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ANAEROBIC TREATMENT OF PALM OIL MILL WASTEWATER UNDER

MESOPHILIC CONDITION

U. Puetpaiboon1

and J. Chotwattanasak2

1Civil Engineering Department, Faculty of Engineering, Prince of Songkla University, Thailand(E-mail: [email protected])2

The Joint Graduate School of Energy and Environment, Kings Mongkut University of Technology Thonburi, Thailand(E-mail:[email protected])

Abstract This paper reports the investigation of performance of the full scale anaerobic digester of

wastewater from production of palm oil at the Asian Palm Oil Co., Ltd. Krabi, Southern Thailand

which has been started operation since December 2001. The dimension of an anaerobic digester is13.5 m in diameter and 15.3 m in height with average volume of 2,100 m

3. Wastewater generation

rate was found to be about 0.5 m3

per ton of fresh fruit bunch. Wastewater from palm oil productionwas pretreated by oil recovery process and primary ponds before feeding to the digester using

centrifugal pump with average flow rate of 300 m3/d. Average volumetric organic loading and HRT

were found to be about 4.53 kgCOD/m3.day and 7 days, respectively. The digester reduces water

pollutants and odor from wastewater and produces energy in form of biogas. The biogas producedfrom anaerobic digester is a cheap source of energy which can be used as a gas engine fuel toproduce electricity and can be sold out as very small power producer in Thailand.

Keywords: Anaerobic treatment process; anaerobic digester; wastewater from palm oil; biogas

Introduction

Palm oil mill is one of the most important agro-industry in the South of Thailand. The oil palm

processing plant releases large amount of organic pollutant wastewater. Discharge of palm oil mill

wastewater without proper treatment will damage environment both in receiving water quality andfoul smell in the neighborhood of factory. The proper wastewater treatment methods should takeadvantage. Usually anaerobic stabilization ponds are widely used for treatment of wastewater from

palm oil production because of their low capital and operating cost. However, foul smell generated

from anaerobic ponds is disturbed the neighborhood community. Another efficient treatment

system is the closed anaerobic digester tank that becomes more popular at present. Furthermore,biogas from the closed treatment system can be further utilized as fuel.

Due to energy and environment are the interested issues for nowadays. The Royal Thai Governmentcreates a policy to promote and subsidize the projects concerning with the energy from biomass

such as a production of biogas from anaerobic digestion in agro-industry section especially


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anaerobic digestion of high concentration of organic matter in wastewater. The full-scale anaerobic

digester for treatment of wastewater from production of palm oil at the Asian Palm Oil Co., Ltd.

Krabi, Southern Thailand is one example of these subsidized projects. The digester has been startedin operation since December 2001. The system reduces pollutants and also produces biogas that

today the mill utilizes as fuel to produce electricity using the internal combustion engine. Themodified diesel engine with induction motor is used to produce electricity and supply to the factoryoffice, labor houses and biogas plant itself.

MethodsThe anaerobic digester with a diameter of 13.5 m and 15.3 m in height and effective volume of2,100 m

3was constructed at the Asian Palm Oil Co., Ltd. Krabi, Southern Thailand. The reactor

comprises of gear motor, torque tube, scrapper set and draft tube as shown in Figure 1. Palm oilmill wastewater from the production process was pre-treated by oil recovery tank and primary

ponds before feeding to the anaerobic digester by centrifugal pump at feeding rate 300 m3/day from

the tank bottom as up flow. The hydraulic retention time of the system was about 7 days with theaverage organic loading rate of about 4.53 kgCOD/m3.day. The flow diagram of anaerobic digestion

system was shown in Figure 2.

Figure 1 Anaerobic digester at Asian palm oil mill, Krabi, Thailand.

Figure 2 Flow diagram of anaerobic digestion system at Asian Palm Oil Mill, Krabi, Thailand.


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Influent and effluent from anaerobic digester were analyzed for BOD5, COD, pH, temperature,

VFA and alkalinity every 3-4 days. The analytical methods used were in accordance with thatspecified in the standard methods for the examination of water and wastewater (APHA, AWWA

and WEF, 1998). Gas volume was monitored by EPI gas flow meter Model: Series 8200 MPNHinsertion style with remote display. Gas composition was determined by gas chromatography.

Results and Discussion

The results of treatment of wastewater from palm oil mill production after pre-treated by oil

recovery and primary ponds before feeding to the anaerobic digester without pH adjustment isshown in Table 1. It can be seen that pH of influent was in a range of 4.69 - 4.85, 4.75 in average,

whereas pH of effluent from the anaerobic digester was in a range of 7.29 - 7.47, 7.36 in average.Alkalinity of influent to the anaerobic digester was in a range of 1,150 - 1,598 mg/L as CaCO3.Alkalinity ofeffluent from digester was in a range of 2,578 - 3,050 mg/L as CaCO3. Since alkalinity

in the anaerobic digester was maintained within a range of 2,500-5,000 mg/L as CaCO3, a goodbuffering capacity was obtained in the anaerobic digester (Polprasert, 1996). This indicated that the

alkalinity of the influent allowed the neutralization of the volatile fatty acids, preventing thereduction of pH from the optimal value (Borja et al., 1996). Temperatures before and after feeding

to anaerobic digester were in a range of 40-45 and 36-40oC, or 42 and 38

oC in average,

respectively. The warm temperature of wastewater from palm oil mill production after pre-

treatment was appropriate to the biochemical reaction in the anaerobic digester which convertedorganic matter to biogas under mesophilic temperature range, between 30 and 38

oC (Metcalf &

Eddy, 2003).

COD in influent and effluent were found to be in a range of 21,560 - 39,200 mg/L and 5,880 -

17,640 mg/L, respectively. Average COD in influent and effluent of the anaerobic digester wereabout 31,687 and 11,270 mg/L, respectively. BOD5 in influent and effluent were in range of 16,950

- 24,600 mg/L and 1,100 - 2,115 mg/L, respectively, and were in average of 20,830 mg/L and 1,564

mg/L, respectively. VFA in influent and effluent was found to be between 6,600-7,200 mg/L as

CaCO3 and 643-1,425 mg/L as CaCO3, respectively. Average VFA in influent and effluent wereabout 6,902 mg/L and 1,220 mg/L as CaCO3, respectively. The average performance of full-scaleof anaerobic digester is shown in Figure 3.

Table 1 Performance of full-scale anaerobic digester treating palm oil mill effluent at Asian Palm

Oil Mill, Krabi, Thailand.

Influent Effluent

Parameter Range Average Range Average

Removal

Efficiency(%)

1. pH 4.69-4.85 4.750.05 7.29-7.47 7.360.06 -

2. Temperature ( oC) 40-45 421 36-40 381 -

3. COD (mg/L) 21,560-39,200 31,6876,371 5,880-17,640 11,2704,100 64

4. BOD5 (mg/L) 16,950-24,600 20,8302,861 1,100-2,115 1,564365 92

4. Alkalinity

(mg/L as CaCO3)

1,150-1,598 1,308165 2,578-3,050 2,845156 -

5. VFA (mg/L) 6,600-7,200 6,902339 643-1,425 1,220288 -


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0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

Alkalinity Volatile Fatty Acid COD BOD5

Influent Effluent

Concentration(mg/L)

BOD5

Figure 3 Performance of full-scale anaerobic digester treating wastewater from palm oil production

at Asian Palm Oil Mill, Krabi, Thailand.

From the operation of the anaerobic digester to treat wastewater from palm oil millproduction at hydraulic retention time of 7 days with the average organic loading rate of about 4.53

kgCOD/m3.day, the average removal efficiencies of COD and BOD5 were found to be about 64%and 92%. Average biogas production rate was found to be 6,000 m

3/day. The results of biogas

composition analysis in Table 2 showed that methane (CH4) concentration was found to be

maximum at about 66 67%, whereas carbon dioxide (CO2) and nitrogen (N2) was found to beabout 29% and 2%, respectively. Oxygen (O2) was found to be less than 1%.

Table 2 Results of biogas composition analysis.

ResultsGas Unit

Sample I Sample II Sample III

CO2 % 29.69 29.37 29.81

O2 % 0.91 0.96 0.72N2 % 2.83 2.97 2.23

CH4 % 66.57 66.70 67.24

At present, biogas produced from anaerobic digester is used in modified diesel engine with

induction motor, which has a capacity of 90 kW as shown in Figure 4. This system produceselectricity and supplies to the office, labor houses and anaerobic digester plant in the factory.

Because it is essential to prevent corrosion to the gas engine, biogas from the anaerobic reactor isfirst scrubbed by lime to reduce H2S. Then removal of water by cyclone is performed to eliminate

water vapor.


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Figure 4 Two modified diesel engines with induction motor at Asian Palm Oil Mill, Krabi,

Thailand.

Conclusions

The anaerobic digester at the Asian Palm Oil Mill can reduce water pollutants and foul smell from

the wastewater and produces energy in form of biogas. At average organic loading rate of 4.53kgCOD /m

3.day and hydraulic retention time of 7 days, BOD5 and COD removal efficiency of up to

92% and 64% was achieved. It was found from biogas production rate that 20 m3

of biogas can be

obtained from 1 m3

of wastewater from palm oil production or at about 0.51 m3

CH4/kgCOD.day.

Biogas was used as fuel in modified diesel engines with induction motor to produce electricity andsupply to the factory office, labor houses and biogas plant itself. Furthermore, the factory plans toinvest in a project called very small power producer (VSPP) in accordance with a renewable energy

policy of the Royal Thai Government.

References

APHA, AWWA and WEF, (1998), Standard Methods for the Examination of Water and

Wastewater, 20th

Edition, Washington D.C., USA.

Biomass One-stop Clearing House (2003). Introduction to conversion of Biomass to Electricity

and Thermal Energy. Available online: http://www.efe.or.th/indexs.htmBorja, R., et al. (1996), Anaerobic treatment of palm oil mill effluent in a two-stage up-flow

anaerobic sludge blanket system,Biotechnology, 45, pp. 125-135.

Food and Agriculture Organization of the United Nations, FAO. (1995),Rural Energy Medium andLarge Scale Biogas System: In the Asia-Pacific Region. 10

thAnniversary issue, Bangkok.

Malasian Palm Oil Board (2000). Oil Palm and the Environment. Available online:

http://mpob.gov.my/oilpalm_env/mill_waste1.htmlMetcalf & Eddy (2003), Wastewater Engineering: Treatment, Disposal, and Reuse, 4

thed.,

McGraw-Hill, Singapore.

Polprasert C., (1996), Organic waste recycling, 2nd

ed., John Wiley & Sons Ltd, England.Rao, Peram M. (1999), Wastewater Management in a Palm Oil Mill: a case study, M.Eng. Thesis,

Asian Institute of Technology, Thailand.

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