Wet oxidation pretreatment for the increase in anaerobic biodegradability of newspaper waste

Bioresource Technology 91 (2004) 273–281

Wet oxidation pretreatment for the increase in anaerobicbiodegradability of newspaper waste

Martin Fox, Tatsuya Noike *

Department of Civil Engineering, Graduate School of Engineering, Tohoku University, Aoba 6, Sendai 980-8579, Japan

Received 23 December 2002; received in revised form 10 May 2003; accepted 30 June 2003

Abstract

Wet oxidation was investigated for its process performance on methane fermentation of newspaper waste. The mechanisms of

solubilization of newspaper waste were investigated using the following criteria: destruction of total COD (TCOD), production of

soluble COD (SCOD), production of volatile fatty acids, production of soluble carbohydrates, production of soluble lignin de-

rivatives (SLD), production of furan (F) and destruction of lignin and cellulose. Wet oxidation was carried out at 170, 190, and 210

�C, with a retention time of 1 h. The highest removal efficiencies of TCOD and cellulose were achieved at 210 �C, approximately 40%

and 69% were destroyed, respectively. On the other hand, highest lignin removal efficiency was achieved at 190 �C in which ap-

proximately 65% was removed. Batch methane fermentation tests were performed in 2–l glass bottles filled with the wet oxidized

newspaper samples. Methane fermentation of newspaper pretreated at 190 �C gave the highest CH4 conversion efficiency (59% of the

initial TCOD was recovered as CH4 gas). Anaerobic cellulose removals varied from 74% to 88%.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Wet oxidation; Holocellulose and lignin solubilization; Methane conversion efficiency; Anaerobic cellulose degradation

1. Introduction

The amount of municipal and industrial waste is in-

creasing year by year. As a result, existing landfill sites

are rapidly running out of space and secondary pollu-tion is becoming a serious problem. Landfilling affects

the environment by leaching and emitting greenhouse

gases (Harf et al., 1999).

Recycling, incineration and other pre-treatment and

treatment processes are necessary to reduce the volume

of municipal and industrial solid wastes. Although, the

cost of recycling or zero waste operations is becoming

competitive with that of such traditional waste disposalmethods as landfilling and incineration, recycling must

be improved through technological developments and

for papers, by a social agreement to prefer paper made

with recycled papers. The utilization of recycled fibers in

Japan is about 53% and is expected to increase up to

55%. Almost 90% of raw materials for paperboard

production are derived from recycled fibers (Meshit-

*Corresponding author. Tel.: +81-22-217-7468; fax: +81-22-217-

7465.

E-mail address: [email protected] (T. Noike).

0960-8524/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2003.06.001

suka, 1999). However, incineration processes as a dis-

posal route are applied to reduce those fractions of

discarded waste paper which are not recyclable any-

more. The ashes generated are then filled into a landfill

(Lay, 1997).In combustion processes, up to 50% of the known

dioxins may be derived from municipal waste incinera-

tors. Therefore, high processing technologies are indis-

pensable to avoid the production of such highly toxic

compounds (Sako, 1997).

New technological efforts are necessary to reduce the

volume of the organic fraction of solid waste. A variety

of biological and thermal processes are available for theconversion of biomass to energy resource (Jerger et al.,

1982), and research on mesophilic anaerobic bioconver-

sion of agricultural and wood residues has contributed to

much of the advancement in the understanding of ap-

plying bioconversion to lignocellulosic wastes (Clarkson

and Xiao, 1999). Crystallinity and lignification are by far

the most important factors of the susceptibility of cell-

ulosic materials to enzymatic and bacterial conversions(Cowling and Kirk, 1976). Therefore, enhancement of

the total accessible surface area of cellulosic substrates

can contribute to the creation of more effective pre-

treatments for lignocellulosic materials.

mail to: [email protected]

274 M. Fox, T. Noike / Bioresource Technology 91 (2004) 273–281

The process in which an organic material is oxidized

with gaseous oxygen in water is called �wet oxidation’

(Sorensen et al., 1990). In the case of lignocellulosic

materials, the wet oxidation process disrupts and oxi-

dizes cellulose and lignin to CO2, H2O, and carboxylic

acids (Bjerre et al., 1996).

This paper reports the effectiveness of wet oxidation

on the methane fermentation of newsprint and theevaluation of it as a new alternative treatment process.

2. Methods

2.1. Wet oxidation pretreatment experiment

Tests were conducted to study the reaction mecha-

nisms and to characterize the products of the wet oxi-

dation process. Ordinary newsprint was used as paperwaste material for the evaluation of wet oxidation. One

of the popular newspapers in Japan �Asahi’ was used.

This material in a dry weight basis contains 54% cellu-

lose and 16% lignin (Fox, 2000). Hand shredded news-

print (20 g) was added individually to 1–l distilled water.

Subsequently, using a blender machine the preparation

was homogenized to form a slurry. Table 1 shows the

different slurry volumes filled into the autoclave. In or-der to improve the contact between gas and liquid

phase, mixing of the reactor was achieved by means of a

shaft impeller. Heating was performed using a heating

jacket. Pressurized air was supplied from a compressor

and during all the runs, the autoclave internal pressure

was maintained above the vapor pressure of the liquid in

order to ensure that the reaction would only occur in the

liquid phase. After the processing period, a sample wasremoved from the vessel and a portion was filtered in a

Kiriyama Roto apparatus using a 4 lm pore size filter.

The filtrate (liquor) was collected and kept for analysis

Table 1

Reaction temperature, autoclave total pressures, slurry volumes, stoichiometr

batch experiments

W.O. run label Newsprint

T (�C) a.t.p.a (kg/cm2) Slurries (ml) O

R-1 170 31.0 335 8

R-2 190 40.9 366 8

R-3 210 51.1 379 9

Batch label Initial pH TCOD (g/l) S

R-1 B-1 7.03 9.72 1

R-2 B-2 7.03 9.96 2

R-3 B-3 7.02 9.86 3

aAutoclave total pressure.bOxygen demand of the slurry (liquid phase).cGas phase volume of the autoclave.dOxygen supplied for oxidation (oxygen dose).e Stoichiometric oxygen factor.

of volatile fatty acids, sugars, and heterocyclic and sol-

uble lignin derivatives. The filtered cake (solid fraction)

was frozen with ethanol at )40 �C, freeze dried, and

dried in a vacuum oven at 40 �C–24 h for lignin and

cellulose determination.

2.2. Anaerobic biodegradability experiment

The anaerobic seed sludge for each batch reactor wastaken from a wastewater treatment plant located in

Yamagata City, Japan. The batch series was performed

in 2–l working volume glass bottles and incubated at

mesophilic condition (35 �C). A total of four batches

including the blank bottle was conducted using the wet

oxidation effluent as the sole substrate. Appropriate

quantities of seed sludge (300 ml) and wet oxidized

newsprint effluents (450 ml) were added to each vial, andsubsequently supplemented with 300 ml of phosphate–

vitamin–mineral solution (medium). Final volumes were

adjusted with distilled water to ensure equal initial total

COD concentration in each run (see Table 1). The

composition of the salt–vitamin mixture was adopted

from Khan (1977). To maintain anaerobic conditions

during the preparation of the batch assays, the bottles

were flushed with 20% CO2 and 80% N2 mixture for 2min. The pH of the vial contents was adjusted to 7. The

control vial was then supplemented with 300 ml digested

sludge and 700 ml distilled water and pH was adjusted

to 7. Following this, gas productions (by syringe mea-

surement) were monitored daily over 60 days until

methane production ceased.

2.3. Analytical methods

Lignin content in newsprint and filtered cakes ob-

tained from the filtration of newsprint wet oxidized ef-

fluents were determined by the Klason 72% sulfuric acid

ic oxygen conditions for newsprint slurries and initial conditions for the

2b (g) g.ph.c (l) O2

d (g) s.o.f.e (%)

.21 0.425 1.63 19.9

.97 0.394 1.76 19.6

.29 0.381 1.82 19.6

COD (g/l) Cellulose (g/l) Lignin (g/l)

.59 2.21 0.742

.34 1.89 0.478

.12 1.43 0.571

M. Fox, T. Noike / Bioresource Technology 91 (2004) 273–281 275

digestion procedure. Once methane fermentation tests

were concluded, bottles were opened and batch liquor

contents were kept for further chemical analysis. Before

the lignin content of batch liquors was determined,

samples were successively extracted using 80% aqueous

ethanol and water at 40 �C. This process removes low

molecular weight carbohydrates, soluble salts and pro-

teins were removed from these extractions (Iiyama et al.,1994). Cellulose concentrations were determined using a

colorimetric method (Updegraff, 1969). Chemical oxy-

gen demand (COD) by a closed reflux method and

suspended solids concentrations were determined ac-

cording to the procedures described in the Standard

Methods (APHA, 1995). Sugar composition of the wet

oxidation liquor was analyzed by high-performance

liquid chromatography (HPLC; Shimadzu, Kyoto,Japan) with refractive index detection (Shimadzu).

Separations were performed on a Biorad (Hercules, CA)

Aminex HPX-87P column (300 · 7.8 mm i.d.) at 85 �Cusing pure water as the mobile phase (0.6 ml/min). The

samples were filtered (0.22 lm) prior to HPLC analysis.

Furfural, 5-hydroxymethyl furfural (HMF) and soluble

lignin derivatives were analyzed at 55 �C using a Biorad

(Hercules, CA) Aminex HPX-87H column (60 �C).Furan is defined as the arithmetic sum of furfural and

HMF. These compounds were separated using a mobile

phase consisting of 0.005 M sulfuric acid (84% v/v) and

CH3CN (16% v/v) at an elution rate of 0.5 ml/min. Total

soluble lignin derivatives is defined as the sum of indi-

vidual peaks eluting in the regions corresponding to the

compounds being determined. These compounds were

separated using the same mobile phase as for furfuraland HMF at an elution rate of 0.35 ml/min. All peaks

were quantified using a variable-wavelength UV detec-

tor (210–300 nm). Methane and carbon dioxide were

measured using a gas chromatograph (GC, Shimadzu

8A) equipped with a thermal conductivity detector

(TCD) and 2 m stainless steel column packed with Po-

rapak T (50/80 mesh). Helium was used as the carrier

gas at a flow rate of 30 ml/min. Volatile fatty acid (VFA)concentrations were determined using gas chromato-

graphy (GC, Shimadzu 14B) with a flame ionization

detector (FID) and a 2 m glass column packed with

Unisole F-200 (30/60 mesh). Cellulose, lignin, total

COD, and soluble COD were determined in triplicate

samples.

3. Results and discussion

3.1. Wet oxidation performance

In order to understand the mechanisms and perfor-mance of the wet oxidation of newsprint, it was neces-

sary to observe (at the temperatures of 170, 190 and 210

�C) the following: destruction of the total chemical oxy-

gen demand (TCOD), production of soluble chemical

oxygen demand (SCOD), generation of organic acids

(volatile fatty acids), production of sugars derived from

holocellulose solubilization, production of soluble com-

pounds derived from lignin solubilization, heterocyclic

compounds like furfural and HMF, and cellulose and

lignin destruction.

In the wet oxidation tests a solution with 2% news-print and an initial TCOD concentration of 24.5 g/l was

used. The reduction of TCOD is shown in Fig. 1A. As

shown here, the gap between the dotted line and the

COD trend represents the destroyed fraction at each of

the respective temperatures. At 170 �C the oxidized

fraction reached approximately 13%; at 210 �C it

reached its highest––40% (see Table 2). Calculation of

the stoichiometric oxygen values for newsprint slurriesrevealed a maximum requirement of approximately 20%

O2. This requirement was given through compressed air,

which was injected into the wet oxidation vessel. As

shown in Table 1, stoichiometric oxygen conditions were

calculated taking into account the O2 available in the

gas phase of the reactor as well as the O2 in the slurry

necessary for complete oxidation (liquid phase). The

reason why the destruction of TCOD at 190 and 210 �Cappears with greater values than those of the stoichio-

metric requirements is perhaps due to the experimental

method of COD for phenolic compounds was underes-

timated. In other words, the potassium dichromate

COD method used in this analysis did not completely

oxidize the soluble monomers resulting in lower values

for the SCOD. Therefore, the contribution to TCOD of

the effluent is lower and therein the removal efficiency ofCOD appears to be overestimated. A solution of this

problem would be to make a balance in the carbon base

instead of the oxygen base.

Fig. 1A shows how newsprint was solubilized at the

tested temperatures (solubilization ratio %). The soluble

fraction at each temperature represents a group of sev-

eral soluble organic compounds produced through the

solubilization of lignin, hemicellulose and cellulose. Fig.1B–E shows organic acids (volatile fatty acids), sugars

(solubilization of the carbohydrate fraction of cellulose

and hemicellulose), soluble lignin derivatives (lignin

solubilization) and heterocyclic (thermal decomposition

of cellulose and hemicellulose). An increase in the sol-

ubilization ratio was expected to be a result of the rise of

the level of acidification of the newsprint slurry inside

the autoclave reactor. According to McGinnis et al.(1983) the softening of wood fiber in subcritical water

exposes lignin, hemicellulose and cellulose to acid reac-

tions. Acidification increases as the temperature rises

and produces more organic acids. As in this work,

Brauns (1952) revealed that a mild hydrolysis condition

of lignocellulose produces acetic and formic acids. These

acids may be formed from the acetyl and formyl groups

of lignin. At 170 �C the generated soluble COD reached

Table 2

Wet oxidation destruction efficiencies for newsprint

Destruction efficiencies (%)

170 �C 190 �C 210 �C

TCOD 13 33 40

Cellulose 38 57 69

Lignin 34 65 61

Fig. 1. Several trends for wet oxidized newsprint. (A) Total, soluble COD and solubilization rate; (B) acetic acid; (C) sugars; (D) lignin soluble

derivatives; (E) furans and (F) cellulose and lignin concentrations after wet oxidation (the results are means of three replicates and error bars

represent the standard deviation).


approximately 4.5 g/l and at 210 �C reached 6.5 g/l. A

solubilization ratio of 29% was attained at 210 �C (Fig.

1A). The final pH value at this temperature was 3. This

result coincides with that by McCarty et al. (1976) in

which organic refuse treated at 200 �C at pH 1 resulted

in 40% of soluble compounds. The result of a low pH

(acid condition) of refuse solubility was attributed to the

fact that the solubilization of cellulose and hemicellulosewas easier than lignin. With regards to the production of

organic acids like acetic, formic, propionic, butyric, iso-

butyric, valeric or iso-valeric, only acetic acid was

identified through the tested temperatures (Fig. 1B).

Concentrations varied from 0.21 g/l at 170 �C to 0.55 g/l

at 210 �C.Hemicellulose exists together with lignin and cellulose

in the fraction of municipal solid wastes through thefraction of paper wastes. The structure of hemicellulose

is thermally or chemically easier to hydrolyze than cel-

lulose (McCarty et al., 1976). Despite the fact that in this

work hemicellulose was not directly determined, it is

believed that the hemicellulose exists together with lig-

nin and cellulose as the main structural polymers of

ordinary newsprint.

From the soluble fraction some sugars were identified

(Fig. 1C). It clearly demonstrated that cellulose and

hemicellulose in low pH were solubilized to result insoluble compounds of lower molecular weight. Cellulose

was probably depolymerized to yield glucose. This is the

final compound of the cellulose hydrolysis. Glucose

forms the cellobiose structure which in turn forms the

cellulose polymer. Cellulose is a linear homopolymer of

several thousand DD-glucose units linked by b-1,4-glu-cosidic bonds (Chynoweth and Pullammanappallil,

1996). Glucose concentration increased from 0.05 g/l at170 �C to 0.5 g/l at 210 �C. It was observed that glucose

linearly increased as the temperature rose, indicating

that the production of this compound depended upon

the degree of acidity of the medium and instability of

cellulose. Hemicellulose could also serve as the source

for glucose production. Regarding the behavior of the

solubilization of the hemicellulose, its presence as a

structural polymer in newsprint was detected by theidentification of several monomers (xylose, arabinose,

and mannose). Bjerre et al. (1996) defined the hemicel-

lulose as a less stable lineal structure in subcritical

conditions than the cellulose. Hemicelluloses are com-

posed of both linear and branched heteropolymers of DD-

xylose, LL-arabinose, DD-mannose, DD-glucose, DD-galactose

and DD-glucuronic acid, and therefore, the products of


depolymerization are the monomers that make up the

hemicellulose polymer (Chynoweth and Pullammanap-

pallil, 1996). Despite that the instability of hemicellulose

at the tested temperatures was not studied, it seemed

that at 190 �C more solubility was achieved. At this

temperature, arabinose, xylose, and mannose reached

their maximum concentrations; 0.05, 0.1 and 0.3 g/l,

respectively.As described before, newsprint led evidence to the

production of heterocyclic compounds. Martinez et al.

(2000) indicated in their work that complex structures of

soluble compounds, which were partly inhibitory in

bacterial cultures, were produced during the acid hy-

drolysis of cellulose and hemicellulose at mild or low

temperatures. These compounds may include furfural

and 5-hydroxymethyl furfural. Yuan and Chen (1999)indicated that some furan compounds, which are kinds

of heterocyclic compounds, were found in several food

systems by thermal decomposition of sugars and

ascorbic acid. HMF and furfural are the main products

of the hydrolysis of hexoses and pentoses, respectively.

Fig. 1E shows these compounds detected at 280 nm. The

arithmetic sum of the concentration of HMF and furf-

ural represents the concentration of furan. This con-centration increased from 0.006 g/l at 170 �C to 0.065 g/l

at 210 �C. Contrary to the work of Bjerre et al. (1996) in

which experimental results showed that wet alkaline

oxidation of wheat straw did not produce furan, news-

print in acid conditions did produce it. This difference

may be due to Bjerre’s addition of alkalinity.

McCarty et al. (1976) pretreated lignocellulosic waste

at several temperatures with sodium hydroxide (pH 13)and chloridric acid (pH 1). The compounds produced at

pH 13 showed a higher solubilization ratio than those at

pH 1. Unfortunately, the nature of the soluble fractions

was not experimentally determined to understand the

behavior of holocellulose and lignin solubilization.

However, in general terms, it can be concluded that

holocellulose solubilization is enhanced at acid pH; on

the other hand, lignin solubilization is enhanced at al-kaline pH.

In this study, the phenolic nature of the soluble

fraction of wet oxidized newsprint was characterized

and determined. These lignin derivatives were quantified

Table 3

Production of soluble lignin derivatives (SLD) and furan (F) detected at 210

g/l (210 nm)

T (�C) vOHa vCb 4-MTLc p-HYAD

170 0.215 0.000 0.020 0.048

190 0.486 0.000 0.016 0.024

210 0.312 0.010 0.021 0.030

aVanillyl alcohol.b Vanillic acid.c 4-Methylcatechol.d p-Hydroxybenzaldehyde.

with an UV-variable wavelength detector at 210 nm.

Fig. 1D shows the total concentration of SLD at the

tested temperatures. This total concentration is given by

the arithmetic sum of each determined compound at

the tested temperature. The following lignin mono-

mers were chosen to characterize the phenolic nature of

the wet oxidized newsprint: hydroquinone, vanillyl al-

cohol, p-hydroxybenzoic acid, vanillic acid, syringicacid, 4-methylcatechol, p-hydroxybenzaldehyde, guaia-

col, vanillin and syringaldehyde. Only four monomers

were identified. These concentrations are shown in Table

3. Vanillyl alcohol presented considerably higher con-

centrations compared with the other compounds, vary-

ing from 0.215 g/l at 170 �C to the maximum

concentration of 0.486 g/l at 190 �C and decreasing to

0.312 g/l at 210 �C. The pathways in which newsprint-lignin was solubilized (or degraded) from its structural

monomers to the phenolic compounds previously de-

termined were unknown. However, despite that these

mechanisms remained uninvestigated, the cause of sol-

ubilization was due to the fact that these soluble prod-

ucts were produced through a cascade of solubilization

reactions. This process gradually changed from neutral

pH towards acid conditions and produced the acid hy-drolysis. With regards to the lignin structure of news-

print, this should be similar to its precursors (i.e.

softwood). Newsprint in Japan is formed mainly by re-

cycled newsprint and when the fiber exhausts its internal

strength after 4 or 5 recycle operations, new fresh fibers

or pulp are replenished for newspaper making. Fresh

fibers are mainly made of thermomechanical pulps and

Kraft pulp (Iiyama, 1999). Lignin characterization isdifficult to accomplish because of the complexity of its

structure. Generally, lignin is formed by phenylpropane-

type groups including coniferyl units or similar units of

guaiacol-propane in the case of softwoods, and both

coniferyl and syringyl units in the case of hardwoods

(Sarkanen and Ludwig, 1971). Brauns (1952) indicated

that hot water played an important role in the behavior

of the solubilization of lignin. For example, an impor-tant degree of solubilization was generated with wood

heated at 100–120 �C. Coniferin, vanillin, methylfurf-

ural, and pyrocatechol were produced at more high

temperatures (180 �C).

and 280 nm

g/l (280 nm)

d SLD Furfural HMF F

0.283 0.003 0.003 0.006

0.551 0.007 0.006 0.013

0.402 0.018 0.047 0.065


The cellulose and lignin which were not hydrolyzed or

oxidized during wet oxidation were determined to cal-

culate the process removal (or destruction) efficiency of

cellulose and lignin (Table 2). Fig. 1F shows these

concentrations after wet oxidation. The dotted lines

represent the initial concentration of each compound

before pretreatment. Cellulose removal increased with

temperature; this agrees with the previous results whichindicated that cellulose solubilization increased in an

acid medium. Lignin removal, on the other hand, at-

tained its maximum value at 190 �C (65%) and later

decreased to 61% (210 �C). It seems reasonable to sup-

pose that the highest solubilization was obtained at 190

�C. Indeed, its maximum production of SLD was ac-

complished at this temperature (Fig. 1D).

The destructive action of wet oxidation on thenewsprint fiber was visualized through several scanning

electron microscope (SEM) plates (or photographs).

Fig. 2 shows the photos taken at different temperatures.

Photo A shows newsprint without wet oxidation. The

long and thin fibers which may be formed by cellulose,

hemicellulose and lignin can be observed here. Photo B

shows wet oxidized newsprint (170 �C). Here a partial

shell off of the outside layer of the fiber-wall is observedwithout major destruction in the fiber itself. Its oblong

form seems to keep untouched. In Photo C, the fiber

treated at 190 �C does not show a shell off like at 170 �C,

Fig. 2. Scanning electron microscope plates for wet oxidized newsprint. A: Ne

40.9 kg/cm2; D: 210 �C, 51.1 kg/cm2. For all pictures ·230, 15 kV, scale: 10

but a greater destruction occurred through segmentation

or if it is understood better, through the shortening of

the oblong fiber as shown in Photo A. In Photo D it is

clear that at 210 �C the fiber has lost almost completely

its oblong form leaving an amorphous remainder with-

out fibrous characteristics. Because the photos are in

black and white, it is not possible to appreciate the

change in color of the micro-fibers. The normal fibers ofnewspaper show a greyish color, whereas the fibers

treated at different temperatures (170, 190 and 210 �C)stain opaque and dark. The higher the temperature of

pretreatment, the darker is the color. Regarding Photo

D, it was necessary to give a gamma correction to be

able to identify it. The effluent presented an intense and

aromatic sweet scent very similar to vanillin. Also the

scent could have been like furfural. Martinez et al.(2000) indicated that syrups of the acid hydrolysis of

bagasse of sugarcane had a rich aroma. Also, solutions

showed a well-tanned color.

3.2. Anaerobic biodegradability performance

In general terms, to degrade anaerobically lignocell-

ulosic materials, these have to be partially delignified or

the microorganisms have the capacity to degrade the

holocellulose and lignin or to break at least their

structural link (Cowling and Kirk, 1976). Khan (1977)

wsprint fiber without wet oxidation, B: 170 �C, 31.0 kg/cm2; C: 190 �C,0 lm.

Table 4

Incubation time, total methane production and conversion efficiencies

(%)

Batch label Incubation

time (days)

CH4 (S.T.P.) (liter) Ce (%)a

Ncb Toc

B-1 60 2.4 4.7 51.0 (51)

B-2 60 2.3 3.9 58.9 (59)

B-3 62 1.8 3.6 50.0 (50)

aConversion efficiency (Nc=To � 100).bNet cumulated volume was corrected for the methane production

attributable to the seed.c Theoretical maximum methane production.


investigated the degradation of cellulosic materials using

mixed culture of bacteria. Newsprint was one of the

tested materials in this study. In a period of 4 weeks, less

than 45% of the cellulose was degraded. According to

the suggestion by the author, an important part of the

holocellulose present in the newspaper was not de-

graded. Moreover, the presence of the lignin affected

negatively the ability of the mixed culture to degradecellulose.

The main objectives to pretreat newspaper by the wet

oxidation process were to improve the biodegradability

of the holocellulose in solubilizing or destroying the

greater amount of lignin leaving the fraction of carbo-

hydrates as intact as possible, and to determine the

optimal pretreatment temperature for the best methane

conversion efficiency. The biodegradable nature of theeffluent of the wet oxidation process was evaluated

through the recovery of its total COD as methane gas.

Likewise, the improvement in the bioavailability of cel-

lulose after wet oxidation was evaluated through re-

moval efficiencies in the batch experiments.

Wet oxidized newspaper at 170, 190 and 210 �C were

used to fill the batch reactors enumerated B-1, B-2 and

B-3, respectively. Based on the total COD of each batchreactor (see Table 1), the maximum theoretical potential

of methane production was calculated from the relation

that indicates that 0.35 liter of methane at standard

temperature and pressure (S.T.P.) is generated from 1 g

of COD (McCarty, 1964). The total cumulated volume

of methane and the methane conversion efficiency for

each batch reactor are shown in Table 4. The cumulated

methane curves were constructed on the basis of dailyvalues until the maximum period of 65 days. Table 5

summarizes the final conditions after 65 days of incu-

bation. The effluents reached relatively high convert-

ibilities of COD. The newsprint treated at 190 �Creached the highest methane conversion. Approximately

59% of the initial COD was recovered as methane gas.

As shown in Fig. 3, net cumulated methane production

curves reached very similar potentials, giving B-3 asmaller one. If lag phases are observed, a retardation

period can be seen 10–15 days before the beginning of

the phase of methane production. The phenolic and

Table 5

Anaerobic removal efficiencies for TCOD, cellulose, lignin, furan and solubl

Run of W.O. Batch label Final pH TCOD (g/l) (%)

R-1 B-1 6.7 5.2 46

R-2 B-2 6.9 5.1 49

R-3 B-3 6.6 5.5 44

SLD (g/l) (%)

Initial Final

R-1 B-1 0.441 0.002 99.5

R-2 B-2 0.552 0.032 94.2

R-3 B-3 0.404 0.066 83.7

heterocyclic nature of the soluble fraction probably

caused this delay in the methane production. Young and

Frazer (1987) indicated that monomers in lignin such as

vanillic acid, vanillin, syringic acid, syringaldehyde,

ferulic acid and cinnamic acid are degraded almost

completely under methanogenic conditions in enrich-

ment cultures of digester sludge. In the studies of ac-

climatization of organisms to phenolic compounds,McCarty et al. (1976) mentioned that the methane fer-

mentation of simple ring structures formed by the heat

treatment of cellulose, hemicellulose and lignin had to

be difficult. Acclimatized cultures were incubated with

different soluble compounds which were supposedly

generated from the heat treatment of lignocellulose.

Biodegradation results indicated that with a suitable

acclimatization all the soluble compounds are anaero-bically degraded and transformed to methane. Between

the tested compounds appeared HMF, furfural, phenol,

catechol, benzoate, benzaldehyde, cinnamic acid and

vanillin. In the present work, a previously conditioned

culture was not used, which could have been one of the

reasons to understand the delay in the production of

methane. Appropriate acclimatization should serve to

enhance the development of particular enzymes of acertain mixed culture to ferment organic molecules to

methane. Table 5 shows the efficiencies of removal of the

phenolic and heterocyclic compounds produced during

wet oxidation. Removal efficiencies varied from 84% (B-

3) to 100% (B-1) for SLD and from 87% (B-3) to 96%

(B-2) for furan.

e lignin derivatives

Cellulose (g/l) (%) Lignin (g/l) (%)

0.58 74 0.67 10

0.24 88 0.42 12

0.33 77 0.49 14

Furan (mg/l) (%)

Initial Final

5.55 0.545 90.2

13.0 0.586 95.5

65.1 8.79 86.5

Fig. 3. Net cumulated methane production for the batch experiments.


The anaerobic degradation of newspaper without an

effective pretreatment leaves an important fraction of

cellulose without degrading. Other studies on anaerobic

bioconversion of paper waste made by Clarkson and

Xiao (1999) indicated that the supposed factor thatlimited a suitable anaerobic conversion of newsprint was

the physical structural association between lignin and

cellulose. In reactors treating paper waste in a period of

300 days, 80% of the available cellulose was degraded. It

seemed that wet oxidation of newspaper changed and

improved the structural relationship between lignin and

cellulose. It most likely made the anaerobic degradation

of holocellulose more vulnerable. As shown in Table 5,cellulose had the greater bacterial degradation in B-2,

where approximately 88% of cellulose was degraded; B-3

and B-1 had 77% and 74%, respectively. With regards to

the percentages of anaerobic removal of lignin, these

results were considered to be inconsistent. Indeed, re-

covered lignin concentrations after methane fermenta-

tion could be inexact due to the quantity of biomass of

the batch reactors for solvent extractions were restrictedto small amounts. If the previous reasons are taken into

account it can be concluded that lignin was not de-

graded anaerobically.

4. Conclusions

Due to a complex lignin–holocellulose structural as-

sociation, the biodegradability of newsprint is especially

low compared with other organic wastes used for

methane recovery. As an attractive renewable source of

energy, its low biodegradability had to be improved witheffective thermal pretreatments in the reduction of lignin

without destroying or degrading the fraction of sugars

(carbohydrates) present in the paper fiber. The following

conclusions can be drawn from this study:

Wet oxidation was a pretreatment technically viable

for the fragmentation and solubilization of newsprint.

The solubilization process on the fibers of paper gener-

ated volatile fatty acids, monosaccharides, heterocyclicand soluble lignin compounds.

Analyzing the pretreatment process from the point of

view of the effectiveness (if conserved much or less intact

the fraction of holocellulose for the later anaerobiolog-

ical process) it can be concluded that the solubilization

of the fiber was enhanced through the natural acidifi-

cation of the wet oxidation process (maximum solubi-

lization ratio was 29% at 210 �C). With regards to the

cellulose removal in the pretreatment process, relatively

high removals at 190 and 210 �C were achieved (57%

and 69%, respectively). Therefore, the remaining cellu-lose for the biological process is low and the requirement

for effectiveness is not totally fulfilled. Nevertheless, it is

thought that due to the thermal and acid effect of sub-

critical water, the fiber changed in some degree its

physical dependency with the lignin and made it bio-

logically more susceptible to be degraded.

The biodegradable nature of newsprint treated in

subcritical water was evaluated through the conversionof the COD to methane using a mixed culture. The

highest methane conversion was obtained with the

fraction of paper pretreated at 190 �C. Fifty nine percentwas converted to methane gas in an approximated pe-

riod of 60 days. The cellulose which was not removed in

the oxidation process served as substrate for the mixed

cultures. Removal efficiencies of cellulose varied be-

tween 74% and 88%. With regards to the lignin, it wasnot degraded anaerobically.

Anaerobic reduction of total lignin derivatives and

furans were accomplished with high efficiencies. Remo-

vals varied between 84% and almost 100% for total

lignin derivatives. Furan removals varied between 87%

and 96%.

Further research is needed in order to optimize the

process for methane fermentation. Adding buffer duringwet oxidation should be an essential solution to prevent

the natural acidification of the process. This should

improve methane fermentation efficiency of wet oxidized

newsprint by leaving more free-holocellulose and solu-

bilizing more lignin.

Acknowledgements

The authors wish to express their profound thanks to

Mr. Takamasa Ohki, Hitachi Zosen Co. Ltd. for the

technical support they offered for the wet oxidation of

paper wastes. They also wish to extend their grate-

ful thanks to Professor Kenji Iiyama, Asian Natu-

ral Environmental Science Center, the University of

Tokyo.

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Wet oxidation pretreatment for the increase in anaerobic biodegradability of newspaper waste

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