. References Adani, F., Genevini, P.L., Tambone, F., 1995. A new index of organic matter stability. Compost Science & Utilization 3, 25–37. Adani, F., Lozzi, P., Genevini, P.L., 2001. Determination of biological stability by oxygen uptake on municipal solid waste and derived products. Compost Science & Utilization 9, 163–178. Adani, F., Gonfalonieri, R., Tambone, F., 2004. Dynamic respiration index as descriptor of the biological stability of organic wastes. Journal of Environmental Quality 33, 1866–1876. Adani, F., Ubbiali, C., Genevini, P.L., 2006. The determination of biological stability of composts using the dynamic respiration index: the results of experience after two years. Waste Management 26, 41–48. AAVV, 1993. Science and engineering of composting. In: Hoitink, H.A.J., Keener, H.M. (Eds.), Science and Engineering of Composting: Design Environmental Microbiological and Utilization Aspects. Ohio State University, Columbus, OH, USA, p. 728. ASTM, 1996. Standard test method for determining the stability of compost by measuring oxygen consumption. American Society for Testing and Materials. ASTM International, West Conshohocken, PA, USA, D 5975-96. Barberis, R., Nappi, P., 1996. Evaluation of compost stability. In: De Bertoldi, M., Sequi, P., Lemmes, B., Papi, T. (Eds.), The Science of Composting. Blackie Academic and Professional, Glasgow, Scotland, pp. 175–184.

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