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Anaerobic batch co-digestion of sisal pulp and fish wastes

Anthony Mshandete a,b, Amelia Kivaisi a,*, Mugassa Rubindamayugi a, Bo Mattiasson b

a Applied Microbiology Unit, Department of Botany, University of Dar es Salaam, P.O. Box 35060, Dar es Salaam, Tanzaniab Department of Biotechnology, Center for Chemistry and Chemical Engineering, University of Lund, P.O. Box 124, S-22100 Lund, Sweden

Received 26 June 2003; accepted 27 January 2004

Available online 6 March 2004

Abstract

Co-digestion of various wastes has been shown to improve the digestibility of the materials and biogas yield. Batchwise digestion

of sisal pulp and fish waste was studied both with the wastes separately and with mixtures in various proportions. While the highest

methane yields from sisal pulp and fish waste alone were 0.32 and 0.39 m3 CH4/kg volatile solids (VS), respectively, at total solid

(TS) of 5%, co-digestion with 33% of fish waste and 67% of sisal pulp representing 16.6% of TS gave a methane yield of 0.62 m 3 CH4/

kg VS added. This is an increase of 5994% in the methane yield as compared to that obtained from the digestion of pure fractions at

5% TS.

2004 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic co-digestion; Batch; Sisal pulp; Fish waste; Sisal waste sludge

1. Introduction

Digestion of biomass in order to produce biogas and

biofertilizer is an attractive mode of treating wastebiomass. The process as such is very complex and it is

catalyzed by a consortium of microorganisms that in a

joined action convert complex macromolecules into low

molecular weight compounds such as methane, carbon

dioxide, water and ammonia. The composition of the

starting material is important in the sense that there is a

need for a suitable ratio between carbon and nitrogen.

Furthermore, main intermediates in the conversion are

volatile fatty acids (VFA). If a high concentration of

VFA is formed, pH will be reduced and that can reach

levels when the methanogenic bacteria are first severely

inhibited and even may die. Therefore, it is important to

have buffering capacity in the system, i.e. products thatwill counteract the effects of the VFA need also to be

formed. It is known that carbohydrate-rich substrates

are good producers of VFA and that protein-rich sub-

strates are yielding good buffering capacity (Nyns, 1986;

Mata-Alvarez et al., 2000). Despite, these co-digestion

benefits, it is not clear whether some wastes have adverse

effects when used in conjunction with another waste

(Callaghan et al., 2002). However, digestion of mixtures

of different wastes is seldom reported (De Baere, 2000).

Based on this information it seems realistic to investigate

the effects of mixing biomass wastes when setting up ananaerobic digestion process with the aim to convert the

biomass into biogas and biofertilizer.

In Tanzania, huge quantities of waste organic matter

are generated. Fifty-two sisal factories produce about

444,000 tons of sisal pulp annually. It has also been

estimated that fish processing industries along lake

Victoria in Mwanza City alone produce 1600 tons of fish

wastes per annum (UNDP, 1993). Currently these

wastes are disposed off untreated hence resulting in

environmental deterioration (Kivaisi and Mtila, 1998).

However, these wastes being abundant and rich in easily

biodegradable substrates, such as carbohydrates, lipids

and proteins are cheap potential feed stocks for anaero-bic digestion in the production of biogas. It is important

to treat organic wastes under controlled conditions in to

order reduce spontaneous dissipation of CH4 to the

atmosphere. Moreover, production of biogas will reduce

the use of fossil fuels, thereby reducing the CO 2 emis-

sion. This is thus in line with Kyoto Summit agreement

(Mata-Alvarez et al., 2000).

For sustainable energy development in the developing

world, low cost low tech renewable energy systems for

rural areas and peri-urban regions, have high potential

for application. To this end, batch systems having the

* Corresponding author. Tel.: +255-22-41-0223; fax: +255-22-241-

0078.

E-mail address: [email protected] (A. Kivaisi).

0960-8524/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2004.01.011

Bioresource Technology 95 (2004) 1924
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simplest designs and being least expensive for use as

solid waste digesters hold great promises. However,

there seems to be insufficient knowledge on the con-

version of substrates other than the traditionally used

cow dung (Foresti, 2001). In this context, knowledge on

solids content and waste to inoculum ratios are impor-

tant. These are variables that affect the experimental

assessment of the potential biodegradability of organic

matter. The solids content is one of the parameters that

has a huge impact on the cost, performance and reli-

ability of the digestion process (Lissens et al., 2001).

Indeed, it has been suggested earlier by other workers

that the ratio of animal waste to other organic residues

must be optimized in order to avoid disturbances in the

fermentation process, notably due to lipids hydrolysis

(Amon et al., 2001). In the literature, co-digestion of

organic fractions with cow dung or with sewage sludge is

frequently reported. This paper, therefore, reports for

the first time, the results on comparison of anaerobic

batch digestion of sisal pulp and fish wastes separatelyas well as the co-digestion of both substrates.

2. Methods

2.1. Waste sources

Sisal pulp (SP), a leafy biomass waste produced

during sisal decortication, was obtained from a sisal

processing factory (Ubena Zomozi, Coast Region,

Tanzania). The fish waste (FW) obtained from the

landing beach in Dar es Salaam City in Tanzania, con-sisted of offals, scales, gills and washing water. After

collection, FW was homogenized using a Moulinex

Masterchef 55 electronic kitchen blender and both SP

and FW were stored at )20 C. Characteristics of the

pure SP, FW and mixed waste fractions used in the

experiments are described in Tables 1 and 2, respec-

tively.

2.2. Inoculum source

The lack of bulk anaerobic inocula in developing

countries like Tanzania has been cited as a bottle neck in

digester start up (Poggi-Varaldo et al., 1997). Thereforein this study, methanogenic activity of sisal wastewater

sludge (SWS) was quantified using acetate and formate

as carbon source. SWS was collected from the sedi-

mentation pond at a depth of about 3 m, using a

VANDORN grab sampler, at Ubena Zomozi sisal fac-

tory. SWS (TS of 10.9% out of which 52.4% being VS)

was used as an inoculum for digester start up.

2.3. Batch bioreactors

The biodegradability and co-digestion of sisal pulp

and fish wastes were tested in 1000 ml bioreactors con-

structed by using conical glass flasks with a working

volume of 600 ml. The biogas produced was collected in

gas tight plastic tubings which were connected to gas

tight aluminium enforced polyethylene bags. The bio-

reactors were fitted with gas sampling septa closed with

n-butyl stoppers and sealed with aluminium caps. Ini-

tially, the digestion mixtures were flushed with nitrogen

for 5 min to replace the air (oxygen) in order to achieve

anaerobic conditions. Subsequently, the mouth of bio-

reactors were closed with n-butyl stoppers to ensure gas

tightness. The bioreactors were kept at an ambient

temperature of 27 1 C and were shaken manually for

1 min twice daily to mix their content.

2.4. Experimental set-up

Digestion of fresh sisal pulp or fish wastes, separately,

was carried out at different wet biomass % (v/v) between

5 and 60. The waste to inoculum ratios were between

0.051.6 and 0.092.5 g VS waste/g VS inoculum, for fishwaste and sisal pulp, respectively. The experimental set-

Table 1

Characteristics of pure fractions of sisal pulp and fish waste used in

anaerobic degradation trials (mean values, n 3)

Analyses Sisal pulp Fish waste

pH 5.6 6.9

Partial alkalinity (PA) (mg CaCO3/l) 530

Total alkalinity (TA) (mg CaCO3/l) 2280

Total solids (TS) (% of fresh sample) 9 32.2Volatile solids (VS) (% of TS) 87.5 55.3

Organic carbon (OC) % (dry wt) 49 51

Total nitrogen (TN) % (dry wt) 1.08 5.85

Total lipids % (dry wt) 5.7 12.6

C:N 45 9

Table 2

Composition of the sisal pulp and fish waste combinations (% of wet weight) used in co-digestion trials (mean values, n 3)

% Wet weight 50(FW):50(SP) 33(FW):67(SP) 25(FW):75(SP) 20(FW):80(SP)

TS % 20.6 16.6 14.8 13.6

VS % of TS 61.8 67.6 71.4 73.6

OC % (dry wt) 49.7 46.6 43.7 38.7

TN % (dry wt) 4.0 2.8 2.3 1.7

Lipids % (dry wt) 7.3 4.8 4.2 3.8

C:N ratio 12 16 18 23

20 A. Mshandete et al. / Bioresource Technology 95 (2004) 1924


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up for the pure sisal pulp and fish wastes consisted of 30

batch bioreactors for each substrate. A control biore-

actor containing only SWS (without waste) was included

and the biogas produced was subtracted from those

registered for the substrates used. All the digestions were

run in triplicates for 25 and 29 days for sisal pulp and

fish waste, respectively. These experiments were termi-

nated when no significant biogas production was ob-

served over a two-week period.

In the co-digestion experiment, mixtures of FW and

SP in varying proportions were digested in 15 bioreac-

tors. The fish waste solid content at which the highest

methane yield was obtained (5% of TS) was kept con-

stant in the preparation of four proportions namely,

50:50%, 33:67%, 25:75% and 20:80% (on wet weight

basis) of FW and SP, respectively. The volume of

inoculum added was kept constant at 590 ml (34 g VS),

the same volume used during digestion of fish waste at

5% TS. In this trial, digestions were run in triplicates for

24 days.

2.5. Analytical methods

The volume of biogas formed was measured by using

a graduated 100 ml gas tight plastic syringe with a

sample lock. The gas composition of 5 ml samples of the

biogas was estimated by the absorption of carbon

dioxide and hydrogen sulphide in concentrated alkaline

solution using serum bottles as described by Erguder

et al. (2001). The bottles were shaken at 250 rpm for 4

min. In this method only CH4 was determined. Methane

yield was calculated as the net amount of methaneproduced per unit VS added to the digester. Acetate, pro-

pionate and butyrate were analyzed by using a Hewlett-

Packard gas chromatograph (type HP 5890). Samples

were centrifuged, 3 ml of the filtrate was acidified with

20 ll conc. H2SO4 and stored at )20 C. Before analysis

the samples were thawed and filtered using 0.45 lm fil-

ters. Samples of 0.9 ml were mixed with 0.1 ml of 100

mM isobutyric acid as an internal standard. Subse-

quently, 0.25 ml 20% ortho-phosphoric acid was added

and, after mixing thoroughly, the samples were allowed

to stand for 30 min. Samples of 0.10.2 ll were injected

into the glass column (1.8 m long and 2 mm internal

diameter) filled with 10% SP1200/1% H3PO4 on 80/chromosorbWAW. Nitrogen was used as a carrier gas at

a flow rate of 40 ml/min. Oven, injection and detection

temperatures were, 130, 170 and 175 C, respectively.

Partial alkalinity (PA), total alkalinity (TA) and pH,

were measured as previously described by Bjornsson

et al. (2000) using a TIM titration manager with an

ABU 901 Autoburette (Radiometer, Copenhagen, Den-

mark). Samples were centrifuged at 6000 rpm for 3 min;

and 6 ml of the supernatant were used. TS and VS were

determined according to APHA Standard Methods

(APHA, 1995). Total nitrogen was determined by the

Kjeldahl method, total lipids were determined by Soxlet

extraction method using petroleum ether solvent

extraction, as described in APHA Standard Methods

(APHA, 1995). The organic carbon was determined by

rapid dichromate oxidation method previously de-

scribed by Nelson and Sommers (1996).

3. Results and discussion

3.1. Biodegradability of pure fish and sisal pulp wastes

The extent on conversion of pure fish waste and sisal

pulp at various substrate concentrations in terms of

methane yield is shown in Fig. 1. Analysis of variance

(ANOVA) for each fish and sisal pulp fractions showed

that there was significant difference in methane yield

when varying the % TS in the incubations (p< 0:0001).

Methane yield decreased with an increase in the totalsolid substrate content. A similar tendency was previ-

ously observed for different types of animal, crop and

organic wastes (Badger et al., 1979; Itodo and Awulu,

1999). In this study the highest methane yield (m3 CH4/

kg VS added) of 0.32 for SP and 0.39 for FW, were

obtained at 5% of TS after 25 and 29 days, respectively.

Similarly, Badger et al. (1979) reported highest methane

yield at 5% of TS when digesting various crops and

organic wastes in 11 batch-reactors during 1736 days

(depending on the material). Batch biomethanation of

leafy biomass, comparable to sisal pulp biomass, gave a

methane yield in range of 0.2710.429 CH4 m3/kg VS

added (Zubr, 1986; Sharma et al., 1988). In the case of

biomethanation of fish wastes per se, literature data on

methane yield are scarce. Ahring et al. (1992) reported

methane yield in the range of 0.4500.500 m3 CH4/kg VS

added from fish oil sludge which is similar to what was

found in the present investigation. The ratio of waste/

inoculum was found to be a critical parameter especially

for incubations with a total solid content higher than

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Total wet biomass content %(v/v).

MethaneyieldCH4

m

/Kg

VSadded

3

Sisal pulp Fish waste

Fig. 1. The effect of different total wet biomass content % (v/v) of sisal

pulp and fish wastes on the methane yield. The sisal pulp had TS of 9%

and fish waste TS of 32%. The values are means standard deviations

(vertical bars, n 3rd deviations).

A. Mshandete et al. / Bioresource Technology 95 (2004) 1924 21


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5%, since the methane yield increased significantly when

the waste/inoculum ratio decreased from 1.6 to 0.05 for

fish and 2.5 to 0.09 for sisal pulp, respectively. These

results are in line with those reported for conversion of

solid poultry slaughterhouse waste (Saliminen et al.,

2000 and Neves et al., 2002). The average approximate

methane content of the biogas increased with incubation

time and reached 59% for sisal pulp and 58% for fish

wastes at the end of digestion. Previous studies on batch

anaerobic digestion of rice straw, maize stalks, cotton

stalks and water hyacinth gave biogas with 6067%

methane content (El-Shinnawi et al., 1989).

3.2. Co-digestion of sisal pulp and fish wastes

The total methane production and methane yield by

the digestions of co-digested fish and sisal pulp wastes

are shown in Table 3. Analysis of variance (ANOVA)

for methane yield for the substrates combinationsshowed that there was significant differences among the

combinations tested (p< 0:0001). The total methane

production and methane yields varied between 0.380.77

l and 0.300.62 CH4 m3/kg VS added, respectively with

values being highest for mixture containing 33%

FW:67% SP and lowest for 50% FW:50% SP. Generally,

based purely on total methane production and methane

yield among mixtures tested, the results suggest that, the

fraction with 33% FW:67% SP wet weight could be

suitable for successful co-digestion for enhanced meth-

ane production. Compared to the methane yields for the

pure sisal pulp and fish waste, respectively co-digestion

of the fish waste and sisal pulp at 33% FW:67% SP wetweight proportions, with 16.6% of TS and a C:N ratio of

16, enhanced the methane yield by 5994%. This could

be due to positive synergism in the digestion medium,

supplying missing nutrients and reducing/diluting of

inhibitory materials in feed stocks by the co-substrates

(Mata-Alvarez et al., 2000). This concurs with a recent

study on batch co-digestion of waste organic solids

which reported that fish offal and brewery solids mixed

with cattle slurry produced an enhancement in the

methane yield compared with that of a control digestion

of using cattle slurry alone (Callaghan et al., 1999).

Furthermore, Kaparaju et al. (2001) reported anenhancement of about 60% in the methane yield with

co-digestion of industry confectionery waste with cow

manure.

The average CH4 content of the biogas produced

from 50:50 was (61%), 33:67 (64%); 25:75 (65%) and

20:80% (58%). The range between 60% and 65% meth-

ane content in this work is closer to the range of 5060%

which is normally obtained from conventional anaero-

bic digestion of organic waste conducted in a single

stage-slurry digesters (Samani et al., 2001).

The C:N ratios of the co-digested sisal pulp and fish

wastes which ranged between 12 and 23 were within the

C:N ratios required for stable biological conversions

reported by others on anaerobic digestion of organic

wastes. Kayhanian and Hardy (1994) reported C:N ra-

tios between 25 and 30 as being optimal. However, some

investigators argue that the C/N of approximately be-

tween 1619 (Nyns, 1986) and 16.818 (Kivaisi and

Mtila, 1998) are optimal for methanogenic performance

if poorly degradable compounds such as lignin are taken

into account. In addition, Gunaseelan (1995) suggesteda C:N of 11 being satisfactory for methanogenic per-

formance using Parthenium, a terrestrial weed, as feed-

stock for the digesters. Furthermore, Itodo and Awulu

(1999) reported successful anaerobic batch digestion for

poultry, cattle and piggery wastes slurries with a C:N of

6:1 and 9:1.

In a well balanced anaerobic digestion process, VFA

levels are low. In this study all the four combinations

examined showed lower levels of VFAs in their digested

slurry which ranged between 3.7 and 6.3 mM for acetic

acid, 0.020.08 mM for propionic acid and 0.0080.05

mM for butyric acid. The initial propionic acid to aceticacid ratio (P/A) at the beginning ranged between 0.007

and 0.01. It has been shown earlier by others examining

anaerobic digestion that increase in P/A ratio greater

than 1.4 and a build-up of acetic acid and butyrate to

above 200 mM as well as 100 mM of proprionate, can

explain process inhibition and ultimate digester failure

(Hill et al., 1987; Ahring et al., 1995).

The pH values and alkalinity before and after the

digestion trials indicate that the pH and alkalinity in

the digesters were conducive for biogas production. The

initial pH ranged between 7.7 and 7.8 while the final pH

values range was 7.37.7 which suggest that souring of

the digesters was not occurring in the co-digestionmixtures tested. Similarly, Anderson and Yang (1992),

reported a range of pH 6.47.6 in a normal functioning

digester, beyond which a state of inhibition may occur

resulting from toxic effects of the hydrogen ions which

are believed to be closely related to the accumulation of

VFAs. The initial partial alkalinity ranged between 3600

and 3800 mg CaCO3/l while the final range was 6700

7700 mg CaCO3/l. The latter, demonstrated an increased

partial alkalinity in the range of 4352% compared to

the initial values before anaerobic digestion. This pro-

vided further evidence that the co-digestion of fish and

Table 3

Total methane production and methane yield at different proportions

of fresh wt% which represents different % of TS of fish and sisal pulp

wastes (mean values, n 3)

% Wet weight % TS Total (CH4) Yield CH4 m3/kgVS

50 FW:50 SP 20.6 0.38 0.31

33 FW:67 SP 16.6 0.77 0.62

25 FW:75 SP 14.8 0.57 0.48

20 FW:80 SP 13.6 0.50 0.44



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sisal pulp proportions studied was successful. Previ-

ously, laboratory studies on mesophilic and thermo-

philic anaerobic sludge digestion reported a range of

20004000 mg CaCO3/l partial alkalinity as being typi-

cal for properly operating digesters (Pohland and

Bloodgood, 1963). The initial values reported in this

study fall within this range. However, the final values

are 24 times higher than the reported values. This in-

crease could be due to generation of NH4 during the

digestion of protein in fish waste which resulted in an

increased digester buffering capacity and hence stability

of the digesters. This is an interesting cost-effective ap-

proach since no external buffer sources were added.

4. Conclusions

This study has shown that anaerobic digestion of

pure sisal pulp and fish wastes is a feasible process.

Furthermore, anaerobic co-digestion of fish waste andsisal pulp is a viable alternative for recovering energy in

the form of biogas with 6065% methane content while

at the same time abating environmental pollution. To

the best of our knowledge anaerobic co-digestion of fish

waste and sisal pulp is being reported for the first time.

The results also indicate that co-digestion with 33% fish

waste and 67% sisal pulp which represented 16.6% of TS

and a C:N ratio of 16 gave the highest methane yield of

0.62 m3 CH4/kg VS added. This was an increase of 59

94% in the methane yield compared to that obtained for

the digestion of pure sisal pulp and fish wastes at 5% of

TS. Therefore, further research is planned to run acontinuous stirred tank reactor to examine the effect of

adding different fish and sisal pulp waste blends to

the system digesting sisal wastewater sludge to gain

more information of the possible scale up of the pro-

cess.

Acknowledgements

This work was supported by Swedish International

Development Agency (SIDA) through the BIO-EARN-

project and their financial support is grate fully

acknowledged.

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