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Page 1: Anaerobic digestion of marine biomass for practical operation

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

Page 2: Anaerobic digestion of marine biomass for practical operation

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

Page 3: Anaerobic digestion of marine biomass for practical operation

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

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Page 4: Anaerobic digestion of marine biomass for practical operation

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

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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

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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

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[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)

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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

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Page 12: Anaerobic digestion of marine biomass for practical operation

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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