relationships between water-soluble carbohydrate and phenol fractions and the humification indices...

$: Relationships between water-soluble carbohydrate and phenol fractions and the humification indices of different organic wastes during composting$
Relationships between water-soluble carbohydrate and phenolfractions and the humi®cation indices of di�erent organic wastes

during composting

M.A. S�anchez±Monedero1, A. Roig*, J. Cegarra, M.P. Bernal

Department of Soil and Water Conservation and Organic Waste Management, Centro de Edafolog�õa y Biolog�õa Aplicada del Segura, CSIC, P.O. Box

4195, E-30080 Murcia, Spain

Received 5 October 1998; received in revised form 16 January 1999; accepted 28 January 1999

Abstract

The present work dealt with the relationships between the degradation and humi®cation processes which the organic matter

underwent during the composting of six di�erent organic-waste mixtures. Four of them were prepared by the Rutgers forced-

ventilation composting system and the other two by the mobile (turn over) pile system. The main components were: sewage sludge,

sorghum bagasse and municipal solid waste.

Di�erent degradation rates were observed for the three main components (cellulose, hemicellulose and lignin) of the organic

matter during composting. In the case of the ®rst two components, the degree of degradation ranged from 70 to 85% during the

whole process, depending on the starting mixture, whereas only 30±50% of the initial concentration of lignin was degraded in the

mixtures prepared with municipal solid wastes (MSW) and lignocellulosic materials. Water-soluble carbohydrate and phenol de-

gradation were studied because they have been proposed as precursors of the humi®cation processes. In the experiment described,

they had di�erent degradation rates during composting depending on the starting mixture and the composting system used. The

water-soluble carbohydrate was the most intensely degraded fraction in the piles prepared with urban refuse although no appre-

ciable degradation was measured in the other three mixtures, whereas there was an appreciable reduction in the water-soluble phenol

fraction of all six mixtures during composting, values of less than 0.1% being reached in the mature composts.

Humi®cation processes were studied by quanti®cation of the extractable humic-like substances and the generally accepted

humi®cation indices: extractable carbon to total organic carbon (EXC/TOC), humic acid carbon to total organic carbon (HAC/

TOC), humic acid carbon to extractable carbon (HAC/EXC) and the humic acid carbon to fulvic acid carbon (HAC/FAC) and by

determining the cation exchange capacity of the mixture during composting. All the indices increased during composting and

followed a similar trend. The humic-like acid fraction was mainly responsible for these changes, showing that the composting in-

volves a process of humi®cation. The cation exchange capacity to total organic carbon ratio showed itself to be a useful humi®cation

index during composting since this index clearly increased more than the others.

Correlations between some of the above humi®cation indices and the concentrations of water-soluble and less polymerised

carbohydrates and phenols indicates the possible in¯uence of these fractions on the humi®cation processes. Signi®cant correlations

were found between the phenols and the HAC/TOC and the HAC/EXC ratios, whereas no signi®cant correlations were recorded

with the carbohydrate fraction. Ó 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Composting; Organic wastes; Compost humi®cation; Compost mineralization

1. Introduction

Recently special attention has been paid to thehumi®cation of the organic matter content of organic

wastes during composting since the knowledge of theparticular processes involved is nowadays rather small.Currently accepted theories on the humi®cation of soilorganic matter have been extended to composts despitethe fact that conditions in the composting pile whichin¯uence the humi®cation process are very di�erentfrom those existing in soil (higher temperature, greatermicrobial population, higher concentration and diver-sity of organic compounds, etc).

Bioresource Technology 70 (1999) 193±201

* Corresponding author. Tel.: +349 68 215 717; fax: +349 68 266 613;

e-mail: [email protected] Present address: Laboratorios ECOSUR S.A.L. Calle Barriomar,

34. E-30010 Murcia, Spain.

0960-8524/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 0 1 8 - 8

One of the most widely accepted theories concerningorganic matter humi®cation in soil is that lignin and itsdegradation products such as phenols, quinones andmore complex compounds are the main precursors inthe formation of humic substances by their polymer-ization and condensation with N-compounds such asproteins, aminoacids, nucleic acids, etc brought aboutby the soil microorganisms (Stevenson, 1982; Scnitzerand Khan, 1972; Tate III, 1987; Dell'Agnola et al.,1993). The contribution of other organic compoundssuch as simple carbohydrates also seems to be relevantbecause their incorporation into the microbial tissuesproduces complex structures which are perhaps relatedto the synthesis of humic substances.

During composting the great degradation su�eredby the main organic components (lignin, cellulose,hemicellulose and proteins) leads to the formation of agreat variety of simple organic compounds such ascarbohydrates, aminoacids, simple peptides and phe-nols of low structural complexity, which can be eitherdegraded by the microorganisms as a source of carbonand energy or used by the microorganisms for thesynthesis of new substances with properties similar tothose of the humic substances of soils or other veryhumi®ed materials.

The aims of the work described in this paper weretwofold: ®rst to monitor the degradation and humi®-cation processes of the main organic matter componentsduring composting and secondly to study the possiblerelationship between both processes since many pre-cursors of the humic substances are released during thedegradation of the main constituents of the organicmatter (lignin, cellulose and hemicellulose).

2. Methods

2.1. Materials

Six di�erent wastes were used: a primary aerobicsewage sludge, cotton waste, sorghum bagasse, pinebark, brewery sludge from the wastewaters of thebrewery factories and the organic fraction of the selec-tively collected municipal solid wastes (MSW).

2.2. Composting performance

Static or turned-pile composting systems were used tocompost six mixtures prepared from seven di�erent or-ganic wastes in the following proportions (fresh weight):

± By the Rutger static-pile system:

Mixture 1: 82% sewage sludge + 18% cotton wasteMixture 2: 88% sorghum bagasse + 11% pine bark +1% urea

Mixture 3: 86% sorghum bagasse + 11% pine bark +3% brewery sludgeMixture 4: 90% MSW + 10% sorghum bagasse

± By the turned-pile system:

Mixture 5: 90% MSW + 10% sorghum bagasseMixture 6: 100% MSW

About 1500±2000 Kg of mixtures 1, 2, 3 and 4 werecomposted in a pilot plant by the Rutgers static-pilecomposting system. This system maintains a tempera-ture ceiling in the pile through the on-demand removalof heat by ventilation. This encourages a high decom-position rate since high temperatures inhibit and slowdown decomposition by reducing microbial activity. Airwas blown from the base of the pile through the holes ofthree PVC tubes, 3 m in length and 12 cm in diameter.The timer was set for 30 s ventilation every 15 min. Theceiling temperature for continuous air blowing was55°C. Mixtures 5 and 6 were composted by the turned-pile system. They were turned every two days during the®rst week, twice during the second week and only onceper week thereafter.

The biooxidative phase of composting (active phase)was considered to be ®nished when the temperature ofthe pile stabilised near to that of the atmosphere (®nalstage), this stage, was reached after 49, 63 and 42 days inmixtures 1, 2 and 3 respectively and after 77 days inmixtures 4, 5 and 6. Air blowing (piles 1, 2, 3 and 4) andturning (piles 5 and 6) were stopped and the mixtureswere then allowed to mature over a period of twomonths (maturation phase). The moisture levels of thepiles were controlled weekly during the biooxidativephase of composting and adjusted by adding the nec-essary amount of water to obtain values between 45±65% of dry matter. One representative sample was takenweekly by mixing six subsamples from six sites of thepile and from the whole pro®le (from the top to bottomof the pile) and then was air dried and ground to 0.5 mmfor analysis.

2.3. Analytical methods

The organic matter (OM) contents of the compostingsamples were analysed by loss on ignition at 430°C for24 h (Navarro et al., 1991). The 0.1 M NaOH- EXC andFAC were calculated after precipitation of the humic-like acids at pH 2.0 (SaÂnchez±Monedero et al., 1996).Lignin and cellulose concentrations were determined bythe American National Standards methods (ANSI andASTM, 1977) and the hemicellulose concentration bysubtracting the cellulose concentration from the deli-gni®ed sample (hollocellulose) obtained by Browning'smethod (Browning, 1967). The HAC was calculated bysubtracting the FAC from the EXC. CEC was deter-

194 M.A. S�anchez±Monedero et al. / Bioresource Technology 70 (1999) 193±201

mined with BaCl2-triethanolamine following the methodof Lax et al. (1986). Water-soluble phenols were deter-mined in the aqueous extract (1:10, w:v) by a modi®edversion of the Folin method (Kuwatsova and Shindo,1973). The water-soluble carbohydrates were analysedin the water extract (1:10, w:v) by the anthrone method(Brink et al., 1959).

3. Results and discussion

3.1. Organic matter degradation

The organic matter of the six mixtures studied wasdegraded much more intensely during the thermophilicphase of composting due to the greater activity of the

microorganisms which had a large quantity of availableeasily-degradable substances. The evolution of the or-ganic matter content of the mixtures during compostingshowed two di�erent behaviours depending on the ma-terials used in the mixtures (Fig. 1). Mixtures 1, 2 and 3for example, which had a high initial lignin contentunderwent a lower organic matter degradation (about40%) because of lignin's resistance to degradation whilethe organic matter contents of the three mixtures con-taining MSW were reduced by more than 70% of theinitial values. This process was particularly pronouncedin mixtures 5 and 6 which were composted by theturned-pile system in which the temperatures reached75°C in the thermophilic stage whereas in the Rutgerssystem (mixture 4) the ceiling temperature was only55°C.

Fig. 1. Absolute levels of the main constituents (cumulative areas) of the organic matter in the composting mixtures at di�erent stages. (I: Initial; T:

Thermophilic; F: Final; M: Mature). n� ash.

M.A. S�anchez±Monedero et al. / Bioresource Technology 70 (1999) 193±201 195

The lignin contents of all six mixtures decreasedduring composting (Fig. 1) (showing the cumulativeareas of the constituents), reaching a degradation be-tween 30 and 50% (dry weigh) of the initial contents bythe end of the maturation phase (two months after thebiooxidative phase had ®nished). This degradationprocess was more intense during the ®rst stages ofcomposting, especially during the thermophilic phase.There was only a slight fall in the lignin content duringthe last stages of the active phase and lignin remainedalmost constant during the maturation stage because themicroorganisms, responsible for this process, need car-bon sources di�erent than that provided by lignin(Harper and Lynch, 1981); these were the carbohydratesresulting from the degradation of cellulose and hemi-cellulose. From the ®rst stages of composting, the de-gradation of lignin, which is decreased during thecomposting process, may have led to the presence ofphenolic and quinonic remains which could be used bythe microorganisms as precursors for humi®cation.

Both cellulose and hemicellulose were strongly de-graded and followed a similar trend in the six mixturesstudied (Fig. 1). Such degradation exceeded 85% in thecase of compost composed of MSW, the degradationbeing greater during the thermophilic phase with a de-gradation of 70% of the initial values. In mixtures 1, 2and 3 the degradation process was less intense than inthose involving MSW, although mixtures 1 and 3 stillled to a degradation of more than 65% of the initialquantity of cellulose. The fact that the degradation inmixture 2 did not exceed 55% may have been due to thehigh concentration of ammonium formed during thehydrolysis of the urea added to the initial mixture (datanot shown) which according to Ko et al. (1974) seems toinhibit the activity of the fungi which cause the degra-dation of cellulose. As a result of this hydrolysis of thecellulose and hemicellulose polymers a less-polymerisedcarbohydrate fraction was obtained.

Fig. 2 shows the water-soluble carbohydrates andphenols evolution in the six mixtures during compost-ing. The method used to determine water-soluble car-bohydrates mainly quanti®ed hexoses and pentoses, thisfraction of the water-soluble organic carbon being themost labile and sensitive in re¯ecting the biochemicaltransformations (Brink et al., 1959). In the mixturesprepared with MSW, the concentration of carbohy-drates fell sharply during the ®rst three weeks by morethan 60% of the initial values although there was verylittle variation during the rest of the composting process,including the thermophilic phase when cellulose andhemicellulose degradations were more intense. The car-bohydrate concentration of mixtures 1, 2 and 3 showedno clear tendency during composting, ranging from 0.3to 0.8% during the process. From these results it can beconcluded that most of the carbohydrates resulting fromthe hydrolysis of cellulose and hemicellulose were used

immediately by the microbial ¯ora and that there was acompensation between the processes of degradation re-sulting from the microbial activity and the formation ofcarbohydrates as a result of the degradation of the cel-lulose and hemicellulose fraction.

The phenolic fraction, unlike the carbohydrate frac-tion, is composed of more complex molecules. Thisphenolic fraction is generated during the partial degra-dation of lignin and is more resistant to degradation dueto its aromaticity. However, the water-soluble phenolfraction, which has a more simple structure and asmaller molecular size, is sensitive to the transforma-tions occurring during composting and has been used asan indicator of the composting process (Saviozzi et al.,1987). This fraction behaved similarly in all six mixtures,although it showed a slight increase in mixture 2 afterthree weeks as was the case with the carbohydratefraction. In all the mixtures there was a gradual fallthroughout composting. Initial values between 0.23 and0.44% fell to less than 0.1%, which suggests that thephenols were used by the microorganisms either as en-ergy source or as precursors for the synthesis of newmolecules. However, Saviozzi et al. (1987) and HaÈninenand Lilja (1994), using a mixture of wastewater sludgefrom a paper-processing factory and chopped wheat-straw and a mixture of slaughter wastes respectively,observed that this fraction behaved otherwise decreasingrapidly during the ®rst months of composting and thenincreasing to levels higher than the initial values. Theirobservations suggested that phenol polymerisation and/or its degradation predominated over the lignin degra-dation since the phenol fraction did not decrease duringthe composting process.

3.2. Humi®cation process

Alkaline extractants are used to isolate and solubilizethe humic substances (HS). However, when these HS areextracted from fresh organic materials and from thoseundergoing composting, along with the humic sub-stances are extracted, others with few humic character-istics. These include lignin residues, quinones,polyphenols, fats, dibenzocarboxylic acids, as well asaliphatic-type residues such as carboxylic and fatty ac-ids, alkanes, polysaccharides, simple peptides, etc. Itwould therefore be more accurate to call these sub-stances extracted by alkaline agents ``humic-like sub-stances'' since they are not strictly humic substances.

An individual study of the EXC, FAC and HAC isuseful for following the evolution of the organic matterduring composting. However, in studying the di�erencesbetween organic mixtures composed of materials ofdi�erent origins, it may be more accurate to consider theratios between these parameters and TOC or betweenthemselves. Senessi (1989) and Iglesias and PeÂrez (1989)used the following parameters:


Figs. 3 and 4 show these indices and the humi®cationdegree de®ned by Roig et al. (1988) as the CEC/TOC,for the six materials studied. Mixtures 1, 2 and 3 withtheir highest lignin content behaved similarly. The highvalues of the HAC/EXC ratio are outstanding and theseincrease in three mixtures from an initial 68±71% to ®nalvalues of 75±80%. The high values recorded for the

humic acids may have been due to the joint extraction orremoval of other materials such as lignin residues whichwould behave in a similar way to the humic acids andprecipitate at pH 2, leading to values which are unusu-ally high for residues of a vegetable nature. The hum-i®cation ratio (EXC/TOC ´ 100) and humi®cationindex (HAC/TOC ´ 100) slightly increases during com-posting although it was almost imperceptible in the caseof mixture 3. The increase noted in the humi®cationratio suggested that the proportion of soluble carbonincreases in an alkaline medium, whereas the rise in thehumi®cation index indicates an increase in the structuralcomplexity of the molecules studied, which thereforeshowed more accentuated humic characteristics.

Humi®cation ratio EXC/TOC ´ 100Humi®cation index HAC/TOC ´ 100Percentage of humic acids HAC/EXC ´ 100Degree of polymerisation HAC/FAC

Fig. 2. Time course of the water-soluble carbohydrates (m, % glucose) and water-soluble phenols (h, % p-cumaric acid) of the composting mixtures

(lsd: least signi®cant di�erence at P < 0.05).


The above ratios behaved di�erently in mixtures 4,5 and 6 which contained mainly or entirely MSW anddid not follow similar trends, emphasising the di�er-ences caused by the addition of a lignocellulosic ma-terial to MSW and by the composting system used. Ingeneral, these three mixtures showed lower humi®cat-ion indices and a more irregular evolution than theother three, probably because they contained a higherproportion of more easily-degradable substanceswhich would be included in the FAC fraction at leastduring the initial stages (Baca et al., 1992; De Nobiliand Petrussi, 1988). This means that the percentage ofhumic acids in these materials varied from an initialvalue of about 40% to ®nal values of nearly 66%

while in the ®rst three mixtures the values rangedfrom 65 to 80%.

Fig. 4 shows the time course of the HAC/FAC andCEC/TOC ratios, which seem to be the most sensitiveindices for following the humi®cation process. Bothclearly increase during composting and have previouslybeen proposed as indicators of maturity (Roig et al.,1988; Iglesias JõÂmenez and PeÂrez GarcõÂa, 1992). Theincrease in the HAC/FAC ratio, also known as the``degree of polymerisation'', re¯ected the formation ofcomplex molecules (HAC) from more simple molecules(FAC) and a diminution in the non-humic componentsof the fulvic acid fraction which are the most easilydegraded by microorganisms. The degree of polymeri-

Fig. 3. Time course of some humi®cation indices (EXC/TOC: n; HAC/TOC: m; HAC/EXC: ´) of the composting mixtures. (lsd: least signi®cant

di�erence at P < 0.05).


sation increases in mixtures 1, 2 and 3 from 1.91, 2.21and 2.50 to 3.14, 3.76 and 3.97 respectively, throughoutthe composting process. The degree of polymerisationalso increases from 0.65 in the mixtures containingMSW to ®nal values of 1.86 and 2.00 in mixtures 4 and 5respectively, values which were very close to that pro-posed by Iglesias JõÂmenez and PeÂrez GarcõÂa (1992) as anindicator of maturity in this type of material. Mixture 6did not behave like the other MSW mixtures since theindex continually ¯uctuated, probably due to the com-petition existing between the degradation, very high inthis mixture and humi®cation processes, with the latterbecoming more predominant in the last two months(maturation phase) when the degree of polymerisationrose from 0.39 to 1.15.

As already mentioned, the CEC/TOC ratio has alsobeen used as an indicator of maturity by Roig et al.(1988) Iglesias JõÂmenez and PeÂrez GarcõÂa (1992), whoproposed a value of 1.7 for manures and 1.9 for com-posts from MSW and MSW mixed with sewage sludge.The values of this ratio rose in the six mixtures studiedfrom initial values of between 0.82 and 0.86, 1.83, 2.45and 2.28 for those containing MSW (mixtures 4,5 and 6)and to 3.50, 2.09 and 2.37 for mixtures 1, 2 and 3, in-dicating that they could be considered as su�cientlymature. Unlike the other humi®cation indices the CEC/TOC ratio increases mainly during the biooxidativephase, although in mixtures 1, 5 and 6 there was also asubstantial increase in the maturation phase. This cri-terion then seems to be suitable for evaluating the degree

Fig. 4. Time course of the HAC/FAC (d) and CEC/TOC (¨) ratios in the composting mixtures. (lsd: least signi®cant di�erence at P < 0.05).


of maturity of any type of compost since apart frommixture 1, the mixtures showed values higher than 1.9 atthe end of the maturation stage.

3.3. Correlations between the humi®cation indices and thewater-soluble carbohydrate and phenol fractions

The possible role of the water-soluble carbohydrateand phenol fractions as precursors in the humi®cationprocess were studied by correlating their amounts withthe humi®cation indices during composting. Althoughno signi®cant correlation between these parameters wasobserved taking the values of all the mixtures, individualmixtures produced statistically signi®cant inversemathematical correlations at a probability level ofP < 0.001 (Table 1).

An inverse correlation between the phenol contentand some of the humi®cation indices considered wasfound in all cases, re¯ecting the humi®cation theories asregards the participation of phenol in the synthesis ofnew ``humic type'' substances (Stevenson, 1982; Scnitzerand Khan, 1972; Tate III, 1987; Dell'Agnola et al.,1993). The most signi®cant correlations were found withHAC/EXC, HAC/FAC and, to a lesser extent, withHAC/TOC, suggesting that the most signi®cant corre-lations occurred with the humi®cation indices where theHA fraction was involved. The other carbon fractions(EXC and FAC) may have been in¯uenced by thepresence of other non-humic substances, as alreadymentioned. The correlations had a lower level of sta-tistical signi®cance in the three MSW-based mixtures inwhich the degradation processes were greatest. Thewater-soluble carbohydrate and phenol fractions wouldhave been preferentially degraded by the microbial ¯ora

as carbon and energy sources, thus diminishing theirrole as precursors in the humi®cation process.

However, with the exception of mixture 5, no type ofcorrelation was observed between the time course of thewater-soluble carbohydrate content and that of theabove humi®cation indices. Because soluble carbohy-drates are the main carbon and energy sources of themicroorganisms, these types of molecule participatemore in the degradation processes and their role asprecursors in the synthesis of humic substances is thusmasked. Moreover the fact that the concentration of thiscomponent did not vary after three weeks in any of themixtures (Fig. 2) indicates the compensation between itsformation as a result of the hydrolysis of more complexpolymers (through the degradation of cellulose andhemicellulose) and its role as energy source for themicroorganisms (utilisation by microorganisms).

4. Conclusions

Both the soluble carbohydrate and phenol fractions,of the soluble organic matter play a very important rolein organic matter degradation. Their concentrations arein continuous equilibrium as a result of their formationfrom more complex polymers and degradation as aconsequence of microbial activity.

Of these two fractions, the phenols in the aqueousextract were inversely correlated with the humi®cationindices in all six mixtures throughout the compostingprocess, which strongly suggests they acted as precursorsin the humi®cation process and implies, therefore, adecrease in concentration with the increase in humic-type substances in the mixtures during composting.

Table 1

Correlations between water-soluble carbohydrates and phenols and the di�erent humi®cataion indices of the six mixtures during composting

Mixture HAC/TOC HAC/EXC EXC/TOC HAC/FAC

1 Carbohydrates NS NS NS NS

Phenols ÿ0.8276 b ÿ0.7884 a NS NS


Phenols ÿ0.8123 b ÿ0.6815 a ÿ0.8075 a ÿ0.7414 b


Phenols ÿ0.7823 a ÿ0.8519 b NS ÿ0.8359 b


Phenols NS ÿ0.8010 c NS ÿ0.7337 b

5 Carbohydrates ÿ0.6486 a ÿ0.8543 c NS ÿ0.7977 b

Phenols ÿ0.8345 c ÿ0.8826 c NS ÿ0.8762 c

6 Carbohydrates NS NS 0.8452 c NS

Phenols NS NS 0.6951 a NS

NS: not signi®cant.a signi®cant at a probability level of P < 0.05.b signi®cant at a probability level of P < 0.01.c signi®cant at a probability level of P < 0.001.


No mathematical correlations were observed betweenthe soluble carbohydrate fraction and the principalhumi®cation indices, probably because such carbohy-drates are the principal carbon sources of the microbial¯ora which are mainly responsible for the degradationwhich takes place. The participation of soluble carbo-hydrates in the humi®cation process is thereforemasked.

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