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a USDA-ARS-AMBL, Beltsville, MD. 20705-2350
b Dep. of Animal and Avian Sciences, Univ. of Maryland, College Park, MD 20742-4452
* Corresponding author (SikoraL{at}ba.ars.usda.gov)
Received for publication March 3, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BVS, biological volatile solids • CEC, cation exchange capacity • VS, volatile solids • WSSC, Washington Suburban Sanitary Commission
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Aerobic composting is the biological oxidative decomposition of organic materials by successive communities of microorganisms under different temperature regimes, which produce a humified end-product. The basic principles of the composting process have been discussed extensively in the literature (Gouleke, 1972; Haug, 1980; Polprasert, 1989, p. 63–80; Miller, 1994; Stratton et al., 1995). The degree of completion of the composting process can be evaluated by measuring various changes in the chemical, physical, and biological properties of the substrate. Although for the purposes of discussion the term "compost maturity" can be used generically to refer to the degree of completion of the composting process, the technical terms "compost maturity" and "compost stability" are not synonymous (Iannotti et al., 1993). Compost maturity refers to the degree of humification of the material. Compost stability refers to the level of activity of the microbial biomass. The degree of compost maturity at the conclusion of the process is critical to the marketability of the product.
One problem associated with immature compost is continued decomposition. The continued decomposition of an immature compost in soil can induce anaerobic conditions as the microbial biomass utilizes oxygen in the soil pores to break down the material. This in turn can deprive plant roots of oxygen, and lead to the generation of hydrogen sulfide (H2S) and nitrite 
(Mathur et al., 1993). Another important problem associated with the application of immature composts to soils is the prevalence of a high (25:1 or greater) carbon to nitrogen ratio in such materials (Iglesias Jimenez and Perez Garcia, 1989). Immature composts with a high C to N ratio can induce nitrogen starvation in plants as microbes scavenge soil N to make up the deficit. Other problems include phytotoxicity due to the presence of organic acids as the intermediate by-products of continuing decomposition. Acetic acid and phenolic compounds, in particular, may suppress seed germination, inhibit root growth, or suppress crop yields (Zucconi et al., 1981a,b; Chanyasak et al., 1983a,b; Hadar et al., 1985).
Cation exchange capacity (CEC) has been heavily investigated as a potential indicator of compost maturity. The CEC in an organic material increases as a function of humification due to the formation of carboxyl and phenolic functional groups (Lax et al., 1986). Carboxyl groups are formed by the oxidation of lateral chains of the aromatic rings or the hydrolysis of esters or lactones (Lax et al., 1986). Harada and Inoko (1980) developed a method for determining the CEC of municipal refuse compost as an indicator of its maturity. These researchers considered a municipal refuse compost with a CEC of greater or equal than 60 cmol kg-1 expressed on an ash-free basis to be sufficiently mature for application to the soil. Harada et al. (1981) found a strong negative correlation between CEC and the solid C to N ratio of municipal refuse compost (r = -0.94). Saharinen (1998) reported that CEC should be calculated per unit of ash since this prevents the overestimation of the number of cation exchange sites on composted materials.
Other researchers have proposed microbial respiration rate as a potential indicator. Willson and Dalmat (1986) proposed an oxygen uptake rate of 20 mg kg-1 h-1 as an acceptable criterion of maturity. Iannotti et al. (1993) found an initial high rate of oxygen consumption (2.0 mg g-1 volatile solids [VS] h-1), which declined to 0.50 mg g-1 VS h-1 for municipal refuse compost by Day 31. Iannotti et al. (1994) reported that oxygen respirometry was highly correlated with composting time (r = 0.80, P = 0.01).
The Dewar flask self-heating test (Niese, 1963; Brinton et al., 1995) evaluates compost stability based on the rise over ambient temperature of compost in an insulated thermoflask. Richard and Zimmerman (1995) attempted to relate oxygen uptake to the maximum reheating potential but found that, whereas respiration rate clearly declined with the increasing age of the municipal refuse compost, temperature was less conclusive in predicting the stabilization process.
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Cold Storage Effects |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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However, Zelles et al. (1991) found that air-drying soil samples prior to storage was more detrimental to retaining biological activity than storing moist samples at 4°C, -18°C, or -140°C up to 20 mo. Stenberg et al. (1998) found that microbial biomass and activity in moist soil samples suffered more pronounced effects under refrigeration for 13 mo at 4°C than at -20°C.
Degradation occurs because psychrotrophs tend to predominate in refrigerated foodstuffs (Prieto et al., 1991; Ken-Yuon and Torres, 1993; Cox et al., 1998), and may be at least partially responsible for the continuing humification of compost under refrigeration. Psychrotrophs are versatile and are able to grow over a wider temperature range than psychrophilic organisms (Russell, 1992). Herbert and Bhakoo (1979), citing Morita (1975), indicate that while psychrotrophs grow at 0°C, their maximum growth temperature exceeds 25°C.
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Experimental Objectives |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Raw Materials
Composting was conducted using the Beltsville extended, aerated, static pile method. The piles were indoors and aerated using negative pressure. Vacuum-filtered, lime-stabilized biosolids (sewage sludge) from the Blue Plains sewage treatment plant in Washington, DC was mixed with wood chips in a ratio of 4:3 parts wood chips to biosolids by volume at the Washington Suburban Sanitary Commission's (WSSC) composting plant (Site 2) in Silver Spring, MD. The mixture was placed on a bed of woodchips containing aeration pipe and composted using the Beltsville static-aerated pile method (Willson et al., 1980). Aeration was monitored by oxygen meters and temperature was monitored by thermocouples placed at a 1.5-m depth from the surface at 6.1-m intervals in each pile.
Composting Method
Individual piles were 3.0 x 6.1 x 30 m (height x width x length) in dimension and contained approximately 549 m3 of material. They were constructed side by side in rows that contained 20 piles per row (approximately three piles were constructed daily). The pile used for sampling was marked off by stakes to separate it from other piles. Two and three tenths meters of fresh material was bedded on 0.3 m of wood chips and covered by 0.5 m of aged compost as cover. The piles were indoors and aerated using negative pressure.
Composting was conducted in two stages. Days 1 through 28 comprised the "actively composting" pile (Stage I). After Day 28, the pile was torn down and sieved through a 12.7-mm sieve to remove large wood chips. The sieved material was moved to the curing building, where a new aerated pile (Stage II) was constructed.
In Stage I, samples were collected at a depth of 0.9 m from the surface. Samples (38 L each) were collected at five sampling locations, every 6.1 m along the 30-m length of the pile at locations where thermocouples had been placed.
The Stage II pile was constructed consisting of material from three actively composting piles, including the sampled pile. The sampled pile was staked off to differentiate it from the other two piles. The dimensions of the Stage II pile were 3.0 x 3.0 x 6.1 m. Due to the much smaller dimensions of the Stage II pile, only three buckets of samples (57 L) were collected at a depth of 0.5 m in 1.2-m intervals per sampling date. After Day 30, the Stage II pile was torn down.
Sampling Methodology
Samples were collected biweekly from the Stage I pile (Days 1 and 15). Five buckets of samples (95 L) were collected per sampling date from the Stage II pile. Due to problems with odor, Stage I was torn down at Day 21 and used as additional cover for other piles. A second Stage I pile at the compost site was then sampled at its Day 15, and was followed through Stage II at Days 29, 36, 43, 50, and 57 (Table 1).
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Test Methods
The biological volatile solids (BVS) content of the composts was obtained after combustion of oven-dry solids at 550°C for 12 h. The Dewar flask reheat test (Niese, 1963; Woods End Research Laboratory, 1993; Brinton et al., 1995) was performed in 2-L insulated Dewar flasks using min-max thermometers. Samples were sieved through a 4-mm sieve, moistened to approximately 50% moisture (wet weight), and placed in the Dewar flask. The maximum daily temperature was recorded as well as the daily ambient temperature over a 5-d incubation period.
The oxygen uptake rate was measured using a modified method based on Iannotti et al. (1993) and the U.S. Composting Council (1995). A refrigerator incubator was used instead of a water bath to incubate the samples at 37°C. Samples were sieved through a 4.0-mm sieve and brought to approximately 50% moisture prior to analysis.
Due to the high lime content of the biosolids compost, a double extraction procedure for calcareous soils using a 0.4 M sodium acetate–0.1 M sodium chloride solution followed by 1.0 M magnesium nitrate was followed to analyze the cation exchange capacity (Polemio and Rhoades, 1977).
Statistical Analysis
The experiment was designed as an incomplete factorial in which the relationship between the dependent variables (maximum rise over ambient temperature, log10 of the oxygen uptake rate, and CEC) and the fixed independent variables (time [expressed in days] and storage [expressed in weeks]) was examined. Because samples were collected from the same individual pile throughout the composting period, correlations among residuals were expected. Potential covariance among the residuals was checked using the repeated measures option in the Mixed procedure of SAS Version 6.12 (Littell et al., 1996). No significant correlation among residuals over time were found. Statistical analysis was performed using two-way mixed models (Littell et al., 1996).
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RESULTS |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Oxygen Respirometry
Oxygen uptake rate is expressed as the log10 in mg O2 g-1 BVS h-1. Significant effects of oxygen uptake were found for both age and storage, with significant linear, quadratic, and cubic effects for age, and linear and quadratic effects for storage (Table 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Due to time limitations, out of the many maturity tests that could have been selected for this study, we chose the Dewar flask self-heating capacity, oxygen respirometry, and CEC because these tests are comparatively simple and inexpensive for a compost site or laboratory to conduct, and are well attested to in the literature. While useful for determining the presence of volatile organic acids that might adversely affect plant growth, plant germination and/or growth response was not tested in this study. As Fig. 1 indicates, temperatures were above 65°C between Days 5 and 15 during Stage I of composting, and declined to about 50°C by the end of Stage I. Although there is some possibility that these high temperatures could have had a suppressive effect on the microflora and created inefficiencies in the process, this does not invalidate the sensitivity of the indicators that measured change throughout the process, regardless of whether maturity was achieved in absolute terms. High temperatures are frequently reached in large-scale composting systems (Willson et al., 1980; Sikora et al., 1981). This is often due to the geometry of pile construction. It is frequently the case that the larger the pile, the more difficult it is to achieve enough aeration to dissipate heat rapidly. Thus, heat will tend to build up and temperatures greater than 65°C are common. These are issues of scale that are rarely addressed in composting studies. Although it is possible that reinoculation did occur between Stages I and II, which might account for the high temperatures reached in Stage II, this would not affect the ability of the indicators to measure changes in the compost over time.
As determined by time parameters set by the WSSC, the compost was in "curing" (Stage II) after Day 25 (Fig. 1). Since we did not attempt to influence the composting operation of the WSSC, we used this terminology to describe Stage II, even if it was concluded that the end product was not fully mature by Day 57, or if the composting process itself was less than optimal. The results of the Dewar flask test revealed that by Day 57 the compost was not yet stable, and the CEC showed continuing humification during refrigerated storage over 12 wk. These two tests are effective maturity indicators even under suboptimal composting conditions. As shown in Fig. 2, BVS declined by 9.4% between Days 1 and 57, which is consistent with other researchers' findings for the composting of lime-stabilized biosolids (Sikora et al., 1981).
Richard and Zimmerman (1995) attempted to correlate the maximum reheating temperature with oxygen respirometry and found that the oxygen respiration rate showed a curvilinear decline from about 15 mg O2 g-1 VS h-1 to 8 mg O2 g-1 VS h-1 over three phases of controlled composting. The maximum reheating temperatures declined from 45 to 37°C, then increased to 42°C in the third phase.
In our study, however, the maximum rise above ambient temperature clearly declined with age over the 8-wk composting period to about 20°C for samples tested fresh. According to the stability criteria of Niese (1963) and Brinton et al. (1995), such compost would be classified as moderately stable. The model suggests that an additional week of curing may stabilize the compost to meet the 10°C "stable" criterion. The effect of cold storage at 4°C on samples varied with compost age. Changes in compost microbiology at different stages in the composting process might explain these interactions. Following the response surface, samples collected between Days 1 and 10 of composting are less affected by cold storage of up to 11 wk (Fig. 3). This indicates that the youngest samples are able to rebound from cold storage with little inhibition of microbial activity. Day 15 seems to show a clear transition, in which samples collected after this time were affected by refrigerated storage. The refrigerated storage effect after Day 15 is clearly curvilinear, indicating that older samples are more strongly affected by cold storage, and lower maximum temperatures can be expected after 2 wk of refrigeration. Day 57 samples were not strongly affected by refrigerated storage, suggesting that readily available nutrients are depleted and mature or near-mature composts are not affected by refrigerated storage.
As temperatures decline and the readily available substrate is used up, actinomycetes and fungi become dominant. Actinomycetes and fungi are important in the degradation of cellulose and lignin, which are more resistant to attack (Polprasert, 1989, p. 63–80). There was less biological activity found in stored samples than in samples tested fresh, which may be due to the decrease in readily available substrate after Day 15 of composting (Fig. 3). Other reasons for reduced biological activity could be (i) the microorganisms were more vulnerable to storage effects and were less able to rebound after cold storage or (ii) a change in microbial population occurred and these organisms are less tolerant of cold storage. Storage effects on unstable composts are small but as composts mature, extended storage will alter the maximum temperature rise, suggesting a more stable compost than recorded for a compost tested fresh. It appears from the CEC data that biodegradation continues under refrigeration, possibly due to the activity of psychrotrophs, which may metabolize the substrate under refrigeration (Prieto et al., 1991; Ken-Yuon and Torres, 1993; Cox et al., 1998).
After 29 d of composting the log10 of the oxygen uptake rate was essentially constant (Fig. 4). These data indicate that this test is not sensitive to change throughout the entire composting process and is not adequate as a sole stability indicator. If this test were used exclusively, it would lead the producer to conclude that the compost had stabilized halfway through the process (Iannotti et al., 1993, 1994), a conclusion that, based on results of the Dewar flask and CEC tests, is clearly erroneous. Application of immature or unstable compost to soil systems can lead to the depletion of available nitrogen and/or the release of volatile organic acids, which are the by-products of the compost's continuing decomposition in the soil. Iannotti et al. (1993) found an initial high rate of oxygen consumption (2.0 mg g-1 volatile solids h-1), which declined to 0.50 mg g-1 VS h-1 for municipal refuse compost by Day 31. Since the composting process was apparently only monitored for 31 d, it would appear that further curing, as indicated by the water extract organic C to organic N ratio of 7.7 (Iannotti et al., 1993), was needed. Similarly, in this study, our values for oxygen uptake declined from about 3.30 mg g-1 BVS h-1 to about 0.50 mg g-1 BVS h-1 about Day 25 of composting. Based on the ratio of organic C to organic N in the water extract, which was above the threshold value of 5.88 (Chanyasak et al., 1982), Iannotti concluded that further curing was needed. In our study, the Dewar flask test showed continuing heating above the threshold value of 10°C (Brinton et al., 1995), and we concluded that readily available carbon was still present and was responsible for the continued heating at the end of the composting process. Results of the CEC test also showed continuing change in the values at the end of the composting process. Because both the CEC and Dewar flask tests showed continued change at the end of the composting process, we conclude that oxygen respirometry may not be an adequate test for Stage II of composting.
Differences between the Dewar flask and oxygen respirometry tests are probably not due to exothermic chemical reactions in the lime-stabilized sludge because although an exothermic reaction occurs quickly when highly reactive Ca materials are mixed with water, it is of relatively short duration (approximately 30 min; Sikora and Francis, 2000). It is likely that the exothermic reaction occurs when Ca(OH)2 is added at the treatment plant during the lime stabilization process and the heat is dissipated before the biosolids arrives at the compost facility. The pH of the lime-stabilized biosolids may still be elevated at pH 11 at the start of composting but, according to laboratory simulation studies, pH drops below 8.0 because CO2 production was detected after 4 d (Sikora et al., 1983). The calcination reaction neutralized Ca(OH)2, producing CaCO3, which would not be sufficiently reactive to result in an exothermic reaction. Therefore, an exothermic reaction may influence the self-heating test for the 1-d-old samples taken in this study, but samples taken later would not have sufficient reactive Ca to interfere with the stability test. Although mean pile temperatures rose to greater than 65°C between Days 5 and 15 in the composting pile and after Day 38 in the curing pile, these elevated pile temperatures did not appear to affect the results of the Dewar flask study. Day 57 compost samples tested without cold storage showed a 15°C rise above ambient temperature, which suggests that microflora were not destroyed as a result of high temperature.
Cation exchange capacity clearly increases with the age of the compost and storage time (Fig. 5). These results appear to indicate that humification continues during storage. During the humification process, complexation and condensation reactions occur, producing high molecular weight, fairly stable compounds (Lax et al., 1986). Either the storage temperature is not sufficiently low to completely inhibit the activity of composting microorganisms, or psychotrophic microorganisms, which are able to metabolize the substrate at lower temperatures, become more competitive under refrigeration. If an ecological transition is occurring between these two communities, then the shift from one active population to another would provide the basis for continuing humification.
Overall, CEC was found to increase from about 6 to about 24 cmol kg-1, at its maximum, an increase of fourfold (Fig. 5). Harada and Inoko (1980) considered a municipal refuse compost with a CEC of greater or equal than 60 cmol kg-1 expressed on an ash-free basis to be sufficiently mature for application to the soil. However, these researchers found a smaller increase for municipal refuse compost (40 to 60 cmol kg-1), an increase of 1.5 times the starting value. Unlike the present study, which used unground samples, they used a finely ground sample (60 mesh or smaller), which had a starting value much higher than that of an unground compost. The CEC values are affected by both preparation techniques and the nature of the substrate, suggesting that one threshold CEC value for maturity is not possible.
There does not appear to be a consensus about the best method for long-term storage, but storage of moist samples at low temperature minimizes disruption of microbial biomass (Bartlett and James, 1980; Ross et al., 1980). One can conclude that during the long-term refrigerated storage of moist compost samples slow microbial degradation of the substrate occurs. Since psychrotrophs are able to metabolize substrate more efficiently at lower temperatures than mesophiles (Margesin and Schinner, 1994; Cox et al., 1998), they could eventually predominate under long-term cold storage conditions. Although degradation may be slower at 4°C than at ambient, under long-term storage degradation can be significant and alter the substrate, changing the test results.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Cation exchange capacity values recorded for biosolids composts suggest that each compost substrate should be treated as unique, and maturity indicator data collected be cross-referenced with values reported in the literature for the same compost produced under similar conditions.
Results of this study indicate the necessity of timely analysis in order to avoid the problem of prolonged sample storage. Since this problem appears to be underreported in the literature (Wu and Ma, 2001), our results provide information on changes in indicator values in samples refrigerated for up to 3 mo. Because these results were obtained from a single biosolids compost operation, future studies should test additional biosolids compost to determine whether the age and storage relationship modeled in this study can be applied to biosolids compost or all composts.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONCold Storage EffectsExperimental ObjectivesMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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