Recycling of separated pig manure: Characterization of maturity and chemical fractionation of elements during composting

e>

Pergamon

Wat Sci. Tech. Vol. 40, No. I. pp. 121-127, 1999

e 1999IAWQ Published by Elsevier Science Ltd Printed inGreat Britain. All rights reserved 0273-1223/99 $20.00 + 0.00

PI!: S0273-1223(99)00372-8

RECYCLING OF SEPARATED PIG MANURE: CHARACTERIZATION OF MATURITY AND CHEMICAL FRACTIONATION OF ELEMENTS DURING COMPOSTING J.-H. Hsu and S.-L. Lo Graduate Institute 0/ Environmental Engineering. National Taiwan University. 7J. Chou-Shan Road. Taipei, Taiwan. ROC

ABSTRACT Composting of separated pig manure(SPM) was studiedto evaluatecriteria indicating compostmaturityand to determine the effect of composting on the fractionationof trace elements in SPM compost. Composting was performed in tum piles and the following parameters were measured in 10 samples during 122 days of compostmg: temperature, CIN ratio, ash content, metal contents, humic substance contents, and fractions (humic acid. fulvicacid, and nonhumic fractions - HA. FA, and NHF, respectively). A sequential extraction scheme was used to partitionCu. Mn, and Zn in SPM compost. The CIN ratio and ash content exhibiteda typicallyhigh rate of changeduring the first 33 days and levelledoff thereafter. The fresh SPM was enriched with Cu, Mn, and Zn due to feed additives. All metal concentrations increasedapproximately 2.6-foldin the final compostdue to decomposition of organic matter. The HA content increased to a maximum at 80 days, representing the degree of humification and maturity of the compost During the composting process, the major portions of Cu. Mil, and Zn were found in the organic, oxide, and carbonate fractions, respectively. Metal distributions in differentchemical fractions were generally independent of composting age and, thus, respective total metal concentrations in the composts. C 1999 1AWQ Published by Elsevier Science. All rightsreserved

KEYWORDS Separated pig manure; composting; maturity; humic substances; sequential extraction. INTRODUCTION About 11 million head of pigs are on feed at any time in Taiwan (T A YB.1995). and 80% of the feeding occurs in the country's southern six counties. Inappropriate disposal of pig manure in regions of intense pig production may pose environmental problems such as the accumulation of heavy metals in soil and pollution of ground and surface waters due to leaching and run-off of nutrients. Separation and composting of the solids from slurry under controlled conditions may provide a better alternative to manure management. The composting process involves biological treatment in which aerobic thermophilic microorganisms use organic matter (OM) as a substrate. The main products of the composting process are fully mineralized materials such as C02. H20. mineral ions. stabilized OM (mostly humic substances). and ash. Wellcomposted manure has the advantage of improving soil structure. increasing soil organic matter. suppressing 121

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soil-borne plant pathogens, and enhancing plant growth (Saviozzi et al., 1988). Noncomposted manure or immature compost applied to agricultural soil may cause phytotoxicity to plants and adversely affect the environment (Garcia et al., 1992). Several tests have been proposed to assess compost maturity and stability: CIN in the solid phase (Jimenez and Garcia, 1992), soluble organic C in water extracts (Inbar et al., 1993), and humification indices (Jimenez and Garcia, 1992; Chefetz et al., 1996). These authors had concluded that several parameters must often be crossbred to define compost maturity. Pig manure is comprehensively characterized with respect to copper (Cu), manganese (Mn), and zinc (Zn) due to feed additives (L 'Herroux et al., 1997). As such, successive application of SPM compost in agricultural soils may cause metals to accumulate to a toxic level. Studies have shown that the chemical form, rather than the total content, of an element is more important in determining its availability for plant uptake or leachability into groundwater (Petruzzelli et al., 1989). Although several studies have assessed the extractability of elements from sewage sludge and municipal solid waste compost (MacNicol and Beckett, 1989; He et al., 1995; Tisdell and Breslin, 1995), very few studies have been conducted on SPM compost. The objectives of this work are to evaluate the criteria indicating compost maturity and to determine the effect of composting on the fractionation of Cu, Mn, and Zn in SPM compost. A sequential extraction scheme was used to determine the phase association of these elements associated with SPM during the composting process. This research may provide useful information for successful utilization of SPM compost. METHODS Composting of separated pig manure The solid fraction or separated pig manure (SPM) obtained from separation of slurry was composted in two piles in an indoor concrete area at a pig farm at Yong-Kong, Tainan County. The raw material was divided into two piles (about 1.5 m 3 each) without forced aeration and composted for 122 d. The compost was turned, mixed, and sampled at 0, 3, 7, 12, 18,25,33,49,80, and 122 d. Water was added immediately after the compost was turned to maintain a moisture content of 50-60% (w/w). At a depth of 0.30 m within the composting piles, the temperature was taken daily during the first 40 d and then once every three days until the end of the process. The samples (4 1) were placed in partially closed polyethylene bags, transported to the laboratory, and then stored at 5°C. Smaller subsamples were air-dried and used for analyses. All measurements were conducted in triplicate for each composting pile. Chemical analyses The moisture content of air-dried composts was determined after drying to a constant weight at 105°C in a forced-air oven. Total C and N were analyzed using a Heraeus CHN-O-RAPID analyzer on composts ground to <0.25 mm. Ash measurements were determined at 400°C for 8 h in a furnace (NEY Model 2-525). For total elemental composition, air-dried samples (1.0 g each) were digested with nitric-perchloric acid. The digest was centrifuged for 25 min at 10K rpm (Kubota, 6800) and the supernatant filtered through 0.45-~m filter membranes. Metals (Cu, Mn, and Zn) analyses of digests were performed using an atomic absorption spectrophotometer (AAS, Perkin-Elmer, 4000). Humic substances For studies on humic substances (HS), 20 g of compost was extracted with 200 ml of 0.1 N NaOH for 24 h. Supernatant solution containing soluble HS was separated by centrifugation at 10K rpm and the residue was resuspended in 0.1 N NaOH. This procedure was repeated eight times. The combined solutions were filtered through 0.45-~m filter membranes and acidified to pH 1 with 3 M H2S04, allowed to stand at room temperature for 24 h, and centrifuged to obtain the fulvic fraction (FF) while discarding the humic acid (HA) fraction. The fulvic fraction was separated into fulvic acid (FA) and nonhumic fraction (NHF) by adsorption of the FA onto Amberlite XAD-8 resin (mesh size 20-60, Sigma, St. Louis). The FF was passed through the

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column at a flow rate of about 12 bed volumes per hour. The column was then washed with 1 bed volume of distilled water. At this stage, the FA adsorbed onto the resin while the NHF was eluted. The combined solutions, FF, and NHF were stored at SoC until analysis for organic C. Organic C analyses were conducted with a total organic carbon analyzer (0· I· Corporation Model 700). Organic C content in the HA fraction of NaOH extracts was not directly analyzed but calculated by taking the difference between total and FF values. The FA content was calculated as the FF minus NHF. Sequential extraction and fractionation The sequential extraction procedure of Tessier et al. (1979) partitions elements into five distinct fractions using multiple extractions of the compost with successively more aggressive leaching solutions. The five fractions are defined as (i) exchangeable metal ions (1 M MgCh at pH 7), (ii) carbonate-bound metal ions (1 M sodium acetate at pH 5), (iii) metal ions bound to iron and manganese oxides (0.04 M hydroxylamine hydrochloride [HONH2HCI] in 25% acetic acid), (iv) metal ions bound to organic matter (0.02 M nitric acid and 30% hydrogen peroxide at pH 2 and 90°C followed by 1.2 M ammonium acetate in 10% nitric acid), and (v) residue-bound metal ions (HN03-HCl04 digestion). After each successive extraction, separation was performed by centrifuging at 10K rpm for 25 min. The supernatant was removed with a pipette, filtered through a 0.45 urn filter membrane, and stored at SoCuntil analysis for Cu, Mn, and Zn. RESULTS AND DISCUSSION Composting of separated pig manure Temperature variations during composting (Fig. 1) followed a typical pattern exhibited by many composting systems (Jimenez and Garcia. 1992; lnbar et al., 1993; Tiquia et al., 1997). Three phases were observed during the process: (a) a thermophilic phase lasting for the first 16 d, during which the temperature rose from 31°C to 48°C within 24 h and increased to a maximum of 68°C; (b) a cooling phase, in which temperature began to drop at Day 16 and leveled off at Day 27; and (c) a stationary phase after Day 27, where the compost temperature equalled that of the ambient with no measurable temperature changes.

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The change in the CIN ratio and ash content reflects OM decomposition and stabilization during composting (Fig. 2). The CIN ratio decreased rapidly from an initial value of21 in the raw material to 11 after only 18 d. The ratio continued to decrease, but less sharply, to 9.3 after 33 d. From this point on, the CIN ratio stabilized at a value of about 8 for the remainder of the process. The initial and final ash contents of the compost were 23% and 41%, respectively (Fig. 2). The change in ash content followed a reverse trend to that of the CIN ratio, exhibiting three phases: (a) Days 0-18, when most of the OM decomposed; (b) Days 18

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to 80; and (c) Days 80 to the end of the experiment, during which the curing stage began and the rate of OM decomposition was extremely low. Temporal variations in total elemental composition Since no leaching and runoff took place during composting, the total concentration of Cu, Mn, and Zn increased with composting time and a corresponding loss of OM (Fig. 3). The total concentrations of Cu, Mn, and Zn in final compost were 791, 1032, and 1562 mg/kg, which were 2.7, 2.8, and 2.4 times the concentrations in raw material, respectively. Comparing with 15-90% increases during cattle manure composting (Inbar et al., 1993) indicates that metals accumulation in SPM compost was drastical and suggests that the type and decomposability ofraw material is of major importance to the metal accumulation during composting. Major changes in the metal contents were recorded during the first 25 d of the process, paralleling the thermophilic stage, OM decomposition, and transformation such as in ash content. Humic substances content Humic substances comprise the most important fraction of OM because of their effects on soil ecology, structure, fertility, and plant growth. NaOH-extracted HS from composts can be separated into HA, FA, and NHF. The levels of HA, FA, and NHF in SPM compost at various stages of the process represent the humification process (Fig. 4). Total HS increased from 27% of the OM in the raw material to 39% of the OM after 25 d and maintained this value until the end of the process. The increasing trend of the HS levels during SPM compo sting agrees with that reported by Inbar et al. (1989) for separated cattle manure composting. The FA level gradually decreased from 8.7% of the OM in the raw material to 5.9% in the mature compost. The HA level increased during the composting process, gradually increasing from 4.0% to 6.2% for the first 18 d, sharply increasing to 14% at Day 25, then gradually increasing to 21% in the mature .compost. In general, fresh composts contain low levels of HA and higher levels of FA (Inbar et al., 1989; Chefetz et al., 1996), a trend also shown in this study. As composting proceeded, the HA content increased, whereas the FA level decreased. Of the humic substances extracted with NaOH, the majority was recovered as NHF for the first 18 d and shifted to HA for the remainder of the process. The NHF increased from 14 to 20% for the first 18 d of compo sting, probably due to a high substrate level and greater biosolid surface area formation during the thermophilic stage, and decreased to 14% in the mature compost because the substrate consisted of polysaccharides and amino acids that readily decomposed after the thermophilic stage. The HAIFA and HAIFF ratios are commonly used to analyze the humification process (Inbar et al., 1989; Jimenez and Garcia, 1992; He et al., 1995). These two humification parameters increased during the composting as follows: (a) the HAIFA ratio remained steady at 0.5-0.7 for the first 18 d, sharply increased to 1.9 at Day 25, then gradually increased to a final value of 3.5 at the end of the process; and (b) the HAIFF ratio remained steady at 0.17-0.22 for the first 18 d, sharply increased to 0.54 at Day 25, and then increased to 1.04 in the mature compost. The increasing trend of these parameters indicates that HA became the main fraction ofHS during composting. 2000

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Sequential extraction and fractionation Although Cu, Mn, and Zn are essential elements for plant growth, its accumulation in soils due to application of SPM compost may cause toxicity. Therefore, contents and chemical forms (thus bioavailability) of these elements are of major concern in assessing the environmental impact of SPM compost. It is believed that metals in the exchangeable fraction are readily available for leaching, the carbonate and oxide-bound fractions are relatively labile and may be potentially bioavailable to the environment, and metals in the organic fraction are relatively immobile and may not be readily bioavailable, whereas metals in the residual fraction are tightly bound and not expected to be released under natural conditions. The differences in the distribution patterns of Cu, Mn, and Zn in the SPM composts studied indicate that the potential mobility and bioavailability ofthese elements vary in the environment. Copper. The distribution of Cu between the different extractants at various stages of composting shows that the greatest amount (53-67%) are found in the organic fraction, followed by the oxide (23-32%) and carbonate (6-13%) fractions, with the smallest amounts of Cu being associated with the exchangeable (1-5%) and residual (1-3%) fractions irrespective of composting age thus metal content (Fig. 5). This result is consistent with that reported by Tisdell and Breslin (1995), who found that the greatest amount of Cu in MSW compost is associated with the organic fraction. The major association ofCu with the organic fraction in these composts may be due to high formation constants of organic-Cu complexes (Stumm and Morgan, 1981).

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Manganese. Like Cu, the distribution of Mn in various chemical fractions was generally independent of composting age and thus metal content. In the samples at various stages of composting, the highest amounts of Mn were present in the oxide (34-60%) and carbonate (23-48%) fractions, followed by the organic (4-16%) and exchangeable (4-13%) fractions, with the lowest being the residual (1-3%) fraction (Fig. 5). However, due to low total Mn content of the mature SPM compost (1032 mglkg) compared with an average soil Mn content of 2500 mglkg (Brady, 1974), it is unlikely that application of SPM composts to soils will cause Mn toxicity to plants. In fact, Chaney and Ryan (1993) showed concern that application of alkaline biosolids can result in Mn deficiency in susceptible crops. Zinc. In the 10 composts at various stages of composting, the major amounts of Zn were found in the carbonate (44-54%) and oxide (35-49%) fractions, a small amount was associated with the organic fraction (4-10%), and the lowest amount found in the exchangeable (1-2%) and residual fractions (1-2%) regardless of compost age and total metal concentration (Fig. 5). Tisdell and Breslin (1995) found that the greatest percentage of Zn was present in the oxide fraction, followed by the carbonate fraction in MSW compost. This result reveals that much ofthe Zn in SPM and MSW composts may be potentially mobile.

CONCLUSIONS Compost maturity is difficult to define by a single parameter, so crossbreeding several parameters is usually needed. The C/N ratio, ash content, metal contents, and humic substances content are all good indicators of SPM compost stability and maturity. All of these parameters exhibited three phases in this study: (a) rapid decomposition during the first 25 d; (b) stabilization until Day 49; and (c) maturation from Day 49 on. Slow decomposition persisted during the stationary phase at ambient temperature. The SPM compost, described in this study, was mature and ready for use as an agricultural substrate after 80 d of composting. Copper, Mn, and Zn contents increased approximately 2.6-fold in the final compost. A sequential extraction scheme was used to fractionate these elements present in SPM during the composting process. It appears to have little influence on composting age and thus metal content on the distribution of Cu, Mn, and Zn. Copper was found primarily in the organic fraction during the composting process. Manganese was mainly associated with the oxide and carbonate fractions. Zinc was most concentrated in the carbonate fraction. The potential mobility of these element in SPM compost was in the order Zn>Mn>Cu. REFERENCES Brady, N. C. (1974). The Nature and Properties ofSoils. 8th ed, Macmillan Pub. Co., New York. Chaney, R. L. and Ryan, J. A. (1993) . Heavy metals and toxic organic pollutants in MSW~ornposts : Research results on phytoavailability, bioavailability, etc. In: Science and Engineering 0/ Composting: Design, Environmental, Microbiological and Utilization aspects, H. A. J. Hoitink and Keener, H. M. (eds), pp. 451-506. Ohio State University, Columbus. ChefelZ, B., Hatcher, P. G., Hadar, Y. and Chen, Y. (1996) . Chemical and biological characterization of organic matter during composting of municipal solid waste. J. Environ. Qual. 2~, 776-785. Garcia, C., Hernandez, T., Costa, F. and Pascual, J. A. (1992). Phytotoxicity due to the agricultural use of urban wastes. Germination experiments. J. Sci. Food Agric. ~9, 313-319. He, X. T., Logan, T. J. and Traina, S. J. (1995) . Physical and chemical characteristics of selected U.S. municipal solid waste composts. J. Environ. Qual. 24, 543·552. Inbar, Y., Chen, Y. and Hadar, Y. (1989). Solid state carbon-13 nuclear magnetic resonance infrared spectroscopy of composted organic matter. Soil Sci. Soc. Am. J. ~3, 1695-1701. Inbar, Y., Hadar, Y. and Chen, Y. (1993). Recycling of cattle manure: The composting process and characterization of maturity. J. Environ. Qual. 22, 857-863. Jimenez, E. I. and Garcia, V. P. (1992) . Determination of maturity indices for city refuse composts. Agric. Ecosyst. Environ. 38, 331-343. L 'Herroux , L., Lc Roux, S., Appriou, P. and Martinez, J. (1997). Behavior of metals following intensive pig slurry applications to a natural field treatment process in Brittany (France). Environ. Pollut. 97(1-2), 119-130. MacNicol, R. D. and Beckett, P. H. T. (1989). The distribution of heavy metals between the principal components of digested sewage sludge. Water Res. 23(2), 199-206. Petruzzelli, G., Szymura, I., Lubrano, Land Pezzarossa, B. (1989). Chemical speciat ion of heavy metals in different size fractions of compost from solid urban wastes. Environ. Technol. Lett. 10, 521-526. Saviozzi, A., Levi-Minzi, R. and Riffaldi, R. (1988) . Maturity evaluat ion of organic waste. BioCycle 29, 54-56. Stumm, W. and Morgan, J. J. (1981). Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in Natural Waters. 2nd ed, John Wiley & Sons, New York. Taiwan Agricultural Year Book (1995). Department of Agriculture and Forestry, Taiwan Provincial Government, Taiwan, R.O.C.

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Tessier, A., Campbell, P. G. C. and Bisson, M. (1979). Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry. 51, 844-851. Tiquia, S. M., Tam, N. F. Y. and Hodgkiss, 1. J. (1997). Composting of spent pig litter at different seasonal temperatures in subtropical climate. Environ. Pollut. 98(1),97-104. Tisdell, S. E. and Breslin, V. T. (1995). Characterization and leaching of elements from municipal solid waste compost. J. Environ. Qual. 24, 827-833.