Chemosphere 59 (2005) 1575–1581

www.elsevier.com/locate/chemosphere

Baseline study of methane emission from open digestingtanks of palm oil mill effluent treatment

Shahrakbah Yacob a,b,*, Mohd Ali Hassan b, Yoshihito Shirai a,Minato Wakisaka a, Sunderaj Subash c

a Department of Biological Functions and Engineering, Graduate School of Life Science and Systems Engineering,

Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japanb Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences,

Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysiac Felda Palm Industries Sdn. Bhd., Balai Felda, Jalan Gurney Satu, 54000 Kuala Lumpur, Malaysia

Received 17 May 2004; received in revised form 5 November 2004; accepted 17 November 2004

Abstract

Anthropogenic release of greenhouse gases, especially CO2 and CH4 has been recognized as one of the main causes

of global warming. Several measures under the Kyoto Protocol 1997 have been drawn up to reduce the greenhouse

gases emission. One of the measures is Clean Development Mechanisms (CDM) that was created to enable developed

countries to cooperate with developing countries in emission reduction activities. In Malaysia, palm oil industry par-

ticularly from palm oil mill effluent (POME) anaerobic treatment has been identified as an important source of

CH4. However, there is no study to quantify the actual CH4 emission from the commercial scale wastewater treatment

facility. Hence, this paper shall address the CH4 emission from the open digesting tanks in Felda Serting Hilir Palm Oil

Mill. CH4 emission pattern was recorded for 52 weeks from 3600 m3 open digesting tanks. The findings indicated that

the CH4 content was between 13.5% and 49.0% which was lower than the value of 65% reported earlier. The biogas flow

rate ranged between 0.8 l min�1 m�2 and 9.8 l min�1 m�2. Total CH4 emission per open digesting tank was

518.9 kg day�1. Relationships between CH4 emission and total carbon removal and POME discharged were also dis-

cussed. Fluctuation of biogas production was observed throughout the studies as a result of seasonal oil palm cropping,

mill activities, variation of POME quality and quantity discharged from the mill. Thus only through long-term field

measurement CH4 emission can be accurately estimated.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Greenhouse gases (GHG); Methane (CH4); Palm oil mill effluent (POME); Anaerobic open digesting tank

0045-6535/$ - see front matter � 2004 Elsevier Ltd. All rights reserv

doi:10.1016/j.chemosphere.2004.11.040

* Corresponding author. Address: Department of Biopro-

cess Technology, Faculty of Biotechnology and Biomolecular

Sciences, Universiti Putra Malaysia, 43400 UPM Serdang,

Selangor, Malaysia. Tel.: +60 3 89468414; fax: +60 3 89430913.

E-mail address: syacob@putra.upm.edu.my (S. Yacob).

1. Introduction

Climate change attributed to the greenhouse gases

(GHG) emissions has been at the forefront of current re-

search in the past decade. These efforts were clearly sta-

ted by Kyoto Protocol with the objective of reducing the

GHG emissions by 2008–2012 (Brown et al., 1998).

ed.

1576 S. Yacob et al. / Chemosphere 59 (2005) 1575–1581

Numerous publications have been published concerning

the effects on climate change, sources and sinks of GHG,

factors and mechanisms that affect the GHG emissions

and controlling strategies for GHG emissions (Bogner

et al., 1995; Sonesson et al., 2000; Avnimelech and

Shechter, 2001; El-Fadel et al., 2001; Le Mer and Roger,

2001; Gonzlez-Gil et al., 2002).

Despite being a developing and carbon sink country,

Malaysia to a certain extent also contributes to the

GHG emission. One of the major sources of GHG in

Malaysia is from the palm oil mill wastewater treatment

system. Briefly, palm oil industry is the highest grossing

crop which was the third largest contributor to Malaysia

Gross Domestic Product at 8% last year. In 2003, the total

income from the palm oil based products export generated

more thanUS$6 billion comprising half of the world palm

oil production. Palm oil is extracted from the mesocarp of

the fruitlets while palm kernel oil is obtained from the ker-

nel. In the process of producing palm oil, a considerable

amount of water is needed (Agamuthu, 1995), leading to

the generation of large volumes of wastewater also known

as palm oil mill effluent (POME).

It was estimated that an average of 32 million tonnes

of POME per year was produced in the 1990s (Ma, 1999),

with average values of 25000 mg l�1 biochemical oxygen

demand (BOD) and 50000 mg l�1 chemical oxygen de-

mand (COD). The main practice of treating POME is

by using ponding and/or open digesting tank systems.

As cited by Ma et al. (1999), the end product of the

anaerobic digestion of POME is a mixture of biogas

(65% CH4, 35% CO2 and traces of H2S) from laboratory

studies and approximately 28 m3 of biogas can be ob-

tained from 1 tonne of POME (Quah and Gillies,

1984). Unfortunately, these gases are being released into

the atmosphere and could have detrimental effects to the

environment. At present, there is no available data on the

GHG emission from the actual waste treatment system.

Therefore this paper will discuss the CH4 emission pat-

tern based on CH4 composition and flow rate from the

commercial anaerobic open digesting tanks. The research

study also demonstrates the influence of mill�s operationand oil palm seasonal cropping on the CH4 emission. It

is anticipated that the information generated from this

study will be used as a guideline in establishing more real-

istic baseline of GHG emission for the palm oil industry.

2. Site descriptions and methods for monitoring

2.1. Serting Hilir Palm Oil Mill

The mill is located in the state of Negeri Sembilan

which approximately 200 km from Kuala Lumpur,

Malaysia. It is owned by the Felda Palm Industries

Sdn. Bhd. (subsidiary of Felda, the largest palm oil

based company in Malaysia). It has the capacity to pro-

cess fresh fruit bunch (FFB) at 54 tonnes h�1. The mill

was commissioned in 1986 to receive and process the

FFB from Felda plantations and its surrounding areas.

To cater for the POME generated from the oil extraction

process, the mill is equipped with an extensive wastewa-

ter treatment facility which occupies 75% of the total

mill land area. The wastewater treatment facility com-

prises of few processes, an anaerobic, facultative anaero-

bic and aerobic (algae) stages.

2.2. Open digesting tank system

The measurement of CH4 emission rate was deter-

mined at the anaerobic treatment using two out of the

six open digesting tanks. The observation was done for

52 weeks to ensure substantial data is collected to indi-

cate the role of seasonal cropping and other factors.

Each digesting tank has the capacity of 3600 m3 of

POME with a hydraulic retention time of 20 days. The

dimension of the open digesting tank is 19.5 m ·12.2 m (diameter · height). The tank was designed to

treat 180 m3 day�1 of raw POME and an equal volume

of treated POME is displaced using gravity flow into

the facultative anaerobic ponds.

2.3. CH4 measurement from open digesting tanks

The biogas produced was collected using a static col-

lection chamber with a surface area of 0.7 m2 and con-

nected to a tube for biogas sampling and detection. In

each open digesting tank a duplicate of static collection

chamber provided a second sampling point. The biogas

flow rate was recorded using a wet gas meter (OSK

14608, Shinagawa Seiki Co.) with a flow rate capacity

of 2 l h�1 to 600 l h�1, while the CH4 gas composition

was determined using gas analyzer (XP-314A, Shin-Cos-

mos Electrics Co. Ltd) plugged to the tubing.

2.4. Chemical oxygen demand (COD)

POME samples were collected daily from the inlet

and outlet of the open digesting tanks to determine the

total carbon removal. COD was measured using the

Standard Methods for the Examination of Water and

Wastewater (APHA, 1992). At the same time the CH4

emission pattern was recorded as described in the previ-

ous section. Correlation between the CH4 and total car-

bon removal was established and plotted.

3. Results

3.1. CH4 emission composition and rate

The average CH4 composition recorded was approx-

imately 36.0%, ranging from 13.5% up to 49.0% (Fig. 1).

0

10

20

30

40

50

60

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51

Week of sampling

CH

4 co

nten

t (%

)

0

5

10

15

20

Bio

gas

flow

rate

(l p

er m

in p

er m

)2

Methane (%) Biogas Flowrate

Fig. 1. Biogas emission pattern over 52 weeks of observation.

S. Yacob et al. / Chemosphere 59 (2005) 1575–1581 1577

Over the period of 52 weeks the average was marginally

higher than data collected in the preliminary study car-

ried out in October 2001 (Shirai et al., 2003). No other

gases such as CO2 and H2S were chemically determined.

This is because of insignificant concentration of H2S

(<2000 ppm) and assuming that the remaining biogas

was mainly CO2. It has been established that CH4 and

CO2 are the main gases produced from POME anaero-

bic biodegradation (Ma, 1999). From Fig. 1, the flow

rate of biogas was negatively correlated to the CH4 com-

position. As evident in week 21 (November 2002) until

week 25 (December 2002) the flow rate dropped from

9.8 l min�1 m�2 to the lowest of 0.9 l min�1 m�2 while

the CH4 composition increased up to 49.0%. Vice versa

from week 44 (April 2003) until week 50 (June 2003),

significant increased in biogas flow rate coupled with

the sudden declined in CH4 composition. These phe-

nomenons were also demonstrated through out the 52-

week observation with minor troughs. An average of

5.4 l min�1 m�2 biogas flow rate was recorded in this

study.

3.2. Total CH4 emission from open digesting tanks

Based on the data collection over 52 weeks, the CH4

emission from the open digesting tanks was influenced

by the activities of the palm oil mill and the seasonal

cropping (Fig. 2a). Commencement of low crop season

in November 2002 was marked by lower volumetric dis-

charge of POME, and coupled with a decline in CH4

emission. The lower emissions continued until May

2003, before increased FFB, POME discharge and

CH4 emission were observed. During this period, a

long year-end public holiday closed the palm oil mill

for a few days, further reducing emissions. In general

1 tonne of POME will be generated from every 2 tonnes

of FFB processed from the mill. As shown in details

in Fig. 2b, the lowest CH4 emission per tank was

recorded in week 25 and week 38 at 0.64 tonne

week�1 and 0.62 tonne week�1 respectively. The decline

in FFB processed was coupled by a decrease in CH4

emission.

An average of 518.9 kg day�1 per tank of CH4 was

emitted from the mill. With a total of 273 days of oper-

ation and six open digesting tanks, it is estimated from

July 2002 until June 2003, 849 tonnes of CH4 was re-

leased to the atmosphere (Table 1).

3.3. Relationship between CH4 emission rate, COD

removal and POME discharged

A correlation between CH4 emission rate and total

COD removal was found and plotted in Fig. 3. An aver-

age of 0.109 kg of CH4 was emitted from a kilogram of

carbon removed. During this observation an average

COD of raw POME was 43288 ± 1924 mg l�1 while

the treated POME was 8327 ± 2049 mg l�1. Based on

these figures the open digesting tank system was able

to remove 34.9 kg of COD per 1 m3 of POME. This indi-

cates approximately 80.7% of COD was removed before

the treated POME being channeled into the facultative

ponds for further treatment. At the facultative ponds,

0

5000

10000

15000

20000

25000

30000

35000

Jul-02 Aug-02 Sep-02 Oct-02 Nov-02 Dec-02 Jan-03 Feb-03 Mar-03 Apr-03 May-03 Jun-03

Week of sampling

FFB

& P

OM

E (

tonn

e pe

r m

th)

0

20

40

60

80

100

120

140

CH

4 (t

onne

per

mth

)

FFB POME Methane

Low crop season

Year-end public holidays

0

100

200

300

400

500

600

700

800

900

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51

Week of sampling

POM

E (

tonn

e pe

r w

eek)

0

5

10

15

20

CH

4 (t

onne

per

wee

k)

POME Methane

(a)

(b)

Fig. 2. (a) Monthly profiles of CH4 emission over the production of FFB and POME. (b) Weekly CH4 emission and amount of POME

discharged.

1578 S. Yacob et al. / Chemosphere 59 (2005) 1575–1581

CH4 emission was still being observed (visual assessment

through bubbling activities). However the volumetric

flow rate was significantly low and cannot be detected

by the wet gas meter.

As shown in Fig. 4, for every tonne of POME trea-

ted, an average of 5.5 kg of CH4 was emitted from the

open digesting tanks. Based on this ratio and total

POME discharged (July 2002–June 2003), it was

estimated that 864 tonnes of CH4 is emitted from the

anaerobic treatment. This value is not far from the ac-

tual CH4 measured during the 52 weeks observation

(Table 1).

4. Discussion

From this study we found out that the CH4 compo-

sition under normal operation of the open digesting

tanks was significantly lower than what was reported

earlier by Ma et al. (1999) which was 65%. Lower CH4

composition and ratio between biogas emission and

amount of POME discharged were largely attributed

to the lack of operational control and high tendency

of O2 contamination in the open digesting tanks, thereby

reducing the anaerobic degradation efficiency. On the

other hand, fully controlled reaction inside the closed

Table 1

Estimation of CH4 emission from July 2002 until June 2003

Total FFB processed 291790 tonnes

Total POME discharged 157035 tonnes

Average CH4 emission rate (per tank) 518.9 kg day�1

Average CH4 composition 36%

No. of days processed 273 days

Approximate CH4 emitted from

six open digesting tanks

849 tonnes

Approximate CO2 emitted from

six open digesting tanks

4672 tonnes

Total CO2 equivalent emitted from

Serting Hilir Palm Oil Milla21652 tonnes

a CH4 is 20 times global warming potential for 100 year

more than CO2 (Milich, 1999).

S. Yacob et al. / Chemosphere 59 (2005) 1575–1581 1579

bioreactor and completely anaerobic condition was

achieved by Ma et al. (1999). In this study, the data

was derived from a commercial open digesting tanks

which governed by factors such as seasonal fluctuations,

quality and quantity of POME and activities of mill.

Hence, measurement of CH4 emission from commercial

activities should be carried out in situ as to prevent any

over estimation of CH4 release into the atmosphere.

We postulated that there is a transfer of O2 from the

atmosphere into the effluent through three main mecha-

nisms. Firstly, O2 can be introduced when fresh POME

is being pumped into the tank causing vigorous mixing

of the effluent. Secondly, slow mixing of effluent through

rising of biogas bubbles and minor eruption of biogas.

These conditions reduce the anaerobic level in the diges-

0

200

400

600

800

1000

1200

1400

4000 4500 5000 5500 60

Total carbon rem

CH

4 em

issi

on (

kg p

er d

ay)

Fig. 3. Relationship between CH4 emi

ter. The third mechanism is the low concentration of

hydrogen in the POME that has escaped into the atmo-

sphere (under the open digesting tank operation).

Hence, the hydrogen is not available for hydrogen-utiliz-

ing homoacetogens and hydrogen-utilizing methanogens

to produce acetate and methane (Lay et al., 1998). There

is also a possibility that the CH4 generated from the

anaerobic process was consumed by methanotrophic

microorganisms as reported in the landfill research by

Bogner et al. (1995).

Attempts to evaluate the affect of seasonal cropping

of oil palm to the biogas flow rate or methane emission

was shown in the second part of the study. Significant

reduction in CH4 emission rate was observed from Octo-

ber 2002 until May 2003 when the amount of POME

discharged declined. This is a normal condition, as the

oil palm will experience low cropping season for

6 months before increasing its production for the next

6 months. Occasional public holidays and closure of

the mill also have an affect on the CH4 emission as no

POME will be loaded into the open digesting tanks.

Therefore lesser organic matter to be converted into

CH4. As shown in Table 1, a total of 157035 tonnes of

POME was discharged from July 2002 until June 2003

whereas the full potential of six open digesting tanks is

295000 tonnes. This indicates when the amount of

POME discharged is below 1080 tonnes day�1 not all

the open digesting tanks will be fed daily or fed at lower

feeding rate (<180 m3 day�1). This in turn reduces the

loading rates for anaerobic process and hence prolongs

the retention of POME in the digester. Long public

00 6500 7000 7500 8000

oved (kg per day)

ssion and total carbon removed.

0

1

2

3

4

5

6

7

8

9

10

200 300 400 500 600 700 800 900

POME (tonne per day)

CH

4 (t

onne

per

day

)

Fig. 4. Relationship between CH4 emission and POME discharged.

1580 S. Yacob et al. / Chemosphere 59 (2005) 1575–1581

holidays as observed in weeks 25 and 26 also have an ef-

fect on the biogas production pattern. This may explain

the decline in the CH4 emission rate.

Other reasons for the fluctuation of emission include

the quality and quantity of POME discharged from the

mill. Upon discharge from the mill, POME is in the form

of highly concentrated dark brown colloidal slurry of

water, oil and fine cellulosic materials from sterilisation

and clarification stages. The final POME would include

hydrocyclone washing and cleaning up processes in the

mill (Agamuthu, 1995). Therefore, the chemical proper-

ties of POME vary widely and depend on the operation

and quality control of individual mill. Our COD results

also support this statement as the POME COD may

vary from 41200 mg l�1 up to 47800 mg l�1 (data not

shown). This may influence the characteristics of POME

discharged from the mill. From Figs. 2a and 4, it can be

seen that the amount of CH4 emitted is a function of

POME discharged from the mill. We also have estab-

lished the ratio between amount of POME discharged

and CH4 emission which is for every tonne of POME,

5.5 kg of CH4 will be emitted. The calculated amount

of CH4 from the ratio is slightly higher than the actual

emission measurement for 52 weeks.

In line with the commencement of Kyoto Protocol in

2008 until 2012, this study provides valuable informa-

tion in establishing the GHG emission particularly in

the palm oil industry. Using the CDM as a platform,

the developed nations would be able to partner with

developing nations in the effort of reducing GHG. Few

technologies are currently being developed and opti-

mized to reduce the GHG emission. Among them are

the utilization CH4 as a renewable energy to generate

electricity, production of organic acids and biodegrad-

able plastic from POME (Hassan et al., 1997; Noraini

et al., 1999). It is expected that through the integration

of such technologies into the POME wastewater treat-

ment system could lead to a substantial GHG reduction.

Additional income could be generated by the production

of value-added products such as electricity, organic acids

and biodegradable plastic. On top of that the project will

generate Certified Emission Reduction (CER) for sale or

export. Then this CER can be used for developed na-

tions commitments to mitigate their GHG emissions.

The palm oil industry can derive new economic, devel-

opment and environment benefits through the imple-

mentation of CDM projects.

5. Conclusion

Results indicated that an average of 36.0% and

5.4 l min�1 m�2 of CH4 composition ad biogas flow rate

respectively was recorded under normal mill operation.

While CH4 could be emitted at 518.9 kg day�1 from

one open digesting tank. A correlation was also estab-

lished between CH4 emission and COD where 0.109 kg

of CH4 for every kilogram of COD removed. While

for every tonne of POME discharged, an average of

5.5 kg of CH4 will be emitted from the anaerobic treat-

ment. The results presented herein indicate that a long-

term observation is crucial to determine the CH4

S. Yacob et al. / Chemosphere 59 (2005) 1575–1581 1581

emission as it is severely governed by the seasonal crop-

ping of oil palm. Secondly, mill�s activities will also influ-

ence the quality and quantity of POME discharged

which in turn affect the anaerobic process. Therefore,

CH4 emission estimation should be based on field mea-

surement and the method used in this study can be used

as a guideline for future baseline study in the other palm

oil mills.

Acknowledgments

The project was sponsored by Kyushu Institute of

Technology, Japan. The authors would like to thank

the management of Serting Hilir Palm Oil Mill for their

cooperation throughout the study.

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