Palm oil mills with wet milling process are accounted for major production of palm oil in Malaysia, Indonesia and Thailand. Besides the main product “crude palm oil”, the mills generate many by-products and liquid wastes which may have a significant impact on the environment if they are not dealt with properly. One ton of fresh fruit bunches (FFB) composed of 230-250 kg of empty fruit bunch (EFB), 130-150 kg of fiber, 60-65 kg of shell and 55-60 kg of kernel and 160-200 kg of crude oil. EFB are bulk solid residues and its use as a fuel for boiler is constrained by its high moisture content and low heating value (<10 MJ/kg dry EFB). Utilization of the EFB as substrate for mushroom cultivation and for the production of particle board should be given first priority. In addition, EFB could be used as organic fertilizer and mulching material. Palm fibers are used mainly as fuel for boilers (heating value of  <5 MJ/kg dry fibers). Other applications of palm fibers include the use as substrate for enzymatic saccharification of or for animal feed. Although palm shell can be used as boiler fuel with heating value of 17 MJ/kg, it causes the black smoke. Alternative use for the production of activated carbon is preferable. Decanter cake can be used as a fertilizer or soil conditioner. Palm oil mill effluent (POME) is the mixture of high polluted effluent (from sterilizer and oil room) and low polluted effluent (steam condensate, cooling water, boiler discharge and sanitary effluent). To minimize overall treatment cost, the different wastewater streams should be collected and treated separately. Oil separated from the wastewater stream by gravity type oil separators is recommended which will contribute to improve production yield and minimize the organic loading for the subsequent biological treatment system. The most appropriate secondary treatment for POME is biological digestion in which the combination of anaerobic and aerobic ponds is used presently. Closed anaerobic system should be used for energy conservation. Application of biologically treated POME for irrigation has been carried out by some palm oil mills having their own plantation nearby. The POME dosage should be based on the fertilizer requirement of plants. Spillage of POME into ground water or into surface water must be avoided. If these wastes are properly managed as described above, the palm oil mill will become an environmental friendly industry.

This paper concerns with the aspects of palm oil production including liquid and solid by-products/residues and wastewater and possible air emission to the atmosphere. Besides the main products “crude palm oil and palm kernel oil”, the mills generate by-products and liquid wastes which may have a significant impact on the environment if  they are not dealt with properly. The paper promotes closed concepts for utilization and disposal regarding the complex of all environmental media of this branch of industry. The relationship between oil palm plantation, palm oil mill and the environment is shown in Figure 1. 

At present the oil palm (Elacis quieensis) plantation using the tanera variety which provides the highest oil content in the fruits of the Fresh Fruit Bunch (FFB) with 21% oil. About 120 Kg/t FFB are nuts including kernels 60 Kg/t FFB (5% of FFB) which contain about 30% oil ~ 30 Kg/t FFB.

If the average production of a medium size palm oil mill is 25 t FFB/h (or 400t/d and 150,000 t/y) and FFB 5 tons per acre per year are harvested, the average oil palm growing area per palm oil mill should be 30,000 acres (50 trees/acre). Good agricultural practices are needed to get high yield of fruits and oil content. Normally oil palm requires fertilizer differently at different stages (Table 1). The transport of the harvested FFB from the plantation to the mill is mostly organised by the farmers and done with open lorries. Normally the oil mills are situated in the neighborhood of the plantations. The average distance to the mill is about 30-50 Km. Anyway the transportation time is about one day.

Standard palm oil extraction processes
There are 3 ways to extract the oil from the fruits of oil palm.

1. Dry process : 
The fruits have to be separated from FFB and are direct heated to stop enzymatic reaction and to evaporate the moisture. After that the fruits are crushed by screw press to get the crude oil. This oil is the mixture of pericarp oil and kernel oil. The palm cake after extraction is used as animal feed. There is no liquid waste from this process.

2. Frying process : 
The FFB are fried in oil under vacuum and the fruits and bunches are pressed to get the kernel oil.
 
 
 
 
 
 

3. Wet process : 
This process is characterized by  steaming the whole FFB in order to inactivate the natural enzymes and loosen the fruits off the bunch and soften the mesocarp, resulting in easier extraction of oil.

 In Thailand there are 6,000 acres of plantation area and yield 3,000,000 tons of FFB. There are 18 mills using wet process which account for 75-80% of palm oil production. The flow diagram of the standard wet process is shown in Figure 2. 

The processing steps of the standard wet process
1. Arrival and storage of FFB
 Soon after harvesting, the FFB must be brought to the mill as quickly as possible to avoid fatty acids production by natural enzymes in the mesocarp. The FFB are unloaded on a ramp and put into containers of 2.5-3.0 tons each.

2. Sterilisation
 Sterilisation of the FFB is done batchwise in an autoclave of 20-30 t FFB capacity (7-10 containers) with the application of live steam at 120-1300C about 1 h (total time of 2 h). The steam condensate or steriliger condensate is the wastewater generated at this step (0.15-0.18 m3 Kg/t FFB).

3. Bunch stripping 
 The containers with the sterilised bunches are empty into a rotary drum thresher where the fruits are separated from the bunch stalk. This processing step generates the empty fruit bunches (EFB) at 230-250 Kg /t FFB. 

4. Digestion
 The separated fruits are carried into digesters and mechanically threated into mash. No residue occurs in this step.

5. Oil extraction and handling of solid wastes
The oily mash is fed into a continuous screw press system. The extracted oil phase is collected and is discharged to the purification section. The remained press cake is transported to a separation system consisting of air classifiers and cyclones for drying and separation of nuts and fibers. Kernels are recovered from nuts in the crackers. The oil is also extracted from kernels by screw press to get palm kernel oil. Fibers and shells are solid residues generated during oil extraction, with the wolds of 145 and 60 Kg/t FFB, respectively. 

6. Oil purification 
6.1 Screening
      To improve oil clarification, hot water is added to the raw oil and passed through a vibrating screen to separate large size solids. The oil after sieving still contains small size solids and water.

6.2 Separation of suspended solids
      The conventional procedure to separate oil from water and suspended solids is the settling tank method. The system is heated by steams. The oil floating on the top is collected by a funnel then sent to crude oil tank.

6.3 Treatment of settling tank underflow 
      The settling tank underflow is collected in the sludge tank and subsequently treated to recover oil. In order to protect the equipment in the subsequent process steps against clogging, the bottom sludge is pre-cleaned by means of microstainer/hydrocyclone of desander. The desarders are cleaned by discharging the accumulated solids to the drain, followed by the injection of fresh water. Desander wastewater is normally around 5 L/t FFB 

6.4 Centrifuging
      The pre-cleaned sludge is collected in a buffer tank and then pumped to a two-phase centrifuge (separator) or a three-phase centrifuge (decanter) for oil recovery. To improve oil separation by the separator it is common practice to add water during centrifugation. Therefore the separator process will generate more wastewater than the decanter process with the quantify of 750 and 300 Kg/t FFB, respectively. The amount of decanter cake is 12 Kg/t FFB. The recovered crude oil is pumped to the settling tank.

6.5 Separation of fine suspended solids
      The crude oil from the settling tank combined with recovered oil from the sludge tank is about 160-200 Kg/t FFB. The final oil purification step is also done by centrifugation to remove fine suspended solids. The low suspended solids content in the crude oil generates not large volumes of solid residues.

6.6 Drying and cooling 
        After centrifugation the crude oil still contains water which is removed by a vacuum evaporation system. Subsequently, the dried crude oil is kept in storage tanks before selling to an oil refinery. Residues of the drying step is cooling water with the quantify of ~ 300 Kg/t FFB 

Characteristics of residues
The entire palm oil milling process does not use any chemicals as a processing aid. Therefore, all substances found in the products, by-products and residues are originated from oil palm. The main residues are EFB, fibers, shells, decanter cake and wastewater. The nutrient contents of these residues are shown in Table 2.

Process Integrated Pollution Prevention and Control Strategy (IPPCS)

In order to manage the production process efficiently, the IPPCS must be applied such as improve production technology, minimize residues and utilize residues as by-product. IPPCS will help to save resources and energy and reduce environmental pollution. 

1. Improvement in production technology
1.1 Plantation management 
      - improve the oil yield of oil palm
      - management of plantation

1.2 Quality of raw material
      - co-operation between the plantations and the palm oil mills

2. Prevention of oil loss in the  production process
- reuse of condensate 
- improve process control and equipment maintenance 
- improve oil extraction process
- improve oil separation process

3. Utilization of solid residues
3.1 Empty fruit bunches (EFB)
3.1.1 Return to plantation 
         The EFB may be used as covering material, organic fertilizer and soil conditioner in the plantation. However, EFB utilization rate is limited by its potassium content. Therefore, as given in Table 3 only 0.2-2.0 t EFB can be used per acre per year. However, exact figures concerning the actual availability of EFB as substrate to the palm trees have not yet been developed and need further investigation. 

3.1.2 Substrate for mushroom cultivation
         EFB could be used directly as a substrate for mushroom cultivation. At present many farmers use EFB to grown straw mushroom.

3.1.3 Fuel for boiler
         The use of EFB as a fuel for boiler is constrained by its high moisture content and low heating value (calorific value of dried EFB ? 10 MJ/Kg). In addition, there are better solid residues available as fuel source at the mills such as dried fibers and shells.

3.1.4 Incineration to recover ash
         Incineration of EFB to reduce its volume and to recover the ash has a great disadvantage of generating excessive air pollution. This method should not be applied because of the wastage of valuable carbon material, which can be useful for soil conditioning.
 

Picture 2: Fibers	Picture 4: Shells

3.2 Fibers
     Around 15% of the FFB are palm fibers. They are used mainly as fuel for boiler (calorific value of dried fibers ? 5 MJ/Kg). Other use is an addition in animal feed. Adding 5-6% NaOH or urea and allowing 2-3 weeks fermentation can improve its quality.
 

3.3 Shells
      Shells can be used as boiler fuel (calorific value of 17 MJ/Kg). However, fibers are more than enough to be used as fuel for boiler. Therefore, the shells are often accumulating in the mill. Another possible use of shells is the production of charcoal and activated carbon which should need further investigation.

3.4 Decanter cake
      For mill using decanter to separate oil from sludge, the generated decanter cake can be used as soil conditioner and fertilizer like EFB. The acceptable annual soil application can be calculated using Table 1 and 2.

4. Utilization of liquid residue
4.1 Low suspended solids (SS) wastewater 
      During palm oil extraction process there are many wastewater streams that contained very low SS. At the steam turbine, there was cooling water of 1 m3/h and at the oil drying step there was 3 m3/h of vacuum condensate. These wastewater can be reused in any step in the mill.

4.2 Steriliser condensate

      Normally the steriliser condensate go is directly into the final oil trap. The  steriliser condensate contains ~ 1% oil which can be recycled into the process at digester,  screw press or settling tank. The reuse of cooling water and condensate will reduce the amount of both water consumption and wastewater for treatment.

4.3 Wastewater from oil room
      The main wastewater from the palm oil extraction by wet process is the wastewater from oil room after separator or decanter. This stream of wastewater combined with sterilizer condensate and cooling water are called palm oil mill effluent (POME). The characteristics of POME from four palm oil mills are shown in Table 4. 

Application of biologically treated POME for irrigation is a method used by many palm oil mills. The application as fertilizer has to be carried out carefully, as overdose will result in nutrient imbalance and lead to undesirable chemical reactions in the soil. Prolonged inadequate utilization of POME may cause the accumulation of magnesium and inhibit the availability of potassium. The POME dosage should be based on the fertilizer requirement of the plants and not on the permissible hydraulic loading rate of the soil. Spillage of POME into ground water or into surface water must be avoided. The amount of POME to be applied under good agricultural practices is given in Table 5. It indicated shat the utilization of POME is limited by the potassium (or magnesium) value. Therefore, only 1.2-18 m3 POME can be used per acre per year for an oil palm plantation. It can be concluded that biological treatment has no signifcant influence on the permissible amount of POME for proper fertilizer. The feasibility utilization of POME must be analysed and tested for practical application. Investigations in Malaysia show that POME used in an oil palm plantation increased the yield by 13 %. Some palm oil mills in Thailand also applied POME in the plantation and found that growth of palm trees and oil yield are increased.

Treatment of wastewater from palm oil mill

1. Primary wastewater treatment

1.1 Segregation of wastewater streams
As shown in the schematic process flow diagram in Figure 2, a palm oil mill has the following effluent streams:

1.1.1 high polluted effluent : effluent from steriliser and oil room

1.1.2 low polluted effluent : steam condensate and cooling water and boiler  house discharge.

1.2.3 Sanitary effluent: wastewater from toilet and canteen.
In order to minimise overall treatment cost the different wastewater steams should be collected and treated separately. The highly polluted wastewater streams from a palm oil mill have different suspended solid contents which influence the effectiveness of the pretreatment system. The steriliger condensate and oil leakage should be collected separately from oil room effluent. 

1.2 Oil separation 
In order to recover the oil from wastewater, the different wastewater streams should be treated separately in gravity type oil separators. The oil removal by means of gravity separator will improve the production yield and minimise the organic loading for the subsequent biological treatment system.

1.2.1 Low suspended solids wastewater
         Since the oil in this type of wastewater is mainly in the free form, oil could be easily removed in gravity type oil separators. The pretreated wastewater could be reused in the mill which may be during digesting and pressing.

1.2.2 High suspended solids wastewater
         This stream mainly comes from centrifuge in the oil room. Since this wastewater is generated by separation equipment with high accelerating force, further oil removal by gravity separation is marginal. However, installation of oil trap is recommended mainly as safety device in case of accidental oil discharge.

2. Secondary wastewater treatment
The most appropriate secondary treatment method for palm oil mill wastewater is biological digestion. Since this wastewater is composed of mainly organic substances and absent of toxic substances. At present, biological treatment systems operated in palm oil mills use a combination of anaerobic and aerobic treatment methods. However, only limited information is available on design figures.

2.1 Effluent cooling
 The optimum temperature for anaerobic treatment is 37C but the palm oil mill effluent has a temperature in the range 75-90C. A cooling step is required prior to biological treatment. The cooling pond shape has to be designed carefully. The depth of the pond should not exceed 1.5 m otherwise the cooling efficiency will decrease. The hydraulic retention time of the cooling pond should be at least 1 day. Despite the high initial temperature of the effluent, biological decomposition of palm oil mill wastewater already starts in the cooling pond. This normally leads to acidifying process and odor-generating substrates are released resulting in bad smell. Control of pH (at about 7.0-7.5) is important to avoid this problem.

In order to improve effluent cooling as well as to get some adjustment of the pH-value and to decrease the substrate concentrations in the initial part of the treatment system, some part of the anaerobically treated effluent can be recycled back to the inlet of the cooling pond. The optimum recycling ratio had to be established for each individual palm oil mill by experiment and should be between 30-70% of the wastewater flow rate. The recycling effluent should come from the spot where highly active anaerobic biomass is available (30-40 days retention time of the overall pond system).

2.2 Anaerobic treatment systems
  These systems have significant advantages over aerobic treatment methods :
- almost energy free operation
- only moderate impact from high organic loads.
- low surplus of sludge formation 

2.2.1 Open anaerobic pond
         The anaerobic digestion system usually used in Thailand is the open pond system consisting of a series of several ponds. During design and operation of the pond system the volume requirement for collection of the settled primary sludge of the raw POME as well as the anaerobic excess sludge have to be considered. Sludge accumulation will result in reduction of pond volume and overall treatment efficiency.

       The decision on whether or not to use this system depends on many factors. The main factors are land price, conditions of the surrounding area and the loss of biogas as a source of energy must be considered as well. However, the biogas is not an important source of energy for palm oil mill at present because the energy produced by combustion of fiber and shell is sufficient. This may change if refinery activities are introduced and more environmental friendly treatment is needed.

 2.2.2 Closed anaerobic digester
       Palm oil mills have sufficient supply of energy from the use of solid residues. Energy surplus and the simplicity as well as low investment and operation cost of the opened pond system make the closed anaerobic digester at present not applicable in palm oil mills. Various systems of closed anaerobic reactors are available such a completely mixed, fixed bed and upflow anaerobic sludge blanket (UASB) reactors. To achieve operational stability in the anaerobic digester, intense fluctuation organic loading has to be avoided. This can be achieved by using the cooling pond as equalization tank for continuous feeding to the anaerobic reactor.

2.3 Aerobic treatment systems

Various aerobic treatment systems for wastewater are readily available in Thailand. The most widely applied system for palm oil mills is the aerobic pond system.

 2.3.1 Aerobic pond systems
       The various aerobic pond systems which differ in the type of oxygen supply and design loading rates are facultative ponds, oxidation ponds, aerated lagoons and polishing ponds. The oxygen supply in most ponds is established by photosynthetic activities of algae and plants and by adsorption of oxygen from the atmosphere except aerated lagoons are artificially aerate. The high temperature of the pond content does enhance the biochemical reactions resulting in more substrate removal.

 2.3.2 Activated sludge process

         In the activated sludge process, oxygen is supplied intensively by aeration and mixing. The concentration of active biomass in the aeration tank must be controlled. The suspended biomass is separated from the treated effluent in a final clarifier. The operation units are aeration tank, clarifier, return sludge system and excess sludge removal and treatment system.

The example of biological treatment of palm oil mill wastewater with combination of anaerobic ponds and aerobic ponds is shown in Table 6. The 500 tons FFB capacity palm oil mill will generated 25 m3/h of wastewater. If the influent has the BOD5 of 30,000 mg/l and the retention of 171 days, the effluent will have BOD5 of 100 mg/l.

Conclusion
 The raw material for palm oil extraction is fresh fruit bunches. If the palm oil mill is located near the oil palm plantation it will reduce the cost of transportation and get good quality raw material. The palm oil mill generates both solid and liquid residues. Process integrated pollution prevention and control strategy is needed for extraction of palm oil by improvement of production technology, prevention of oil loss and utilization of solid and liquid residues. Proper treatment of palm oil mill effluent in combination with utilization as liquid fertilizer will make the zero discharge concept come to work. Then the palm oil mill will become an environmental friendly industry.

Acknowledgement
 Thanks Deutsche Gesselschaft fur Technische Zusammenarbeit (GTZ),  Department of Industrial Works, Ministry of Industry, and Thailand Research Fund for supporting most part, of this work.

References

APHA, AWWA and WOCF. 1989. American Standard Methods for Examination of Water and Wastewater (17th ed.), American Public Health Association, Washington, D. C.

Bureau of Industrial Environmental Technology 1997. Environmental Management Guideline for the Palm Oil Industry. Department of Industrial Works, Ministry of Industry DIW TG 002. 

Cheah, S. C., A. N. Ma, L. C. L. Ooi and A. S. H. Ong. 1988. Biotechnological applications for the utilization of wastes from palm oil mills. Fat Sci. Technol. Vol  :536-540.

E S C A P. Environment and Development Series. 1982. Industry Pollution Control Guidelines IV. Palm Oil Industry. Economic and Social Commission for Asia and the Pacific, United Nations, Bangkok. 

H-Kittikun, A., Prasertsan, P., Srisuwan, S., Jitbunjerdkul, S. and Thonglimb, V.1994. Oil Recovery from Palm Oil Mills Wastewater. Research and Development Institute, Prince of Songkla University, Hat Yai, Thailand.

Plam Oil Research Institute of Malaysia. 1985. Palm Oil Factory Process Handbook Part 1.

Thanh. N. C., S. Muttamara, B. N. Lohani, T. Lee, K. Hum, K. C. Leong, M. A., Kazimi, D. M. Tam and S. Burintratikul. 1980. Palm Oil Wastewater Treatment Study in Malaysia and Thailand. International Development Research Centre., Final Report No.114. Asian Institute of Technology.

Uerkull, H. R. and Fairhurst, T. H.  1991. Fertilizing for High Yield and Quality of Palm Oil. International Potash Institute, Worblaufen IPI-Bulletin No.12