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Biomass and Bioenergy 30 (2006) 892–896

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Kinetic study of thermophilic anaerobic digestion of solid wastesfrom potato processing

Bernd Linke

Leibniz- Institute of Agricultural Engineering Bornim, Max-Eyth-Allee 100, D-14469 Potsdam, Germany

Received 14 May 2004; received in revised form 13 February 2006; accepted 16 February 2006

Available online 4 April 2006

Abstract

Anaerobic treatment of solid wastes from potato processing was studied in completely stirred tank reactors (CSTR) at 55 1C. Special

attention was paid to the effect of increased organic loading rate (OLR) on the biogas yield in long-term experiments. Both biogas yield

and CH4 in the biogas decreased with the increase in OLR. For OLR in the range of 0.8 gl�1 d�1–3.4 gl�1 d�1, biogas yield and CH4

obtained were 0.85 l g�1–0.65 l g�1 and 58%–50%, respectively. Biogas yield y as a function of maximum biogas yield ym, reaction rate

constant k and HRT are described on the basis of a mass balance in a CSTR and a first order kinetic. The value of ym can be obtained

from curve fitting or a simple batch test and k results from plotting y/(ym�y) against 1/OLR from long-term experiments. In the present

study values for ym and k were obtained as 0.88 l g�1 and 0.089 d�1, respectively. The simple model equations can apply for dimensioning

completely stirred tank reactors (CSTR) digesting organic wastes from food processing industries, animal waste slurries or biogas crops.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic digestion; Thermophilic; CSTR; Biogas; Potato wastes; Kinetic model

1. Introduction

Anaerobic digestion of wastewater as well as wastes fromfood processing industries and energy crops has attractedmuch interest in recent years. This technology offers greatpotential for rapid disintegration of organic matter toproduce biogas and save fossil energy [1–3]. The UASBconcept has been successfully applied in practice fortreating high-polluted wastewater from potato processing[4]. The performances of anaerobic hybrid reactors withvarious nylon fibre densities for treatment of cassava starchwastewater have shown direct correlation between CODremoval, densities per packed bed volume and number ofmethanogens [5]. By comparison with mesophilic treat-ment, the results of both a pilot study for wastewaterstreams, deriving from steam blanching of potatoes [6] andanaerobic digestion of ethanol stillage [7] suggest thatthermophilic anaerobic treatment should be favoured. Foranaerobic treatment of solid wastes from potato proces-sing, the major disposal route is common anaerobic

e front matter r 2006 Elsevier Ltd. All rights reserved.

ombioe.2006.02.001

ess: blinke@atb-potsdam.de.

treatment with sewage sludge [8–10]. Potato pulp as asingle substrate was investigated in batch experimentswhich resulted in ultimate biodegradability in the range of86–91% [11]. A two-step process for treatment of potatopeelings consisting of liquefaction digesters coupled with amethane fixed-film digester is described in [12]. The mixtureof the acidogenic effluents was 80% degraded in amethanation reactor and overall organic matter removalreached a level of 87%.There are three main tendencies in anaerobic modelling for

predicting the reactor behaviour [13]. Based on kineticequations such as Monod or Contois, an unstructurednonsegregated model [14,15] and an unstructured segregatedmodel [16] are proposed. Structured kinetic models fordynamic simulation of the anaerobic degradation on thebasis of a complex matrix with kinetic constants for differentdegradation steps of organic material in completely stirredtank reactors (CSTR) make it possible to predict real processresponse to specific operating conditions [17–19]. However,for dimensioning the fermenter size of CSTRs both theorganic loading rate (OLR) and the hydraulic retention time(HRT) are the parameters applied most frequently in

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Notation

TS Total solids (g kg�1)VS Volatile solids (g kg�1)TKN Total Kjeldahl nitrogen (g kg�1)NH4-N Ammonia-nitrogen (g kg�1)VFA (C2–C6) Volatile fatty acids (g kg�1)pH pH-value (dimensionless)HRT Hydraulic retention time (d)OLR Organic (VS) loading rate (g l�1 d�1)

VR Volume of the reactor (l)m0 mass flow of the input (kg d�1)c0 VS-concentration of the input (g kg�1)cE VS-concentration in the reactor (g kg�1)r(c) substrate removal rate as function of c

(g l�1 d�1)k first order reaction rate constant (d�1)y VS biogas yield (l g�1)ym maximum VS biogas yield (l g�1)

gas meter

stirrer

heating bathcirculator

feedslurry

digestedslurry

gas bag

Fig. 1. Schematic diagram of semi-continuous lab-scale experiments.

Table 1

Chemical compositions of inoculum and substrates used

Parameter Inoculum Potato waste samples

1 2 3

pH — 7.36 4.03 3.84 3.75

TS g l�1 22.3 142 113 128

VS g l�1 14.2 131 101 119

VFA g l�1 3.44 1.98 2.73 2.20

NH4-N g l�1 0.76 0.17 0.20 0.14

TKN g l�1 1.99 2.49 2.27 2.81

B. Linke / Biomass and Bioenergy 30 (2006) 892–896 893

practice. As methane yield was found to decrease approxi-mately in a straight line with the increase in OLR anddecrease in temperature, a simple approach can be used.Methane yield at any HRT is a function of a critical HRT atwhich the reactor fails, the maximum methane yield and atemperature term calculated from a derived Arrhenius-equation [20]. Presently only limited data are available onthe anaerobic treatment of solid wastes from potatoprocessing in CSTR and the kinetics of biogas productionat thermophilic temperatures. In the present study the effectof increasing OLR on the biogas yield at 55 1C was examinedin long-term experiments. Furthermore, a kinetic model foranaerobic digestion combining mass balance equations of aCSTR with biogas yield from volatile solids is submitted.

2. Materials and methods

2.1. Semi-continuously fed reactor experiments

Solid wastes obtained from a full-scale potato processingplant were used as substrate in a CSTR. The temperatureof the 2.5 l mechanically stirred glass reactor was kept at55 1C by circulating heated water through the jacket. Thereactor was operated in a fill-and-draw mode with sixfeedings per week and mixed slowly of about 100min�1 for15min every 2 h. (Fig. 1). For start-up the reactor wasinoculated with effluent of a 3200m3 reactor digesting solidwastes from a potato processing plant at an OLR of 1.5 kgVS m�3 d�1. Beginning with an OLR of 0.8 g VS 1�1 d�1,the OLR was increased stepwise and maintained for eachOLR step of about 50 days. Within the experiment threesamples of potato waste were analysed (Table 1). Sampleswere taken fresh from the potato processing plant andstored at 4 1C. Different solid potato waste samples 1, 2and 3 were used, from 0–110 days, 111–170 days and171–205 days, respectively. The biogas produced wasmeasured daily using a multi-chamber rotor gas meter(RITTER) and collected in gas bags (LINDE).

2.2. Analytical methods

Methane content of the biogas was analysed twice aweek by infrared detection (PRONOVA). Samples of the

reactor effluent were taken once a week and determinedfor VFA, TS and VS. Values of OLR and biogas yieldare average values of a seven-day period. For example,biogas yield was calculated from the biogas productionderived from the VS-load of one week. Biogas wasnormalised at the standard temperature and pressure(0 1C, 1013mbar). Analyses for pH, TS, VS, VFA, NH4-N and TKN were performed according to Germanstandard methods [21].

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0.00

0.20

0.40

0.60

0.80

1.00

0.0 1.0 2.0 3.0 4.0

OLR (gl-1d-1)

y (l

g-1

)

0

20

40

60

80

100

CH

4 (V

ol.%

)

0.88

Fig. 3. Effect of OLR on y (J) and CH4 (+) from semi-continuous�1

B. Linke / Biomass and Bioenergy 30 (2006) 892–896894

3. Results and discussion

3.1. Semi-continuous reactor experiments

The reactor performance data in the course of time, withspecial emphasis on biogas yield, OLR, VS and VFAconcentration in the effluent (Fig. 2), clearly demonstratethe effect of OLR upon other parameters. After start-up asharp decrease of VFA concentration was observed. Thiseffect resulted from the fact that the OLR in the lab-scalereactor was lower than that in the full-scale reactor fromwhich the inoculum was obtained. After 50 days the OLRwas increased to about 2 gl�1 d�1. This step resulted in adecreased biogas yield and increase of VFA. When OLRwas adjusted to 2.5 gl�1 d�1 VFA, biogas yield and VSconcentration in the effluent cE were observed to be about2.5 gl�1 , 0.75 lg�1 and 2.5%, respectively.

The next increase in OLR to 3.2 gl�1d�1 resulted in afurther increase of VFA and VS concentration in theeffluent and indicates the beginning of reactor failure dueto a critical OLR. However, plotting of all observed biogasyields and CH4 in the biogas against the correspondingvalues of OLR results in both decrease of y and CH4 withincrease of OLR (Fig. 3). The maximum biogas yield canbe obtained from curve fitting according to (9) and resultsto 0.88 l g VS�1.

3.2. Development of the kinetic model

The simple model presented here describes the biogasproduction process for a CSTR. The mass balanceequation with equal mass flow of input and output m0

(mass of biogas is neglected) can be written as

VRdc

dt¼ m0 � c0 �m0 � cþ VR � rðcÞ. (1)

The substrate removal rate r(c) as a function of c isexpressed as first order kinetic with

�dc

dt¼ rðcÞ ¼ �k � c. (2)

0.00

1.00

2.00

3.00

4.00

5.00

0 50 100 150 200 250Time t (days)

OL

R (

gl-1

d-1

); V

FA

(g

l-1);

cE (

%)

0.00

0.20

0.40

0.60

0.80

1.00

y (l

g-1

)

Fig. 2. Semi-continuous reactor performance data y (J), OLR (’), VFA

(� ) and VS concentration (%) in the effluent cE (W).

By combining (1) and (2) with VR ¼ m0 �HRT at steadystate for VR � ðdc=dtÞ ¼ 0 we obtain

HRT ¼1

k�

c0

c� 1

� �. (3)

The overall correlation between substrate concentration c

and biogas yield y at time t is shown in Fig. 4. Thebiodegradable fraction of the complex organic substrate isdisintegrated to biogas according to (4)

c0 � cðtÞ

c0¼

yðtÞ

ym

(4)

and (5), respectively,

c0

c¼

ym

ym � y. (5)

When the term c0/c in (3) is replaced by ym=ðym � yÞ thehydraulic retention time of a CSTR can be described by thefollowing equation:

HRT ¼1

k�

y

ym � y

� �(6)

and

y ¼HRT � k � ym

HRT � k þ 1, (7)

experiments with residues from potato processing (c0 ¼ 117710.5 g l ,

35 1C), ym ¼ 0.88 l g�1.

time t

sub

stra

te

con

cen

trat

ion

c

bio

gas

yie

ld y

ymc0

y (t)

c (t)

c0 - c (t)

t

Fig. 4. Correlation between substrate degradation and biogas production

in course of time.

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0

4

8

12

16

20

0.0 0.4 0.8 1.2 1.61/BR (l d g-1)

y /

(ym

- y

)

Fig. 6. Graph of y/(ym�y) ¼ 10.42 � 1/OLR (J) from experiments with

residues from potato processing (c0 ¼ 117710.7 g l�1, 35 1C), slope:

k � c0 ¼ 10.42 g l�1d�1, k ¼ 0.089 d�1.

B. Linke / Biomass and Bioenergy 30 (2006) 892–896 895

respectively. For dimensioning the fermenter size of CSTRsboth the OLR and the HRT are the most appliedparameters in practice. With OLR ¼ c0/HRT equation(6) can be written as

OLR ¼k � c0

y�

ym � y� � (8)

and

y ¼ ym

k � c0

k � c0 þOLR, (9)

respectively. For calculating the fermenter size by means ofHRT or OLR, detection of ym and k is essential. Whereasym yields from a simple batch test, detection of k can beobtained from long- term experiments in a CSTR. Thevalue of k can be obtained by plotting y=ðym � yÞ againstHRT or 1/OLR. The slope of the straight line yields k ork � c0; respectively. Therefore, biogas yield y can beexpressed as an absolute proportion p of ym, HRT and p

results from (10) and (11), respectively.

HRT ¼p

k � ð1� pÞ, (10)

p ¼HRT � k

HRT � k þ 1, (11)

A graph of p for different values of HRT and k (Fig. 5)indicates that HRT decreases with increase of k. Forexample, in order to obtain 80% of ym, for k ¼ 0:1 theHRT required is 40 days.

3.3. Application of the kinetic model

Results from long term thermophilic anaerobic digestionexperiments with solid wastes from potato processing(c0 ¼ 117710.7 g l�1, 35 1C) as described above were usedto apply the model. On the basis of ym, k and c0 bothreactor size and reactor performance data can be calcu-lated. The maximum biogas yield ym is equivalent to theultimate anaerobic biodegradability and results when theOLR value is near zero. Considering the curve fitting on

0.7

0.75

0.8

0.85

0.9

0.95

1

0 20 40 60 80 100tm (d)

p o

f y m

(-) 0.150.2

0.30.4

0.5

0.125 0.1

0.09 0.080.07 0.06

k = 0.05 d-1

Fig. 5. Absolute proportion p of ym for different values of HRT and k.

the base of (9) we obtain ym ¼ 0:88 lg�1 for OLR ¼ 0 (Fig.3). The reaction rate constant k results from the plot ofy=ðym � yÞ against 1/OLR and the slope of k � c0 ¼

10:42 g l�1 d�1 to k ¼ 0:089 d�1 (Fig. 6). However, bymeans of this parameter reactor performance data can becalculated. For example, in order to obtain 80% and 90%of ym, the required HRTs result from (10) to 45 days and101 days, respectively (Fig. 5).

4. Conclusions

In long term lab-scale experiments it could be demon-strated that thermophilic anaerobic digestion is applicablefor treatment of solid wastes from potato processing withVS and TKN concentrations in the range of about10%–13% and 2.3%–2.8%, respectively. Results haveshown that with an increase of the organic loading ratein the range of 0.8–3.4 gl�1 d�1, biogas yield decreases. Themaximum biogas yield ym can be obtained from curvefitting of equation (9) at OLR ¼ 0. Alternatively, detectionof ym is possible by means of a simple batch-test. Modelequations are developed on the basis of mass balanceequations for a completely stirred tank reactor (CSTR) anda first order kinetic. For given values of reaction rateconstant k, maximum biogas yield ym and its favouredproportion p in reactor performance, both the requiredhydraulic retention time HRT and the organic loading rateOLR can be calculated by means of a few parameters. Thevalues of k and ym are specific parameters for differentsubstrates used. In order to obtain reliable data for k, long-term experiments are essential. Special emphasis should beplaced on reactor performance at steady state and at OLRthat does not result in reactor failure. The simple modelequations may be used for dimensioning completely stirredtank reactors (CSTR) digesting organic wastes from foodprocessing industries, animal waste slurries and biogascrops.

ARTICLE IN PRESSB. Linke / Biomass and Bioenergy 30 (2006) 892–896896

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