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TECHNICAL REPORTS
Waste Management

Relationship between Compost Stability and Extractable Organic Carbon

Department of Soil and Water Science, Univ. of Florida, Gainesville, FL 32611-0290

* Corresponding author (lqma{at}ufl.edu)

Received for publication June 8, 2001.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Establishing a simple yet reliable compost stability test is essential for a better compost quality control and utilization efficiency. The objective of this study was to examine the relationship between extractable organic carbon (OC) and compost stability based on 18 compost samples from five composting facilities. The compost samples were extracted sequentially with water for 2 h [water_(2h)] and 0.1 M NaOH for 2 and 24 h [NaOH_(2h) and NaOH_(24h), respectively]. The extractable OC was further separated into fulvic acid (FA) and humic acid (HA) fractions by adjusting the pH to <2. The mass specific absorbance (MSA) of OC in the six fractions was measured. Compost stability was estimated with a CO₂ evolution method. The extractable OC concentration was influenced by the total volatile solids and decreased with curing time for compost with a high level of extractable OC. The OC levels in each fraction were significantly correlated (p < 0.05) to each other except for the water_(2h)–extractable HA. In addition, all the FA and HA fractions except for water_(2h)–extractable HA were highly (P < 0.01) and linearly correlated to CO₂ evolution, but multiple regression showed that NaOH_(24h)–extractable OC was insignificant for CO₂ evolution. The relatively high slope of NaOH_(2h)–extractable FA versus CO₂ evolution suggests that this fraction may contribute the most to compost CO₂ evolution. The water_(2h)– and/or NaOH_(2h)–extractable FA tests are recommended for measuring compost stability because of their high correlation with CO₂ evolution. This estimation can be obtained through a simple photometric method covering a wide range of carbon concentrations up to 4000 mg L^-1.

Abbreviations: FA, fulvic acid • HA, humic acid • MSA, mass specific absorbance • OC, organic carbon

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
HUMIFICATION is widely considered an important process during the composting of organic materials where humic substances form and nonhumic substance decompose (Bernal et al., 1998; Hsu and Lo, 1999; Leita and Denobili, 1991; Miikki et al., 1997; Sanchez-Monedero et al., 1999; Schnitzer et al., 1993). As composting progresses, the percentage of humic substances is expected to increase relative to the total dry mass or the total organic matter. As a result, humification-related parameters have been examined to represent compost stability and maturity (Adani et al., 1997; Garcia et al., 1991; Jimenez and Garcia, 1992; Veeken et al., 2000).

The method commonly used to extract humic substances from composts is similar to that used to extract organic matter from soils, where humic substances are extracted with a dilute sodium hydroxide solution, sodium pyrophosphate solution, or a mixture of the two. The fraction that precipitates at pH < 2 is referred to as humic acid (HA), whereas the fraction remaining soluble at pH < 2 is defined as fulvic acid (FA). Contrary to what is expected, concentrations of HA either decreased during composting or in some cases the differences were not significant (Bernal et al., 1996; Garcia et al., 1991; Jimenez and Garcia, 1992). Adani et al. (1995) speculated that this discrepancy is due to the fact that NaOH-extractable organic carbon (OC) contains a considerable biodegradable nonhumus fraction, especially during the initial stages of composting.

Chefetz et al. (1998a) developed the concept of a core-HA fraction, which was obtained by removing the nonhumic substance from the HA fraction via successive extractions with an organic solvent followed by sulfuric acid treatments, and finally by an alkaline solution. The process is believed to be inert with respect to the core-HA fraction. Their research found that the OC content of the core HA relative to total dry mass increased while the HA decreased steadily with composting. Adani et al. (1995) found that the difference in OC between HA and core-HA fractions decreased during composting and proposed to use the ratio of core HA to HA as a new compost stability index (organic matter evolution index, OMEI), expressed as a number between zero and one. Though the relation of core HA to HA sheds light onto OC evolution during the composting process, it cannot be used as a practical indicator for compost stability, due to the extensive laboratory work needed to calculate this index.

In addition to the OMEI, the ratio of HA to FA, another humification-related parameter, has been used for assessing organic matter stability. The ratio of HA to FA invariably increased during composting, especially when the fraction of NaOH-soluble OC at pH < 2 is treated as FA (Garcia et al., 1991). This is because the FA fraction decreased more than the HA fraction during composting. However, due to the variability among composts of different material sources, the suggested critical value of this index covers a large range for different composts, even within the same composting system and time (Bernal et al., 1998). As a result, it is not feasible to use the ratio of HA to FA as the sole indicator of compost stability.

The primary objective of this study was to examine the relationship between water- and NaOH-extractable OC, and compost stability estimated by the CO₂ evolution rate. The extracted OC was further separated into FA and HA fractions to examine the relationship of these fractions to compost stability. The ultimate goal of this research is to develop a simple and reliable way to estimate the stability of composts from different source materials.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Compost Sample Collection and Preparation
Biosolids compost samples were collected from full-scale composting facilities (sample sets identified as Meadow, Register A, Register B, Winslow, Sunset, and Puerto Rico) in Florida and Puerto Rico. The Register A and B samples were collected from the same composting facility at different seasons. The composition and ratios of biosolids to other feedstock materials varied greatly from one facility to another but were consistent within each facility. Two to four samples were collected at each facility from different stages of the curing process as defined by the facility (Table 1) . Collected samples were placed in polyethylene bags, packed with ice in a cooler and shipped to the lab on the same day. Upon arrival, the samples were sieved through a 9.5-mm screen to remove large particles and air-dried. The Puerto Rico samples were collected and freeze-dried before shipping to our lab. Compost mass is expressed on a dry weight basis in this paper. The chemical and physical properties of the compost samples are listed in Table 1. For details on measurement of these parameters, please see Wu et al. (2000).

Organic carbon concentrations in the aqueous solutions were determined with a Shimadzu (Kyoto, Japan) TOC-5050A carbon analyzer. The OC concentration of the HA fraction was calculated as the difference between total OC and FA. The pellet containing HA was re-dissolved with 0.1 M NaOH. The absorbance of FA and HA solutions at 420 nm was measured with a Shimadzu UV160U UV-visible spectrophotometer with samples diluted to approximately 500 mg C L^-1 for FA and 100 mg C L^-1 for HA solutions. The readings were then used to calculate the mass specific absorbance (MSA), which is defined as the absorbance per unit mass of DOC (L mg^-1 m^-1).

Carbon Dioxide Evolution
Compost stability was measured based on CO₂ evolution rate, a measure of the microbial respiration of the compost samples using a modified procedure of Iannotti et al. (1994). Approximately 10 g dry weight of compost sample at 60% (w/w) moisture content was sealed in a 0.5-L vessel along with a beaker containing a known volume of 0.5 M NaOH solution. The samples were incubated for 7 to 8 d at room temperature (24 ± 2°C). During the incubation, the released CO₂ was captured by the NaOH solution, which was then analyzed titrimetrically at regular intervals. Since there is a large variation in the evolution of CO₂ during incubation, the peak CO₂ evolution rate was used to represent compost stability.

Statistical Analyses
The SAS (SAS Institute, 1996) procedure GLM with LSMEAN and PDIFF options was used to compute the p value of statistical differences between samples. Since compost stability and maturity degrees differed greatly from one facility to another, samples from each facility were treated as nested by facility. Correlation coefficients between parameters were calculated with the CORR procedure. The stepwise multiple correlation procedure was used to derive the multiple regression between CO₂ evolution rate and other parameters.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Compost samples used in this study varied greatly in their physical and chemical properties (Table 1), which directly affect compost stability measurement. In general, the Sunset samples had the highest water holding capacity and C to N ratio as a result of their high volatile solids and OC, whereas the Register samples had relatively high total nitrogen and OC. On the other hand, the Meadow and Puerto Rico samples had generally low values for all the parameters determined in this study. These differences among compost samples were mainly due to the differences in their source materials (Table 1).

Carbon Dioxide Evolution Rate
Generally speaking, CO₂ evolution rates decreased with curing (Table 2) . The decrease in CO₂ evolution rates was significant (p < 0.01) in the Register samples, which suggested that these samples were not stable. In comparison, decreases in the remaining samples were minor, indicating that they were relatively stable compost samples (Table 2). Hue and Liu (1995) used the mean plus two standard deviations of the CO₂ evolution rate from 14 commercial composts as the threshold level for a stable compost, which is 120 mg CO₂ kg^-1 h^-1 based on the average of the last 2 d of a 3-d incubation test. Based on this standard, all compost samples in the current study were stable except for the five samples from the Register facility having CO₂ evolution rates > 120 mg CO₂ kg^-1 h^-1 (Table 2). This was consistent with the conclusions drawn from the changes in CO₂ evolution rate with curing. The average CO₂ evolution rate for the remaining samples was 55 mg CO₂ kg^-1 h^-1 with a standard deviation of 21 mg CO₂ kg^-1 h^-1. A value of 98 mg CO₂ kg^-1 h^-1 was obtained as the threshold level based on this current study following the method of Hue and Liu (1995), which was close to 120 mg CO₂ kg^-1 h^-1. This suggests that an index based on CO₂ evolution rate may be a viable parameter to use in screening for unstable composts with diverse source materials.

As expected, the distribution of extractable OC (sum of HA and FA) across the three extractions increased in the order water_(2h), NaOH_(2h), and NaOH_(24h), which accounted for 2 to 34, 27 to 40, and 39 to 70% of the total extractable OC, respectively. This direct correlation with extraction intensity can be attributed to the proportional increase in HA (Table 2), raising the ratio of HA to FA. It is generally believed that FA is more soluble, and thus more easily extractable than HA.

It is reasonable to assume that the water-extractable OC would be more degradable than the NaOH-extractable OC, which was confirmed by our data. With curing, the water-extractable OC decreased much more significantly than the NaOH-extractable OC (Table 2). In the Register A and B samples, for example, reduction in total water-extractable OC was 39 and 46% compared with 26 and 28% for the NaOH-extractable OC, respectively. This is partially because of higher ratios of HA to FA in the NaOH-extractable OC than in the water-extractable OC. However, no consistent trend with curing was observed between the NaOH_(2h)– and NaOH_(24h)–extractable OC [i.e., the NaOH_(2h)–extractable OC was not necessarily more degradable than the NaOH_(24h)–extractable OC] (Table 2).

As discussed earlier, total extractable OC in each extraction decreased with curing (Table 2). The reduction was more pronounced in samples with high extractable OC, such as the Register samples. The decrease of water-extractable OC with composting has been reported previously (Bernal et al., 1998; Chefetz et al., 1998b; Hue and Liu, 1995). Hue and Liu (1995) suggested using 10 g of water-extractable OC per kg dry matter as the threshold value for stable compost. Use of this value in the current study yields results similar to those obtained based on CO₂ evolution rate (i.e., all compost samples, except for Register, were stable) (Table 2).

Composting is believed to be a humification process, thus, concentration of humic substances is expected to increase with composting. Due to the differences in HA and FA, they behave differently during composting. Using a single NaOH extraction, inconsistent changes in HA have been reported (Bernal et al., 1998; Calace et al., 1999; Pascual et al., 1997; Sanchez-Monedero et al., 1999). On the other hand, significant and consistent reduction in FA concentrations was observed in almost all studies. Our results are consistent with these previous studies. The ratios of HA to FA generally increased with curing primarily due to the consistent and substantial reduction of FA during curing. However, such changes were inconsistent, which agreed well with data published by Bernal et al. (1998). This was most likely because our samples were collected from different curing ages, which span a relatively short period of time (<30 d). Thus, changes in the ratio of HA to FA during this period were probably insignificant. The ratios of HA to FA also varied greatly with composting source materials, and seemed to be less correlated with other parameters that measure compost stability.

Similar to the changes in extractable OC, the ratios of HA to FA increased in the following order: water_(2h) (0.11 to 0.99), NaOH_(2h) (0.41 to 3.48), and NaOH_(24h) (1.03 to 4.01), or with increased extraction intensity. It is apparent that these three fractions contain OC of different degrees of polymerization and aromaticity, which can be evaluated based on mass specific absorbance (MSA). The MSA has previously been used to characterize the degree of humification of a water-soluble humic substance (Battin, 1998). The greater the MSA, the higher the degree of polymerization and aromaticity. Thus, MSA of HA is expected to be greater than that of FA, and MSA of the NaOH-extractable HA and FA greater than that of the water-extractable HA and FA (Table 3) . In general, MSA of HA and FA increased with curing (with some exceptions), indicating an occurrence of humification during composting (Table 3).

Relation of Extractable Organic Carbon to Carbon Dioxide Evolution Rate
Concentration of OC in each of the nine fractions was highly (p < 0.01) and linearly correlated to CO₂ evolution rate (r = 0.59 to 0.89), except for the water_(2h)–extractable HA (r = 0.27) (data not shown). The NaOH_(2h)–extractable FA had the highest correlation coefficient (r = 0.95, p < 0.001) followed by the water_(2h)–extractable FA fraction (r = 0.89, p < 0.001) (Fig. 1) . The combination of the water_(2h)–extractable FA and NaOH_(2h)–extractable FA was also highly correlated with CO₂ evolution rate, with r = 0.93 (p < 0.001). Thus, FA concentrations of NaOH_(2h) or water can be used to indicate compost stability. In addition, FA can be easily measured by a simple photometric method, since the MSA of FA is within a relatively narrow range of 0.05 to 0.18 L mg^-1 m^-1 (Table 3). In fact, FA concentration was highly correlated to its absorption with r = 0.95 (p < 0.001) for the water_(2h)–extractable FA and r = 0.87 (p < 0.001) for NaOH_(2h)–extractable FA fraction (Fig. 2) . Concentrations of OC in these compost extracts covered a wide range up to 4 g L^-1 C, which makes it possible to determine OC concentration in a simple fashion.

View larger version (15K): [in this window] [in a new window]   Fig. 1. Relationship between compost CO2 evolution and organic carbon concentration. Water(2h)–extractable fulvic acid and NaOH(2h)–extractable fulvic acid represent samples sequentially extracted for 2 h with water and NaOH, respectively.

View larger version (13K): [in this window] [in a new window]   Fig. 2. Relationship between organic carbon concentration of fulvic acid and absorbance at 420 nm. Water(2h)–extractable fulvic acid and NaOH(2h)–extractable fulvic acid represent samples sequentially extracted for 2 h with water and NaOH, respectively.

[1]

The regression model includes water_(2h) and NaOH_(2h) fractions, which account for 95% (p < 0.01) of the variability, and indicates that the NaOH_(24h)–extractable OC does not significantly contribute to CO₂ evolution. It may also indicate that the NaOH_(24h)–extractable OC is either a stable humic substance or a lignin-type material that is resistant to decomposition, although the percentage of total extractable OC is the highest for this fraction.

It is interesting to note that the water_(2h)–extractable HA had a negative effect on CO₂ evolution (Eq. [1]), while both NaOH_(2h) fractions contributed positively to CO₂ evolution. Based on fractionation and NMR data, Chefetz et al. (1998b) concluded that one fraction of water-extractable HA was related to real HA and this fraction represents an intermediate state in the humification process. The negative correlation between CO₂ evolution rate and the OC content of water_(2h)–extractable HA leads one to speculate that the water_(2h)–extractable HA fraction was not an easily available carbon source for microbial growth. Rather, it may ultimately polymerize and precipitate into water-insoluble HA as the compost further matures.

The ratio of HA to FA is not significantly related to CO₂ evolution rate if all of the samples are considered together, although it was significantly (p < 0.01) correlated to CO₂ evolution rate for Register samples (data not shown). A similar observation was made by Jimenez and Garcia (1992) and Bernal et al. (1998). They attributed this inconsistency to the dependency of this parameter on the differences in compost source materials.

	CONCLUSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Extractable OC decreased with curing, which was influenced by the composition of compost source materials. Except for water_(2h)–extractable HA, the OC contents in the FA and HA fractions of each fraction correlated to each other and the CO₂ evolution rate, but multiple regression data showed that the NaOH_(24h) fraction is insignificant in contributing to the total CO₂ evolution. The water_(2h)–extractable FA and/or NaOH_(2h) tests are recommended as indicators for compost stability due to their high correlation with compost stability estimated by CO₂ evolution. The FA concentrations can be estimated by a simple photometric method covering a wide range of C concentrations.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Approved for publication as Florida Agricultural Experiment Station Journal Series no. R-08752.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (14) Citing Articles via Google Scholar Google Scholar Articles by Wu, L. Articles by Ma, L.Q. Search for Related Content PubMed PubMed Citation Articles by Wu, L. Articles by Ma, L.Q. Agricola Articles by Wu, L. Articles by Ma, L.Q. Related Collections Humic Substances Other Waste Management