Journal of Environmental Quality 30:246-253 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
WASTE MANAGEMENT
Chemical Characterization of Synthetic Soil from Composting Coal Combustion and Pharmaceutical By-Products
Christopher A. Guesta,
Cliff T. Johnstona,
James J. Kingc,
James E. Allemanb,
Jody K. Tishmackb and
L.Darrell Nortond
a Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907
b Dep. of Civil Engineering, Purdue Univ., West Lafayette, IN 47907
c Eli Lilly and Company Tippecanoe Laboratories, Lafayette, IN 47902
d National Soil Erosion Lab., USDA-ARS, West Lafayette, IN 47907
Corresponding author (clays{at}purdue.edu)
Received for publication February 7, 2000.
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ABSTRACT
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Land application of coal combustion by-products (CCBs) mixed with solid organic wastes (SOWs), such as municipal sewage sludge, has become increasingly popular as a means of productively using what were once considered waste products. Although bulk chemical and physical properties of several of these CCB–SOW materials have been reported, detailed information about their synthesis and mineralogy of the CCB–SOW materials has not been reported. In this paper, chemical and mineralogical properties of a soil-like material obtained from composting a mixture of CCBs with a pharmaceutical fermentation by-product (FB) were investigated at the laboratory and field scale. All starting materials and products were characterized by X-ray diffraction (XRD), fourier transform infrared (FTIR) spectroscopy, and elemental analyses. The results showed that the FB was strongly bound to the CCBs and could not be removed by washing. Within 2 wk of the start of a composting study, there was a rapid drop in pH from 12 to 8, an increase in temperature to 70°C, and a reduction in the dissolved oxygen content, attributed to the rapid establishment of a highly active microbial population. Composting produced a soil-like material with high levels of plant nutrients, a high nutrient retention capacity, and metal contents similar to median levels of those metals reported for soils. The levels of boron and soluble salts are such that sensitive plants may initially show toxicity symptoms. However, with adequate rainfall, leaching should rapidly remove most of the B and soluble salts. With care, the material produced is safe for use as a synthetic topsoil.
Abbreviations: CCB, coal combustion by-product • FB, fermentation by-product • FBC, fluidized bed combustion • FTIR, fourier transform infrared • SOW, solid organic waste • XRD, X-ray diffraction
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INTRODUCTION
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LAND application of CCBs mixed with SOW, such as municipal sewage sludge, has become increasingly popular as a means of productively using what were once considered waste products. Initially, CCB–SOW mixtures were used to aid revegetation of severely contaminated sites (Soper and McMahon, 1987). In recent years, however, mixtures of coal ash and biosolids (municipal sewage sludge) have become commercially available as soil amendments or soil substitutes.
Applied alone, high application rates of CCBs can adversely affect soil properties and crop growth (Carlson and Adriano, 1993; Kalra et al., 1997). Negative affects resulting from high application rates of CCBs include reduced microbial activity (Pichtel, 1990), reduced N mineralization (Garau et al., 1991), B toxicity (Aitken et al., 1984), and increased cementation and compaction of the soil (Carlson and Adriano, 1993). Co-application of coal ash and biosolids or related SOWs, however, has been shown to alleviate many of the short-term negative effects of both coal ash and sewage sludge application to soils (Adriano et al., 1982; Pichtel, 1990; Pichtel and Hayes, 1990; Wong, 1995; Norton et al., 1998).
Several mixtures of CCBs and biosolids are commercially available as soil amendments and soil substitutes. N-Viro (Toledo, OH) soil, for example, is a by-product of anaerobic curing of digested biosolids with fly ash and kiln dust (Logan et al., 1995). A second commercial product, Ecoloam (Horsehead Resource Development, Monaca, PA), is a mixture of fly ash, sewage sludge, and variable amounts of potash and lime (Jarrett et al., 1994). In addition, Wong and coworkers have published a series of papers on the manufacture and properties of an artificial soil obtained by mixing CCBs with sewage sludge (Wong, 1995; Wong and Lai, 1996; Wong and Su, 1997a,b). More recently, O'Brian and Barker (1998) published work on the acidification of lime-stabilized biosolids to develop synthetic topsoil for turfgrass production.
Concern about the long-term effect of land application of CCB–SOW mixtures has been expressed (McBride, 1995; Wong, 1995). One of the main areas of concern is the level of trace metals in the materials applied to the land (McBride, 1995), as land application of CCBs can increase the trace metal content of a soil (Furr et al., 1979; Francis et al., 1985). The limiting factor for land application of Australian CCBs is the B content of the ash (Aitken et al., 1984; Aitken and Bell, 1985), which induces B phytotoxicity.
A feature common to these synthetic soil-like materials is that they all represent a physical mixture between an inorganic CCB-based substrate with an SOW. Depending on the nature of the starting materials, various chemical and biochemical processes take place that partially transform the SOW and possibly the inorganic matrix (e.g., composting, aerobic and anaerobic degradation, including both mesophilic and thermophilic reactions, as well as dewatering). During these processes, the SOW material interacts with the inorganic substrate, which results in some form of attachment of the organic material to the CCB components. Although bulk chemical and physical properties of several of these commercial CCB–SOW materials have been reported (Logan and Harrison, 1995), detailed information about their synthesis, mechanism(s) of SOW attachment to the inorganic CCB components, and mineralogy of the CCB–SOW materials has not been reported.
In this study, we continue the research of Norton et al. (1998), and report on the synthesis of a soil-like material obtained from composting CCBs with an FB. The objectives of this study were to (i) determine the proportions of CCB and FB best suited to large-scale mechanical mixing; (ii) use C analysis, FTIR spectroscopy, and XRD analysis to examine the nature of the organic material and the strength of its attachment to the inorganic CCB matrix; (iii) characterize the composted CCB–FB mixture for major and trace elements; and (iv) discuss possible uses of this CCB–FB material (synthetic topsoil or soil amendment).
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MATERIALS
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The CCBs used in this study were obtained from Wade Power Plant at Purdue University, which uses a fluidized bed combustion (FBC) furnace and a traditional chain grate furnace. Approximately 80% of the 36000 Mg of ash produced annually at the Wade Power Plant is generated by the FBC process. A circulating fluidized bed furnace reduces SO2 emissions by injecting crushed limestone into the furnace and co-combusting the limestone with the coal. This process reduces SO2 emissions to statutory levels, but it also produces larger quantities of ash than traditional coal-burning processes. Approximately one-third of a megagram of ash is produced for every megagram of coal burned. Fluidized bed combustion ash is highly alkaline with a pH > 12, and has some cementitious properties. Stoker ash, produced through the combustion of low sulfur coal in the chain grate furnace, has a neutral pH, and is primarily crystalline silicates and amorphous silicate glasses. The elemental composition and pH of the FBC and stoker CCBs are given in Table 1. In line with the relative proportions of FBC and stoker ash produced annually by the power plant, the CCBs referred to in this study are approximately 80% FBC ash and 20% stoker ash.
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Table 1. Elemental analysis of Purdue fluidized bed combustion (FBC) ash, Purdue stoker ash, and Eli Lilly fermentation by-product (FB)
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The FB used in this study was obtained from Eli Lilly and Company Tippecanoe Laboratories (Lafayette, IN), and is from the production of Tylosin, an antibiotic used to increase feed efficiency and to prevent or treat bacterial infections in agricultural operations. The microorganisms that produce this antibiotic are grown in a carefully controlled nutrient solution containing soybean flour, peanut meal, fish meal, beet molasses, lard oil, starch or dextrin, soybean oil, corn glutin, and dextrose. When the antibiotic content of the nutrient solution reaches a maximum, the solution is harvested and the antibiotic is separated from the nutrient solution. The microorganisms that produce the antibiotic can only exist in a very specific set of nutritional and temperature conditions. Once the antibiotic is harvested, these conditions no longer exist, and the microorganisms die. The resulting FB contains the depleted nutrient mixture and dead microorganisms. Since the process that leads to its formation is very highly controlled, the quality of the FB is very consistent. Annual production of FB from Eli Lilly and Company Tippecanoe Laboratories is 25000 Mg, corresponding to a dry mass of FB of 5000 Mg (the solids content of this FB is 20%). The elemental composition of the Tylosin-based FB used in this study is listed in Table 1.
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METHODS
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Small-Scale Mixing
Mixtures of CCBs and FB were prepared on a small scale (100-g quantities) to evaluate the proportions of CCBs and FB best suited to mechanical mixing in a large-scale field study and to study the stability of the CCB–FB mixtures. The CCB–FB mixtures were prepared by mixing 10, 25, and 50 g of CCBs with 90, 75, and 50 g of FB (fresh weight basis), respectively. This produced CCB to FB ratios of 10:90, 25:75, and 50:50. The FB was collected fresh from Eli Lilly (within 1 h of antibiotic removal) and was either used on the day of collection or the following day after overnight storage in a refrigerator at 4°C. The CCB–FB mixtures were air-dried, then lightly ground to break up any lumps, and stored in plastic containers prior to analysis. The suitability of the different CCB–FB mixtures for mechanical mixing was qualitatively assessed by their degree of liquefaction during mixing, estimated by visual indexing. The influence of preheating the CCBs to 100 and 200°C was also evaluated by mixing the preheated CCBs with FB at the same CCB to FB ratios. Fourier transform infrared (FTIR) spectroscopy, XRD, and C and N analyses were used to study the CCB–FB mixtures.
Washing
To evaluate the strength of the attachment of the FB components to the inorganic CCB matrix, the CCB–FB mixtures were shaken in water overnight. One-half gram of the air-dried CCB–FB mixtures was shaken in 25 mL of distilled-deionized water under an N2 atmosphere in 50-mL polyethylene test tubes for 20 h at 20°C. The resulting suspension was centrifuged for 15 min at 22000 x g, and dried in an oven at 30°C. Fourier transform infrared (FTIR) spectroscopy, XRD, and C and N analyses were used to study the effect of washing on the CCB–FB mixtures.
Large-Scale Mixing and Composting
A front-end loader and a large pug mill were used to construct four large compost piles in windrows, each containing 25 Mg of CCBs and 25 Mg of FB (fresh weight basis). Wood chips, from chipped pallets, were used to increase the bulk of the compost piles and to allow airflow through the compost piles. Further aeration of the piles was achieved by turning. The ratio of wood chips to other materials was 2:1 by volume. For 90 d, the composting piles were monitored for temperature, water content, O2 content, C and N concentrations, and pH. The water content was determined by drying in an oven at 103°C. The O2 content was measured using a portable oxygen analyzer with a 1.2-m hollow copper sampling tube. The temperature of the piles was measured using a dial thermometer with a 1.2-m stem. To obtain representative values, oxygen content and temperature were measured at a depth of 0.9 m, at various points along the piles, and an average was calculated. The pH was measured after vigorously mixing 25 g of compost in 150 mL water for 20 min. The water and O2 contents were used to determine when the piles required watering and turning, to maintain >5% O2, 40 to 65% water, and 49 to 60°C, considered by Rynk et al. (1992) to be optimum conditions for composting. Turning was done using a windrow turner. The temperature, C, N, and pH measurements were used to confirm that microbial composting was actually occurring in the piles. After 90 d of composting the CCB–FB–wood chip mixtures, the piles were mechanically sifted (12.5-mm mesh) to remove the wood chips, and the four piles were combined.
Characterization of Materials and Product
The major element composition of the FBC ash was determined using American Society for Testing and Materials (ASTM) Method C311-88 (American Society for Testing and Materials, 1989), and the major element composition of the stoker ash was determined by X-ray fluorescence at Commercial Testing and Engineering Co. (Lombard, IL). The trace element contents of the FBC ash, stoker ash, and the composted mixture were determined by inductively coupled plasma (ICP) analyses after digestion of the samples in HNO3–HCl–HF. The B content of the composted mixture was determined by ICP following digestion of the sample in HNO3. All samples were analyzed in triplicate, and a certified soil standard was digested and analyzed along with the other samples. Eli Lilly supplied the elemental analyses of the FB, and also the Hg content of the composted mixture and the stoker ash. The composted mixture was analyzed for extractable plant nutrients by A & L Great Lakes Laboratories (Fort Wayne, IN) using the methods described by Dahnke (1988). Total carbon and nitrogen were determined using a Fisons Instruments NA 1500NC analyzer using EAGER 200 software (Fisons Instruments, Beverly, MA).
Powder XRD patterns were obtained of the CCBs and mixed CCB–FB materials using Co K
radiation at 35 kV, 25 mA on a Philips 3520 vertical diffractometer (Philips Analytical, Natick, MA). The FTIR spectra were obtained using a PerkinElmer (Norwalk, CT) 1600 FTIR spectrometer equipped with a deuterated triglycerine sulfate (DTGS) detector. Samples were prepared as KBr pellets, with 2 mg of sample in 200 mg of infrared grade KBr, and pressed under vacuum at a pressure of 70 MPa for 5 min. Spectra were obtained in the 450 to 4000 cm-1 range at an optical resolution of 4 cm-1 using weak apodization.
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RESULTS AND DISCUSSION
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Small-Scale Mixing
Of the three CCB–FB ratios evaluated (10:90, 25:75, 50:50), the 50:50 CCB–FB mixture presented fewest problems, either with mixing or with drying. Upon air-drying, the mixture contained only a few lumps, and required little grinding. Increasing the proportion of FB to 75% or 90%, however, caused the mixtures to liquefy when mixed, forming a sludge with a gelatinous consistency. These mixtures (10:90 and 25:75) dried as a solid mass and were difficult to grind. Heating the CCBs before mixing did not alter the behavior of the mixtures. Based on the qualitative mixing and drying behavior of these mixtures, the ratio of CCBs to FB of 50:50 was selected for the large-scale mixing–composting study. Use of the either the 10:90 or 25:75 mixtures would be likely to clog the equipment used for mixing.
The crystalline components of the CCBs are anhydrite (CaSO4), lime (CaO), and hematite (Fe2O3), with a trace of quartz (Fig. 1a) . Addition of the FB to the CCBs had little influence on the overall mineralogy of the CCB components (Fig. 1b). Comparison of Fig. 1b with Fig. 1a shows that the main effect of mixing the CCBs with the FB is the dissolution of almost all of the lime present in the CCBs, as shown by the reduction in size of the lime peaks. There also appears to be a slight increase in the amount of anhydrite present, relative to the amount of hematite and quartz.
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Fig. 1. X-ray diffraction (XRD) patterns of the coal combustion by-products (CCBs) (a), the 50:50 CCB–fermentation by-product (FB) mixture (b), and the composted mixture (c). Hm = hematite, Ah = anhydrite, Q = quartz, Cc = calcite, D = dolomite, L = lime
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In contrast to the inorganic, crystalline nature of the CCBs, the composition of the FB used in this study is a complex mixture of amorphous organic constituents. Powder XRD methods detect only crystalline components, so this method does not provide any direct information about the nature of the FB in the CCB–FB mixtures. Fourier transform infrared spectroscopy (FTIR) was used to investigate the amorphous organic FB components of the CCB–FB mixtures. The FB material (Fig. 2a)
has a broad band in the 3600 to 3200 cm-1 region, mainly from the OH stretching bands of water. This material contains approximately 80% water. The C–H stretching bands are readily observed by the intense bands in the 3000 to 2800 cm-1 region. The bands at 2920 and 2850 cm-1 correspond to the C–H stretching frequencies of aliphatic carbon bonds, although some unsaturated C–H bonds are also present. The band at 1740 cm-1 corresponds to the carbonyl stretching frequency of protonated carboxylic acid groups. The strong, broad band at 1650 cm-1 contains a contribution from the H–O–H bending (
2 mode) band of water. However, other organic compounds have IR bands in this region (Johnston et al., 1994). Numerous other spectral features are present in the region below 1600 cm-1, although unambiguous assignment of these spectral features are not known at the present time.
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Fig. 2. Fourier transform infrared (FTIR) spectra of the fermentation by-product (FB) (a), the coal-combustion by-products (CCBs) (b), and the 50:50 CCB–FB mixture after washing (c), in the 450 to 4000 cm-1 region, prepared as KBr pellets
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The FTIR spectrum of the CCBs (Fig. 2b) is characterized by a sharp, well-resolved band at 3640 cm-1, which is assigned to the (OH) band of Ca(OH)2 (Farmer, 1974). The material contains little water as evidenced by the low intensity in the 3600 to 3100 cm-1 region. Of relevance to this study, there is essentially no intensity in the C–H stretching region, indicating that little organic material is present on the CCBs. The broad band at about 1420 cm-1 is assigned to carbonate. The lower frequency bands below 1300 cm-1 have not been assigned.
Washing
To remove any potentially "free" FB from the CCB–FB mixtures, the samples were vigorously shaken in water overnight. The mineralogy of the CCB–FB mixtures remained nearly constant during the washing procedure (data not shown). Changes included the partial dissolution of any remaining lime, and the formation of small quantities of portlandite (Ca(OH)2) and ettringite (Ca6Al2(SO4)3(OH)12·25H2O). The presence of organic material on the CCBs in the 50:50 CCB–FB mixture after washing is clearly indicated by the presence of the C–H stretching bands at 2920 and 2850 cm-1 (Fig. 2c). Although the (CH) bands indicate the presence of the FB, or its degradation product, the lower frequency region is very different from the original FB. Of particular interest is the loss of the carbonyl stretching band at 1740 cm-1. Upon mixing the FB with the alkaline CCBs, the pH of the resulting mixture is increased to a pH > 11 (Norton et al., 1998). This pH increase is consistent with the observed spectra. As the carboxylate groups respond to the higher pH, the carbonyl band of the protonated carboxylic acid groups is shifted to a lower frequency corresponding to the carboxylate stretching bands (Deacon and Phillips, 1980). The position of the red-shifted carboxylate band cannot be determined due to the interference from water and other bands in the 1650 to 1550 cm-1 region. In addition to the loss of the 1740 cm-1 band, a much stronger band at 1430 cm-1 indicates that the carbonate content of the 50:50 CCB–FB mixture has increased. Most of this increase can be attributed to the increase in the pH of the sample on mixing the CCBs with the FB, as increasing the pH increases the activity of CO-2 (Lindsay, 1979).
The CCB–FB mixtures were analyzed for total carbon concentration before and after the washing procedure (Fig. 3)
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14% C) correspond to the 50:50 CCB–FB mixtures, while the high carbon concentrations (35% C) correspond to the 10:90 CCB–FB mixtures. A regression line through the data points has a slope of 0.91, an intercept of 1.28, and an r2 of 0.904. The 95% confidence interval for the regression line (Fig. 3) shows that the intercept is not significantly different from zero. This slope and intercept indicate that very little carbon is lost from the samples during the washing procedure. These results mean that the FB is not easily separated from the inorganic substrate of the CCBs.
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Fig. 3. Total carbon contents of the coal combustion by-product (CCB)–fermentation by-product (FB) mixtures before and after washing
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Composting
The compost piles were monitored for temperature, pH, O2 content, water content, percent C, and percent N for 90 d. The O2 and water contents were used to determine the timing of watering and turning the piles. The temperature results (Fig. 4a)
show a rapid increase in temperature, with the piles reaching a temperature of 66°C 6 d after mixing, and fluctuating between 43 and 60°C for the first month of composting. The pH (Fig. 4b) of the piles stayed above 12 for the first 4 d after mixing, and by Day 9, the pH had dropped to 8.6, and then continued a gradual decline to a pH of 7.6 at the end of the trial. Over the course of the composting trial, the N concentration of the piles decreased from 1.0 to 0.5%, and the C concentration decreased from 20 to 16% (Fig. 5)
. In comparison, N-Viro soils have a mean C concentration of 12%, and a mean N concentration of 0.89% (Logan and Harrison, 1995), similar to the C and N concentrations of the composted mixture obtained here.
The rapid increase in temperature is attributed to microbial activity in the compost piles, and the decrease in percent C confirms the presence of a highly active microbial population. A considerable decrease in the N concentration of the piles occurred as composting progressed. Initially, this decrease may have been due to release of NH3 because of the high pH. In the latter half of the composting, turning of the piles was less frequent, so the center of the piles may have been anaerobic, resulting in N release. The drop in pH is attributed to the neutralization of the residual alkalinity in the ash by carbonic acid (H2CO3), resulting from increased microbial respiration. The initial sharp temperature increase may have been due to the heat of hydration of some of the ash components. However, the rapid drop in pH was not detected in the laboratory mixtures (Norton et al., 1998) and cannot be explained purely in terms of inorganic processes. The rapid increase in temperature, sharp drop in pH, and the drop in C concentration of the piles indicate rapid establishment of an active microbial population in the compost piles.
Composted Mixture
The plant nutrient analysis results of the composted mixture (Table 2) are classified as high or very high (Vitosh et al., 1995), indicating that this material is fertile. The cation exchange capacity (CEC) result (Table 2) given is probably an overestimate of the actual value. The ammonium acetate (1 M, pH 7) extraction method for CEC measurement is inappropriate for the composted mixture, as the sample contains soluble calcium (calcite and anhydrite). Ammonium acetate dissolves carbonates, so the extracted solution contains both exchangeable and soluble Ca, resulting in an overestimation of the CEC.
The concentrations of Cr, Cu, Pb, Mo, Ni, and Zn in the composted mixture (Fig. 6)
are lower than the respective concentrations of those metals in the starting CCBs. Note also that the metals concentrations in the composted mixture are within the range of concentrations of those metals in soils, and close to their median concentrations in soils (Fig. 6; Holmgren et al., 1993; Sparks, 1995).
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Fig. 6. Comparison of the elemental composition of the coal combustion by-products (CCBs) and the composted mixture with the range of elements found in soils and the USEPA 503 regulations
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The minerals found in the CCBs dominate the mineralogy of the composted mixture. The XRD analysis of the composted mixture shows that it is primarily anhydrite, with quartz, calcite, hematite, and dolomite (Fig. 1c). Mixing the CCBs with FB and composting results in the dissolution of lime and the formation of calcite and dolomite.
Potential Environmental Impact
Trace Metals
To assess the potential long-term effect of land application of the composted mixture, the trace metal and extractable nutrient contents were determined. The trace metal results are compared with the USEPA (1995) regulations for land application of sewage sludge and to the range of selected trace elements found in soils (Holmgren et al., 1993; Sparks, 1995) as a guide to levels that may be acceptable for a soil amendment or a synthetic topsoil. The trace metal content of the composted mixture falls well below the threshold levels given in the USEPA (1995) regulations. That means that if this material were a sewage sludge, there would be no restrictions on the amounts that could be land applied (based on the metal concentrations). Comparison with the range and median concentrations of those elements in soils (Fig. 6) shows that Cu, Mo, and Ni are present in the composted mixture at levels a little higher than the median found in soils. Lead is present at about median levels, and Cr, Hg, and Zn are present at below median levels. Based on the trace element content of this material, there appears to be little risk to using it as a soil amendment or as a topsoil, according to the USEPA (1995) regulations and in comparison of the composted mixture with existing data of the trace metal contents of soils (Holmgren et al., 1993; Sparks, 1995).
Potential Boron Phytotoxicity
The composted mixture has a total boron content of 121 mg kg-1, of which 46 mg kg-1 is extractable in water (Table 2). Murphy and Walsh (1972) showed that soybean [Glycine max (L.) Merr.] exhibits mild B phytotoxicity at soil solution levels of 2 mg kg-1 and almost 100% yield reduction at 10 mg kg-1, whereas other plants (cotton [Gossypium hirsutum L.] and sugar beet [Beta vulgaris L.]) are less sensitive to B toxicity. The B content of Australian CCBs is a limiting factor for land application (Aitken et al., 1984; Aitken and Bell, 1985). However, B is water soluble and readily removed by leaching (Aitken and Bell, 1985; Ghodrati et al., 1995). In the composted mixture, almost one-half of the total B is extractable, implying that if used as a synthetic topsoil, the composted mixture may cause B toxicity in sensitive plants for the first year after land application. In subsequent years, after B removal by leaching, the B content of the composted mixture should be below levels that could affect sensitive plants.
Soluble Salts
In addition to the high B content, the composted mixture has a soluble salt content (EC) of 0.3 S m-1 (Table 2). This soluble salt content is high enough to adversely affect the growth of some plants, and may reduce yields by as much as 50% in highly sensitive crops (Bohn et al., 1985). Soluble salts have been cited as the primary limiting factor to vegetation of power plant ash deposits (Adriano et al., 1980). Recent research has addressed the concern associated with soluble salt from fly ash applied to croplands. Any increase in soil salinity from application of fly ash to soil is expected to be greatly diminished after 2 to 3 yr (Adriano et al., 1980). In mixtures of excessively drained sandy soil and fly ash, potentially phytotoxic EC levels were reduced to nonphytotoxic levels after 700 mm of rainfall (Ghodrati et al., 1995; Gangloff et al., 1997). In regions that receive in excess of 700 mm of rainfall annually, soluble salt levels in the composted mixture may initially affect sensitive plants, but this effect should be negligible within 3 yr. In arid or semiarid regions where annual rainfall is less than 400 mm, removal of the soluble salts by leaching is likely to be slow, so salt-sensitive species should be avoided.
Regulatory Concerns
The total N content of the composted mixture is higher than that found in most soils (Fig. 6; Sparks, 1995). Due to concerns about the effect of nitrate leaching on surface and ground water, the Indiana Department of Environmental Management (IDEM) has proposed regulating the composted mixture as if it were a manure, and basing application rates on available nitrogen. Based on the IDEM calculations (Sutton et al., 1994), the annual application rate of the composted mixture would be between 60 and 112 Mg ha-1, depending on the mineralization factor (the fraction of organic N that will mineralize in the first year) used in the calculations. Assuming that only 20% of the N present mineralizes in the first year [the lowest mineralization factor listed by Sutton et al. (1994)], the maximum application rate would be 112 Mg ha-1. This means that the composted mixture could be applied to a limiting thickness of 1 cm (assuming a bulk density of 1.3 g cm-3).
Potential Uses for the Composted Mixture
There are many potential uses for CCB–SOW mixtures like the one produced here, depending on the properties of the mixture. High-pH materials can be used as a substitute for agricultural lime, whereas lower-pH materials can be used as fertilizer, as a soil amendment for land reclamation or landscaping, as an ingredient in synthetic topsoil, or as a substitute for soil for landfill cover (Logan and Harrison, 1995). The suitability of CCB–SOW mixtures for each of these uses depends on their pH, nutrient content, and heavy metal content. The initial material produced during this research from mixing CCBs and FB has a high pH, possibly making it suitable for use as a liming material. The material produced after composting has a lower pH than the original mixture, so it is not suitable for use as a liming material. However, the composted mixture contains high levels of plant nutrients, and due to the high organic matter content, has a high nutrient retention capacity. The trace metal levels of the composted mixture are comparable with median levels found in soils (Holmgren et al., 1993; Sparks, 1995). Based on this information, the composted mixture is suitable for use as synthetic topsoil, as well as for use as a soil amendment in reclamation or landscaping. The only potential problems come from the B and soluble salt contents, both of which are present at levels that may cause phytotoxicity symptoms in sensitive plants. Due to the solubility of B and the ease of leaching of both B and the soluble salts, the levels of both should rapidly decline to low levels that would not even affect sensitive plants.
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CONCLUSIONS
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Mixing the Purdue CCBs and Eli Lilly FB produces a mixture that does not readily separate into its component parts. The fresh mixture has a pH that is too high for use as a plant growth medium, although it could be used as a liming material. When the material is composted, the pH drops to around 8 in less than 1 wk. Composting produces a synthetic soil-like material that contains high levels of plant nutrients and, due to the high organic matter content, has a high nutrient retention capacity. The synthetic soil material contains trace metals at levels that are comparable with median levels found in soils. The levels of boron and soluble salts found in the synthetic soil and their leaching characteristics indicate that sensitive plants may suffer if grown in the synthetic soil during the first 3 yr after land application. With adequate rainfall, the B and soluble salt contents should rapidly drop below levels that affect sensitive plants. Before this material can be used as a topsoil, more research is required to assess N mineralization rates and nitrate leaching rates from up to 0.15 m thicknesses (commonly assumed cultivation depth) of the synthetic soil material applied under field conditions.
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ACKNOWLEDGMENTS
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Thanks to R. Altieri for his help in getting the project started, to C. Brauch for his work in obtaining samples and taking the measurements of the compost piles, and to Dr. D.G. Schulze and Dr. B. Joern for help and advice with the research. Thanks also to Eli Lilly, Purdue University, and the Indiana Department of Commerce for providing the funding for this research.
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REFERENCES
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ABSTRACT
INTRODUCTION
