Waste Management 34 (2014) 653–660

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Waste Management

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Chemical and spectroscopic characterization of organic matterduring the anaerobic digestion and successive composting of pig slurry

0956-053X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.wasman.2013.12.001

⇑ Corresponding author.E-mail address: mariarosaria.provenzano@uniba.it (M.R. Provenzano).

Maria Rosaria Provenzano a,⇑, Anna D. Malerba a, Daniela Pezzolla b, Giovanni Gigliotti b

a Dipartimento di Scienze del Suolo, della Pianta e degli Alimenti, University of Bari, Bari, Italyb Dipartimento di Ingegneria Civile e Ambientale, Università di Perugia, Perugia, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 July 2013Accepted 3 December 2013Available online 9 January 2014

Keywords:Anaerobic digestionCentrifuged digestate compostingChemical analysisFTIREEM fluorescence spectra

In this work, anaerobic digestion of pig slurry and successive composting of the digestate after centrifu-gation were studied by means of chemical analysis, FTIR and fluorescence spectroscopy as excitation–emission matrix (EEM). Chemical analysis highlighted the organic matter transformation occurring dur-ing the processes. A decrease of volatile solids and total organic carbon were observed in the digestatewith respect to the fresh pig slurry as a consequence of the consumption of sugars, proteins, amino acidsand fatty acids used by microorganisms as a C source. Water Extractable Organic Matter (WEOM) wasobtained for all samples and fractionated into a hydrophilic and a hydrophobic fraction. The highestWEOM value was found in the pig slurry indicating a high content of labile organic C. The digestate cen-trifuged and the digestate composted showed lower hydrophilic and higher hydrophobic contentsbecause of the decrease of labile C. Total phenolic content was lower in the digestate with respect to freshpig slurry sample (36.7%) as a consequence of phenolic compounds degradation. The strong decrease oftotal reducing sugars in the digestate (76.6%) as compared to pig slurry confirmed that anaerobic processproceed mainly through consumption of sugars which represent a readily available energy source formicrobial activity. FTIR spectra of pig slurry showed bands indicative of proteins and carbohydrates. Adrop of aliphatic structures and a decrease of polysaccharides was observed after the anaerobic processalong with the increase of the peak in the aromatic region. The composted substrate showed an increaseof aromatic and a relative decrease of polysaccharides. EEM spectra provided tryptophan:fulvic-like fluo-rescence ratios which increased from fresh substrate to digestate because of the OM decompostion. Com-posted substrate presented the lowest ratio due to the humification process.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

A recent editorial of Waste Management journal (Iacovidouet al., 2013) is entitled: ‘‘Anaerobic digestion in municipal solidwaste (MSW) management: Part of an integrated, holistic and sus-tainable solution’’. Anaerobic digestion (AD) is defined as ‘‘a pro-cesses in which microorganisms break down biodegradablematerial in the absence of oxygen. AD converts organic matter intobiogas (consisting primarily of methane and carbon dioxide), arenewable source of energy, and digestate, a potentially valuablefertiliser and soil conditioner’’.

MSW represents a significant but small percentage of the bio-mass produced daily all over the world. In Europe an increasinginterest has grown around green energy production through ADof agricultural feedstock. Pig manure has been determined as oneof the most significant contributors raising negative impacts onthe environment in terms of global warming, eutrophication and

acidification (Bayo et al., 2012). However, pig waste is not only‘‘a waste’’ but also a valuable resource in terms of nutrients forcrops and energy. To reduce the environmental impacts from thepig manure management, many efforts have focused on variousstrategies aiming at energy and nutrient recoveries, effective nutri-ent controls and reductions in greenhouse gas and ammonia emis-sions (Hutching et al., 2007; Monteny et al., 2006; Moller et al.,2004).

In general, animal manure is a complex solid/liquid system. Thecontents of fiber (included hemicellulose, cellulose, and lignin) andcrude protein in manure are of most concern, because they are themajor components that can be converted into value-addedproducts. Swine manures contains about 40% of fiber (includinghemicellulose, cellulose, and lignin) whereas proteins an amountat about 1=4 of dry matter (Washington University report, 2003).

Anaerobic digestion of animal wastes not only produces electricand thermal energy, but helps reducing greenhouse gas emissionsto the atmosphere (Cuèllar and Webber, 2008). Compared to theraw animal slurries, the residual solids of anaerobic digestion ofanimal manure are significantly less odorous and have lower

654 M.R. Provenzano et al. / Waste Management 34 (2014) 653–660

organic pollution potential so could serve as a good soil amend-ment. This is an additional argument to promote anaerobicdigestion as a first rank process for sustainability by returnig thedigested matter back to the soil. Several AD plants processingagricultural feedstock have been constructed in Italy in the lastfew years (Menardo et al., 2011). They are generally built insidelivestock farms and are fed mostly with liquid and solid animalmanure and energy crops. It is also important to consider thepossibility to separate the digestate into a liquid and a solidfraction and in fact in some Italian AD plants digested slurry ismechanically separated. The liquid fraction, rich in soluble nutri-ents such as nitrogen and potassium, is generally used for fieldfertigation near AD plants, whereas the solid fraction, whichretains a great amount of volatile solids and phosphorus, is soldas organic fertilizer (Liedl and Shafflelf, 2006). Moreover, theseparated solid fraction would capture residual methane and con-sequently could reduce GHG emissions (Amon et al., 2006) duringits storage. As reported by some authors (e.g. Hartmann et al.,2000), the organic matter of the digestate solid fraction is repre-sented in great measure by fibers (hemicellulose and cellulose)and lignin, which are compounds minimally digestible by bacteria(Menardo et al., 2011). Hence, the digestate, for its chemical prop-erties, might be also used as a substrate for the composting processfor improving the quality of the end products (Bustamante et al.,2013). The compost quality refers often to compost stability,defined as the rate or degree of organic matter decomposition ex-pressed in function of microbial activity and is evaluated by meansof respirometric measurements (Adani et al., 2003; Said-Pullicinoet al., 2007a). Also, it is important to consider the quality of WaterExtractable Organic Matter (WEOM) added by the organic amend-ment, because its rate of degradation decreases with progressingorganic matter stabilization. Therefore, the chemical characteriza-tion of the water-soluble organic compounds in the final compostcould allow to characterize the organic matter added to the soilthrough compost application and the impact on soil–plant–system(Said-Pullicino et al., 2007b).

Fluorescence and infrared spectroscopy have proven to be avaluable tool widely used to investigate the content of the mainbiochemical components such as carbohydrates, proteins, fats, lig-nin and cellulose of soil organic matter (Chen, 2003; Provenzanoet al., 1998), to describe the transformation of organic matter dur-ing a composting process or compost maturity (Ouatmane et al.,2000; Smidt et al., 2002), to analyze compost extracts (Carballoet al., 2008), humic and fluvis acids (Amir et al., 2005; Gonzalez-Vilaet al., 1999), to study microbial and fungal biomass (Grube et al.,1999) and relative amounts of proteins, fats, lignins, carbohydratesin organic matter (Orhan and Buyukgungor, 2000) and in wastesdegradation (Calderon et al., 2006; Fakharedine et al., 2006; Wonet al., 2006; Pognani et al., 2010).

The aim of the present work is to study the organic matter evo-lution during AD of pig slurry and successive composting of the so-lid fraction of digestate by chemical characterization of the organicmaterials and by means of Fourier transform infrared spectroscopy(FTIR) and fluorescence spectroscopy as excitation–emission ma-trix (EEM).

2. Materials and methods

2.1. Pig slurry anaerobic digestion plant and composting process

The plant is located near Perugia, Central Italy, in an area char-acterized by outstanding livestock activities and productivity. Theplant has been operating since 1987 and collects slurries from sev-eral nearby pig farms. During the years, the plant has undergonerelevant improvements and currently it processes about155,000 Mg y�1 of pig slurry under mesophilic conditions at

37 �C. The loading is continuous and computerized and the hydrau-lic retention time is about 25 days. The biogas produced, consti-tuted by about 600 ml l�1 CH4, 400 ml l�1 CO2 and traces of H2

and H2S, is delivered to a power co-generator to be converted intoelectricity (3,800,000 kWh per year) and heat energy. The anaero-bic plant is integrated with an aerobic treatment plant so thatthe digestate is centrifuged to obtain two by-products: a liquidfraction, directly used as a nitrogen fertilizer, and a solid fractiondelivered to the composting plant. Composting was carried out un-der aerobic conditions and involved a thermophilic phase ofapproximately 28 d during which the mixture was subjected todaily turnings, followed by a curing phase in piles for approxi-mately 3 additional months. The feedstock was composed of thesolid fraction after centrifugation and yard trimmings from prun-ing activities (70:30 on fresh weight of digestate and ligno-cellu-losic materials, respectively). During the active phase, thefeedstock was fed at one end of a rectangular, concrete, aeratedbay 21 m long and 3 m wide and allowed to accumulate in a layerfrom 2.0 to 2.7 m deep. Three large screws mounted on a bridgecrane served to ‘‘air’’ the biomass and gradually convey the com-posting mixture from the loading to the unloading side of thebay, by about 0.8 m per day for a theoretical period of 28 days. Aer-obic conditions were optimized by means of forced aeration. Cur-ing was carried out in piles for approximately 3 months, onspecial floored areas laterally bound by reinforced concrete wallsand equipped with an aeration system.

From this plant pig slurry (PS), digestate before centrifugation(D) and the solid fraction after centrifugation (S) were sampled.In addition, the stabilized compost (CS) was used to evaluate theorganic matter transformation during the aerobic treatment. Foreach sampling, several sub-samples of about 2 kg were randomlyretrieved, thoroughly mixed and homogenized to obtain a repre-sentative sample of about 1 kg.

2.2. Chemical analysis

Moisture content was determined by weight loss upon drying at105 �C in an oven for 24 h. Electrical conductivity and pH weredetermined on the fresh samples for PS and D, whereas dried sam-ples of S and CS were used for the water extraction (1:5 w/v)(ANPA, 2001). Total volatile solids (VS) were determined by weightloss upon ashing at 550 �C for 24 h in a muffle furnace. Total organ-ic carbon (TOC) was determined by an elemental analyser (EA 1110Carlo Erba, Milan, Italy; ANPA, 2001). Fresh samples were used fordetermination of total Kjeldahl-N (TKN) by means of Kjeldahl dis-tillation method (ANPA, 2001).

To determine the water-extractable organic matter (WEOM),fresh samples were used for PS and D filtering through a 0.45 lmmembrane filter; whereas, dry samples of S and CS were extractedwith deionized water (1:20 w/v) and filtered through a 0.45 lmmembrane filter. The hydrophilic (HI) and hydrophobic (HO) frac-tions of WEOM were then obtained as described in Said-Pullicinoet al. (2007a), and C content both in the total extract (WEOM)and the HI fraction was measured by using Pt-catalysed, high tem-perature combustion (680 �C) followed by infrared detection ofCO2 (TOC-5000A, Shimadzu Corp., Tokyo, Japan). C content in theHO fraction was obtained by difference between water extractableorganic C (WEOC) and C concentration in the HI fraction.

Total phenolic compounds (TPC) in the water extracts weredetermined by using a modified version of the Folin–Ciocalteaumethod (Box, 1983) as described by Said-Pullicino and Gigliotti(2007). To 2.5 ml of water extracts, 0.2 ml of Folin–Ciocalteau re-agent and 0.4 ml of 2 M sodium carbonate solution were added.After mixing, the color was allowed to develop for 1 h at room tem-perature and the absorbance was measured at 760 nm against acontrol. Concentrations were calculated against a calibration curve

M.R. Provenzano et al. / Waste Management 34 (2014) 653–660 655

prepared by measuring the absorbance of six different concentra-tions of vanillic acid (from 1 to 6 lg ml�1) and results are ex-pressed in mg C l�1 vanillic acid-C equivalents. Total reducingsugars (TRS) in the aqueous extracts were determined using a phe-nol reagent (Dubois et al., 1956). Aliquots of 0.5 ml water extractswere treated with 0.5 ml of the phenol solution (0.53 M in distilledwater) and mixed. Then 2.5 ml of conc. H2SO4 were quickly addedunder continuous shaking. The mixtures were left for 10 min atroom temperature and incubated in a water bath at 30 �C for20 min. Then the absorbance was read at 490 nm against a control.A standard curve was prepared by measuring the absorbance of sixdifferent concentrations of glucose-C (from 5 to 50 lg C ml�1). Re-sults for TRS are expressed in mg C l�1 glucose-C equivalents.

Humic-like substances were extracted and purified as describedby Ciavatta et al. (1990). The dried organic materials were ex-tracted with a 0.1 M NaOH and 0.1 M Na4P2O7 solution (1:50 w/v) under N2 at 65 �C for 24 h. The suspensions were centrifugedat 12000 rpm for 20 min, and the supernatants were filteredthrough a 0.45 lm membrane filter. An aliquot of the extractswas acidified to pH 2 with concentrated H2SO4 to separate humic(HA) from fulvic acids (FA). Coagulated humic acids (HA) were col-lected, while the supernatants containing the fulvic acids (FA) werefurther purified on 10–12 cm3 of insoluble polyvinylpyrrolidoneresin (Aldrich, Germany) previously equilibrated in 0.005 MH2SO4 (Petrussi et al., 1988). The eluate contained the nonhumifiedfraction (NH), characterized by the presence of organic compoundssuch as carbohydrates, free amino acids, and peptides which areco-extracted in alkaline solutions (Businelli et al., 2007). The NHfraction was discarded, while the fraction retained was eluted with0.5 M NaOH and represented the purified FA. Total extractable C(TEC) concentration of the filtered alkaline extract, as well as thatof the purified FA fractions, were determined using the elementalanalyser previously described. The HA fraction was obtained bytaking the difference between TEC and FA. The degree of humifica-tion (DH%) was also calculated as the percentage of the ratio(HA + FA)/TEC. Microbial respiration of the composting matrixbased on oxygen consumption was measured using a procedureoriginally described by Lasardi and Stentiford (1998) and modifiedby Adani et al. (2003). The specific oxygen uptake rate (SOUR) andoxygen demand (OD) were determined by measuring the changesin the concentration of dissolved oxygen in an aqueous compostsuspension under conditions ensuring optimum microbial activityand maximum reaction rates over a period of 24 h. The SOUR wascalculated by means of a linear regression analysis of dissolvedoxygen against time over repeated measuring intervals of 15 minand expressed as mg O2 g�1 VS h�1.

All analyses were carried out in triplicate and standard error(SE) was calculated.

2.3. Spectroscopic analysis

The FT-IR spectra were obtained in the 4000–400 cm�1 wave-length range by a Nicolet 5PC FTIR spectrophotometer, with a2 cm�1 resolution and 64 scans/min. 1 mg sample and 400 mgKBr, spectrometry grade, were homogenized thoroughly in anagate mortar and pellets of 1 cm diameter and about 1 mm thick-ness were obtained by pressing, under vacuum, about 200 mg ofmixture with precaution taken to avoid moisture uptake.

Fluorescence spectra were obtained on water solution of sam-ples at a concentration of 100 mg L�1 after overnight agitationand equilibration at room temperature, successive filtration withWhatman n� 1 paper and adjustment top pH 8 with 0.05 N NaOH.Excitation emission matrix (EEM) spectra were obtained by settingthe emission (Em) wavelength in the range from 400 to 600 nm,while the excitation (Ex) wavelength was increased from 300 to500 nm in 5 nm steps. The EEM contour maps were obtained so

that each different fluorophore was characterised by an Ex/Emwavelength pair. In order to standardize the fluorescence intensity(FI), the Raman signal of deionised water at 348 nm ex (emitted be-tween 395 and 400 nm) was measured and all results were stan-dardized to a mean Raman peak of 20 intensity units (Baker,2002). All measurements were carried out at lab temperature of22 ± 2 �C.

3. Results and discussion

3.1. Chemical analysis

In Table 1 chemical data of organic materials used in the exper-iment are reported.

Several factors influence the anaerobic digestion of an organicsubstrate, e.g. C/N ratio, the water content of the raw materials,pH, loading rate and temperature (Abbasi et al., 2012). Dry mattercontent of organic wastes is then a significant parameter of the AD,since the water content can affect the waste degradation and themicrobial activity. In our experiment, PS sample, with total solidsconcentration below 10%, represented a typical substrate used for‘‘wet’’ digestion systems (García-Bernet et al., 2011). D sample ob-tained at the end of the process had a value slightly higher than theinitial raw material, suggesting that the anaerobic treatment of pigslurry does not lead to a decrease of the water content, whereas thetotal solids increased in S and CS, i.e. after centrifugation and thesubsequent composting of the solid fraction of digestate. A de-crease of VS and TOC was observed in D sample (�12.5% and�12.1%, respectively) as compared to PS. In fact, during the anaer-obic digestion microorganisms use sugars, proteins, amino acidsand fatty acids as a C source, resulting in a decrease of VS andTOC contents (Tambone et al., 2013). The organic matter (OM) lossduring the process may be expressed according to the equationproposed by Paredes et al. (1996) and recently used by Altieriet al. (2011) and Gigliotti et al. (2012) for describing the OM lossduring composting:

OM� loss% ¼ 100� 100½X1ð100� X2Þ�=X2ð100� X1Þ

where X1 and X2 represent here the ash content of the pig slurry andits digestate before the centrifugation, respectively.

By this equation it was estimated an OM loss of 44.8% duringthe anaerobic digestion. The much lower decrease of VS and TOCobserved in CS sample (�2.4% and �10.8%, respectively) with re-spect to S suggested that the computable OM loss could easily beunderestimated because of the addition of ligno-cellulosic materi-als before the composting process. The initial pH value of PS washigher than the optimum pH of a feed-in material that should becomprised between 6.5 and 7.5 (Deublein and Steinhauser,2008). This was probably due to the low initial C/N ratio of PS thatcaused the release of nitrogen and the accumulation as ammoniahence rising the pH (Abbasi et al., 2012). The pH value did notchange after the process whereas a small decrease was observedfor S sample. The highest value was measured for CS sample reach-ing almost the optimal value expected by the Italian law limit (6.0–8.5) for compost commercialization (DLgs 75/10). The TKN contentdid not change in D, whereas a decrease was observed from PS to S,although in D and S it was lower than expected. In fact, it has beenreported (Orzi et al., 2010) that volatile solid degradation causesnitrogen to concentrate in the digestate. Although the TKN contentdid not change after the anaerobic treatment, a C/N ratio decreasewas observed in D because of organic matter degradation. After-wards, liquid/solid separation by centrifugation caused the transferof the ammonia from D to the liquid phase, resulting in an increaseof C/N ratio in S. After the composting process, C/N ratio of CSreached the value of about 10, lower than that of 13.4 observed

Table 1Chemical data of organic materials used in the experiment.

Parameter PS D (before centrifugation) S (after centrifugation) CS

Total solids (%) 4.9 ± 0.6 6.0 ± 1.2 31.8 ± 2.4 69.5 ± 3.3VS (%) 77.1 65 75 64.2pH 8.27 ± 0.03 8.29 ± 0.01 7.74 ± 0.02 8.83 ± 0.05CE (mS cm�1) 1.4 ± 0.1 0.3 ± 0.1 0.7 ± 0.1 3.2 ± 0.1TOC (%) 34.5 ± 4.5 22 ± 0.2 34.2 ± 0.5 31.8 ± 1.4TKN (%) 4.42 ± 0.90 4.20 ± 0.29 2.90 ± 0.02 3.18 ± 0.10C/N ratio 7.8 5.2 11.8 10.0WEOC (%) 9.44 ± 0.01 1.71 ± 0.20 2.29 ± 0.54 3.13 ± 0.09HiWEOC (%) 72.5 91.2 23.3 30.8HoWEOC (%) 27.5 8.8 76.7 69.2Ho:Hi 0.4 0.10 3.6 2.2TEC (%) 23.5 ± 1.7 13.6 ± 1.7 13.2 ± 2.4 12.1 ± 0.1HA+FA (%) 8.0 ± 1.5 8.6 ± 1.6 6.6 ± 2.2 6.9 ± 0.3NH (%) 15.5 5.0 6.6 5.2DH (%) 21.4 63.1 40.96 48.3SOUR (mg O2 g�1 SV h�1) 70.2 ± 4.8 11.4 ± 2.1 9.5 ± 1.1 4.4 ± 1.3TPC (mg g�1 vanillic acid equivalents) 4.90 ± 0.24 3.1 ± 0.3 1.10 ± 0.03 3.90 ± 0.04TRS (mg g�1 C-glucose) 4.02 ± 0.10 0.98 ± 0.07 1.92 ± 0.77 5.13 ± 0.11

All data are expressed on a dry basis and represent the average of 3 replicates ± SE.VS, volatile solids; CE, electrical conductivity; TOC, total organic carbon; TKN, total Kjeldahl nitrogen; WEOC, water extractable organic carbon; HiWEOC, hydrophilic fraction;HoWEOC, hydrophobic fraction; TEC, total extractable carbon; HA, humic acids; FA, fulvic acids; NH, nonhumified fraction; DH, degree of humification; SOUR, specific oxygenuptake rate; TPC, total phenolic compounds; TRS, total reducing sugars.

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by Puyuelo et al. (2011) for mature and refined compost obtainedfrom digested and composted organic fraction of MSW.

PS sample showed the highest WEOC value indicating a highcontent of labile organic C in the untreated material. The reductionin D sample was due to consumption of the labile C. When consid-ering the different fractions of WEOM, higher HI fraction was ob-tained in PS and D. The lower HI content and higher HO contentin S and CS is clearly associated to the decrease of labile C in thesolid fraction after centrifugation and then in the final compost.Consequently, centrifugation can be considered a key factor forobtaining a more stable material that can act as a very suitablesubstrate for the composting process. As for CS sample, similar re-sults were obtained by Zhou et al. (2000) who found that thehydrophobic fraction of WEOM increased during composting. Also,Said-Pullicino et al. (2007a) during the curing phase of a compost-ing process of source-separated municipal solid waste, yard trim-mings from pruning activities and foliage residues from thetobacco agro-industry (55%, 15% and 30% w/w, respectively) foundthat the percentage of water-soluble organic carbon in the HO frac-tion increased. In the same paper, Said-Pullicino et al. (2007a) pro-posed to use the ratio of HO to HI carbon content as an appropriateindicator of compost stability. A value of HO/HI ratio >1 for CS sam-ple confirmed the correct evolution of the composting process.

Opposite to the results of WEOC characterization, the humic-like substances (HA + FA) content was higher in PS and D, resultingin the greatest DH value in the digested pig slurry. This suggestedthe presence of stable organic C after the anaerobic digestion, asalso observed by De Neve et al. (2003), who found that stable Cin the digested slurry was higher compared to different compostedmaterials. Also, Tambone et al. (2009) stated that the anaerobicdigestion determines the relative concentration of more recalci-trant organic molecules and a higher degree of biological stabilityof digestate with respect to the starting mixture, resulting in stableand partially hygienic organic product. Although, the HA + FA con-tent both in S and CS were not different with respect to D sample,the lowest NH observed in the digestate before centrifugation re-sulted in a major DH. Results of SOUR showed the following orderPS > D > S > CS confirming that its decrease can be used as an indexof compost stability (Adani et al., 2003; Said-Pullicino et al.,2007a).

Analysis of phenols and sugars in the WEOM may provide inter-esting results since low-molecular weight compounds are involved

in the organic matter degradation during anaerobic digestion (Caoet al., 2013). Results of TPC content showed a decrease in D (�1.8%)with respect to PS sample as a consequence of phenolic com-pounds degradation operated by the microbial biomass in the reac-tors (Levén and Schnürer, 2005; Cao et al., 2013). The decrease ofTPC was particularly evident in S (�3.8%) compared to PS sample,suggesting that most of TPC remained in the liquid fraction. Thehigher amount of TPC in CS with respect to D sample may be ex-plained both by the degradation of lignin present in the materialadded before composting and the greater contribution of micro-bial-derived carbohydrates (Said-Pullicino et al., 2007b). Thestrong decrease of TRS in D as compared to PS indicated that theanaerobic process proceed mainly through consumption of sugarswhich represent a readily available energy source for microbialactivity although this seems not to apply to CS that exhibited thehighest TRS value.

3.2. FTIR spectra

The assignments of infrared bands has been based on long-established bibliography (Wu et al., 2011; Cuetos et al., 2010;Pognani et al., 2010). FTIR spectra of pig slurry (Fig. 1a) shows abroad band around 3300–3400 cm�1 (H bonds, OH groups), twodistinct peaks at 2930 and 2850 cm�1 (CAH asymmetric, CAHstretch of ACH), and peaks at: 1640 cm�1 (C@@O of primary amides,ketone and quinone groups and C@@C double bond); 1560 cm�1

(CAN of secondary amides); 1432 cm�1 (phenolic OH) and at1020 cm�1 (CAO stretch of polysaccharide, SiAO stretch). Hsuand Lo (1999) reported that pig manure spectra resemble soil HAclassified as Type III by Stevenson and Goh distinguished by thepresence of a strong absorption band near 1650 cm�1, moderatelystrong absorption at 1540 cm�1, strong absorption near 1050 cm�1

and relatively pronounced absorption near 2900 cm�1. A uniquefeature of these spectra is the presence of bands indicative ofproteins and carbohydrates. Similar results were obtained byCuetos et al. (2010) when studying by FTIR the transformationsundergone by the organic fraction of slaughterhouse waste duringanaerobic digestion. Based on their evidence they stated thatfresh substrate ‘‘reflected a high aliphaticity degree verifying a highcontent in nitrogen-containing compounds’’. Distinct changes areevident when comparing pig slurry spectra with those of theproducts after anaerobic digestion (Fig. 1b). In particular, bands

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at 2920–2930 cm�1 and at 2851 cm�1 associated with fats and lip-ids tend to decrease as a result of the drop in the levels of aliphaticstructures. Similar results were obtained by Pognani et al. (2010)when analyzing by DRIFT spectroscopy the organic fraction ofMSW. After the AD process they found a strong reduction of the ali-phatic fraction contents associated to the transformation of volatilefatty acids into CH4 and CO2. They concluded that their resultsshowed ‘‘the expected degradation of the organic matter and thechange to other structures more stable’’. The 1560 cm�1 peak isno longer evident whereas the peak in the polysaccharides regiondecreases as a result of the biodegradation of amino chain and

sugars, respectively. Further, the relative height of the aromaticregion at 1640 cm�1 raises as the anaerobic process proceeded.Similar results were obtained by Hsu and Lo (1999) who, aftercomposting pig manure for 122 days, found an increase in aromaticand a decrease in carbohydrates indicating that easily degradableorganic matter constituents, such as aliphatic and amide compo-nents, polysaccharides and alcohols are chemically or biologicallyoxidized and therefore the mature compost contained morearomatic structures of higher stability. The composting processinduces organic matter degradation under aerobic condition sothat chemical transformations are expected to be different from

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Fig. 4. Tryptophan:fulvic-like fluorescence ratio.

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those occurring during an anaerobic digestion. However, in bothcases the first step is the degradation of easily degradable mole-cules and the final product is characterized by higher stabilityand greater chemical complexity as compared to the correspond-ing starting materials. Similarly, Gomez et al. (2007) when evaluat-ing the stability of digestates obtained from different biowastesstated that ‘‘easily degradable organic matter constituents, suchas aliphatic components, amino-compounds and polysaccharidesare degraded and the volatile content of wastes is reduced,increasing the aromaticity degree’’. Conversely, Cuetos et al.(2010), when comparing spectra of digestates obtained fromslaughterhouse waste co-digested with the organic fraction ofdomestic solid waste, found an increase of aromatic at1630 cm�1 along with an increase of primary and secondaryamides at 1530 cm�1 and of polysaccharides at 1030 cm�1. Theyexplained that the enrichment in amide and aromatic structuresmay be accounted for by carbon lost and digestion process, wherean important part of carbon was emitted as CH4 and CO2. Further,as Fig. 1b shows, a number of peaks in the 1440–1370 cm�1 regionascribed to alkyl groups of fiber (hemicellulose, cellulose, andlignin) which do not degrade during the process are evident onFTIR spectra of digestates. Our results are coherent with those ofAmir et al. (2005) who showed that sludge decomposition duringcomposting begins by the lipid, protein and carbohydrate compo-nents. Similar results were also obtained by Marcato et al. (2008)who studied anaerobic digestion of pig slurry and found that rawand digested slurry FTIR spectra exhibited the same absorbanceareas, but they differed in the intensity of some peaks. In particu-lar, in digested slurry, they observed a remarkable decrease ofaliphatic structures, lipids and polysaccharides. These decreasesrepresented the biodegradation of the labile fraction into biogas

(Smidt et al., 2002), with a relative increase in more resistantand stable compounds. Marcato et al. (2008) also compared FTIRspectra of pig slurry and digestates to those of humic (HA) and ful-vic acids (FA) extracted from pig slurry and observed that pig slurryspectra were quite similar to HA spectra while digestates spectralooked like FA spectra due to the reduction of aliphatic groups at2900 cm�1 and 1460 cm�1. From these results they concluded that‘‘anaerobic stabilization of organic matter is mainly due to thebuild-up of more stable compounds in the dry matter rather thanhumification process. The same authors assigned the band at1420 cm�1 in pig slurry spectra to the stretching of carbonategroup and reported that: ‘‘The digested slurry FTIR spectra revealedan increase in carbonates probably due to OM mineralizationduring anaerobic digestion. Indeed, the bioconversion of organicmatter into biogas led to the release of compounds such as Ca2+

which reacts with carbonate ion and precipitates’’. The presenceof calcium carbonate in pig slurry has been considered alsoby other authors. Plaza et al. (2002) studied the effect of PS

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amendment on the main properties of soils and found that, withrespect to the control soil, the PS-amended soils had greater pH.They linked the soil pH increase to the large content of CaCO3 inthe PS. In our case, no CaCO3 was added to pig slurry so the peakat 1420 cm�1 has to be assigned to phenolic OH. Digestates aftercentrifugation (Fig. 1c) exhibit simpler spectra with absorptionsat 1640 cm�1, 1420 cm�1, 1020 cm�1 and a sharp peak at1260 cm�1 assigned to aromatic C. As compared to D, S samplesspectra shows very different relative intensity of peaks with a de-crease of the 1640 cm�1 peak and an increase of the 1020 cm�1

peak. CS spectra (Fig 1d) shows a trend similar to that of S samplebut a relative increase of aromatic and a relative decrease of poly-saccharides are evident as a result of the composting processwhich builds up aromatic structures and degrades sugars. The sub-tracted spectrum of S and CS (Fig. 2) highlights transformationsundergone by S samples after composting.

3.3. EEM fluorescence spectra

According to Baker (2001, 2002), Peak A occurs at an excitationwavelength of 320–350 nm and emission wavelength of 400–450 nm and has been related to fulvic-like substances, peak B oc-curs at an excitation wavelength of 340–390 nm and emissionwavelength of 440–500 nm and has been related to humic-likesubstance, whereas peak C is a fluorophore at 270–280 nm excita-tion and 304–315 nm emission is attributed to tyrosine-like fluo-rescence, peak D at 270–285 nm excitation and 340–360 nmemission is attributed to tryptophan-like fluorescence.

Baker (2002) analyzed by EEM freshly collected, not-filtered pigslurry samples and found only one peak of group D (tryptophan)and no group C (tyrosine) fluorescence peak. In our case (Fig. 3),pig slurry samples show a group A fluorescence peak which isdetectable also on EEM spectra of digestates and digestates com-posted. Digestates show in addition a group D fluorescence peakof higher intensity. Digestates centrifuged shows a peak locatedat much lower wavelength pair that that of D samples. As expected,digestates composted exhibit the highest FI. Livestock faeces typi-cally comprise 15–25% protein in wet manure; within this 3–16 gtrue protein N is tryptophan and tyrosine (Baker, 2002) so thatthe tryptophan:fulvic-like fluorescence ratios (Baker, 2002) maybe used to evaluate the evolution of the chemical process. Ratiosof all samples are illustrated in Fig. 4. A tryptophan:fulvic-like fluo-rescence ratio increase is observed from PS to D samples whichmay be explained by the OM decomposition during the anaerobicprocess. The highest ratio of S is associated to the centrifugationprocedure which concentrates fibers in the digestate. The lower ra-tio observed for CS sample is easily explained by the humificationprocess which took place in the composted substrate.

4. Conclusions

Results obtained in the present work provided chemical andspectroscopic evidence of the organic matter evolution during ADof pig slurry and successive composting of the solid fraction of dig-estate. Chemical analysis evidenced a consumption of sugars, pro-teins, amino acids and fatty acids during the AD process. Pig slurryshowed the highest content of labile organic C, whereas the lowerhydrophilic content and higher hydrophobic content in the solidfraction after centrifugation and in the stabilized compost wasassociated to the decrease of labile C. The greatest DH value wasfound in the digested pig slurry suggesting the presence of stableorganic C after the anaerobic digestion. Results of TPC and TRS indi-cated phenolic compounds degradation and sugars consumptionduring the AD process. FTIR spectra suggested a drop of aliphaticstructures, a decrease of polysaccharides and an increase of the

peak in the aromatic region after the AD process, whereas the com-posting process induced a further increase of aromatics.

EEM spectra provided tryptophan:fulvic-like fluorescence ratioswhich indicated OM decomposition during the anaerobic processand OM stabilization after the composting process.

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