Bioresource Technology 97 (2006) 47–56
Conversion of municipal solid waste to carboxylic acids
using a mixed culture of mesophilic microorganisms
Cateryna Aiello-Mazzarri a, Frank K. Agbogbo b, Mark T. Holtzapple
a
b,*
Universidad del Zulia, Facultad de Ingenierı́a, Coordinación de Ciclo Básico, Departamento de Quı́mica, Maracaibo, Venezuela
b
Texas A&M University, Department of Chemical Engineering, College Station, TX 77840-3122, USA
Received 22 October 2004; received in revised form 10 February 2005; accepted 11 February 2005
Available online 12 April 2005
Abstract
Waste biomass was anaerobically converted to carboxylate salts by using a mixed culture of acid-forming microorganisms. Municipal solid waste (MSW) was the energy source (carbohydrates) and sewage sludge (SS) was the nutrient source (minerals, metals,
and vitamins). Four fermentors were arranged in series and solids and liquids were transferred countercurrently in opposite directions, which allows both high conversions and high product concentrations. Fresh biomass was added to Fermentor 1 (highest carboxylic acid concentration) and fresh media was added to Fermentor 4 (most digested biomass). All fermentations were performed
at 40 C. Calcium carbonate was added to the fermentors to neutralize the acids to their corresponding carboxylate salts. Iodoform
was used to inhibit methane production and urea was added as a nitrogen source. Product concentrations were up to 25 g/L, with
productivities up to 1.4 g total acid/(L liquid d). Mass balances with closure between 93% and 105% were obtained for all systems.
Continuum particle distribution modeling (CPDM) was applied to correlate batch fermentation data to countercurrent fermentation
data and predict product concentration over a wide range of solids loading rates and residence times. CPDM for lime-treated MSW/
SS fermentation system predicted the experimental total acid concentration and conversion within 4% and 16% respectively.
2005 Published by Elsevier Ltd.
Keywords: MixAlco process; Municipal solid waste; Sewage sludge; Fermentation; Carboxylic acids; Mixed acids
1. Introduction
Many energy experts are concerned about the depletion of petroleum, the main source of liquid fuels and
many chemicals. Oil prices are volatile and supplies
are unstable. Oil combustion releases carbon dioxide,
the most important greenhouse gas implicated in global
warming (Sterzinger, 1995; Klass, 1998). In contrast,
biomass is a source of liquid fuels that does not result
in a net increase of carbon dioxide in the atmosphere
(Holtzapple et al., 1997; Hileman, 1999) because biomass growth removes from the atmosphere the same
*
Corresponding author. Tel.: +1 979 845 9708; fax: +1 979 845 6446.
E-mail addresses: [email protected] (C. Aiello-Mazzarri), m-holtzapple@
tamu.edu (M.T. Holtzapple).
0960-8524/$ - see front matter 2005 Published by Elsevier Ltd.
doi:10.1016/j.biortech.2005.02.020
amount of carbon dioxide that biomass combustion
generates (Bungay, 1981; Sterzinger, 1995).
Another environmental problem is the generation
and accumulation of the following wastes: Municipal
solid waste (MSW)—in the United States (US), approximately 220 million tons of MSW were generated
in 1998, four million tons more than in 1997 (EPA,
1998). Municipal solid waste contains about 65% biodegradable components, such as paper, food scraps, and
yard waste (EPA, 1998). For the purpose of this
research, we will refer to MSW as the biodegradable
organic fraction, which excludes glass, metal, plastic,
ceramic, rock, or soil also found in MSW. Sewage
sludge (SS)—SS is the residual solids from conventional
aerobic or anaerobic sewage treatment. The Environmental Protection Agency (EPA, 1999) estimated that
48
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
6.9 million tons of dry SS were generated in 1998 in the
US of which about 60% were land applied, composted,
or used as landfill cover. The remaining 40% was discarded with no attempt to recover nutrients.
MSW is an excellent energy source, but lacks nutrients. SS has nutrients, but lacks energy-yielding carbohydrates. In this research, MSW and SS were combined, so
they complement each other, making the process an
attractive alternative for managing two different streams
that are produced in every community.
Holtzapple et al. (1997) have developed a process to
obtain fuels and chemicals from biomass, the MixAlco
process (Fig. 1). In this process, biomass is first pretreated with lime, and then a mixed culture of acid-forming anaerobic microorganisms produces carboxylate salts.
These salts are subsequently concentrated and thermally
converted to mixed ketones and finally hydrogenated
to mixed alcohols. The advantages of this process
include the use of low-value substrates without the
use of a sterile environment or the need for enzyme addition.
A countercurrent fermentation (Fig. 2) allows the
least reactive biomass to contact the lowest carboxylic
acid concentration, which in batch fermentations could
not be digested because of accumulating carboxylic acid.
As the solids are transferred from one fermentor to the
next upstream fermentor (i.e., from F1 to F2, F2 to F3,
and F3 to F4), the biomass becomes less reactive and the
carboxylate salt concentration drops. This flow arrangement reduces the inhibitory effect from the accumulating
Waste
Biomass
Pretreatment
Fermentation
Concentration
Thermal
Conversion
Mixed
Alcohols
Hydrogenation
Fig. 1. MixAlco process.
carboxylate salts by adding fresh liquid to the most digested biomass. Both high conversions and high product
concentrations are possible by using countercurrent
operation.
Long residence times are associated with countercurrent fermentations. Obtaining a wide range of operating
conditions experimentally would be tremendously time
consuming. Mathematical modeling of the countercurrent fermentations was performed using continuum
particle distribution modeling (CPDM) developed by
Loescher (1996). CPDM can predict the operating conditions for the countercurrent fermentations from batch
fermentation data.
2. Methods
2.1. Fermentor
Each fermentor consisted of a 1-L plastic centrifuge
bottle (Beckman # 355676, Fisher, PA), which was
capped with a rubber stopper with a glass tube inserted
through the stopper (Ross, 1998). The glass tube was
capped with a rubber septum for gas sampling and volume measurement. A large hole was drilled in the centrifuge bottle cap, so that when the cap was replaced it
could fit over the glass tube, and still be tightened to
hold the stopper in the bottle. Inside the bottle there
were two stainless steel (0.25-in. welded 304) tubes with
welded ends that mixed the components of the fermentation. The fermentors were placed on a Wheaton Modular Cell Production Roller Apparatus (Model III,
Fisher, PA) located in an incubator and rotated at
1 rpm.
2.2. Substrates
2.2.1. Municipal solid waste (MSW)
To ensure uniform substrate composition through all
the MSW experiments, 136 kg of MSW were prepared.
To simulate the organic fraction of landfill waste (Holtz-
Product
Liquid
Fresh
Liquid
F1
Fresh
Biomass
F2
F3
F4
Product
Solid
Fig. 2. Four-stage countercurrent system fermentation (F1: Fermentor 1, F2: Fermentor 2, F3: Fermentor 3, and F4: Fermentor 4).
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
apple et al., 1992). The components of MSW were collected (Bryan, College Station, TX), sun-dried, ground
and passed through a 10-mm screen in a hammer mill
(Forest Science Research Laboratory, Texas A&M University). Once combined, the MSW was ground and
passed through a 6-mm screen to ensure a uniform mixture. Fats and oils were not added to the MSW to prevent spoiling during storage.
49
material from Bee Creek Park (College Station, TX),
and compost material from domestic and commercial
piles. The compost samples were taken from the middle
of the pile to ensure a large quantity of anaerobic microorganisms. To minimize exposure to oxygen, the swamp
material and compost were collected into bottles filled
with deoxygenated distilled water.
2.6. Methanogens inhibitor
2.2.2. Sewage sludge
Aerobically treated sewage sludge was obtained from
Bryan Wastewater Treatment Plant Number 3 (Bryan,
TX). The sewage had undergone activated sludge treatment in which the incoming sewage stream was sent to
an aeration basin for approximately 15 d. After leaving the aeration basin, the sludge was digested aerobically for approximately 40 d. Upon removal from
the digestor, the sludge was coagulated, dried for 10 d,
and ground in a hammer mill fitted with a 3-mm screen.
2.3. Lime treatment
The components of lignocellulose are complex and
include solubles, cellulose, hemicellulose, lignin, and
ash. As biomass digests, the remaining fraction is less
reactive (Gizjen et al., 1988), which is logical considering
that biomass is heterogeneous; some components are
highly digestible whereas others are almost inert. To
enhance digestibility, lime treatment was performed at
100 C for 1 h with loadings of 0.1 g Ca(OH)2/g dry biomass and 10 mL of distilled water/g dry biomass. After
treatment, CO2 was bubbled through the biomass slurry
to neutralize the lime and the slurry was dried at 105 C.
Detailed procedures of the lime treatment are available
in Aiello-Mazzarri (2002).
2.4. Media and nutrients
The fermentation media consisted of deoxygenated
distilled water, 0.28 g sodium sulfide/L distilled water,
and 0.28 g cysteine hydrochloride/L distilled water.
Dry nutrients were added to the fermentations. The
dry nutrient mixture used in all the experiments corresponds to the modified Caldwell and Bryant medium
(Caldwell and Bryant, 1966).
2.5. Inocula
Rumen fluid was used as the main inoculum. Rumen
contents were removed from a fistulated steer (University Nutrition and Field Laboratory, Texas A&M University), filtered through four layers of cheesecloth into
1-L propylene centrifuge bottles, transported to the laboratory, and inoculated (5% v/v) into each fermentor
within 1 h after collection to ensure microorganism viability. Other sources of microorganisms included swamp
Iodoform (CHI3) was used as a methanogen inhibitor
in all fermentations. An iodoform solution (20 g CHI3/L
ethanol) was added individually to each reactor continuously throughout the fermentations. Iodoform is light
and air sensitive, so the solution was kept in ambercolored glass bottles and special care was taken to
replace the cap immediately after use.
2.7. Batch experiments
Batch experiments were performed to determine
experimental reproducibility, and to collect the necessary data for continuum particle distribution modeling
(CPDM). These data were used to develop a model rate
equation for the countercurrent fermentation. The procedure involved operating five different fermentors with
different initial substrate concentrations (20, 40, 70,
100 g dry substrate/L liquid). An additional fermentor
with high substrate concentration (100 g dry substrate/
L liquid) and high carboxylic acid concentration
(approximately 20 g carboxylic acids/L liquid) was evaluated. Samples were taken by removing the fermentor
from the incubator, collecting the gas, removing the
stopper, purging with nitrogen, measuring pH, centrifuging for 10 min, collecting a liquid sample under nitrogen purge, resealing the fermentor, and placing it back
in the roller apparatus. For the CPDM experiments,
the resulting carboxylic acid concentrations were converted into acetic acid equivalents to be used in the
modeling equations developed by Loescher (1996).
The batch carboxylic acid concentrations can be converted to acetic acid equivalents (a)
a ðmol=LÞ ¼ acetic ðmol=LÞ
þ 1:75 propionic ðmol=LÞ
þ 2:5 butyric ðmol=LÞ
þ 3:25 valeric ðmol=LÞ
þ 4:0 caprioc ðmol=LÞ
þ 4:75 heptanoic ðmol=LÞ
ð1Þ
On mass basis, the acetic acid equivalent can be expressed as
Ae ðg=LÞ ¼ 60:05 ðg=molÞ a ðmol=LÞ
ð2Þ
50
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
2.8. Countercurrent fermentations
In countercurrent operation, liquid and solids flow in
opposite directions in four-fermentor trains. In the laboratory scale, the fermentors operate in a semi-continuous
manner, whereas on an industrial scale, the fermentation
would be continuous. Countercurrent fermentations were
initiated as batch cultures under anaerobic conditions by
adding the substrates, calcium carbonate, urea, nutrients,
and inocula to deoxygenated water media in each fermentor. The experiments were conducted as batch fermentations until the culture was established (7–10 d).
Countercurrent operation was initiated with the transfer
of liquid and solids occurring every 1, 2, or 3 d. Countercurrent fermentations were conducted at varying liquid
residence times (LRT) and volatile solid loading rate
(VSLR). The operating parameters for each fermentation
with lime-treated MSW/SS are shown in Table 1.
2.9. Reaction conditions
The fermentations were performed under anaerobic
conditions at 40 C. Every 3 d, after each liquid/solid
transfer, 2.0 g calcium carbonate was added to each fermentor to neutralize carboxylic acids. To maintain
anaerobic conditions, nitrogen from a high-pressure
liquid-nitrogen cylinder (Praxair, Bryan, TX) was flushed
whenever the fermentors were open to the atmosphere.
The solid and liquid transfer procedures are detailed in
Aiello-Mazzarri (2002).
2.10. Analytical methods
The fermentor broth was analyzed by gas chromatography to measure the concentration of carboxylic acids.
Table 1
Operating parameters for lime-treated MSW/SS countercurrent
fermentations
Fermentation trains
A
B
C
D
E
LRT (d)
VSLR (g VS/(L liquid in
all fermentors d))
VS feed at each
transfer (g VS)
Solid feed at each
transfer (g dry)
Liquid feed to F4 at
each transfer (L)
Frequency of transfer
Iodoform addition rate
(mg iodoform added/L
liquid fed to F4)
Nutrients addition rate
(g dry nutrients added/L
liquid fed to F4)
Urea addition rate
(g urea added/L
liquid feed to F4)
20.7
3.0
19.0
4.0
22.7
8.1
20.7
3.1
17.0
6.4
11.6
19.3
24.1
11.6
15.4
14.4
24.0
30.0
14.4
19.2
0.15
0.20
0.10
0.15
5.33
8
1.33
1.00
2.00
1.33
2.00
1.0
0.75
1.5
1.0
1.5
Every 3 d
8
5.33
0.10
8
The broth was mixed with equal parts of an internal
standard (4-methyl-n-valeric acid) and 3-M H3PO4.
The analysis was performed using an Agilent 6890 series
gas chromatograph (Agilent Technologies, Palo Alto,
CA) equipped with a flame ionization detector (FID)
and a 7683 series injector. A 30-m fused-silica capillary
column (J&W Scientific, Model # 123-3232 CX, Agilent
Technologies, CA) was used. The column head pressure
was maintained between 90 and 103 kPa (13–15 psig).
At every sample injection, the gas chromatograph temperature program allowed the temperature to rise from
50 C to 200 C at a 20 C/min rate. The temperature
was subsequently held at 200 C for 10 min. Helium
was used as carrier gas, and the total run time per sample was 17 min. The fermentation broth consists of a
mixture of carboxylate salts and carboxylic acids. This
analytical procedure converts all salts to their corresponding acids, allowing product concentrations to be
reported as g carboxylic acid/L.
Gases produced during fermentation were accumulated within the reactor. Every sampling day, the volume
of gas produced since the last transfer session was measured. The volume was measured by displacing water in
an inverted glass graduated cylinder apparatus that was
filled with 30% CaCl2 solution. The CaCl2 minimized
microbial growth in the water tank, and reduced water
evaporation. The CaCl2 solution had an acidic pH
(5.6), which prevented CO2 adsorption. To check methanogen inhibition, every 2 or 3 d the gas from the
fermentation was analyzed for CH4 by gas chromatography. A 5-mL sample was taken through the reactor
septum and analyzed using an Agilent 6890 series gas
chromatograph. The chromatograph was equipped with
a thermal conductivity detector (TCD). A 4.6-m stainless steel packed column with 2.1-mm ID (60/80 Carboxen 1000, Supelco 1-2390 U, Agilent Technologies,
CA) was used. Samples were injected manually. The
inlet temperature was fixed at 230 C, and the detector
temperature was set at 250 C. The oven temperature
was maintained at 225 C for 5 min. Helium was used
as carrier gas. The total elution time for a sample was
5 min.
Volatile solids in the initial substrates and solid fermentation residues were determined by first drying the
material at 105 C and then ashing the material at
550 C for at least 3 h. Volatile solid determination
in the liquid fermentation broth followed the same
heating procedure above; except prior to drying, the
liquid was mixed with lime to ensure that the carboxylic acids would not volatize and alter the measurement.
2.11. Mass balance
For all the countercurrent fermentation experiments,
a complete mass balance was obtained on the entire
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
train during a steady-state period. Closure is defined as
follows:
conversion ðxÞ ¼
VS digested
VS fed
closure ¼
mass out
mass in þ water of hydrolysis
closure ¼
undigested VS þ dissolved VS þ carboxylic acids produced þ biotic CO2 þ CH4
mass in þ water of hydrolysis
ð3Þ
To calculate the water of hydrolysis, it was assumed that
the biomass could be represented as cellulose, which has
a monomer weight of 162 g/mol. When cellulose is
hydrolyzed, it gains one molecule of water per monomer; therefore, the water of hydrolysis is calculated as
18
water of hydrolysis ¼ VS digested 162
ð5Þ
yield ðyÞ ¼
total carboxylic acids produced
VS fed
total acid selectivity ðsÞ
total carboxylic acids produced
¼
VS digested
51
ð9Þ
ð10Þ
ð4Þ
ð11Þ
total acid productivity ðpÞ
¼
total carboxylic acids produced
L liquid in all reactors time
ð12Þ
2.13. Statistical analyses
2.12. Operational parameters
The liquid residence time determines how long the
liquid remains in the system, which affects the final product concentration. Long liquid residence times allow
high product concentrations whereas shorter liquid
residence times allow lower product concentrations
(Holtzapple et al., 1999). Liquid residence time is calculated as
TLV
liquid residence time ðLRTÞ ¼
ð6Þ
Q
where, Q = flowrate of liquid out of the fermentor set
(L/d), TLV = total liquid volume.
X
TLV ¼
ðK i w þ F i Þ
ð7Þ
The statistical analyses were performed using Excel
software. Analysis of variance for fermentations A and
D were performed using the student t-test at 5% level
of significance. The mean and standard deviations of
the total acid concentrations for fermentations A and
D were determined from the steady-state operational
data from days 126 to 257. Correlations between volatile
solid loading rate (VSLR) and acid productivity (p),
selectivity (s), yield (y), and conversion (x) were obtained by fitting experimental results to linear equations
using Excel software.
3. Results and discussion
i
where, K i ¼ average wet mass of solid cake in Fermentor
i ðgÞ; w = average liquid fraction of solid cake in
Fermentor i (L liquid/g wet cake); F i ¼ average
volume of free liquid in Fermentor i ðLÞ.
The volatile solids loading rate is calculated as
volatile solids loading rate ðVSLRÞ ¼
VS fed=d
TLV
ð8Þ
At a low VSLR, the solid residence time increases,
allowing for more complete digestion.
Biomass is composed of volatile solids (VS) and ash,
and except for the lignin most VS are reactive. The
digestion process converts part of the VS into gas and
liquid products, with some solids remaining undigested.
In the liquid products, VS consist of carboxylic acids,
extracellular proteins and energy-storage polysaccharides (Ross, 1998). The following terms are used
throughout this paper.
3.1. Batch reproducibility
For the following experiments, simulated MSW and
SS were combined in an 80:20 ratio. To determine the
reproducibility of MSW/SS batch experiments, two sets
of three fermentors were operated simultaneously under
identical conditions. Untreated MSW/SS was used as
the substrate for one set, whereas lime-treated MSW/
SS was used for the other. The fermentors contained
the substrate at a concentration of 100 g dry substrate/
L liquid. So that the microorganisms would already be
adapted to the substrate, the inoculum used in these
experiments was inocula from countercurrent experiments with the same substrate.
The initial pH for all fermentations was approximately
6.5. During the fermentations, the pH varied from 5.4 to
6.5. The off-gas was analyzed for methane every other
day, but no methane was found. The combination of
52
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
18
16
Total carboxylic acid
concentration (g/L)
14
12
10
8
6
4
Treated MSW/SS
Untreated MSW/SS
2
0
0
2
4
6
8
10
12
14
16
18
20
Time (days)
Fig. 3. Average carboxylic acid for untreated MSW/SS and treated
MSW/SS batch fermentations at 100 g substrate/L liquid, error bars ±1
standard deviation.
low pH and iodoform addition effectively inhibited methanogenesis. Average total carboxylic acids concentrations for the untreated and lime-treated MSW/SS are
shown in Fig. 3 along with one-standard-deviation errors
bars. The variation was less as the beginning and
increases as the fermentation progresses. The largest
standard deviation was 0.46 g/L untreated MSW/SS
and 0.62 g/L for treated MSW/SS. This experiment demonstrated that batch experiments using lime-treated or
untreated simulated MSW/SS as substrate system were
very reproducible.
3.2. Countercurrent fermentations
The results from the lime-treated countercurrent fermentations are shown in Table 2. Fermentations A and
D were initiated using the same substrate (80% lime-
treated MSW/20% SS) and the same inocula mixture.
Both fermentations were started on the same day and
were operated identically to determine the reproducibility of the countercurrent fermentations.
In Fig. 4, the total carboxylic acid concentration as a
function of time during fermentations A and D are presented. Both fermentations have the same behavior
through the operation time (258 d). A t-test was conducted over the steady-state operation (days 126–257),
and the results showed that there were no significant
differences between these data set at 5% level of significance. The results show excellent reproducibility for
countercurrent fermentations.
Fermentation C had the highest total carboxylic acid
concentration obtained for lime-treated MSW/SS
(26.0 g carboxylic acids/L liquid) which surpassed the
economic goal of 22 g carboxylic acid/L liquid (Holtzapple et al., 1999). This fermentation was operated at pH
5.7, LRT = 22.7 d, and VSLR = 8.1 g VS/(L liquid d)
for 403 d, and also had the highest acid productivity
(1.36 g total acids/(L liquid d)). The conversion was
0.302 g VS digested/g VS fed, the yield was 0.18 g carboxylic acid/g VS digested, and the selectivity was
0.577 g VS digested/g VS fed. Fermentation D (pH
5.9, LRT = 20.7 d, and VSLR = 3.1 g VS/(L liquid d))
had the highest conversion (0.449 g VS digested/g VS
fed). The total carboxylic acid productivity was 1.12 g
total acids/(L liquid d), the yield was 0.264 g carboxylic
acid/g VS digested, and the total carboxylic acid productivity was 0.577 g total acids/(L liquid d).
The correlations between volatile solid loading rate
(VSLR) and acid productivity (p), selectivity (s), yield
(y), and conversion (x) are shown in Figs. 5–8. The results show that at higher volatile solids loading rates,
higher total acid productivities are obtained. Conversion, selectivity, and yield decrease with increasing
Table 2
Results for lime-treated MSW/SS countercurrent fermentations
Fermentation trains
A
B
C
D
E
Average pH in all fermentors
Total carboxylic acid concentration (g/L)
Acetic acid (wt%)
Propionic acid (wt%)
Butyric acid (wt%)
Valeric acid (wt%)
Caproic acid (wt%)
Heptanoic acid (wt%)
Conversion (g VS digested/g VS fed)
Yield (g total acids/g VS fed)
Selectivity (g total acids/g VS digested)
Total carboxylic acid productivity (g total acids/(L liquid d))
Biotic CO2 productivity (g CO2/(L liquid d))
Methane productivity (g CH4/(L liquid d))
Mass balance closure (g VS digested/g VS in)
5.96 ± 0.15
16.90 ± 0.77
39.57 ± 4.72
15.70 ± 3.57
23.31 ± 5.00
8.04 ± 1.41
9.47 ± 2.49
1.61 ± 0.86
0.432
0.276
0.639
0.832
0.390
0.002
0.96
5.79 ± 0.02
16.29 ± 1.52
40.55 ± 4.30
14.18 ± 2.10
21.33 ± 3.71
7.77 ± 1.44
12.33 ± 2.96
2.64 ± 1.27
0.366
0.221
0.603
0.886
0.251
0.001
101.9
5.68 ± 0.06
25.99 ± 1.46
40.40 ± 3.10
14.17 ± 1.40
22.15 ± 2.99
6.97 ± 0.96
11.09 ± 2.54
1.41 ± 0.74
0.302
0.175
0.577
1.360
0.372
0.007
100.3
5.93 ± 0.20
17.07 ± 0.99
45.06 ± 4.48
17.81 ± 2.39
18.69 ± 3.63
7.35 ± 1.49
6.98 ± 2.22
1.70 ± 1.12
0.449
0.264
0.589
0.829
0.290
0.002
0.93
5.85 ± 0.11
18.62 ± 0.97
43.89 ± 4.40
12.24 ± 2.21
22.08 ± 3.31
7.25 ± 1.01
9.35 ± 2.82
1.45 ± 0.83
0.392
0.237
0.558
1.125
ND
ND
ND
Note: All errors are ±1 standard deviation.
ND = not determined.
8 mg iodoform/L
fed to F4
Total carboxylic acid
concentration (g/L)
25
20
15
Fermentation A
Fermentation D
Iodoform/Bromoform
8 mg/L fed to F4
10
Steady-state operation
Switch to 4 mg iodoform/L
fed to F4
5
Conversion
(g VS digested/g VS fed)
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
53
0.7
0.6
0.5
0.4
0.3
Experimental data
Best line fit
0.2
0.1
0
0
2
4
6
8
10
VSLR (g VS/(L liquid in all fermentors·d))
Fig. 8. Correlation of conversion with volatile solid loading rate.
0
10
30
50
70
90
110 130 150 170 190 210 230 250 270
Time (days)
Fig. 4. Total acid concentration for lime-treated MSW/SS fermentation A and D.
p ¼ 0:0965VSLR þ 0:5317
ð13Þ
s ¼ 0:0090VSLR þ 0:6274
ð14Þ
1.5
x ¼ 0:0263VSLR þ 0:5037
ð15Þ
1
y ¼ 0:0178VSLR þ 0:3086
ð16Þ
Experimental data
Total acid productivity
(g total acids/g/(L liquid·d))
2
Best Line Fit
0.5
0
0
2
4
6
8
10
VSLR (g VS/(L liquid in all fermentors·d))
Fig. 5. Correlation of total acid productivity with volatile solid
loading rate.
Selectivity
(g total acids/g VS digested)
VSLR. These correlations are described by the following
equations:
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Experimental data
Best line fit
0
2
4
6
8
10
VSLR (g VS/(L liquid in all fermentors·d))
Yield
(g total acids/g VS fed)
Fig. 6. Correlation of selectivity with volatile solid loading rate.
0.4
0.3
0.2
Experimental data
Best line fit
0.1
0
0
2
4
6
8
10
VSLR (g VS/(L liquid in all fermentors·d))
Fig. 7. Correlation of yield with volatile solid loading rate.
The substrate system, lime-treated MSW/SS, is a heterogeneous lignocellulosic material in which some of the
components are more digestible by the microorganisms
(Ross, 1998). MSW is the source of carbon and energy,
and SS is the source of micronutrients. At high VSLR,
more substrate is available for the microorganisms,
which means that more nutrients and easily digested
substrate are present. The mixed culture of microorganisms digested the primarily accessible fraction of the
substrate; however, less carboxylic acids were produced
per unit of digested biomass. Part of the digested substrate is non-acid soluble volatile solids (dissolved VS).
When substrate and nutrients are abundantly available,
the microorganisms may produce energy-storage compounds that can be used in periods of starvation as suggested by Domke (1999).
However, at low VSLR, acid productivity was lower,
but conversion, selectivity, and yield were higher. At
lower VSLR, less substrate was added, so fewer nutrients and less easily digestible fractions of the substrate
were available for the microorganisms. As a consequence, the microorganisms not only would consume
the more-available fractions, but also the less-available
fractions of the substrate. In natural environments, such
as rumen, substrate concentrations are low, and therefore not enough for the microorganisms to reach maximum growth rates (Russell and Baldwin, 1979). At these
conditions, per unit of digested biomass, the microorganisms may produce more carboxylic acid and less
energy-storage components.
The CPDM method developed by Loescher (1996)
has been used to predict the product acid concentration
and conversions for countercurrent fermentation systems (Ross, 1998; Domke, 1999; Thanakoses, 2002;
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
Chan, 2002; Aiello-Mazzarri, 2002). CPDM utilizes
batch data to predict the results of the countercurrent
fermentations.
The acetic acid equivalents (Ae) from each of the five
batch experiment was fit to the equation
Ae ¼ a þ
bt
1 þ ct
ð17Þ
where t is the time (d) of fermentation, and a, b, and c
are constants fit by least squares analysis. The rate r
was obtained from Eq. (18).
r¼
dðAe Þ
b
¼
dt
ð1 þ ctÞ2
ð18Þ
The specific rate, ^r (g Ae produced/(g VS d)) was determined from Eq. (18) by dividing it by the initial amount
of substrate concentration, S0 (g VS/L) in each of the
five fermentors
r
^r ¼
ð19Þ
S0
The predicted rate, ^rpred , was obtained from Eq. (20);
where the rate of acid production depends on volatile
solids conversion (x) and product concentration (Ae)
eð1 xÞ
^rpred ¼
f
h
1 þ g½/Ae ð20Þ
Least square analysis was used to determine the empirical parameter constants e, f, g, and h for ^rpred (Eq.
(20)) from the specific rate ^r (Eq. (19)). Ae was converted
back to carboxylic acid concentration by / (the ratio of
total grams of actual acids to grams of Ae). The (1 x)
term in the numerator of Eq. (20), is the conversion penalty function by South and Lynd (1994).
The conversion, x was calculated using
xðtÞ ¼
Ae TLV
M0 r
ð21Þ
where, r = selectivity (g Ae produced/g VS digested);
M0 = the initial substrate mass (g VS).
The selectivity r, for Eq. (21) was calculated from the
selectivity s (g total acids produced/g VS digested) determined in the countercurrent experiment.
s ¼ /r
From the CPDM Mathematica program, the values
of the predicted total carboxylic acid concentration
and conversion for the countercurrent fermentation at
various LRT and VSLR were obtained. CPDM predictions for conversions and product concentrations for the
lime-treated MSW/SS countercurrent fermentations
(substrate = 200 g VS/L liquid) are presented in the
‘‘map’’ in Fig. 9. The experimental carboxylic acid concentrations and conversions from the lime-treated
MSW/SS countercurrent fermentations are compared
with the predicted values from the CPDM ‘‘map’’
shown in Fig. 9. As shown in Table 3, the average absolute error between the experimental and predicted
total carboxylic acid concentration and conversion
for lime-treated fermentations was 4.4% and 15%,
respectively.
The error between the predicted conversion, xpred and
the experimental conversion, xexp as a function of selectivity is shown in Fig. 10. Linear regression of the data
gave the following correlation:
Error ¼
ð22Þ
The selectivity (s) used in the CPDM Mathematica program was 0.60 g total acids/g VS digested, the average
value of selectivity for the lime-treated MSW/SS countercurrent fermentations with terrestrial inocula. Selectivity slightly decreases with increasing VSLR (Fig. 6)
and was described by Eq. (14). Eq. (20) was used in
a Mathematica program (Aiello-Mazzarri, 2002) to
predict acetic acid equivalent concentration (Ae) and
conversion (x) for the countercurrent fermentation at
various VSLR and LRT. Ae was converted back to
carboxylic acid concentration by multiplying by /.
xpred xexp
¼ 2:663s 1:418
xexp
ð23Þ
Future improvements to the CPDM model should
incorporate a varying selectivity, r, to better describe
the countercurrent fermentation process.
Higher substrate concentrations would be allowed if
the process were done on a large scale (Holtzapple
et al., 1999). A higher VS concentration should result in
higher total carboxylic acid concentrations. CPDM
was used to predict the conversion and product concentrations for a VS concentration of 300 g VS/L liquid; the
CPDM ‘‘map’’ is shown in Fig. 11. As observed in the
CPDM ‘‘map,’’ total acid concentrations as high as
40 g/L can be reached at LRT of 35 d and VSLR of
12 g/(L d). Also, conversions as high as 82% can be
achieved at LRT of 10 d and VSLR of 2 g/(L d). Both,
high conversions (>70%) and high product concentrations (>30 g/L) can be achieved at LRT of 35 d and
VSLR between 2 and 3 g/(L d).
35
Total carboxylic acid
concentration (g/L)
54
30
LRT (days)
25
35
30
20
25
15
12
10
VSLR
(g/(L·d))
5
8
6
5
15
4
3
2 10
0
0
0.2
0.4
0.6
0.8
1
Conversion (g VS digested/g VS fed)
Fig. 9. CPDM ‘‘map’’ for lime-treated MSW/SS countercurrent
fermentation (200 g VS/L liquid).
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
55
Table 3
Comparison of experimental and predicted carboxylic acid concentration and substrate conversion for lime-treated MSW/SS countercurrent
fermentations A–F
Fermentation trains
A
B
LRT (d)
20.7
19.0
VSLR (g VS/L d)
3.0
4.0
Experimental carboxylic acid concentration (g/L)
16.90
16.29
Predicted (CPDM) carboxylic acid concentration (g/L)
16.12
17.54
4.6
7.7
Errorb (%)
Experimental conversion
0.43
0.37
Predicted (CPDM) conversion
0.54
0.49
Errorb
20.3
24.3
P
a
Average absolute error ¼ ðjðpredicted experimentalÞ=experimentaljÞ=6.
b
Error = ((predicted experimental)/experimental) · 100.
D
E
F
22.7
8.1
25.99
23.53
9.5
0.30
0.29
3.3
20.7
3.1
17.07
17.10
0.2
0.45
0.56
24.3
17
6.4
18.62
18.39
1.2
0.39
0.36
7.7
19.7
5.7
20.66
19.97
3.3
0.33
0.38
15.1
30
4.4
15.8
60
20
15
10
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Selectivity
(g total acid/g VS digested)
Total carboxylic
acid concentration (g/L)
25
Error (%)
Averagea (%)
C
Mesophilic
-------- Thermophilic
50
40
30
LRT (days)
35
30
20
25
12
VSLR
(g/(L·d))
10
8
15
6 6 5
4 3
4
10
3
2
0
Fig. 10. Correlation of error between experimental and predicted
conversion and selectivity for lime-treated MSW/SS fermentations
with terrestrial inocula.
0.2
0.4
0.6
0.8
1
Conversion (g VS digested/g VS fed)
Fig. 12. CPDM ‘‘maps’’ for lime-treated MSW/SS four-stage countercurrent fermentation under mesophilic and thermophilic conditions
(80/20 MSW to SS ratio, and 300 g VS/L liquid for both conditions.
Thermophilic data from Chan, 2002).
45
Total carboxylic acid
concentration (g/L)
0
40
35
LRT (days)
30
4. Conclusions
35
30
25
20
25
15
VSLR
(g/(L·d)
10
12
8
6 5
15
4
10
3
2
5
0
0
0.2
0.4
0.6
0.8
1
Conversion (g VS digested/g VS fed)
Fig. 11. CPDM ‘‘map’’ for lime-treated MSW/SS countercurrent
fermentation for substrate concentration 300 g VS/L liquid.
The CPDM ‘‘maps’’ for lime-treated MSW/SS countercurrent fermentations under mesophilic and thermophilic conditions (Chan, 2002) at 300 g substrate/L
are compared in Fig. 12. For the same LRT and VSLR
lower than 8 g VS/(L d), using mesophilic conditions
(40 C) higher acid concentrations and conversions can
be achieved than with thermophilic conditions (55 C).
At high VSLR using thermophilic conditions, acid concentrations as high as 60 g acids/L can be obtained, but
conversions may be lower than 40%.
The batch and countercurrent fermentations using
lime-treated simulated MSW/SS as substrate were very
reproducible. MSW/SS countercurrent fermentations
have been well characterized, and countercurrent fermentations can be operated for extended periods of time
under steady-state conditions. Addition of 4–8 mg iodoform/(L liquid added to F4) inhibited methane generation. The highest total carboxylic acid concentration
obtained for lime-treated MSW/SS with terrestrial inocula was 26.0 g carboxylic acids/L liquid, which surpassed the economic goal of 22 g carboxylic acid/L
liquid (Holtzapple et al., 1999). The fermentation was
operated at pH 5.7, LRT = 22.7 d and VSLR = 8.1 g
VS/(L liquid d) for 403 d. Industrial-scale fermentors
with higher substrate concentrations would achieve
higher product acid concentrations and conversions.
The CPDM model predicts product concentrations
and conversions within 4% and 16%, respectively. The
CPDM model was a useful tool in predicting product
concentration and conversion. The predicted values
56
C. Aiello-Mazzarri et al. / Bioresource Technology 97 (2006) 47–56
can be used to estimate the costs and process economics
of industrial-scale fermentors.
References
Aiello-Mazzarri, C., 2002. Conversion of municipal solid waste to
carboxylic acids by anaerobic countercurrent fermentation. Ph.D.
Dissertation. Texas A&M University, College Station, Texas.
Bungay, H.R., 1981. Energy, the Biomass Options. John Wiley &
Sons, New York.
Caldwell, D.R., Bryant, M.P., 1966. Medium without rumen fluid for
non-selective enumeration and isolation of rumen bacteria. Appl.
Microbiol. 14, 794–801.
Chan, W., 2002. Thermophilic anaerobic fermentation of waste
biomass for producing acetic acid. Ph.D. Dissertation. Texas
A&M University, College Station, Texas.
Domke, S.B., 1999. Fermentation of industrial biosludge, paper fines,
bagasse, and chicken manure to carboxylate salts. Ph.D. Dissertation. Texas A&M University, College Station, Texas.
EPA, 1998. Municipal solid waste generation, recycling and disposal in
the United States: facts and figures for 1998. US EPA 530-F-00024, Washington, DC. Environmental Protection Agency. Available from: <http://www.epa.gov/garbage/pubs/mswfinal.pdf>.
EPA, 1999. Biosolids generation, use and disposal in the United States,
1977. US EPA 530-R-99-009, Washington, DC. Environmental
Protection Agency. Available from: <http://www.epa.gov/oigearth/
ereading_room/BIOSOLIDS_FINAL_REPORT.pdf>.
Gizjen, H.J., Zwart, K.B., Teunissen, M.J., Voegels, G.D., 1988.
Anaerobic digestion of cellulose fraction of domestic residue by
means of rumen microorganisms. Biotechnol. Bioeng. 32, 749–755.
Hileman, B., 1999. Case grows for climate change. Chem. Eng. News
77, 16–23.
Holtzapple, M.T., Lundeen, J.E., Sturgiss, R., Lewis, J.E., Dale, B.E.,
1992. Pretreatment of lignocellulosic municipal solid waste by
ammonia fiber explosion (AFEX). Appl. Biochem. Biotechnol. 34,
5–21.
Holtzapple, M.T., Ross, M.K., Chang, N.S., Chang, V.S., Aldelson,
S.K., Brazel, C., 1997. Biomass Conversion to Mixed Alcohol
Fuels Using the MixAlco Process. In: Saha, B.C., Woodward, J.
(Eds.), ACS symposium series 666. ACS, Washington, DC, pp.
130–142.
Holtzapple, M.T., Davison, R.R., Ross, K., Aldrett-Lee, S., Nagwani,
M., Lee, C.M., Lee, C., Adelson, S., Karr, W., Gaskin, D., Shiraga,
H., Chang, N.S., Chang, S., Loescher, M., 1999. Biomass conversion to mixed alcohol fuels using the MixAlco process. Appl.
Biochem. Biotechnol. 77–79, 609–631.
Klass, D.L., 1998. Biomass for Renewable Energy, Fuels, and
Chemicals. California Academic Press, San Diego, pp. 651–653.
Loescher, M.E., 1996. Volatile fatty acids fermentation of biomass and
kinetic modeling using the CPDM method. Ph.D. Dissertation.
Texas A&M University, College Station, Texas.
Ross, M.K., 1998. Production of acetic acid from waste biomass.
Ph.D. Dissertation. Texas A&M University, College Station,
Texas.
Russell, J.B., Baldwin, R.L., 1979. Comparison of substrate affinities
among several rumen bacteria: a possible determinant of rumen
bacterial competition. Appl. Environ. Microbiol. 37, 531–536.
South, C.R., Lynd, L.R., 1994. Analysis of conversion of particulate
biomass to ethanol in continuous solids retaining and cascade
bioreactors. Appl. Biochem. Biotechnol. 45–46, 467–481.
Sterzinger, G., 1995. Making biomass energy a contender. Technol.
Rev. 98, 34–40.
Thanakoses, P., 2002. Conversion of bagasse and corn stover to mixed
carboxylic acids using a mixed culture of mesophilic microorganisms. Ph.D. Dissertation. Texas A&M University, College Station,
Texas.