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

Published by Elsevier Science LtdPrinted inGreat Britain. All rights reserved

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RECYCLING OF SEPARATED PIGMANURE: CHARACTERIZATION OFMATURITY AND CHEMICALFRACTIONATION OF ELEMENTSDURING 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 compostmaturityandto determine the effect of composting on the fractionationof trace elements in SPM compost. Compostingwas performed in tum piles and the following parameters were measured in 10 samplesduring 122 days ofcompostmg: temperature, CIN ratio, ash content, metal contents, humic substance contents, and fractions(humicacid. fulvicacid, and nonhumic fractions - HA. FA, and NHF, respectively). A sequential extractionschemewas used to partitionCu. Mn, and Zn in SPM compost. The CIN ratio and ash content exhibitedatypicallyhighrate of changeduring the first 33 daysand levelledoff thereafter. The fresh SPMwas enrichedwith Cu, Mn, and Zn due to feed additives. All metal concentrations increasedapproximately 2.6-foldin thefinal 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, themajor portionsof Cu. Mil, and Zn were found in the organic, oxide, and carbonate fractions, respectively.Metaldistributions in differentchemical fractions were generallyindependent of composting age and, thus,respective total metal concentrations in the composts. C 1999 1AWQ Published by Elsevier Science. Allrightsreserved

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 (TAYB.1995). and 80% of the feedingoccurs in the country's southern six counties. Inappropriate disposal of pig manure in regions of intense pigproduction may pose environmental problems such as the accumulation ofheavy metals in soil and pollutionof ground and surface waters due to leaching and run-off of nutrients. Separation and composting of thesolids from slurry under controlled conditions may provide a better alternative to manure management.

The composting process involves biological treatment in which aerobic thermophilic microorganisms useorganic matter (OM) as a substrate. The main products of the composting process are fully mineralizedmaterials 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

122 I.-H. HSU and S.-L. LO

soil-borne plant pathogens, and enhancing plant growth (Saviozzi et al., 1988). Noncomposted manure orimmature compost applied to agricultural soil may cause phytotoxicity to plants and adversely affect theenvironment (Garcia et al., 1992).

Several tests have been proposed to assess compost maturity and stability: CIN in the solid phase (Jimenezand 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 mustoften 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 inagricultural soils may cause metals to accumulate to a toxic level. Studies have shown that the chemicalform, rather than the total content, of an element is more important in determining its availability for plantuptake or leachability into groundwater (Petruzzelli et al., 1989). Although several studies have assessed theextractability 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 theeffect of composting on the fractionation of Cu, Mn, and Zn in SPM compost. A sequential extractionscheme was used to determine the phase association of these elements associated with SPM during thecomposting process. This research may provide useful information for successful utilization of SPMcompost.

METHODS

Composting of separated pig manure

The solid fraction or separated pig manure (SPM) obtained from separation of slurry was composted in twopiles in an indoor concrete area at a pig farm at Yong-Kong, Tainan County. The raw material was dividedinto two piles (about 1.5 m3 each) without forced aeration and composted for 122 d. The compost wasturned, mixed, and sampled at 0, 3, 7, 12, 18,25,33,49,80, and 122 d. Water was added immediately afterthe compost was turned to maintain a moisture content of 50-60% (w/w). At a depth of 0.30 m within thecomposting piles, the temperature was taken daily during the first 40 d and then once every three days untilthe end of the process. The samples (4 1)were placed in partially closed polyethylene bags, transported tothe laboratory, and then stored at 5°C. Smaller subsamples were air-dried and used for analyses. Allmeasurements 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 aforced-air oven. Total C and N were analyzed using a Heraeus CHN-O-RAPID analyzer on compostsground 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 through0.45-~m filter membranes. Metals (Cu, Mn, and Zn) analyses of digests were performed using an atomicabsorption 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 wasresuspended in 0.1 N NaOH. This procedure was repeated eight times. The combined solutions were filteredthrough 0.45-~m filter membranes and acidified to pH 1 with 3 M H2S04, allowed to stand at roomtemperature 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 adsorptionof the FA onto Amberlite XAD-8 resin (mesh size 20-60, Sigma, St. Louis). The FF was passed through the

Recycling of separated pig manure 123

column at a flow rate of about 12 bed volumes per hour. The column was then washed with 1 bed volume ofdistilled water. At this stage, the FA adsorbed onto the resin while the NHF was eluted. The combinedsolutions, FF, and NHF were stored at SoC until analysis for organic C. Organic C analyses were conductedwith a total organic carbon analyzer (0· I· Corporation Model 700). Organic C content in the HA fraction ofNaOH extracts was not directly analyzed but calculated by taking the difference between total and FFvalues. 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 fractionsusing multiple extractions of the compost with successively more aggressive leaching solutions. The fivefractions are defined as (i) exchangeable metal ions (1 M MgCh at pH 7), (ii) carbonate-bound metal ions (1M sodium acetate at pH 5), (iii) metal ions bound to iron and manganese oxides (0.04 M hydroxylaminehydrochloride [HONH2HCI] in 25% acetic acid), (iv) metal ions bound to organic matter (0.02 M nitric acidand 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 wasperformed by centrifuging at 10K rpm for 25 min. The supernatant was removed with a pipette, filteredthrough 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 compostingsystems (Jimenez and Garcia. 1992; lnbar et al., 1993; Tiquia et al., 1997). Three phases were observedduring the process: (a) a thermophilic phase lasting for the first 16 d, during which the temperature rosefrom 31°C to 48°C within 24 h and increased to a maximum of 68°C; (b) a cooling phase, in whichtemperature 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.

5 10o 20 40 60 80 100 120 140

Composl ing TIme (days)

25 50

-+- 0Jnp:1t: 20 40~Ambitt 0

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~ -o-C/N ~15 30 ~

~ _ash <10 20

aJ 40 00 eo 100 1aJ 140

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100'-----'--'----'--'----'--'---'

o

Figure 1. Compost and ambient temperatureduring SPM composting.

Figure 2. The C/N ratio and ash contentduring SPM composting.

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 ratiostabilized at a value of about 8 for the remainder of the process. The initial and final ash contents of thecompost were 23% and 41%, respectively (Fig. 2). The change in ash content followed a reverse trend tothat of the CIN ratio, exhibiting three phases: (a) Days 0-18, when most of the OM decomposed; (b) Days 18

124 J.-H. HSU and S.-L. LO

to 80; and (c) Days 80 to the end of the experiment, during which the curing stage began and the rate of OMdecomposition 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 Znincreased 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 theconcentrations in raw material, respectively. Comparing with 15-90% increases during cattle manurecomposting (Inbar et al., 1993) indicates that metals accumulation in SPM compost was drastical andsuggests that the type and decomposability ofraw material is ofmajor importance to the metal accumulationduring 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, andNHF. The levels of HA, FA, and NHF in SPM compost at various stages of the process represent thehumification process (Fig. 4). Total HS increased from 27% of the OM in the raw material to 39% of theOM after 25 d and maintained this value until the end of the process. The increasing trend of the HS levelsduring SPM compo sting agrees with that reported by Inbar et al. (1989) for separated cattle manurecomposting. The FA level gradually decreased from 8.7% of the OM in the raw material to 5.9% in themature compost. The HA level increased during the composting process, gradually increasing from 4.0% to6.2% 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 recoveredas NHF for the first 18 d and shifted to HA for the remainder of the process. The NHF increased from 14 to20% for the first 18 d ofcompo sting, probably due to a high substrate level and greater biosolid surface areaformation during the thermophilic stage, and decreased to 14% in the mature compost because the substrateconsisted ofpolysaccharides 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 thecomposting as follows: (a) the HAIFA ratio remained steady at 0.5-0.7 for the first 18 d, sharply increased to1.9 at Day 25, then gradually increased to a final value of 3.5 at the end of the process; and (b) the HAIFFratio remained steady at 0.17-0.22 for the first 18 d, sharply increased to 0.54 at Day 25, and then increasedto 1.04 in the mature compost. The increasing trend of these parameters indicates that HA became the mainfraction ofHS during composting.

__ Humic acid__ Fulvicacid-.-Nonhwrucfraction-e-Total e:xIracted

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i 1500

I roou __OJ

'3 500 __Mn......-Zo::a

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o 20 40 eo 80 100 120 140

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Figure 3. Variation ofCu, Mn, and Zn contentduring SPM composting.

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Figure 4. Content ofHA, FA, and NHF as % oforganic matter during SPM composting.

Recycling of separated pig manure

Sequential extraction and fractionation

125

Although Cu, Mn, and Zn are essential elements for plant growth, its accumulation in soils due toapplication of SPM compost may cause toxicity. Therefore, contents and chemical forms (thusbioavailability) of these elements are of major concern in assessing the environmental impact of SPMcompost. It is believed that metals in the exchangeable fraction are readily available for leaching, thecarbonate and oxide-bound fractions are relatively labile and may be potentially bioavailable to theenvironment, 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 naturalconditions. The differences in the distribution patterns of Cu, Mn, and Zn in the SPM composts studiedindicate 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 thatthe greatest amount (53-67%) are found in the organic fraction, followed by the oxide (23-32%) andcarbonate (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 resultis consistent with that reported by Tisdell and Breslin (1995), who found that the greatest amount of Cu inMSW compost is associated with the organic fraction. The major association ofCu with the organic fractionin these composts may be due to high formation constants of organic-Cu complexes (Stumm and Morgan,1981).

::s 100%l.iVi 80%aE 60%0u;:; 40%9

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Figure 5. Distribution of'Cu, Mn, and Zn in the various fractions by sequential extraction.

126 J.-H. HSU and S.-L. LO

Manganese. Like Cu, the distribution of Mn in various chemical fractions was generally independent ofcomposting age and thus metal content. In the samples at various stages of composting, the highest amountsof 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 averagesoil Mn content of 2500 mglkg (Brady, 1974), it is unlikely that application of SPM composts to soils willcause Mn toxicity to plants. In fact, Chaney and Ryan (1993) showed concern that application of alkalinebiosolids 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 thecarbonate (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%) regardlessof compost age and total metal concentration (Fig. 5). Tisdell and Breslin (1995) found that the greatestpercentage 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 usuallyneeded. The C/N ratio, ash content, metal contents, and humic substances content are all good indicators ofSPM compost stability and maturity. All of these parameters exhibited three phases in this study: (a) rapiddecomposition during the first 25 d; (b) stabilization until Day 49; and (c) maturation from Day 49 on. Slowdecomposition persisted during the stationary phase at ambient temperature. The SPM compost, described inthis 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 wasused to fractionate these elements present in SPM during the composting process. It appears to have littleinfluence on composting age and thus metal content on the distribution of Cu, Mn, and Zn. Copper wasfound primarily in the organic fraction during the composting process. Manganese was mainly associatedwith the oxide and carbonate fractions. Zinc was most concentrated in the carbonate fraction. The potentialmobility of these element in SPM compost was in the order Zn>Mn>Cu.

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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.

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Tessier, A., Campbell, P. G. C. and Bisson, M. (1979). Sequential extraction procedure for the speciation of particulate tracemetals. 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 insubtropical 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.