Biogasification of Garbage Slurry by Methane Fermentation

Journal of JSAM 62 (6): 116•`124, 2000Research Paper

Biogasification of Garbage Slurry by Methane

Fermentation System (Part 2)-Characteristics of Gasification Process-

Junye JIA*1, Yutaka KITAMURA*1, Tateshi FUJIURA*1,

Kouichi TAKEYAMA*1, C.L. HANSEN*2

Abstract

Characteristics of the 2nd phase of a methane fermentation system or gasification process were

evaluated experimentally. The substrate for the 2nd phase was supernatant from the effluent of

the 1st or acidogenic phase of an anaerobic process treating garbage slurry. The mesophilic

bench scale apparatus consisting of four CSTR's performed as well as a thermophilic gasification

process in regard to steady state data on pH, substrate consumption, biogas production rates and

methane yield. Kinetic simulations on the microbe growth and methane production determined

critical HRT was 5.4 days, HRT to facilitate maximum microbe density was 8.3 days, and HRT for

the maximum methane production rate was 6.7 days, respectively. Based on these HRT's, the total

vessel volume needed to handle the garbage produced by one person would be 82.5 L and the

methane produced per person would be 13.2L/d.

[Keywords] gasification process, kinetic evaluation, biogas production, methane yield, fundamental design

I Introduction

As the food industry has prospered and diets

have improved, there has been an increase in

the amount of garbage produced1). A process

needed to be developed to utilize precious or-

ganic resources and energy by disposing of

garbage without producing dioxin via in-

cineration or polluting water with organic

wastes. A methane fermentation system is a

method that will degrade garbage biologically

and produces biogas as alternative energy. It

has been applied successfully to the treatment

of agricultural and municipal sewage2). This

process would be especially applicable for

treatment of garbage from multi-story hous-

ings or for when homes are close enough to-

gether to provide low cost feed stock to a

communal system and easy utilization of the

biogas produced. Thermophilic3) and meso-

philic4) methane fermentation with high effi-

ciency reactors have been studied for their

applicability to garbage treatment. However,

there is little fundamental data on mesophilic

methane fermentation employing a conven-

tional reactor that is easily maintained and

relatively low cost.

In a former report5), we reported characteris-

tics of garbage solids degradation and volatile

acid (VA) formation in the acidogenic process

or the 1st phase of a 2 phase methane fermen-

tation system using synthetic garbage slurry6).

The acidogenic process kinetic parameters

were: microbe yield, 0.55 g-TS/g-TS; product

*1JSAM Member, Faculty of Life and Environmental

Science, Shimane University, Matsue-shi, 690-8504

Japan*2College of Agriculture, Utah-State University,

Logan, Utah, 84322-8700 USA

JIA, KITAMURA, FUJIURA, TAKEYAMA, HANSENBiogasification of Garbage Slurry by Methane Fermentation System (Part 2) 117

yield, 0.51 g-VA/g-TS; and critical hydraulic

retention time (HRTcri), 1.5 days. This article

reports the characteristics of acid consump-

tion and biogas production in the gasification

phase of the methane fermentation system

using effluent from the acidogenic process.

Garbage slurry was influent for the acidogenic

process7). In this paper, we report results of

using settled effluent from the acidogenic pro-

cess as influent for a biogasification system to

produce methane.

II Materials and Methods

1. Experimental apparatus

Figure 1 shows a diagram of the schematic

apparatus. The experimental gasification

system consisted of four glass flask reactors

(Sibata, Tokyo), each with 2.5L working

volume (12cm outside diameter and 40cm total

height), an acrylic water bath (52cm•~44cm•~

25 cm) equipped with a temperature controller

(Yamato BF-200, Tokyo), the gas stir system

composed of suction pumps (NRK UP2, Tokyo)

regulated by an on-off timer and gas distribu-

tion pipes, gas collection equipment with dry

test gas meters (Shinagawa DC-1A, Tokyo)and

gas bags (PINOCCHIO, Tokyo, 35cm diameter).

Stirring of reactors was conducted by blowing

generated biogas through the reactor contents

for 15 minutes at 6 hours interval. In order to

assure that the reactors worked as CSTR's,

feed and withdraw operations were conducted

by following the fill and draw method after

manual stirring was performed. The reactors

were set in a water bath to be maintained in

the mesophilic range (36•}1•Ž). Liquid paraffin

floated on the bath to prevent the water evap-

oration.

2. Materials and procedures

(1) Seeding sludge

Anaerobic digested sludge from a sewage

center (East Sewage Plant at Lake Shinji,

Shimane) was filtered with a 1.00mm steel

sieve (Maruto, Tokyo) to remove coarse solids,

and 2.5L of filtrate was added to each reactor.

Reactors were initially purged with N2 for

Fig. 1 Experimental apparatus of gasification process

118 Journal of the Japanese society of agricultural machinery Vol. 62, No. 6 (2000)

quick start-up.

(2) Propagation and acclimation

To propagate the methanogenic microbes

for the gasification process, synthetic waste-

water containing acetic acid 3.3g-VA/L as the

substrate and nutritions5) was fed to reach

reactor with a 15 day HRT. After the propaga-

tion, effluent from the acidogenic process7)

with a 5 day HRT was added to each

methanogenic reactor as substrate for the gas-

ification process with the same HRT. The feed

for the gasification process was prepared by

settling the acidogenic effluent for about an

hour and taking only the supernatant, the sed-

iment sludge was returned to each reactor.

The supernatant occupied about three-fifth of

the total effluent under steady state. Charac-

teristics of this supernatant were; pH, 5.1;

substrate concentration, 2.6g-VA/L. Garbage

solids concentration was nearly equal to 0

g-TS/L. According to Chang et al.8), an opera-

tional period of four to six times HRT is suffi-

cient to bring reactors to steady state. Thus,

the operation of reactors for propagation and

acclimation was carried out for 3 months, re-

spectively.

(3) Experimental runs

To assure that the gasification process

reached steady state, pH, substrate concentra-

tion, S (g-VA/L) and microbe density, X (g-TS/

L) were measured for the supernatant liquid or

influent to the reactors and the fermented

liquid or effluent from the reactors. Because

the garbage solids concentration was nearly 0

g-TS/L in the acidogenic supernatant, sus-

pended solids in the reactor was presumed to

be organisms or microbe density X. Produc-

tion rate and composition of biogas were also

measured as parameters of gasification proc-

ess. Following the propagation and acclima-

tion period in the gasification reactors, super-

natant feeding was continued to give 14,12,10

and 8 day HRT's (RUN 1 through 4 respective-

ly) for about three months.

3. Measurements

pH, S and X were determined by the proce-

dure mentioned in our first report5). Biogas

production rate was monitored everyday by

the gas meter. The biogas composition was

analyzed by gas chromatograph (Shimadzu

GC-14A, Tokyo) to know methane and carbon

dioxide concentrations according to the

method of Maekawa et al.9). H2S concentration

was determined by use of Kitagawa-type gas

detective tubes.

III Results and Discussion

1. Steady state characteristics of the gas-

ification process

Operational HRT, VA loading rate, (LVA) and

water quality parameters for each reactor are

presented in Table 1. The parameters were

Table 1 Experimental conditions and steady state parameters on water

quality

(Subscript: in; influent, out; effiuent)

JIA, KITAMURA, FUJIURA, TAKEYAMA, HANSEN:Biogasification of Garbage Slurry by Methane Fermentation System (Part 2) 119

arithmetic mean values of 5 to 8 data points

that were obtained after steady state had been

established. Influent substrate concentration

(Sin) was held constant at 2.6g-VA/L. As HRT

decreased from 14 to 8 days, LVA increased to

0.32 from 0.18 g-VA/L• d. The influent pH

(pHin) was acidic at 5.1, whereas the effluent pH

(pHout) was in the neutral range from 7.2 to 7.3.

The pHout values were within the pH values of

7.0 to 7.4 which Li et al.10) observed for high-

solids methane fermentation and close to the

pH of 7.4 reported by Chun et al.11) for garbage

methane fermentation under mesophilic con-

ditions. Reactor microbe density (Xout) and

effluent substrate concentration (Sout) in-

creased respectively from 0.48 to 0.56g-TS/L

and 0.12 to 0.24g-VA/L with reductions in the

HRT. Because the Sin was constant, these in-

creases resulted in lessening of the substrate

consumption ratio ((Sin Sout)/Sin•~100%) from

95.4 to 90.8% and the rising of substrate con-

sumption rate, rs (=(Sin-Sout)/HRT) from 0.18 to

0.30g-VA/L• d. These dependencies of Xout and

Sout on the HRT conformed to the characteris-

tics reported by Chang et al.8) for mesophilic

methane fermentations fed acetic acid as the

sole substrate.

Table 2 shows total HRT, TS loading rate,

LTs and gas parameters for the experimental

reactors. The total HRT is for the whole meth-

ane fermentation system including 5 days of

HRT for the acidogenic phase. Biogas produc-

tion rate is the amount of biogas produced per

unit volume of reactor per day and yield is the

amount of biogas produced per unit mass of

consumed substrate. Biogas production rate

increased from 0.18 to 0.31L/L•Ed with the

decrease in the HRT and the obtained yield

was maintained between 0.98 and 1.02L/g-VA.

H2S was not detected (data not shown). The

methane concentrations were almost constant

between 71 and 72% despite changes in the

HRT, so that the methane production rate, rm

increased from 4.13 to 0.22L/L • d and the

methane yield, Ym ranged from 0.71 to 0.73 L/

g-VA. The biogas yields and methane concen-

trations calculated by Buswell et al.12) on the

stoichiometry are 0.75 L/g-VA and 50.0% for

acetic acid, 0.91L/g-VA and 58.3% for propion-

ic acid, 1.02L/g-VA and 62.5% for butyric acid

and 1.42L/g-VA and 72.2% for stearic acid,

respectively. Comparing these theoretical

values with the experimental biogas yields

from 0.98 to 1.02L/g-VA and methane concen-

trations from 71 to 72%, the influent ac-

idogenic supernatant was assumed to include

mostly longer chain volatile acids. Methane

yields, Ym' (=ƒÁm/LTs) based on TS balance

were also determined to range from 0.33 to 0.40

L/g-TS. The Ym' of 0.38L/g-TS obtained by

Sasaki et al.13) for the thermophilic garbage

methane fermentation under 15 day HRT was

within these values, so our mesophilic meth-

ane fermentation system is found to have as

Table 2 Experimental conditions and steady state parameters on biogas

production

*Garbage slurry: 7.2gTS/L


high productivity of biogas as a thermophilic

fermentation due to the employment of the

phase separation method. Based on the

effluent water quality and the biogas produc-

tion, all the reactors were concluded to work

well as gasification processes.

2. Kinetic evaluation for gasification pro-

cess

For CSTR under steady state conditions, ƒÊ

(1/d), the specific growth rate of microbes was

generally assumed to be equal to the dilution

rate, D (1/d)14). However, in the case of an

anaerobic gasification process, methane mi-

crobe death occurs at every contact with

oxygen in the influent and must be ac-

counted for in the growth constants15). ƒÊ for

the gasification process of CSTR can be

modeled as follows

(1)

Where, Kd is self-decay constant (1/d). By

using the microbe growth rate, ƒÁx (=ƒÊ•EXout)

g-TS/L• d, the microbe yield, YG (=ƒÁx/ƒÁs)

g-TS/g-TS and the expression (1), a following

equation is derived:

(2)

Based on equation (2), the relationship be-

tween 1/D or the operational HRT for each

reactor and (Sin-Sout)/Xout was plotted using the

steady state data in Table 1 to show a linear

function as in Figure 2. According to the

values of intercept on the Y axis and the slope

for the obtained regression curve, YG and Kd

were determined to be 0.33g-TS/g-VA and 0.05

1/d respectively as the growth constants for

the gasification. These values were in accord-

ance with YG=0.3 and Kd=0.04 reported by

Sono16) for methane microbes under the phase

separation control.

Based on the Monod growth model of ƒÊ=

ƒÊ max•ESout/(Ks+Sout), a relationship between ƒÊ

and Sout is presented in Figure 3 which is a

Lineweaver-bark plot. According to the

Fig. 2 Plot for the determination of growth

constants

Fig. 3 Plot for the determination of kinetic

constants

values for slope and Y axis intercept for the

regression line, maximum specific growth rate

ƒÊ max=0.25(1/d) and substrate saturation con-

stant K=0.14(g-VA/L) were obtained, respec-

tively. Making a comparison of these values

with ƒÊmax=0.40 and Ks=0.05 for methane mi-

crobes with phase separation15), the growth

activity of methane microbes in this process

was 40% lower and the substrate saturation

constant was 3-fold worse than the reference.

It was considered to be due to the influent

substrate composition that consisted of sever-

al volatile acids with different consumption

characteristics by microbes.


Fig. 4 Simulation of relationship between

HRT and Xout

Fig. 5 Simulation of relationship betweenHRT and CH4 production rate

3. Simulation on the microbe growth and

methane production

Combining equation (2) with the Monod

growth model yielded an equation showingthe relationship between Xout and D:

(3)

Using equation (3), variation of Xout with 1/D

or the HRT was plotted in Figure 4. The X

axis intercept indicates the critical HRT of 5.4

days when wash out of methane microbes

from the reactor occurred. Numerical analysis

for the differential form of equation (3) derived

the HRT of 8.3 days to have the maximum

microbe density of 0.54g-TS/L.

Using the definition of methane yield Ym(=

ƒÁm/ƒÁ5), equation (3) was transformed to the fol-

lowing equation which presents the relation-

ship between ƒÁm and D:

(4)

Substitution of the mean Ym of 0.72(L/g-VA)

obtained from Table 2 to equation (4), give a

relation between ƒÁm and 1/D or the HRT. This

was simulated in Figure 5. This curve also

indicated the HRTcri , of 5.4 days, which was in

accordance with the HRTcri determined in

Figure 4. As the result of numerical analysis

of the differential form of equation (4), the

HRT for the maximum methane production

rate was 6.7 days, and the maximum ƒÁm was

0.22L/L• d.

4. Fundamental design of methane fer-

mentation system

Based on the simulation results above, opti-

mum HRT (HRTopt) to have maximum meth-

ane production for the gasification process

was set to be 6.7 days or 1.3 days longer than

the HRTcri . of 5.4 days. A system was designed

to treat the garbage produced by one person

according to the HRTopt, and the characteristic

values for both acidification and gasification

processes. The fundamental design indices for

the anaerobic fermentation system were a 5

day HRT for the acidogenic process and 6.7

day HRT for the gasification process as shown

in Table 3. The garbage production rate was

based on data by Sankai6). The working

volume of each reactor was determined ac-

cording to the formula, V=HRT• F(V: Reac-

tor volume, L; F: Flow rate, L/d). The practi-

cal or usable volume was calculated by assum-

ing one third of the total volume as the gas

space. According to our results7), acidogenic

slurry from the acidogenic reactors was

separated 3: 2 to produce the supernatant and

the sediment respectively. Therefore, the


Table 3 Design parameters for methane fermentation

system

Fig. 6 Methane fermentation system for a 400 people condominium

slurry flow rate to obtain the acidogenic super-

natant of 5.25L/d must be 8.75L/d (divide 5.25

by 3/5). Accordingly, the sedimentation tank

volume was calculated to be about 0.5 L or

8.75/24•~4/3=0.49 when the withdrawing of

acidogenic slurry from the reactor was set at

24 times a day.

A flow diagram of a methane fermentation

system for a hypothetical condominium of 100

families with four persons each is shown in

Figure 6. Required volumes for the acidogenic

reactor (HRT=5 day), the sedimentation tank

and the gasification reactor were 14, 0.2 and

18.8m3 respectively, and the amount of meth-

ane transformed from the garbage by the

system was estimated to be 5.28m3/d. The

energy of the methane produced becomes 2.1•~

105 kJ based on its heating value17) of 4.0•~104

kJ/Nm3. This much energy will increase the

temperature of about 5m3 water by 10•Ž. Ac-

cording to Sasaki et al.18), the excess energy

that can be used is about 70% of the produced

biogas in the case of methane fermentation for

swine manure. Besides, Maekawa et al.19)

described that it varies with depending on the

fermentation method or the shape and materi-

al of the reactor. So that the excess energy of

the methane fermentation system here was

estimated by determining the amount of

energy required to increase the garbage slurry

temperature to the fermentation temperature

or 36•Ž. The estimation revealed that about

67% (16•Ž•~2.1m3/d•€(10•Ž•~5.0m3/d)•~2.1•~

105kJ=1.4•~105kJ) of the produced biogas is

necessary to heat the slurry when it is as-

sumed to be drained at 20•Ž and its specific

heat is the same as the water. Considering the

amount of energy to maintain the reactor and


the sedimentation tank at constant tempera-

ture, the excess energy will be about 10 to 20%

of the total produced biogas. It suggests that

this system is difficult to stand alone as an

energy production process.

On the other hand, much residual organics

were found in the effluent from the gasific-

ation reactor in this experiment. Its concentra-

tion was 0.51g/L as total volatile acid and 0.55

g/L as total solid. When COD is assumed to be

about 10 times of the total volatile acid20), the

organic concentration of effluent was es-

timated to be about 5.1g/L as COD. Also for

the residual concentrations of nitrogen and

phosphonium, Tsuji et al.21) reported that they

were 18 to 27mg-TN/L and 6.2 to 8.9mg-TP/L

respectively for the garbage methane fermen-

tation and their removal abilities were not

shown. Therefore, a secondary treatment such

as aerobic processes involving nitrification or

dephosphorization process would be necessary

for the effluent from the biogasification

system.

IV Conclusions

The characteristics of acid consumption,

biogas production and kinetic parameters in

the gasification process or the 2nd phase of the

methane fermentation system were evaluated

experimentally. A fundamental data to design

a biogasification system to treat garbage bio-

mass by methane fermentation was also deter-

mined.

(1) As HRT decreased from 14 to 8 days, pH

values, reactor microbe density, effluent sub-

strate consumption ratio, biogas production

rate, methane concentration and methane

yield changed from 7.3 to 7.2, 0.48 to 0.56g-TS/L,95% to 91%, 0.18 to 0.31L/L d, 71 to 72%

and 0.33 to 0.40L/g-TS, respectively.

(2) Kinetic evaluation for gasification proc-ess was determined by applying the material

balance and the Monod model. The following

were determined: microbe yield, 0.33(g-TS/

g-VA); self-decay constant, 0.05(l/d); maxi-

mum specific growth rate, 0.25(l/d) and sub-

strate saturation constant, 0.14(g-VA/L).

(3) The simulated relationship between

HRT and reactor microbe density or methane

production rate indicated that the critical HRT

was 5.4 days, the HRT for the maximum mi-

crobe density was 8.3 days and the HRT for the

maximum methane production rate was 6.7

days for the gasification process.

(4) Based on the simulation results above,

the fundamental design volumes to treat the

garbage produced by one person were deter-

mined as 35 L for the acidogenic reactor, 0.5L

for the sedimentation tank, and 47 L for the

gasification reactor. This system would pro-

duce methane of 13.2L/d.

References

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2) Switzenbaum, M.S.: Obstacles in the Implementationof Anaerobic Treatment Technology, BioresourceTechnology, Vol. 53, 255-262, 1995

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6) Sankai, T.: Development of Wastewater Purifier Corre-spond with Disposer for Garbage Recycling System,Water and Waste, Vol. 38 (7), 55-61, 1996

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8) Chang, J.E., Noike, T., Matsumoto, J.: Effect of Reten-tion Time and Feed Substrate Concentration onMethanogenesis in Anaerobic Digestion, Proceedingsof the Japan Society of Civil Engineers, No. 320(4), 67-76, 1982


9) Maekawa, T., Yamazawa, S., Yoshikawa, S. Hanaoka,

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Agricultural Structure, Japan, Vol. 15 (1), 7-21, 1985

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Mineral Nutrient Requirements for the High-Solids

Methane Fermentation of Organic Fraction of Munici-

pal Solid Waste, 32nd Proceedings of Japan Society on

Water Environment, 337, 1998

11) Chun, F.C., Kataoka, N., Miya, A., Suzuki, T.: Character-

istics of Methane Fermentation with Garbage under

Mesophilic and Thermophilic Conditions, 32nd Pro-

ceedings of Japan Society on Water Environment, 336,

1998

12) Buswell, A.M., Solo, Jr.F.W.: The Mechanism of Meth-

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1778-1780,1948

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Characteristics of High Solid Methane Fermentation

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14) Yamane, T.: Biological Reaction Engineering, 2nd

press, Sangyo Tosho Co. Ltd., Tokyo, Japan, 198515) Stronach MS., Rudd T., Lester N.J.: 1. The Biochemis-

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(Received: 11. May. 2000-Question time limit: 31. Jan-uary. 2001)

「研究論文」

メタン発酵システムによる生ごみスラリーのバイ

オガス変換(第2報)

-ガス化プロセス-

質俊業*1・北村豊*1・藤浦建史*1・竹山光一*1・

C.L.ハンセン*2

要旨

生ごみスラリーを原料とする汚泥返送式酸生成

プロセスからの流出液を用いたメタン変換システ

ムの第二分解過程(相皿)であるガス化プロセス

の特性を動力学的に明らかにした。4基の完全混

合リアクタの各滞留時間(HRT=8,10,12,14

日)において,ガス化プロセスのpH,有機酸消費

及びバイオガス生成速度,メタン濃度メタン収

率等特性値を測定した結果,ガス化プロセスとし

て正常に機能したこと及び高温発酵と同等のバイ

オガス生産能力を持ち得ることがわかった。また

菌体増殖・メタン生成特性のシミュレーシュンの

結果より,菌体を流出させる臨界HRTcri=5.4

日,菌体濃度を最大とするHRTXmax=8.3日,メ

タン生成速度を最大とするHRTMmax=6.7日が求

められた。以上の結果に基づいて,1日1人あた

り250gの生ごみから13.2Lのメタン生成量が求

められた。

[キーワード]ガス化プロセス,動力学的評価,バイオガス

生成,メタン収率,基本設計

*1会員,島根大学生物資源科学部(〒690-8504 島根県松

江市西川津町1060番地 TEL 0852-32-6546)*2ユタ州立大学農学部(ユタ州ローガン84322-8700アメ

リカ合衆国)

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Biogasification of Garbage Slurry by Methane Fermentation