anaerobic digestion of marine biomass for practical operation
TRANSCRIPT
ORIGINAL ARTICLE
Anaerobic digestion of marine biomass for practical operation
Kana Kuroda • Yuu Akiyama • Yuichiro Keno •
Naoki Nakatani • Koji Otsuka
Received: 15 November 2012 / Accepted: 28 November 2013
� JASNAOE 2013
Abstract Marine biomass such as seaweed and fishery
waste is a potential energy source although it has not yet
been practically utilized. This work focuses on renewable
energy technologies, particularly anaerobic digestion, for
the utilization of various organic wastes as energy resour-
ces. Primarily fundamental data for the efficient operation
of an anaerobic digestion plant with marine biomass was
obtained and a practical operation method was proposed
using a simple model based on the carbon mass balance.
Batch-processing experiments for seaweed, fishery waste
and vegetable waste were carried out and each digestion
characteristic was presented. The results indicate that
fishery waste is the most efficient even though the accu-
mulation of ammonium ions may inhibit methane produc-
tion. Seaweeds are not efficient in either the production rate
or the yield of methane gas. We investigated the efficient
continuous operation for 6 biomass input scenarios with the
proposed model. The results show that seaweed can be a
useful supplement for the efficient operation.
Keywords Seaweed � Green tide � Food waste � Fishery
waste � Anaerobic digestion � Mathematical model
1 Introduction
Japanese industry depends on foreign countries for most
of its energy supply, and, therefore, it has made intensive
efforts in leading renewable energy development under a
government R&D program called the Sunshine and New
Sunshine Project that adheres to increasing global envi-
ronmental concerns. Despite such efforts, the amount of
renewable energy which has been practically utilized
remains limited [1]. Renewable energy has attracted much
attention again especially after the Tohoku area was
struck by an earthquake in 2011. Among renewable
energy technologies, the Japanese government promotes
biomass utilization with ‘‘Biomass Nippon Strategy,’’
which is the Japanese fundamental policy in biomass
utilization. Biomass utilization is considered ‘‘carbon
neutral,’’ which leads to the replacement of energy and
products derived from fossil fuels with alternatives. Jap-
anese biomass resources are limited and production scale
is small [2]. Although such candidates are the biomass
produced mainly in land, marine biomass can also be a
good candidate since Japan is surrounded by sea. The
potential for marine biomass production may be greater
than that of land on the basis that vast areas are available
for growth and the availability of water may not limit
growth rate, indicating the possibility of obtaining high
productivity [3].
Ulva species, called sea lettuce, a kind of seaweed, often
propagates explosively and piles up in shallows. Such
phenomenon has been reported in both the Japanese
enclosed sea and world eutrophic shallow water areas [4,
5], and is called ‘‘green tide’’. This is because nutrient
levels in an enclosed sea near big industrial cities are very
high since the pollution load is much higher than the nat-
ural purification capacities. Since such seaweeds are either
K. Kuroda (&) � Y. Keno � N. Nakatani � K. Otsuka
Graduate School of Engineering, Department of Marine System
Engineering, Osaka Prefecture University, 1-1 Gakuen-cho,
Naka-ku, Sakai, Osaka 599-8531, Japan
e-mail: [email protected]
Y. Akiyama
Advanced Design Section, Development & Research Dept.
Technical Division, Sanoyas Shipbuilding Corporation,
2767-21 Shionasu, Kojima, Kurashiki,
Okayama 711-8588, Japan
123
J Mar Sci Technol
DOI 10.1007/s00773-013-0247-9
burned or buried in the ground after harvest, they have
been regarded as waste and not as energy resources.
Seaweeds can be expected to reduce nutrient levels since
they absorb carbon and nutrients by photosynthesis. Thus,
they can play an important role in protecting the environ-
ment and as energy resources if the technology, which
converts seaweed to energy, is practically available. One of
the techniques to convert seaweed to usable energy is
anaerobic digestion. Anaerobic digestion is a biological
conversion process, in which organic matter is converted to
biogas (consisting mainly of methane and carbon dioxide)
as a product. The produced methane could then be utilized
for heat or electricity generation, thus replacing fossil fuels
and concomitantly reducing carbon dioxide emissions.
Seaweeds can serve as an excellent feedstock for the pro-
duction of methane owing to their abundance and being an
easily biodegradable renewable resource [6]. Their disad-
vantage, however, is that biomass yield seasonally changes
causing an unstable supply to an anaerobic digestion plant.
Therefore, we mix vegetable waste and fishery waste,
which is supplied stably to the city, with seaweeds to
overcome the disadvantage.
Literature about lab-scale experiments of anaerobic
digestion of seaweeds aim to evaluate potential methane
yields and to understand the effect regarding to pre-treat-
ment, chemical composition of seaweeds and so on [7–11].
With regard to feasible studies, the Japan Ocean Industries
Association (JOIA) [12] advanced the concept of practical
plant based on fundamental experiments from 1980 to
1983. They concluded that the generation of high value by-
products is necessary for an economically feasible opera-
tion. Kelly and Dworjanyn [13] reviewed the anaerobic
digestion of seaweed biomass carried out in Morocco,
France and Japan and concluded that its viability for pro-
duction of biogas under practical conditions. There are,
however, few commercial operating plants using marine
biomass and, therefore, studies about practical aspects of
anaerobic digestion are very important.
Generally, practical operation requires careful consid-
eration to the chemical condition of the digestion tank
rather than potential methane yields since continuous bio-
mass input can cause low pH [14] and the accumulation of
ammonia nitrogen [15] in digestion tank. It can prohibit the
digestion process and makes the digestion less efficient.
Also, the performance of anaerobic digestion depends
largely on the types of organic matter [16], and thus it is
very important to understand the anaerobic digestion
characteristics of the targeted organic matter for the effi-
cient operation. Here, we introduce a mathematical model
as a tool to investigate an efficient operation. The repre-
sentative model is ‘‘Anaerobic Digestion Model No.1
(ADM1)’’, which was developed by the International
Water Association (IWA) [17] in 2002. The model
describes substances in each biochemical process based on
chemical oxygen demand (COD) balance and has been
developing though it has not been used for a practical
application. One of the reasons is that the model employs a
large number of constants and coefficients with difficulty
of calibration model parameters [18].
In this study, we investigate an effective continuous
biomass (seaweeds, vegetable wastes and fishery waste)
input to the anaerobic digestion plant. First, we conducted
fundamental experiments to understand characteristics of
anaerobic digestion of each biomass. Secondly, we devel-
oped a simpler mathematical model than ADM1 based on
carbon mass balance and propose an effective operation of
the biomass input to the digestion tank.
2 Marine biomass utilization system
In many Japanese eutrophic shallow water areas, green
tide, caused by an explosive propagation of green algae,
often occurs in the summer season. Dead green algae
accumulate at the bottom of the sea or on the beach
resulting in a large amount of spoilage and thus damaging
the benthic ecosystem. By convention, they are burned up
in a combustion plant with the consumption of fossil fuels.
However, the green tide species, particularly Ulva sp., have
a high growth rate and, therefore, effectively fix carbon and
nutrients. Thus, if they can be utilized before sedimenta-
tion, then consequently, this could lead to a reduction in the
nutrient concentration of seawater and cause low-carbon
effects such as reduction in the supplementary fuels
required for combustion. Eutrophication is caused by loads
of organic matter and nutrients owing to our living activ-
ities. In other words, the utilization of biomass belonging to
the green tide species is a kind of recycle process of car-
bon, nitrogen and phosphorus waste (CNP).
The concept of the utilization system based on a CNP
recycle using marine biomass is illustrated [19] in Fig. 1.
Marine biomass produced by green tide is harvested before
their accumulation and decomposition. Seaweeds, which
are promptly cultivated in the system, absorb carbon and
nutrients and, therefore, can prevent eutrophication, which
causes the red and blue tides. The seaweed cultivation
system which promotes the seaweed yield and food waste
is also available in the anaerobic digestion system.
The process of anaerobic digestion of marine biomass is
schematically presented in Fig. 2. Harvested marine bio-
mass and food wastes are pretreated to provide a slurry
state. Biogas, which is a mixture of methane and carbon
dioxide, is produced in the anaerobic digestion process.
The produced methane gas can be used for generating
electricity or directly as city gas. The digestive sludge
could be utilized either as compost or as a liquid fertilizer.
J Mar Sci Technol
123
Thus, the marine biomass utilization system is a renewable
biomass system and contributes to improving the benthic
ecosystem.
Anaerobic digestion consists mainly of three steps:
hydrolysis, acidogenesis and methanogenesis as shown in
Fig. 3. Specific bacteria at each process generate bio-
chemical reaction and then organic compounds are diges-
ted. First, hydrolysis is the process which reduces complex
organic polymers to simple soluble molecules. Carbohy-
drates, proteins and lipid polymers are hydrolyzed to sug-
ars, amino acids and long-chain fatty acids, respectively.
Ammonia is produced with the decomposition of amino
acids. Second, long-chain fatty acids are further converted
to a mixture of short chain volatile fatty acids (VFAs) and
other products such as carbon dioxide, hydrogen and acetic
acid. Finally, methane, the final product of the anaerobic
digestion, is produced by consuming acetic acid, carbon
dioxide and hydrogen. Therefore, the flux of produced and
consumed substances at each route is one of the key factors
for an efficient anaerobic digestion [20].
3 Batch-processing experiments
3.1 Materials
Three kinds of biomass were prepared: seaweed, vegetable
waste and fishery waste. Table 1 presents the composition
of each biomass. Seaweed paste composed of a mixture of
four kinds of seaweeds, namely, Ulva pertusa (Sea lettuce),
Ulva compressa (‘‘Hira-aonori’’ in Japanese), Ulva prolif-
era (‘‘Suji-aonori’’ in Japanese) and Ulva meridionalis
(‘‘Hosoeda-aonori’’ in Japanese). Ulva pertusa and Ulva
compressa were harvested at Tarui Southern Beach in
December 2010 and at Rinku Park in February 2011,
respectively, both of which are located near Kansai Inter-
national Airport in Osaka Bay. Ulva prolifera, cultivated in
deep ocean water, was provided by Umi No Kenkyusha
Co., Ltd. (Kochi prefecture, Japan) whereas Ulva merid-
ionalis was cultivated by Kochi University. Ulva prolifera
and Ulva meridionalis were candidates for cultivation at a
seaweed cultivation plant (Fig. 1). Vegetable waste con-
stituents are a mixture of six kinds of vegetables, namely,
onion, cabbage, radish, Chinese cabbage, potato and carrot,
assumed to be food waste since they are in the top six
amongst the most transported to Osaka Municipal Whole-
sale Market. The ratio of the vegetables in each mixture
was based on wet weight. The fishery waste mixture con-
sisted of starfish and blue mussel in a 1:1 ratio based on
total solid (TS) weight (dry weight) and was obtained from
Osaka Bay in summer 2011. Table 2 shows chemical
characteristics of each biomass: total solids, carbon in TS,
Fig. 1 A concept of the marine
biomass utilization system
Fig. 2 Schematic representation of the process of anaerobic digestion
in the marine biomass utilization system
J Mar Sci Technol
123
and the ratio of carbon to nitrogen and phosphorus. Sea-
weed contains water the most among three biomass. Fish-
ery waste contains more nitrogen than the others, because
of its protein composition. It also contains more phospho-
rus than the others. Each biomass sample was ground by an
electric blender and was well mixed as pre-treatment.
3.2 Experiments and analysis
A sealed 1-l glass bottle filled with sewage sludge (800 ml)
was used as a digester as depicted in Fig. 4. The sludge was
procured from Higashi-Nada sewage plant, Hyogo, Japan,
which treats sewage sludge together with municipal waste.
The bottle was tightly stoppered with a rubber stopper and
was equipped with two tubes for gas stock and for
removing sludge. All four digesters were charged with
seaweeds, vegetable waste, fishery waste and no biomass.
On incubation at 35 �C in a temperature controlled
chamber under the conditions for mesophilic methane
fermentation, they were mixed at 400 rpm with a magnetic
stirrer. The rate of addition of organic load was 3.0 kg-
TS m-3. The biogas produced was stored in a gas sampling
bag located in the chamber, and the volume of the biogas
was measured with a 50 ml syringe. Gas analyses (N2,
CH4, CO2) were carried out by gas chromatography
(SHIMADU, GC-8A) using a thermal conductivity detector
and a 2.0 9 2.0 mm stainless-steel column packed with
Parapak Q with helium as a carrier gas. Detector and col-
umn were kept 80 and 50 �C, respectively. The pH was
continuously measured by pH glass electrode (MOTHER-
TOOL CO., LTD, PH-230SD), which was dipped inside
the sludge. Digestion liquid was removed for the analyses
of ammonium ion (NH4?–N) and dissolved organic carbon
(DOC) with 10 ml syringe. We pretreated samples with
0.20 micrometer membrane filter to get only dissolved
organic sample. DOC was analyzed by Total Organic
Fig. 3 Process flow of the
degradation of organic matter
through anaerobic digestion
Table 1 Content and mixture ratio of seaweed, vegetable waste and
fishery waste
Content Ratio [%] (wet weight)
Seaweed
Ulva pertusa (Sea lettuce) 17
Ulva compressa (Hira-aonori) 17
Ulva prolifera (Suji-aonori) 33
Ulva meridionalis (Hosoeda-aonori) 33
Vegetable waste
Onion 20
Cabbage 19
Radish 16
Chinese Cabbage 16
Potato 16
Carrot 14
Fishery waste
Starfish 50 (TS based)
Blue mussel 50 (TS based)
Table 2 Chemical characteristics of seaweed, vegetable waste and
fishery waste
TS (%) C (%-TS) C/N (–) C/P (–)
Seaweed 5.3 31.1 11.1 163.7
Vegetable waste 9.3 36.2 22.6 103.4
Fishery waste 17.7 28.3 4.4 44.2
J Mar Sci Technol
123
Carbon Analyzer for Laboratory (TORAY Engineering
Co., Ltd., TOC-150). Volatile fatty acids (VFA) were
analyzed by a high performance liquid chromatograph,
HPLC, (SHIMADU, LC-20) with an electric conductivity
detector (COD-10A VP) and a 8.0 9 300 mm of column
(Shim-pack SCR-102H) using 0.8 ml min-1 H3PO4 as a
mobile phase. The liquid samples pretreated with a
0.45 lm membrane filter were centrifuged at 3,000 rpm for
3 h. The samples for VFA detection were kept in a freezer
at -20 �C and were centrifuged at 2,000 rpm for 10 min
before the analysis. The amount of biogas, its composition
and NH4?–N were measured or analyzed once a day in the
first week from the experiment started and every 3 days
after the second week. DOC and VFA were measured in
the first 4 days and day 7.
3.3 Digestion characteristics
Figures 5 and 6 show the biogas and the methane pro-
duction rate, respectively. Vegetable waste and fishery
waste have a rapid increase in both biogas and methane just
after the experiments started. The biogas production rate of
the vegetable waste and the fishery waste reaches a maxi-
mum at day 3, while the maximum methane production
rate of the vegetable waste and the fishery waste appeared
at day 5 and day 4, respectively. Seaweed produces biogas
and methane much slower than the other biomass and
reaches a maximum at day 6. The methane production rate
of vegetable waste and fishery waste drops significantly
after they reached their production peak. Vegetable waste
finishes the gas production around day 10, while gas pro-
duction lasted until day 22 in both fishery waste and sea-
weed. Fishery waste produces methane much faster than
vegetable waste at day 1, while the biogas production rate
is almost same. This indicates that methanogenesis in
fishery waste starts faster than vegetable waste and allows a
better methane production rate.
Fig. 4 Schematic diagram of a 1-l anaerobic digester
Fig. 5 Biogas production rate during the experiments
Fig. 6 Methane production rate during the experiments
Fig. 7 Time series of dissolved organic carbon for the first 7 days
J Mar Sci Technol
123
Dissolved organic concentration of three samples each
reaches the highest at day 1 and declined after day 4 as
shown in Fig. 7. The vegetable waste kept highest among
three samples in the first 4 days. These results indicate that
hydrolysis starts within a day and continue in the first
4 days.
Figure 8 represents acetic acid, which is converted to
carbon mass unit. The acetic acid of vegetable waste and
fishery waste reaches the peak at day 1 and is consumed by
day 4. The seaweed reaches the peak at day 2 and the
concentration gradually decreases. This means that sea-
weed takes more time to consume acetic acid than vege-
table waste and fishery waste, implying that it causes the
time delay of the maximum methane production rate.
Figure 9 represents time series of pH. The pH of veg-
etable waste and fishery waste declines rapidly within a
day, indicating that the production of acids, which are
essential substances for methane production, starts. The pH
drop of the fishery waste is smaller than the vegetable
waste. The pH of seaweed decreases slowly until day 5 and
then gradually increases.
Attention will now be given to the concentration of
NH4?–N as shown in Fig. 10. The concentration of NH4
?–
N in fishery waste rapidly increases in the first 4 days and
reaches 1,300 mg l-1, while the seaweed and vegetable
waste is almost stable between 1,025 and 1,100 mg l-1 all
the time. As shown in Table 2, fishery waste contains more
nitrogen, a component of protein, than the other biomass.
Therefore, the rapid increase of NH4?–N results from the
decomposition of protein and influences the pH drop.
Table 3 summarizes the obtained yield of biogas and
methane per carbon and per total solids in the biomass and
carbon recovery rate, which is defined by the ratio of
carbon in produced methane to carbon in the biomass.
Among the three biomass, it is found that fishery waste
obtained the best carbon recovery rate. As for seaweed and
vegetable waste, seaweed obtained slightly more methane
yield and slightly better carbon recovery rate than vege-
table waste. Some literature represent methane gas yield of
Ulva sp., as below. Matsui and Koike [21] reported that the
methane gas yield is 125 N ml g TS-1 at thermophilic
condition (55 �C) in a pilot-scale. The methane gas yield
obtained by Habig and Ryther [8] ranges from 70 to
130 Nml gTS-1 at mesophilic conditions (28 �C). Hansson
Fig. 8 Time series of acetic acid for the first 7 days
Fig. 9 Time series of pH during the experiments
Fig. 10 Time series of NH4?–N during the experiments
Table 3 Total biogas and methane yield and carbon recovery rate
Biogas
(Nml g-C-1)
Methane
(Nml g-C-1)
Methane
(Nml g-TS-1)
Carbon
recovery
rate
Seaweed 911 658 205 0.35
Vegetable
waste
940 616 223 0.33
Fishery
waste
1,202 876 248 0.47
J Mar Sci Technol
123
[9] obtained methane gas yield ranged from 170 to
220 Nml gTS-1 at mesophilic conditions (35 �C) with a
green algae mixture of Ulva, Cladophora and Chaeto-
morpha. The values above are calculated from the VS
based methane gas yield in the literature. Our results are
sufficient compared to the literature though the methane
fermentation conditions are different.
4 Anaerobic digestion model for appropriate operation
4.1 Model formulation
We developed a mathematical model of an anaerobic
digestion reactor based on carbon mass balance as shown
in Fig. 11. The model has eight substances flowing in and
out the reactor: particles organic matter (POC), degradable
organic matter (POCe), dissolved organic matter (DOC),
dissolved organic matter except for acetic acid (DOCa),
acetic acid (Acd), ammonia nitrogen (NH4), methane and
carbon dioxide. The proposed model consists of three
reaction processes: hydrolysis, acetogenesis and
methanogenesis.
4.1.1 Hydrolysis
We assume that the produced substances at hydrolysis are
proportional to the amount of both hydrolysis bacteria and
degradable organic matter.
Flux of dissolved organic matter and the amount of
degradable particle organic matter are shown as follows.
F1 ¼ VpBpPOCe ð1Þ
POCe ¼ ðecÞPOC ð2Þ
where Vp is the coefficient of hydrolysis, Bp is the amount
of hydrolysis bacteria and ec is the ratio of degradable
organic matter to the organic one.
4.1.2 Acidogenesis
Flux related to acidogenesis is F2a, F2b and FNH4. We
assume that acidogenic bacteria produce acetic acid, car-
bon dioxide and ammonia nitrogen which is a by-product
when dissolved organic matter of the amino group is
decomposed. The produced substances are assumed to be
proportional to the amount of acidogenic bacteria shown as
F2a ¼ VaBaDOCa ð3Þ
Chemical equation of decomposition of organic matter
is expressed as
C6H12O6 þ 2H2O! 2CH3COOHþ 2CO2 þ 4H2: ð4Þ
And, therefore, the flux of produced carbon dioxide can
be written as
F2b ¼1
2F2a ð5Þ
where Va is the coefficient of acidogenesis, Ba is the
amount of acidogenic bacteria. Ammonia nitrogen is
assumed to be produced with the decomposition of the
dissolved organic matter. We assume that nitrogen in
DOCa is decomposed to ammonia nitrogen, which is
proportional to the flux of acetic acid by the ratio of
nitrogen to carbon of DOCa as follows
FNH4¼ N
C
� �F2a: ð6Þ
4.1.3 Methanogenesis
Flux related to methanogenesis is F3a, F3b and F4. Metha-
nogenic bacteria have two paths to produce methane: from
acetic acid (F3a) and from carbon dioxide (F4). When
consuming acetic acid, they produce carbon dioxide (F3b) as
a by-product. Flux with consuming acetic acid is written as
F3a ¼ VcBcAcd ð7Þ
where Vc is the coefficient of methanogenesis and Bc is the
amount of methanogenic bacteria.
Chemical equation of the decomposition of acetic acid is
expressed as below
CH3COOH! CH4 þ CO2: ð8Þ
Therefore, the flux from acetic acid to carbon dioxide
can be written as
F3b ¼ F3a ð9Þ
and flux with consuming carbon dioxide is shown as
F4 ¼ VdCO2 ð10Þ
where Vd is a methanogenesis constant related to carbon
dioxide.
Fig. 11 Schematic diagram of anaerobic digestion reactor and the
substances through the digestion
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4.2 Parameter determination
The coefficient (Va, Vc, Vd, Vp) was determined by using a
least-squares fit of the methane gas production, the con-
centration of acetic acid, pH and the content of ammonium
nitrogen obtained from the experiments. It was assumed
that the amount of bacteria (Ba, Bp, Bc) is constant at
10 mgC l-1 and that 70 % of the produced methane gas
comes from acetic acid and 30 % of that from carbon
dioxide [22]. The values of parameters for the calculation
are described in Table 4.
We compare the calculation and experimental results
as shown in Figs. 12, 13, 14, 15, 16, 17, 18, 19 and 20.
The measurement error is -12.4 to ?12.4 % for acetic
acid, -7.9 to ?7.9 % for CH4 and -8.4 to ?8.4 % for
NH4. The model has a good agreement with acetic acid,
methane gas and NH4?–N of all substances. With regard
to pH, the calculated results are much lower than the
experimental results though it can follow the trend such
as rapid drop in a day. The discrepancy can be related to
seawater content in biomass since the model equation
does not assume that seawater can make the sludge
alkalinized. The proposed model can follow the time
series of the substances though we should consider the
effect of seawater to improve the accuracy especially
about pH.
Table 4 Values of model parameters used in the simulations
ec Vp (/day) Va (/day) Vc (/day) Vd (/day)
Seaweed 0.63 0.150 0.016 0.031 0.062
Vegetable waste 0.60 0.297 0.022 0.050 0.071
Fishery waste 0.80 0.100 0.041 0.075 0.057
0
50
100
150
200
250
300
350
400
450
500
0 1 2 3 4 5 6 7
Acetic acid (Calculation)
CH4 (Calculation)
Acetic Acid (Measurement)
CH4 (Measurement)
Time (day)
Car
bon
(mgC
l-1)
Fig. 12 Comparison between measured and calculation values for
seaweeds (CH4 and acetic acid)
900
950
1,000
1,050
1,100
1,150
1,200
1,250
1,300
1,350
1,400
0 1 2 3 4 5 6 7
Calculation
Measurement
Time (day)
NH
4+ -N
(m
gN l-1
)
Fig. 13 Comparison between measured and calculation values for
seaweeds (NH4?–N)
Fig. 14 Comparison between measured and calculation values for
seaweeds (pH)
0
50
100
150
200
250
300
350
400
450
500
0 1 2 3 4 5 6 7
Acetic acid (Calculation)
CH4 (Calculation)
Acetic Acid (Measurement)
CH4 (Measurement)
Time (day)
Car
bon
(mgC
l-1)
Fig. 15 Comparison between measured and calculation values for
vegetable waste (CH4 and acetic acid)
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5 Simulation of continuous operation
5.1 One month continuous operation
Here, we apply the proposed model to 1 month of contin-
uous operation to investigate the effect of continuous bio-
mass input to methane production, pH and NH4?–N.
Table 5 describes the scenarios of two primary biomass
with various biomass input to the fermentation tank. We
choose vegetable waste and fishery waste as primary bio-
mass since their methane production is more efficient than
the one of seaweed. Primary biomass of Cases 1 to 3 and
Cases 4 to 6 are vegetable waste and fishery waste,
respectively. Case 1 is only vegetable waste while Case 2 is
vegetable input with dilution water and Case 3 is the one
with seaweed. In Cases 2 and 3, vegetable waste and the
900
950
1,000
1,050
1,100
1,150
1,200
1,250
1,300
1,350
1,400
0 1 2 3 4 5 6 7
Calculation
Measurement
Time (day)
NH
4+ -N
(m
gN l-1
)
Fig. 16 Comparison between measured and calculation values for
vegetable waste (NH4?–N)
Fig. 17 Comparison between measured and calculation values for
vegetable waste (pH)
0
50
100
150
200
250
300
350
400
450
500
0 1 2 3 4 5 6 7
Acetic acid (Calculation)
CH4 (Calculation)
Acetic Acid (Measurement)
CH4 (Measurement)
Time (day)
Car
bon
(mgC
l-1)
Fig. 18 Comparison between measured and calculation values for
fishery waste (CH4 and acetic acid)
900
950
1,000
1,050
1,100
1,150
1,200
1,250
1,300
1,350
1,400
0 1 2 3 4 5 6 7
Calculation
Measurement
Time (day)
NH
4+ -N
(m
gN l-1
)
Fig. 19 Comparison between measured and calculation values for
fishery waste (NH4?–N)
Fig. 20 Comparison between measured and calculation values for
fishery waste (pH)
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Table 5 Scenarios of continuous biomass input for simulation
Primary
biomass
Biomass input
Case 1 Vegetable waste Vegetable waste: daily
Case 2 Vegetable waste: every other day
Dilution water: every other day but
not same day with vegetable waste
Case 3 Vegetable waste: every other day
Seaweed: every other day but not
same day with vegetable waste
Case 4 Fishery waste Fishery waste: daily
Case 5 Fishery waste: daily
Dilution water: every 3 days
Case 6 Fishery waste: daily
Seaweed: every 3 days
Fig. 21 Simulated results of CH4 for vegetable waste (Cases 1–3)
Fig. 22 Simulated results of NH4?–N for vegetable waste (Cases
1–3)
Fig. 23 Simulated results of pH for vegetable waste (Cases 1–3)
Fig. 24 Simulated results of CH4 for fishery waste (Cases 4–6)
Fig. 25 Simulated results of NH4?–N for fishery waste (Cases 4–6)
J Mar Sci Technol
123
other biomass are alternately put into the fermentation
tank. Cases 4 to 6 are daily input of fishery waste and Cases
5 and 6 contain dilution water and seaweed, which are put
into the fermentation tank every 3 days, respectively.
Calculation profiles have been shown in Figs. 21, 22, 23,
24, 25 and 26.
The total yield of methane gas in Case 1 and 4 for
1 month are higher than the other individual cases of
dilution water and seaweeds (Figs. 21, 24). Thus, addition
of dilution water results in decreased production of meth-
ane gas.
The change in NH4?–N content of vegetable-based
biomass (Cases 1–3) and fishery-based biomass (Cases
4–6) as a function of time (Figs. 22, 25, respectively)
exhibit different characteristics. NH4?–N from fishery
wastes accumulates daily even though the sludge is daily
withdrawn. Case 4 recorded 2,400 mgN l-1 on the 30th
day, which is still well within the toxicity level of
4,000 mg l-1 [23]. However, it was evident that continu-
ous biomass input causes inhibition by ammonia after
30 days. Notably, with regards to dilution water and sea-
weed, seaweed is slightly more effective in buffering the
accumulation of NH4?–N.
As observed from Figs. 23 and 26, all the cases display
the same characteristic drop in the pH caused by aceto-
genesis. Cases 1 and 4 show a significant pH drop within
5 days and reach around 6.5, which is generally considered
to be the lower limit for a proper anaerobic digestion [18].
As the calculation does not consider the effect of pH,
practical operation of Cases 1 and 4 seems to yield smaller
amount of methane gas than the calculated value. The pH
of seaweeds is lower than that of the dilution water since
seaweed itself is also digested. The results indicate that the
dilution water and seaweed can be used for preventing the
drop in pH.
5.2 Biomass input for practical operation
Experimental results and calculations indicate the impor-
tance in arranging the biomass input operation considering
biomass digestion characteristics. Vegetable waste and
fishery waste can be good candidates for the rate of pro-
duction as well as the yield of methane. The only draw-
back, however, pertains to the resulting drop in pH. Fishery
waste may cause an accumulation of NH4?–N owing to its
high nitrogen composition. Simulation results indicate that
dilution water and seaweed are effective in overcoming this
disadvantage. In practice, seaweeds could be used as a
supplementary biomass since they do not have high
anaerobic digestion efficiency thus denying them the
opportunity to be considered as the main biomass of the
anaerobic digestion plant. Seaweeds can also contribute in
saving the consumption of dilution water.
6 Conclusions
In this study, we have introduced a marine biomass utili-
zation system equipped with an anaerobic digestion system
for treating seaweeds, vegetable waste and fishery waste.
We investigated the characteristics of anaerobic digestion
for each biomass and found that seaweed took the most
time for methane production. On the other hand, the results
of vegetable waste and fishery waste showed good effi-
ciency of anaerobic digestion. High value of NH4?–N in
fishery waste indicates that a careful control is necessary to
prevent inhibition due to ammonia.
We developed a simple model based on the carbon mass
balance as a continuous operation method and found that it
agrees reasonably with the experimental results. We sim-
ulated a 1 month continuous operation with seaweeds,
vegetable waste and fishery waste. The results from the
simulation showed that the vegetable and fishery waste
without dilution water may cause a rapid drop in pH and
the accumulation of ammonium ion, respectively indicat-
ing a need for dilution water for preventing those disad-
vantages. It was also suggested that seaweed could be an
alternative to dilution water owing to its high water con-
tent. The proposed scenario is effective based on the fact
that seaweeds cannot become a main biomass of the plant
since seaweed yield varies seasonally.
Acknowledgments This study was supported by Grants-in-Aid for
Scientific Research (Grand No. 22246110) under the Ministry of
Education, Culture, Sports, Science and Technology in Japan and by
Fundamental Research Developing Association for Shipbuilding and
Offshore (REDAS). We are grateful to Prof. Masanori Hiraoka, Kochi
University, Dr. Takayuki Kusakabe, Research Institute of Environ-
ment, Agriculture and Fisheries, Osaka Prefectual Government and
Mr. Kotaro Goto, Nihon Mikuniya Co. Ltd., for providing cultivated
Fig. 26 Simulated results of pH for fishery waste (Cases 4–6)
J Mar Sci Technol
123
seaweed (Ulva meridionalis), starfish and blue mussel, respectively.
Our sincere thanks to Prof. Takasada Ishii, Osaka Prefecture Uni-
versity, for the HPLC analysis. Thanks are due to Mr. Nobuyuki
Kotera for assistance with the experiments.
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