C H I S Hwang

Chun

D H Woo

J K Chung

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Monitoring of biogas production from
fermentation of rice straw using a bubble
counting sensor
I 1 2 ID.H. woo, C.H. HWANG. I.S. CHUNG and J.K. CHUN
JDeparfmellf of Food Science and Technology. College of Agricultural and Life Sciences. Seolll Nmional University: and
2Depar,mef1f a/Genetic Engineering. KyulIg /-lee Unil'ersiry. SU\\'OIJ. 44/-744, Korea. Received 24 October, /995: accepted
7.//111' 1997.
10mm~ r-
MATERIALS AND METHODS
l'
Rubber block
Air
Threaded bolt
Syringe
Handle
Phololnlerrupler
Correlation belween OJ'ifice diameter and l.wbble diameter
The bubble sensing apparatus was constructed of a glass
column of 10 mm diamcler and 100 mm height, an acrylic
orifice plale (replaceable), (wo photoil1lerrupters (EESB5),
and an air-tight syringe. as shown in Fig. 1. A set of experi-
ments was carried out in triplicate at 25°C with a known
volumc or air (4 mL) and various diameters of acrylic drillcd
orifice plates to establish the correlation between orifice
diameter and bubble diameter. Air was injected by a cali-
brated gas-tight syringe into the column and the air now rate
\Vas controlled by adjusting the speed or the plunger. The
ascending bubbles were detected with the photointerrupter.
The acrylic plates of I mm thickness had orifice diameters of
2.3,2.7,3.2.3,25,3.8,4.2, and 4.8 mm. At a given diameter
or orifice, the avcrage ascending bubble velocity (1/1) was
calculated by dividing the distance bctwccn the two photoin-
tcrrupters by the timc taken for the bubble to travel.
Assuming the bubble to bc spherical and uniform in size at a
givcn asccnding velocity, the volume of a bubble was deter-
mined by injected air volume and bubble counts at a fixed air
flow rate. Average bubble diameter (DOl') \Vas calculated by:
D",. = (611/rr) 1/3 (I)
Fig. I. Schematic diagram for bubble sensing apparatlls.
Woo. D.H., Hwang, C.H .. Chung, I.S. and Clwn. J,K. 1997. MOlli-
torin~ of biog:ls production from fermentation of rice straw
using it bubble counting sensor. Can. Agric. Eng. 39:203-206. A
bubble counting sensor with an on-line monitoring system was ap-
plied 10 biogas production from fenncl1l<ttion of rice straw, This
simple and low-cosl sensor was capable of measuring bubble diame-
ter and counls by a photointcrruptcr and a clock cycle (2441 Hz). Thc
dctcclable rangc of gas production rate was 0.067 - 10 mL/min. The
on-line monitoring system buill with a olle-chip microcomputcr was
opcrated for over 100 days and dcmonstrated to bc promising in
monitoring gas production during fermcntation proccsscs. Kcy-
words: biogas production. rice straw. bubble counting sensor. on-linc
monitoring. fennel1tation.
Un cnplcur-cornpteur de bulles cquipc d'un systcmc de surveil-
lancc cn temps reel a etc utilise pour lllcsurcr la production dc biogas
(I partir de la fermcntation de la paille de riz. Avec cet appareil simple
et PCll couteux. on cst capable de mesurcr Ie nombrc et Ie dianu::lrc
dc bullcs par Ie photointerruptcr etle cycle d'horlogcric (2441 Hz).
Lc garnmc Illcsurablc du tau x de production du gaz a etc dc 0.067 ;'1
10 mLlmin. L'cl1semble du systcme a fonctionne pcndant plus de
100 jOllrs et a montrc un potclltiel promcllcur pour la surveillancc de
1:1 production du gaz pour tout procede. de fcnncntalion. MOl c1es:
production de biogas, paillc de riz, captcur pour compter Ic bullcs.
surveillance cn lignc fermcntation.
Mcthane fermentation of' agricultural wastes is becoming of
economic and environmental importance. Traditional mcth-
ane fermentation is a slow process and can be quite sensitive
to environmental changes (Chynoweth and Jerger 1988:
Clausen et al. 1979). Ln methane fermentation, inherent prob-
lems exist due to unstable characteristics or the process
(Renard et al. 1988; Salm 1984). In this respect. an efficient
monitoring system for the methane fennentalion process is
required. This can be achieved by on-line detection or the
biogas during the fermentation process.
The gas production rate has been successfully monitored
using a bubble counting sensor in aerobic and facultativc
processes such as for yeast and Kimchi fermentation (Lee
and Chun 1994: Choi and Chun 1996). However. there is no
information available all monitoring gas production by this
type or sensor in an anaerobic proccss. In this study, the
development and performance of the scnsor wcrc studied for
on·line monitoring of gas production in methane fermenta-
tion of rice straw.
INTRODUCTION
CANADIAN AGRICULTURAL ENGINEERING Vol. 39. No.3, July/Augusl/Scplcmber 11)97 203
where \I = volume of a bubble. The photo interrupter pro-
duced square pulses in response to the bubble passage. Based
on the hypothesis that the bubble diameter (Dh) should be
equal to the width of the square pulse, its width was calcu-
lated using a known clock of 2441 Hz (cycle width
0.052x I0" mm) as described by:
where:
Wpp = width of a square pulse (at high logic duration)
produced from the phot.ointcrrupter (mm).
f = multiplying factor (number of clock cyclcs counted
during onc pulse duration of thc photointerrupter
signal), and
Wee = width of clock cycle.
Modified bubble counting sensor
The bubble sensing apparatus was further modified 10 fabri-
catc a practical sensor for mcasuring the biog,ls production
rates in methane fermentation. Figure 2 shows the modified
bubble- counting sensor consisting of bubble formation (A)
and count (B) sections. The weir was provided to prevent
liquid from flowing in a reverse direction when reduced
pressure developed at the gas inlet during fermentation. Sec-
tion A was filled with distilled water and the liquid height
was maintained at 10 111m water over the orifice plate. The
orifice dia111eter of the sensor was 3.25 mm. Section B was
equipped with the photointerrupter for detection of bubbles.
This bubble cOllnting sensor was also calibrated with the
known gas volume in a way similar to that described above.
r
Bubble sensor
./ gas outletVent hole
Thermister probe
so: ~ersealing ~
eir n 1"-~ f--- Hater
f-:: f-
- vessel
Fixer " - /" "l r
Water L W Fermente
~ vessel
(20 L)
Heater
Wat
w
Fig. 3. Schcmatic diagram of fcrmenter.
Anal.ysis of biogas
The gas composition was analyzed by a gas chromatograph
(Pye Unieam PU4500. Eindhoven, The Netherlands)
equipped with FID. The column used was Porapak Q 80/100
(stainless, 1.2 111. 0.32 111111 00). The temperature of the
injector, column, and detector were 40°C, 40°C, and 100ae,
respectively. The carrier gas was nitrogen and its flow rate
was 15 mL/min.
Substrate prcparation and inoculation
Five hundred grams of rice straw were cut into length of 30
mm and treated with 5 liters of I N sodium hydroxide solution
at 55°C for 24 h to make it more digestible (Hashimoto 1986:
Pavlostthis and Gossett 1985). The pretreated rice straw WilS
adjusted to p1-l7.0 and was supplemented wilh urea for a C/N
ratio of 30 to 40. The mixture was placed in the fennenter
maintained at 35°C where pig manure was subsequently
seeded by 20% of the total volume.
(2)
(B)
Pholo-
Interrupter
Gas oul.,.
Orifice
Waler
Mirror
Bubble "
Gas oul
t
Dh = Wpp =/Wcc
RESULTS AND DISCUSSION
Fig. 2. Schematic diagram of modified bubble counting
sensor.
Fermenter and monitoring s)'stem
A fermenter with working volume of 20 L was used for the
experiments (Fig. 3). The fermcnter had an outer vessel,
which was used as a water jacket with an electrical heater
(500W). The fennenter lid had a venl hole and 150 ml11-
deplh-weir. The sensor was mounted at the lid of thc
l'ermenter and the tygon tubing (100 111m) was conneclCd
betwcen the biogas oullet and the sensor inlet (Fig.3). The
biogas was eluted via the vent hole into the bubble counting
sensor since the weir was used to prevent gas leakage. The
one-chip microcomputer-based controller was built with a
mieroeol1troller (MC68705R3, Motorola, 1983) to acquire
the bubble count and control the tcmperalUrc of water jacket
on a realtime basis. Modified bubble counting sensor
The bubble sensing system was further modified (Fig. 2) to
where do = orifice diameter.
It was also found Ollt Ihat the mcasured bubble diameter is
clo.se to thc average bubble diameter with a percent dilTer-
ence being within 2% (Table I). Thus. it should bc mel1lioned
that the method of bubble counting by this sensor is qui Ie
reliable.
(3)for 2.7 < do < 4.8D",· = 0.1654 do + 4.449
Correlation between orifice diameter and bubble diameter
Ascending bubble velocity (If/) was inversely proportional to
orifice diameter (Fig. 4) and it was not affected by the heighl
of water in the range of 10 to 50 111m. 11 is apparent that the
asccnding bubble velocity was constant at a fixed orifice
diameter of the sensor.
The average diameter or air bubble (D(lI') is related to the
orifice diametcr at 25°C by the expression (Fig. 5):
Side viewFront view
2W woo. HWANG. CHUNG and CHUN
800400 600
Bubble number
200
70
60
I 50
Q) 40E
::J
'0 30>
en
co 20(!)
10
3.80 4.20
Vb =-11.161 do + 150.33
r = 0.9909 ""
3.20 3.552.702.30
130
90
1.60
~
§, 120
~
u
o
Q)
~ 110
t:
:c
t:
CD
~ 100«
Orifice diameter (mm) Fig. 6. Relationship between gas volume and bubble
number at 2SoC. The orifice diameter was
3.2S mm.
(4)v = 0.06696 N
where:
V =gas volume (mL), and
N =bubble count.
The detectable range of this sensor was 0.067 - 10 mL/min
with an orifice diameter of 3.25 mm since within this range
no overlapping pulses were observed. Our finding indicates
that the bubble count can be used as a real time indicator of
gas production from fennentation of rice straw.
measure the biogas production rates in methane fennenta-
tion. This was identical to the system in Fig. I except that the
liquid height was maintained at 10 mm over the orifice plate
and the air inlet resistance by weir exist. The modified bubble
counting sensor was also calibrated with the known gas
volume as shown in Fig. 6. The volume was proportional to
bubble counts with the linear relationship:
4.70
Dav = 0.1654 do +4.4493
r =0.9907--
3.20 3.55 3.80 4.20
Orifice diameter (mm)
5.6
E 5.4
§, 5.2...
~ 5.0
~ 4.8
~ 4.6
:g 4.4
ffi 4.2
4.0
2.70
Fig. S. Relationship between average bubble diameter
and orifice diameter. The apparatus shown in
Fig. 1 was used at 2SoC.
Fig. 4. Relationship between ascending bubble velocity
and orifice diameter. The apparatus shown in
Fig. 1 was used at 2SoC.
Monitoring of biogas production from methane
fermentation
As shown in Fig. 7, the gas production rate was successfully
monitored for over 100 days. The gas production rate gradu-
ally increased up to 40 days and reached the maximum level
Table I: Comparison of measured bubble diameters with predicted values at various orifice sizes
Bubble diameter (mm)Orifice diameter
(mm)
Bubble count I
(count)
Bubble volume2
Vb. (mm3)
Measured (Db) Predicted (DC/I') with Eq. 33
2.85
3.25
3.80
63.8 ± 0.922
58.4 ± 0.933
53.9 ± 0.555
62.7
68.5
74.2
4.93
5.07
5.21
4.92
5.07
5.21
Bubble counts for 4 mL of air volume (being injected sufficiently slowly as not to form overlapping bubbles). Values are mean ± SD
for number of runs = 60.
2 Calculated by known air volume (4 mL) divided by bubble counts (Fig. I ).
3 Regression coefficient between orifice diameter and bubble diameter was 0.9907** (significant at I% level).
CANADIAN AGRICULTURAL ENGINEERING Vol. 39. No.3. July/August/September 1997 205
Fig. 7. Gas production rate in the fermentation system as
a function of time. The fermenter was maintained
at 35°C and pig manure was seeded by 20%
of total volume.
on day 43. This result reveals that this sensor can be applica-
ble for monitoring the gas production rate during the
extended period of fennentation.
Another experiment was conducted to analyze the gas
production and composition during a 30 day period of fer-
mentation of rice straw. The gas production rate increased
abruptly at day 2 and became negligible on the 10th day.
Thereafter, the rate increased sharply from 0 to a peak 10 Lid
on the 14th day and remained relatively high until the 21st
day. Gas analysis showed that the onset of actual methano-
genesis had occurred about the II th day after the initial
fennentation stage. Methane concentration in biogas was
negligible at the early stage, became relatively constant after
the onset of methanogenesis, and remained high at about
80% of the total biogas during the stage of methanogenesis.
There also appeared to be an initial lag time of 24 h in the
bubble fonnation. This was probably due to the time needed
for the pressure to build up and form the first bubble in the
sensor. It turned out that the onset of methanogenesis can be
easily detected with this bubble counting sensor. Therefore,
it is apparent that the sensor with on-line monitoring system
would be a useful device for monitoring methane fermenta-
tion processes.
The manufacturing cost of this gas sensor was less than
$10, which would be much cheaper than the one on the
market. The limitation of this sensor lies in its narrow detect-
able ranges. To apply it to a large scale digester, the
refinement of this gas sensor must be further investigated.
5
:0-
;t 4
Q)
1ii
~
3c:
0
U
:3 2'0
0
~a.
~
(!J
20 40 60
Time (d)
80 100
However, our findings indicate that this simple and low-cost
bubble counting sensor seems promising for monitoring the
gas production rate of an anaerobic system such as methane
fennentation. This work also demonstrates that a one-chip
microcomputer-based controller was suitable for data acqui-
sition and monitoring of methane fermentation.
ACKNOWLEDGMENT
This work was supported by a research grant from Korea
Energy Management Cooperation. One of the authors (ISC)
acknowledges partial support from the Research Center for
New Bio-Materials in Agriculture.
REFERENCES
Choi, N.E. and J.K. Chun. 1996. Development of microcon-
troller-based process controller for the monitoring of
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Chynoweth, D. and D. Jerger. 1988. Energy production from
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Clausen, E.C., O.C. Sitton and J.L. Gaddy. 1979. Biological
production of methane from energy crops. Biotechnology
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Hashimoto, A.G. 1986. Pretreatment of wheat straw for fer-
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Pavlostathis, S.G. and J.M. Gossett. 1985. Alkaline treatment
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206 WOO. HWANG, CHUNG and CHUN


Monitoring of biogas production from fermentation of rice straw using a bubble counting sensor I 1 2 I

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Monitoring of biogas production from fermentation of rice straw using a bubble counting sensor I 1 2 I

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