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

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<ul><li><p>tioss</p><p>niei, Ita</p><p>Article history:Received 24 July 2013Accepted 3 December 2013Available online 9 January 2014</p><p>Keywords:Anaerobic digestionCentrifuged digestate compostingChemical analysisFTIREEM uorescence spectra</p><p>In this work, anaerobic digestion of pig slurry and successive composting of the digestate after centrifu-</p><p>fertiliser and soil conditioner.MSW represents a signicant but small percentage of the bio-</p><p>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 signicant contributors raising negative impacts onthe environment in terms of global warming, eutrophication and</p><p>06; Moller et al.,</p><p>iquid systee, and lign</p><p>crude protein in manure are of most concern, because theymajor components that can be converted into valueproducts. Swine manures contains about 40% of ber (includinghemicellulose, cellulose, and lignin) whereas proteins an amountat about 1=4 of dry matter (Washington University report, 2003).</p><p>Anaerobic digestion of animal wastes not only produces electricand thermal energy, but helps reducing greenhouse gas emissionsto the atmosphere (Cullar and Webber, 2008). Compared to theraw animal slurries, the residual solids of anaerobic digestion ofanimal manure are signicantly less odorous and have lower</p><p> Corresponding author.</p><p>Waste Management 34 (2014) 653660</p><p>Contents lists availab</p><p>an</p><p>elsE-mail address: (M.R. Provenzano).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 valuable</p><p>sions (Hutching et al., 2007; Monteny et al., 202004).</p><p>In general, animal manure is a complex solid/lcontents of ber (included hemicellulose, cellulos0956-053X/$ - see front matter 2013 Elsevier Ltd. All rights reserved. Thein) andare the-added1. Introduction</p><p>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 dened as a pro-</p><p>acidication (Bayo et al., 2012). However, pig waste is not onlya 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-gation were studied by means of chemical analysis, FTIR and uorescence spectroscopy as excitationemission 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 conrmed 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 uo-rescence ratios which increased from fresh substrate to digestate because of the OM decompostion. Com-posted substrate presented the lowest ratio due to the humication process.</p><p> 2013 Elsevier Ltd. All rights reserved.a r t i c l e i n f o a b s t r a c tChemical and spectroscopic characterizaduring the anaerobic digestion and succe</p><p>Maria Rosaria Provenzano a,, Anna D. Malerba a, DaaDipartimento di Scienze del Suolo, della Pianta e degli Alimenti, University of Bari, BarbDipartimento di Ingegneria Civile e Ambientale, Universit di Perugia, Perugia, Italy</p><p>Waste M</p><p>journal homepage: www.n of organic matterive composting of pig slurry</p><p>la Pezzolla b, Giovanni Gigliotti b</p><p>ly</p><p>le at ScienceDirect</p><p>agement</p><p>evier .com/locate /wasman</p></li><li><p>Manorganic pollution potential so could serve as a good soil amend-ment. This is an additional argument to promote anaerobicdigestion as a rst 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 eldfertigation near AD plants, whereas the solid fraction, whichretains a great amount of volatile solids and phosphorus, is soldas organic fertilizer (Liedl and Shafelf, 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 bers (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,dened 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 nal compostcould allow to characterize the organic matter added to the soilthrough compost application and the impact on soilplantsystem(Said-Pullicino et al., 2007b).</p><p>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 uvis 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).</p><p>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 uorescence spectroscopy as excitationemission ma-trix (EEM).</p><p>2. Materials and methods</p><p>2.1. Pig slurry anaerobic digestion plant and composting process</p><p>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-</p><p>654 M.R. Provenzano et al. /Wasteeral nearby pig farms. During the years, the plant has undergonerelevant improvements and currently it processes about155,000 Mg y1 of pig slurry under mesophilic conditions at37 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 l1 CH4, 400 ml l1 CO2 and traces of H2and 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 oored areas laterally bound by reinforced concrete wallsand equipped with an aeration system.</p><p>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.</p><p>2.2. Chemical analysis</p><p>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 mufe 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).</p><p>To determine the water-extractable organic matter (WEOM),fresh samples were used for PS and D ltering through a 0.45 lmmembrane lter; whereas, dry samples of S and CS were extractedwith deionized water (1:20 w/v) and ltered through a 0.45 lmmembrane lter. 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.</p><p>Total phenolic compounds (TPC) in the water extracts weredetermined by using a modied version of the FolinCiocalteaumethod (Box, 1983) as described by Said-Pullicino and Gigliotti(2007). To 2.5 ml of water extracts, 0.2 ml of FolinCiocalteau re-agent and 0.4 ml of 2 M sodium carbonate solution were added.</p><p>agement 34 (2014) 653660After 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</p></li><li><p>8.5) for compost commercialization (DLgs 75/10). The TKN content</p><p>Manprepared by measuring the absorbance of six different concentra-tions of vanillic acid (from 1 to 6 lg ml1) and results are ex-pressed in mg C l1 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 ml1). Re-sults for TRS are expressed in mg C l1 glucose-C equivalents.</p><p>Humic-like substances were extracted and puried 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 lteredthrough a 0.45 lm membrane lter. An aliquot of the extractswas acidied 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 puried on 1012 cm3 of insoluble polyvinylpyrrolidoneresin (Aldrich, Germany) previously equilibrated in 0.005 MH2SO4 (Petrussi et al., 1988). The eluate contained the nonhumiedfraction (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 puried FA. Total extractable C(TEC) concentration of the ltered alkaline extract, as well as thatof the puried FA fractions, were determined using the element...</p></li></ul>