Conventional methods and emerging wastewater polishing technologies for palm oil mill effluent...

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Journal of Environmental Management 149 (2015) 222e235

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Journal of Environmental Management

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Review

Conventional methods and emerging wastewater polishingtechnologies for palm oil mill effluent treatment: A review

Wai Loan Liew a, Mohd. Azraai Kassim a, *, Khalida Muda a, c, Soh Kheang Loh b,Augustine Chioma Affam c

a Water Research Alliance, Level 2, Block C07, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysiab Malaysian Palm Oil Board, 6, Persiaran Institusi, Bandar Baru Bangi, 43000 Kajang, Selangor, Malaysiac Faculty of Civil Engineering, Department of Environmental Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

a r t i c l e i n f o

Article history:Received 6 February 2013Received in revised form23 September 2014Accepted 14 October 2014Available online

Keywords:Palm oilPalm oil mill effluentPolishing technologiesSustainabilityTertiary treatment

* Corresponding author. Tel.: þ6(07)553 0244; fax:E-mail address: [email protected] (Mohd.A. Kassim)

http://dx.doi.org/10.1016/j.jenvman.2014.10.0160301-4797/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Malaysian palm oil industry is a major revenue earner and the country is ranked as one of the largestproducers in the world. However, growth of the industry is synonymous with a massive production ofagro-industrial wastewater. As an environmental protection and public health concern, the highlypolluting palm oil mill effluent (POME) has become a major attention-grabber. Hence, the industry istargeting for POME pollution abatement in order to promote a greener image of palm oil and to achievesustainability. At present, most palm oil mills have adopted the ponding system for treatment. Due to thesuccessful POME pollution abatement experiences, Malaysia is currently planning to revise the effluentquality standards towards a more stringent discharge limits. Hence, the current trend of POME researchfocuses on developing tertiary treatment or polishing systems for better effluent management.Biotechnologically-advanced POME tertiary (polishing) technologies as well as other physicochemicalmethods are gaining much attention as these processes are the key players to push the industry towardsthe goal of environmental sustainability. There are still ongoing treatment technologies being researchedand the outcomes maybe available in a while. However, the research completed so far are compiledherein and reported for the first time to acquire a better perspective and insight on the subject with aview of meeting the new standards. To this end, the most feasible technology could be the combinationof advanced biological processes (bioreactor systems) with extended aeration, followed by solids sepa-ration prior to discharge. Chemical dosing is favoured only if effluent of higher quality is anticipated.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The palm oil industry has been an important economiccontributor for countries like Malaysia, Thailand, Indonesia,Colombia and other tropical developing regions. While impressiveexport and production figures are widely reported in almost allpalm oil or POME treatment literature, it is not astonishing that amassive production of the effluent has turned out to be a mainsource of water pollution. InMalaysia, it is estimated that at least 60million tonnes of POME was generated in the year 2009 alone (Nget al., 2011). For each tonne of fresh fruit bunch (FFB) processed,large quantities of POME containing 29e33 kg of 30 �C, 3-daysBiochemical Oxygen Demand (BOD3) are discharged into the wa-ter bodies (Thanh et al., 1980).

þ6(07)557 1700..

In Malaysia, a lot of efforts on research and development hadmade the industry dotted with significant successful pollutionabatement history. Being the pioneer in targeting sustainable palmoil industry, Malaysia has gained valuable experiences in devel-oping technologies for both upstream and downstream processing.In the year 2011, the palm oil processing mills attained 95.5%compliance to the effluent discharge limits (DOE, 2011). Thisachievement will continue to soar higher when further aim for theimplementation of greater environmental management initiativessuch as Cleaner Production (CP), biogas capture for Clean Devel-opment Mechanism (CDM) (PEMANDU, 2010; Ng et al., 2011),Roundtable for Sustainable Palm Oil (RSPO) (Basiron, 2007), andpossibly towards zero discharge is made.

Previous reviews have been published emphasizing the currentconventional POME treatment methods and state-of-the-art labo-ratory treatability studies. Some papers emphasize reusing theeffluent and the industry's solid wastes to attempt resource re-covery. As the current scenario in palm oil research focuses on

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W.L. Liew et al. / Journal of Environmental Management 149 (2015) 222e235 223

tertiary/polishing treatment of POME to progress towards thechallenging 20 mg L�1 BOD3 discharge limit, this paper aims toconcisely review and report the POME tertiary/polishing technol-ogies which have not been compiled before so as to gain an insightfor better effluent management.

1.1. The palm oil industry at a glance

Development of the oil palm sector is marked as an industrialsuccess story. As the main plantation commodity in Malaysia, thetotal plantation area has expanded from amere 400 ha planted area(year 1920) to 54,000 ha (year 1960); subsequently to 2,692,286 ha(year 1996) and the most recent statistic at year 2013 reviewed is5,229,739 ha of oil palm planted area (DOE, 1999; MPOB, 2014).Travelling across the country, oil palm trees are seen nearly in everybit of unfilled land. According to a stature report by Basiron (2007),forest and oil palm recorded 61.82 and 13.20%, respectively for landcoverage in Malaysia. From such a huge area of oil palm plantation,the country is capable of producing more than 94 million tonnes ofFFB to be processed by palm oil mills which are spread across thenation (MPOB, 2014). Up to year 2011, Malaysia recorded a total of426 palm oil mills in operation (DOE, 2011; MPOB, 2012), with 250mills operated in Peninsular Malaysia and the remaining 176 millsin Sabah and Sarawak.

Fig. 1 illustrates the distribution of the palm oil mills throughoutthe country. In the Peninsular, Pahang state has the highest numberof mills while Sabah state tops the country at 123mills in operation.Despite having the highest number of palm oil processing mills,Sabah state nevertheless is well-known for its biodiversity lush-ness. Major rivers in Sabah state like the Kinabatangan River, theSegaliud River, the Muanad River, the Segama River, the PangBurong River, and the Kalumpang River are important to the localcommunities, tourism activities, and are getting severe exertionson conservation. Hence, most environmental concerns allied with

Fig. 1. An overview of the Malaysian palm oil industry in year 2011. A e The number of p(tonnes); C e The estimated POME production (tonnes); D e Sterilizer condensate (tonnes

the palm oil industry accentuates in the Sabah state, such as theimplementation of the 20 mg L�1 BOD3 discharge limit. Seeing theabundant number of palm oil processing mills in Malaysia, 30additional mills are indeed under planning and constructionthroughout the country while 3 existing mills are not in operation(MPOB, 2012). With data on the amount of FFB processed by mills,the quantity of POME production can be projected. In Malaysia, therecorded national production rate for POME is 0.67 cubic metersper tonne of FFB processed by mills (DOE, 1999; Ma, 1999; Ng et al.,2011). POME can be further divided into the sterilizer condensate,clarification wastewater, and hydrocyclone wastewater in a ratio of9:15:1 (Wu et al., 2010). The information in Fig. 1 shows thebooming Malaysian palm oil industry (year 2011), but in contrast, alarge amount of liquid wastes are produced requiring appropriatetreatment before discharge.

1.2. Palm oil factory processes as sources of pollution

In Malaysia, the wet palm oil milling process is typically applied.The method uses hot water to leach out the oil, which also explainsthe large consumption of water resources for milling processes andthe concomitant large production of wastewater. A less popular drymilling method uses mechanical presses on the digested mash tosqueeze out the crude oil. In short, the crude palm oil extractionprocess starts with collection of the FFB from the oil palm planta-tions. In the palm oil processing mills, fresh bunches are deliveredinto horizontal sterilizers (commonly used in modern factoriescompared to the vertical sterilizers) or pressure vessels where a livesteam is applied against the fruits at approximately 100e140 �C for25e30 min (small bunches, 3e6 kg) or 50e75 min (larger bunches,17 kg) to cook the palm fruits. The reported pressure used in ster-ilizers was 35e45 psi. The primary objective of sterilizing the freshbunches is to deactivate and henceforth inhibit the enzyme activity(lipolytic enzymes) of palm fruits. The fat-splitting or lypolitic

alm oil processing mills in operation; B e The fresh fruit bunches processed by mills); E e Clarification wastewater (tonnes); F e Hydrocyclone wastewater (tonnes).

W.L. Liew et al. / Journal of Environmental Management 149 (2015) 222e235224

enzymes can result in the breaking down of oil into free fatty acids(FFA). A rise in FFA could lead to low oil yield as the fat-splittingenzymes would hydrolyse much of the oil during the fruit pulp-ing process. Other motives of sterilization include preparation offruit pericarp for subsequent processing and pre-conditioning thenuts to lessen kernel breakage during both pressing and nutcracking. Moreover, sterilization helps to smoothen the latter pro-cess of mechanical stripping/threshing to free the palm fruits fromthe bunches. Mechanical stripping is facilitated when sufficientmoist heat in the form of steam during sterilization can penetrate tothe points of attachment between the fruits and bunches, henceallowing hydrolysis at these points. These sterilized fruits are thensubsequently smashed to press out the crude palm oil. The oil isfurther treated in purifiers and vacuum dried for storage and export(Ab Rahman et al., 2011; Thanh et al., 1980; PORIM, 1985).

In the process of crude palm oil extraction, huge quantities ofwater is required and is typically obtained from the adjacentfreshwater resources, i.e., the rivers, which incurs very little treat-ment and pumping costs. A detailed palm oil milling process,describing the sources and quantities of water and its subsequentwastewater generation, as well as the products and by-productsalongside the milling processes is presented in Fig. 2. The millingprocess coupled with the information of water, wastewater, and by-products were results obtained from a survey of seven local palmoil mills. The factories involved are located in different locationsaround the Johore state, which are Kota Tinggi, Bukit Besar, Pen-ggeli, Paloh, Kahang, Segamat, and one factory in Seri Ulu Langat(Selangor state). To process 1 tonne (1000 kg) of FFB, about 1.50cubic meters of water source are extractedmainly for the operationof boilers and the hydrocyclone separator (DOE, 1999; Ma, 1999;Chavalparit et al., 2006). In Malaysia, all palm oil mills have at

Fig. 2. Typical palm oil milling processes in addition to the sources of water pol

least one boiler with steam generation capacity varying between15,000 and 30,000 Ib per hour. Water-tube (WT) boilers and fire-tube (FT) boilers are two common types used in local palm oilmills but the WT boilers are favoured due to higher steam gener-ation capacity (Rashid et al., 1998). The natural water is treated andmade suitable as boiler feedwater to prevent corrosion of the boilermetal, scale formation, foaming, and priming. In theWT boilers, theboiler feed water is evaporated into steam under the influence bysteam pressure, steam temperature, steam quality, and feed watertemperature. At a process throughput level of 60 tonnes FFB perhour, about 410e455 kg tonne�1 h�1 of steam is required (Cooper,1983). The hydrocyclone separators on the other hand use the flowof water to separate two components of different densities bycentrifugal force. Palm kernels have lower density compared to thepalm shells. Using a large quantity of water, the equipment sepa-rates wet kernels for further processing in the kernel silo and wetshells. The wet shells, combining with other palm biomasses (dryshells and fibres) are used as boiler fuels for fuel and electricitygeneration for the milling processes (Ma et al., 1993). An alternativeto the hydrocyclone separator is the more conventional clay bathseparator which requires lower power consumption and capitalcosts (PORIM, 1985; Ab Rahman et al., 2011).

In the milling processes, about 50% (0.75 cubic meters) of thewater source eventually became POME while the other 50% turnout to be usedwater. The usedwater is discharged into the drains orrivers without going neither into the effluent stream nor thewastewater treatment system. The generation of POME is illus-trated in Fig. 2 and the three main wastewater sources whichcombine tomake up the POME is represented as Source 1 (sterilizercondensate), Source 8 (clarification wastewater), and Source 10(hydrocyclone wastewater). Typical characteristics of this type of

lution and by-products generation from the processing of one tonne of FFB.


wastewater is shown in Table 1 (DOE, 1999; Khalid and WanMustafa, 1992; Ma et al., 1993; Rupani et al., 2010; Wu et al., 2010).

The sterilizer condensate (or sterilizer waste), wastewaterSource 1 in Fig. 2 resulted from the FFB sterilization process in ahorizontal sterilizer or horizontal cylindrical autoclaves (DOE,1999). Steam at 3 bars is admitted to the sterilizer with the pur-pose of air removal and to rapidly raise the temperature of the FFBto the optimum value. In a single peak sterilization cycle, theminimum steam consumption is 190 kg h�1 tonne�1 of FFB whilethe peak rate steam consumption is 500 kg h�1 tonne�1 of FFB. For amultiple peak sterilization cycle, the minimum steam consumptionis 240 kg h�1 tonne�1 of FFB and the peak steam consumption willbe themaximum that the steam system can support when pressureis being brought up after a blow-off (Whiting, 1983). Althoughthese figures might vary among the palm oil mills due to thevariation in type of sterilizers and the introduction of new tech-nologies would have lowered the steam consumption rate, thereport do deliver a guide to the consumption of steam by the palmoil mills. The sterilization process results in the production of ahuge amount of condensate which must be removed speedily toavoid flooding of the bearings of the cage bogies as well ascontamination of the sterilizer condensate with the hydrocarbonlubricant (Ab Rahman et al., 2011). The increasing level ofcondensate will have an effect on the contact between the fruitswith thewastewater and oil will bewashed out from the bunches inexcessive quantities. For every tonne of FFB processed, approxi-mately 0.27 cubic meters of sterilizer condensate is formed, or 36%of the total POME is made up of the sterilizer condensate (DOE,1999; Wu et al., 2010).

The largest portion of wastewater contributing to the POME(about 60%) is the clarification wastewater (or separator sludge)discharged from the process operation of clarification of theextracted crude palm oil, denoted as wastewater Source 8 in Fig. 2.Pulp press is used to extract crude palm oil from the digested palmfruits. However, the crude oil produced at this point is a mixture ofpalm oil (35e45%), water (45e55%), and impurities consisting ofeither soluble or insoluble vegetable matter in varying proportions(DOE,1999). Through themechanism of settling and centrifuging ina clarification tank, the clarified palm oil on top of the tank iscontinuously skimmed-off and further purified in a vacuum dryerprior to sending into the storage tanks. Meanwhile, the oily sludgesettles at the bottom and it is delivered to a sludge separator torecover the remaining oil (PORIM, 1985; Thanh et al., 1980). Theleft-over would be the sludge waste which is discharged into an oiltrap and further delivered in a waste stream to the wastewatertreatment system. It is reported that for every tonne of crude palmoil produced, approximately 1.5 tonnes of sludge waste consistingof water and fibrous debris is attained (Wu et al., 2010). In the

Table 1Characteristics of different sources of wastewater combining to produce the POME.

Parameter Sterilizercondensate

Clarificationwastewater

Hydrocyclonewastewater

pH 5.0 4.5 eb

Oil and grease 4000 7000 300Biochemical oxygen

demand (BOD)a23,000 29,000 5000

Chemical oxygendemand (COD)

47,000 64,000 15,000

Suspended solids 5000 23,000 7000Dissolved solids 34,000 22,000 100Total nitrogen 600 1200 100Ammoniacal-nitrogen 20 40 eb

All units in mg L�1 except pH.a Sample incubated for 3 days at 30 �C.b No values were reported for the parameter.

context of FFB processed, about 0.45 cubic meters of the clarifica-tionwastewater is produced for every tonne of FFB processed in thepalm oil mill. In comparison with another two wastewater sourceswhich make up the POME, the solids content in clarificationwastewater is much higher due to the presence of a higher pro-portion of both soluble and insoluble carbohydrate constituents inthe wastewater (Ho et al., 1984).

The thirdwastewater source (no.10 in Fig. 2) is the hydrocyclonewastewater which makes up around 4% of the POME (DOE, 1999;Wu et al., 2010). Residue (press cake) from the press machineconsists of a mixture of fibre and nuts. While the fibres will beseparated and sent to the boiler house, the remaining nuts are sentto the nut cracker and subsequently to the hydrocyclone separator(Lam and Lee, 2011). The process separates the cracked mixture ofkernel and empty shell based on their differences in specific gravity.Employing a separation medium of clay suspension or salt solutionwith a specific gravity of 1.12, the hydrocyclone can effectivelyseparate the kernels and the shells with specific gravity of 1.07 and1.15e1.25 respectively (DOE, 1999; PORIM, 1985). The generationrate of hydrocyclone wastewater is 0.03 cubic meters per tonne ofFFB processed.

Other than the three main waste streams, there are minorsources of relatively clean wastewater which constitute the POME.Since the volume is unpredictable and the effect is inconsequential,these wastewater sources are often neglected and very seldomreported. The quantity of this wastewater depends on the palm oilmill operation. A survey of the palm oil mills identified 9 additionalsources of wastewater generation, other than the typically reportedsterilizer condensate, clarification wastewater, and hydrocyclonewastewater. They are presented in Fig. 2 as wastewater source 2, 3,4, 5, 6, 7, 9, 11, and 12.

Wastewater Source 2 is generated as a result of floor washing inthe digester and pulp press operation site. In the digester, soft andstripped palm fruits are mashed by the central rotating shaft car-rying a number of mechanical arms. The high temperature rotatingmechanism loosen the fruit's outer covering from nuts beforedelivering the homogenized oil mash to squeeze out the crudepalm oil (Embrandiri et al., 2012; Lam and Lee, 2011). The producedcrude palm oil with entrained impurities requires oil separationand purification. This is done in the oil room where clarification ofthe crude oil occurs. Floor washing in the oil room contributes towastewater Source 3 (Embrandiri et al., 2012). Also in the oil room,the purification of crude oil produces a minor effluent which isdenoted by wastewater Source 4. The purification process is mainlyaimed to achieve a high degree of moisture removal (Chungsiripornet al., 2006; Embrandiri et al., 2012; Prasertsan and Prasertsan,1996). Vacuum drier further purifies the crude oil until it reachesmoisture and dirt content below 0.1 and 0.01% respectively (DOE,1999; Embrandiri et al., 2012). The occurrence of water over-flowing from the vacuum dryer machineries contributes to waste-water Source 5.

Purification of crude oil in the horizontal or vertical clarificationtank helps in maintaining oxidative stability besides preventinggums deposition (Igwe and Onyegbado, 2007). Besides the proce-dure of refinement of oil skimmed-off from the top of the clarifi-cation tank, the bottom phase processing contributes to somesources of wastewater. In the oil room, the bottom phase which isgenerally underflow sludge will go through oil recovery. Minorsources of effluent come from the strainer process (wastewaterSource 6) and the desander process (wastewater Source 7). Straineris a pre-treatment process for the underflow sludge whereby thesolid bits are removed. The diluted crude oil or sludge is hencetreated in desander for the purpose of sand removal prior tofeeding into the high speed centrifuge to achieve oil recovery(PORIM, 1985).


Aside the oil room, there are several sources of wastewatergeneration as denoted by wastewater Sources 11 and 12. Whilelarge quantities of water are used in the form of hot steam, thesteam turbine is an important section in the palm oil mill for steam(heat) generation as well as electricity as a by-product. Cooling theturbine generates a certain volume of wastewater to be treated inthe wastewater treatment system (Chungsiriporn et al., 2006). Theboiler too is found to contribute to some wastewater from thesteam condensate and boiler blow down. Besides, there are alsofloor washing in the premise at locations where empty fruitbunches (EFB) are delivered for screw pressing. This is denoted aswastewater Source 9 in Fig. 2.

Due to various sources of wastewater, the resulting POME hasfluctuating characteristics. The characterization of POME collectedfrom different batches or days from different mills too will givedifferent analytical results. Moreover, different oil extraction tech-niques, quality of the palm fruits, the factory requirement oneffluent discharge limit, climate as well as the condition of the palmoil processing are thinkable reasons for the dissimilarity. Differentcropping season of the oil palm andmill activities have also affectedthe quantity and quality of the POME produced (Wu et al., 2010).

The waste streams from the hot water and steam usually de-livers sterilized condensate, and the clarification wastewater aretypically segregated and collected in different oil pits for residualoil recovery (Khalid and Wan Mustafa, 1992). The recovered sludgeoil is poor in quality and will not be included in the production ofoil. Instead, these oils are drummed and sold as technical oils fornon-edible applications (Ma, 1991, 1999). After oil recovery, thetwo waste streams mix together with the hydrocyclone waste-water to form a mixed wastewater or the POME. In general, thePOME when fresh is hot, acidic, thick and viscous brownish or greyslurry. It has very high organic matter and is said to be 100 times aspolluting as the domestic sewage (Ma and Augustine Ong, 1985;Khalid and Wan Mustafa, 1992). POME is a colloidal mixture ofwater, oil, and fine suspended components. The suspended com-ponents are mainly vegetative matters like cell walls, organelles,short fibres, water-soluble carbohydrates ranging from hemi-celluloses to simple sugars (glucose, reducing sugars, and pectin),nitrogenous compounds (from proteins to amino acids), freeorganic acids, lipids, as well as the assembly of minor organic andmineral constituents (Edewor, 1986; Foo and Hameed, 2010; Ma,1991). Wu et al. (2009) further reported the presence of Pentose,which is a building unit of insoluble carbohydrate. The suspendedsolids in POME slurry are mainly cellulose matter mixed with smallportions of residue oil. The effluent is also reported to be non-toxicas the entire milling process does not use any chemical (Ma andAugustine Ong, 1985; Khalid and Wan Mustafa, 1992; Igwe andOnyegbado, 2007).

Nonetheless, the remaining 50% or about 0.75 cubic meters ofwater used in the milling of palm oil are used water which does notcombine in the POME effluent streams. Having quite a similarquantity with the POME, the used water could cause loss of water tothe environment, either to the drains, adjacent rivers of the palm oilmills, or through evaporation. Piping leakages and wash waters usedto flush the machineries are other sources of water loss to theenvironment. Indeed, a large portion of water are actually lost in theform of steam/vapour. Steam is a main form of energy used in thepalm oil mills (Sommart and Pipatmanomai, 2011). Most processesrely on the consumption of steams and hot water during crude palmoil extraction. Steam lost through exhaust gases from the sterilizerwas reported by Chavalparit et al. (2006) and DOE (1999).

With a large amount of water loss in the palm oil milling pro-cesses, several literature suggested ways to reduce water con-sumption and wastewater discharge from the factories. The palmoil industry can adopt source reduction, which is a highly desired

effort and the highest rank in the federal hierarchy of integratedsolid waste management (Tchobanoglous et al., 1993). In thecontext of water and wastewater, water conservation and mini-mization of effluent discharge are becoming highlighted issues,particularly in the palm oil industry. Among the driving forces to abetter water management are the increasingly stringent environ-mental regulations, the palm oil industry's commitment to achievesustainable palm oil, an intense scrutiny over the long-term healtheffects on human and nature, as well as the availability of cleanwater resources in the country is of paramount importance.Reducing the quantity of POME can reduce the delivery of thiswastewater to the treatment system to ease or lessen the cost of theultimate treatment (Tchobanoglous et al., 2004). Several optionsavailable for water conservation and wastewater reduction in thepalm oil industry include milling process modifications, house-keeping practices, and utilization or recycling of the POME.

Most palm oil milling process modifications were found toaccomplish maximum oil recovery or to recover as much oil aspossible because the higher content of oil and grease characteriseda higher strength of the wastewater (Schuchardt et al., 2007; Thanhet al., 1980). Thanh et al. (1980) reported a closed-systemwhich caneliminate the release of effluent from the oil clarification room. Adecanter as an extra unit between the vibrating screen and oilclarifier was used to separate the minor solid materials from thecrude palm oil. The settled oil sludge are delivered to the three-phase nozzle centrifuge for oil recovery, the recovered oil are sentback to the clarifier, the sludge will have land application while theliquid phase are recycled back to press and oil clarifier to make upthe dilution water. Thanh et al. (1980) also described the dry pro-cess where a wind silo is used to separate kernels from crackedshells by gravity. No water consumption is involved, but the vari-ation of electricity voltage occurred in mills resulted in high kernelloss. On another hand, Schuchardt et al. (2007) suggested theapplication of a new sterilization process which does not generatecondensate. The conventional autoclave sterilizer and a new oilrecovery process can produce 0.45 cubic meters of POME per tonneof FFB processed while the new sterilizer process and new oil re-covery process only produce 0.25 cubic meters of POME per tonneof FFB processed.

There are several in-plant control and housekeeping measuressuggested by the Malaysian Department of Environment to opti-mize water use (DOE, 1999). Regular maintenance to avoid leaks inwater pipelines and valves, skilled operation of the hydrocyclonesto reduce water usage, control of overflow in the press room andclarification room, as well as regular equipment maintenance toprevent excessive wash-down are some of the cleaner productionmeasures described (Thanh et al., 1980). Installation of the triggerrelease valve on the end of each hose is a cost effective approach inwater-saving. Palm oil mill staff should be well-informed about theimportance of water conservation. Simple practices like shutting offvalves, water taps, and water hoses immediately after use inaddition to ceasing of flushing out spillages of oil and remnants intodrains can avoid the profligacy of water (Thanh et al., 1980). Otherapproaches of CP include control of oil clarification temperature,proper design and operation of oil traps, separation of effluent andstorm water drainage systems, as well as proper interim storage ofsolid waste materials (DOE, 1999; Thanh et al., 1980). In Malaysia,the national average POME generation rate from palm oil mills are0.67e0.75 cubic meters per tonne of FFB processed (Ma, 1999; Nget al., 2011). If the factory is well-managed and good house-keeping practises are implemented, the amount can be auxiliaryreduced to only 0.5 cubic meters for the handling of one tonne ofFFB (DOE, 1999). The national average for Thailand on the otherhand is 0.60e0.89 cubicmeters of POME per tonne of FFB processed(Chavalparit et al., 2006; Chungsiriporn et al., 2006).

Table 2Parameter limits for watercourse discharge.

Parameters Limits according to periods of discharge

1-7-1978e30-6-1979 1-7-1979e30-6-1980 1-7-1980e30-6-1981 1-7-1981e30-6-1982 1-7-1982e31-12-1983 1-1-1984 andthereafter

Biochemical oxygendemand (BOD) 3 days,30 �C; mg L�1

5000 2000 1000 500 250 100

Chemical oxygen demand(COD); mg L�1

10,000 4000 2000 1000 eb eb

Total solids; mg L�1 4000 2500 2000 1500 eb eb

Suspended solids; mg L�1 1200 800 600 400 400 400Oil and grease; mg L�1 150 100 75 50 50 50Ammoniacal-nitrogen; mg L�1 25 15 15 10 150a 150a

Total nitrogen; mg L�1 200 100 75 50 300a 200a

pH 5.0e9.0 5.0e9.0 5.0e9.0 5.0e9.0 5.0e9.0 5.0e9.0Temperature, �C 45 45 45 45 45 45

a Value of filtered sample.b No values were reported for the parameter.


Another option is to utilize or recycle the POME, where theeffluent is seen as a cheap renewable residue rather than a kind ofwastes to be disposed (Salihu and Alam, 2012). Indeed, it is awasteful exercise to spend money to treat this high amount ofPOME followed by releasing the effluent without furtherconstructive applications. Treated effluent that is good in qualitycan be directly used as feed water for boiler or hydrocyclone(DOE, 1999; Loh et al., 2013). Turbines cooling water can be recir-culated in-plant while the steam condensates can be reused for thepurpose of processing, cleaning, and washing (Thanh et al., 1980). Astudy which recycles the sterilizer condensate and cooling water tothe processing unit has reduced to 65% freshwater consumptionand 67% wastewater generation. The clarification wastewater toowas channelled to the processing unit asmixingwater, the decanteras blending water, and to the separator as balancing water(Chungsiriporn et al., 2006). Besides, partially-treated POME whichcontained substantial quantities of valuable nutrients is useful forcropland application, particularly in fruits and vegetables' growth(Abdullah, 1992; DOE, 1999; Lim et al., 1991; Lim and Zaman, 1992;Salihu and Alam, 2012; Sulaiman et al., 2011). Hence, use of theinorganic fertilizers in the oil palm plantations or other croplandscan be reduced. Schuchardt et al. (2007) also suggested the utili-zation of EFB and POME in a combined-composting process whichcan fulfil the demand of sustainable palm oil production. The pro-cess involved biological drying of the POME to produce much ofcompost for agricultural applications.

Other than the information on liquid POME, Fig. 2 also presenteda mass balance for the processing of 1000 kg of palm fruits (Chan,1999; DOE, 1999; Ma et al., 1993; PORIM, 1985). The steam sterili-zation process results in 900 kg of sterilized FFB whereby 100 kgwere lost asmoisture and another 0.3 kgwere oil in the condensate.The subsequent stripping/threshing mechanism for fruit-stalkseparation can produce 666 kg of fruitlets and 234 kg of EFB.Indeed, the generation of EFB is represented as 23% of FFB pro-cessed in the palm oil mills (Ma, 1991; Prasertsan and Prasertsan,1996; Sridhar and AdeOluwa, 2009). The EFB contained 152 kg ofwater, 82 kg of non-oily solids, and oil of less than 1 kg. Under highpressure, the crude oil is extracted from the digested mash offruitlets by use of the screw presses. The crude oil slurry is fed to aclarification systemwhereby the mass balance denotes further useof 173 kg of diluted water in the extraction of crude palm oil of225 kg (approximately 20% from FFB) (Sridhar and AdeOluwa,2009). The bottom-phase or sludge waste is 180 kg. On the otherhand, the press cake is conveyed to a depericarper whereby presscake fibre of 180 kg and nuts are segregated. Press fibre cake

generation is about 13.5% of FFB processed and constitutes 74 kgwater, 97 kg non-oily solids, and 9 kg oil (Ma, 1991; Prasertsan andPrasertsan, 1996; Sridhar and AdeOluwa, 2009). Further processingof nuts will produce total shell of 73 kg (water 15 kg, non-oily solids57 kg, oil 1 kg) and total kernels of 67 kg (water 14 kg, non-oilysolids 19 kg, oil 34 kg). In terms of percentage of waste genera-tion in palm oil processing, shells and kernels are 5.5% and 6% of FFBprocessed respectively (Ma, 1991; Prasertsan and Prasertsan, 1996;Sridhar and AdeOluwa, 2009). Finally, potash ash generation isreported to be 0.5% of FFB (DOE, 1999).

1.3. Laws and legislations governing the industry

In the 1970s, exponential growth of the palm oil industryresulted in severe environmental problems. The Department ofEnvironment (DOE) is hence liable for implementing and enforcingenvironmental regulations against the industry. Among the regu-lations applied to palm oil industries' waste management practicesare:

(1) Environmental Quality (Prescribed Premises) (Crude PalmOil) Regulations, 1977

(2) Environmental Quality (Clean Air) Regulations, 1978(3) Environmental Quality (Scheduled Waste) Regulations, 2005

Among the listed regulations, the Environmental Quality (Pre-scribed Premises) (Crude Palm Oil) Regulations, 1977, is predomi-nantly discussed. The regulations had specified detailed provisionsto be complied. Most attention is drawn to the limits for parametersof effluent to be discharged into a watercourse or onto land. Anincreasingly stringent discharge limit into a watercourse is shownin Table 2 as in the Second Schedule, under Regulation 12(2) and12(3). For discharges onto land, the BOD3 value should not surpass5000 mg L�1 (Legal Research Board, 2008).

To progress towards a cleaner environment, the DOE is planningto revise a more stringent discharge limit to be imposed on effluentmanagement. As stated in Regulation 12(4) e for watercoursedischarge and 13(6) e for discharge onto land, the Director Generalcan impose a more stringent limit if it is necessary. Above andbeyond, there is an attempt to impose 20 mg L�1 BOD3 dischargelimit on crude POME. The scope of the discharge limit coversenvironmentally sensitive areas such as tourism areas of EastMalaysia (Sabah and Sarawak States), and those locations in closeproximity to water intake points. Since then, the 20 mg L�1

discharge limit concerns have been circulating among the


administrators, millers, and researchers, as it is a challenge to beaccomplished.

2. Liquid waste treatment

As the paper emphasizes POME tertiary/polishing treatment,typical or conventional effluent treatment systems adopted bypalm oil millers are briefly reported. Past review works on POMEtreatment technologies are divided into categories such as aerobicdigestion, anaerobic digestion, and physicochemical methods. Thispaper departs from typical treatment methods based on means ofeffluent discharge and hope to provide a different view for a betterappreciation and understanding of these well-established systems.

2.1. Discharge into inland watercourse

Types of wastewater treatment options selected are based onthe miller's preferences, the mill location, and land availability tocater for the wastewater treatment plant (Ma, 1999). Out of 416licensed prescribed premises, 267 mills have been given thepermission to discharge their liquid wastes into inland watercoursein the year 2009 (DOE, 2009). Normally, (1) the ponding system aswell as (2) the open tank digester and extended aeration systemeffluent are discharged into inland watercourse. The criteria lead-ing to the grant of licence for giving permission includes (LegalResearch Board, 2008):

a) Whether it would be practical to adapt the existing equip-ment, control equipment or industrial plant to conform togiven conditions;

b) The economic life of the existing equipment, control equip-ment or industrial plant, having regard to the date ofpurchase;

c) The quantity or degree of cut-back of emission, discharge ordeposit of wastes to be achieved;

d) The estimated cost to be incurred by the licensee to complyto the given conditions; and

e) The nature and size of the trade, process or industry beingcarried out in the premises.

The specification of the characteristics of these wastes is as lis-ted in Table 2 (Legal Research Board, 2008).

The ponding system is the most popular method adopted bymore than 85% of palm oil mills (Ma, 1999). Ponding in generalincludes wastes stabilization lagoons (ponds) and oxidation ponds.Oxidation ponds on the other hand can be loosely categorized asaerobic, facultative, and maturation ponds, otherwise as facultativeponds when oxygen is deficient (Wong, 1980). The ponding systemnormally includes sand and oil traps, cooling ponds, acidificationponds, anaerobic ponds, facultative ponds, and aerobic ponds (indescending order). Manually operated sand and oil traps are pre-treatment unit operations. In cooling ponds, the raw POME iscooled down to lower than 35 �C prior to feeding into subsequentponds (Thanh et al., 1980). Aerobic ponds are usually constructedup to 1e1.5 m deep while the anaerobic ponds are usually 5e7 mdeep, both in earth structure without lining. The organic loadingrate differs from 0.2 to 0.35 kg BOD m�3 day�1 (Ma, 1999). Anaer-obic ponding can digest high amount of solids and is inexpensive,but requires long retention times and a huge land area, with solidsaccumulation frequently reported (Hojjat et al., 2009). Solidsreaching 4.8 kg volatile solids m�3 day�1 can be digested inanaerobic ponds (Khalid andWanMustafa, 1992). Retention time is20 days in anaerobic ponds (Yacob et al., 2006a). In due course ofoperation, solid sludge will accumulate at the bottom of the pondwhere desludging/desilting is eventually required. In brief, theponding system entails low maintenance, it is economical, it offers

process and operational simplicity, and is a feasible means oftreating high strength organic wastewater. However, a huge landarea (1 hae5 ha) and long hydraulic retention times (HRT 40e200days) are required (Thanh et al., 1980; Wong, 1980). In terms ofdimension, a typical size of an anaerobic pond is60.0� 29.6� 5.8 m (length�width� depth), taking a palm oil millwhich has a processing capacity of 54 tonnes per hour as anexample. Size of pond depends on the capacity of the palm oil millas well as the area available for ponds (Yacob et al., 2006a). Solidsaccumulated may reach 31,500 mg L�1 before ponding begins, butare removed during the process by suction pumps. After thedecanter-dryer and ponding treatment, about 130 mg L�1 could bereached at the final discharge (Ma and Ong, 1985).

Another common system applied is the conventional anaero-biceaerobic system (open tank digestion and extended aeration)(Chan et al., 2010). The two-phased anaerobic digestion consists ofmild steel-made digesters which are open at the top and leftunstirred. POMEwith organic loading rates from 0.8 to 1.0 BODm�3

day�1 undergoes approximately 20 days of HRT in the anaerobicdigesters. Open tank digesters are capable of removing chemicaloxygen demand (COD) by 81% (Yacob et al., 2006b). Subsequently,the effluent is further treated in extended aeration ponds foradditional reduction in COD and BOD. The HRT of the extendedaeration pond is 10 days (Ugoji, 1997). Mechanical surface aeratorsare used to supply air to the treatment plant with HRT of about 40days (Ma, 1999; Chan et al., 2010). Supernatant is then dischargedinto a close by watercourse.

2.2. Land disposal

The secondmost favoured POME dischargemanner is accountedfor 96 licensed prescribed premises that practiced land disposal(DOE, 2011). Supernatant from the (1) ponding system (typicallyanaerobic ponds) or (2) treatment ponds in the decanter-driersystem, (3) stirred-tank digested POME, as well as (4) aerobic and(5) anaerobic digester bottom sludge are sources of POME utilizedfor controlled land application techniques (Ma, 1991, 1999; Zakariaet al., 2000). Through pond digestion processes, the nutrient con-tent particularly nitrogen and potassium will go down throughleaching and settling of sludge solids at the pond underneath. Incontrast, tank digestionwith agitation and stirring of effluent causenegligible effects to nutrient content. This mechanism breaks downcomplex organic solids for enhancing nutrient uptakes by plants(Zakaria et al., 1995).

In general, closed tank digesters coupled with biogas recoveryfacilities are associated with land application (Ma, 1999). In com-parison with open tank anaerobic digesters, closed tank anaerobicdigesters are more efficient in removing COD (>95%) at a lower HRTof 17 days (Yacob et al., 2006b). Closed systems also generate bio-gases which are eventually either converted to electricity in abiogas engine (Loh et al., 2013) or burned into gas flare. In somecases, biogas can be used as fuel in modified diesel engines withinduction motors to produce electricity (Puetpaiboon andChotwattanasak, 2004). On the other hand, effluent from an opentank digester and extended aeration discussed in the precedingsession are also discarded to land application, mainly for irrigation.The raw or partially treated POME is applied to land either bydischarging to overland flow or applying directly for irrigation(Thanh et al., 1980). Commercial scales of land application systemshave been highlighted (Zakaria et al., 1995; DOE, 1999; Lim et al.,1999) as below and their corresponding advantages and disad-vantages are shown in Table 3:

� Sprinkler/pipe irrigation system,� Furrow/gravity flow system,

Table 3Advantages and disadvantages of various types of land application of POME.

Types of landapplication

Advantages Disadvantages

Sprinkler/pipeirrigationsystem

Can reach higher planesand undulating area

It often requiresexceedingly long sectionsof drip line

High energy, capital andmaintenance costs

Large areas may requirethe installation of a waterpump or additionalpiping to maintainenough pressurethroughout the drip linesystem

Suitable for large expanseof landSuitable to all types of soilexcept heavy clayElimination of thechannels for conveyance,therefore no conveyanceloss

Furrow/gravityflow system

Flow rate is dependent onthe size of furrow and aretypically dictated bysiphons or bank cuts

The efficiency of furrowirrigation is generally<65%

Most suited to clay soilswhere the potential forleaching is far less

It has a higher risk ofhuman exposure andlabour requirement

Low energy use It cannot be used on steepland

Tractor/tanker/pumpsystem

It has a simple design Requires more energy foroperation of centrifugalpumpmounted on tractor

It has no moving parts High investment cost isrequired

Cathodic protectionsystems are an economicalternative to periodicrepainting and theassociated downtime forrepair

Corrosion may occurinside the tanker

Flatbed system This consists of series ofshallow bunded-beds ofabout 15 cm depth

Cost for provision ofdrainage

It is good for a flat terrain Recirculation system isrequired for reuse ofwastewater

It can be used for soilsthat do not disperseeasilyA large supply ofwastewater can behandled easily

Longbed system It is similar to flatbedexcept that each bed maybe as much as 70 m inlength

Cost for provision ofdrainage

It is good for a flat terrain Recirculation system isrequired for reuse ofwastewater

It can be used for soilsthat do not disperseeasily


� Tractor/tanker/pump system,� Longbed system, and� Flatbed system.

With regard to environmental concern, research has publicizedthat land application of POME at controlled loading rates hasinsignificant impact on soil and water resources (Zakaria et al.,

1995). Although POME is highly polluting, no toxicity is reporteddue to zero chemical addition throughout the wet milling process.POME nutrients andminerals could be accumulated by carrying outa stand-alone or off palm oil mills wastemanagement strategy suchas evaporation. The concentrated effluent is rich in plant nutrients,especially nitrogen (950 mg L�1), phosphorous (150 mg L�1) andpotassium (1960 mg L�1) (DOE, 1999). It is therefore a good rawmaterial for making fertilizer (Panda, 2013). Research also hasrevealed that no heavy metal accumulation has been detected inthe soil, water and the crop in long-term POME receiving lands(DOE, 1999).

2.3. Composting

According to DOE (2011), 12 licensed prescribed premises inMalaysia were granted the permission to practice solitary com-posting. In most cases, anaerobic liquor from anaerobic ponds, andanaerobic sludge from open or closed anaerobic digested tanks areused for composting treatment at field scale. EFB are pressed andshredded upon collection for crude palm oil recovery. The pressed-shredded EFB are brownish in colour with a length size of15e20 cm. EFB act as the main carbon source for the compostingtreatment due to its high cellulose and hemicelluloses content. Bypercentage of EFB, cellulose and hemicelluloses were 52.8 and14.8%, respectively (Baharuddin et al., 2010); while the anaerobicPOME sludge is a nutrient source reported for composting withhigh concentrations of nitrogen (3600 mg L�1), phosphorus(1200 mg L�1), potassium (2400 mg L�1), and magnesium(1500 mg L�1) (Ma, 1991).

The thickened anaerobic POME sludge from the bottom part ofthe clarifier tank is used for composting treatment. The total POMEanaerobic sludge added into the EFB compost throughout theprocess is about one tonne (1:1 ratio) (Baharuddin et al., 2010). Acompleted composting process using shredded EFB and partiallytreated POME from an open anaerobic pond takes 80 days with afinal C/N ratio of 12.5. However, the characteristics of maturedcompost are reported to show some discrepancy due to variation ofpartially treated POME (Baharuddin et al., 2009). A more constantmicrobial seeding and nitrogen source from closed tank anaerobicPOME sludge is hence favoured with 40 days of composting periodand a final C/N ratio of 12.4 (Baharuddin et al., 2010). To cater forthe concern on groundwater and nearby freshwater resourcespollution, several mills are facilitated with leachate collectiondrains where run-over POME were channelled back to anaerobicponds or digester tanks for further treatment.

2.4. Other methods

Above and beyond the three key distinctive disposal approach asreported above, other mills' practised subsequent disposal schemes(DOE, 2011):

� Discharge into inland watercourse and land disposal (32licensed prescribed premises)

� Discharge into inland watercourse and composting (3 licensedprescribed premises)

These mills applied a combination of disposal methods. Withsuch combination of the disposal processes, it offers process flexi-bility and perhaps better control of each of the treatment unit op-erations. Whatever treatment options are in operation, records ofenforcement have shown that the Malaysian palm oil industry isgenerally complying with the prevailing national regulation of100 mg L�1 BOD3 for inland watercourse disposal and 5000 mg L�1

BOD3 for land disposal. Successful pollution abatement and


emerging demand to fulfil the local requirement for better envi-ronmental protection has urged the DOE to further revise the na-tional regulation to a more stringent BOD3 discharge limit.

3. POME polishing/tertiary treatment technologies

As satisfying results are shown by the existing POME primary/secondary treatment technologies, POME polishing systems pullattention to further remove organic matters, total suspended solids,and colour reduction. Since the 20 mg L�1 BOD3 discharge limitcame into enforcement, most polishing technologies are foundincapable of performing consistently and continuously. Theenforcement record is showing 95.5% compliance and in order toattain 100% compliance, the DOE discharge standard may need tobe reviewed (DOE, 2011). Total suspended solids are frequentlyregarded as contributing to high BOD in the effluent. As such, at-tempts to filter out suspended solids through application ofmembrane separation technologies are rapidlymade, optimisticallyto reduce BOD and suspended solids concomitantly in the finaldischarge. On the other hand, colour removal is attaining

Table 4Summary of technologies/systems in treatability studies for POME tertiary treatment/po

Technologies/systems Description Removal efficie

Membrane filtration processes Pre-treatment e chemicalcoagulation (Envifloc 40 L, 0.05v/v), flocculation (Envifloc 20S,0.015 v/v), sedimentation, andadsorption; followed bymembrane processes eultrafiltration (UF) and reverseosmosis (RO)

AN 99%; BOD3

Colour 100%; O100%; Turbidityrecovery 78%.

Membrane filtration processes Ultrafiltration membraneprocess with hollow fibremembrane configuration; madeby polyethersulphone material

AN 62%; BOD5 998%; TKN 54%;

Advanced Oxidation Processes(AOPs)

Technology to produceHydroxyl radicals (HO*) eHydrogen Peroxide/UV (H2O2/UV)

COD 7e61%; Co

Advanced Oxidation Processes(AOPs)

Technology to produceHydroxyl radicals (HO*) eFenton oxidation process(1) Ambient Fenton e insidelaboratory under ambient light(2) Solar Fenton e open spacedirectly under sunlight

Ambient FentoColour 92%.Solar Fentone

95%.

Hybrid membrane bioreactorsystem

Phase 1: anaerobic e activatedsludge process; phase 2: anoxice activated sludge process;phase 3: aerobic (submerged-membrane).

COD 94%; SS 9896%; TP 64%.

Sequencing batch reactor (SBR),with suspended activatedsludge (SAS)

96 h SBR operation (Fill 5 min,React 95 h, Settle 15 min,Decant 10 min).

COD 27e39%; C

Sequencing batch reactor (SBR),with activated sludge e

granular activated carbon(ASGAC)

96 h SBR operation (fill 5 min,react 95 h, settle 15 min, decant10 min).

COD 59e70%; C

Sequencing batch reactor (SBR) 22 h SBR operation (fill, react20 h, settle 2 h, decant).

BOD3 97e98%;98e99%; SVI 65

Membrane filtration processes Pre-treatment consists ofchemical coagulation andflocculants aid (FeCl3 andC3H5NO), adsorption, followedby ultrafiltration membraneprocess.

COD >93%; ColTurbidity >99%

Abbreviations: AN e Ammoniacal-Nitrogen; BOD3 e Biochemical Oxygen Demand (3-dayOxygen Demand; OG e Oil and Grease; SS e Suspended Solids; SVI e Sludge Volume IndeTP e Total Phosphorus.

consideration as the final effluent is still in dark brownish colour.From an aesthetic point of view, colour treatment is receivingconsiderable attention particularly in tourism and environmentallysensitive areas.

Tables 4 and 5 present a synopsis of POME polishing/tertiarytreatment technologies. Such attempt to gather the sporadic POMEpolishing/tertiary treatment research for the first time may none-theless offer the palm oil industry an insight for better effluentmanagement.

Treatability studies to polish POME accentuates on producinghigher quality effluent while full scale polishing plants emphasizethe discharge limit compliance. As most of the technologies intreatability studies are more advanced and costly, they are seen toprogress towards water reclamation and recycling. Membranetechnology is frequently reported to produce a higher qualitytreated effluent capable of recycling boiler feed water and to evenbe reclaimed as drinking water. Nik Sulaiman and Chea (2004)applied ultrafiltration (UF) with various membranes of molecularweight cut-off (MWCO) to polish biologically treated POME. Thestudy suggested that membrane with higher MWCO produced

lishing.

ncies Organicloadingrates

Hydraulicretentiontimes

References

99%; COD 99%;dour 100%; OG100%; Water

N/A N/A Ahmad et al., 2003, 2006

7%; COD 98%; SSTurbidity 80%.

N/A N/A Nik Sulaiman and Chea, 2004

lour 7e61%. N/A N/A Abdullah, 2008

n e COD 75%;

COD 82%; Colour

N/A N/A Aris et al., 2008

%; TN 83%; TOC 1.77e1.87 kgCOD m�3 d�1

22 h Ahmad et al., 2009

olour 0%e14%. N/A 96 h Zahrim et al., 2009

olour 28e41%. N/A 96 h Zahrim et al., 2009

COD 95e96%; SSmL g�1

1.8e4.2 kgCOD m�3 d�1

20 h Chan et al., 2010

our >99%;.

N/A N/A Idris et al., 2010

s @ 30 �C); BOD5 e Biochemical Oxygen Demand (5-days @ 20 �C); COD e Chemicalx; TKN e Total Kjeldahl Nitrogen; TN e Total Nitrogen; TOC e Total Organic Carbon;

Table 5Summary of full scale technologies/systems for POME tertiary treatment/polishing.

Technologiesa Description Removal efficiency References

Membrane bioreactortechnology (MBR)

Phase 1 e Anoxic sector(activated sludge process);phase 2 e aerobic sector(activated sludge process); andphase 3 e ultrafiltrationmembrane process.

BOD3 99%; COD 98%; SS 99%. Sulong et al., 2007

Biologicalephysicochemicaltreatment processes

Phase 1 e biological treatment(aerobic suspended growthprocesses e activated sludgeprocess); phase 2 e chemicaltreatment (chemicalcoagulationeflocculation); andphase 3 e physical treatment(screening and sedimentation).

Ammonia, BOD3, colour, andresidual SS removal.

Sulong and Abdul Wahab, 2008

Sequencing batch reactor (SBR)e constructed wetlandsystem

Phase 1 e sequencing batchreactor process for extendedaeration; and phase 2 e

constructed wetland systemwith aquatic processing units(Cat Tails plant).

BOD3 < 20 mg L�1 Chong and Tan, 2010

Suspended packing in activatedsludge aeration tank, withcomplete mixing

Phase 1 e clarifier and hencegradual acclimatization ofultra-aerobic microbes incompletely mixed activatedsludge reactor; phase 2 e

extended aeration of suspendedpacking; phase 3 e clarifier;and phase 4 e post-treatment(physicochemical treatment).

BOD3 < 20 mg L�1 Tin, 2010

Extended aeration, coupledwith fixed packing inactivated sludge aerationtank

Phase 1 e extended aerationprocess followed by clarifier;phase 2 e placement of packingmaterials in support racks(activated sludge reactor);phase 3 e clarifier withreturned activated sludge(RAS); and phase 4 e polymerdosing.

BOD3 < 20 mg L�1 Idris, 2010

Biologicalephysicochemicaltreatment processes

Phase 1 e activated sludgesystem (activated sludgereactor) with extended aerationprocess; phase 2 e chemicalcoagulation and flocculation;and phase 3 e filtrationprocess.

BOD3 96%; SS 20%. Chong, 2010

Aerobic suspended andsubmerged attached growthbiological treatmentprocesses

Phase 1 e extended aerationprocess in activated sludgesystem; phase 2 e clarifier;phase 3 e aerated submergedfixed bed reactor; and phase 4e sand filtration process.

BOD3 < 20 mg L�1 Mohd Siran, 2010

Aerobic suspended andattached growth biologicaltreatment processes

Phase 1 e extended aerationprocess in activated sludgesystem; phase 2 e clarifier;phase 3 e fixed bed reactor;and phase 4 e clarifier and sandfiltration process.

BOD3 50e75%. Lu, 2010

Combination of ozone andsubmerged fixed filmbiological process

Phase 1 e polymer/dissolvedair floatation (polymer/DAF)process; phase 2 e ozonesystem; phase 3 e submergedfixed film biological reactorwith bio-media; and phase 4 e

sand and carbon filter.

BOD3 90%; sludge removal 50%. Jurgensen, 2010

Attached growth - roughingfilter solid contact (RFSC)technology

Phase 1 e roughing filtertower; phase 2 e solid contacttank; and phase 3 e clarifier.

BOD3 < 20 mg L�1; organics75%.

Shahrudin, 2010

Membrane bioreactortechnology (MBR)

Phase 1 e anaerobic submergedMBR coupled with biogasrecovery; and phase 2 e

aerobic submerged MBR.

BOD3 99% (approximate) Moro, 2010

Physicochemical treatmentprocesses

Phase 1 e chemicalcoagulation; phase 2 e

flocculation; and phase 3 e flocparticles separation.

BOD3 < 20 mg L�1 Barr, 2010

Abbreviations: BOD3 e Biochemical Oxygen Demand (3-days @ 30 �C); COD e Chemical Oxygen Demand; SS e Suspended Solids.a Substitution of commercial names to more general technology/system terms are done referring to Tchobanoglous et al. (2004).


higher fluxes but lower MWCO produced better quality permeate.However, colour removal was ineffectivewhen applyingmembranetechnology alone after the conventional ponding system. Findingsfrom a more recent study by Idris et al. (2010) suggested that pre-treatment of biologically-treated POME before tertiary treatmentusing UF membrane resulted in better permeate quality.Biologically-treated POME from ponding systems was subjected tophysical pre-treatment processes, namely coagulation andadsorption. Both processes were commonly applied to reduce SSfrom effluent. Permeate from UF membrane was found successfulin COD, colour, and turbidity reduction.

Besides applying membrane technology in biologically-treatedPOME, the technology was also attempted on raw effluent(Ahmad et al., 2003, 2006). Raw POME was subjected to chemicaland physical pre-treatment, consisting of coagulation (ferric chlo-ride) and flocculation aid (acrylamide), sedimentation, andadsorption using activated carbon. Effluent was further treatedusing UF membrane and reverse osmosis (RO) membrane. Thetechnology was successful in producing high quality effluent forwater recycling. All reported cases above recommended thatmembrane technology is practical but only after it is integratedwith pre-treatment processes. When more unit processes areincluded, the desired effluent quality is reachable but the criticalquestionwill be the costs required. Palm oil processing is a low costactivity and the applicability of a higher cost waste processingsystem will be unconvincing to palm oil millers. Moreover, opera-tions of membrane technology often have fouling issues and fluxreduction phenomenon. These problems can be effectively over-come by using pre-treatment such as coagulation, adsorption, andpre-oxidation. In addition, modest operation methods such asrunning modes, rinsing modes, chemical cleaning, and air scouringcan also achieve the desired goal (Goa et al., 2011).

Another technology attempted for tertiary treatment of POMEwas advanced oxidation processes (AOPs). The technology is basi-cally chemical treatment using the highly reactive hydroxyl radicals(OH�). A study conducted by Aris et al. (2008) applied Fentonoxidation to polish biologically treated POME. Two processes usedwere ambient-Fenton and solar-Fenton. The better performedsolar-Fenton used oxidation mechanism and resulted in rapiddegradation of organic compounds in POME which subsequentlylead to colour reduction in the wastewater. Another study reportedby Abdullah (2008) used the hydrogen peroxide photolysis method.In short, solar-Fenton performedwell in colour reduction comparedto COD whereas hydrogen peroxide photolysis performed better inCOD reduction compared to colour. Both studies were still atlaboratory-scale and important operating conditions such as reac-tion time, optimum pH and light intensity, dosage, as well asworking volumes are subjected to further detailed exploration.Nonetheless, the viable results on COD and colour removal offeredinsights on the feasibility of AOPs as one of the POME polishingtechnologies.

Other than physical and chemical approaches, biologicalmethods were also attempted to polish biologically-treated POME.POME from the conventional ponding systems had gone throughactivated sludge processes in anaerobic, anoxic, and aerobic con-ditions. Biological polishing systems applied more advanced bio-logical processes such as bioreactor systems, attached growthsystems, and granular sludge technologies. Zahrim et al. (2009)applied suspended activated sludge system in sequencing batchreactor (SBR) to polish anaerobically-treated POME. Colour removalwas attained at about 0e14% while organic matter removal was atabout 27e39% only. The study later applied an attached growthsystemwhere granular activated carbon was seeded with activatedsludge in an SBR. The system was described to remove colour atabout 28e41% while organic matter removal was at about 59e70%.

High percentage of colour and COD removal in the latter systemwas reported due to the adsorption of colour compounds into thegranular activated carbon while biofilm growth on the surfacefurther oxidise the colour compounds. Biodegradation andadsorption mechanisms contributed 50% each in recalcitrantpollutant removal in this case. Chan et al. (2010) similarly appliedSBR for the post-treatment of anaerobically-treated POME. Thetechnology was positively applied (at laboratory scale) to accom-plish significant COD, BOD, and SS removal due to the factors likehigh biomass acclimation during bioreactor start-up, high retentionof mixed liquor volatile suspended solids (MLVSS), and good set-tleability of activated flocs.

In addition to independent biological processes, a more recentand popular technology to integrate biological and physical sepa-ration was frequently reported in POME tertiary treatment. Forinstance, Ahmad et al. (2009) evaluated a hybrid membranebioreactor (MBR) system to sequentially treat raw POME in a seriesof activated sludge processes of anaerobic, anoxic, and aerobic re-actors followed by membrane separation in the aerobic zone.However, the membrane system experienced fouling in long-termoperation and membrane cleansing did not restore the initialpermeability. On the whole, the membrane technology is stillcapable of treating POME significantly and produce a good qualityeffluent.

Essentially, almost all kind of wastewater treatment technolo-gies established have been attempted to be applied in POMEtreatment and polishing treatability studies (either laboratory orpilot-scales). Physicochemical and biological processes are per-forming satisfactorily to manage POME. The key issues are still theapplicability and practicality of these technologies after consideringseveral factors such as cost-effectiveness, operational consistency,and system sustainability if operated as full-scale treatment plantsin palm oil mills in the future. Furthermore, POME's unique char-acteristic as organic, non-toxic, highly coloured and existence ofrecalcitrant organic compounds in the wastewater are crucial fea-tures to be considered in order to implement the full-scale tech-nologies beyond lab-developed researches. Extensive research isneeded to narrow down the practicality gap between treatabilitystudies and full-scale treatment plants. Among some attention-grabbing issues could be, but not limited to, are scale-up studies,operational troubleshooting in full-scale treatment plants, funda-mental wastewater and technological researches to gain betterunderstanding of unit processes, process optimization, as well asoperational flaws identification.

Compared to treatability studies, full-scale treatment plants aslisted in Table 5 are more matured tertiary treatment technologiesready for commercialization. While no previous reports or sum-maries of all the commercially available tertiary treatment tech-nologies in Malaysia were found, information of the technologiesreported in this paper were collectively taken from notes andpresentations of seminars andworkshops, as well as through verbalsurvey to the participated palm oil millers and sporadic internetresources. Also, these technologies reported are offered by thetechnology providers, no information and laboratory data areavailable however, on the system performance. The purpose of theassortment of polishing technologies here is to provide importantand first-hand information on available options, typical processesused, and appreciation of these readily-available technologies as away towards better management of the POME.

The use of MBR has been reported by Sulong et al. (2007). Themembrane installation is aimed to replace the secondary clarifierafter biological treatment in anoxic and aerobic activated sludgeprocesses. Among the benefits of membrane instead of clarifier aresmaller footprint, the ability for high solids concentration reten-tion, and producing effluent appropriate for reuse in the palm oil


mills. The cost reported is about RM 2.3 million for a typical 40tonnes FFB h�1 mill. Other MBR offer was reported by Moro (2010)where submerged membrane was installed in both anaerobic andaerobic tanks. Besides, a biologicalephysicochemical treatmentprocess technology was also disclosed (Sulong and Abdul Wahab,2008). The combined and compact biologicalechemicalephysicaltreatment process offered SS, ammonia, and colour reductionthrough its odour-free treatment to produce treated effluentaccomplished for water reuse. A small footprint plant (only 40%compared to conventional activated sludge processes) producinghigh quality and reusable effluent is probable at the costs of aboutRM 1.4 million for a 30 tonnes FFB h�1 mill.

The POME tertiary treatment (polishing) technology is expectedto be consistent, sustainable, and inexpensive. Most systemsoffered by the technology providers ranged in costs of between RM1.5 to 2.0 million. To cater to the criteria of sustainability, chemicaladdition or processes which required high energy consumption arehence unfavourable. Resource recovery of the palm oil milling by-products as well as water reclamation is usually incorporated as amarketing strategy in the commercial polishing systems. Thecriteria of providing a system with consistent performance are theremaining concern to address. Enquiries made to the MalaysianPalm Oil Board (MPOB) were able to inform that several technol-ogies for technological transfer from pilot tests to actual palm oilmills installation are still ongoing and will be published whencompleted.

The POME is generally an organic waste, where presence ofrecalcitrant organic compounds makes the effluent obstinate totreat. Thus, most full-scale treatment technologies are actuallyadvanced biological processes or biological-based technology in-tegrated with chemical or physical treatments. For instance, anSBR-constructed wetland was introduced by Chong and Tan (2010)to bring the BOD3 of treated effluent below 20 mg L�1 and as apotential system for water recycling. Partially-treated effluent fromanaerobic digestionwith BOD:N:P ratio of 100:46:4.0 (20 days HRT)entered the SBRs. SBRs represented secondary treatment and theprocess is similar in concept with activated sludge system. Treatedeffluent was reported to contain approximately 27 mg L�1 of BOD3and 213 mg L�1 of SS. The effluent then entered an aquatic pro-cessing unit containing cattails plant. The system was a form ofconstructed wetland using the sub-surface flow system as a finalpolishing stage. Final discharge was recorded at 17 mg L�1 BOD3and 60mg L�1 SS. A system of this kind is a full biological treatment,where no chemical addition is required and power requirementonly occurred for aeration purposes in the SBRs. Other full biolog-ical systems are briefly described in Table 5 (Tin, 2010; Idris, 2010;Mohd Siran, 2010; Lu, 2010; Shahrudin, 2010).

Besides full biological systems, Chong (2010) introduced a ter-tiary treatment system to be incorporated with the conventionalponding system. The biological system was applied, followed bychemical coagulation (400e700mg L�1), flocculation (4e7mg L�1),and physical treatment (sedimentation). Also the addition of lacticacid bacteria, yeast and photosynthetic bacteria during the bio-logical phase was reported. The final stage was a multi-mediafiltration treatment to reduce SS in the final effluent. Bacteriadecomposed both organic and inorganic materials in addition toreducing the organic matter to a soluble state, henceforth turninginto food sources for other microbes. Enzymes catalyst secreted bythe microbes will break down complex organic material intosmaller substances. Further oxidation and purification processesensure a stabilized effluent suitable for final polishing in the sub-sequent chemical and filtration treatment. Jurgensen (2010) alsoreported a combined biologicalechemical technology to treatPOME. The working principle is laid on application of ozone tech-nology as pre-treatment to the submerged fixed-film bioreactor.

Ozone is an important oxidizer used to break down the hardlybiodegradable organic matters in POME. Once the organic com-pounds were oxidized, the left-over organic matter was readilybiodegradable in the bioreactor with an active biofilm surface areaof about 300 m2 m�3. The only technology reported without anybiological component was described by Barr (2010). The technologycomprised of 4 phases, namely coagulation, flocculation, separa-tion, and discharge. An important feature of the technology was inthe control of the sludge settleability. With the addition of aspecifically-tailored coagulant, agglomeration of the destabilizedparticles occurred rapidly to form flocs. An efficient separationprocess holds the key to pollution abatement, where sludge wereseen settled at the bottom of the tank in a recorded time of only3 min. The technology was described to be able to contribute to thefinal treated POME of 20 mg L�1 BOD3.

In a glance, suspended and attached growths advanced bio-logical processes are principally employed in full-scale waste-water treatment plants due to the organic-based wastewatercharacteristics. Attached growth systems are usually favouredbecause the system demonstrates higher organic loading capa-bilities, hence increasing the potential for treating high organicwaste loads per unit volume. Higher density of bacterial popula-tion, better tolerance to process upset, smaller footprint, lessexpensive, and quick start-up are other advantageous features ofthe system in comparison with suspended growth processes(Steiner, 2000). Membrane filtration processes give very clearcolour effluent, and are usually integrated for water reclamationand reuse. Moreover, physicochemical treatment approaches areallied with colour removal and bio-solids/sludge reduction in thefinal effluent.

Efforts for better environmental management of the Malaysianpalm oil industry have been consecutively executed over the de-cades. It is known that the ponding system used for POME treat-ment is not sufficient, thus various researches have been conductedand almost all kinds of technologies have been reported to treatPOME. Surprisingly, the new BOD3 20 mg L�1 discharge limit is sochallenging to the industry in spite of all the technologiesattempted. The major research issue is to meet the 20 mg L�1 BOD3discharge standard consistently and this has been a challenge withthe amount of research reported on treatment of POME. Therefore,BOD, SS, and colour are still parameters to be removed and are is-sues before an attempt to reach zero discharge is achieved.

High or low processing of FFB seasons in a year produceddifferent loadings of incoming POME into the polishing plants.Incompetent operation and maintenance worsen the situationwhere some technologies were unable to perform optimally.Sludge removal and subsequent beneficial application of thesebiomasses should be granted more consideration. Colour causingcompounds in the wastewater are known. Whether applyingspecific microbial agent can break down these organic compoundsis a challenge. Apparently some plant constituents such as caro-tene, phenolic compounds, polyphenols, tannin and lignin, ormelanoidin are sources of the colour in POME (Neoh et al., 2013;Oswal et al., 2002; Limkhuansuwan and Chaiprasert, 2010).Another point in support of the argument is, it is noticed that mostPOME research nowadays are referring to old information such asthe milling process and wastewater characteristics. As the in-dustry progress vigorously over the years, many factors should beinvestigated in order to get further insights of POMEmanagement.Introduction of hybrid palm fruits and several other new species,improved milling processes due to technological advances, as wellas water resources used in palm oil mills (supplied by watercompanies, self-treated, or recycled) could be clues to new evi-dence on the exploration and innovation for a breakthrough inpalm oil wastewater research.


4. Conclusions

Intensive and dynamic research on POME pollution abatementhas paid off efficaciously, and the Malaysian palm oil industry isleading the trend towards sustainable palm oil production. As aroadmap to palm oil zero emissions, POME polishing should beaccomplished either for water reclamation, reuse, or absolutepollution abatement. Apart from the recognised problems such asnot being cost-effective and system inconsistency, a polishing/ter-tiary treatment technology which exhibits reliable, sustainable andaffordable features is most awaited. The technology is expected tobe able to comply with the new 20 mg L�1 BOD3 discharge limit.Unsatisfactory performance of the wastewater treatment plant isoften due to the lack of competent personnel or operators andinfluential factors like design, operations, loading, influent andweather. From current surveys, the combination of bioreactor sys-tems with extended aeration and solids separation seems practi-cable. However, more research efforts are needed for theconfirmation of its appropriateness. While we are working onPOME treatment via end-of-pipe processes, it is perhaps a goodmove to consider a CP, and ultimately zero emissions in the in-dustry for promoting a greener image and implementing greenmarketing on the country's largest revenue earner e the palm oilindustry.

Acknowledgements

The authors gratefully acknowledge the financial support fromUniversiti Teknologi Malaysia (UTM) and Ministry of Education(Q.J130000.2508.01H53). The authors would also like to thank theDepartment of Environment (DOE Putrajaya Office and Johor BahruBranch) and all palm oil millers for facilitating this study. Specialacknowledgement is extended to Gustaf Olsson (Emeritus Profes-sor) from Lund University for proof reading the article.

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