Anaerobic digestion of biomass for methane production: A review

Biomassand Biornergy Vol. 13, Nos. l/2, pp. 833114,1997 mc 1997Published by Elsevier Science Ltd. All rights reserved

Pergamon

Printed in Great Britain

PII: SO961-9534(97)00020-2

ANAEROBIC

0961-9534/97$17.00 + 0.00

DIGESTION OF BIOMASS FOR METHANE PRODUCTION: A REVIEW V. NALLATHAMBIGUNASEELAN

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

2. AD PROCESSES FOR BIOMASS

1. INTRODUCTION

2.1. Conventional single stage digestion

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. Bioenergy will be the most significant renewable energy source in the next few decades until solar or wind power production offers an economically 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.

2.1.1. Continuully fed digesters. In these the rate of feeding should be digesters, 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 continuously 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 temperature. 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 RefCOM, SOLCON, dry anaerobic cornposting (DRANCO), KWU-Fresenius, BIOCEL and 83

84

V. NALLATHAMBIGUNASEELAN

sequenced batch anaerobic cornposting (SEBAC) are the dry fermentation processes using MSW as the substrate, some of which were discussed in a recent review.16 The SEBAC process have been developed at the University of Florida for conversion of organic fraction of MSW (OF-MSW) to CH, and compost. It employs three stages for enhanced conversion of MSW to CH,. The SEBAC system, a promising concept for the AD of MSW, is described elsewhere.‘?, I3 2.1.3. BIOGAS and BIOMETprocesses. The “BIOGAS” process has been developed at the Institute of Gas Technology (IGT), U.S.A. This concept combines the treatment of sewage sludge (SEW) at 2-3% TS and solid wastes (MSW at 55% TS) resulting in a substrate concentration of about 10% TS.” A similar co-digestion process called the “BIOMET”‘” has been studied at pilot scale in Sweden. 2.1.4. BIOTIIERMGAS process. The BIOTHERMGAS process carried out by the IGT, U.S.A., combines biological and thermochemical unit operations into a scheme that can convert the biomass efficiently (regardless of moisture and nutrient contents) to CH, with minimum process residues.” Results of the preliminary systems analyses using Bermuda grass and MSW as feedstocks indicate that this process is technically superior to either biological or thermochemical processes and economically feasible. 2.1.5. Plug-Jlow digester. In tubular plugflow digester, a volume of the medium with a suitable inoculum enters at one end of the tube and, if the rate of passage of the medium is correct, by the time the medium reaches the other end the digestion is completed. For continuous operation, some of the digested effluent flowing from the end of the tube is separated and returned to the influent substrate. 2.1.6. The anaerobic j3ter. This is primarily meant for digestion of easily fermentable factory waste waters produced in large quantities. Even a 6-day retention time would mean an impossibly large digester. Hence, in order to prevent washout, the bacteria are allowed to attach to a solid support, such as stones packed inside a tank and the waste water flows upward through the tank. This process requires a retention time of only a few hours and the gas is collected from the top. In a fluidized-bed digester, a modified form of anaerobic filter, the

bacteria are attached to small glass spheres which are freely suspended in the up-flowing feed. 2.2. Two-stage and two-phase digesters In a two-stage digester, the residual substrates from the first stage can be reduced at the second-stage digester, carrying out the same reactions as the first stage but running at a different retention time. For quickly fermentable wastes, a two-stage reactor can have a lower overall retention time than a single stage. The second stage could be a stirred tank or a plug-flow digester or an anaerobic filter. A two-phase digester is a mechanically similar system of two stirred-tank digesters. In this process, fermentation and methanogenesis are separated by using different retention times. Liquefaction and acidification of the substrate is accomplished in a first reactor, while only methanogenesis takes place in the second reactor. It was first promoted by Ghosh et al.*’ for the combined digestion of SEW and MSW. The total digestion time was considerably lower than the conventional single-stage digestion. Some kinetic considerations argue in favour of the two-phase approach when optimal growth conditions for hydrolytic and methanogenic bacteria are considered.*’ Colleran et al.,** Verrier et a1.,23 Mata-Alvarez24 and Viturtia et a1.‘5 proposed this process for the digestion of agricultural solid wastes. Two-phase AD of OF-MSW was studied by Hofenk et a1.,26 who concluded that there was no difference in the biogas yields between single-stage and twophase systems. Unless the hydrogen produced in the fermentative phase can be collected and transferred to the methanogenic phase, a loss of potential CH, occurs. This process is technologically feasible, but an assessment of the economic feasibility is more complex and has to be reviewed for any given situation. 3. BIOCHEMICAL

METHANE ASSAY

POTENTIAL

(BMP)

The BMP assay was developed to determine the ultimate CH, yield (B,) of organic substrates and for monitoring anaerobic toxicity.*’ B, of a variety of biomass were determined using a modified method of Owen et a1.28m33 The BMP is a valuable, quick and inexpensive method for determination of the potential extent and rate of conversion of biomass and wastes to CH,. A similar assay has otherwise been named as

Anaerobic

anaerobic say.34

biogasification

4. POTENTIAL

SOURCES

digestion

potential

of biomass

(ABP) as-

FOR METHANE

A wide range of biomass have considered as potential sources for production (Fig. 1).

been CH,

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 fraction of MSW (MS-OF MSW) used to feed the

for methane

production:

85

a review

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

WOODS

-I

GRASSES I

I

I

FRESHWATER BIOMASS

MARINEBIOMASS

1ORGANIC

I

AQUATIC 1

Fig. 1. Selected

types of methane

yielding

biomass

I

I

Yeast

HS-OF MSW Levy-l country FL VS = 92.5% TS Yard waste samples Grass, VS = 88.1% TS Leaves, VS = 95% TS Branches, VS = 93.9% TS Blend, VS = 92% TS

TRF: predigested SEW 4.76:1 (VS basis) MS-OF MSW, Sumter country Fresh, VS = 79.7% TS Dried, VS = 84.1% TS

Processed MSW (TRF): extract/minerals. 5.78: l(VS basis)

OF MSW (simulated): 80:20 TS basis Cont. = 6.6% TS VS=91% TS

NA

NA

NA

NA 35

BMP assay

NA

NR

NR

3.9

2.1-6.9

l)$

0.209(0.005)$ 0.123(0.005)$ 0.134(0.006)$ 0.143(0.004)f

0.205(0.01

0.222(0.014): 0.215(0.013)$

0.294(0.038) 0.307(0.037)

0.336(0.067)

0.324(0.043)

0.290

0.390

0.390 0.360 0.430 0.430

1.5 2 4

I

0.230

0.187*

0.260* 0.264* 0.235*

0.230 0.290

NA

20 14

14

20

14

9925

14420

temperatures

CH, yield? (m’ kg-’ VS,)

2.6 1.6

13.7

15

10.0 12.1 13.2

OLR (kg VSm-’ d -’)

solid waste feeds at mesophilic

35

35

37

37

33-37

33-37

35

19 21

3742

14-21

16-21

HRT (days)

15

(“C)

with municipal

37

3540

3540

Temp.

performance

BMP assay

BMP assay

CSTR Semi continuous 3.5 1

CSTR Semi continuous 3.5 I

CSTR 2.2 m’

CSTR 3 m’

CSTR Laboratory

HS-OF MSW Cont. = 3-5.6% TS VS = 82-87% TS

HS-OF MSW Cont. = 6.4% TS, VS = 89.9% TS

CSTR 20 m’ BIOMET process

MS-OF MSW: SEW 85:lS TS basis Cont. = 7710% TS

plant

Pilot plant 500 m’ Valorga process

MS-OF MSW Cont. = 35% TS, VS = 58.6% TS

FL

Pilot plant 60 m3 Dranco process

MS-OF MSW Corm. = 25-35%

SEW

Laboratory plant 0.035 m’ Dranco process

MS-OF MSW Cont. = 30&35% TS

TS

Fermenter

1. Digester

Feed

Table

NR

NR

NA

NR

NR

NR

70-75

63-69

NR

41 48

45

NR

NR

VSr (%)

NA

NA

0.69(0.17)* 1.04(0.23)*

1.14(0.40)*

0.77(0.18)*

1.59*

0X2-2.02*

0.39 0.55 0.87 1.70

0.58 0.46

2.6*

2.8

2.6 3.2 3.1

CH, PRP (m’ rnmJ d-‘)

[321

f321

[321

[371

[401

[391

[381

I181

[91

PI

[71

Reference

E

6 2 g R

E 2; F 8

2

g

Anaerobic

a

digestion

of biomass

for methane

production:

a review

87

55

Pilot plant 0.060 m3

Pilot plant 0.060 m’

Pilot plant 0.060 m’

Laboratory plant 0.015 m1 Valorga process

Pilot plant SEBAC process

Pilot plant SEBAC process

Pilot plant SEBAC process

Pilot plant 3 m3 Semi-dry fermentation

MS-OF MSW Cont. = 30% TS

MS-OF MSW Cont. = 35% TS

MS-OF MSW Cont. = 40% TS

MS-OF MSW Cont. = 30% TS

MS-OF MSW Sumter country, VS = 81% TS

HS-OF MSW Levy Country, VS = 93% TS

Yard waste

MS-OF MSW fresh Cont. = 1622% TS VS = 44.8% TS

55.8 55.5 56.2

55

55

55

60

55

55

Pilot plant 0.060 m’ 55

SC-55

(“C)

with municipal

Temp.

performance

MS-OF MSW Cont. = 25% TS VS = 47% TS

Laboratory plant 0.035 m’ Dranco process

MS-OF MSW Cont. = 3&35%

TS

Fermenter

Feed

Table 2. Digester

5.7 7.4 II.7

42

21

42 21

8 IO 15 30 8 10 I5 30 8 IO 15 30 8 IO 15 30 9 12

16-21

HRT (days)

17.8 12.9 9.7

NR

6.4

3.2 6.4

10.0 12.1 13.2 14.9 14.7 11.8 7.8 3.9 17.5 14.0 9.3 4.7 20.6 16.5 10.9 5.4 23.5 18.8 12.4 6.2 18-20 16.5

OLR (kg VSm-3d-‘)

solid waste feeds at thermophilic

40.6

1.06

NR

3.35* 3.01* 1.73*

0.190

0.070(0.02)

0.192* 0.215* 0.179*

NR

19.0

49.7 36.0

50 46

NR

NR

NR

NR

NR

0.61 I .02

2.86* 3.41* 3.74* 4.62* 1.92 1.74 1.44 0.96 2.33 2.52 2.20 1.50 2.88 3.34 2.64 1.56 0.00 2.40 I .98 I .68 3.644.48* 3.3*

0.286* 0.282* 0.283* 0.310* 0.131* 0.147* 0.185* 0.246* 0.133* 0.180* 0.236* 0.319* 0.140* 0.202* 0.242* 0.289* o.ooo* 0.128* 0.160* 0.27 I * 0.220 0.200

VSr (%)

0.190 0.160

CH, PRt (ml m-l d-‘)

CH, yield? (m’ kg-’ VS,)

temperatures

[411

[I31

(111

UOI

[71

Reference

5

c z $ F 1 $ 0 2 % R

g

55

CSTR pilot plant 3 m3 semi-dry

fermentation

CSTR pilot plant 3 m3 semi-dry fermentation

MS-OF MSW Cont. = 17% TS; VS = 44% TS

MS-OF MSW: Algae (9:l TS basis) Cont. 20% TS

7.5

8.5

6.1 7.8 11.7

13.4

13.4

19.9 13.5 6.9

0.212*

0.188*

0.131* 0.159* 0.254;

2.80*

2.53*

2.68* 2.17* 1.73*

HRT = hydraulic retention time, OLR = organic loading rate, VS, = VS added, CH, PR = methane production rate, VS., = VS reduction. MS-OFMSW organic fraction of municipal solid waste, HS-OFMSW = hand sorted organic fraction of MSW, NA = not applicable, NR = not reported. *Values calculated from the data reported. tValues in parentheses are SD.

55

51.5 54.6 54.8

Pilot plant 3 m’ Semi-dry fermentation

MS-OF MSW pre-composed Cont. = 1622%TS VS = 44% TS

= mechanically

NR

NR

NR

sorted

f421

5. 0 a 09 Q g 2 r?,

> I

90

V. NALLATHAMBI~UNASEELAN

parameters should make SEBAC a promising content of above 82% corresponds to the HS, concept for AD of MSW. SS, SC or simulated 0F-MSW.‘3,36,38 40 The According to Mata-Alvarez et a1.4’ the second refers to most of the data for MS-OF of the semi-dry process is very MSW with VS content less than 60%.9~‘o~‘3~36.4’.42performance healthy and allows very high yields and Given these characteristics, higher biodegradproduction rates. CH, PR of 3.35 m3 m-3 d-’ at ability and consequently higher yields are 6-day HRT is a very high figure for CSTR and expected from the AD of HS or SS-OF MSW. it is quite comparable with those reported in the 4.1.2. OF-MS W digestion at mesophilic temliterature for dry digestion systems at therperature. Considering the biodegradation of mophilic conditions (Table 2). OF-MSW in a CSTR-type digesters at 35°C a It has been demonstrated that the algae from maximum CH, yield ranging from 0.39 to the Venice lagoon can be co-digested with the 0.43 m3 kg-’ VS added was reported for HS-OF OF-MSW under semi-dry thermophilic conMSW without paper and wood36.3R.39and VS ditions. This approach will contribute to the reduction (VSr) ranged from 63 to 69% disposal of harvested algae from the lagoon of (Table 1). The methane yield of MS-OF MSW Venice.42 ranged from 0.11 to 0.16 m3 kg-’ VS added and Cecchi et af.43 proposed the step diffusional VSr was around 30% due to its high ash value.36 model to describe substrate utilization during The CH, yields reported for MS-OF MSW at AD of the SS-OF MSW. The new model is high-solids (Table 1) ranged from 0.18 to found to show a better fit to the experimental 0.26 m3 kg-’ VS added with a VSr of 45%;’ 9 result than those obtained with other models. however, methane production rate (CH,PR) of 4.1.4. Partial cornposting prior to digestion, 3.2 m3 mm3 d-’ was achieved at loading rate Ten Brummeler and Kosterls reported that the (OLR) of 12 kg VS mm3 d-’ and retention time start-up of the dry anaerobic batch digestion (HRT) of 1621 days.’ The OLR applied in the (BIOCEL process) of the OF-MSW at 30°C can Dranco process8 is the highest, whereas that be accelerated by partial aerobic cornposting for applied by Pauss et a1.3X is the lowest. The 2 weeks. A major drawback was a loss of 40% potential of AD of OF-MSW increases in of the potential CH, yield during cornposting. A systems in which co-digestion of MSW and shorter partial cornposting period might be SEW is carried out.“. 36.40 more feasible. According to Mata-Alvarez et 4.1.3. OF-MS W digestion at thermophilic a/.4’ pre-cornposting process surely removes the temperature. In the thermophilic high-solids easily degradable fraction of the organics in the digestion studies (Table 2) higher OLR and MSW causing the lower digester performance. CH,PR could be achieved at reduced HRT as However, at the same time during the expected and the CH, yields of MS-OF MSW cornposting process some of the large molecules, were around 0.2 m3 kg-’ VS added.“.” Despite which are difficult to degrade, are broken down the fact that the OF-MSW from Sumter and making them more easily available for the Levy countries were differently sorted and anaerobic hydrolytic bacteria of the digester. varied widely in content, their percentages of VS Thus, at long HRT ( > 12 days) this effect is were above 81%. SEBAC of the Sumter and noticeable in the case of pre-composted MS-OF Levy sources of OF-MSW showed that for the MSW, increasing the CH, yield, whereas at 21-day runs, a CH, yield of 0.16 and shorter HRT (68 days) there is no time to 0.19 m’ kg-’ VS added and VSr of 36% and degrade the de-polymerized compounds and 41%, respectively were achieved.12,13 The dataonly the contrary effect is more heavily observed base on extent and rates of the major (Table 2). Further research is needed to test the biodegradable organic components of MSW32 validity of this hypothesis as it is reported that (Table 1) showed that BMP of paper samples the methanogenic potential of the waste from ranged from 0.08 to 0.37 m3 kg-’ VS added, but the S. Giorgio di Nogaro plant, which was the types of paper that comprised the Sumter pre-composted aerobically, was considerably and Levy sources were not reported.‘2,‘3 The reduced (0.14 m3 kg-’ VS) when the reactor presence of high proportions of slowly operated at 21-day HRT36 (Table 1). biodegradable lignocellulosic material like 4.2. Sewage sludge and industrial efluent paperI would have resulted in partial biodegradation in 21-day runs. The potential for further A considerable amount of information has improvements by optimizing several operational been gathered over the performance of sewage

Anaerobic

digestion

of biomass

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) leaves

and

4.3.1. FVS W. These wastes are characterised by high percentages of moisture ( > 80%) and VS ( > 95%) and have a very high biodegradability. They are transported to municipal dump et al.76 have referred sites and Mata-Alvarez 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 a1.5othe 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 applepulp, 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 alkalinity 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 a1.53evaluated the AD potential of

for methane

production:

a review

91

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 a1.54examined 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,” deoiled (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. et a1.57 demonstrated the AD Sharma 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

waste

waste

CSTR Semicontinuous 5.5 1

CSTR Semicontinuous 60

Fruit wastes (tomato, mango, orange peel with oil, deoiled orange, pineapple, banana and jack fruit wastes in succession)

processing

I

CSTR Semicontinuous 60

Fruit and vegetable wastes mixture (orange, cauliflower, cucumber, lettuce, tomato and water-melon mixture)

Tomato

Up-flow anaerobic sludge bedanaerobic filter reactor; Twophase AD; Hydrolyzer (H) 1.3 1; methanizer (M) 0.5 I

1

CSTR Semicontinuous

With sludge recycling

CSTR Semicontinuous 1881

CSTR Continuous with solids recycling 10 I

Apricot fibre Corn cobs Apple cake Apple waste Asparagus waste Sugarbeet pulp Pineapple pressings

Papaya fruit processing Without sludge

CSTR Semicontinuous 11

Fermenter

35

28-32

28-32

35

30

30

35-37

33

Temp.

(“C)

d-‘)

3.8 3.8 3.8 3.8 3.8 3.8 5.7 7.6 9.5 4.3

0.030 0.090 0.250 0.370 0.320 0.270 0.190 0.110 0.040 0.420

0.510

NR

H-2.6 M-l

8 12 16 20 24 16 16 16 16 24

0.383

0.353* 0.255*

0.169* 0.245* 0.321* 0.357*

0.286; 0.267* 0.252* 0.228* 0.230* 0.263* 0.335*

0.316* 0.219* 0.343* 0.261* 0.308* 0.281* 0.417* 0.310*

CH, yield” (m’ kg-’ VS.,)

NR

1.06(0.21)

0.85(0.09)

0.81(0.20) 0.28(0.07)

1.39(0.24)

2.61(0.55)

3.74* 3.90* 3.88* 3.43* 4.17* 4.06* 3.87*

0.83-1.18 0.741.06 0.961.15 1.02-1.15 1.02~1.60 0.83-1.15 0.80-0.90 0.87-1.25

OLR (kgVSm-’

0.136* .0.297* 0.557* 0.701* 0.502* 0.637* 0.835* 0.551* 0.218* 0.8

NR

NR

0.30 0.27

0.44(0.12) 0.34(0.09) 0.26(0.26) 0.10(0.02)

NR

NR

CH, PRh (m’ m-j d-‘)

NR

NR

NR

98.5

90

61.3 54.3

78.8 64 57.7 51.2

96.3 95.7 93.4 88.1 89.7 95.2 93.2

70 40 70 50 40 60 75 75

VSr (%)

[601

[591

t251

[541

1511

1501

Reference

retention time; OLR, organic loading rate; VS,, VS added; from the data reported; b values in parentheses are s.d.)

H-7.5 M-3

12 9.6

15 15 15 15

NR

32

HRT (days)

with fruit, vegetable solid waste and leaf feeds (Temp, temperature; HRT, hydraulic rate; VS,, VS reduction; NA, not applicable; NR, not reported; * values calculated

Spinach-waste Asparagus peels French bean-waste Strawberry-slurry Apple-pulp Apple-slurry Carrot-waste Green pea-slurry

Feed

Table 3. Digester performance CH, PR, methane production E

Anaerobic digestion of biomass for methane production: a review

2

2

93

1

1

I

I

I

0.1

3

5

5

5

3

Batch 4 1

Batch

Batch

Calolropis procera leaves, CPL CPL

Batch

Batch

Batch

Gliricidia maculata leaves

Ipomoea Jistulosa stem (IFS) IFS, 0.4 mm size IFS, 40 days incubation with water

Ipomoea fistulosa leaves 0.088 mm size 0.4 mm size 1.0 mm size 6.0 mm size 150 x 100 mm size

Mirabilis jalapa leaves 0.088 mm size 0.4 mm size 1.0 mm size 6.0 mm size 30 x 50 mm size

Batch

JA tops Fresh Silage

3 1 3 I

I

CSTR Semicontinuous 10

Batch Batch

No. 1168, fresh

1

3 1

3

CSTR Semicontinuous 10

Batch

Batch

Fermenter

Topinanca variety, ensiled variety No. 1168, ensiled

Variety ensiled

Helia Helianthus tuberosus, stem + leaves (Jerusalem artichoke, JA) Topinanca variety, fresh ensiled

Symphytum asperum tops (Comfrey tops) Fresh Silage

Rheum rhaponticum tops (Rhubarb tops) Fresh Silage

Feed

I

I

30

35

29-35

37

31

37

NA

NA

NA

NA

NA

NA

NA

NA NA

21 27 35

46 44

37

NA

NA

HRT (days)

50 59

(“C)

37

35

35

Temp.

Table 3-Continued

NA

NA

NA

NA

NA

NA

NA

NA NA

2.6 2.5

2.2 2.5

NA

NA

OLRb (kgVSm 3 d - ‘)

NR

0.280

0.181(0.034)

0.361 0.426

0.429(0.002) 0.427(0.001) 0.421(0.005) 0.413(0.001) 0.387(0.002)

0.339(0.002) 0.341(0.001) 0.329(0.001) 0.327(0.002) 0.290(0.004)

0.309 0.301

0.338* 0.354*

0.307 0.28 1

0.250* 0.265*

0.334 0.323

0.316 0.345

CH, yieldb (m’ kg-’ VS,)

1.624(0.087)

NA

NA

NA

NA

NA

NA

NA NA

NR

NR

NA

NA

CH, PR” (m’ me3 d-‘)

NR

64.5

37.5

50.7 59.1

55.3 55.0 54.2 53.1 49.9

45.0 45.3 43.7 43.3 38.5

NR

NR NR

61 66

NR 67

NR

NR

vsr (%)

1671

[661

[651

[641

1571

[571

[631

tz22;

[621

[621

[631

1631

Reference

??

Anaerobic digestion of biomass for methane production: a review

3.8 kg VS mm’ d-‘. The CH, yield was slightly lower for the 16-day HRT. 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 microflora 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, proteolytics, 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. 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 production of biogas possible all the year round independent of the seasonal availability of raw materials. 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,h4on 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. 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

95

leaves, such as Mirabilis, IpomoeaJistulosa, etc., entire leaves can also be used without shredding. Mahamat et a1.‘j6were 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) 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 X2have a substantial effect on CH, yield from grasses. 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 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

Pennisetum purpureum (Napier grass, NG) Age: 120 days 180 days 330 days NG tops Before micronutrient addition After micronutrient addition NG, PI 300086 NG, Fresh-R-3 Fresh-R-2 40% moisture-R-3 20% moisture-R-5 NG, Fresh-PI 300086 Ensiled-PI 300086 NG, PI 300086, Harvest frequency 3 times/year 2 times/year 1 time/year NG, PI 300086 whole plant leaves stems NG, N-75 Whole plant leaves stems NG, S44 whole plant leaves stems NG, 551 whole plant NG, S42, whole plant Energycane Ball milled 0.8 mm particle size 8.0 mm particle size Energycane, fresh ensiled

Feed

Table 4. Digester performance rate; VS,, VS reduction;

NA

NA

” ” ” ”

”

0.294(0.034) 0.258(0.020) 0.238(0.004) 0.243(0.002) 0.262(0.001) 0.242(0.001) 0.296(0.019) 0.284(0.026) 0.298(0.011)

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

NA

35

35

35

35

35

35

35

35

BMP assay

BMP assay

BMP assay

BMP assay

BMP assay

BMP assay

BMP assay

0.320 ’ 0.240 ’ 0.290 ’ 0.245(0.001) 0.265(0.007)

0.304(0.014) 0.306(0.004) 0.287(0.022) 0.342(0.017) 0.322(0.026)

”

”

” ” ”

”

”

” ”

” ”

”

”

”

NR

NR

NR

NA

NA

NR

NR

NR

NR

NR

NA

NA

NA

NA

NR NR NR

0.194 NA NA

0.158* 0.288(0.004) 0.274(0.010) 0.191(0.014) 0.255(0.013) 0.247(0.014) 0.260(0.014) 0.310(0.008)

BMP assay

NR

0.139

0.113*

NA NA

NR

NA

VSr (X)

0.310 0.260 0.240

NA NA

CHdPR h (m3 m-’ d-‘)

35 35 ” ” ”

CH, yield b (m’ kg-’ VS,)

1.23

d-‘)

20

OLR (kgVSm-’

35

(days)

CSTR Semicontinuous 4 I BMP assay BMP assay

HRT

NA

(“C)

NA

Temp.

35

BMP assay

Fermenter

1691

1701

Reference

with grass feeds (Temp, temperature; HRT, hydraulic retention time; OLR, organic loading rate; VS,, VS added; CH, PR, methane production are s.d.; ” ultimate CH, yield) NA, not applicable; NR, not reported; *values calculated from the data reported; b values in parentheses

g

Grass mixture Triticum aestivum (Wheat straw, WS) 20 mm size 0.5 mm size WS, 0.5 mm size

Biomass grown in flooded soils Alemangrass-6A Alemangrass-7A Paragrass- 1P Paragrass-3P Saccharum robusturn Sugarcane hybrids: US 72-1288 US 84-1008 US 84-1009 US 84-1018 Turf grass Floritum St. Aug. Seville St. Aug. Rye grass straw (1-3 cm size)

Without external nutrients With external nitrogen addition With external nitrogen and phosphorus addition BG, 0.088 mm particle size 0.4 mm particle size 1.0 mm particle size 6.0 mm particle size 30.0 mm particle size

Energycane, L79- 1002 Harvest frequency 3 times/yr 2 times/yr I time/yr Cynodan dactylon (Bermuda grass, BG)

11

CSTR semicontinuous 20

Batch

I

33-37

CSTR semicontinuous 20 1 CSTR

33-37

35-39

35

35

37

20

NA

16

” ” ” ” 0.277(0.028) 0.261(0.013) 0.292(0.005) 0.251(0.007)

2.36

0.255* 0.327* 0.259*

0.190*

1.94* NA

0.136*

” ”

” ” ” 0.242(0.011) 0.238(0.009) 0.281(0.010)

0.332(0.025) 0.247(0.004)

” ”

” ” ”

0.298(0.001) 0.293(0.006)

0.226(0.004) 0.228(0.002) 0.214(0.005) 0.205(0.003) 0.137(0.003)

0.218

0.112 0.219

0.294(0.027) 0.261(0.016) 0.246(0.004)

2.02

NA

NA

20

NA

NA

1.6

1.6 1.6

NA

NA

NA

12

35

BMP assay

I

12 12

NA

35 35

35

5

I

35

BMP assay

Batch

CSTR Semicontinuous 7

BMP assay

NR

NA

0.370*

NR

NA

NA

NA

NR

0.350;

0.192* 0.351*

NA

79 91 65

NR

76

NR

NR

NR

30.0 30.2 28.4 27.1 18.2

38.1

20 37.5

NR

Continued

WS, ball milled gamma ray irradiation 0 M rad 1 M rad 5 M rad 10 M rad 50 M rad 100 M rad NH,OH pretreatment NH, OH-80”C-24 h 0 g OH kg-’ VS 34 g OH kg-’ VS NH,OH Pretreatment NaOH-90”C-1 h OgOHkg-‘VS 34 g OH kg-’ VS WS, ball milled NaOH-34 g OH kg-’ VS-95”C-1 h NaOH-34 g OH kg-’ VS-95%I h Ca(OH),-34 g OH kg- ’VS-95’C-1 h Untreated (control) WS, ball milled. I/S ratio (VS basis) I/S ratio 0.07 I/S ratio 0.16 I/S ratio 0.19 I/S ratio 0.25-10.9 WS, 0.088 mm size 0.4 mm size 1.0 mm size 6.0 mm size 30 x 5 mm size WS, No.1 WS, No.2 Oryza sativa (paddy straw) 0.088 mm size 0.4 mm size 1.0 mm size 6.0 mm size 30 x 5 mm size Millet straw, dried 3 x 3 mm size Pearl millet, fresh ensiled

Feed

37

35 35

1

Batch 100 ml BMP assay

Batch

35

BMP assay

5

NA NA

NA

NA

NA

37

1

Batch

5

NA

35

5

120 ml

55

Batch

Semi-continuous 4 dm3

NA

54-56

Batch

4 dm’

NA

54-56

(days)

Batch 4 dm3

HRT NA

(“C)

54-56

Temp.

Batch 4 dm’

Fermenter

Table &Continued

NA NA

NA

NA

NA

NA

NR

NA

NA

NA

OLR (kgVSm-’ d-‘)

0.390 0.257(0.016) 0.304(0.013)

0.365(0.001) 0.367(0.001) 0.358(0.002) 0.347(0.002) 0.241(0.004)

0.013 y 0.033 u 0.018 y 0.299-0.331 0.249(0.001) 0.248(0.001) 0.241(0.001) 0.227(0.001) 0.162(0.003) 0.302(0.008) 0.333(0.006)

NR

0.300(0.020) 0.383(0.016)

0.318(0.008) 0.362(0.002)

0.304(0.001) 0.314(0.002) 0.278(0.001) 0.3 1S(O.002) 0.275(0.005) 0.21 l(O.027)

”

”

’

” ”

” ”

” ” ” ” ” ”

CH, yield b (m3 kg-’ VS,)

NA NA

NA

NA

NA

NA

3.82(0.03) 3.46(0.06) 3.21(0.07) 3. I l(O.07)

NA

NA

NA

CH,PR ’ (m’ mm3 d-‘)

63.0 NR

55.6 56.0 54.6 52.9 36.8

38.7 38.5 37.4 35.2 25.0 NR

NR

50 48.6 44.4 38.4

66.5 70.7

56.8 57.3

59.4 64.0 65.0 66.4 55.7 39.7

VSr (X)

[661 [331

[571

~291

[571

[761

[751

[751

[751

Reference

k

2 p

1 Q

[

2

.':

20 mm size l-3 cm size

day feeding

Sorghum hicolor: alpha cellulose (1: 1 VS basis) Dry fermentation (28%TS) Sorghum bicolor: alpha cellulose Sorghum Sorghum 1.6 mm particle size 8.0 mm particle size Sorghum

Alternate

Daily feeding

Corn stover Sorghum cultivars (i) High energy Atlas x RT x 430 AT x 623 x Wray AT x 623 x Rio (ii) Sweet BMR-12 Rio (iii) Grain Giza 114 RS 610 Sorghum-Rio cultivar Daily feeding

Maize crop, 20 mm size Maize crop 1-3 cm size

Oats crop, Oats crop,

28

28 15

35

35 55

Non-mixed vertical flow reactor 10 1 Occasionally stirred reactor

0.380 0.360 0.410 ” 0.420 ’ 0.311(0.008)

NA NA

NA NA

35 35

BMP assay BMP assay

0.310 24.1

21.7

NA NA

0.320

17.1

29.8

NA NA

55

35 35

20 1

”

NA

NA

NA NA

NR

NR

NR

90.7 82

NR

NR

1.80 0.370(0.012)

4.8

NR

1.6 3.2 4.0 3.2

0.40 0.83 0.96 1.20

73.8 83.5 NR

0.280(0.002) 0.310(0.003) 0.260(0.009) 0.260(0.008) 0.250(0.013) 0.360(0.009)

93.6 92.8 91.3

NA

92.3 92.0

NR

81.0 80.0

66.0 55.0

NA

NA NR

NA NR

0.350(0.003) 0.400(0.001)

BMP assay BMP assay

CSTR

3

1

CSTR

0.360(0.001) 0.340(0.001) 0.340(0.006)

NA

NA

35

BMP assay

0.342; 0.253* 0.360(0.003)

NA 2.3

0.295* 0.274*

NA

NA 20

NA 2.3

NA

‘35-39 33-37

NA 20

35

I

I

35-39 33-37

Batch I 1 CSTR semicontinuous 20 Batch 1 I CSTR semicontinuous 20 BMP assay

100

V.NALLATHAMBIGUNASEELAN

of CH, production and high VFA concentrations. Daily addition of a solution containing nickel, cobalt, molybdenum, selenium and sulphate increased the CH, production by 40% and significantly decreased VFA concentrations. 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. Biodegradability can be increased by physical pre-treatment, which includes size reduction and pre-incubation with water. Particle size reduction provides high surface area for the cellulosic materials. Sharma et aZ.57demonstrated 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 hypothesized 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 untreated 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-treatment 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 laboratoryscale 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 mm3d-’ loading, a 28day HRT and a 75-day SRT. This

Anaerobic digestion of biomass for methane production: a review

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 mm3d-’ 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 semicontinuous 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 biodegradability 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 conditions, harvest age and year of harvest may

101

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 overgrazed 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

Batch

HRT = hydraulic retention time, OLR = organic *Values in parentheses are SD, tultimate CH, yield.

Ipomoea Jistulosa Stem (IFS) IFS, 0.4 mm size IFS, 6mm size IFS. 40 days incubation with water

I

loading

5

CSTR semi-continuous 21

rate, VS, = VS added,

37

25 35 45 55 25 35 45 55

CSTR semi-continuous 21

treated

35

BMP assay

chlorite

35

BMP assay

sodium

35

BMP assay

Vine shoots,

35

BMP assay

(days)

VS, = VS reduction,

NA

20

20

NA

NA

NA

NA

NA

35

BMP assay

HRT NA

(“C)

35

Temperature

BMP assay

OLR

biomass

NA

I .o

1.0

NA

NA

NA

NA

NA

NA

0.361 0.182 0.426

0.330t 0.330t 0.300t 0.109 0.154 0.164 0.193 0.180 0.273 0.283 0.315

0.140 0.310 0.220 0.290 0.210

(O.Ol)(O.Ol).t (0.020) (O.Ol)? (o.olo)t

0.220 0.320 0.320 0.063 0.014 0.240 0.280 0.042 (0.003)

CH, yield* (m’ kg-’ VS,)

NR = not reported.

(kg VS mm3 d-‘)

feeds

NA = not applicable,

with woody

Fermenter

performance

Populus deltoides (cotton wood) Populus sp. (Hybrid poplar) Platanus occidentalis (Sycamore) Pinus taeda (Loblolly pine) Eucalyptus sp. Black alder Red alder Abies concolor (White fir) All Salix sp. (Willow) Stem and bark 0.8 mm particle size All Populus sp. (Poplar) stem and bark 0.8 mm particle size Liyuidambar styrac$ua (Sweet gum) Poplar wood 0.003 mm size 0.8 mm size 8.0 mm size Vine shoots, untreated

5. Digester

Feed

Table

50.7 25.6 59.1

NR

NR

NR

NR

NR

NR

32.3 53.8 56.7 3.6 1.0 32.5 48.4 NR

VSr (%)

1641

[941

[941

[331

[28, 331

~291

[931

Reference

treated,

PH, NaOH feeding

alternate

loading

1 NA

NA

NA

rate, VS, = VS added,

29-3

I

Batch

3

28-31

3 1

24-28

10 10 10 10 20 20 20 20

22-26 35 40 45 22-26 35 40 45

HRT

28-32 22-26 22-26

(‘C)

6. Digester

5 IO 20 40 10 10 20

28-32

Temperature

Batch

I

HRT = hydraulic retention time, OLR = organic *Values calculated from the data reported. tValues in parentheses are SD.

t

day

daily feeding

PH, Untreated, I/S = 67 (vol/vs) Fresh Dried PH, dried, heat treated PH, dried, HCI treated PH, dried, NaOH treated Lantana camera, NaOH treated CM (50:50 w/w) Ageratum, partially decomposed

treated,

PH, NaOH

2

Semicontinuous CSTR 3 1 Semicontinuous CSTR 3 1

PH, untreated, day feeding

Batch

Semicontinuous CSTR 31

Parthenium hysterophorus (PH) PH, untreated, daily feeding

alternate

Fermenter

Feed

Table

CH,PR

(days)

production

NA

NA

NA

4.12 4.12 4.12 4.12 2.06 2.06 2.06 2.06

4.95 2.48 1.24 0.62 4.12 4.12 2.06

(0.002) (0.002) (0.004) (0.001) (0.003) (0.003) (0.007) (0.003)

(0.002) (0.005) (0.001) (0.011) (0.005) (0.002) (0.005)

(rn’ kg-’ VS,)

rate, VS, = VS reduction,

0.241*

0.147 (0.012) 0.140 (0.008) 0.157 (0.015) 0.203 (0.009) 0.236 (0.008) 0.236*

0.095 0.172 0.214 0.060 0.173 0.202 0.214 0.112

0.034 0.117 0.115 0.101 0.110 0.039 0.086

CH, yieldt

weed feeds

(kgVS m-’ d-‘)

with terrestrial

= methane

OLR

performance (rn’m_W)

NA

NA

NA

(0.006) (0.006) (0.014) (0.003) (0.006) (0.005) (0.015) (0.001)

(0.012) (0.011) (0.014) (0.007) (0.02) (0.009) (0.011)

NA = not applicable,

0.393 0.709 0.883 0.248 0.357 0.417 0.442 0.232

0.167 0.290 0.143 0.062 0.456 0.160 0.179

CH, PRt

NR NR = not reported.

1991

2.

P, x2

$

WI

65.9 NR

5 ;:

z 3 P 5

z I

% s

a B 8. s

E a _$

>

$ ,c.

[771

[961

Reference

42.8 60.3

39.8 36.4

65.8 66.7 NR

35.3 62.6 65.1 NR 62.4

25.9 42.9 42.1 NR 45.7 17.9 35.5

VSr (%)

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% increase 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 Parthenium 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 production, 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 decomposition 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-dimensional, 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, Laminaria and Ascophyllum, green algae Ulva, Cladophora and Chaetomorpha and red algae Gracilaria (Table 7).33.42,7’ ‘oo~‘ , 08Macrocystis 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 complexes 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 fucoidin 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 theoretical yield for biomethanation of kelp was found to be 0.51m3 kg-’ VS added. ??

Table 7. Digester performance with marine biomass feeds Temp. (‘C)

Feed

Fermenter

Macrocystis pyrijera

CSTR

35

(raw kelp, RK) RK

Batch 2 dm’

28.3 35.0 39.6 44.0 49.3 54.6 59.7 35

RK

CSTR Semi-continuous 2 dmi/10 dm’

RK CSTR 2 dm’ Baseline treated kelp (BLTK) BLTK: kelp juice (VS basis) 4:l 3:2 BLTK CSTR 2 dm’ BLTK: kelp juice (VS basis) 4:l 3~2 Kelp juice RK CSTR 2 dm’ Daily feeding Semi-continuous Alternate day feeding BLTK CSTR 2 dm’ Intermittent mixing Continuous mixing RK CSTR 2 drnl Without external nutrient With external nutrient (N and P) Laminaria saccharina CSTR Laminaria saccharina CSTR Semi-continuous

HRT OLR (days) (kg VS m-’ d-‘)

VSr (%) Reference

18

1.6

0.277-0.310

NR

11001

NA

NA

NR

[loll

18

1.6

0.021 0.103 0.113 0.055 0.150 0.142 0.028 0.278

50.8

35 55 55 35 35

IO 18 1 18 18

1.6 1.6 3.2 1.6 1.6

0.215 0.149 0.134 0.264 0.218

38.6 31.2 27.3 47 41

35 35 35

18 18 IO

1.6 1.6 1.6

0.235 0.211 0.210

35.6 42.0 34.4

35 35 35

IO 10 10

1.6 1.6 1.6

0.202 0.197 0.116

24.0 43.5 23.3

35

18

1.6

0.243

NR

35

18

1.6

0.250

NR

35 35

18 18

1.6 1.6

0.231 0.206

NR NR

35

12

1.6

0.239

45.1 42.6

0.233 37 35

Laminaria hyperborea Ascophyllum nodosum Sargassum f&iitans

Bladders Blades Stipe Whole plant

CH, yield* (m’ kg-’ VS,)

25 24

1.1-1.6 1.65

24 24

1.65 1.75

0.280 0.110

0.2054.220 0.230

49-53 NR

11031

[102]

BMP assay

35

NA

NA

0.178(0.014)t 0.143(0.004)~ 0.182(0.018)t 0.165(0.008)t

NR

[331

BMP assay

35

NA

NA

0.171(0.004)t 0.148(0.007)?

NR

t331

75.1

HO41

80.1 85.7 26-48

[106]

Sargassum pieropleuron

Bladders Blades Stipe Whole plant

0.1 i9[0.004jj

0.145(0.001)t

Gracilaria tikvahiae (GT)

High tissue nitrogen Moderate tissue N Nitrogen deficient CiT Ulva sp. High tissue nitrogen Moderate tissue N Nitrogen deficient

’

Batch 2 1

29-35

NA NA NA 3G-60

NA

0.220 0.230 0.190 0.130-0.200

CSTR

25-35

Batch 2 I

29-35

NA NA NA

NA

0.220 0.230 0.330

35

20 15 12-15 NA

1.0 1.0

0.212 0.203 0.250-0.350 0.350-0.480

0.54

70.1 77.3 86.7

[1041

63

uo71

Ulva rigida (go-90%) + Gracilaria confervoides

Washed Unwashed Ulca + Cladophora + Chaetomorpha

Semicontinuous 180 1 Semi-continous 2 I Batch I 1

35 35 35

2-2.5 NA

525 NR

HRT = hydraulic retention time, OLR = organic loading rate, VS, = VS added, VS, = VS reduction, applicable, NR = not reported. *Values in parentheses are SD. tultimate CH, yield. 105

[IO81

NA = not

Salvinia molesta Azolla pinnata Cerafopteris thalictroides Seirpus grosses Cyperus papyrus Utricularia reticulata Hydrilla verticillata Azolla pinnata (AP) not exposed

to metals

WH, untreated WH, steam treated WH, particle size reduction WH, NaOH treated WH, Miss, IM-B WH, Miss, 2MB WH, FL, IM-8 WH, Miss + NH, Cl, IM-4 WH, Miss + mixed nutrient 2M-3 WH, Miss, IT-5 WH, Miss + NHp Cl, IT-8 WH, Miss + NH, Cl, IT-IO WH, shoots WH, roots WH, high lignin WH, low lignin Lagarosiphon major (lake weed) Pistiu stratiotes BMP assay

Batch

Batch

Batch Batch

2

1

11 3 1

37

35-39 29.5 33.0 37.5 37

NA

NA

NA NA

NA

NA

NA NA

NA

35

BMP assay

CSTR 71

35

NA

1.6 1.6 1.6 1.6 1.6 2.4 3.36 4.8 NA

12 12 12 12 12 16.7 12 6 NA

CSTR 71

ABP assay

35

35 35 35 35 35 55 55 55 35

CSTR

sludge

WH:Primary (3:l)

biomass

OLR (kg VSm-’ d-l)

with freshwater HRT (days) 1.60 2.00 2.64 3.41 1.60 2.66 3.72 4.52 NA

(“C)

performance

15 12 10 8.5 15 10 8.5 7 NA

CSTR

Eichhornia crassipes (Water hyacinth, WH)

Temp.

8. Digester

35

Fermenter

Feed

Table

0.190 0.170 0.170 0.170 0.280 0.240 0.250 0.250 0.316(0.022) 0.319 0.311(0.019) 0.362 0.182 0.185 0.098 0.176 0.173 0.156 0.143 0.122 0.320 0.180 0.196(0.003): 0.213(0.004)$ 0.269* 0.369* 0.346* 0.410’ 0.242 0.132 0.204 0.066 0.038 0.132 0.081 0.117*

-xeldt (m’ kg - ’VS.,)

feeds

NA

NA

NA NA

NA

0.307* 0.312* 0.166* 0.297* 0.293* 0.396* 0.508* 0.615* NA

0.300 0.350 0.440 0.550 0.450 0.640 0.920 1.110 NA

CH, PRt (m’ m-’ d-‘)

NR

62 89 83 99 NR

NR

28.8 29.8 17.0 28.5 28.9 27.4 24.6 21.3 NR

49.8* 43.8* 37.9* 33.4* 56.0* 46.8* 44.1* 44.0* NR

VSr (%)

[‘I81

[I 161

I73 [1151

[’101

Reference

Anaerobic

N

digestion

of biomass

for methane

production:

a review

107

V.NALLATHAMBIGLJNASEELAN

Methane yields were in the range 0.310.34 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 digesters 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 Laminaria 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 mm3kg-’ 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 semidry 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 Chaetomorpha 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

freshwater) habitats have been generally used as CH, prod~~tion33~70.73,llO~llS for substrates (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 conditions under conventional evaluated (CSTR, mesophilic, low OLR and high HRT) separately and as part of a mixed feedstock.“’ production data were 0.17The CH, 0.30.19 m’ kg-’ VS added and 0.55 m3 m-3 day-’ for WH and and 0.455 0.240.28 m’ kg-’ VS added 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 performance 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.‘13and Mallik et a1.97have suggested WH as an additive with CM for biogas digesters.

109

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.,‘18Azofla pinnata and Lemna minor are currently being considered for Non-contaminated wastewater treatment. 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 (Chlorophyta) 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, conversion 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 Table 9. Literature

data

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 biodegradable 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

of biomass

with high yields of methane CH, yield* (m’ kg-’ VS added)

Biomass 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 2. Water hyacinth (NaOH treated) HS = hand sorted, SS = source sorted, *Values in parentheses are SD. tUltimate CH, yield.

SC = separated

Reference

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 0.390 0.383 (0.016)t 0.367 (0.001) 0.360 (0.003) 0.342 (0.017)t

[331 [661 [751 [571

v91 [331

0.426 0.361 0.330 0.315 0.241 0.236 (0.008) 0.236

collection,

[991 [771 1801

0.480 0.330 0.310

[to81

0.410 0.362

Ill51 [l101

SEW = sewage sludge.

[IO41 [tOOI

Anaerobic

digestion

of biomass

digestion process is very healthy as it allows very high production rates. 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 conversion 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. 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. 5. Hybrid poplar and sycamore produced the highest CH, yield of 0.32 m’ kg-’ VS added, whereas eucalyptus, loblolly pine and white fir exhibited poor degradability. Biphasic curves of CH, production indicated that BMP assays of woody biomass, conducted for less than 50 days may underestimate B,. 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-tobark ratio and toxic components influence the AD of wood. 6. Weeds trap a significant amount of solar energy and grow in natural stands without inputs and irrigation. Large-scale utilization is one of the best strategies for weed management. NaOH-treated Parthenium, NaOH-treated Lantana and cattle manure blend and partially decomposed Ageratum are potential sources for CH, production. 7. The productivity is greater in water than on land because three-dimensional harvest is possible in water. Kelp is the only biomass that needs no pre-treatment as it lacks the refractory lignocellulosic complex. N deficient Ulva,

for methane

production:

III

a review

Cladophora and Chaetomorpha blend and kelp are high CH, producers. 8. The concept of using aquatic plants for water treatment and the harvested biomass as an energy source is gaining attention throughout 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-toCH, conversion economically viable, the number of unexplored genera to be screened in still enormous. 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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