Kinetic study of thermophilic anaerobic digestion of solid wastes from potato processing
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ga mconstant k and HRT are described on the basis of a mass balance in a CSTR and a rst order kinetic. The value of ym can be obtained
potential for rapid disintegration of organic matter toproduce biogas and save fossil energy . The UASB
anaerobic digestion of ethanol stillage  suggest thatthermophilic anaerobic treatment should be favoured. Foranaerobic treatment of solid wastes from potato proces-
methane xed-lm digester is described in . The mixtureof the acidogenic efuents was 80% degraded in a
degradation steps of organic material in completely stirredtank reactors (CSTR) make it possible to predict real processresponse to specic operating conditions . However,
ARTICLE IN PRESSsing, the major disposal route is common anaerobic 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
0961-9534/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
E-mail address: firstname.lastname@example.org has been successfully applied in practice fortreating high-polluted wastewater from potato processing. The performances of anaerobic hybrid reactors withvarious nylon bre densities for treatment of cassava starchwastewater have shown direct correlation between CODremoval, densities per packed bed volume and number ofmethanogens . By comparison with mesophilic treat-ment, the results of both a pilot study for wastewaterstreams, deriving from steam blanching of potatoes  and
methanation reactor and overall organic matter removalreached a level of 87%.There are three main tendencies in anaerobic modelling for
predicting the reactor behaviour . Based on kineticequations such as Monod or Contois, an unstructurednonsegregated model [14,15] and an unstructured segregatedmodel  are proposed. Structured kinetic models fordynamic simulation of the anaerobic degradation on thebasis of a complex matrix with kinetic constants for differentstudy values for ym and k were obtained as 0.88 l g1 and 0.089 d1, 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
Anaerobic digestion of wastewater as well as wastes fromfood processing industries and energy crops has attractedmuch interest in recent years. This technology offers great
treatment with sewage sludge . Potato pulp as asingle substrate was investigated in batch experimentswhich resulted in ultimate biodegradability in the range of8691% . A two-step process for treatment of potatopeelings consisting of liquefaction digesters coupled with afrom curve tting or a simple batch test and k results from plotting y/(ymy) against 1/OLR from long-term experiments. In the presentBiomass and Bioenergy 3
Kinetic study of thermophilic anfrom potat
Leibniz- Institute of Agricultural Engineering Borni
Received 14 May 2004; received in revised for
Anaerobic treatment of solid wastes from potato processing was
attention was paid to the effect of increased organic loading rate (O
and CH4 in the biogas decreased with the increase in OLR. For
obtained were 0.85 l g10.65 l g1 and 58%50%, respectively. Bio2006) 892896
erobic digestion of solid wastesprocessing
Max-Eyth-Allee 100, D-14469 Potsdam, Germany
3 February 2006; accepted 16 February 2006
4 April 2006
died in completely stirred tank reactors (CSTR) at 55 1C. Special) on the biogas yield in long-term experiments. Both biogas yield
R in the range of 0.8 gl1 d13.4 gl1 d1, biogas yield and CH4s yield y as a function of maximum biogas yield y , reaction rate
ARTICLE IN PRESS
k rst order reaction rate constant (d1)y VS biogas yield (l g1)ym maximum VS biogas yield (l g
Fig. 1. Schematic diagram of semi-continuous lab-scale experiments.
Chemical compositions of inoculum and substrates used
Parameter Inoculum Potato waste samples
1 2 3
pH 7.36 4.03 3.84 3.751
ioenpractice. 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 . 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 ll-and-draw mode with sixfeedings per week and mixed slowly of about 100min1 for15min every 2 h. (Fig. 1). For start-up the reactor wasinoculated with efuent of a 3200m3 reactor digesting solid
TS Total solids (g kg1)VS Volatile solids (g kg1)TKN Total Kjeldahl nitrogen (g kg1)NH4-N Ammonia-nitrogen (g kg
1)VFA (C2C6) Volatile fatty acids (g kg
1)pH pH-value (dimensionless)HRT Hydraulic retention time (d)OLR Organic (VS) loading rate (g l1 d1)
B. Linke / Biomass and Bwastes from a potato processing plant at an OLR of 1.5 kgVS m3 d1. Beginning with an OLR of 0.8 g VS 11 d1,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 0110 days, 111170 days and171205 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 theVR Volume of the reactor (l)m0 mass ow 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 l1 d1)
ergy 30 (2006) 892896 893reactor efuent 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 .
TS g l 22.3 142 113 128
VS g l1 14.2 131 101 119VFA g l1 3.44 1.98 2.73 2.20NH4-N g l
1 0.76 0.17 0.20 0.14TKN g l1 1.99 2.49 2.27 2.81
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 efuent (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 gl1 d1. This step resulted in adecreased biogas yield and increase of VFA. When OLR
By combining (1) and (2) with VR m0 HRT at steadystate for VR dc=dt 0 we obtain
HRT 1k c0
The overall correlation between substrate concentration cand biogas yield y at time t is shown in Fig. 4. Thebiodegradable fraction of the complex organic substrate isdisintegrated to biogas according to (4)
and (5), respectively,
ym y. (5)
When the term c0/c in (3) is replaced by ym=ym y thehydraulic retention time of a CSTR can be described by thefollowing equation:
HRT 1k y
y HRT k ymHRT k 1 , (7)
ARTICLE IN PRESS
B. Linke / Biomass and Bioenergy 30 (2006) 892896894was adjusted to 2.5 gl1 d1 VFA, biogas yield and VSconcentration in the efuent cE were observed to be about2.5 gl1 , 0.75 lg1 and 2.5%, respectively.The next increase in OLR to 3.2 gl1d1 resulted in a
further increase of VFA and VS concentration in theefuent 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 tting according to (9) and resultsto 0.88 l g VS1.
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 ow of input and output m0(mass of biogas is neglected) can be written as
dt m0 c0 m0 c VR rc. (1)
The substrate removal rate r(c) as a function of c isexpressed as rst order kinetic with
rc k c. (2)
0 50 100 150 200 250Time t (days)
); c E
-1 )Fig. 2. Semi-continuous reactor performance data y (J), OLR (), VFA
( ) and VS concentration (%) in the efuent cE (W).0.00
0.0 1.0 2.0 3.0 4.0OLR (gl-1d-1)
%)Fig. 3. Effect of OLR on y (J) and CH4 (+) from semi-continuous
experiments with residues from potato processing (c0 117710.5 g l1,35 1C), ym 0.88 l g1.
c0 - c (t)
tFig. 4. Correlation between substrate degradation and biogas production
in course of time.
respectively. For dimensioning the fermenter size of CSTRsboth the OLR and the HRT are the most appliedparameters in practice. With OLR c0/HRT equation(6) can be written as
OLR k c0yym y (8)
y ymk c0
k c0 OLR, (9)
respectively. For calculating the fermenter size by means ofHRT or OLR, detection of ym and k is essential. Whereasy yields from a simple batch test, detection of k can be
in the range of 0.83.4 gl1 d1, biogas yield decreases. Themaximum biogas yield ym can be obtained from curvetting of equation (9) at OLR 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 rst 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 specic 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 food
ARTICLE IN PRESSB. Linke / Biomass and Bioenm
obtained from long- term experiments in a CSTR. Thevalue of k can be obtained by plotting y=ym y 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 presults from (10) and (11), respectively.
HRT pk 1 p , (10)
p HRT kHRT k 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 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 117710.7 g l1, 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 tting on
0 20 40 60 80 100tm (d)
0.125 0.10.09 0.08
0.07 0.06k = 0.05 d-1Fig. 5. Absolute proportion p of ym for different values of HRT and k.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 ratethe base of (9) we obtain ym 0:88 lg1 for OLR 0 (Fig.3). The reaction rate constant k results from the plot ofy=ym y against 1/OLR and the slope of k c0 10:42 g l1 d1 to k 0:089 d1 (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).
0.0 0.4 0.8 1.2 1.61/BR (l d g-1)
Fig. 6. Graph of y/(ymy) 10.42 1/OLR (J) from experiments withresidues from potato processing (c0 117710.7 g l1, 35 1C), slope:k c0 10.42 g l1d1, k 0.089 d1.
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ARTICLE IN PRESSB. Linke / Biomass and Bioenergy 30 (2006) 892896896
Kinetic study of thermophilic anaerobic digestion of solid wastes from potato processingIntroductionMaterials and methodsSemi-continuously fed reactor experimentsAnalytical methods
Results and discussionSemi-continuous reactor experimentsDevelopment of the kinetic modelApplication of the kinetic model