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Kinetic study of mesophilic anaerobic digestion of pulp& paper sludge

Yunqin Lin, Dehan Wang*, Qing Li, Lijian Huang

Key Laboratory of Soil Environment and Waste Reuse in Agriculture of Guangdong High Education Institutions,

South China Agricultural University, Guangzhou 510640, China

a r t i c l e i n f o

Article history:

Received 26 September 2010

Received in revised form

5 October 2011

Accepted 10 October 2011

Available online 8 November 2011

Keywords:

Pulp & paper sludge

Anaerobic digestion

Methane

Mesophilic

Kinetic

* Corresponding author. Tel.: þ86 20 8528030E-mail addresses: yqlin@scau.edu.cn (Y.

0961-9534/$ e see front matter ª 2011 Elsevdoi:10.1016/j.biombioe.2011.10.001

a b s t r a c t

Anaerobic digestion of pulp and paper sludge (PPS) and monosodium glutamate waste

liquor (MGWL) was studied in completely stirred tank reactors (CSTR) at 37 � 2 �C. This

work focused on the effect of increased organic loading rate (OLR) on the methane

production in long-term experiments. For OLR in the range of (1.5e5.0) kg m�3 d�1 based on

VS fed, VFA and SCOD concentrations decreased for the first 10 days and then kept stable at

about 2.3 kg m�3 and 4.0 kg m�3 respectively until to the critical OLR of 5.0 kg m�3 d�1; and

the methane generation rate enhanced to 0.838 m3 m�3 d�1 during this period until to the

reactor failure. Additionally, reaction rate constant k and sludge retention time (SRT) are

described on the basis of a mass balance in a CSTR followed a first order kinetic equation.

In the present study, values for ym and k were obtained as 0.733 m3 kg�1 of removed VS and

0.07 d�1, respectively. The simple model can apply for dimensioning a CSTR digesting of

organic wastes from pulp and paper industries, food processing industries, sewage treat-

ment plants or biogas crops.

ª 2011 Elsevier Ltd. All rights reserved.

1. Introduction China, the yield of pulp & paper sludge (PPS) was 2628 Mt at

Anaerobic digestion of wastewater as well as organic wastes

has attracted much interest in recent years. This technology

offered great potential for rapid disintegration of organic

matter to produce biogas and save fossil energy [1e3] which

could supply energy in an environmentally friendly, sustain-

able way. The batch or semi-continuous system has been

successfully applied in practice for treating wastewater and

solid wastes [4e6]. By comparison with mesophilic treatment,

the application of high temperature (thermophilic) would

produce more free ammonia, consumemore energy and need

more sensitive system control. Hence, mesophilic anaerobic

digestion was widely used to treat organic wastes [7,8].

For the pulp & papermaking industry, it is believed to be

the largest manufacturing user of wastes in future [9]. In

2; fax: þ86 20 85287672.Lin), dehanwang@scau.edier Ltd. All rights reserve

a water content of 800 g kg�1 of sludge in 2007, and an esti-

mated increase to 3088 Mt with the same water content is

expected in 2020. So large amount of PPS will make great

environmental pollution unless it is treated properly. On the

other hand, wood, one of the major raw materials used in the

pulp and paper industry, is composed of cellulose fibers,

carbohydrates such as starch and sugars, as well as lignin

which acts as an adhesive substrates for the cellulose fibers.

The pulp and paper industry breaks down the wood to sepa-

rate the cellulose from the non-cellulose substances by

chemical process or mechanical process. So there are more

fines, carbohydrate (sugars, salts, etc.) and proteins in PPS [10],

which is suitable for anaerobic treatment.

As formonosodium glutamate waste liquor (MGWL), it was

another solid residue from the production of monosodium

u.cn (D. Wang).d.

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glutamate by concentration and distillation. The yield of of

MGWLwas 1575Mt at a water content of 550 g kg�1 of liquor in

2008, which was the largest in the world. Most of MGWL was

disposed of by landfilling, which was a waste of land and also

easy to make the secondary pollution, especially when large

quantity of leachate producing. However, there was more

abundant nitrogen inMGWL (e. g. the total nitrogen content of

theMGWLused in this study attained to 118.3 g kg�1 (see Table

1)), which was suitable for biological treatments. A batch

process for treatment of pulp & paper sludge (PPS - rich in

carbon) and monosodium glutamate waste liquor (MGWL e

rich in nitrogen) was described to attain the organics removal

efficiency in the range of 50e60% [11].

The anaerobic conversion process of a biological waste to

methane involved several biological reaction steps [12e15]. In

this work, three stages have been considered: (1) the complex

biopolymerswerehydrolytically converted to lower-molecular-

weightcompoundsable tobeusedassubstratesbycells [13e15];

(2) the hydrolyzedwastewas converted to volatile organic acids

by an anaerobic microflora e acetogenic microorganisms; (3)

finally, methane was produced from volatile organic acids by

methanogenic microorganisms. For organic wastes rich in

higher-molecular-weight compounds (e. g. cellulose, hemi-

cellulose, proteins), some authors [6,7] proposed that the time

required for complete digestion was long because the waste

dissolution and its hydrolysis to lower-molecular-weight

compounds were the rate-limiting steps in the anaerobic

process [16,17].

Structured kinetic models for dynamic simulation of the

anaerobic degradation on the basis of a complex matrix with

kinetic constants for different degradation steps of organic

material in completely stirred tank reactors (CSTR) made it

possible to predict real process response to specific operating

conditions [18e20]. However, for dimensioning the fermenter

size of a CSTR, both the organic loading rate (OLR) and the

sludge retention time (SRT) were the parameters appliedmost

frequently in practice. As methane yield was found to

decrease approximately in a straight line with the increase of

OLR and decrease of temperature, a simple approach could be

used. Except for a critical SRT at which the reactor failed,

methane yield was a function of SRT during digestion, and the

maximum methane yield could calculate from a derived

Arrhenius equation [21] or it could get from a batch

experiment.

Presently, only limited data have been available on the

anaerobic digestion of PPS and MGWL in a CSTR and the

kinetics of biogas production at mesophilic temperatures. In

this work, the effect of increasing OLR on themethane yield at

Table 1 e General characterization of the different waste used

TS(g kg�1)

VS(g kg�1 of TS)

pH

Pulp & paper sludge 314.5 623.0 7.8

Monosodium glutamate

waste liquor

430.0 685.0 5.3

Inoculum sludge 91.7 532.0 7.8

37 � 2 �C was examined in long-term experiments. Further-

more, a kineticmodel formass balance equations of anaerobic

digestion in a CSTR with the biogas production from volatile

solids was submitted.

2. Materials and methods

2.1. Materials collection

PPS samples were collected from the secondary clarifiers

(normally settling tanks) of the Pulp & Paper Plant on 10 July,

2009. The Pulp & Paper Plant was at No. 40 Guangzhi Road,

Haizhu District, Guangzhou City, China. There are two

processes in this plant: one is bleaching chemi-thermo-

mechanical pulp (BCTMP) made from Masson pine (Pinus

massoniana Lamb.); the other is papermaking fromwaste paper

after de-inking. Wastewater arises from three sections e

pulping, papermaking and de-inking e and is usually dewa-

tered to moisture content of (700e800) g kg�1 of sludge at the

end process of waste water treatment. Seed sludge was ob-

tained from the sewage tank (near Building 4 in the South

China Agricultural University, Guangzhou, China) and was

acclimatized with PPS in the laboratory for 3 months until to

the OLR ¼ 1.5 kg m�3 d�1 based on VS fed. The total solid (TS)

content of seed sludge was 100 g kg�1 of sludge after accli-

matization. In order to get the optimal ratio of carbon to

nitrogen (C/N), monosodium glutamate waste liquor (MGWL)

was applied, which was collected from Ao-Sang Monosodium

Glutamate Factory (Guangzhou, China).

All kinds of samples were collected prior to the experi-

ment, stored in the refrigerator (0e4 �C) and analyzed for total

solids (TS), volatile solids (VS), organic carbon (OC), total

nitrogen (TN) and pH according to the Standard Methods for

the Examination of Water and Wastewater [22] (Table 1).

2.2. Semi-continuously fed reactor experiment procedure

A bench-scale anaerobic digester stirred reactor with 2 L

working volume was used. The anaerobic digester was oper-

ated in a fill-and-drawmode with feedings per day andmixed

slowly about 100 r min�1 for 10 min every 2 h on 37 � 2 �C.For start-up, the initial fermentation condition of anaerobic

digestion experiment was determined with the inoculum ratio

of 100 g kg�1 of TSfed according to an earlier feasibility study in

batch reactors [11]. Based on the former condition, the feed-

stock for the start-up of anaerobic digestion composed of PPS

454.6 g, MGWL 222.9 g, inoculum sludge 200 g and distilled

in the anaerobic test.

OC(g kg�1, basedon dry weight)

TN(g kg�1, basedon dry weight)

C/N ratio

2 327.5 10.9 30.05

6 295.0 118.3 2.49

5 267.0 7.1 37.61

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water 1322.5 g. Beginning with an OLR of 1.5 kg m�3 d�1 based

on VS fed after start-up, the OLR was increased stepwise and

maintained a certain time for each OLR step; different dosages

of each material were fed into the reactors (Table 2). Samples

were taken fresh from the processing reactor andmeasured for

pH, TS, VS, SCOD, and VFA. The methane production was

measured daily by water displacement method; and the

methane concentration in the biogas was measured by gas-

chromatography [23].

2.3. Development of the kinetic model

The simple model presented here describes the biogas

production process for a CSTR. The mass balance equation

with the equal mass flow of input and output at steady state

under a first order kinetic can be written as

SRT ¼ 1k

�coc� 1

�(1)

(where k means the reaction rate constant; co means the

original concentration of substrate; c means the substrate

concentration at any time.)

Based on the overall correlation between substrate

concentration c and biogas yield y at time t [24], the hydraulic

retention time of a CSTR can be described by the following

equation:

SRT ¼ 1k$

�y

ym � y

�(2)

and

y ¼ SRT$k$ym

SRT$ðkþ 1Þ (3)

respectively. (Where ym means the maximum production of

biogas; y means the biogas production at any time.)

For dimensioning the fermenter size of a CSTR, both the

OLR and the SRT are the most applied parameters in practice.

With OLR ¼ co=SRT; Eq. (2) can be written as

OLR ¼ k$coy=

�ym � y

� (4)

and

y ¼ ym$k$co

k$co þOLR(5)

Table 2 e Organic loading rate (OLR) design and composition o

OLR(kg m�3 d�1)

SRT (d) Dosage for

Pulp & papersludge

Monosw

1.5 28.9 16.1

2.0 21.6 21.6

2.5 17.3 27.1

3.0 14.4 32.5

3.5 12.4 37.5

4.0 10.8 43.4

4.5 9.6 48.9

5.0 8.7 53.2

respectively. For calculating the fermenter size by means of

SRT or OLR, detection of ym and k is essential.Whereas, ym gets

from a simple batch test, and k can be obtained from long-

term semi-continuous experiments in a CSTR. The value of k

can be obtained by plotting y=ðy� ymÞ against SRT or 1/OLR.

The slope of the straight line yields k or k$co.

Therefore, biogas yield y devided by ym can be expressed as

a proportion p ðp ¼ ðy=ymÞÞ, the correlation between SRT and p

results from Eq. (6):

SRT ¼ pk$ð1� pÞ (6)

Eq. (6) indicates that SRT decreasedwith the increase of k for

a constant p. For example, in order to obtain 80% of ym, for

k ¼ 0.1d�1, the SRT required is 40 days.

2.4. Analytical methods

Samples of the reactor effluent were taken once a day and

determined for the routine parameters in triplicate. Analyses

for TS, VS, pH, and SCOD were performed according to the

standard methods [22], while SCOD was measured by the

potassium dichromate method. VFA were analyzed by

a distillation-titration method and the results were expressed

as acetic acid concentration [25].

3. Results and discussions

3.1. Semi-continuous experimental results

The reactor performance data in the course of time, with

special emphasis on the methane production, pH values, VFA

and SCOD concentrations in the effluent (Fig. 1), clearly

demonstrated the effect of OLR upon other parameters. For

the first stage of the semi-continuous experiment after start-

up (on days 0e10, OLR ¼ 1.5 kg m�3 d�1 based on VS fed),

a sharp decrease of VFA and SCOD concentrations was

observed, from 4.618 to 1.975 kgm�3 and 9.203 to 4.916 kgm�3,

respectively; this effect resulted from the fact that the OLR in

the lab-scale reactor was lower than that in the full-scale

reactor from which the inoculum was usually obtained and

there were some VFA and SCOD remained in the reactor when

the start-up stage ended, which caused to the greater

f the feedstock used to the semi-continuous experiment.

each material (g) Total dosage fordraw/fill materials

(g d�1)odium glutamateaste liquor

Distilledwater

1.6 51.5 69.2

2.1 68.8 92.6

2.2 85.9 115.6

3.1 103.3 138.9

3.8 120.0 161.3

4.1 137.7 185.2

4.6 154.8 208.3

5.6 171.1 229.9

Fig. 1 e The variation of methane production, pH values, VFA and SCOD concentrations with organic loading rate (OLR)

increased in the semi-continuous anaerobic experiment.

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consumption of VFA and SCOD than generation in the first

stage [26]. However, an obvious increase of methane produc-

tion and pH value, with the rate of 1663% and 20% (see Fig. 1),

respectively, were obtained on the first 10 days of semi-

continuous experiment, and then a smooth increase of

methane production and a stable value of pH were observed

until to days 120, which indicated that the system was stable

after 10 days when a fill-and-draw mode with one feeding per

day began with an OLR of 1.5 kg m�3 d�1 based on VS fed. The

start-up phase of this process lasted for a shorter time (10

days) than that (14 days) reported by Zupancic [4] when the

process fully adapted to the substrate.

After 29 days, the OLR was increased to 2.0 kg m�3 d�1,

resulting in a further increase of methane production and

a stability of pH value, VFA and SCOD concentrations; the same

resultswere got before OLR attained to a value of 5.0 kgm�3 d�1;

themaximumofmethaneproduction ratewas0.838m3m�3d�1

as well as the methane yield of 0.475 m3 kg�1 based on VS

removed,whichwasattainedondays120 (OLR¼ 4.5 kgm�3 d�1)

with theVS removal efficiency of 57% (theVS removal efficiency

in the range of 47e61% during the semi-continuous digestion);

the maximummethane yield obtained in this work was higher

than that (0.42 m3 kg�1) got by Riau in the mesophilic digester

[27], but lower than that (0.598 m3 kg�1) attained from the

mesophilic co-digestion of thickened waste activated sludge

(TWAS) and fat, oil and grease (FOG) because of the more

optimal mixing substrate for methane production [28]; the pH

values were stable in the range of 7.5e8.5, which was a little

higher than the optimal pHvalue (6.8e7.2) for themethanogens

mainly due to the greater consumption of the organic acids

generating from the substances and the furthermore formation

of the ammonia in the reactor, because the PPS was alkaline

(pH¼ 7.82, seeTable1) andnoacidicmaterialwasused toadjust

the pH value in the reactor during the long-term semi-contin-

uous experiment; the VFA concentrations were always about

2.0 kg m�3 during this stage, which were suitable for the ace-

togenic and methanogenic microorganisms (in the range of

1.0e5.0 kg m�3) and the similar VFA concentration range was

achieved withmesophilic anaerobic digesting of sewage sludge

by Riau [27,29]; and the SCOD concentrationsmaintained in the

rangeof 3.0e6.0 kgm�3whichwas consistent to theSCODtrend

in the digester 3, 4 obtained by Salminen [30].

However, for the next increase of OLR to 5.0 kg m�3 d�1, an

obvious increase of VFA concentrations (from 2.92e14.082

kgm�3), SCOD concentrations (from 5.934e16.912 kg m�3) and

pH values (from 8.05 to 8.79) in the effluent was observed, and

the methane generation rate decreased significantly from

0.838 to 0.39 m3 m�3 d�1 as well as the methane yield changed

Fig. 4 e The effect of SRT on the biogas production

efficiency.Fig. 2 e The effect of OLR on biogas ( y) and methane (CH4)

production from semi-continuous experiments with pulp

& paper mill sludge (co [ 43.25 kg mL3, T [ 37 ± 2 �C,ym [ 0.733 m3 kgL1).

b i om a s s an d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 6 2e4 8 6 74866

from 0.475 to 0.177 m3 kg�1, respectively, at the same time

(Fig. 1), which indicated that the beginning of the failure in the

anaerobic digestion system due to a loaded OLR. In this study,

the critical OLR was 4.5 kg m�3 d�1 with a 10 day SRT.

According to Hobson and Wheatley [31], loading rate and SRT

for sewage sludge and manure digesters were typically in the

range of (2e6) kg m�3 d�1 with a 15 day SRT and 3 kg m�3 d�1

with a (10e20) day SRT, respectively. It indicated that this

process was more efficient and steady to treat PPS and MGWL

formethane production. Nevertheless, plotting of all observed

methane yield and biogas production ( y) against the corre-

sponding values of OLR resulted in both decrease of methane

yield and ywith the increase of OLR (Fig. 2). Themaximumgas

yield could be obtained from the curve fitting (Fig. 2) according

to Eq. (5) (see part 2.3) and resulted to 0.733 m3 kg�1 of biogas

production and 0.434 m3 kg�1 of methane yield based on VS

removed.

3.2. Application of the kinetic model

Results from the long-term mesophilic anaerobic digestion

with PPS and MWGL processing (co ¼ 43.25 kg m�3,

T¼ 37� 2 �C) as described abovewere used to apply themodel.

On the basis of ym, k and co, both reactor size and reactor

performance data can be calculated. The maximum biogas

yield ym is equivalent to the ultimate anaerobic biodegrad-

ability and results when the OLR value is near zero. Consid-

ering the curve fitting on the base of Eq. (5), ym ¼ 0.733 m3 kg-1

was obtained for OLR ¼ 0 (Fig. 2). The reaction rate constant k

resulted from the plot of y=ðy� ymÞ against 1=OLR, and the

Fig. 3 e The graph of y/( ymLy) and 1/OLR from semi-

continuous experiments with PPS and MWGL

(co [ 43.25 kg mL3, T [ 37 ± 2 �C).

slope of k$co ¼ 2.9786 kg m�3 d�1 as well as k ¼ 0.07 d�1 due to

co ¼ 43.25 kg L-3 (Fig. 3). The value of k (0.07 d�1) in this

experiment was a little lower than 0.089 d�1 obtained by Linke

[24], because potato waste with higher biodegradation was

used in the latter to obtain the greater reaction rate. However,

by means of this parameter, the reactor performance data can

be calculated. For example, in order to obtain 80% and 90% of

ym, the required SRT resulted from Eq. (6) was 57 days and 129

days, respectively (Fig. 3).

As shown in Fig. 4, it could be concluded that biogas

production ( y) decreased with SRT increased, which demon-

strated the correlation between y and SRT ðSRT ¼ ðco=OLRÞÞ onthe basis of constant ym and k as well as Eq. (5) described.

However, longer SRT will certainly reduce the utilization effi-

ciencyof thereactoranddecrease thebiogasproductionperday.

So it is necessary to choose anoptimal SRT to obtain thehighest

p and the reactor utilization efficiency in practice. Taking the

VFA and SCOD concentrations, pH values, methane daily

production (m3 m�3 d�1) into consideration (Fig. 1), the optimal

SRT for anaerobic digestion of PPS and MGWLwas 10 days.

4. Conclusions

In long-term bench-scale experiments, it could be demon-

strated that mesophilic anaerobic digestionwas applicable for

treatment of PPS and MWGL with the TS concentration of

100 g kg�1 of sludge. Results have shown that with the organic

loading rate (OLR) in the range of (1.5e5.0) kgm�3 d�1 based on

VS fed, the methane generation rate and methane yield

increased firstly and then decreased. The maximum biogas

yield ym (0.733 m3 kg�1 of removed VS) and the maximum

methane yield (0.434 m3 kg�1 of removed VS) could be ob-

tained from curve fitting of Eq. (5) at OLR ¼ 0. The values of k

was 0.07 d�1 based on a first order kinetic. The created kinetic

model equations could be used for dimensioning completely

stirred tank reactor digesting organic wastes from pulp and

paper industries, food processing industries, sewage treat-

ment plants or biogas crops.

Acknowledgments

The authors would like to thank the Nature Natural Science

Foundation of China (Project No: 51108195) for financially

supporting this research.

b i om a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 4 8 6 2e4 8 6 7 4867

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Notations

PPS: pulp & paper sludgeMGWL: monosodium glutamate waste liquorTS: total solidsVS: volatile solidsC/N: the ratio of carbon to nitrogenOC: organic carbonTN: total nitrogenSCOD: soluble chemical oxygen demandVFA: volatile fatty acidSRT: sludge retention timeOLR: organic loading rateCSTR: completely stirred tank reactorBCTMP: bleaching chemi-thermo-mechanical pulpMC: moisture contentA. D.: Anno DominVR: volume of the reactorQ0: mass flow of the feedc0: initial VS concentration of the feedc(t): VS concentration in the reactorr: substrate removal rate in the reactork: first order reaction rate constanty: biogas yieldym: maximum biogas yieldp: proportion of y to ym