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Biomass residues from palm oil mills in Thailand: An overview on quantity and potential usage
Pergamon PII:
Biomass and Bioenergy Vol. 1I, No. 5, pp. 387-395, 1996 Copyright 0 1996 published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 80961-9534(%)ooo34-7 0961-9534/96 $15.00 + 0.00
BIOMASS RESIDUES FROM PALM OIL MILLS IN THAILAND: AN OVERVIEW ON QUANTITY AND POTENTIAL USAGE S. PRAsERTsAN*t tDepartment IDepartment
and P. PRASERTSAN~
of Mechanical Engineering, Prince of Songkla University, Hat Yai, 90110, Thailand of Industrial Biotechnology, Prince of Songkla University, Hat Yai, 90110, Thailand (Received 2 January 1996; revised 7 April 1996; accepted 1 May 1996)
Abstract-Palm oil production is one of the major industries in the south of Thailand. A study of the quantity and potential usage of palm oil mill wastes was carried out. Sixteen palm oil mills in the region generate 386,930 tons/yr 165,830 tons/yr and 110,550 tons/yr of empty fruit bunches, palm press fiber and palm kernel shell, respectively. In addition, 1,202,260 tons/yr of waste water is being treated anaerobically. Only the p&carp fiber is used for boiler feed. Empty fruit bunches and the shell are disposed of by the land filling method, which is very costly. In some factories the empty fruit bunches are burnt in the furnaces, which causes air pollution. The study suggests potential usages of the solid and liquid wastes. Copyright 0 1996 published by Elsevier Science Ltd. Keywords-Palm
oil residues; waste utilization; waste management; biomass.
1. INTRODUCTION
Palm oil is one of the major agroindustries in the south of Thailand. Oil palm planting area for the country totals 150,000 hectares, solely in the south. The oil palm (Hueis guineensis) was introduced to Thailand in 1968, after it successfully replaced the natural rubber trees in Malaysia. As the climatic conditions in the south are suitable for palm trees, the oil palm plantation area has expanded ever since. The planting area and palm oil production in the southern region are given in Table 1. At present, there are 16 factories employing the standard oil extracting process (normally called the wet process) and 24 factories the dry process. In the standard milling process, which is the process used in the factories of a milling capacity over 10 tons of raw material per hour, water is added into a digester. Raw material supplied to the mills consists of fresh fruit bunches (FFB). In 1993 the yield of FFB was 1.5 x lo6 tons’ and the crude palm oil production was 305,000 tons. It was estimated that about 1.18 x lo6 tons of organic waste was released from the palm oil mills. The stringent environmental measures implemented at present and those that will be introduced in the near *Author to whom correspondence
should be addressed.
future will inevitably affect the waste disposal practice of the mills. An increasing public concern about the environment has forced some factories to close down. This is because they failed to meet the effluent discharge standard and the bad odor (from the anaerobic pond) and dust (from burning the solid wastes such as empty fruit bunches and palm shells) offended people living nearby. Only a fraction of solid waste is used for steam and power generation. This paper reports a result of a field survey to determine the amount and types of palm oil mill wastes and proposes means to properly treat or utilize these wastes. Since 8590% of the palm oil is produced in the standard process factories, this paper deals with the standard process only.
2. FSI’IMATION OF WASTES FROM PALM OIL MILLS
The amount of wastes from palm oil mills were presented in a study in Malaysia.3 The palm varieties, growth conditions and plantation management in Thailand are different from those in Malaysia and undoubtedly result in a different quantity of solid wastes. Furthermore, the milling process in Thailand consumes more water,4 hence has a higher waste water discharge rate. In order to obtain precise data, the estimation of wastes was carried out by 387
S. PRASERT~AN and P. PRASERTSAN
388
questionnaires, observation, monitoring and interviewing. The FFB price varies according to its quality. The purchasing officers of the mills take samples from the suppliers and analyse them for the percentage of oil and shell (some varieties have big nuts and thick shells). The data collected by the mills were obtained during the survey and interviewing, and were used to double check the data obtained from the questionnaires. There are 16 standard process mills and 24 dry process mills in the south of Thailand. In total, 10 and five questionnaires were returned from standard and dry process mills, respectively. A survey of 11 standard mills and three dry process mills was carried out.
3. WASTES FROM MILLING PROCESSES
The survey data of the dry process mills showed no significant difference in the details of the process. However, for the standard wet process mills, they can be classified into two groups based on the oil separation techniquethose which use decanters (five mills) and separators (five mills). It was found that there is one mill which has installed both the decanter and the separator in parallel, in the separation line. Only the separator is operated in the normal season. The additional decanter is operated to accommodate the high production rate in the peak season. In the dry process, the fruits are dried by hot air generated from the burning of firewood, after which the oil from both the pericarp and the kernel is extracted by a screw press. The residue from the process is the crushed fruit, known as palm cake which can be sold for animal feed. The palm cake contains 8-10% residual oil and about 9% moisture content. Although this process leaves no waste, it is not used in the large scale production because the
mixed oil is not suitable for the downstream processes. In large factories, steam and water are used as shown in Fig. 1 and hence, generate waste water. The fresh fruit bunches are sterilized in a horizontal direct contact steam sterilizer at 140°C for 50 min to inactivate the lipolytic enzymes and loosen the fruit still attached to the bunches. The sterilized bunches are fed into a rotary drum thresher to separate the fruits from the bunches. The empty fruit bunches (EFB) are conveyed to the dumping ground. The fruits are fed into a digester where water at 80°C is added. The homogenous oil mash from the digester is pushed through a screw press, and later passes through a vibrating screen, a hydrocyclone and decanters (or separators) to remove fine solids and water. Decanter effluent and decanter cake are the major wastes at this stage. Centrifugal and vacuum driers are used to further purify the oil before pumping it to a storage tank. Fiber and nuts from the screw press are separated in a cyclone. The nuts are cracked in a centrifugal cracker. The kernels are packed and sold to kernel oil mills. Shells are normally left unused. There are various forms of solid and liquid wastes from the mills. These include empty fruit bunches (EFB), palm press fiber (PPF), palm kernel cake (PKC), palm kernel shell (PKS), sludge cake (SC) and palm oil mill effluent (POME). Only EFB, PPF, PKS and POME appear in large quantities and are considered as wastes. The others can be sold for animal feed or fertilizer. The quantity of the wastes depends on the quality of the raw material as illustrated in Fig. 2. Taking the average percentage of the FFB composition found from the survey (28% EFB, 12% PPF and 8% PKS), the solid wastes could be estimated as 386,930 tons of EFB, 165,830 tons of PPF and 110,550 tons of PKS (Table 2). Waste water was found to be 0.44-l. 18 tons/ton FFB.4 If the average figure of 0.87 tons/ton FFB is used for the calculation,
Table 1. Oil palm plantation and production of palm oil Year
Planting area (ha)
Yielding area (ha)
Yield of FFB ( x 10’tons)
Palm oil” production ( x lo3 tons)
1987 1988 1989 1990 1991 1992 1993
98,390 109,190 128,610 140,050 146,360 153,280 152,640
69,160 82,840 90,940 95,990 103,240 108,000 114,560
728 885 1098 1191 1315 1352 1526
145.66 117.02 219.63 238.35 263.20 270.40 305.20
Source: Office of Agriculture Economics. ‘Calculation based on 20% oil content in the fresh fruit bunches.
Biomass residues from palm oil mills in Thailand
389
FFB
Condensate
.* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-
Sterilizer
1
Thresher
Steam (14O”C, 50 mim)
.*~~.~-.+
EFB
Fruits
t Digester
-
Water (80-9O’C)
Oil mash I Screw press
Effluent *d.
.
.
.
.
Liquid .
Decanter . . . -* Cake
1 w.....
i
Solid Cyclone
. . ..w
PPF
1
Oil
Nut
Purifier
Nut cracker
. . . . . .)
PKS
I
Kernel
i Treatment
Pack for kernel
ponds
Oil mill Fig. 1. Palm oil milling process. (-
the annual discharge of waste water is 1,202,260 tons. 4. CURRENT AND POTENTIAL USAGE OF WASTES AND RECOMMENDATION FOR TREATMENT
4.1. Empty fruit bunches (EFB) EFB is the major component of all solid wastes. Steam from the sterilization process results in a moisture content in the EFB as high as 60%, which makes it unsuitable as fuel. It was reported that the EFB has 42% C, 0.8% N, 0.06% P, 2.4% K and 0.2% Mg.’ The survey found that the EFB have been used as raw material for mushroom cultivation only recently. Without any treatment, the EFB are pressed to form rectangular blocks. Mushroom spores are inoculated into the EFB blocks which were in turn covered by a black plastic sheet to retain moisture and limit sunlight. Mushrooms are a good source of protein and are considered a valuable product, with the sale price ranging from US!§ 2400-4000 per ton of Vovariella vohacea. A further advantage of mushroom cultivation is the residue, which now can be
) Process; (- - -) waste.
easily decomposed to produce another valueadded product, namely organic fertilizer. This type of fertilizer is much more suitable for the soil than the original EFB itself. Possible deficiencies in phosphate can easily be overcome through the addition of some waste water. The readily available and digestible residue obtained after mushroom cultivation with or without additional cornposting would allow easier transportation and fertilization as compared to the original EFB. This is particularly important as the bulky nature of EFB makes direct application difficult and pollutes the air if incinerated in furnaces. In some plantations, the EFB are left to decompose under the trees. Besides the palm trees, the EFB are also applied to fruit orchards using 20 FFB per tree. It helps retain moisture and returns organic matter to the soil. However, the EFB cannot be stacked in more than two layers around the tree or they attract harmful insects. The bulky nature of the EFB causes a high land-fill disposal cost. The mills, therefore, burn the EFB down to ash. Particulates and gas (SO,, CO*, CO and NO,) emitted from the furnaces sometimes cause air pollution to the nearby
S. PRASERTSAN and P. PRASERTSAN
390
4.2. Palm press fiber (PPF) The oil retained in its cell wall makes the PPF a good combustible material. In factories which produce both steam and electrical power, all of the PPF is used. However, only 30% is consumed if power is not produced. Therefore, in some factories, about 70% of PPF is considered as waste. The PPF ash contains 1.7-6.6% P, 17-25% K and 7% Ca.5 It could therefore be used as source of minerals for plants. Although PPF is similar to rice straw, it contains a higher percentage of fiber and lignin which cannot be digested easily by animals. However, its similarity to rice straw makes it a good substrate for mushroom cultivation.6 It is interesting to note that a study by Okiy’ found that PPF is suitable for the pulp and paper industry.
communities and results in public protests. Burning a ton of EFB produces 4 kg of ash. Because of the high mineral content of the ash (especially potassium) it can be sold at 1 Baht per kilogram (1 US$ = 25 Baht). It is interesting to note that there is now at least one factory which processes the EFB into fibrous material. The EFB fiber is coarser and stronger than that obtained from the pericarp. By adding a binding agent, such as rubber latex, the EFB fiber can be used for cushion filling material. One of the most promising products manufactured from the EFB is the medium density fiber (MDF) board which will be on the production line as soon as a factory is set up. The EFB has a great potential in these applications. It should be noted that products like coir fiber, fiber board, cement board, roofing tile and card paper can be produced from the EFB or fiber. However, the oil palm waste-based products received little interest in Thailand since other agricultural wastes in the country, such as rubber wood and bagasse, are readily available in larger quantities and the technical knowledge is already wellestablished at an industrial level scale. Furthermore, the use of oil palm waste as raw material requires oil separation (from the waste) process and, consequently, generates waste water as experienced in one factory.
4.3. Palm kernel shell (PKS) PKS is the most difficult waste to decompose. The shell size is uniform and is not as bulky as the EFB. They are usually left unused in the factory or disposed of by the land-fill method. In terms of energy, PKS is an energy intensive substance. Local industries that require process heat (or steam) generally have furnaces (or boilers) designed for firewood or fuel oil. However, substantial modification of the
FFB (100%)
EFB (20-30%)
Fruit (70-74%)
[59%]
Moisture
Dry EFB
+=l Pericarp
Nut
(12-14%)
(14-16%)
(51-551)
(18.9-19.2%)
[9.5%]
[49.5%]
[26.7%]
[ 14.3%]
r-d
Oil
Fiber
Moisture
(25-28s)
(12-13%)
(13-14%)
[ll.S%]
[ 13.9%]
[1.3%]
d-l
Shell
Kernel
Moisture
(6.8-7.4%)(8.5-8.4%)(3.3-3.4%) [10.6%]
[l%l
[2.7%]
Fig. 2. Composition of fresh fruit bunch. Figures in brackets are the percentage of FFB. () = high-quality FFB and fl = low-quality FFB.
3 3 5 3 1 1 16
No. of mills
250,400 457,000 395,000 244,000 18,000 17,500 1,381,900
EFB ( x 10’tons/yr) 70.11 127.96 110.60 68.32 5.04 4.90 386.93
Waste water ( x 10’tons/yr) 217.85 397.59 343.65 212.28 15.66 15.23 1 202.26
Table 2. Wastes from palm oil mills by provinces in 1993
Production capacity (tons FFB/yr)
Waste water was calculated based 0.87 tons/ton FFB, an average figure obtained from monitoring of four factories. EFB, PPF and PKS were calculated based on 28%, 12% and 8% of FFB, respectively.
Chum-porn Surat-thani Krabi Trang Satoon Songkla Total
Province
30.05 54.84 47.40 29.28 2.16 2.10 165.83
PPF ( x 10’tons/yr)
20.03 36.56 31.60 19.52 1.44 1.40 110.55
PKS ( x lo-’tons/yr)
$ J z! !z g J E v) 6’ el B
a z. g
392
S. PRASERTSAN and P. ~ASERTSAN
furnaces is needed if the conventional fuel is to be replaced by PKS. Therefore, many factories are still reluctant to use PKS as fuel unless they are economically forced to do so. There is a possibility that the PKS can be used for activated carbon productions*9 or charcoal.3 PKS contains 20.3% of fixed carbon and is physically similar to the coconut shell, which has been used to produce the activated carbon successfully. It is anticipated that the stringent environment control measures will increase the demand for activated carbon in the future. It is possible that activated carbon can be applied for the decolorization of the unacceptably dark-colored effluent of the palm oil mills. At present, there are 110,550 tons of PKS available annually, at no cost, at 16 mills. Some factories have shown an interest in incorporating activated carbon production in the milling operation. Many mills have installed co-generation plants, i.e. generate both heat and electrical power. High-pressure steam passes through a back-pressure steam turbine to generate electricity sufficient for the mill consumption. The exhausted steam is used as heat source for the milling process. Only the (pericarp) fiber and sometimes a small amount of shell is fed into the boiler furnace. As private power production is being encouraged in Thailand, it is therefore recommended that the surplus shell should be used to generate electricity and be sold to the grid. The heating value of the shell is 17.4 MJ/kg.” It is appropriate to assume that the overall thermal efficiency of the co-generation system is 25%. Therefore, the electrical energy sold to the grid is 2 x lo9 MJ/yr, which is equivalent to that generated from a 79 MW power plant operating for 300 days per year (which is the normal operating duration of the palm oil mills in Thailand). In practice, if the PKS is used for electricity generation, 16 small co-generation systems have to be installed. Extensive study of co-generation (heat and power) in the Thai palm oil industrylO indicated that the heat-to-power ratio of the system was about 12 during off-peak periods and 8 during peak periods. The analysis showed that the existing co-generation system was not properly designed, and that the low efficiency of the system was due to a large difference of enthalpy between the steam required to generate electricity and process steam and as a result, excessive steam blow-out occurred (39-47% of total steam input). To lessen the imbalance of
electricity and process heat, an energy conservation measure to utilize the excess steam to preheat the boiler feed water appears to be very cost effective. In addition, the boiler flue gas which is at about 330°C should be used to preheat the combustion air in order to improve the thermal efficiency.” Many local cottage industries such as lime making and brick firing are now faced with energy shortage problems. Usually, heat for the process is obtained from firewood. Natural rubber is another important industry in the south of Thailand. Rubber trees cut down for replanting (after their economic life), which are the only source of firewood, are no longer a cheap fuel source. At present, the rubber wood price is soaring because the number of rubber-wood-based furniture factories is continuously increasing. This inevitably has an adverse effect on the local industries which rely on the rubber wood for their energy supply. The PKS can substitute the rubber wood if the furnaces are properly modified. 4.4. Palm oil mill efluent (POME) There are three major sources of waste water, namely sterilizer condensate (17%), decanter or separator sludge (75%) and hydrocyclone water (8%). Monitoring at four factories revealed that the milling process produces waste water in the range of 0.44-l. 18 m’/ton FFB with the average figure of 0.87 m3/ton FFB4 Waste water is treated anaerobically in a series of ponds. Over half of the land has to be spared for the waste water treatment ponds. There is a concern that this waste water might pollute underground water. As a discharge standard of effluent from the palm oil industry has not been set in Thailand yet, the general standard for any effluent is enforced. Since the final effluent is of brownish colour, it cannot be discharged into the natural waterways. There is, therefore, an urgent need to investigate the decolorization process. This is not limited to the use of activated carbon mentioned earlier, but includes the biological method using microorganisms such as Phanerochaete chrysosporium.‘* However, environmental guidelines for the palm oil industry are being developed and the effluent standard is anticipated to be implemented by the year 2000. It was observed during the visit to the palm oil mills that if algae is present in the final pond, the effluent has a very light brown colour which is considered acceptable for discharge into the
Biomass residues from palm oil mills in Thailand
river. Therefore, the role of algae on decolorization and the use of algae as a by-product should be investigated. The waste water characteristics have been reported in many publications.2*4. ‘XI4 The high organic load of waste water in terms of BOD ( w 50,000 mg/l) or COD ( N 80,000 mg/l) is equivalent to the load generated by a population of 3 million people.4 While the palm oil production in Malaysia is 20.6 times that of Thailand’s, the waste water organic load in terms of population equivalent is only 2.4 times.13 These figures clearly reflect the lack of control and inefficient treatment in Thailand. Furthermore, the specific waste water from the palm oil mills in Thailand, which is 0.87 m3/ton FFB, is substantially higher than the figure 0.6 m3/ton FFB quoted in Malaysia.” It was reported that the minimum discharge achievable can be as low as 0.3 m3/ton FFB.16 The use as fertilizer and water supply for the palm trees are the most common methods for the waste water utilization. As most of the carbon source (with a proper nutrient ratio in terms of BOD:N:P = 1OO:l.1:0.2) is used by the microorganisms when the waste water is treated anaerobically, nutrients such as N, P and K are left over for the plants. Substances yielded from the drying of the waste water can be used as an animal feed supplement”.” or fertilizer.19 However, the energy cost of drying must be very low or obtained freely such as solar energy. Commercially available single cell protein for animal feed (Prolima and Centriplus) can be cultivated from the POME.’ Biotechnological products such as cellulase, xylanase, betaglucosidase and pectinase enzymes can be obtained niger ATCC 6275.*’ from cultivating Aspergih Furthermore, the enzymes can then be used for the saccharification of palm cake and palm fiber to produce sugars (glucose, xylose, arabinose, mannose, etc.). The enzymes can also be applied to enhance the oil recovery from press cake fiber*’and a laboratory experiment revealed that it can separate oil and suspended solids from the POME. Anaerobic digestion is an effective method for treatment of wastewater with high organic content. During the treatment process, microorganisms hydrolyse fat, oil and carbohydrate (in the form of sugar, starch, pectin and pentosan) to volatile fatty acids (acetic, butyric, etc.) and isobutyl alcohol. These compounds have offensive smells and are regarded as air pollution. It was found recently that commercial products
393
claiming to mitigate the bad smell are available, and many mills apply the substance in the pond. This practice does not only result in an additional treatment cost but is also found not to work in the long term. Alternatively, the treatment should be carried out in a reactor in which the biogas is obtained. Biogas could be produced from mesophilic anaerobes at a rate of 0.57 m3/kg COD utilized per day, and contained 6&69% methane gas.*’ The production of biogas was also conducted on a pilot scale using a thermophilic contact process at a temperature of 45”-60°C with an organic loading of 3.0 kg BOD/m3/day. The highest yield of biogas was seen at 65°C and contained less than 10 ppm of hydrogen gas. The treatment efficiency was 96%.23 With the average output of 40 kg (40,000 ppm) organic matter, the potential production of biogas is 12 m3/m3 effluent.24 For a mill with handling capacity of 20 tons FFB/h and 10 h working time per day, the daily discharge of the effluent is 174 m3/day (calculated from 0.87 m3/ton FFB). Assuming a very conservative long fermentation time of 20 days, the digester volume is 3480 m3. It can be anticipated that after 20 days of treatment the BOD should be reduced by 90%, the COD by 75% and the total solids by 70%. Biogas produced from the digester is calculated as 2088 m3/day or, in terms of heat, 48 GJ/day. 5. DISCUSSION
The palm oil industry contributes an appreciable level of economic growth to the southern region of Thailand. Thousands of people are engaged in the upstream and downstream processes of the milling. Although the mills generate a great amount of waste, the waste is a natural product, and non-toxic, because no chemicals are used in the process. The waste, if properly managed, can be returned to the soil as fertilizer without any harmful effect. This means that the waste should not be kept in a place where the concentration is too high for the environment. However, dumping the waste by scattering evenly over 150,000 ha of plantation is economically not feasible. It is worth mentioning the potential of incorporating integrated farming in the ponding treatment system of a palm oil mill. Because of the nutrient-rich effluent, some vegetables could be grown in the treatment ponds. Bamboo rafts can be used as growing platforms. In the final pond where algae is present, phytoplankton-
394
S. F~ASERTSAN and P.
feeding fish such as tilapia can be cultured. A frozen seafood factory in Hat Yai, Songkla province, discovered that an unpleasant odor developed in fish cultured in the final treatment pond, unless the fish were kept in clean, fresh water for few weeks. If the effluent is dried out, many associated problems vanish. The water in the effluent may be vaporized by the heat obtained from burning the solid wastes. One ton of FFB generates 870 kg of waste water at 66°C. Taking the total solid content of 60 kg (derived from Table 2) the amount of water to be evaporated is 8 10 kg, which requires about 1900 MJ. One ton of FFB gives 80 kg of PKS, 120 kg of PPF and 280 kg of EFB. It was reported that the heating values of the dried shell, fiber and EFB are 17.4, 4.6 and 9.6 MJ/kg, respectively.2s If the heat conversion efficiency is 80%, about 2200 MJ is obtainable from the solid wastes. It is possible, therefore, to eliminate both liquid and solid wastes simultaneously by this method. However, the factory has to install a heat generator with a capacity of 470 kW for every ton FFB/h production rate. For the process to be feasible, the capital and maintenance cost must be competitive with the overall negative value of the wastes (both solid and liquid). It is worth to note that at present the mills have to invest 2-2.5 million Baht for a furnace to burn the EFB. This money can be used for a new design furnace that incorporates the evaporation of POME by the EFB and PKS. The investment cost might be offset by the by-product (fertilizer in a form of dry sludge) and by the benefit resulting from the waste-free condition (neither waste disposal cost nor land for waste water ponds). However, the technology for complementary waste management has to be developed along with the economic feasibility study. It is not an intention of this study to suggest the definitive method of waste utilization or management. The brief analysis given above is merely to demonstrate one possibility for waste management. There are many other possible ways including the use of solid wastes for co-generation, and the sale of excessive electricity to the grid, which was extensively studied by Wibulswas and Thavornkit.” However, the drying of waste water by energy from the solid wastes is the only method which can make the mills free from waste. The dry sludge can be used as fertilizer. Burning of EFB and shell generates no net CO, emission because the CO*
F'RASERTSAN
is reused by the palm trees to complete the carbon cycle. 6. CONCLUSION
The quantity of palm oil mill wastes is presented and methods of waste disposal and utilization are proposed. Waste disposal is usually a costly practice which imposes a so-called “negative value” on the waste. Waste utilization requires investment and technology to produce “positive value” products from the waste and at the same time mitigates the environmental problems caused by the waste. However, the feasibility of waste utilization depends on the cost-benefit analysis, the competitiveness of the process and products, etc. Acknowledgements-This work was financially supported by the National Center for Genetic Engineering and Biotechnology, NSTDA (Thailand) to whom the authors are grateful.
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23. S. C. Cheah, A. N. Ma, L. C. L. Ooi and A. S. H. Ong, Biotechnological applications for the utilization of wastes from palm oil mills. Fat Science Technology 90, 536540
(1988).
24. G. Petitpierre. Palm oil effluent treatment and production of biogas. Oleagineux 37, 372-373 (1982). 25. A. A. Aziz, Gaseous fuel from palm fruit shells for heating and power generation. Proceedings of the 5th ASEAN Conference on Energy Technology. Bangkok, 1994, pp. 197-204.
Biomass and Bioenergy Vol. 1I, No. 5, pp. 387-395, 1996 Copyright 0 1996 published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 80961-9534(%)ooo34-7 0961-9534/96 $15.00 + 0.00
BIOMASS RESIDUES FROM PALM OIL MILLS IN THAILAND: AN OVERVIEW ON QUANTITY AND POTENTIAL USAGE S. PRAsERTsAN*t tDepartment IDepartment
and P. PRASERTSAN~
of Mechanical Engineering, Prince of Songkla University, Hat Yai, 90110, Thailand of Industrial Biotechnology, Prince of Songkla University, Hat Yai, 90110, Thailand (Received 2 January 1996; revised 7 April 1996; accepted 1 May 1996)
Abstract-Palm oil production is one of the major industries in the south of Thailand. A study of the quantity and potential usage of palm oil mill wastes was carried out. Sixteen palm oil mills in the region generate 386,930 tons/yr 165,830 tons/yr and 110,550 tons/yr of empty fruit bunches, palm press fiber and palm kernel shell, respectively. In addition, 1,202,260 tons/yr of waste water is being treated anaerobically. Only the p&carp fiber is used for boiler feed. Empty fruit bunches and the shell are disposed of by the land filling method, which is very costly. In some factories the empty fruit bunches are burnt in the furnaces, which causes air pollution. The study suggests potential usages of the solid and liquid wastes. Copyright 0 1996 published by Elsevier Science Ltd. Keywords-Palm
oil residues; waste utilization; waste management; biomass.
1. INTRODUCTION
Palm oil is one of the major agroindustries in the south of Thailand. Oil palm planting area for the country totals 150,000 hectares, solely in the south. The oil palm (Hueis guineensis) was introduced to Thailand in 1968, after it successfully replaced the natural rubber trees in Malaysia. As the climatic conditions in the south are suitable for palm trees, the oil palm plantation area has expanded ever since. The planting area and palm oil production in the southern region are given in Table 1. At present, there are 16 factories employing the standard oil extracting process (normally called the wet process) and 24 factories the dry process. In the standard milling process, which is the process used in the factories of a milling capacity over 10 tons of raw material per hour, water is added into a digester. Raw material supplied to the mills consists of fresh fruit bunches (FFB). In 1993 the yield of FFB was 1.5 x lo6 tons’ and the crude palm oil production was 305,000 tons. It was estimated that about 1.18 x lo6 tons of organic waste was released from the palm oil mills. The stringent environmental measures implemented at present and those that will be introduced in the near *Author to whom correspondence
should be addressed.
future will inevitably affect the waste disposal practice of the mills. An increasing public concern about the environment has forced some factories to close down. This is because they failed to meet the effluent discharge standard and the bad odor (from the anaerobic pond) and dust (from burning the solid wastes such as empty fruit bunches and palm shells) offended people living nearby. Only a fraction of solid waste is used for steam and power generation. This paper reports a result of a field survey to determine the amount and types of palm oil mill wastes and proposes means to properly treat or utilize these wastes. Since 8590% of the palm oil is produced in the standard process factories, this paper deals with the standard process only.
2. FSI’IMATION OF WASTES FROM PALM OIL MILLS
The amount of wastes from palm oil mills were presented in a study in Malaysia.3 The palm varieties, growth conditions and plantation management in Thailand are different from those in Malaysia and undoubtedly result in a different quantity of solid wastes. Furthermore, the milling process in Thailand consumes more water,4 hence has a higher waste water discharge rate. In order to obtain precise data, the estimation of wastes was carried out by 387
S. PRASERT~AN and P. PRASERTSAN
388
questionnaires, observation, monitoring and interviewing. The FFB price varies according to its quality. The purchasing officers of the mills take samples from the suppliers and analyse them for the percentage of oil and shell (some varieties have big nuts and thick shells). The data collected by the mills were obtained during the survey and interviewing, and were used to double check the data obtained from the questionnaires. There are 16 standard process mills and 24 dry process mills in the south of Thailand. In total, 10 and five questionnaires were returned from standard and dry process mills, respectively. A survey of 11 standard mills and three dry process mills was carried out.
3. WASTES FROM MILLING PROCESSES
The survey data of the dry process mills showed no significant difference in the details of the process. However, for the standard wet process mills, they can be classified into two groups based on the oil separation techniquethose which use decanters (five mills) and separators (five mills). It was found that there is one mill which has installed both the decanter and the separator in parallel, in the separation line. Only the separator is operated in the normal season. The additional decanter is operated to accommodate the high production rate in the peak season. In the dry process, the fruits are dried by hot air generated from the burning of firewood, after which the oil from both the pericarp and the kernel is extracted by a screw press. The residue from the process is the crushed fruit, known as palm cake which can be sold for animal feed. The palm cake contains 8-10% residual oil and about 9% moisture content. Although this process leaves no waste, it is not used in the large scale production because the
mixed oil is not suitable for the downstream processes. In large factories, steam and water are used as shown in Fig. 1 and hence, generate waste water. The fresh fruit bunches are sterilized in a horizontal direct contact steam sterilizer at 140°C for 50 min to inactivate the lipolytic enzymes and loosen the fruit still attached to the bunches. The sterilized bunches are fed into a rotary drum thresher to separate the fruits from the bunches. The empty fruit bunches (EFB) are conveyed to the dumping ground. The fruits are fed into a digester where water at 80°C is added. The homogenous oil mash from the digester is pushed through a screw press, and later passes through a vibrating screen, a hydrocyclone and decanters (or separators) to remove fine solids and water. Decanter effluent and decanter cake are the major wastes at this stage. Centrifugal and vacuum driers are used to further purify the oil before pumping it to a storage tank. Fiber and nuts from the screw press are separated in a cyclone. The nuts are cracked in a centrifugal cracker. The kernels are packed and sold to kernel oil mills. Shells are normally left unused. There are various forms of solid and liquid wastes from the mills. These include empty fruit bunches (EFB), palm press fiber (PPF), palm kernel cake (PKC), palm kernel shell (PKS), sludge cake (SC) and palm oil mill effluent (POME). Only EFB, PPF, PKS and POME appear in large quantities and are considered as wastes. The others can be sold for animal feed or fertilizer. The quantity of the wastes depends on the quality of the raw material as illustrated in Fig. 2. Taking the average percentage of the FFB composition found from the survey (28% EFB, 12% PPF and 8% PKS), the solid wastes could be estimated as 386,930 tons of EFB, 165,830 tons of PPF and 110,550 tons of PKS (Table 2). Waste water was found to be 0.44-l. 18 tons/ton FFB.4 If the average figure of 0.87 tons/ton FFB is used for the calculation,
Table 1. Oil palm plantation and production of palm oil Year
Planting area (ha)
Yielding area (ha)
Yield of FFB ( x 10’tons)
Palm oil” production ( x lo3 tons)
1987 1988 1989 1990 1991 1992 1993
98,390 109,190 128,610 140,050 146,360 153,280 152,640
69,160 82,840 90,940 95,990 103,240 108,000 114,560
728 885 1098 1191 1315 1352 1526
145.66 117.02 219.63 238.35 263.20 270.40 305.20
Source: Office of Agriculture Economics. ‘Calculation based on 20% oil content in the fresh fruit bunches.
Biomass residues from palm oil mills in Thailand
389
FFB
Condensate
.* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-
Sterilizer
1
Thresher
Steam (14O”C, 50 mim)
.*~~.~-.+
EFB
Fruits
t Digester
-
Water (80-9O’C)
Oil mash I Screw press
Effluent *d.
.
.
.
.
Liquid .
Decanter . . . -* Cake
1 w.....
i
Solid Cyclone
. . ..w
PPF
1
Oil
Nut
Purifier
Nut cracker
. . . . . .)
PKS
I
Kernel
i Treatment
Pack for kernel
ponds
Oil mill Fig. 1. Palm oil milling process. (-
the annual discharge of waste water is 1,202,260 tons. 4. CURRENT AND POTENTIAL USAGE OF WASTES AND RECOMMENDATION FOR TREATMENT
4.1. Empty fruit bunches (EFB) EFB is the major component of all solid wastes. Steam from the sterilization process results in a moisture content in the EFB as high as 60%, which makes it unsuitable as fuel. It was reported that the EFB has 42% C, 0.8% N, 0.06% P, 2.4% K and 0.2% Mg.’ The survey found that the EFB have been used as raw material for mushroom cultivation only recently. Without any treatment, the EFB are pressed to form rectangular blocks. Mushroom spores are inoculated into the EFB blocks which were in turn covered by a black plastic sheet to retain moisture and limit sunlight. Mushrooms are a good source of protein and are considered a valuable product, with the sale price ranging from US!§ 2400-4000 per ton of Vovariella vohacea. A further advantage of mushroom cultivation is the residue, which now can be
) Process; (- - -) waste.
easily decomposed to produce another valueadded product, namely organic fertilizer. This type of fertilizer is much more suitable for the soil than the original EFB itself. Possible deficiencies in phosphate can easily be overcome through the addition of some waste water. The readily available and digestible residue obtained after mushroom cultivation with or without additional cornposting would allow easier transportation and fertilization as compared to the original EFB. This is particularly important as the bulky nature of EFB makes direct application difficult and pollutes the air if incinerated in furnaces. In some plantations, the EFB are left to decompose under the trees. Besides the palm trees, the EFB are also applied to fruit orchards using 20 FFB per tree. It helps retain moisture and returns organic matter to the soil. However, the EFB cannot be stacked in more than two layers around the tree or they attract harmful insects. The bulky nature of the EFB causes a high land-fill disposal cost. The mills, therefore, burn the EFB down to ash. Particulates and gas (SO,, CO*, CO and NO,) emitted from the furnaces sometimes cause air pollution to the nearby
S. PRASERTSAN and P. PRASERTSAN
390
4.2. Palm press fiber (PPF) The oil retained in its cell wall makes the PPF a good combustible material. In factories which produce both steam and electrical power, all of the PPF is used. However, only 30% is consumed if power is not produced. Therefore, in some factories, about 70% of PPF is considered as waste. The PPF ash contains 1.7-6.6% P, 17-25% K and 7% Ca.5 It could therefore be used as source of minerals for plants. Although PPF is similar to rice straw, it contains a higher percentage of fiber and lignin which cannot be digested easily by animals. However, its similarity to rice straw makes it a good substrate for mushroom cultivation.6 It is interesting to note that a study by Okiy’ found that PPF is suitable for the pulp and paper industry.
communities and results in public protests. Burning a ton of EFB produces 4 kg of ash. Because of the high mineral content of the ash (especially potassium) it can be sold at 1 Baht per kilogram (1 US$ = 25 Baht). It is interesting to note that there is now at least one factory which processes the EFB into fibrous material. The EFB fiber is coarser and stronger than that obtained from the pericarp. By adding a binding agent, such as rubber latex, the EFB fiber can be used for cushion filling material. One of the most promising products manufactured from the EFB is the medium density fiber (MDF) board which will be on the production line as soon as a factory is set up. The EFB has a great potential in these applications. It should be noted that products like coir fiber, fiber board, cement board, roofing tile and card paper can be produced from the EFB or fiber. However, the oil palm waste-based products received little interest in Thailand since other agricultural wastes in the country, such as rubber wood and bagasse, are readily available in larger quantities and the technical knowledge is already wellestablished at an industrial level scale. Furthermore, the use of oil palm waste as raw material requires oil separation (from the waste) process and, consequently, generates waste water as experienced in one factory.
4.3. Palm kernel shell (PKS) PKS is the most difficult waste to decompose. The shell size is uniform and is not as bulky as the EFB. They are usually left unused in the factory or disposed of by the land-fill method. In terms of energy, PKS is an energy intensive substance. Local industries that require process heat (or steam) generally have furnaces (or boilers) designed for firewood or fuel oil. However, substantial modification of the
FFB (100%)
EFB (20-30%)
Fruit (70-74%)
[59%]
Moisture
Dry EFB
+=l Pericarp
Nut
(12-14%)
(14-16%)
(51-551)
(18.9-19.2%)
[9.5%]
[49.5%]
[26.7%]
[ 14.3%]
r-d
Oil
Fiber
Moisture
(25-28s)
(12-13%)
(13-14%)
[ll.S%]
[ 13.9%]
[1.3%]
d-l
Shell
Kernel
Moisture
(6.8-7.4%)(8.5-8.4%)(3.3-3.4%) [10.6%]
[l%l
[2.7%]
Fig. 2. Composition of fresh fruit bunch. Figures in brackets are the percentage of FFB. () = high-quality FFB and fl = low-quality FFB.
3 3 5 3 1 1 16
No. of mills
250,400 457,000 395,000 244,000 18,000 17,500 1,381,900
EFB ( x 10’tons/yr) 70.11 127.96 110.60 68.32 5.04 4.90 386.93
Waste water ( x 10’tons/yr) 217.85 397.59 343.65 212.28 15.66 15.23 1 202.26
Table 2. Wastes from palm oil mills by provinces in 1993
Production capacity (tons FFB/yr)
Waste water was calculated based 0.87 tons/ton FFB, an average figure obtained from monitoring of four factories. EFB, PPF and PKS were calculated based on 28%, 12% and 8% of FFB, respectively.
Chum-porn Surat-thani Krabi Trang Satoon Songkla Total
Province
30.05 54.84 47.40 29.28 2.16 2.10 165.83
PPF ( x 10’tons/yr)
20.03 36.56 31.60 19.52 1.44 1.40 110.55
PKS ( x lo-’tons/yr)
$ J z! !z g J E v) 6’ el B
a z. g
392
S. PRASERTSAN and P. ~ASERTSAN
furnaces is needed if the conventional fuel is to be replaced by PKS. Therefore, many factories are still reluctant to use PKS as fuel unless they are economically forced to do so. There is a possibility that the PKS can be used for activated carbon productions*9 or charcoal.3 PKS contains 20.3% of fixed carbon and is physically similar to the coconut shell, which has been used to produce the activated carbon successfully. It is anticipated that the stringent environment control measures will increase the demand for activated carbon in the future. It is possible that activated carbon can be applied for the decolorization of the unacceptably dark-colored effluent of the palm oil mills. At present, there are 110,550 tons of PKS available annually, at no cost, at 16 mills. Some factories have shown an interest in incorporating activated carbon production in the milling operation. Many mills have installed co-generation plants, i.e. generate both heat and electrical power. High-pressure steam passes through a back-pressure steam turbine to generate electricity sufficient for the mill consumption. The exhausted steam is used as heat source for the milling process. Only the (pericarp) fiber and sometimes a small amount of shell is fed into the boiler furnace. As private power production is being encouraged in Thailand, it is therefore recommended that the surplus shell should be used to generate electricity and be sold to the grid. The heating value of the shell is 17.4 MJ/kg.” It is appropriate to assume that the overall thermal efficiency of the co-generation system is 25%. Therefore, the electrical energy sold to the grid is 2 x lo9 MJ/yr, which is equivalent to that generated from a 79 MW power plant operating for 300 days per year (which is the normal operating duration of the palm oil mills in Thailand). In practice, if the PKS is used for electricity generation, 16 small co-generation systems have to be installed. Extensive study of co-generation (heat and power) in the Thai palm oil industrylO indicated that the heat-to-power ratio of the system was about 12 during off-peak periods and 8 during peak periods. The analysis showed that the existing co-generation system was not properly designed, and that the low efficiency of the system was due to a large difference of enthalpy between the steam required to generate electricity and process steam and as a result, excessive steam blow-out occurred (39-47% of total steam input). To lessen the imbalance of
electricity and process heat, an energy conservation measure to utilize the excess steam to preheat the boiler feed water appears to be very cost effective. In addition, the boiler flue gas which is at about 330°C should be used to preheat the combustion air in order to improve the thermal efficiency.” Many local cottage industries such as lime making and brick firing are now faced with energy shortage problems. Usually, heat for the process is obtained from firewood. Natural rubber is another important industry in the south of Thailand. Rubber trees cut down for replanting (after their economic life), which are the only source of firewood, are no longer a cheap fuel source. At present, the rubber wood price is soaring because the number of rubber-wood-based furniture factories is continuously increasing. This inevitably has an adverse effect on the local industries which rely on the rubber wood for their energy supply. The PKS can substitute the rubber wood if the furnaces are properly modified. 4.4. Palm oil mill efluent (POME) There are three major sources of waste water, namely sterilizer condensate (17%), decanter or separator sludge (75%) and hydrocyclone water (8%). Monitoring at four factories revealed that the milling process produces waste water in the range of 0.44-l. 18 m’/ton FFB with the average figure of 0.87 m3/ton FFB4 Waste water is treated anaerobically in a series of ponds. Over half of the land has to be spared for the waste water treatment ponds. There is a concern that this waste water might pollute underground water. As a discharge standard of effluent from the palm oil industry has not been set in Thailand yet, the general standard for any effluent is enforced. Since the final effluent is of brownish colour, it cannot be discharged into the natural waterways. There is, therefore, an urgent need to investigate the decolorization process. This is not limited to the use of activated carbon mentioned earlier, but includes the biological method using microorganisms such as Phanerochaete chrysosporium.‘* However, environmental guidelines for the palm oil industry are being developed and the effluent standard is anticipated to be implemented by the year 2000. It was observed during the visit to the palm oil mills that if algae is present in the final pond, the effluent has a very light brown colour which is considered acceptable for discharge into the
Biomass residues from palm oil mills in Thailand
river. Therefore, the role of algae on decolorization and the use of algae as a by-product should be investigated. The waste water characteristics have been reported in many publications.2*4. ‘XI4 The high organic load of waste water in terms of BOD ( w 50,000 mg/l) or COD ( N 80,000 mg/l) is equivalent to the load generated by a population of 3 million people.4 While the palm oil production in Malaysia is 20.6 times that of Thailand’s, the waste water organic load in terms of population equivalent is only 2.4 times.13 These figures clearly reflect the lack of control and inefficient treatment in Thailand. Furthermore, the specific waste water from the palm oil mills in Thailand, which is 0.87 m3/ton FFB, is substantially higher than the figure 0.6 m3/ton FFB quoted in Malaysia.” It was reported that the minimum discharge achievable can be as low as 0.3 m3/ton FFB.16 The use as fertilizer and water supply for the palm trees are the most common methods for the waste water utilization. As most of the carbon source (with a proper nutrient ratio in terms of BOD:N:P = 1OO:l.1:0.2) is used by the microorganisms when the waste water is treated anaerobically, nutrients such as N, P and K are left over for the plants. Substances yielded from the drying of the waste water can be used as an animal feed supplement”.” or fertilizer.19 However, the energy cost of drying must be very low or obtained freely such as solar energy. Commercially available single cell protein for animal feed (Prolima and Centriplus) can be cultivated from the POME.’ Biotechnological products such as cellulase, xylanase, betaglucosidase and pectinase enzymes can be obtained niger ATCC 6275.*’ from cultivating Aspergih Furthermore, the enzymes can then be used for the saccharification of palm cake and palm fiber to produce sugars (glucose, xylose, arabinose, mannose, etc.). The enzymes can also be applied to enhance the oil recovery from press cake fiber*’and a laboratory experiment revealed that it can separate oil and suspended solids from the POME. Anaerobic digestion is an effective method for treatment of wastewater with high organic content. During the treatment process, microorganisms hydrolyse fat, oil and carbohydrate (in the form of sugar, starch, pectin and pentosan) to volatile fatty acids (acetic, butyric, etc.) and isobutyl alcohol. These compounds have offensive smells and are regarded as air pollution. It was found recently that commercial products
393
claiming to mitigate the bad smell are available, and many mills apply the substance in the pond. This practice does not only result in an additional treatment cost but is also found not to work in the long term. Alternatively, the treatment should be carried out in a reactor in which the biogas is obtained. Biogas could be produced from mesophilic anaerobes at a rate of 0.57 m3/kg COD utilized per day, and contained 6&69% methane gas.*’ The production of biogas was also conducted on a pilot scale using a thermophilic contact process at a temperature of 45”-60°C with an organic loading of 3.0 kg BOD/m3/day. The highest yield of biogas was seen at 65°C and contained less than 10 ppm of hydrogen gas. The treatment efficiency was 96%.23 With the average output of 40 kg (40,000 ppm) organic matter, the potential production of biogas is 12 m3/m3 effluent.24 For a mill with handling capacity of 20 tons FFB/h and 10 h working time per day, the daily discharge of the effluent is 174 m3/day (calculated from 0.87 m3/ton FFB). Assuming a very conservative long fermentation time of 20 days, the digester volume is 3480 m3. It can be anticipated that after 20 days of treatment the BOD should be reduced by 90%, the COD by 75% and the total solids by 70%. Biogas produced from the digester is calculated as 2088 m3/day or, in terms of heat, 48 GJ/day. 5. DISCUSSION
The palm oil industry contributes an appreciable level of economic growth to the southern region of Thailand. Thousands of people are engaged in the upstream and downstream processes of the milling. Although the mills generate a great amount of waste, the waste is a natural product, and non-toxic, because no chemicals are used in the process. The waste, if properly managed, can be returned to the soil as fertilizer without any harmful effect. This means that the waste should not be kept in a place where the concentration is too high for the environment. However, dumping the waste by scattering evenly over 150,000 ha of plantation is economically not feasible. It is worth mentioning the potential of incorporating integrated farming in the ponding treatment system of a palm oil mill. Because of the nutrient-rich effluent, some vegetables could be grown in the treatment ponds. Bamboo rafts can be used as growing platforms. In the final pond where algae is present, phytoplankton-
394
S. F~ASERTSAN and P.
feeding fish such as tilapia can be cultured. A frozen seafood factory in Hat Yai, Songkla province, discovered that an unpleasant odor developed in fish cultured in the final treatment pond, unless the fish were kept in clean, fresh water for few weeks. If the effluent is dried out, many associated problems vanish. The water in the effluent may be vaporized by the heat obtained from burning the solid wastes. One ton of FFB generates 870 kg of waste water at 66°C. Taking the total solid content of 60 kg (derived from Table 2) the amount of water to be evaporated is 8 10 kg, which requires about 1900 MJ. One ton of FFB gives 80 kg of PKS, 120 kg of PPF and 280 kg of EFB. It was reported that the heating values of the dried shell, fiber and EFB are 17.4, 4.6 and 9.6 MJ/kg, respectively.2s If the heat conversion efficiency is 80%, about 2200 MJ is obtainable from the solid wastes. It is possible, therefore, to eliminate both liquid and solid wastes simultaneously by this method. However, the factory has to install a heat generator with a capacity of 470 kW for every ton FFB/h production rate. For the process to be feasible, the capital and maintenance cost must be competitive with the overall negative value of the wastes (both solid and liquid). It is worth to note that at present the mills have to invest 2-2.5 million Baht for a furnace to burn the EFB. This money can be used for a new design furnace that incorporates the evaporation of POME by the EFB and PKS. The investment cost might be offset by the by-product (fertilizer in a form of dry sludge) and by the benefit resulting from the waste-free condition (neither waste disposal cost nor land for waste water ponds). However, the technology for complementary waste management has to be developed along with the economic feasibility study. It is not an intention of this study to suggest the definitive method of waste utilization or management. The brief analysis given above is merely to demonstrate one possibility for waste management. There are many other possible ways including the use of solid wastes for co-generation, and the sale of excessive electricity to the grid, which was extensively studied by Wibulswas and Thavornkit.” However, the drying of waste water by energy from the solid wastes is the only method which can make the mills free from waste. The dry sludge can be used as fertilizer. Burning of EFB and shell generates no net CO, emission because the CO*
F'RASERTSAN
is reused by the palm trees to complete the carbon cycle. 6. CONCLUSION
The quantity of palm oil mill wastes is presented and methods of waste disposal and utilization are proposed. Waste disposal is usually a costly practice which imposes a so-called “negative value” on the waste. Waste utilization requires investment and technology to produce “positive value” products from the waste and at the same time mitigates the environmental problems caused by the waste. However, the feasibility of waste utilization depends on the cost-benefit analysis, the competitiveness of the process and products, etc. Acknowledgements-This work was financially supported by the National Center for Genetic Engineering and Biotechnology, NSTDA (Thailand) to whom the authors are grateful.
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