Aiello Agbogbo y Hotzapple BTech 2006

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Conversion of municipal solid waste to carboxylic acidsusing a mixed culture of mesophilic microorganisms

Cateryna Aiello-Mazzarri a, Frank K. Agbogbo b, Mark T. Holtzapple b,*

a Universidad del Zulia, Facultad de Ingeniera, Coordinacion de Ciclo Basico, Departamento de Qumica, Maracaibo, Venezuelab 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. Muni-

cipal 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 direc-

tions, which allows both high conversions and high product concentrations. Fresh biomass was added to Fermentor 1 (highest car-

boxylic 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 deple-

tion 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 globalwarming (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 bio-

mass growth removes from the atmosphere the same

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), ap-

proximately 220 million tons of MSW were generated

in 1998, four million tons more than in 1997 (EPA,1998). Municipal solid waste contains about 65% biode-

gradable 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 Environ-

mental Protection Agency (EPA, 1999) estimated that

0960-8524/$ - see front matter 2005 Published by Elsevier Ltd.

doi:10.1016/j.biortech.2005.02.020

* 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).

Bioresource Technology 97 (2006) 4756
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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apple et al., 1992). The components of MSW were col-

lected (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 Uni-

versity). Once combined, the MSW was ground and

passed through a 6-mm screen to ensure a uniform mix-

ture. Fats and oils were not added to the MSW to pre-vent spoiling during storage.

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 treat-

ment in which the incoming sewage stream was sent to

an aeration basin for approximately 15 d. After leav-

ing the aeration basin, the sludge was digested aero-

bically 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 bio-

mass and 10 mL of distilled water/g dry biomass. After

treatment, CO2 was bubbled through the biomass slurryto 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 corre-

sponds 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 (Univer-

sity Nutrition and Field Laboratory, Texas A&M Uni-

versity), filtered through four layers of cheesecloth into

1-L propylene centrifuge bottles, transported to the lab-

oratory, and inoculated (5% v/v) into each fermentor

within 1 h after collection to ensure microorganism via-

bility. Other sources of microorganisms included swamp

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 micro-

organisms. To minimize exposure to oxygen, the swamp

material and compost were collected into bottles filled

with deoxygenated distilled water.

2.6. Methanogens inhibitor

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 contin-

uously throughout the fermentations. Iodoform is light

and air sensitive, so the solution was kept in amber-

colored 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 neces-

sary data for continuum particle distribution modeling

(CPDM). These data were used to develop a model rate

equation for the countercurrent fermentation. The pro-

cedure 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 eval-

uated. Samples were taken by removing the fermentorfrom the incubator, collecting the gas, removing the

stopper, purging with nitrogen, measuring pH, centri-

fuging for 10 min, collecting a liquid sample under nitro-

gen purge, resealing the fermentor, and placing it back

in the roller apparatus. For the CPDM experiments,

the resulting carboxylic acid concentrations were con-

verted into acetic acid equivalents to be used in the

modeling equations developed by Loescher (1996).

The batch carboxylic acid concentrations can be con-

verted to acetic acid equivalents (a)

a mol=L acetic mol=L

1:75 propionic mol=L

2:5 butyric mol=L

3:25 valeric mol=L

4:0 caprioc mol=L

4:75 heptanoic mol=L 1

On mass basis, the acetic acid equivalent can be ex-

pressed as

Ae g=L 60:05 g=mol a mol=L 2

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2.8. Countercurrent fermentations

In countercurrent operation, liquid and solids flow in

opposite directions in four-fermentor trains. In the labo-

ratory scale, the fermentors operate in a semi-continuous

manner, whereas on an industrial scale, the fermentation

would be continuous. Countercurrent fermentations wereinitiated as batch cultures under anaerobic conditions by

adding the substrates, calcium carbonate, urea, nutrients,

and inocula to deoxygenated water media in each fer-

mentor. The experiments were conducted as batch fer-

mentations until the culture was established (710 d).

Countercurrent operation was initiated with the transfer

of liquid and solids occurring every 1, 2, or 3 d. Counter-

current 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 fer-

mentor 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 chromatog-

raphy to measure the concentration of carboxylic acids.

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, AgilentTechnologies, CA) was used. The column head pressure

was maintained between 90 and 103 kPa (1315 psig).

At every sample injection, the gas chromatograph tem-

perature 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 sam-

ple was 17 min. The fermentation broth consists of a

mixture of carboxylate salts and carboxylic acids. This

analytical procedure converts all salts to their corre-

sponding acids, allowing product concentrations to be

reported as g carboxylic acid/L.

Gases produced during fermentation were accumu-

lated within the reactor. Every sampling day, the volume

of gas produced since the last transfer session was mea-

sured. 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 meth-

anogen inhibition, every 2 or 3 d the gas from the

fermentation was analyzed for CH4 by gas chromatog-

raphy. A 5-mL sample was taken through the reactor

septum and analyzed using an Agilent 6890 series gaschromatograph. The chromatograph was equipped with

a thermal conductivity detector (TCD). A 4.6-m stain-

less steel packed column with 2.1-mm ID (60/80 Carbo-

xen 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 fer-

mentation 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 carbox-

ylic acids would not volatize and alter the measure-

ment.

2.11. Mass balance

For all the countercurrent fermentation experiments,

a complete mass balance was obtained on the entire

Table 1

Operating parameters for lime-treated MSW/SS countercurrent

fermentations

Fermentation trains A B C D E

LRT (d) 20.7 19.0 22.7 20.7 17.0

VSLR (g VS/(L liquid in

all fermentors d))

3.0 4.0 8.1 3.1 6.4

VS feed at each

transfer (g VS)

11.6 19.3 24.1 11.6 15.4

Solid feed at each

transfer (g dry)

14.4 24.0 30.0 14.4 19.2

Liquid feed to F4 at

each transfer (L)

0.15 0.20 0.10 0.15 0.10

Frequency of transfer Every 3 d

Iodoform addition rate

(mg iodoform added/L

liquid fed to F4)

5.33 8 8 5.33 8

Nutrients addition rate

(g dry nutrients added/L

liquid fed to F4)

1.33 1.00 2.00 1.33 2.00

Urea addition rate

(g urea added/L

liquid feed to F4)

1.0 0.75 1.5 1.0 1.5

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train during a steady-state period. Closure is defined as

follows:

closure mass out

mass in water of hydrolysis3

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 mono-

mer; therefore, the water of hydrolysis is calculated as

water of hydrolysis VS digested 18162 5

2.12. Operational parameters

The liquid residence time determines how long the

liquid remains in the system, which affects the final prod-

uct 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 calcu-

lated as

liquid residence time LRT TLVQ

6

where, Q = flowrate of liquid out of the fermentor set

(L/d), TLV = total liquid volume.

TLV X

i

Ki w Fi 7

where, Ki average wet mass of solid cake in Fermentori g; w = average liquid fraction of solid cake inFermentor i (L liquid/g wet cake); Fi averagevolume of free liquid in Fermentor i L.

The volatile solids loading rate is calculated as

volatile solids loading rate VSLR VS fed=dTLV

8

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 polysaccha-

rides (Ross, 1998). The following terms are used

throughout this paper.

conversion x VS digested

VS fed9

yield y total carboxylic acids produced

VS fed10

total acid selectivity s

total carboxylic acids produced

VS digested11

total acid productivity p

total carboxylic acids produced

L liquid in all reactors time12

2.13. Statistical analyses

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 ob-

tained by fitting experimental results to linear equations

using Excel software.

3. Results and discussion

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 experi-

ments 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

closure undigested VS dissolved VS carboxylic acids produced biotic CO2 CH4

mass in water of hydrolysis4



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low pH and iodoform addition effectively inhibited meth-

anogenesis. Average total carboxylic acids concentra-

tions 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 dem-

onstrated 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 fer-

mentations 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 reproducibil-

ity of the countercurrent fermentations.

In Fig. 4, the total carboxylic acid concentration as a

function of time during fermentations A and D are pre-

sented. Both fermentations have the same behaviorthrough the operation time (258 d). A t-test was con-

ducted over the steady-state operation (days 126257),

and the results showed that there were no significant

differences between these data set at 5% level of sig-

nificance. 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 (Holtzap-

ple 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 car-

boxylic 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 produc-

tivity 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. 58. The re-

sults show that at higher volatile solids loading rates,

higher total acid productivities are obtained. Conver-

sion, selectivity, and yield decrease with increasing

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10 12 14 16 18 20

Time (days)

Totalcarb

oxylicacid

concentra

tion

(g/L)

Treated MSW/SS

Untreated MSW/SS

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.

Table 2

Results for lime-treated MSW/SS countercurrent fermentations

Fermentation trains A B C D E

Average pH in all fermentors 5.96 0.15 5.79 0.02 5.68 0.06 5.93 0.20 5.85 0.11

Total carboxyl ic aci d c onc ent ration (g/L) 16. 90 0.77 16.29 1.52 25. 99 1. 46 17.07 0.99 18.62 0. 97

Acetic acid (wt%) 39.57 4.72 40.55 4.30 40.40 3.10 45.06 4.48 43.89 4.40

Propionic acid (wt%) 15.70 3.57 14.18 2.10 14.17 1.40 17.81 2.39 12.24 2.21

Butyric acid (wt%) 23.31 5.00 21.33 3.71 22.15 2.99 18.69 3.63 22.08 3.31

Valeric acid (wt%) 8.04 1.41 7.77 1.44 6.97 0.96 7.35 1.49 7.25 1.01

Caproic acid (wt%) 9.47 2.49 12.33 2.96 11.09 2.54 6.98 2.22 9.35 2.82

Heptanoic acid (wt%) 1.61 0.86 2.64 1.27 1.41 0.74 1.70 1.12 1.45 0.83

Conversion (g VS digested/g VS fed) 0.432 0.366 0.302 0.449 0.392

Yield (g total acids/g VS fed) 0.276 0.221 0.175 0.264 0.237

Selectivity (g total acids/g VS digested) 0.639 0.603 0.577 0.589 0.558

Total carboxylic acid productivity (g total acids/(L liquid d)) 0.832 0.886 1.360 0.829 1.125

Biotic CO2 productivity (g CO2/(L liquid d)) 0.390 0.251 0.372 0.290 ND

Methane productivity (g CH4/(L liquid d)) 0.002 0.001 0.007 0.002 ND

Mass balance closure (g VS digested/g VS in) 0.96 101.9 100.3 0.93 ND

Note: All errors are 1 standard deviation.

ND = not determined.



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VSLR. These correlations are described by the following

equations:

p 0:0965VSLR 0:5317 13

s 0:0090VSLR 0:6274 14

x 0:0263VSLR 0:5037 15

y 0:0178VSLR 0:3086 16

The substrate system, lime-treated MSW/SS, is a hetero-

geneous 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 microorgan-

isms digested the primarily accessible fraction of the

substrate; however, less carboxylic acids were producedper unit of digested biomass. Part of the digested sub-

strate is non-acid soluble volatile solids (dissolved VS).

When substrate and nutrients are abundantly available,

the microorganisms may produce energy-storage com-

pounds that can be used in periods of starvation as sug-

gested 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 nutri-

ents and less easily digestible fractions of the substrate

were available for the microorganisms. As a conse-

quence, 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 there-

fore not enough for the microorganisms to reach maxi-

mum growth rates (Russell and Baldwin, 1979). At these

conditions, per unit of digested biomass, the micro-

organisms 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 sys-

tems (Ross, 1998; Domke, 1999; Thanakoses, 2002;

0

5

10

15

20

25

10 30 50 70 90 110 130 150 170 190 210 230 250 270

Time (days)

Totalc

arboxylicacid

concentration(g/L)

Fermentation A

Fermentation D

Steady-state operation

Switch to 4 mg iodoform/L

fed to F4

8 mg iodoform/Lfed to F4

Iodoform/Bromoform

8 mg/L fed to F4

Fig. 4. Total acid concentration for lime-treated MSW/SS fermenta-

tion A and D.

0

0.5

1

1.5

2

0 2 4 6 8 10

VSLR (g VS/(L liquid in all fermentorsd))

Totalacidproductivity

(gtotalacids/g/(Ll

iquidd))

Experimental data

Best Line Fit

Fig. 5. Correlation of total acid productivity with volatile solid

loading rate.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10


Selectivity

(gtotalacids/gVSdigested)

Experimental data

Best line fit

Fig. 6. Correlation of selectivity with volatile solid loading rate.

0

0.1

0.2

0.3

0.4

0 2 4 6 8 10


Yield

(gtotalacids/gVSfed)

Experimental data

Best line fit

Fig. 7. Correlation of yield with volatile solid loading rate.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10


Conversion

(g

VSdigested/gVSfed)

Experimental data

Best line fit

Fig. 8. Correlation of conversion with volatile solid loading rate.



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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 a bt

1 ct 17

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).

rdAe

dt

b

1 ct2

18

The specific rate, r (g Ae produced/(g VS d)) was deter-

mined from Eq. (18) by dividing it by the initial amount

of substrate concentration, S0 (g VS/L) in each of the

five fermentors

r rS0

19

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)

rpred e1 x

f

1 g/Aeh

20

Least square analysis was used to determine the empir-

ical 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 pen-

alty function by South and Lynd (1994).

The conversion, x was calculated using

xt Ae TLV

M0 r21

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) deter-

mined in the countercurrent experiment.

s /r 22The selectivity (s) used in the CPDM Mathematica pro-

gram was 0.60 g total acids/g VS digested, the average

value of selectivity for the lime-treated MSW/SS coun-

tercurrent fermentations with terrestrial inocula. Selec-

tivity 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 /.

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 predic-

tions for conversions and product concentrations for the

lime-treated MSW/SS countercurrent fermentations

(substrate = 200 g VS/L liquid) are presented in themap in Fig. 9. The experimental carboxylic acid con-

centrations 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 abso-

lute 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 selec-

tivity is shown in Fig. 10. Linear regression of the data

gave the following correlation:

Error xpred xexp

xexp 2:663s 1:418 23

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 concen-trations 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 concentra-

tions (>30 g/L) can be achieved at LRT of 35 d and

VSLR between 2 and 3 g/(L d).

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Conversion (g VS digested/g VS fed)

Totalcarboxylicacid

concentration(g/L)

234

56812

10

15

2530

35

LRT (days)

VSLR

(g/(Ld))

Fig. 9. CPDM map for lime-treated MSW/SS countercurrent

fermentation (200 g VS/L liquid).



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The CPDM maps for lime-treated MSW/SS coun-

tercurrent fermentations under mesophilic and thermo-

philic 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 con-

centrations as high as 60 g acids/L can be obtained, but

conversions may be lower than 40%.

4. Conclusions

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 fer-

mentations can be operated for extended periods of time

under steady-state conditions. Addition of 48 mg iodo-

form/(L liquid added to F4) inhibited methane genera-

tion. The highest total carboxylic acid concentration

obtained for lime-treated MSW/SS with terrestrial in-

ocula was 26.0 g carboxylic acids/L liquid, which sur-

passed 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

Table 3

Comparison of experimental and predicted carboxylic acid concentration and substrate conversion for lime-treated MSW/SS countercurrent

fermentations AF

Fermentation trains A B C D E F Averagea (%)

LRT (d) 20.7 19.0 22.7 20.7 17 19.7

VSLR (g VS/L d) 3.0 4.0 8.1 3.1 6.4 5.7

Experimental carboxylic acid concentration (g/L) 16.90 16.29 25.99 17.07 18.62 20.66

Predicted (CPDM) carboxylic acid concentration (g/L) 16.12 17.54 23.53 17.10 18.39 19.97Errorb (%) 4.6 7.7 9.5 0.2 1.2 3.3 4.4Experimental conversion 0.43 0.37 0.30 0.45 0.39 0.33

Predicted (CPDM) conversion 0.54 0.49 0.29 0.56 0.36 0.38

Errorb 20.3 24.3 3.3 24.3 7.7 15.1 15.8

a Average absolute error P

jpredicted experimental=experimentalj=6.b Error = ((predicted experimental)/experimental) 100.

0

5

10

15

20

25

30

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)

Error

(%)

Fig. 10. Correlation of error between experimental and predicted

conversion and selectivity for lime-treated MSW/SS fermentations

with terrestrial inocula.

0

5

10

15

20

25

30

35

40

45

0 0.2 0.4 0.6 0.8 1


Totalcarboxylicacid

concentration(g/L)

23

4568

12

10

15

2530

35

LRT (days)

VSLR

(g/(Ld)

Fig. 11. CPDM map for lime-treated MSW/SS countercurrent

fermentation for substrate concentration 300 g VS/L liquid.

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1


Totalcarboxylic

acidconcentration(g/L)

23

4568

12

10

15

2530

35

346

Mesophilic

-------- Thermophilic

VSLR(g/(Ld))

LRT (days)

Fig. 12. CPDM maps for lime-treated MSW/SS four-stage coun-

tercurrent 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).



10/10

can be used to estimate the costs and process economics

of industrial-scale fermentors.

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