Fractionation and characterisation of dissolved organic matter fromcomposting green wastes

A. de Guardiaa,*, S. Bruneta, D. Rogeaua, G. Matejkab

a Cemagref, Livestock and Municipal Wastes Management Research Unit, 17 avenue de Cucill�ee, 35 044 Rennes c�eedex, Franceb National Engineering School of Limoges, Parc d’Ester-Technopoole, 87 068 Limoges c�eedex, France

Received 1 May 1999; received in revised form 22 November 2001; accepted 28 November 2001

Abstract

A new fractionation procedure using membrane ultrafiltration (UF), followed by chemical characterisation – measurement of

total organic carbon (TOC), chemical oxygen demand (COD) and organic nitrogen and spectroscopic study – was applied to

aqueous extracts of composting green wastes. Three membranes of molecular weight (MW) cut-offs of 1, 10 and 100 kDa were used.

The study demonstrated the first step of the transfer of organic matter from the solids to the aqueous biofilm surrounding the solids.

The microbiological consumption of the dissolved organic matter mainly used molecules smaller than 1 kDa, while the aromati-

sation of the organic matter, observed after 100 days composting, involved molecules larger than 10 kDa. � 2002 Elsevier Science

Ltd. All rights reserved.

Keywords: Composting; Aqueous extract; Organic matter fractionation; Membrane ultrafiltration; Humification; Green wastes

1. Introduction

Composting becomes more and more an alternativetreatment method of organic wastes. On the contrary toothers, it allows treatment of small or big amounts ofwastes and remains a cheap and robust process. In orderto control the composting organic wastes transforma-tions, drying, stabilisation, pathogen destruction, con-trol of nitrogen content, etc. – it is necessary to improvethe understanding of the phenomena ruling the process.These phenomena include microbiological metabolismand mass and heat transfers. Assuming microorganismsrequire enough moisture to live, it is often consideredthat most organic matter biodegradation occurs in athin liquid phase, the ‘‘biofilm’’, constituted by moisturesurrounding and impregnating the solids (Inbar et al.,1990).

At the beginning of a composting treatment, the mi-croorganisms, living in the wastes, start utilising thebiodegradable organic matter which is dissolved in thebiofilm. That consumption is responsible for heat pro-duction and temperature increase in the system. Con-

currently, the microorganisms produce enzymes whichattack the solid organic fraction. This enzymatic attackleads to enrichment of biofilm in molecules which themicroorganisms can in turn easily utilise. Physicaltransfer from solid phase to liquid phase may be re-sponsible for this enrichment too. Thus, the initial stageis the microbiological consumption of biodegradableorganic matter. Some unfavourable conditions such asoxygen depletion, insufficient moisture or very hightemperature, may limit that microbiological activity.Depletion of nutrients, which means organic matterstabilisation, could also lower microbiological activity.

The biofilm surrounding the solids has often beenextracted and analysed particularly in order to identifyparameters indicative of organic matter stabilisation(Chanyasak and Kubota, 1981; Chanyasak et al., 1982;H€aanninen et al., 1995; Jimenez and Garcia, 1989). Thesestudies exhibit evolutions which can be explained bymeans of the different phenomena described above.Fractionation of the organic matter dissolved in thebiofilm followed by chemical characterisation of eachfraction may also favour the understanding of thesephenomena.

Dissolved organic matter may be fractionated bymacroreticular exchange resins which leads to separa-tion and quantification of hydrophobic and hydrophilic

Bioresource Technology 83 (2002) 181–187

*Corresponding author. Tel.: +33-2-2348-2133; fax: +33-2-2348-

2115.

E-mail address: amaury.de-guardia@cemagref.fr (A. de Guardia).

0960-8524/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0960-8524 (01 )00228-0

fractions. So, Wershaw et al. (1995) and Mejbri et al.(1996) characterised leachates derived from compostingpiles. That method has also been used to fractionatehumic acids from composts (Chefetz et al., 1996; DeNobili and Petrussi, 1988; Aoyama, 1996). Zhou et al.(2000) compared dissolved organic matter from sewagesludge and composted sludge on the basis of that frac-tionation.

The gel permeation chromatography and the mem-brane ultrafiltration (UF) allow both fractionations asfunction of molecular sizes or weights in the biofilm. Thefirst method leads to a chromatogram on which eachsignal corresponds to a liquid fraction collected at theexit of the permeation column. So Chanyasak et al.(1980, 1982) and Prudent et al. (1995) showed that themolecular weights (MW) of peptides, polysacharidesand humic substances increased while composting.Trubetskaya et al. (2001) used that method to comparehumic acids derived from soil and sewage sludge com-post.

The UF method uses a membrane with a specific MWcut-off. It permits the separation of a liquid phase in onepart containing molecules with MWs higher than cut-offand another one containing molecules with MWs lowerthan cut-off. Agbekodo and Legube (1995) used mem-brane UF to study distribution of dissolved organiccarbon and its biodegradable part in natural surfacewater. They showed that biodegradable dissolved or-ganic carbon was low in the fractions with MW higherthan 10 kDa whereas fractions with MWs lower than500 Da accounted for the first 30% of total biodegrad-able dissolved organic carbon. Burdige and Gardner(1998) applied the same method to pore waters fromestuarine and continental margin sediments. Gourdonet al. (1989) and Tr�eebouet et al. (1996) both showed thatin landfill leachates, molecules with MWs lower than 1kDa accounted for most of the chemical oxygen demand(COD). Vidal et al. (2001) and Barker et al. (1999) in-vestigated the MW distributions of several effluentstreated by anaerobic digestion. Garcia et al. (1993)studied hydrolase activity in the organic matter fractionsof composting sewage sludge.

The purpose of our study was to investigate if biofilmUF could contribute to a better understanding of thecomposting process. The low biodegradation rate ofcomposting green wastes led us to choose such sub-strates in our experiment. After characterisation of theglobal solid transformations occurring during theircomposting (De Guardia et al., 1998), the biofilm sur-rounding the solids has been extracted and fractionatedby UF. The UF fractions have been chemically char-acterised by measurement of pH, total organic carbon(TOC), COD, Kjeldahl, ammonium and organic nitro-gen concentrations. These concentrations have beenexpressed in terms of mass evolution and we propose toexplain here these evolutions.

2. Methods

2.1. Green wastes composting process

Green wastes were brought to the composting plantby the inhabitants of the nearest agglomerations. Theywere composed of branches, leaves, weeds, and grasscuttings. The branches had low biodegradation ratesand their structure would favour the pile aeration. Theleaves, weeds and grass cuttings were fine solids; they arewetter and richer in nitrogen and their biodegradationrates are higher, than branches.

After a few days of storage, the green wastes wereground and stacked in windrows on a cemented un-covered area. Each windrow had a trapezoidal cross-section (30 m� 10 m and 3 m high) and an initial massof 500–1500 t. The windrows were turned each 15–21days by a mobile equipment. For the first three months,in case of insufficient moisture, water was sprayed fromabove. After six to seven months of composting, theproduct was separated using one rotative sieve of 10mm, the undersized material being the compost and theoversized material being recycled in grinding at the be-ginning of the process.

Biofilm transformations occurring in one windrowstacked in April 1997 were characterised. The experi-ment lasted eight months.

2.2. On site sampling

The windrow was sampled during each turning (DeGuardia et al., 1998).

When the mobile equipment was going through thewindrow, it turned about 30 cm width of the pile. About30 passages were necessary to turn the whole windrow.At each passage, we collected manually about 10 kg ofproduct in the cross-trapezoidal section. At the end, allthe collected samples were mixed together and thecomposite sample (ca. 300 kg) was reduced, by succes-sive division in identical portions, to about 30 kg.

2.3. Aqueous solid–liquid extraction

Considering the heterogeneity brought about by thebiggest pieces of branches, the wet product was frac-tionated by sieving through a 25 mm square mesh. Theoversized fraction always accounted for around 10% inweight. This constant and small percentage led us toconsider the undersized fraction (< 25 mm) as the re-active product. About 100 g of the wet undersizedfraction (< 25 mm) were extracted three times. Eachextraction consisted in mixing the product (100 g) with500 ml deionised water (pH ¼ 5:5, R > 10 MX cm).After 4 h agitation, the product was filtered and mixedonce again with 500 ml deionised water. Finally, thethree extracts were mixed together. The solution ob-

182 A. de Guardia et al. / Bioresource Technology 83 (2002) 181–187

tained was centrifuged at 3000 rpm for 10 min and thesupernatant was filtered through a 8 lm membrane(Millipore MF SCWP) then through a 0.22 lm one(Millipore MF GSWP). The final solution was calledF0.22 lm.

2.4. Organic matter ultrafiltration

Ultrafiltrations were performed using a 400 ml cy-lindrical cell equipped with magnetic agitation (Milli-pore UFXF) to reduce polarisation at the surface ofmembranes. These were flat, circular membranes 76 mmdiameter with MW cut-offs of 1, 10 and 100 kDa. Themembranes made of regenerated cellulose (MilliporeYM) offer a low proteic adsorption rate and are con-sidered non-ionic. The range of cut-offs was alreadytested by Garcia et al. (1993, 1995) and Tr�eebouet et al.(1996). Nitrogen was applied to pressurise the UF cell at3 bars for membranes with MW cut-off of 1 and 10 kDaand at 0.5 bar for membrane with MW cut-off of 100kDa. The water flow as a function of pressure wasmeasured for each membrane and their resistances werecalculated as follows:

J ¼ DP=ðRm � lÞ;

where J is the water flow ðm3 m�2 s�1Þ, DP the pressure(Pa), Rm the membrane resistance ðm�1Þ, l is the dy-namic viscosity of water (Pa s).

The value of Rm is 8:68� 1013 m�1 for 1 kDa MWcut-off membrane, 1:66� 1013 m�1 for 10 kDa and3:04� 1012 m�1 for 100 kDa.

The procedure of UF was as described in Fig. 1. Two-hundred ml of F0.22 lm was introduced in the UF cellequipped with 100 kDa MW cut-off membrane. After175 ml was fractionated, 25 ml of deionised water wereadded to the 25 ml of F0.22 lm remaining and 25 ml ofthat mixture were subsequently fractionated. So weobtained 200 ml permeate and the addition of 175 mlwater to the retentate, agitated for 10 min, also led to200 ml additional retentate. The retentate R100 kDa

contained mostly molecules ð> 90%Þ with MW greaterthan 100 kDa and the permeate contained moleculeswith MW less than 100 kDa. With the same procedure,the permeate was fractionated on the membrane withMW cut-off of 10 kDa and it led to a retentate R10 kDacontaining molecules less than 100 kDa but greater than10 kDa. The permeate was fractionated and it led to aretentate, R1 kDa, containing molecules with MW lessthan 10 kDa but greater than 1 kDa and to a permeate,F1 kDa, containing molecules smaller than 1 kDa.

2.5. Organic matter characterisation

Each fraction obtained was characterised by mea-suring the following parameters: pH, TOC, COD,Kjeldahl and ammonium nitrogen concentrations. ThepH was measured with a pH-meter (Hanna type HI8424) and a pH-probe (Metrohm type 6.0232.500). TOCconcentrations were measured with the analyser O.I.Analytical model 1010. COD concentrations were de-termined by dichromate oxidation according to theprocedure described in norm NF T 90–101 (Oct. 1998).Total ammoniacal nitrogen was determined by steamdistillation using MgO, followed by back titration of theboric acid distillates using sulfuric acid (0.1 M). Kjeldahlnitrogen was digested using the Kjeldahl procedure anddistilled with NaOH (30%) (NF EN 25663, January1994). Organic nitrogen was obtained by subtraction ofammoniacal nitrogen from Kjeldahl nitrogen.

3. Results and discussion

3.1. Biofilm mass evolutions

Considering the different phenomena interfering inthe process, microbiological consumption and mass andheat transfers, it seems important to describe the biofilmtransformations as exactly as possible. The expression ofbiofilm transformations only in terms of concentrationsis not satisfactory. For instance, as we can observe anincrease in concentration, its expression in mass canshow a decrease. The expression in mass accounts forthe real transformations which can help understandingof the process, whereas evolution in concentration, in-crease or decrease, depends not only on product trans-formations but also on water evaporation. We chose togive both expressions, in concentration and in mass.

To determine mass evolution as function of thecomposting time, the concentration (mg/g biofilm) hasto be multiplied by the total mass of biofilm in thewindrow (mBt: kg) at every time t. That mass is the sumof the mass of dissolved dry matter plus the mass ofwater. At a first approximation the mass of dissolveddry matter can be neglected compared with the mass ofwater. By measure of organic matter of the sample atFig. 1. Membrane UF procedure.

A. de Guardia et al. / Bioresource Technology 83 (2002) 181–187 183

time t (OMt: �) and considering the total mineral massremains constant over the treatment, the total mineralmass at time t (mMMt: kg) remains equal to the initialtotal mineral mass (mMM0: kg), we can calculate thetotal dry mass of the windrow (mSt: kg). Then measureof moisture at time t (Ht: �) allows calculation of thetotal biofilm mass mBt. Measure of initial wet mass ofthe windrow (mH0: kg) and measure of its moisture H0(�) and organic matter OM0 (�) allow calculation ofthe total mineral mass mMM0 (kg).

mMM0 ¼ mH0� ð1�H0Þ � ð1�OM0Þ;mMMt ¼ mMM0 ¼ mSt� ð1�OMtÞ;then

mSt ¼ mMM0=ð1�OMtÞ;mBt ¼ mSt�Ht=ð1�HtÞ:

Table 1 gives moisture and biofilm mass as functionsof the composting time. The small decrease of moistureand biofilm mass could be explained by atmosphericprecipitation which hid water evaporation caused byincrease of compost temperature.

3.2. Control of fractionation procedure

Measure of TOC in the different fractions F1 kDa,R1 kDa, R10 kDa, R100 kDa and addition of the cor-responding masses allows calculation, to within 2–5%,mass of TOC found in F0.22 lm. This control allowsvalidation of the fractionation procedure.

3.3. pH evolutions

Fig. 2 shows the pH evolutions in each UF fraction.The fraction F0.22 lm exhibited an evolution alreadyobserved in composting (De Nobili and Petrussi, 1988;Inbar et al., 1993; Jimenez and Garcia, 1989; Sharmaet al., 1997). At the beginning the pH was around 7–8,then, it decreased rapidly to reach 5 at 30 days and after80 days treatment, the pH increased, first rapidly thenmore slowly to reach its initial value. Such evolution ofthe pH was observed for each fraction. We suggest thatthe pH decreased when solid–liquid transfer and mi-

crobiological degradation of dissolved organic matterincreased. Transfer and biodegradation would be re-sponsible for the acidification of the biofilm. Moleculestransferred were mostly acids as the first step of biode-gradation would produce acids, but these would soon beoxidised to carbon dioxide. In a second step the decreaseof transfer and the slowing down of biodegradationfavour the return increase of pH. Fig. 2 shows that theleast acid fraction is F1 kDa, that is the fraction con-taining the smallest molecules. The formation of car-bonate and ammonium ions may explain why F1 kDa ismore basic than the other fractions. The formation ofcomplexes linking ions and organic matter might havemodified their specific acidity and this could partly ex-plain the observed changes.

3.4. TOC and COD

The changes of TOC concentrations and masses aredescribed in Figs. 3(a) and (b). These evolutions are verysimilar. Yet the TOC mass evolution in F0.22 lm de-creased between 80 and 114 days, whereas the TOCexpressed in concentration slightly increased during thatsame period. The decrease in mass corresponded to a

Table 1

Moisture and biofilm mass as functions of the composting time

Composting time t

(days)

Moisture Ht

())Biofilm mass mBt

(kg)

15 0.519 293 410

29 0.458 214 490

80 0.501 257 380

114 0.429 183 810

133 0.441 188 960

148 0.476 187 830

182 0.491 213 880

203 0.501 197 600

Fig. 2. pH evolution for each UF fraction.

Fig. 3. (a) and (b). Evolutions of TOC in concentrations and in masses

for each UF fraction.

184 A. de Guardia et al. / Bioresource Technology 83 (2002) 181–187

real elimination of organic carbon whereas the concen-tration increase was caused by water evaporation; that isa decrease in mass of biofilm. However, Fig. 3(b) showsTOC mass in F0.22 lm first increased then it slowlydecreased to its initial value after 120 days and it in-creased towards the end of the treatment. The F1 kDafraction was responsible for the variations of F0.22 lmup to 120 days, it accounted for the largest part of thebiofilm. The molecules smaller than 1 kDa are the mostsensitive to aerobic biodegradation. The other fractionsaccounted for small and similar parts of TOC mass inthe biofilm and remained fairly constant over thetreatment, with the exception of a slight increase ofR100 kDa after 150 days, which could be explained byaccumulation in the biofilm of molecules of MW > 100kDa. This accumulation could have been caused by asolid–liquid transfer or polymerisation of smallest mol-ecules (Chanyasak et al., 1980).

In the same way as for TOC, COD concentrationsand masses had virtually the same evolution (Figs. 4(a)and (b)). COD mass in F0.22 lm first increased thendecreased quickly until 120 days and the decrease wenton more slowly till the end of treatment. Once again theF1 kDa fraction was mostly responsible for the varia-tions of F0.22 lm and accounted for 30–50% of this.The COD masses in the other fractions remained fairlyconstant over the treatment. COD masses evolutionsconfirm the previous interpretation. The moleculestransferred from solid to the biofilm then consumed bymicroorganisms were mostly molecules smaller than 1kDa. COD indicates the oxidation level of these mole-cules too. So whereas TOC masses exhibited a slightincrease after 120 days, COD masses slightly decreased,and this may prove stabilisation of organic matter tobiological degradation. The diminution of COD/TOCratio had the same meaning (Fig. 5).

3.5. Absorption spectra of the water extracts

Absorption spectra of the UF fractions in the ultra-violet and visible light regions exhibited a high absorp-tion in the low wavelength region which regularlydecreased as wavelength increased. The same patternwas noted by Prudent et al. (1995) for humic substances.

Absorptions of aqueous extracts at 280, 465, 665 nmand the ratio Abs. 465/Abs. 665 have been widelystudied by Chen et al. (1977), Mathur et al. (1993) andGressel et al. (1995). Optical density at 254 nm (OD) hasbeen studied in order to characterise aromatisation oforganic matter in natural surface water (Martin-Mous-set et al., 1997) or in landfill leachates, (Mejbri et al.,1996). Both groups studied the ratios Abs. 254 nm/TOCor COD/Abs. 254 nm which removes mass variations.Fig. 6 shows evolution of ratio Abs. 254 nm/TOC, TOCin g l�1, for each UF fraction.

At the beginning there is, for each fraction, a decreaseof ratio Abs. 254 nm/TOC which might have been ex-plained by increase of TOC in the biofilm caused byincrease of transfer from solid to biofilm. From the 30thday to the end of treatment, the ratio slightly increasedfor the fractions F1 kDa and R1 kDa but remained thesmallest. At the 80th day, the ratio of R100 kDa in-creased but decreased after 120 days, whereas for frac-tion R10 kDa the ratio sharply increased from the 120thto the 200th day. The increase of ratio Abs. 254 nm/TOC in fractions R100 kDa and R10 kDa implied aro-matisation of organic matter in these fractions.

Figs. 7(a) and (b) give evolution in concentrationsand in masses of organic nitrogen in each fraction.Ammonium ions were collected in F1 kDa fractions,thus organic nitrogen corresponded to Kjeldahl nitrogen

Fig. 4. (a) and (b). Evolutions of COD in concentrations and in

masses for each UF fraction.

Fig. 5. COD/TOC ratio evolution for each UF fraction.

Fig. 6. Evolution of ratio Abs. 254 nm/TOC for each UF fraction.

A. de Guardia et al. / Bioresource Technology 83 (2002) 181–187 185

in fractions R100 kDa, R10 kDa, R1 kDa and toKjeldahl nitrogen minus ammonium ions in F0.22 lmand F1 kDa fractions.

Organic nitrogen (Figs. 7(a) and (b)) and COD(Fig. 4) exhibit quite similar variations for each frac-tion. Once again the fraction F1 kDa accounts for thechanges observed in F0.22 lm. The increase of organicnitrogen in the first 30 days was caused by solid–liquidtransfer and its diminution has often been explained byconsumption of amino acids and proteins by micro-organisms (Chanyasak et al., 1980, 1982; Iannotti et al.,1994).

4. Conclusions

The study reported here shows that fractionation ofdissolved organic matter by membrane UF is a usefulmethod for a preliminary fractionation study of biofilmsurrounding the solids in composting organic wastes.That fractionation with membranes of MW cut-off of100, 10, 1 kDa followed by the chemical characterisationof the fractions, i.e. measure of TOC, COD and organicnitrogen expressed preferably in masses, confirmed thatthe organic matter dynamics occurring during compo-sting definitely involve the biofilm. Changes observed inTOC, COD and organic nitrogen can be explained bysolid to liquid transfer, enzymatic attack and micro-biological consumption of dissolved organic matter.Membrane UF showed that the smaller the organicmolecules are, the more sensitive they seem to be to theaerobic treatment. It would be interesting to identifywhich molecules are concerned in these transformations.The study of absorption at 254 nm of the different

fractions also exhibited aromatisation of the biggestmolecules, which mechanism will have to be investi-gated.

Acknowledgements

The authors would like to thank the Angers Ag-glomeration District and particularly Mr Brisset and hiscolleagues from green wastes composting treatmentplant. The advice and financial support from the WastesTechniques and Agriculture and Food Departments ofThe French Agency For Environment and EnergyManagement are gratefully acknowledged.

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