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TECHNICAL REPORTS

Waste Management

An Integrated Chemical, Thermal, and Microbiological Approach to Compost Stability Evaluation

^a Istituto Sperimentale per la Nutrizione delle Piante, Via Trieste 23, I-34170 Gorizia, Italy
^b Istituto Sperimentale per la Nutrizione delle Piante, Via della Navicella 2/4, I-00184 Rome, Italy

^* Corresponding author (claudio.mondini{at}isnp.it).

Received for publication July 26, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The evaluation of compost stability is of the utmost importance for the reliability of composting as a recycling strategy. To date there is no single parameter that can give a sure indication of the stability of composts from different starting materials. This paper investigates different methods of evaluating the dynamics of transformation of materials and the stability level of the end products in a composting process. The following parameters were determined on compost samples of different ages from cotton (Gossypium herbaceum L.) cardings and yard wastes: humification index (HI), degree of humification (DH), thermogravimetry (TG) microbial biomass C (B_C), and ninhydrin-reactive N (B_NIN). Finally, from TG, derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC) thermal stability parameters were deduced. Humification parameters in the end products (0.2 and 81% for HI and DH, respectively) showed the effective stability reached by the organic matter (OM). Thermal analysis evidenced the presence of two main organic pools with different thermal stability. During composting a relative increase in the more stable organic pool was indicated by the variation of the thermostability index R1 from 0.41 to 0.74. The parameter R1 was significantly correlated with both HI (r = -0.94; P < 0.05) and DH (r = 0.97; P < 0.05). Microbial biomass content dynamics reflected the availability of readily decomposable substrates. The ratio between B_NIN and total N in the end product was 0.96%, indicating a good stability level. The simultaneous application of different approaches, considering different properties of composting materials, provides a more complete description of the stability and quality reached by the organic materials.

Abbreviations: B_C, microbial biomass carbon • B_NIN, microbial biomass ninhydrin-reactive nitrogen • C_org, organic carbon • DH, degree of humification • DSC, differential scanning calorimetry • DTG, derivative thermogravimetry • FA, fulvic-like acids • FE, fumigation-extraction • HA, humic-like acids • HI, humification index • NH, nonhumic fraction • N_TOT, total nitrogen • OM, organic matter • R1, thermostability index • S_TOT, total sulfur • TEC, total extracted carbon • TG, thermogravimetry

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE QUALITY OF COMPOST mainly depends on the level of OM stability (Wu et al., 2000). In fact, application on soil of nonstabilized organic materials could affect both crops and the environment because of the presence of phytotoxic compounds (Butler et al., 2001). During composting the most biodegradable organic compounds are broken down and part of the remaining organic material is converted into humic-like substances (Hsu and Lo, 1999; Sanchez Monedero et al., 1999; Wu and Ma, 2002). Several indexes and methods have been proposed for evaluating compost stability (Bernal et al., 1998; Itävaara et al., 2002; Wu et al., 2000; Wu and Ma, 2001). However, to date, there is no single method that can be successfully used alone on compost from different organic residues (Barberis and Nappi, 1996; Chen et al., 1996; Itävaara et al., 2002) due to the widely different chemical characteristics of organic wastes. The utilization of different parameters and indexes that address different properties of composting materials can give a more complete picture of the degree of transformation achieved by the organic materials.

Humification parameters, based on the fractionation of the extractable humic-like and nonhumic organic C, have been successfully used for evaluating the stability level of compost (Dell'Abate et al., 1998; Hsu and Lo, 1999; Tittarelli et al., 2002). However, the extraction and fractionation of OM could have drawbacks, such as the alteration of some organic constituent (Stevenson, 1994) and the presence of substances interfering with the determination of humic-like substances (Adani et al., 1995). Additional knowledge on the degree of transformation of the OM could be obtained from other analytical techniques, such as thermal methods. Thermal analysis has the advantage of being simple, fast, reproducible, and can be performed on the whole sample without requiring pretreatment. Thermogravimetry and DSC were used to assess compost stability and maturity (Blanco and Almendros, 1994, 1997; Dell'Abate et al., 1998, 2000; Dell'Abate and Tittarelli, 2002). Composting of organic materials is basically a microbially mediated process, so parameters related to microbial amount and activity could be useful for characterizing the process. The holistic concept of soil microbial biomass, considering the microbial community as a unique functional entity, has made it possible to evaluate the amount and activity of microorganisms in a substrate by means of easily determined parameters. Application of these parameters to compost substrates has been shown to provide valuable information on the dynamics of the process and evaluation of the end product quality (De Nobili et al., 1996; Mondini et al., 1997, 2002).

Integrating chemical parameters, such as those related to both the humification process and thermal analysis, with microbiological parameters could be useful for a better definition of compost stability, but also to study the influence of starting materials and different composting strategies on the quality of the end products (Dell'Abate et al., 1998). Knowledge on the factors affecting the process could also lead to its optimization, thus implying a better quality of the end product and lower production costs.

The aim of this work was to study the dynamics of transformation of organic materials during composting and determine the stability level of the end product by integrating chemical, thermal, and microbiological approaches.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Composting Process and Sampling
Cotton carding residues and yard wastes (brush, leaves, and grass clippings) were carefully mixed in a proportion of 1.1:1 v/v corresponding to 1:1.5 w/w. The C/N ratios of starting materials were 23.5 and 101 for cotton and yard wastes, respectively. The mixture was brought to a C/N ratio of 24.5 and moisture content of 65% by the addition of 1.2% (w/v) urea solution. The mixture was settled in three plastic boxes with holes (106 by 90 by 42 cm deep; 0.4 m³) placed outdoors under a shed. Temperature (daily) and water content (weekly) of the composting mixture were measured throughout the process. Temperature was collected by inserting a thermocouple into the pile. During the composting process, adequate aeration and water content were provided by manual turning and supplying water. Manual turning was done twice a week in the first 2 wk and weekly thereafter. Watering was done on Days 7 and 14 to bring the mixture to approximately 65% water content. Five subsamples of composting material (about 1 kg total) were taken from each box on Days 0, 5, 19, 60, and 103 (C₀, C₅, C₁₉, C₆₀, and C₁₀₃) and carefully mixed. An aliquot of each sample was air-dried for about 10 d before chemical and thermal analysis, while the remaining portion was immediately analyzed for microbial biomass content.

Organic Carbon, Total Nitrogen, and Total Sulfur Analysis
Organic C (C_ORG), total N (N_TOT), and total S (S_TOT) were determined in duplicate for each sample by a dry combustion method with an NA 1500 elemental analyzer (Fisons, CE Instruments, Milan, Italy) on aliquots of dried samples ground to <0.5 mm. To optimize the analytical conditions independent measures of S_TOT were taken, while N_TOT and C_ORG were determined simultaneously.

Humification Parameters
The extraction of the organic C was performed at 65°C for 24 h using a 0.1 M NaOH + 0.1 M Na₄P₂O₇ solution. The samples were then centrifuged at 5000 x g and the supernatant filtered through a 0.20-µm Millipore filter (Millipore, Billerica, MA) (total extracted C, TEC). The humic-like acid fraction (HA) was separated from the fulvic-like acid (FA) and nonhumic fractions (NH) by precipitation after acidification of the alkaline solution at pH <2. The NH was separated from the FA by chromatography onto a column of polyvinylpyrrolidone (PVP, Aldrich, Germany). The FA was then combined with the HA (HA + FA). The organic C content of each fraction (TEC, NH, HA + FA) was determined according to a wet dichromate oxidation procedure (Walkley and Black, 1934).

Extraction and fractionation were done in a single replicate on each sample, while determination of the organic content in the extracts was performed in triplicate.

The following two humification parameters were calculated:

1) Humification Index (Sequi et al., 1986):

2) Degree of Humification (Ciavatta et al., 1990):

Thermal Analysis
Differential scanning calorimetry and TG methods are based on programmed heating of the sample in a controlled atmosphere. Different components in the sample, which undergo transformations at different temperatures, produce a graph whose shape reflects the chemical composition and structure of the sample and the energy involved in the reaction. Basic treatment of these techniques can be found in MacKenzie (1970) and Speyer (1994). Thermal experiments were duplicated for each sample using an STA 409 Simultaneous Analyzer (Netzsch, Selb, Germany) equipped with a TG/DSC sample carrier supporting a type S thermocouple constituted by an alloy of Pt and Rh 10% and Pt (PtRh10-Pt). In DSC measurement, the temperature difference between sample and reference material is recorded as a direct measure of the difference in the heat-flow rates; in TG, the weight change of a sample is measured during the thermal program. The first derivative of the TG trace represents the weight loss rate (expressed as % min^-1): its calculation allows the distinction among subsequent decomposition steps which are evaluated on the basis of the onset and end peak temperatures of the DTG curve.

The whole samples were analyzed after grinding in a micro-mill (Culatti IKA, Janke & Kunker, Germany) with a 1.00-mm sieve and then manually in an agate mortar. Thermal runs were also done on cotton, separated from the input materials. The conditions were as follows: heating rate of 10°C min^-1 from 20 to 900°C, static air atmosphere, alumina crucible, calcined kaolinite as reference, sample weight about 10 mg. The thermobalance was calibrated for buoyancy effects to obtain a quantitative estimate of weight changes. Heat production in the heat-flux DSC was calibrated under the same conditions by using a sapphire standard and subtracting a baseline obtained by an additional run for the empty crucibles. The Netzsch applied software, SW/c_p/311.01, was used for data processing.

Microbial Biomass Carbon and Ninhydrin-reactive Nitrogen
Microbial biomass C and B_NIN content were determined in triplicate for each sample using the fumigation-extraction (FE) method (Vance et al., 1987). Moist samples (corresponding to approximately 25 g of compost dry weight) were fumigated with ethanol-free CHCl₃ for 24 h at 25°C in a desiccator. After fumigant removal, the samples were extracted for 30 min with 400 mL of 0.5 M K₂SO₄ solution and then filtered through Whatman No. 42 filter paper (Whatman International Ltd., Maidstone, UK). Non-fumigated samples were extracted as above at the time fumigation started.

The K₂SO₄–extractable organic C was determined by an automated ultraviolet (UV) persulphate oxidation procedure (Wu et al., 1990). Microbial biomass C was calculated as:

where E_C = [(C from fumigated samples) - (C from nonfumigated samples)].

The K₂SO₄–extractable ninhydrin-reactive N (N_NIN) was determined as described by Joergensen and Brookes (1990). Microbial biomass ninhydrin-reactive N was calculated as:

Statistical Analysis
All results were expressed on an oven-dry sample basis (105°C, 24 h) and are the mean of three or six sample replicates. Data underwent univariate ANOVA and sample means were compared using the Student–Newman–Keuls test (Snedecor and Cochran, 1980). The relationships between thermostability index R1 and HI and DH were analyzed by the Pearson correlation coefficient. All statistical analyses were performed using SPSS Version 9.0 statistical package (SPSS Inc.,1999).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Organic Carbon, Total Nitrogen, and Total Sulfur Content
Organic C of mixed starting materials was 51.5% and slightly decreased during the process to a value of about 49% in the end products (Table 1). The opposite was recorded for N_TOT and S_TOT, which doubled their relative amounts during the process. The N_TOT and S_TOT contents at the end of the process were 4.3 and 0.43%, respectively. The relative amounts of C_ORG and N_TOT in the end product were far higher than those reported in the literature for similar products (Centemero et al., 1999; Giandon et al., 1999).

The different dynamics of the three elements were reflected by the C_ORG/N_TOT, C_ORG/S_TOT, and N_TOT/S_TOT ratios. The C_ORG/N_TOT ratio decreased from 24.5 at the start of the process to 11.4 in the end product: this value is considered optimal for soil application to avoid microbial immobilization of N. The value of the C_ORG/S_TOT ratio decreased from more than 200 at the start, to about 100 at the end of the process, reaching the value range (60–130) normally found in soil OM (Stevenson and Cole, 1999). The ratio between N_TOT and S_TOT remained practically constant with a value of about 10 (Table 1).

Humification Parameters
The TEC in the NaOH + Na₄P₂O₇ alkaline solution remained practically constant during the process (Table 2), whereas the dynamics of the humification parameters indicated a significant change in the quality of extracted C, with the transformation of part of the nonhumified C into more stable chemical forms of C (humic-like substances). The extractable organic C in the end product reached a high level of stability, as indicated by values of 0.2 and 81% for HI and DH, respectively. Ciavatta et al. (1990) suggested 0.5 as the threshold value for the HI parameter in well-stabilized compost, whereas the DH value was higher than those normally found in end products of compost (Ciavatta et al., 1993; Govi et al., 1993, 1994).

View this table: [in this window] [in a new window]   Table 2. Humification parameters of compost samples with different levels of stability.

Thermal Analysis
In Fig. 1 , thermograms (DSC, TG, and DTG traces) of the cotton sample and samples taken at 0, 5, 19, 60, and 103 d are reported, while in Table 3 the deduced thermal parameters are summarized. The initial sample C₀, constituted by the mixed input materials, and sample C₅, taken 5 d after the start of the composting process, gave thermal profiles strongly characterized by the presence of the cotton material, thus indicating that very little transformation of OM occurred during the first days. In particular, the initial sample C₀, after an endothermic dehydration reaction at low temperature (up to 180°C), showed on the DSC curve a three-steps pattern of thermal oxidation in the 200 to 560°C temperature range (Fig. 1): it began with an intense sharp exothermic reaction, with a peak on the DSC trace at about 315°C, followed by a less intense one at about 400°C and a third intense effect in the range 450 to 540°C. The first reaction was associated with a consistent weight loss (60.3%), recorded by the TG trace and due to labile OM oxidation, which reached its maximum rate (DTG peak, 8.4% min^-1) at about 300°C. Samples at more advanced stages of the composting process (C₁₉, C₆₀, and C₁₀₃) showed thermal patterns progressively differentiated from the starting input materials (Fig. 1). The thermal behavior of sample C₁₉ was quite different from those of C₀ and C₅: OM oxidation occurred in a wider temperature range (from 200 to 620°C) and involved two main organic pools, which gave well-resolved exothermic peaks on the DSC curve at 315 and 490°C, respectively; the first one less intense than the second. On the DTG trace a lower weight loss rate was registered for the first oxidative decomposition reaction, which had a maximum value of 4.6% min^-1 at about 303°C. Finally, at about 780°C an endothermic carbonate decomposition reaction was observed.

View larger version (42K): [in this window] [in a new window]   Fig. 1. Differential scanning thermogravimetry (DSC), thermogravimetry (TG), and derivative thermogravimetry (DTG) thermograms of cotton separated from input materials and of samples C0, C5, C19, C60, and C103 taken at different stages during the composting process.

The distinct weight losses registered on the TG curves permitted the deduction of the thermal parameters reported in Table 3. Sample C₅ showed a high amount of total OM (83.4%), 69% of which was constituted of the more thermally labile fraction. These values decreased in C₁₉ samples to 75.1 and 59%, respectively. The samples taken after 60 d (C₆₀) were characterized by less combustible OM (70.1%), 56.4% of which contributed to the first exothermic reaction. Similar results were obtained with the samples taken at the end of the process (C₁₀₃). These results again indicate that OM transformation occurred mainly during the first 19 d of the process. However, changes of thermal stability in organic materials detected during this composting process can be considered slight if compared with previous results (Dell'Abate et al., 1998, 2000). The index R1 (Table 3), which is the ratio between the weight losses associated with the second and first exothermic reactions, increased from 0.41 (sample C₀) to 0.66 in sample C₁₉ and 0.77 in sample C₆₀. A slight (but not statistically significant) decrease to 0.74 was observed after the curing phase (sample C₁₀₃), probably due to rearrangements of humic-like molecules as found previously (Dell'Abate and Tittarelli, 2002). Finally, the last column of Table 3 reports the relative amount of residues from the combustion process, consisting of inorganic ash: values increased with composting time according to the decrease in total organic matter content of the sample.

The results of the thermal analysis were in agreement with those of humification parameters, in fact a significant correlation between parameter R1 with both HI (r = -0.94; P < 0.05) and DH (r = 0.97; P < 0.05) was found (Fig. 2) .

View larger version (12K): [in this window] [in a new window]   Fig. 2. Correlation between the thermostability index R1 and (a) humification index (HI) and (b) degree of humification (DH) in compost samples with different levels of stability.

View this table: [in this window] [in a new window]   Table 4. Microbiological parameters of compost samples with different levels of stability.

The low values of B_C and B_NIN recorded 5 d after the start of the process could be due to the fact that the microorganisms were not well established on the fresh materials and/or to the inadequacy of the FE method when applied to compost samples in the early stages of the process. A correct interpretation of microbial biomass data depends on extraction efficiency, so a different microbial extractability from the fresh or from the decomposed material could have affected the results. Horwath and Elliott (1996) recorded a variation in the extraction of microbial C over N according to incubation time in ryegrass (Lolium perenne L.).

The close relationship between N_TOT and microbial biomass N in soil has been well established. In addition, the percentage of biomass N with respect to N_TOT in soils at equilibrium varies within a quite narrow range (2–6%) depending on soil type and management (Brookes et al., 1985). Higher percentages of soil microbial biomass with respect to N_TOT were recorded when the content of readily decomposable substrate was increased due to soil amendment with OM (Powlson et al., 1987). Therefore, in the early stages of composting, with a high content of easily available substrates, it could be expected that the microbial N content represents a high percentage of N_TOT. Conversely, in stabilized compost, where the easily available substrate content is reduced, it is reasonable to assume that the percentage of microbial N with respect to N_TOT is lower and varies within a narrow range. Therefore, assuming that the microbial biomass content in compost reflects the quantity of available substrate, calculation of the B_NIN/N_TOT ratio could be useful to evaluate the stability level of the compost. In addition, the standardization of B_NIN data with respect to the N_TOT content allows a more correct comparison with different processes and starting materials. The dynamics of this ratio in 10 different composting processes (more than 100 processing days) showed a decreasing trend, with values in end products of between 0.5 and 1.1% (De Nobili et al., 1996; Mondini et al., 1997 and Mondini, unpublished data, 2001). Excluding the sample C₅, because of the potential limitation to microbial determination outlined above, the B_NIN/N_TOT ratio in this work showed a decreasing trend, with a value recorded at the end of the process of about 1% (Table 4).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The results obtained from the different parameters used to evaluate the degree of transformation of cotton and yard wastes during composting (C/N ratio, humification parameters, thermostability index R1, B_NIN/N_TOT ratio) demonstrated the effective stability reached by the end product.

Moreover, considering different properties of composting substrates was useful for elucidating specific traits of the degradation process and to support results obtained with a single parameter. Dynamics of thermal and humification parameters during the composting process showed the presence of two main organic pools with different stability and different rates of OM transformation. Comparison of thermal patterns of samples with those of starting materials allowed a more complete description of the degree of transformation reached by the organic substrates. The reliability of B_NIN/N_TOT ratio as indicator of readily available substrate level was confirmed by dynamics of humification and thermal properties, in particular of the extractable nonhumic substances content and the weight loss associated with the first OM exothermic reaction, representing the thermally more labile fraction of OM.

Composting as a reliable strategy for the sustainable recycling of organic wastes mainly depends on the stability level of the end products. However, evaluating compost stability is difficult due to the widely different properties of the organic substrates. The results of this work showed that the simultaneous application of different approaches, including chemical, microbiological, and thermal characteristics of composting materials, provides a more thorough description of the stability level reached by organic wastes during the composting process.

	ACKNOWLEDGMENTS

 
We thank Dr. F. Fornasier for elemental analysis and Mrs. E. Vida for valuable technical support. This research was supported by a grant from the Italian Ministry for Agricultural and Forestry Policies (MIPAF), Project Parsifal, General Series, Paper no. 6.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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