anaerobic digestion of biomass for methane production: a review
TRANSCRIPT
Pergamon
Biomass and Biornergy Vol. 13, Nos. l/2, pp. 833114, 1997 mc 1997 Published by Elsevier Science Ltd. All rights reserved
Printed in Great Britain PII: SO961-9534(97)00020-2 0961-9534/97 $17.00 + 0.00
ANAEROBIC DIGESTION OF BIOMASS FOR METHANE PRODUCTION: A REVIEW
V. NALLATHAMBI GUNASEELAN
Department of Zoology, PSG College of Arts and Science, Coimbatore, 641 014, India
(Received 24 April 1996: revised 3 March 1997; awepred 31 Mnrch 1997)
Abstract-Biological conversion of biomass to methane has received increasing attention in recent years. Hand- and mechanically-sorted municipal solid waste and nearly 100 genera of fruit and vegetable solid wastes, leaves, grasses, woods, weeds, marine and freshwater biomass have been explored for their anaerobic digestion potential to methane. In this review, the extensive literature data have been tabulated and ranked under various categories and the influence of several parameters on the methane potential of the feedstocks are presented. Almost all the land- and water-based species examined to date either have good digestion characteristics or can be pre-treated to promote digestion. This review emphasizes the urgent need for evaluating the inumerable unexplored genera of plants as potential sources for methane production. c 1997 Published by Elsevier Science Ltd
Keywords-Biomass; methane yield; municipal solid waste; fruit and vegetable solid waste; grasses; woody biomass; weeds; aquatic biomass; anaerobic digestion; biochemical methane potential; renewable energy. anaerobic digesters
1. INTRODUCTION
Biomass has been defined as contemporary plant matter formed by photosynthetic capture of solar energy and stored as chemical energy.’ The recent oil crisis and the consequent price rises have spawned considerable interest in the exploration of renewable energy sources. Bioen- ergy will be the most significant renewable energy source in the next few decades until solar or wind power production offers an economi- cally attractive large-scale alternative. The energy that biomass contains can be reclaimed by various methods.’ The criteria for selection of the conversion process and the advantages of anaerobic digestion (AD) are outlined by Chynoweth et al.’ This paper surveys the primary biomass sources for methane (CH,) production reported in the literature. Animal manures, sewage sludges and effluents from biomass-based industries, which are secondarily derived from the vegetation are outside the scope of this review. Most of the data reported do not contain any statistical information on variability, only the mean values. A few of the data from the literature lack homogeneity in conditions of measurement, units, etc. and, in some cases, the data given by individual research groups are inadequate and are not included in this outline.
2. AD PROCESSES FOR BIOMASS
2.1. Conventional single stage digestion
2.1.1. Continuully fed digesters. In these digesters, the rate of feeding should be continuous for maximum efficiency, but for practical reasons the digesters are usually fed intermittently; the most common period being once a day. In climatically-heated continuous digesters, there are temperature fluctuations between day and night or between days, resulting in poor performance. In the continu- ously stirred tank reactor (CSTR), an influent substrate concentration of 3-8% total solids (TS) is added daily and an equal amount of effluent is withdrawn. The digester is maintained constantly at mesophilic or thermophilic tem- perature. The addition of large amounts of water requires large reactor volume and high post-treatment costs for the digester residue. In semi-dry digestion. substrate concentration in the range of 16-22% TS is used.
2.1.2. High solids anaerobic digestion. This process takes place at a TS concentration of more than 25% and is also called “dry anaerobic fermentation”. Most of the high solids AD studies have been confined to municipal solid waste (MSW).” ” The Ref- COM, SOLCON, dry anaerobic cornposting (DRANCO), KWU-Fresenius, BIOCEL and
83
84 V. NALLATHAMBIGUNASEELAN
sequenced batch anaerobic cornposting (SE- bacteria are attached to small glass spheres BAC) are the dry fermentation processes using which are freely suspended in the up-flowing MSW as the substrate, some of which were feed. discussed in a recent review.16 The SEBAC process have been developed at the University 2.2. Two-stage and two-phase digesters
of Florida for conversion of organic fraction of In a two-stage digester, the residual substrates MSW (OF-MSW) to CH, and compost. It from the first stage can be reduced at the employs three stages for enhanced conversion of second-stage digester, carrying out the same MSW to CH,. The SEBAC system, a promising reactions as the first stage but running at a concept for the AD of MSW, is described different retention time. For quickly fer- elsewhere.‘?, I3 mentable wastes, a two-stage reactor can have a
2.1.3. BIOGAS and BIOMETprocesses. The lower overall retention time than a single stage. “BIOGAS” process has been developed at the The second stage could be a stirred tank or a Institute of Gas Technology (IGT), U.S.A. This plug-flow digester or an anaerobic filter. concept combines the treatment of sewage A two-phase digester is a mechanically similar sludge (SEW) at 2-3% TS and solid wastes system of two stirred-tank digesters. In this (MSW at 55% TS) resulting in a substrate process, fermentation and methanogenesis are concentration of about 10% TS.” A similar separated by using different retention times. co-digestion process called the “BIOMET”‘” Liquefaction and acidification of the substrate is
has been studied at pilot scale in Sweden. accomplished in a first reactor, while only
2.1.4. BIOTIIERMGAS process. The BIO- methanogenesis takes place in the second
THERMGAS process carried out by the IGT, reactor. It was first promoted by Ghosh et al.*’
U.S.A., combines biological and thermochemi- for the combined digestion of SEW and MSW.
cal unit operations into a scheme that can The total digestion time was considerably lower
convert the biomass efficiently (regardless of than the conventional single-stage digestion.
moisture and nutrient contents) to CH, with Some kinetic considerations argue in favour of
minimum process residues.” Results of the the two-phase approach when optimal growth
preliminary systems analyses using Bermuda conditions for hydrolytic and methanogenic
grass and MSW as feedstocks indicate that this bacteria are considered.*’ Colleran et al.,**
process is technically superior to either biologi- Verrier et a1.,23 Mata-Alvarez24 and Viturtia et
cal or thermochemical processes and economi- a1.‘5 proposed this process for the digestion of
cally feasible. agricultural solid wastes. Two-phase AD of
2.1.5. Plug-Jlow digester. In tubular plug- OF-MSW was studied by Hofenk et a1.,26 who
flow digester, a volume of the medium with a concluded that there was no difference in the
suitable inoculum enters at one end of the tube biogas yields between single-stage and two-
and, if the rate of passage of the medium is phase systems. Unless the hydrogen produced in
correct, by the time the medium reaches the the fermentative phase can be collected and
other end the digestion is completed. For transferred to the methanogenic phase, a loss of
continuous operation, some of the digested potential CH, occurs. This process is techno-
effluent flowing from the end of the tube logically feasible, but an assessment of the
is separated and returned to the influent economic feasibility is more complex and has to
substrate. be reviewed for any given situation.
2.1.6. The anaerobic j3ter. This is primarily meant for digestion of easily fermentable 3. BIOCHEMICAL METHANE POTENTIAL (BMP)
factory waste waters produced in large quan- ASSAY
tities. Even a 6-day retention time would mean The BMP assay was developed to determine an impossibly large digester. Hence, in order to the ultimate CH, yield (B,) of organic substrates prevent washout, the bacteria are allowed to and for monitoring anaerobic toxicity.*’ B, of a attach to a solid support, such as stones packed variety of biomass were determined using a inside a tank and the waste water flows upward through the tank. This process requires a
modified method of Owen et a1.28m33 The BMP is a valuable, quick and inexpensive method for
retention time of only a few hours and the gas determination of the potential extent and rate of is collected from the top. In a fluidized-bed conversion of biomass and wastes to CH,. A digester, a modified form of anaerobic filter, the similar assay has otherwise been named as
Anaerobic digestion of biomass for methane production: a review 85
anaerobic biogasification potential (ABP) as- say.34
4. POTENTIAL SOURCES FOR METHANE
A wide range of biomass have been considered as potential sources for CH, production (Fig. 1).
4.1. Organic fraction of MSW
4.1.1. MSW composition. MSW has been identified as a heterogeneous material in which the composition varies widely. The composition of MSW is affected by various factors, including regional differences, climate, extent of recycling, collection frequency, season, cultural practices, as well as changes in technology.35 The qualities of the OF-MSW are influenced not only by the sorting system but also by various methods used for quantifying the OF-MSW. According to Mata-Alvarez et al.” in mechanically-sorted MSW (MS-MSW), large amounts of suspended, non-biodegradable solids and unavoidable small pieces of plastic, wood, paper, etc. are present. The mechanically-sorted organic frac- tion of MSW (MS-OF MSW) used to feed the
I I
WOODS
GRASSES
1 ORGANIC I
digester in Treviso contained (on a TS basis) 50% putrescible fraction, 6% paper, 1% wood, 2% plastic and 36% inert fraction. The percentage of VS of the waste was 43%. These non-biodegradable solids are not present in the source-sorted organic fraction of MSW (SS-OF MSW) or hand-sorted organic fraction of MSW (HS-OF MSW) or in the organic fraction of MSW from a separated collection (SC-OF MSW). Consequently, the VS content of the waste was 88°h.36 However, the MS-OF MSW from Sumter country contained (on a TS basis) 47% paper, 11% cardboard, 10% plastic, 6% yard waste and 23% miscellaneous and its VS content was 81°h.‘2.‘3 The HS-OF MSW from Levy country contained (on a TS basis) 92% paper and the percentage of VS was 93%.“,‘3 Rivard et a1.37 reported that most MSW processing technologies result in the separation and removal of the food and yard waste fraction to produce refuse-derived fuel (RDF). This results in the reduction of the nutrient value of the processed MSW as a feedstock for AD. Nevertheless, considering the percentages of VS of OF-MSW presented in Tables 1 and 2, two groups can be denoted. The first, with a VS
-I FRESHWATER BIOMASS I
I I
MARINE BIOMASS
1 AQUATIC
Fig. 1. Selected types of methane yielding biomass
Feed
MS-
OF
MSW
C
ont.
= 30
&35
%
TS
Tabl
e 1.
Dig
este
r pe
rform
ance
w
ith
mun
icip
al
solid
w
aste
fe
eds
at
mes
ophi
lic
tem
pera
ture
s
HR
T O
LR
CH
, yi
eld?
C
H,
PRP
VSr
Fe
rmen
ter
Tem
p.
(“C)
(d
ays)
(k
g V
Sm-’
d -’
) (m
’ kg
-’ V
S,)
(m’
rnm
J d-
‘)
(%)
Ref
eren
ce
g
3540
16
-21
NR
La
bora
tory
pl
ant
0.03
5 m
’ D
ranc
o pr
oces
s
[71
MS-
OF
MSW
C
orm
. =
25-3
5%
TS
Pilo
t pl
ant
60 m
3 D
ranc
o pr
oces
s
3540
14
-21
10.0
12
.1
13.2
15
0.26
0*
0.26
4*
0.23
5*
0.18
7*
2.6
3.2
3.1
2.8
NR
PI
MS-
OF
MSW
Pi
lot
plan
t C
ont.
= 35
%
TS,
500
m’
VS
= 58
.6%
TS
V
alor
ga
proc
ess
MS-
OF
MSW
: SE
W
85:lS
TS
ba
sis
Con
t. =
7710
%
TS
CST
R
20
m’
BIO
MET
pr
oces
s
HS-
OF
MSW
C
ont.
= 3-
5.6%
TS
V
S =
82-8
7%
TS
CST
R
Labo
rato
ry
plan
t
HS-
OF
MSW
C
ont.
= 6.
4%
TS,
VS
= 89
.9%
TS
C
STR
3
m’
OF
MSW
(s
imul
ated
): SE
W
80:2
0 TS
ba
sis
Con
t. =
6.6%
TS
V
S=91
%
TS
CST
R
2.2
m’
Proc
esse
d M
SW
(TR
F):
Yea
st
extra
ct/m
iner
als.
5.78
: l(V
S ba
sis)
CST
R
Sem
i co
ntin
uous
3.
5 I
TRF:
pr
edig
este
d SE
W
4.76
:1
(VS
basi
s)
MS-
OF
MSW
, Su
mte
r co
untry
FL
Fr
esh,
V
S =
79.7
%
TS
Drie
d,
VS
= 84
.1%
TS
CST
R
Sem
i co
ntin
uous
3.
5 1
BM
P as
say
HS-
OF
MSW
Le
vy-l
coun
try
FL
VS
= 92
.5%
TS
Y
ard
was
te
sam
ples
G
rass
, V
S =
88.1
%
TS
Leav
es,
VS
= 95
%
TS
Bra
nche
s, V
S =
93.9
%
TS
Ble
nd,
VS
= 92
%
TS
BM
P as
say
BM
P as
say
37
15
13.7
0.
230
2.6*
45
[9
1
3742
19
2.
6 0.
230
0.58
41
21
1.
6 0.
290
0.46
48
I1
81
35
33-3
7
33-3
7
37
37
35
35
35
1442
0
9925
I 0.
390
0.39
1.
5 0.
360
0.55
2
0.43
0 0.
87
4 0.
430
1.70
2.1-
6.9
0.39
0 0X
2-2.
02*
NR
63-6
9
14
3.9
0.29
0 1.
59*
70-7
5
20
14
20
14
NA
NR
0.
324(
0.04
3)
0.77
(0.1
8)*
1.14
(0.4
0)*
0.69
(0.1
7)*
1.04
(0.2
3)*
NA
NR
[381
2 E 2;
F
[391
8 6 2
[401
g R
E
[371
0.33
6(0.
067)
NR
0.
294(
0.03
8)
0.30
7(0.
037)
N
R
NA
0.
222(
0.01
4):
0.21
5(0.
013)
$ N
R
NA
N
A
0.20
5(0.
01
l)$
NA
N
R
NA
N
A
0.20
9(0.
005)
$ 0.
123(
0.00
5)$
0.13
4(0.
006)
$ 0.
143(
0.00
4)f
NA
N
R
[321
f321
[321
Anaerobic digestion of biomass for methane production: a review 87
a
Tabl
e 2.
D
iges
ter
perfo
rman
ce
with
m
unic
ipal
so
lid
was
te
feed
s at
th
erm
ophi
lic
tem
pera
ture
s g
Feed
Fe
rmen
ter
Tem
p.
(“C)
H
RT
OLR
C
H,
yiel
d?
CH
, PR
t V
Sr
(day
s)
(kg
VSm
-3d-
‘)
(m’
kg-’
VS,
) (m
l m
-l d-
‘)
(%)
Ref
eren
ce
MS-
OF
MSW
C
ont.
= 3&
35%
TS
La
bora
tory
pl
ant
0.03
5 m
’ D
ranc
o pr
oces
s
Pilo
t pl
ant
0.06
0 m
’
SC
-55
16-2
1 10
.0
0.28
6*
2.86
* 12
.1
0.28
2*
3.41
* 13
.2
0.28
3*
3.74
* 14
.9
0.31
0*
4.62
* 14
.7
0.13
1*
1.92
11
.8
0.14
7*
1.74
7.
8 0.
185*
1.
44
3.9
0.24
6*
0.96
17
.5
0.13
3*
2.33
14
.0
0.18
0*
2.52
9.
3 0.
236*
2.
20
4.7
0.31
9*
1.50
20
.6
0.14
0*
2.88
16
.5
0.20
2*
3.34
10
.9
0.24
2*
2.64
5.
4 0.
289*
1.
56
23.5
o.
ooo*
0.
00
18.8
0.
128*
2.
40
12.4
0.
160*
I .
98
6.2
0.27
I *
I .
68
18-2
0 0.
220
3.64
4.48
* 16
.5
0.20
0 3.
3*
NR
MS-
OF
MSW
C
ont.
= 25
%
TS
VS
= 47
%
TS
55
NR
MS-
OF
MSW
Pi
lot
plan
t C
ont.
= 30
%
TS
0.06
0 m
3 55
N
R
MS-
OF
MSW
Pi
lot
plan
t C
ont.
= 35
%
TS
0.06
0 m
’ 55
8 IO
15
30 8 10
I5
30 8 IO
15
30 8 IO
15
30 9 12
NR
MS-
OF
MSW
Pi
lot
plan
t C
ont.
= 40
%
TS
0.06
0 m
’ 55
N
R
MS-
OF
MSW
C
ont.
= 30
%
TS
Labo
rato
ry
plan
t 0.
015
m1
Val
orga
pr
oces
s
60
50
46
MS-
OF
MSW
Su
mte
r co
untry
, V
S =
81%
TS
Pilo
t pl
ant
SEB
AC
pr
oces
s 55
42
3.
2 0.
190
0.61
49
.7
21
6.4
0.16
0 I .
02
36.0
HS-
OF
MSW
Le
vy
Cou
ntry
, V
S =
93%
TS
Pilo
t pl
ant
SEB
AC
pr
oces
s 55
21
6.
4 0.
190
1.06
40
.6
Yar
d w
aste
Pi
lot
plan
t SE
BA
C
proc
ess
55
42
NR
0.
070(
0.02
) N
R
19.0
MS-
OF
MSW
fr
esh
Pilo
t pl
ant
3 m
3 55
.8
5.7
17.8
0.
192*
3.
35*
Con
t. =
1622
%
TS
Sem
i-dry
55
.5
7.4
12.9
0.
215*
3.
01*
VS
= 44
.8%
TS
fe
rmen
tatio
n 56
.2
II.7
9.
7 0.
179*
1.
73*
NR
[71
UO
I
c z $ F 1 $ (1
11
0 2 %
R
[I31
5
[411
MS-
OF
MSW
pr
e-co
mpo
sed
Con
t. =
1622
%TS
V
S =
44%
TS
Pilo
t pl
ant
3 m
’ Se
mi-d
ry
ferm
enta
tion
51.5
6.
1 19
.9
0.13
1*
2.68
* N
R
54.6
7.
8 13
.5
0.15
9*
2.17
* 54
.8
11.7
6.
9 0.
254;
1.
73*
MS-
OF
MSW
C
STR
pilo
t pl
ant
Con
t. =
17%
TS;
3
m3
sem
i-dry
V
S =
44%
TS
fe
rmen
tatio
n
55
8.5
13.4
0.
188*
2.
53*
NR
f4
21
MS-
OF
MSW
: A
lgae
(9
:l TS
ba
sis)
C
ont.
20%
TS
CST
R
pilo
t pl
ant
3 m
3 se
mi-d
ry
ferm
enta
tion
55
7.5
13.4
0.
212*
2.
80*
NR
> I
HR
T =
hydr
aulic
re
tent
ion
time,
O
LR
= or
gani
c lo
adin
g ra
te,
VS,
= V
S ad
ded,
C
H,
PR
= m
etha
ne
prod
uctio
n ra
te,
VS.
, = V
S re
duct
ion.
M
S-O
FMSW
=
mec
hani
cally
so
rted
orga
nic
fract
ion
of
mun
icip
al
solid
w
aste
, H
S-O
FMSW
=
hand
so
rted
orga
nic
fract
ion
of
MSW
, N
A
= no
t ap
plic
able
, N
R
= no
t re
porte
d.
5.
0 *V
alue
s ca
lcul
ated
fro
m
the
data
re
porte
d.
a tV
alue
s in
par
enth
eses
ar
e SD
. 09
Q
g 2 r?
,
90 V. NALLATHAMBI~UNASEELAN
content of above 82% corresponds to the HS, SS, SC or simulated 0F-MSW.‘3,36,38 40 The second refers to most of the data for MS-OF MSW with VS content less than 60%.9~‘o~‘3~36.4’.42 Given these characteristics, higher biodegrad- ability and consequently higher yields are expected from the AD of HS or SS-OF MSW.
4.1.2. OF-MS W digestion at mesophilic tem- perature. Considering the biodegradation of OF-MSW in a CSTR-type digesters at 35°C a maximum CH, yield ranging from 0.39 to 0.43 m3 kg-’ VS added was reported for HS-OF MSW without paper and wood36.3R.39 and VS reduction (VSr) ranged from 63 to 69% (Table 1). The methane yield of MS-OF MSW ranged from 0.11 to 0.16 m3 kg-’ VS added and VSr was around 30% due to its high ash value.36 The CH, yields reported for MS-OF MSW at high-solids (Table 1) ranged from 0.18 to 0.26 m3 kg-’ VS added with a VSr of 45%;’ 9 however, methane production rate (CH,PR) of 3.2 m3 mm3 d-’ was achieved at loading rate (OLR) of 12 kg VS mm3 d-’ and retention time (HRT) of 1621 days.’ The OLR applied in the Dranco process8 is the highest, whereas that applied by Pauss et a1.3X is the lowest. The potential of AD of OF-MSW increases in systems in which co-digestion of MSW and SEW is carried out.“. 36.40
4.1.3. OF-MS W digestion at thermophilic temperature. In the thermophilic high-solids digestion studies (Table 2) higher OLR and CH,PR could be achieved at reduced HRT as expected and the CH, yields of MS-OF MSW were around 0.2 m3 kg-’ VS added.“.” Despite the fact that the OF-MSW from Sumter and Levy countries were differently sorted and varied widely in content, their percentages of VS were above 81%. SEBAC of the Sumter and Levy sources of OF-MSW showed that for the 21-day runs, a CH, yield of 0.16 and 0.19 m’ kg-’ VS added and VSr of 36% and 41%, respectively were achieved.12,13 The data- base on extent and rates of the major biodegradable organic components of MSW32 (Table 1) showed that BMP of paper samples ranged from 0.08 to 0.37 m3 kg-’ VS added, but the types of paper that comprised the Sumter and Levy sources were not reported.‘2,‘3 The presence of high proportions of slowly biodegradable lignocellulosic material like paperI would have resulted in partial biodegra- dation in 21-day runs. The potential for further improvements by optimizing several operational
parameters should make SEBAC a promising concept for AD of MSW.
According to Mata-Alvarez et a1.4’ the performance of the semi-dry process is very healthy and allows very high yields and production rates. CH, PR of 3.35 m3 m-3 d-’ at 6-day HRT is a very high figure for CSTR and it is quite comparable with those reported in the literature for dry digestion systems at ther- mophilic conditions (Table 2).
It has been demonstrated that the algae from the Venice lagoon can be co-digested with the OF-MSW under semi-dry thermophilic con- ditions. This approach will contribute to the disposal of harvested algae from the lagoon of Venice.42
Cecchi et af.43 proposed the step diffusional model to describe substrate utilization during AD of the SS-OF MSW. The new model is found to show a better fit to the experimental result than those obtained with other models.
4.1.4. Partial cornposting prior to digestion, Ten Brummeler and Kosterls reported that the start-up of the dry anaerobic batch digestion (BIOCEL process) of the OF-MSW at 30°C can be accelerated by partial aerobic cornposting for 2 weeks. A major drawback was a loss of 40% of the potential CH, yield during cornposting. A shorter partial cornposting period might be more feasible. According to Mata-Alvarez et a/.4’ pre-cornposting process surely removes the easily degradable fraction of the organics in the MSW causing the lower digester performance. However, at the same time during the cornposting process some of the large molecules, which are difficult to degrade, are broken down making them more easily available for the anaerobic hydrolytic bacteria of the digester. Thus, at long HRT ( > 12 days) this effect is noticeable in the case of pre-composted MS-OF MSW, increasing the CH, yield, whereas at shorter HRT (68 days) there is no time to degrade the de-polymerized compounds and only the contrary effect is more heavily observed (Table 2). Further research is needed to test the validity of this hypothesis as it is reported that the methanogenic potential of the waste from the S. Giorgio di Nogaro plant, which was pre-composted aerobically, was considerably reduced (0.14 m3 kg-’ VS) when the reactor operated at 21-day HRT36 (Table 1).
4.2. Sewage sludge and industrial efluent
A considerable amount of information has been gathered over the performance of sewage
Anaerobic digestion of biomass for methane production: a review 91
sludge digesters. Both primary and secondary sludges are fed into anaerobic digesters, mainly as a means of sludge reduction and gas production. Chynoweth et al.” have reported a BMP of 0.59 m3 kg-’ VS for the primary sludge. Effluents from breweries and distilleries and palm-oil (oil produced from the palm tree, Elaeis guineensis) mill and solid waste from instant coffee industry have been tested on laboratory or large-scale anaerobic digesters. In most cases, pollution control is a major factor, along with, or to the exclusion of, gas energy production. For more detailed information on these aspects, the reader may consult the book44 and original papers.“’ 49
4.3. Fruit andz?egetahle solid waste (FVSW) and leaves
4.3.1. FVS W. These wastes are characterised by high percentages of moisture ( > 80%) and VS ( > 95%) and have a very high biodegrad- ability. They are transported to municipal dump sites and Mata-Alvarez et al.76 have referred these wastes as SC-OF MSW. As can be seen in Table 3, the CH, yield of FVSW is very high. Data from the literature indicate the AD potential of FVSW, most of which refer to laboratory trials.
According to Knol et a1.5o the maximum OLR for stable digestion of a variety of FVSW ranged from 0.8 to 1.6 kg VS mm3 d-’ with 32-days HRT. The French bean waste and the carrot waste were very well digested and the lower biodegradability of the asparagus peels could be due to their woody structure. For carbohydrate-rich substrates, like the apple- pulp, alkali addition and the use of mixed substrates have proved to be suitable correction measures.
However, Lane” found that recovery of settled solids from the discharged digester effluents and their return to the digester enables S&96% VS removal, provided adequate alka- linity levels are maintained. For balanced digestion, alkalinity (mg ll’) of 0.7 x volatile fatty acids (VFA, mg ll’) is required and it should not be less than 1500. The performance of digestion of asparagus waste was stable at OLR of 4.2 kg VS mm’ d-’ with 90% removal of vs.
Inadequate alkalinity levels appear to have been the cause of digestion failure of peach waste at 3 kg mm3 d-’ with 20-days HRT in experiments reported by Hills and Roberts.5’
Radhika et a1.53 evaluated the AD potential of
coconut pith (CP, the dust particles that fall away during the separation of fibres from coconut husk) and cattle manure (CM). Performance of several blends of the two feedstocks indicated that CP and CM mixture in the ratio 3 : 2 (dry wt. basis), respectively, showed enhanced biogas production with S&85% CH,.
Yang et a1.54 examined at 30°C biogasification of papaya processing wastes and found that with sludge recycling HRT was reduced, while maintaining effective anaerobic performance at OLR of 0.85-l .06 kg VS mm3 d-’ with SRT near 25 days.
According to Gollakota and Meher,” de- oiled (oil expelled) cake of non-edible oil seeds, such as castor (Ricinus communis) could be considered as substrate for biogas production at a loading rate of 8 kg TS me3 d-‘, 15-days HRT and 37°C with intermittent mixing.
Viturtia et a1.,2s studied at laboratory scale the performance of a two-phase AD of a mixture of FVSW in the mesophilic range using a hybrid up-flow anaerobic sludge bed-anaerobic filter (UASB-AF) reactor. When the systems were operated at hydrolyzer and methanizer HRT of 2.6 and 1 day, respectively, CH, yield as high as 0.51 m3 kg-’ VS was achieved.
Stewart et a1.s6 measured biogas yields from AD of banana (fruit and stem damaged by wind) and potato waste (peelings and rejects) in 20 1 continuous digesters at 35°C. The high CH, yields obtained from the digested wastes resulted from almost complete destruction of the VS. For a HRT of 20 days with OLR 2.5 kg TS mm’ d-‘, the CH, yield for banana waste was 0.53 m’ kg-’ VS added at 100% VS conversion.
Sharma et a1.57 demonstrated the AD potential of banana peeling (Musa paradisica). According to them, particle sizes of 0.088 and 0.4 mm produced an almost equal quantity of biogas, thus grinding below 0.4 mm would seem to be uneconomical.
Ghanem et al.,5x examined the digestibility of beet pulp, a waste product from sugar industry and found that it could be utilized efficiently for biogas production when treated with 1% NaOH.
The harvest of fruits and crops varies with season. In order to operate the digester throughout the year with any of the FVSW available, Viswanath et af.59 investigated the effect of successive addition of various FVSW on digester performance. Performance was stable at 16- and 20-day HRT with an OLR of
Tabl
e 3.
D
iges
ter
perfo
rman
ce
with
fr
uit,
vege
tabl
e so
lid
was
te
and
leaf
fe
eds
(Tem
p,
tem
pera
ture
; H
RT,
hy
drau
lic
rete
ntio
n tim
e;
OLR
, or
gani
c lo
adin
g ra
te;
VS,
, V
S ad
ded;
C
H,
PR,
met
hane
pr
oduc
tion
rate
; V
S,,
VS
redu
ctio
n;
NA
, no
t ap
plic
able
; N
R,
not
repo
rted;
*
valu
es
calc
ulat
ed
from
th
e da
ta
repo
rted;
b
valu
es
in
pare
nthe
ses
are
s.d.)
E
Feed
Fe
rmen
ter
Tem
p.
(“C)
H
RT
OLR
C
H,
yiel
d”
CH
, PR
h V
Sr
(day
s)
(kgV
Sm-’
d-‘)
(m
’ kg
-’ V
S.,)
(m’
m-j
d-‘)
(%
) R
efer
ence
Spin
ach-
was
te
Asp
arag
us
peel
s Fr
ench
be
an-w
aste
St
raw
berry
-slu
rry
App
le-p
ulp
App
le-s
lurry
C
arro
t-was
te
Gre
en
pea-
slurry
Apr
icot
fib
re
Cor
n co
bs
App
le
cake
A
pple
w
aste
A
spar
agus
w
aste
Su
garb
eet
pulp
Pi
neap
ple
pres
sing
s
Papa
ya
frui
t pr
oces
sing
was
te
With
out
slud
ge
With
sl
udge
re
cycl
ing
Frui
t an
d ve
geta
ble
was
tes
mix
ture
(o
rang
e,
caul
iflow
er,
cucu
mbe
r, le
ttuce
, to
mat
o an
d w
ater
-mel
on
mix
ture
)
Frui
t w
aste
s (to
mat
o,
man
go,
oran
ge
peel
w
ith
oil,
deoi
led
oran
ge,
pine
appl
e,
bana
na
and
jack
fr
uit
was
tes
in
succ
essi
on)
Tom
ato
proc
essin
g w
aste
CST
R
Sem
i- co
ntin
uous
1
1 33
32
0.
83-1
.18
0.74
1.06
0.
961.
15
1.02
-1.1
5 1.
02~1
.60
0.83
-1.1
5 0.
80-0
.90
0.87
-1.2
5
CST
R
Con
tinuo
us
with
so
lids
recy
clin
g 10
I
35-3
7
CST
R
Sem
i- co
ntin
uous
18
81
CST
R
Sem
i- co
ntin
uous
Up-
flow
an
aero
bic
slud
ge
bed-
an
aero
bic
filte
r re
acto
r; Tw
o-
phas
e A
D;
Hyd
roly
zer
(H)
1.3
1;
met
hani
zer
(M)
0.5
I
CST
R
Sem
i- co
ntin
uous
60
1
CST
R
Sem
i- co
ntin
uous
60
I
CST
R
Sem
i- co
ntin
uous
5.
5 1
30
30
35
28-3
2
28-3
2
35
NR
15
2.61
(0.5
5)
0.16
9*
15
1.39
(0.2
4)
0.24
5*
15
0.81
(0.2
0)
0.32
1*
15
0.28
(0.0
7)
0.35
7*
12
0.85
(0.0
9)
0.35
3*
9.6
1.06
(0.2
1)
0.25
5*
H-7
.5
M-3
N
R
0.38
3
H-2
.6
M-l
3.74
* 0.
286;
N
R
96.3
3.
90*
0.26
7*
95.7
3.
88*
0.25
2*
93.4
3.
43*
0.22
8*
88.1
4.
17*
0.23
0*
89.7
4.
06*
0.26
3*
95.2
3.
87*
0.33
5*
93.2
NR
0.
510
NR
98
.5
8 3.
8 0.
030
0.13
6*
NR
12
3.
8 0.
090
.0.2
97*
16
3.8
0.25
0 0.
557*
20
3.
8 0.
370
0.70
1*
24
3.8
0.32
0 0.
502*
16
3.
8 0.
270
0.63
7*
NR
16
5.
7 0.
190
0.83
5*
16
7.6
0.11
0 0.
551*
16
9.
5 0.
040
0.21
8*
24
4.3
0.42
0 0.
8 N
R
0.31
6*
NR
70
0.
219*
40
0.
343*
70
0.
261*
50
0.
308*
40
0.
281*
60
0.
417*
75
0.
310*
75
0.44
(0.1
2)
78.8
0.
34(0
.09)
64
0.
26(0
.26)
57
.7
0.10
(0.0
2)
51.2
0.30
61
.3
0.27
54
.3
NR
90
1501
1511
[541
t251
[591
[601
Anaerobic digestion of biomass for methane production: a review 93
2 2
Tabl
e 3-
Cont
inue
d
Feed
Fe
rmen
ter
Tem
p.
(“C)
H
RT
OLR
b (d
ays)
(k
gVSm
3 d
- ‘
) C
H,
yiel
db
CH
, PR
” vs
r (m
’ kg
-’ V
S,)
(m’
me3
d-
‘)
(%)
Ref
eren
ce
??
Rhe
um r
hapo
ntic
um t
ops
(Rhu
barb
to
ps)
Fres
h Si
lage
Sym
phyt
um
aspe
rum
top
s (C
omfre
y to
ps)
Fres
h Si
lage
Hel
ia H
elia
nthu
s tu
bero
sus,
stem
+
leav
es
(Jer
usal
em
artic
hoke
, JA
) To
pina
nca
varie
ty,
fres
h en
sile
d
Var
iety
N
o.
1168
, fr
esh
ensi
led
Topi
nanc
a va
riety
, en
sile
d va
riety
N
o.
1168
, en
sile
d
JA
tops
Fr
esh
Sila
ge
Mir
abili
s ja
lapa
lea
ves
0.08
8 m
m
size
0.
4 m
m
size
1.
0 m
m
size
6.
0 m
m
size
30
x 5
0 m
m
size
Ipom
oea
fistu
losa
lea
ves
0.08
8 m
m
size
0.
4 m
m
size
1.
0 m
m
size
6.
0 m
m
size
15
0 x
100
mm
si
ze
Ipom
oea
Jist
ulos
a st
em
(IFS
) IF
S,
0.4
mm
si
ze
IFS,
40
day
s in
cuba
tion
with
w
ater
Glir
icid
ia m
acul
ata
leav
es
Calo
lrop
is p
roce
ra
leav
es,
CPL
C
PL
Bat
ch
3 1
35
NA
N
A
Bat
ch
3 1
35
NA
N
A
CST
R
Sem
i- co
ntin
uous
10
I
37
50
59
37
46
44
21
NA
27
N
A
35
NA
2.2
0.25
0*
2.5
0.26
5*
CST
R
Sem
i- co
ntin
uous
10
I
Bat
ch
3 1
Bat
ch
3 I
2.6
2.5
NA
N
A
Bat
ch
3 I
NA
Bat
ch
5 I
37
NA
N
A
0.31
6 0.
345
NA
N
R
1631
NA
N
R
[631
NR
N
R
67
[621
NR
NA
N
A
61
66
NR
N
R
NA
N
R
[621
tz22
;
[631
NA
45
.0
45.3
43
.7
43.3
38
.5
[571
NA
55
.3
55.0
54
.2
53.1
49
.9
1571
NA
50
.7
59.1
[6
41
NA
37
.5
NA
64
.5
[651
[661
1671
0.33
4 0.
323
0.30
7 0.
28 1
0.33
8*
0.35
4*
0.30
9 0.
301
0.33
9(0.
002)
0.
341(
0.00
1)
0.32
9(0.
001)
0.
327(
0.00
2)
0.29
0(0.
004)
Bat
ch
5 I
31
NA
N
A
0.42
9(0.
002)
0.
427(
0.00
1)
0.42
1(0.
005)
0.
413(
0.00
1)
0.38
7(0.
002)
Bat
ch
5 1
37
NA
N
A
0.36
1 0.
426
Bat
ch
3 1
29-3
5 N
A
NA
0.
181(
0.03
4)
Bat
ch
0.1
I 35
N
A
NA
0.
280
Bat
ch
4 1
30
NA
N
A
NR
1.
624(
0.08
7)
NR
Anaerobic digestion of biomass for methane production: a review 95
3.8 kg VS mm’ d-‘. The CH, yield was slightly leaves, such as Mirabilis, IpomoeaJistulosa, etc., lower for the 16-day HRT. entire leaves can also be used without shredding.
Sarada and Joseph6’ studied the influence of HRT, OLR and temperature on CH, PR and yield during AD of tomato-processing waste (TPW). For the 24-days HRT, 4.3 kg VS me-’ d-’ and 35°C a CH, yield of 0.42 m’ kg-’ VS added and CH, PR of 0.8 m3 rn~-’ d-’ were achieved.
Sarada and Joseph” enumerated the microfl- ora that developed during the AD of TPW. In the batch process, the methanogen count decreased possibly due to the decrease in the pH of the slurry. In semi-continuous processes, the cellulolytics, xylanolytics, pectinolytics, prote- olytics, lipolytics and methanogens increased with increase in the HRT. The numbers of methanogens were almost proportional to the HRT and this seemed to be reflected in the CH, content of the biogas. The xylanolytics and lipolytics were predominant organisms.
Mahamat et a1.‘j6 were of the opinion that the low CH, potential of Calotropis leaves may be due to the presence of some toxic compound, which may partly inhibit the digestion process. Calotropis is known to contain a strong cardiotonic, the inhibitory properties of which on AD is not known. Traore,67 however, by batch digestion experiments, showed that Calotropis is a good substrate for biogas production.
Shyam and Sharma’* conducted high-solids digestion experiments with mango leaves and CM in 1.2 m’ batch type digesters. The biogas yield of the blend was higher than CM alone.
4.4. Grasses (gramineae)
It is worthwhile including the unused parts of vegetable plants in this section as they are often seen among the FVSW. Gunnarson et a1.62 demonstrated that the biogas production was approximately equal for both fresh and ensiled Jerusalem artichoke (JA) and, thus, the crop can be stored as silage until used for AD. The variety No. 1168, a hybrid between JA and sunflower produced higher CH, yield than the Topinanca variety.
Zubr”’ had the same view that the yields and rates of biogas production from fresh and ensiled materials were not significantly different. The use of a separate ensiling followed by methanogenic fermentation would make pro- duction of biogas possible all the year round independent of the seasonal availability of raw materials.
The literature base for evaluating AD potential of grasses (Table 4) showed that Napier grass3”” energy cane (ball milled),33 Alemangrass-6A,33 turf grass (Floritum St. Aug),33 wheat straw,‘9,73 76 paddy straw,” millet straw,33. ” oats crop,” maize crop,” corn stover29 and sorghum3?. 7R. l9 exhibited CH, yields as high as 0.3 m3 kg-’ VS added without pre-treatment. Jerger et al.” observed the highest CH, yield of 0.4 m3 kg-’ VS added and VSr of 92% for sweet sorghum (Rio cultivar). Different plant parts,‘-’ harvesting frequency,33 plant age,” clonal variations,” nutrient addition,“‘,” particle size reductionS7.” and alkali treatment74- “. “. ‘a X2 have a substantial effect on CH, yield from grasses.
4.3.2. Leafy biomass. It has been postulated that CH, yields and kinetics were generally higher in leaves than in stems.” The data of Sharma et a1.57,h4 on AD of Ipomoea jistulosa leaves and stem also confirmed the above concept.
According to Gunaseelanh5 Gliricidia leaves are used for green-leaf manuring in India. Consequently, the vast energy converted through photosynthesis is lost. AD of Gliricidia leaves resulted in a CH, yield of 0.18 m3 kg-’ VS added and a digester residue of high manurial value.
4.4.1. Plant parts, harvest ,fi-equency, age, ensiling and clonal variations. In Napier grass, CH, yields and kinetics were generally higher in leaves than in stems. Substantial differences were observed in CH, yield and conversion kinetics within the same species (different clones). The BMP of the 551 variety was higher than PI 300086, N-51, N-75, S42, S44 varieties. Post-harvest conditions, such as ensiling or drying did not have a substantial effect on the BMP of energy cane, Napier grass and pearl millet. However, CH, yields and kinetics increased with harvest frequency with Napier grass.” Age of Napier grass at harvest time influenced the CH, yield. Young tissues produced more methane than the old tissues, probably because younger tissues are less lignified.‘O
Besides the required particle size of 0.4 mm for agricultural residues such as straw, Sharma et al.” indicated that in the case of succulent
4.4.2. Nutrient addition. Wilkie et al. Oy demonstrated that mesophilic AD of mature Napier grass (PI 300086) supplemented with nitrogen and phosphorus resulted in a low rate
Tabl
e 4.
D
iges
ter
perfo
rman
ce
with
gr
ass
feed
s (T
emp,
te
mpe
ratu
re;
HR
T,
hydr
aulic
re
tent
ion
time;
O
LR,
orga
nic
load
ing
rate
; V
S,,
VS
adde
d;
CH
, PR
, m
etha
ne
prod
uctio
n g
rate
; V
S,,
VS
redu
ctio
n;
NA
, no
t ap
plic
able
; N
R,
not
repo
rted;
*v
alue
s ca
lcul
ated
fro
m
the
data
re
porte
d;
b va
lues
in
par
enth
eses
ar
e s.d
.; ”
ultim
ate
CH
, yi
eld)
Feed
Fe
rmen
ter
Tem
p.
(“C)
H
RT
(day
s)
OLR
C
H,
yiel
d b
CH
dPR
h
VSr
(k
gVSm
-’ d-
‘)
(m’
kg-’
VS,
) (m
3 m
-’ d-
‘)
(X)
Ref
eren
ce
Penn
iset
um p
urpu
reum
(N
apie
r gr
ass,
NG
) A
ge:
120
days
18
0 da
ys
330
days
N
G
tops
B
efor
e m
icro
nutri
ent
addi
tion
Afte
r m
icro
nutri
ent
addi
tion
NG
, PI
30
0086
N
G,
Fres
h-R
-3
Fres
h-R
-2
40%
m
oist
ure-
R-3
20
%
moi
stur
e-R
-5
NG
, Fr
esh-
PI
3000
86
Ensi
led-
PI
3000
86
NG
, PI
30
0086
, H
arve
st
frequ
ency
3
times
/yea
r 2
times
/yea
r 1
time/
year
N
G,
PI
3000
86
who
le
plan
t le
aves
st
ems
NG
, N
-75
Who
le
plan
t le
aves
st
ems
NG
, S4
4 w
hole
pl
ant
leav
es
stem
s N
G,
551
who
le
plan
t N
G,
S42,
w
hole
pl
ant
Ener
gyca
ne
Bal
l m
illed
0.
8 m
m
parti
cle
size
8.
0 m
m
parti
cle
size
En
ergy
cane
, fr
esh
ensi
led
BM
P as
say
35
NA
N
A
0.31
0 0.
260
0.24
0
0.11
3*
0.15
8*
0.28
8(0.
004)
0.
274(
0.01
0)
” 0.
191(
0.01
4)
” 0.
255(
0.01
3)
” 0.
247(
0.01
4)
” 0.
260(
0.01
4)
” 0.
310(
0.00
8)
”
0.29
4(0.
034)
”
0.25
8(0.
020)
”
0.23
8(0.
004)
”
0.24
3(0.
002)
”
0.26
2(0.
001)
”
0.24
2(0.
001)
”
0.29
6(0.
019)
”
0.28
4(0.
026)
”
0.29
8(0.
011)
”
0.30
4(0.
014)
”
0.30
6(0.
004)
”
0.28
7(0.
022)
”
0.34
2(0.
017)
”
0.32
2(0.
026)
”
0.32
0 ’
0.24
0 ’
0.29
0 ’
0.24
5(0.
001)
”
0.26
5(0.
007)
”
NA
N
R
1701
0.13
9 N
R
0.19
4 N
R
NA
N
R
NA
N
R
1691
NA
N
R
NA
N
R
NA
N
R
NA
N
R
NA
N
R
NA
N
R
NA
N
R
NA
N
R
CST
R
Sem
i- co
ntin
uous
4
I B
MP
assa
y B
MP
assa
y
35
35
35
20
1.23
NA
N
A
NA
N
A
BM
P as
say
35
NA
N
A
BM
P as
say
35
NA
N
A
BM
P as
say
35
NA
N
A
BM
P as
say
35
NA
N
A
BM
P as
say
35
35
35
35
NA
N
A
NA
N
A
NA
N
A
NA
N
A
BM
P as
say
BM
P as
say
BM
P as
say
Ener
gyca
ne,
L79-
100
2 H
arve
st
frequ
ency
3
times
/yr
2 tim
es/y
r I
time/
yr
Cyn
odan
da
ctyl
on
(Ber
mud
a gr
ass,
BG
)
With
out
exte
rnal
nu
trien
ts
With
ex
tern
al
nitro
gen
addi
tion
With
ex
tern
al
nitro
gen
and
phos
phor
us
addi
tion
BG
, 0.
088
mm
pa
rticl
e si
ze
0.4
mm
pa
rticl
e si
ze
1.0
mm
pa
rticl
e si
ze
6.0
mm
pa
rticl
e si
ze
30.0
mm
pa
rticl
e si
ze
BM
P as
say
CST
R
Sem
i- co
ntin
uous
7
I
Bat
ch
5 I
35
NA
N
A
0.29
4(0.
027)
”
0.26
1(0.
016)
”
0.24
6(0.
004)
”
NA
N
R
0.19
2*
0.35
1*
0.35
0;
NR
NA
20
37.5
38.1
30.0
30
.2
28.4
27
.1
18.2
NR
NA
N
R
NA
N
R
NR
76
0.37
0*
NR
NA
NR
79
91
65
35
35
35
12
12
12
1.6
0.11
2 1.
6 0.
219
1.6
0.21
8
37
NA
N
A
0.22
6(0.
004)
0.
228(
0.00
2)
0.21
4(0.
005)
0.
205(
0.00
3)
0.13
7(0.
003)
0.29
8(0.
001)
”
0.29
3(0.
006)
”
0.24
2(0.
011)
”
0.23
8(0.
009)
”
0.28
1(0.
010)
”
0.27
7(0.
028)
”
0.26
1(0.
013)
”
0.29
2(0.
005)
”
0.25
1(0.
007)
”
0.33
2(0.
025)
”
0.24
7(0.
004)
”
0.13
6*
0.19
0*
35
NA
N
A
Bio
mas
s gr
own
in
flood
ed
soils
A
lem
angr
ass-
6A
Ale
man
gras
s-7A
Pa
ragr
ass-
1 P
Pa
ragr
ass-
3P
Sacc
haru
m
robu
stur
n Su
garc
ane
hybr
ids:
US
72-1
288
US
84-1
008
US
84-1
009
US
84-1
018
Turf
gr
ass
Flor
itum
St
. A
ug.
Sevi
lle
St.
Aug
. R
ye
gras
s str
aw
(1-3
cm
si
ze)
Gra
ss
mix
ture
Tr
iticu
m
aest
ivum
(W
heat
str
aw,
WS)
20
mm
si
ze
0.5
mm
si
ze
WS,
0.
5 m
m
size
BM
P as
say
BM
P as
say
CST
R
sem
i- co
ntin
uous
20
1
CST
R
35
NA
N
A
33-3
7
35
20
16
2.02
1.94
*
Bat
ch
1 1
35-3
9
33-3
7
NA
N
A
20
2.36
0.25
5*
0.32
7*
0.25
9*
CST
R
sem
i- co
ntin
uous
20
I
Con
tinue
d
Feed
Fe
rmen
ter
Tabl
e &C
ontin
ued
Tem
p.
(“C)
H
RT
(day
s)
OLR
C
H,
yiel
d b
CH
,PR
’
VSr
(k
gVSm
-’ d-
‘)
(m3
kg-’
VS,
) (m
’ m
m3
d-‘)
(X
) R
efer
ence
NA
59
.4
64.0
65
.0
66.4
55
.7
39.7
[751
NA
56
.8
57.3
[7
51
NA
66
.5
70.7
3.82
(0.0
3)
50
3.46
(0.0
6)
48.6
3.
21(0
.07)
44
.4
3. I
l(O.0
7)
38.4
NA
N
R
NA
.':
2 [7
51
[ 1 Q 2 [7
61
p k
[571
NA
38.7
38
.5
37.4
35
.2
25.0
N
R
NA
55
.6
56.0
54
.6
52.9
36
.8
~291
[571
NA
63
.0
[661
N
A
NR
[3
31
WS,
ba
ll m
illed
ga
mm
a ra
y irr
adia
tion
0 M
ra
d 1
M
rad
5 M
ra
d 10
M
rad
50 M
rad
10
0 M
ra
d N
H,O
H
pret
reat
men
t N
H,
OH
-80”
C-2
4 h
0 g
OH
kg
-’ V
S 34
g O
H
kg-’
VS
NH
,OH
Pr
etre
atm
ent
NaO
H-9
0”C
-1
h O
gOH
kg-‘V
S 34
g O
H
kg-’
VS
WS,
ba
ll m
illed
N
aOH
-34
g O
H
kg-’
VS-
95”C
-1
h N
aOH
-34
g O
H
kg-’
VS-
95%
I h
Ca(
OH
),-34
g
OH
kg
- ’ V
S-95
’C-1
h
Unt
reat
ed
(con
trol)
WS,
ba
ll m
illed
. I/S
ra
tio
(VS
basi
s)
I/S
ratio
0.
07
I/S
ratio
0.
16
I/S
ratio
0.
19
I/S
ratio
0.
25-1
0.9
WS,
0.
088
mm
si
ze
0.4
mm
si
ze
1.0
mm
si
ze
6.0
mm
si
ze
30 x
5 m
m
size
W
S,
No.
1 W
S,
No.
2 O
ryza
sa
tiva
(pad
dy
stra
w)
0.08
8 m
m
size
0.
4 mm
si
ze
1.0
mm
si
ze
6.0 m
m
size
30
x 5
mm
si
ze
Mill
et
straw
, dr
ied
3 x
3 m
m
size
Pe
arl
mill
et,
fres
h en
sile
d
Bat
ch
4 dm
’ 54
-56
NA
N
A
0.30
4(0.
001)
”
0.31
4(0.
002)
”
0.27
8(0.
001)
”
0.3
1 S(O
.002
) ”
0.27
5(0.
005)
”
0.21
l(O
.027
) ”
0.31
8(0.
008)
”
0.36
2(0.
002)
”
Bat
ch
4 dm
3 54
-56
NA
N
A
Bat
ch
4 dm
’ 54
-56
NA
N
A
0.30
0(0.
020)
”
0.38
3(0.
016)
”
55
5 N
R
NR
Se
mi-c
ontin
uous
4
dm3
Bat
ch 12
0 m
l 35
N
A
NA
0.
013
y 0.
033
u 0.
018
y 0.
299-
0.33
1 ’
0.24
9(0.
001)
0.
248(
0.00
1)
0.24
1(0.
001)
0.
227(
0.00
1)
0.16
2(0.
003)
0.
302(
0.00
8)
0.33
3(0.
006)
0.36
5(0.
001)
0.
367(
0.00
1)
0.35
8(0.
002)
0.
347(
0.00
2)
0.24
1(0.
004)
0.39
0 0.
257(
0.01
6)
” 0.
304(
0.01
3)
”
Bat
ch
5 1
37
NA
N
A
35
37
NA
N
A
NA
N
A
BM
P as
say
Bat
ch
5 1
Bat
ch
100
ml
BM
P as
say
35
NA
N
A
35
NA
N
A
Oat
s cr
op,
20 m
m
size
O
ats
crop
, l-3
cm
si
ze
Mai
ze
crop
, 20
mm
si
ze
Mai
ze
crop
1-
3 cm
si
ze
Cor
n st
over
So
rghu
m
culti
vars
(i)
H
igh
ener
gy
Atla
s x
RT
x 43
0 A
T x
623
x W
ray
AT
x 62
3 x
Rio
(ii
) Sw
eet
BM
R-1
2 R
io
(iii)
Gra
in
Giz
a 11
4 R
S 61
0 So
rghu
m-R
io
culti
var
Dai
ly
feed
ing
Dai
ly
feed
ing
Alte
rnat
e da
y fe
edin
g
Sorg
hum
hi
colo
r: al
pha
cellu
lose
(1
: 1
VS
basi
s)
Dry
fe
rmen
tatio
n (2
8%TS
) So
rghu
m
bico
lor :
alph
a ce
llulo
se
Sorg
hum
So
rghu
m
1.6
mm
pa
rticl
e si
ze
8.0
mm
pa
rticl
e si
ze
Sorg
hum
Bat
ch
I 1
CST
R
sem
i- co
ntin
uous
20
I
Bat
ch
1 I
CST
R
sem
i- co
ntin
uous
20
I
BM
P as
say
BM
P as
say
CST
R
3 1
35
28
Non
-mix
ed
verti
cal
flow
re
acto
r 10
1
Occ
asio
nally
st
irred
re
acto
r
CST
R
20 1
BM
P as
say
BM
P as
say
BM
P as
say
35-3
9 N
A
NA
0.
295*
33
-37
20
2.3
0.27
4*
‘35-
39
33-3
7
35
35
NA
N
A
0.34
2;
20
2.3
0.25
3*
NA
N
A
0.36
0(0.
003)
NA
N
A
0.36
0(0.
001)
0.
340(
0.00
1)
0.34
0(0.
006)
35
28
55
15
55
29.8
21.7
35
35
NA
35
NA
NA
35
NA
1.6
3.2
4.0
3.2
4.8
0.37
0(0.
012)
17.1
0.
320
NR
N
R
24.1
0.
310
NA
0.
380
NA
90
.7
NA
0.
360
NA
82
NA
0.
410
” 0.
420
’ N
A
NR
NA
0.
311(
0.00
8)
” N
A
NR
0.35
0(0.
003)
0.
400(
0.00
1)
0.28
0(0.
002)
0.
310(
0.00
3)
0.26
0(0.
009)
0.
260(
0.00
8)
0.25
0(0.
013)
0.
360(
0.00
9)
BM
P as
say
NA
N
R
NA
N
R
NA
NA
0.40
0.
83
0.96
1.
20
1.80
66.0
55
.0
81.0
80
.0
NR
93.6
92
.8
91.3
92.3
92
.0
73.8
83
.5
NR
NR
NR
100 V.NALLATHAMBIGUNASEELAN
of CH, production and high VFA concen- trations. Daily addition of a solution containing nickel, cobalt, molybdenum, selenium and sulphate increased the CH, production by 40% and significantly decreased VFA concen- trations.
Ghosh et al.” reported that Bermuda grass (BG) is deficient in nitrogen and phosphorus. Accordingly, several mesophilic digestion runs were conducted with BG at HRT = 12 days and OLR = 1.6 kg VS m-3 d-l, with and without external nitrogen and phosphorus additions. It was found that supplementing the feed with NH, Cl increased the CH, yield by 96% and cellulose conversion by 33 %. Nitrogen addition appeared to decrease hemicellulose conversion and phosphorus addition had no effect on hemicellulose conversion or CH, production. It was speculated that the metabolism of the breakdown product (glucose) of cellulose requires the least investment of enzymes and energy. The CH,, yield from grass mixture’* was higher than that of BG without external nutrient addition”.
4.4.3. Particle size reduction. Biodegradabil- ity can be increased by physical pre-treatment, which includes size reduction and pre-incu- bation with water. Particle size reduction provides high surface area for the cellulosic materials. Sharma et aZ.57 demonstrated that in BG, wheat straw and paddy straw, particle sizes of 0.088 and 0.40 mm produced an almost equal quantity of biogas and, thus, grinding below 0.40 mm would seem to be uneconomical. However, Chynoweth et a1.33 conducted tests with sorghum and energy cane and hypoth- esized that particle sizes in the millimetre to centimetre range would not significantly expose more surface area and would, thus, exhibit similar kinetics. Particle size reduction below 1 mm would also be uneconomical to obtain on a commercial basis.
4.4.4. Alkali treatment. Pavlostathis and Gossett74 reported greater than 100% increase in CH, yield from wheat straw (WS) pre-treated with 500 g NaOH kg-’ TS for 24 h at room temperature (26 + 2°C) compared with un- treated WS. They suggested a solids separation and filtrate recycle scheme to recover excess alkali for reuse. However, Hashimoto75 using laboratory-scale batch fermenters, evaluated the effects of pre-treating WS with y-ray irradiation, NH,OH and NaOH on CH, yield. Results showed that CH, yield increased as pre-treat- ment alkali concentration increased, with the
highest yield being 37% over untreated straw for the pre-treatment consisting of NaOH dosage of 34 g OH- kg-’ VS at 90°C for 1 h. NaOH is more effective than NH,OH in pre-treating straw, and y-ray irradiation had no significant effect on CH, yield. Semi-continuous fermentations of straw-manure mixtures confirmed the relative effectiveness of NaOH, and Ca(OH)2 pre-treatment had no beneficial effect on CH; PR.
4.4.5. Inoculum/substrate (Z/S) ratio. For successful digestion, pH of the digester should be within the optimum range and be carefully monitored. This is tedious and-consequently it has been shown that with a large inoculum size, batch digestion can be successfully completed without pH adjustment and also CH, PR is accelerated.” Batch fermentation experiments using WS showed that B,, was drastically lower at I/S ratios (on a VS basis) below 0.25. CH, PR increased at a decreasing rate up to an I/S ratio of 2, after which it remained relatively constant.76 The inoculum-to-feed ratio (I/F) on the standard BMP procedure is approximately 1 (VS basis). Chynoweth et af.33 determined the effect of increasing the I/F ratio on kinetics of CH, production from cellulose, Napier grass and energy cane in order to optimize rates of CH, production in the BMP assays. The data suggest that, for an estimate of the maximum rate of CH, production using the BMP, increasing the I/F ratio may be needed for some type of substrates. Chynoweth et a1.33 have, therefore, modified the I/F ratio of the BMP procedure to 2.
4.4.6. Sorghum. Sorghum bicolor yielded 20-30 Mg ha-’ in the north temperate zones and the high biomass yield makes it attractive as potential feedstocks for CH, production.69 Jerger et al.‘* examined several sorghum cultivars including sweet, grain and high energy, to determine their anaerobic biodegradability using BMP assay. The highest CH, yield of 0.4 m’ kg-’ VS added was obtained from sweet sorghum cultivar, Rio. CH, yields from the other cultivars ranged from 0.27 to 0.36 m3 kg-’ VS added. Subsequently, experiments were conducted using the Rio cultivar in laboratory- scale CSTR, non-mixed vertical flow reactor (NMVFR) and occasionally-stirred reactor (OSR) to determine the operating conditions and reactor most suitable for large-scale digesters. A CH, yield of 0.36 m3 kg-’ VS added was achieved in NMVFR at a 3.2 kg VS mm3 d-’ loading, a 28day HRT and a 75-day SRT. This
Anaerobic digestion of biomass for methane production: a review 101
represented a 36% increase in the CH, yield over a CSTR operated at the same loading and HRT. The CH, PR from a thermophilic OSR operated with 75day HRTjSRT and 4.8 kg VS m-’ d-’ was 1.8 m3 mm3 d-’ in comparison with a CH, PR of 1.2 m3 me3 d-’ from the mesophilic NMFVR.
Richards et ~1.‘~ performed high-solids AD of sorghum (Sorghum bicolor) and sorghum and cellulose mixture (1 : 1 VS basis) using semi- continuous feed-and-mixed systems at 55’C. CH, PR ranged from 5.7 to 7.5 1 kg-’ d-’ and is some of the highest reported volumetric productivity for biomass feedstocks.
4.4.7. Manure-grass mixture. AD of several blends of manure and grasses has been carried out by several authors68,80 ” and all have reported enhanced CH, production.
4.5. Woods
Anaerobic digestion of woody biomass has not been considered technically feasible without pre-treatment. It has been proposed that many factors may influence the anaerobic biodegrad- ability of wood: low moisture content; relative lignin; cellulose and hemicellulose content; proportion of structural and non-structural carbohydrates; cellulose crystallinity; degree of association between lignin and carbohydrates; particle size; wood-to-bark ratio; and toxic components.29~88~93 An inverse linear relationship between VS reduction and lignin content was showed in the anaerobic biodegradability of woody speciesa The anaerobic biodegradability of several woody species was determined using BMP assay. The highest CH, yield of 0.32 m3 kg-’ VS added was achieved from hybrid poplar and sycamore9j (Table 5), whereas eucalyptus, loblolly pine and white fir exhibited poor degradability29,93 with CH, yields of 0.014, 0.063 and 0.042 ms kg-’ VS added, respectively. These results were attributed to long-term fermentation of feedstock solids and adaptation of a wood-degrading inoculum. However, Turick et ~1.‘~ have demonstrated high rates, and B, used BMP in a study to evaluate the biodegradability of 32 woody samples from 15 biomass species without pre-treatment other than particle size reduction. Approximately two-thirds of the samples tested gave biphasic curves of CH, production, indicating that BMP assays of woody biomass conducted for less than 50 days may underestimate B,. Genetic (clonal) differences, environmental growth con- ditions, harvest age and year of harvest may
influence the BMP of woody biomass.1*.‘3 Willow and poplar clones represent an excellent choice for commercial CH, production (Table 5). Jimenez et ~1.~~ reported an estimated 700 000 t of vine shoots produced annually in Spain. The crude vine shoots had a lignin content of 21%. Lignin was removed by 1% sodium chlorite treatment at 80°C for 3 h. Anaerobic digestion using CSTR at 35°C 20-day HRT and OLR of 1 g VS 1-l d-’ produced 0.154 and 0.273 m’ CH, kg--’ VS added for crude and treated vine shoots, respectively. Sharma et ~1.~ found that 0.4mm particles of Ipomoea $stulosa (IFS) stem produced 98% more CH, than the 6 mm particles. When IFS was pre-incubated in water for 40 days, the CH, yield was 0.426 m3 kg-’ VS.
4.6. Terrestrial weeds
The use of weedy plants as a potential source of biomass is a rather recent concept. These non-conventional crops on wastelands can be considered as potential biomass and used as feedstocks for biogas digesters, because:
?? Weeds have ability to trap a significant amount of solar energy.
?? Weeds are capable of growing on soils generally unsuitable for conventional crop production.
?? The genetic base of weeds is such that many can grow under a wide range of cultural and climatic conditions.
?? Weeds have a few serious known pests. ?? Weeds grow in natural stands without
inputs and irrigation. ?? Large-scale utilization is one of the best
strategies for weed management.
Parthenium hysterophorus,h8. “. 95.9h Lantana camera,%’ Cannabis sativa,” Eupatorium odoratum,” Ageratum” are some of the weeds studied as sources for CH, production (Table 6).
Parthenium hysterophorus L. is a minor weed in tropical North and South America, South Africa, Indo China and is a major problem in India as well as Australia. It is an aggressive, invasive weed of sugar-cane and sunflower cropland, wasteland and over- grazed pasture. Approximately 2 000 000 ha of land in India have been infested with this weed.96 Gunaseelan95 reported that anaerobic digestion of mixtures of CM and Parthenium (flowering stage) enhanced CH, production in batch digesters. Anaerobic digestion of Parthenium in CSTR at 30°C IO-day HRT and
Feed
Fe
rmen
ter
Tabl
e 5.
D
iges
ter
perfo
rman
ce
with
w
oody
bi
omas
s fe
eds
Tem
pera
ture
(“
C)
HR
T (d
ays)
O
LR
(kg
VS
mm
3 d-
‘)
CH
, yi
eld*
(m
’ kg
-’ V
S,)
VSr
(%
) R
efer
ence
Popu
lus
delto
ides
(c
otto
n w
ood)
Po
pulu
s sp
. (H
ybrid
po
plar
) Pl
atan
us
occi
dent
alis
(S
ycam
ore)
Pi
nus
taed
a (L
oblo
lly
pine
) Eu
caly
ptus
sp
. B
lack
al
der
Red
al
der
Abie
s co
ncol
or
(Whi
te
fir)
All
Salix
sp
. (W
illow
) St
em
and
bark
0.
8 m
m
parti
cle
size
A
ll Po
pulu
s sp
. (P
opla
r)
stem
an
d ba
rk
0.8
mm
pa
rticl
e si
ze
Liyu
idam
bar
styr
ac$u
a (S
wee
t gu
m)
Popl
ar
woo
d 0.
003
mm
si
ze
0.8
mm
si
ze
8.0
mm
si
ze
Vin
e sh
oots
, un
treat
ed
BM
P as
say
35
NA
N
A
0.22
0 32
.3
[931
BM
P as
say
0.32
0 0.
320
0.06
3 0.
014
0.24
0 0.
280
0.04
2 (0
.003
)
0.14
0 (O
.Ol)-
0.
310
(O.O
l).t
0.22
0 (0
.020
) 0.
290
(O.O
l)?
0.21
0 (o
.olo
)t
53.8
56
.7
3.6
1.0
32.5
48
.4
NR
NR
~291
[28,
331
NR
NR
NR
[3
31
NR
[9
41
NR
[9
41
NA
N
A
0.36
1 50
.7
0.18
2 25
.6
0.42
6 59
.1
1641
35
NA
N
A
35
NA
N
A
35
NA
N
A
35
NA
N
A
BM
P as
say
BM
P as
say
BM
P as
say
BM
P as
say
35
NA
N
A
0.33
0t
0.33
0t
0.30
0t
0.10
9 0.
154
0.16
4 0.
193
0.18
0 0.
273
0.28
3 0.
315
CST
R
sem
i-con
tinuo
us
21
CST
R
sem
i-con
tinuo
us
21
25
35
45
55
25
35
45
55
37
20
1.0
Vin
e sh
oots
, so
dium
ch
lorit
e tre
ated
20
I .o
Ipom
oea
Jist
ulos
a St
em
(IFS
) IF
S,
0.4
mm
si
ze
IFS,
6m
m
size
IF
S.
40 d
ays
incu
batio
n w
ith
wat
er
Bat
ch
5 I
HR
T =
hydr
aulic
re
tent
ion
time,
O
LR
= or
gani
c lo
adin
g ra
te,
VS,
=
VS
adde
d,
VS,
= V
S re
duct
ion,
N
A
= no
t ap
plic
able
, N
R
= no
t re
porte
d.
*Val
ues
in p
aren
thes
es
are
SD,
tulti
mat
e C
H,
yiel
d.
Feed
Fe
rmen
ter
Tabl
e 6.
D
iges
ter
perfo
rman
ce
with
te
rres
trial
w
eed
feed
s
Tem
pera
ture
(‘
C)
HR
T (d
ays)
O
LR
(kgV
S m
-’ d-
‘)
CH
, yi
eldt
(r
n’ k
g-’
VS,
) C
H,
PRt
(rn’
m_W
) V
Sr
(%)
Ref
eren
ce
Part
heni
um h
yste
roph
orus
(P
H)
Sem
i- PH
, un
treat
ed,
daily
co
ntin
uous
fe
edin
g C
STR
31
PH,
untre
ated
, al
tern
ate
day
feed
ing
PH,
NaO
H
treat
ed,
daily
fe
edin
g
PH,
NaO
H
treat
ed,
alte
rnat
e da
y fe
edin
g
PH,
Unt
reat
ed,
I/S =
67
(vol
/vs)
Fr
esh
Drie
d PH
, dr
ied,
he
at
treat
ed
PH,
drie
d,
HC
I tre
ated
PH
, dr
ied,
N
aOH
tre
ated
La
ntan
a ca
mer
a,
NaO
H
treat
ed
t C
M
(50:
50
w/w
) A
gera
tum
, pa
rtial
ly
deco
mpo
sed
Sem
i- co
ntin
uous
C
STR
3
1 Se
mi-
cont
inuo
us
CST
R
3 1
Bat
ch
2 I
24-2
8 N
A
NA
Bat
ch
3 1
28-3
1 N
A
NA
0.09
5 (0
.002
) 0.
172
(0.0
02)
0.21
4 (0
.004
) 0.
060
(0.0
01)
0.17
3 (0
.003
) 0.
202
(0.0
03)
0.21
4 (0
.007
) 0.
112
(0.0
03)
0.14
7 (0
.012
) 0.
140
(0.0
08)
0.15
7 (0
.015
) 0.
203
(0.0
09)
0.23
6 (0
.008
) 0.
236*
N
A
Bat
ch
3 I
29-3
1
NA
N
A
0.24
1*
NA
28-3
2
28-3
2 22
-26
22-2
6
22-2
6 10
4.
12
35
10
4.12
40
10
4.
12
45
10
4.12
22
-26
20
2.06
35
20
2.
06
40
20
2.06
45
20
2.
06
5 IO
20
40
10
10
20
4.95
0.
034
(0.0
02)
0.16
7 (0
.012
) 2.
48
0.11
7 (0
.005
) 0.
290
(0.0
11)
1.24
0.
115
(0.0
01)
0.14
3 (0
.014
) 0.
62
0.10
1 (0
.011
) 0.
062
(0.0
07)
4.12
0.
110
(0.0
05)
0.45
6 (0
.02)
4.
12
0.03
9 (0
.002
) 0.
160
(0.0
09)
2.06
0.
086
(0.0
05)
0.17
9 (0
.011
)
0.39
3 (0
.006
) 0.
709
(0.0
06)
0.88
3 (0
.014
) 0.
248
(0.0
03)
0.35
7 (0
.006
) 0.
417
(0.0
05)
0.44
2 (0
.015
) 0.
232
(0.0
01)
NA
25.9
[9
61
42.9
>
42.1
N
R
E
45.7
a
17.9
_$
35.5
a B 8.
s 35
.3
62.6
%
s
65.1
N
R
z 62
.4
I
65.8
z
66.7
3
NR
P 5
39.8
[7
71
36.4
5 ;:
42.8
60
.3
$ ,c.
65.9
$
NR
WI
P,
x2
N
R
1991
2.
HR
T =
hydr
aulic
re
tent
ion
time,
O
LR
= or
gani
c lo
adin
g ra
te,
VS,
=
VS
adde
d,
CH
,PR
=
met
hane
pr
oduc
tion
rate
, V
S, =
VS
redu
ctio
n,
NA
=
not
appl
icab
le,
NR
=
not
repo
rted.
*V
alue
s ca
lcul
ated
fro
m
the
data
re
porte
d.
tVal
ues
in p
aren
thes
es
are
SD.
104 V. NALLATHAMBIGUNASEELAN
4.13 kg VS m-3 d-’ produced CH, yield of 0.11 m3 kg-’ VS added and volumetric CH, productivity of 0.46 m3 me3 d-‘.y6 Results on pre-treatment showed greater than 95% in- crease in CH, production from NaOH-treated Parthenium than untreated Parthenium. It has been postulated that at low room temperature feeding could be performed on alternate days, which established a HRT of 20 days, and OLR of 2.06 kg VS m-’ d-‘. At 35 and 40°C feeding should be daily at a HRT of 10 days and OLR of 4.13 kg VS m--3 d-‘. The estimated energetic analysis indicate anaerobic digestion of Parthe- nium to be technically feasible.96 It has been shown that batch digestion of fresh Parthenium for 35 days at 26 f 2°C at an I/S ratio (on a volume/VS basis) of 67, produced 0.147 f 0.012 m3 CH, kg-’ VS added and that of dried Parthenium at the same operating conditions produced 0.140 f 0.008 m’ CH, kg-’ VS added. At I/S ratios below 67, the yields were drastically low. A high volume of inoculum accelerated the rate of biogas pro- duction, leading to the possibility of short-term batch fermentation of Parthenium. Batch digestion of Parthenium confirmed the relative effectiveness of Na OH pre-treatment.”
Lantana camera, a weed growing abundantly on the Himalayan slope, India, was treated with NaOH and mixed with CM to feed batch digesters. AD for 37 days at 28-31°C produced 62% higher CH, yield than CM alone.*”
Cannabis sativa was used as an additive with poultry litter and CM for biogas production. Use of fresh Cannabis at 31% of the mixture completely stopped gas production, probably due to the presence of high amounts of alkaloids.97
According to Jagadeesh et a1.,98 fresh Eupatorium odoratum L. contains methanogenic inhibitors and pre-treatment in slaked lime for 24 h, leaching and partial aerobic decompo- sition for 6 days make Eupatorium a fit candidate for biogas production.
Partially decomposed Ageratum under aerobic conditions for 5 days can be used as a substrate with and without CM for biogas production. The 56-days CH, yield from Ageratum alone was calculated to be 0.24 m3 kg-’ VS added in batch digesters at 30 f l”C.99
4.7. Aquatic biomass
The potential of aquatic biomass production may be greater than that of land on the basis of
the vast areas available for growth and the availability of water may not limit growth rates, suggesting the possibility of obtaining high productivity. Moreover, terrestrial biomass production is only two-dimensional, which includes production along length and breadth. Aquatic biomass production is three-dimen- sional, where the “height” element is also added.
4.7.1. Marine biomass. Recent studies on bioconversion of marine macroalgae as potential sources for CH, include brown algae Macrocystis pJ@era, Sargassum, Lami- naria and Ascophyllum, green algae Ulva, Cladophora and Chaetomorpha and red algae Gracilaria (Table 7).33.42,7’, ‘oo~‘08 Macrocystis pyrifera (California giant brown kelp) is a perennial floating plant and can grow to a length of up to 61 m. It is a primary producer of organic matter and over 2000 species of marine flora and fauna are associated with kelp beds along the central and southern Californian coast. It was selected for IGT’s work on “Marine Biomass Program” sponsored by the Gas Research Institute (GRI) and the U.S. Department of Energy (DOE). The results of kelp digestion studies conducted at the IGT’oo~‘o’ are summarized below:
?? Kelp has high water and ash content. Elemental analysis showed that nitrogen content varied from 0.96 to 2.2 wt%, corresponding to a C/N range of 2414, respectively and C/P ratio of 83. The major organic components are mannitol, protein and cellulose and minor components are laminarin and fucoidin. Kelp should be highly biodegradable because it does not contain the refractory lignocellulosic com- plexes typical of terrestrial biomass forms.
?? Chopped raw kelp (RK), baseline treated kelp (BLTK; prepared by treating chopped RK with CaCl, and pressing the mixture to obtain a dewatered cake) and kelp juice (the pressate during BLTK preparation) were used as feeds.
?? Mannitol and algin were the most biodegradable and protein and cellulose the least biodegradable. Laminarin and fu- coidin have only minimal influence on the overall component balance.
?? The empirical formula of kelp was C,,,, H, 73 0 1.48. Based on stoichiometry, the theoreti- cal yield for biomethanation of kelp was found to be 0.51m3 kg-’ VS added.
Table 7. Digester performance with marine biomass feeds
Feed Fermenter Temp. HRT OLR CH, yield* VSr
(‘C) (days) (kg VS m-’ d-‘) (m’ kg-’ VS,) (%) Reference
Macrocystis pyrijera (raw kelp, RK)
RK
CSTR
Batch 2 dm’
35
28.3 35.0 39.6 44.0 49.3 54.6 59.7 35
35 55 55 35 35
18 1.6
NA NA
CSTR Semi-continuous 2 dmi/10 dm’
CSTR 2 dm’
18 1.6
IO 1.6 18 1.6 1 3.2 18 1.6 18 1.6
BLTK: kelp juice (VS basis) 4:l 3:2 BLTK CSTR 2 dm’ BLTK: kelp juice (VS basis)
CSTR 2 dm’ Semi-continuous
CSTR 2 dm’
CSTR 2 drnl
35 18 1.6 35 18 1.6 35 IO 1.6
35 IO 1.6 35 10 1.6 35 10 1.6
35 18 1.6
35 18 1.6
35 18 1.6 35 18 1.6
35 12 1.6
CSTR CSTR Semi-continuous
37 35
BMP assay 35
25 1.1-1.6 24 1.65
24 1.65 24 1.75
NA NA
BMP assay 35 NA NA
Batch 2 1 29-35 NA NA NA
3G-60
NA
CSTR 25-35
Batch 2 I 29-35 NA NA NA
0.54
NA
Semi- 35 continuous 180 1 35 Semi-continous 2 I 35
20 15
12-15
1.0 0.212 1.0 0.203
2-2.5 0.250-0.350 NA 0.350-0.480
0.277-0.310 NR 11001
NR [loll 0.021 0.103 0.113 0.055 0.150 0.142 0.028 0.278 RK 50.8
0.215 38.6 0.149 31.2 0.134 27.3 0.264 47 0.218 41
RK Baseline treated kelp
(BLTK)
0.235 35.6 0.211 42.0 0.210 34.4
4:l 3~2 Kelp juice
RK Daily feeding
Alternate day feeding BLTK
Intermittent mixing Continuous mixing
RK Without external nutrient With external nutrient
(N and P) Laminaria saccharina Laminaria saccharina
0.202 24.0 0.197 43.5 0.116 23.3
0.243 NR
0.250 NR
0.231 NR 0.206 NR
0.239 45.1 0.233 42.6
0.2054.220 49-53 [102] 0.230 NR 11031
0.280 0.110
0.178(0.014)t 0.143(0.004)~ 0.182(0.018)t 0.165(0.008)t
NR [331
Laminaria hyperborea Ascophyllum nodosum Sargassum f&iitans
Bladders Blades Stipe Whole plant
Sargassum pieropleuron Bladders Blades Stipe Whole plant
Gracilaria tikvahiae (GT)
0.171(0.004)t 0.148(0.007)?
NR t331
0.1 i9[0.004jj 0.145(0.001)t
0.220 0.230 0.190 0.130-0.200
High tissue nitrogen ’ Moderate tissue N Nitrogen deficient CiT
Ulva sp. High tissue nitrogen Moderate tissue N Nitrogen deficient
Ulva rigida (go-90%) + Gracilaria confervoides
Washed Unwashed
Ulca + Cladophora + Chaetomorpha
75.1 HO41 80.1 85.7
26-48 [106]
70.1 [1041 77.3 86.7
0.220 0.230 0.330
63 uo71
525 [IO81 NR Batch I 1 35 NA
HRT = hydraulic retention time, OLR = organic loading rate, VS, = VS added, VS, = VS reduction, NA = not applicable, NR = not reported.
*Values in parentheses are SD. tultimate CH, yield.
105
Tabl
e 8.
D
iges
ter
perfo
rman
ce
with
fr
eshw
ater
bi
omas
s fe
eds
Feed
Fe
rmen
ter
Tem
p.
(“C)
H
RT
OLR
-x
eldt
C
H,
PRt
VSr
(d
ays)
(k
g V
Sm-’
d-l)
(m’
kg -
’ V
S.,)
(m’
m-’
d-‘)
(%
) R
efer
ence
Eich
horn
ia c
rass
ipes
(W
ater
hy
acin
th,
WH
)
WH
:Prim
ary
slud
ge
(3:l)
WH
, un
treat
ed
WH
, st
eam
tre
ated
W
H,
parti
cle
size
red
uctio
n W
H,
NaO
H
treat
ed
WH
, M
iss,
IM-B
W
H,
Mis
s, 2M
B
WH
, FL
, IM
-8
WH
, M
iss
+ N
H,
Cl,
IM-4
W
H,
Mis
s +
mix
ed
nutri
ent
2M-3
W
H,
Mis
s, IT
-5
WH
, M
iss
+ N
Hp
Cl,
IT-8
W
H,
Mis
s +
NH
, C
l, IT
-IO
W
H,
shoo
ts
WH
, ro
ots
WH
, hi
gh
ligni
n W
H,
low
lig
nin
Laga
rosi
phon
m
ajor
(la
ke
wee
d)
Pist
iu s
trat
iote
s
Salv
inia
mol
esta
A
zolla
pin
nata
Ce
rafo
pter
is
thal
ictr
oide
s Se
irpu
s gr
osse
s Cy
peru
s pa
pyru
s U
tric
ular
ia r
etic
ulat
a H
ydri
lla v
erti
cilla
ta
Azo
lla p
inna
ta (
AP)
no
t ex
pose
d to
m
etal
s
CST
R
35
15
1.60
12
2.
00
10
2.64
8.
5 3.
41
15
1.60
10
2.
66
8.5
3.72
7
4.52
N
A
NA
0.19
0 0.
170
0.17
0 0.
170
0.28
0 0.
240
0.25
0 0.
250
0.31
6(0.
022)
0.
319
0.31
1(0.
019)
0.
362
0.18
2 0.
185
0.09
8 0.
176
0.17
3 0.
156
0.30
0 49
.8*
0.35
0 43
.8*
0.44
0 37
.9*
0.55
0 33
.4*
0.45
0 56
.0*
0.64
0 46
.8*
0.92
0 44
.1*
1.11
0 44
.0*
NA
N
R
[’ 10
1
0.30
7*
0.31
2*
0.16
6*
0.29
7*
0.29
3*
0.39
6*
0.50
8*
0.61
5*
NA
NA
NA
N
A
28.8
29
.8
17.0
28
.5
28.9
27
.4
24.6
21
.3
NR
NR
62
89
83
99
NR
I73
[115
1
NA
[I
161
NA
N
R
[‘I81
CST
R
35
AB
P as
say
35
35
35
35
35
35
55
55
55
35
35
12
1.6
12
1.6
12
1.6
12
1.6
12
1.6
16.7
2.
4 12
3.
36
6 4.
8 N
A
NA
NA
N
A
CST
R
71
CST
R
71
BM
P as
say
BM
P as
say
Bat
ch
1 1
Bat
ch
3 1
0.14
3 0.
122
0.32
0 0.
180
0.19
6(0.
003)
: 0.
213(
0.00
4)$
0.26
9*
0.36
9*
0.34
6*
0.41
0’
0.24
2 0.
132
0.20
4 0.
066
0.03
8 0.
132
0.08
1 0.
117*
35-3
9 29
.5
33.0
37
.5
37
NA
N
A
NA
N
A
Bat
ch
NA
N
A
Bat
ch
2 1
37
NA
N
A
Anaerobic digestion of biomass for methane production: a review 107
N
V.NALLATHAMBIGLJNASEELAN
Methane yields were in the range 0.31- 0.34 m’ kg-’ VS added, which is 55% of the theoretically expected yield. Seasonal fluctuations in the nitrogen content of kelp occur and are related to nutrient content of the surrounding waters. Other nutrients including phosphorus and trace elements had no significant effect on kelp digester performance. Particle size reduction below 0.05 cm did not result in improved conversion of kelp. The salt effect data indicate that the methanogenic organisms were retarded by the hypertonic kelp slurries at short retention time. Methane yields and digestion efficiencies at the optimum thermophilic range (55C) were higher than those at the optimum mesophilic range (40°C) in batch studies. Thermophilic (55°C) semicontinuous di- gesters exhibited lower CH, yields than those of the mesophilic (35°C) digester. At 35°C the semi-continuous CH, yield was about three times that of the batch yield. In contrast, at 55°C both were about the same. For raw kelp, increasing the retention time from 10 to 18 days at 35°C increased the CH, yield by about 29%. Little beneficial effect was derived by increasing the retention time in thermophilic digestion. Charging large doses of kelp during daily feeding at short retention times inhibit CH, fermentation. At a 18-day HRT and OLR of 1.6 kg VS m-’ d-‘, there was little difference in CH, productions with daily and alternate day feeding frequency. Mesophilic CH, yields were lower with continuous mixing than with intermittent mixing. Biomethanation of raw kelp was not limited by the selected nutrients such as nitrogen and phosphorus.
CH, yields from the two Laminaria species were about double that of Ascophyllum, and Lami- naria hyperborea seemed better suited for CH, production.‘“2, lo3
The BMP of different parts of Sargarssum j?uitans33 indicated that the CH, yield from the stipe was the highest among the different plant parts, whereas in Sargassum pteropleuron the CH, yield from the stipe was the lowest. The BMP of the whole plant of S.JEuitans was higher than that of S. pteropleuron.
Habig et al.‘” raised both Gracilaria tikvahiae and Ulva on three different nitrogen regimes ranging from nitrogen enrichment to nitrogen starvation and investigated the effects of nitrogen content on CH, fermentation in batch digesters. Low nitrogen classes of each species had greater soluble carbohydrate content than the enriched classes. Low nitrogen Gracilaria contained very high neutral fibre fraction, but the crude fibre fraction was similar for each Gracilaria class. On the other hand, Ulva classes possessed a similar neutral fraction, but the crude fibre content decreased with decrease in nitrogen content. Nitrogen deficient Ulva out-performed the more enriched classes in terms of total biogas and CH, production, CH, yield and VS reduction, whereas nitrogen deficient Gracilaria produced almost similar CH, yields. Contrary to a previous report by Fannin et al.,“’ it has been indicated that some nitrogen deficient seaweed species constitute a very satisfactory methanogenic substrate. The mesophilic batch CH, yields from Ufva ranged from 0.22 to 0.33 mm3 kg-’ VS added and Gracilaria tikvahiae from 0.19 to 0.23 me3 kg-’ VS added.
According to Hanisak’06 0.13-0.2 m’ CH, kg-’ VS added was produced from Gracilaria tikvahiae in semi-continuous digesters at 30°C.
Few experiments have been carried out on the anaerobic digestion of macroalgae in the Venice lagoon, blends of Ulva rigida and Gracilaria confervoides. In conventional digesters at an OLR of 1 kg VS m-’ d-‘, 20-day HRT and 35°C washed, dried and comminuted 90% Ulva and 10% Gracilaria produced 0.21 m’ CH, kg-’ VS added and 63% VS reduction.“’ The algae, when co-digested with OF-MSW under semi- dry thermophilic conditions, resulted in 0.21 m’ CH, kg-’ VS added and 2.8 m’ CH, m-’ day-‘.42
Hansson”* obtained a methane yield of 0.2550.35 m3 kg-’ VS added in semi-continuous fermentation of Ulva, Cladophora and Chaeto- morpha mixture at 35°C whereas the CH, yields in batch cultures were higher, 0.35-0.48 m3 kg-’ VS added compared with semi-continuous fermentations.
4.7.2. Freshwater biomass. Among water that moves via hydrological cycle, it has been estimated that 13 200 x 10” kg is in oceans, 1.25 x 10” kg is in freshwater lakes and ponds and 0.013 x 10” kg in rivers.“’ Free-floating hydrophytes, rooted emergent plants and rooted submerged vegetation from lentic (standing
Anaerobic digestion of biomass for methane production: a review 109
freshwater) habitats have been generally used as substrates for CH, prod~~tion33~70.73,llO~llS
(Table 8). These aquatic macrophytes have been the subject of great interest for the past few years because of their potential uses in waste-water treatment and as a feed supplement for aquatic and terrestrial animals. The concept of using aquatic plants for water treatment and the harvested biomass as an energy source is gaining attention throughout the world.
The prolific growth of water hyacinth (WH) and the ease of harvest techniques make it a suitable feedstock for biological conversion to CH,. Anaerobic digestion of WH has been evaluated under conventional conditions (CSTR, mesophilic, low OLR and high HRT) separately and as part of a mixed feedstock.“’ The CH, production data were 0.17- 0.19 m’ kg-’ VS added and 0.3- 0.55 m3 m-3 day-’ for WH and 0.240.28 m’ kg-’ VS added and 0.455 1.11 rn’ rnmi day-’ for WH/primary sludge 3 : 1 blend. The ultimate CH, yield from WH, based on ABP assay was 0.34 m3 kg-’ VS added. Alkaline treatment with 50% NaOH increased the ultimate biodegradability by approximately 15%, and neither particle size reduction nor steam treatment exhibited any effects. CSTR digester with recycle of 30% of the solids and an unmixed up-flow solids digester achieved about 20% higher CH, yields than that observed in the CSTR digesters without recycle.“’
Klass and Ghosh”’ found that Mississippi WH grown in a sewage-fed lagoon produced more CH, than Florida WH harvested from a freshwater pond. Mississippi WH differed in the C/N, C/P, hemicellulose content, pH and buffering capacity from that of the Florida WH. Little change was observed in digester perform- ance with nutrient addition at mesophilic temperatures and there was no apparent benefit of nitrogen additions on the CH, yield. Biogas production rate at 55°C was higher than that at 35°C.
The ultimate CH, yields from WH, based on BMP assay, showed that CH, yields were higher in shoots than in roots.”
The addition of nickel at 2.5 ppm either to CM alone or to CM and WH blends increased biogas production. This was attributed to the activity of the nickel-dependent metalo-enzymes involved in biogas production.“’
Deshpande et a1.‘13 and Mallik et a1.97 have suggested WH as an additive with CM for biogas digesters.
Moorhead et af.‘14 evaluated the growth characteristics of WH in diluted and undiluted anaerobic digester effluents obtained from digesters with WH as feedstock. The highest gain in plant dry weight (18 g m-* day-‘) was noted for the diluted effluent having an initial NH,-N concentration of 65 mg 1-l.
The 17-day CH, yield from Lagarosiphon in batch digesters at 37°C was found to be 0.27 m3 kg-’ VS added.73
The high biodegradability of Pistia (83-99% of VS) and the 30-day CH, yield ranged from 0.35 to 0.41 m kg-’ VS added in mesophilic batch digesters indicated that Pistia is an excellent substrate for biogas production.“’
Abbasi et al.‘16 suggested that periodic harvesting and utilization is apparently the best strategy for keeping freshwater biomass under control. Among the common aquatic plants, anaerobic digestion of Salvinia and Ceratopteris produced CH, as high as 0.2 m3 kg-’ VS added.
Balasubramanian and Kasthuri Bail” suggested that WolfJia and Lemna sp. grown in digested CM slurry from a biogas plant could be recycled along with fresh CM for biogas production.
According to Jain et a1.,‘18 Azofla pinnata and Lemna minor are currently being considered for wastewater treatment. Non-contaminated Azolla and Lemna plants were exposed to heavy metals and subsequently utilized for biogas production in batch digesters at 37°C for 42 days. It was found that iron or manganese did not affect biogas production at a metal content of 1100 pg g-’ dry matter. Copper, cobalt, lead and zinc contained in the biomass decreased biogas production. Cadmium and nickel at a metal content of about 600 and 400 pg g-’ dry matter, respectively, showed a favourable effect on biogas production and its CH, content.
5. FUTURE PERSPECTIVE
Available data on AD of biomass indicate that nearly 100 genera of plants have been evaluated as potential sources for biogas production. Table 9 lists the biomass identified as excellent substrates for methane production. There are approximately 12 500 genera of angiosperms, 70 gymnosperms, 260 ferns, 400 red algae (Rhodophyta), 190 brown algae (Phaeophyta) and 360 green algae (Chloro- phyta) living on the earth at present.“’ Plants vary in size from structurally simple microscopic organisms to large structurally complex plants,
110 V. NALLATHAMBIGUNASEELAN
such as the California redwood trees which may attain heights of over 120 m and diameters of 10 m. Considering biomass yield as one of the parameters that make biomass-to-CH, conver- sion economically and technically feasible, the number of unexplored genera to be screened is still enormous.
6. CONCLUSIONS
Laboratories throughout the world are continuing research on AD to evaluate different types of waste streams and biomass feedstocks as substrates for various reactor configurations and to develop processes with improved reaction kinetics and CH, yield. The database developed from the literature is used to derive the following conclusions:
1. Almost all the land- and water-based species examined to date either have good
digestion characteristics or can be pre-treated to promote digestion.
2. The qualities of the OF-MSW (processed MSW) is influenced not only by the sorting system but also by various methods used for quantifying OF-MSW. The AD potential of OF-MSW may be classified based on the VS content and the percentage of poorly biodegrad- able materials, such as paper, wood etc. Consequently, the CH, yields from OF-MSW may be classified into three groups. CH, yield from hand-sorted or source-selected OF-MSW with a range of 0.3990.43 m’ kg-’ VS added, mechanically-sorted OF-MSW with CH, yield in the range of 0.18-0.26 m’ kg-’ VS added and that of pre-composted OF-MSW with less than 0.14 m’ kg-’ VS added. The AD potential of OF-MSW increases in systems in which co-digestion of MSW and sewage sludge is carried out. The performance of the semi-dry
Table 9. Literature data of biomass with high yields of methane
Biomass CH, yield* (m’ kg-’ VS added) Reference
Organic fraction qf municipal solid wnste (OF-MSW) I. HS-OF MSW 2. SC-OF MSW: SEW 2. SS-OF MSW 4. HS-OF MSW
Fruit and vegetable solid waste (FVS W) und lecf 1. Banana (fruit and stem) 2. FVSW mixture 3. Ipomoea leaves 4. Potato waste 5. Cauliflower leaves 6. Tomato processing waste 7. Carrot waste 8. Banana peeling
Grass I. Sorghum 2. Millet straw 3. Wheat straw (NaOH treated) 4. Paddy straw 5. Corn stover 6. Napier grass 551
Woody biomass 1. Ipomoea stem
40 days pre-incubated in water 0.4 mm particle
2. Poplar wood 3. Vine shoot (pre-treated)
Terrestrial weed 1. Ageratum (partially decomposed) 2. Parthenium (NaOH treated) 3. Lanrana (NaOH treated) + Cattle manure
Marine biomass 1. Ulva, Clirdophora and Chartomorpho 2. Ulva (N deficient) 3. Macrocyslis pyrffera
Freshwater biomass I. PisIia
0.430 0.403 0.399 0.390
0.529 0.510 0.429 (0.002) 0.426 0.423 (0.001) 0.420 0.417 0.409 (0.002)
0.420-F [331 0.390 [661 0.383 (0.016)t [751 0.367 (0.001) [571 0.360 (0.003) v91 0.342 (0.017)t [331
0.426 0.361 0.330 0.315
0.241 [991 0.236 (0.008) [771 0.236 1801
0.480 0.330 0.310
0.410 0.362
[to81 [IO41 [tOOI
Ill51 [l101 2. Water hyacinth (NaOH treated)
HS = hand sorted, SS = source sorted, SC = separated collection, SEW = sewage sludge. *Values in parentheses are SD. tUltimate CH, yield.
Anaerobic digestion of biomass for methane production: a review III
digestion process is very healthy as it allows very Cladophora and Chaetomorpha blend and kelp high production rates. are high CH, producers.
3. The French bean waste and the carrot waste were very well digested. For balanced digestion, alkalinity (mg 1-l) of 0.7 x VFA (mg 1-l) is required and it should not be less than 1500. The highest CH, yield of 0.53 m’ kg-’ VS added with 100% VS conver- sion has been reported for damaged banana. Ensiling or drying has no effect on CH, yield and kinetics. CH, yields and kinetics were generally higher in leaves than in stems. The practice of directly applying Gliricidia leaves for green leaf manuring, results in loss of vast energy converted through photosynthesis. AD of these leaves yield CH, as well as residue of high manurial value.
8. The concept of using aquatic plants for water treatment and the harvested biomass as an energy source is gaining attention through- out the world. Water hyacinth, and Pistia are considered as excellent substrates for methane production.
9. Available data on AD of biomass indicate that nearly 100 genera of plants and their wastes have been evaluated. Considering biomass yield as one of the parameters that makes biomass-to- CH, conversion economically viable, the num- ber of unexplored genera to be screened in still enormous.
4. Among grasses, the performance of the fermenters with sweet sorghum (Rio cultivar) feeds showed the highest VS reduction of 92% and CH, yield of 0.4 m’ kg-’ VS added. Different plant parts, harvesting frequency, plant age, clonal variations, nutrient addition, particle size reduction and NaOH pre-treatment have a substantial effect on CH, yield from grasses. However, particle sizes in the millimetre to centimetre range would not significantly expose more surface area and, thus, would exhibit similar kinetics.
Acknowledgements-The author wishes to thank Mr D. K. P. Varadarajan (Secretary), and Dr B. Sampathkumar (Principal), PSG College of Arts and Science for their encouragement.
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