Pollutant in palm oil production process

Abstract

Palm oil mill effluent (POME) is a by-product of the palm industry and it releases large amounts of greenhouse gases (GHGs). Water systems are also contaminated by POME if it is released into nonstandard ponds or rivers where it endangers the lives of fish and water fowl. In this paper, the environmental bottlenecks faced by palm oil production were investigated by analyzing the data collected from wet extraction palm oil mills (POMs) located in Malaysia. Strategies for reducing pollution and technologies for GHG reduction from the wet extraction POMs were also proposed. Average GHG emissions produced from processing 1 ton of crude palm oil (CPO) was 1100 kg CO₂eq. This amount can be reduced to 200 kg CO₂eq by capturing biogases. The amount of GHG emissions from open ponds could be decreased from 225 to 25 kg CO₂eq/MT CPO by covering the ponds. Installation of biogas capturing system can decrease the average of chemical oxygen demand (COD) to about 17,100 mg/L and stabilizing ponds in the final step could decrease COD to 5220 mg/L. Using a biogas capturing system allows for the reduction of COD by 80% and simultaneously using a biogas capturing system and by stabilizing ponds can mitigate COD by 96%. Other ways to reduce the pollution caused by POME, including the installation of wet scrubber vessels and increasing the performance of biogas recovery and biogas upgrading systems, are studied in this paper.

Implications: Around 0.87 m³ POME is produced per 1 ton palm fruit milled. POME consists of around 2% oil, 2–4% suspended solid, 94–96% water. In palm oil mills, more than 90% of GHGs were emitted from POME. From 1 ton crude palm oil, 1100 kg CO₂eq GHGs are generated, which can be reduced to 200 kg CO₂eq by installation of biogas capturing equipment.

Introduction

Industry is the foundation of any human economy and it depends heavily on fossil fuels. Fossil fuel consumption has increased rapidly due to industrial development. Petroleum is the most commonly used fuel in transportation, agriculture, and industry (Hosseini and Wahid, 2012; Hosseini and Wahid, 2013a). More than 80% of the energy demand of the world is provided by fossil fuel, of which around 58% alone is dedicated to transportation systems (Escobar et al., 2009). Currently, one of the main concerns is a fuel crisis due to fossil fuel resource depletion. In addition to a possible fuel crisis, the toxic emissions released by fossil fuel combustion are responsible for many environmental problems (Bucksch and Egebäck, 1999; Su et al., 2012). The effects of greenhouse gases (GHGs), climate change, increasing sea levels, receding glaciers, and diminishing biodiversity are some of the consequences of creating more fossil fuel pollution (Nigam and Singh, 2011). In order to solve these problems, new sustainable and renewable biofuel fuel resources, such as animal waste, agricultural products, wastewater effluent, and municipal solid wastes, have been introduced (Fine and Hadas, 2012). Bioethanol, biomethanol, biodiesel, and biohydrogen are the main products of biomass. Palm oil–based biofuel is currently being developed in tropical countries where this resource is widely available. The environmentally sound characteristics of palm oil biofuel combustion promises to provide a clean alternative fuel (Khan et al., 2007; Hosseini and Wahid et al., 2013b). The positive properties of palm oil have attracted global attention, even though the production, utilization, and export of palm oil have been developed for tropical countries (Sawangkeaw et al., 2011). Palm oil is one of the primary sources of biodiesel cultivated in Malaysia, Indonesia, Thailand, and some tropical countries in Africa and South America due to their appropriate equatorial climate. Palm oil accounts for 28% of total vegetable oil production per year and it is one of the most commonly used vegetable oils in the world (Hasen et al., 2012). Figure 1 demonstrates the contribution of different bioresources in the global market and Figure 2 shows the most important palm oil producers in the world (Organisation for Economic Co-operation and Development–Food and Agriculture Organization [OECD-FAO], 2013).

Figure 1. The contribution of different bioresources in the global market.

Figure 2. The most important palm oil producers in the world (million ton).

Despite its many beneficial qualities, there are a few negative aspects associated with the production of palm oil. Nongovernmental organizations (NGOs) point to deforestation and the destruction of animal habitat as serious problems caused by biofuel production and palm oil plantations (Laurance et al., 2010). Large amounts of palm oil mill effluent (POME) released from palm oil mills (POMs) can jeopardize the environment and biogas released from POME during anaerobic digestion is serious challenges created by current production processes (Yacob et al., 2006). Indeed, biogas production from POME is intensified significantly by adding solid residues such as empty fruit bunches (EFB) to the POME (Fang, 2011; Hosseini and Wahid, 2013). The components of biogas are GHGs that absorb or emit specific wavelengths of radiation in the thermal infrared spectrum that contribute to climate change (Vinneras et al., 2006). Carbon dioxide (CO₂), methane (CH₄), water vapor (H₂O), nitrous oxide (N₂O), and ozone (O₃) are the most significant GHGs. GHGs contribute to climate change because of the ratio of heat captured by 1 unit mass of GHG to 1 unit mass of CO₂ in a specific period of time (Hosseini et al., 2013). Some researchers claim that without GHG, the temperature of the earth would be 33 °C colder than the present (Vijaya et al., 2010). The potential of different GHGs to contribute to climate change are shown in Table 1.

Table 1. GW potential of different GHGs

GHG	GW Potential for 100 Years
CO2	1
CH4	23
N2O	296
HFC-134a	1300
HFC-23	12000
SF6	22200

POMs that lack appropriate strategies for biogas collection contribute to the production of GHG. Despite their potential to harm the environment, the number of mills continues to increase due to global interest to palm oil–based biodiesel. For instance, palm oil production in Malaysia has increased drastically, from 4.1 million tons in 1985 to 18.9 million tons in 2011 (Malaysian Palm Oil Broad, 2010). Several studies have indicated that POMs have the potential to decrease GHG emissions (Basiron and Weng, 2004; Stichnothe and Schuchardt, 2010; Yacob et al., 2006). However, the negative effects of POMs on the environment and effective methods for mitigating GHG released by POME using a biogas recovery and upgrading system have never been investigated. In order to develop the sustainability of palm oil production process, POME treatment should be arranged properly and GHG emission from POME should be captured. The objective of this study was to develop methods to reduce the pollution caused by POMs that use a wet extraction processes.

Palm Oil Mill Process

POMs use either a dry or wet extraction processes. The most common method for extracting palm oil is a wet palm oil milling mechanism (Ahmad et al., 2011). In this study, 20 Malaysian POMs located in Johor, Melaka, and Selangor were selected. The data were collected from POMs that milled between 40 and 100 ton/hr oil palm fresh fruit bunches (FFB) and included information such as electricity demand, transportation systems, distance from plantations, fossil fuel consumption, the quantity and quality of POME, the amount of FFB, and POME treatment methods. Life cycle assessments (LCAs) are commonly used as a tool to examine GHG emissions and to measure the impact of palm oil–based biofuel production. The LCA used to estimate GHG emissions in this study was a gate-to-gate analysis (Chuen and Yusoff, 2012). The scope of this study was limited to the milling process described in Figure 3.

Figure 3. Boundary of milling process.

FFB from plantations or jungles are transported to POMs. The wet extraction process for crude palm oil (CPO) includes sterilizing, stripping, digestion and pressing, clarifying, separating the fiber and nuts, and oil extraction and purification. The CPO is the primary product created by POMs. Palm kernels (PKs), shells, and fibers are the solid residues and POME is the most significant liquid by-product (Ho and Ofomaja, 2005). Empty fruit brunches (EFB) and decanter cakes are also classified as waste. In some POMs, the PK pressing process produces PK oil. Figure 4 depicts the palm oil milling procedure in Malaysian factories (Mahlia et al., 2001; Wu et al., 2010).

Figure 4. Palm oil extraction process.

Sterilization

Sterilization is the first step of the palm oil process and it is done to prevent the rapid formation of free fatty acids during the pulping process and to remove any attached fruit. During the sterilization process, FFB are sterilized at 140 °C and at 3 × 10⁵ Pa for 50 min. Table 2 shows the effluent characteristics for different types of POMs.

Table 2. The properties of POME released from different parts of palm oil mills

Parameters^a	Sterilization Condensate	Separator Sludge	Hydrocyclone Wastewater
pH	5	4.5	—
BOD3^b	23200	28700	5700
COD	47200	63800	14700
TSS	5000	23300	7800
Total dissolved solid	36100	22000	400
Total nitrogen	600	1200	90
Ammoniacal nitrogen	22	48	—
Oil and grease	4200	6900	300

Notes: ^aAll parameters are in units of mg/L expect pH. ^bThe sample for BOD analysis is incubated at 27 °C for 72 hr.

Stripping

The sterilized fruits are separated from the bunch stalks during the stripping process by a rotary drum thresher. The EFB are used as fertilizer on plantations, used to generate heat for the factory, or they are used as a fuel.

Digestion and pressing

The digestion of sterilized fruit and any attached calyx leaves is completed in a vessel heated to between 80 and 90 °C to remove the mesocarp from the nuts before they are pressed. The oil mash is separated from nuts and the mesocarp during the pressing process.

Clarification

The CPO extracted in pressing process consists of water and vegetable impurities in the shape of either dissolved materials or indissoluble solids suspended in water. The water in the CPO is removed from subordinate parts through a settling and centrifuging process. The water removed during this process is transferred to a centrifuge or sludge separator. The characteristics of the sludge are illustrated in Table 2. As shown in Table 2, the total dissolved solids in the clarified sludge are significantly more than in the hydrocylone wastewater (Ho et al., 1984).

Fiber and nut separation

A cake is produced when the oil extracted after the fruit digestion pressing is used to separate the nut from the fiber, which is completed using an air stream or mechanical separation.

Extraction and drying of kernel

The PK is created when the nuts are processed in a hydrocyclone mechanism that separates the kernels from the empty shells after the nuts are cracked. The characteristics of the hydrocyclone wastewater have been depicted in Table 2.

Results and Discussion

A great deal of fresh water is used in palm oil production, especially when the palm fruit bunches are sterilized and clarified. Consequently, large amounts of POME are generated in POMs. Typically, 0.87 m³ of POME is produced for every ton of palm fruit that is milled. The POME is commonly composed of approximately 2% oil, 2–4% suspended solids, and 94–96% water. During the milling process, the palm fruit mesocarp debris appears as suspended solids from hydrocyclone waste, sludge separator, and sterilizer condensate. A mixture consisting of hydrocyclone wastewater, separator sludge, and sterilizer condensate in a ratio of 1:15:9 creates a colloidal suspension of POME. For every ton of CPO production, 2.5–3 tons POME are generated. Therefore, during the production of palm oil, huge amounts of contaminated materials are released into the environment (Wu et al., 2007). Chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in POME are significant causes of water pollution if they are released into water systems. Table 3 depicts the characteristics of POME that have been released into rivers (Malaysian Palm Oil Board, 2010).

Table 3. The standards for POME discharged to rivers

Parameter	Discharge Standard (1 January 1984 and Thereafter)
Temperature (°C)	45
pH	5.0–9.0
Oil and grease	50
BOD3^b	100
COD	—
Total solid	—
Suspended solid	400
Total volatile solid	—
Total nitrogen	200^c
Ammoniacal nitrogen	150^c

Notes: ^aAll parameters are in mg/L except temperature and pH. ^bSample is incubated for 3 days at 30 °C.

^cValue of filtered sample.

Oceans, rivers, and streams are threatened when nonstandard POME, which contains dissolved solids, is released. The presence of soluble organic matters in discharged POME decreases the rate of oxygen in the water due to increased bacteria and this phenomenon endangers the life of fish and other marine creatures (Tchobanoglous and Burton, 1991). Recent reports show that the majority of palm oil mills do not observe environmental standards and river pollution continues to increase rapidly due to POME discharged from these factories (Ahmad and Chan, 2009). Pretreatments such as screening, grit chambers, sedimentation, and floatation units should be employed to eliminate pollutants such as suspended solids from effluents before they are released into the environment. However, biological treatments should also be completed to remove soluble solids from POMEs (Rodriguez-Roda et al., 2000). The properties of POME depend on the palm oil processing mechanism, the rate of discharge, the type and the age of the fruit, different batches, and the climate (Ahmad et al., 2005). Some researchers have noted that the oil palm cropping season, as well as factory activities such as mill shut downs, public holidays, and quality control, has significant effects on the biological treatment of the POME (Yacob et al., 2005). POME has the potential to release CH₄ into the atmosphere. The effect of CH₄ on climate change is 23 times more significant than the effect of CO₂. Each ton of POME releases 28.13 m³ of biogas, which has the potential to be an excellent source of renewable and sustainable energy if wastewater treatment management can be instigated (Lam and Lee, 2011). Currently, the best sustainable method used by some POMs is capturing released biogas and using it as the feedstock in their boilers (Basri et al., 2010). In this study, the gate-to-gate analysis started when the FFB were received and continued until they were stored in the CPO storage tanks. GHG emissions, energy consumption, and the raw material used were taken into account when examining the production of 1 ton of CPO. The annual amount of FFB used in this study was approximately 2,851,890 million tons and approximately 541,860 million tons of CPO was produced, leading to an average oil extraction rate (OER) of 19%. The average life cycle inventory (LCI) for the POMs generated by each ton of CPO is shown in Table 4. Table 5 shows the average of GHG emissions for each ton of CPO produced.

Table 4. The average of life cycle inventory (LCI) from the studied POMs for 1 ton CPO generation

Measured Parameters	Unit	Amount
Electricity from turbine	MJ	412.10
Electricity from grid	MJ	3.04
Diesel	MJ	184.90
FFB	Ton	5.63
EFB as boiler fuel	Ton	1.37
Shell as boiler fuel	Ton	0.20
Mesocarp fiber as boiler fuel	Ton	0.73
CH4 as boiler fuel	Ton	0.044
Process water	Ton	3.78
Boiler water	Ton	2.83
Boiler ash	Ton	0.018
PK	Ton	0.66
POME	Ton	3.19
Required steam for turbine	Ton	2.51
Required steam for sterilization	Ton	2.88

Table 5. The average of GHG emission for 1 ton CPO production

GHG Resource	kg CO2eq
Electricity from grid	0.505
Boiler	64.56
Biogas	1011.20
Mill	11.98
Transportation	14.08
Total	1102.325

GHGs were generated at various points in the system, such as emissions from the production of inputs (FFB, chemicals, fossil fuels, and electricity), emissions from the transportation of the raw material to the POM, the combustion of fuel in boilers, production processes, and POME generation. In POMs, more than 90% of GHGs were emitted by POME. Only 6% was generated by the boilers and 1.3% was generated by transportation of raw materials from plantations that were more than 5 km from the factory. OER can be increased by augmenting processes, meaning that GHG emissions decreased when the OER increased. The pollution related to transportation was also reduced when OER increased. In 2010, only 10% of Malaysian POMs installed the infrastructure required to capture biogas and reuse it to fuel their processes (Vijaya et al., 2010). As shown earlier in Table 5, the majority of GHGs created by POMs was related to POME and was due to a lack of appropriate strategies to capture biogas. More than 90% of released biogas from POME can be captured in closed systems. Therefore, the rate of GHG emission released from 1 ton CPO can decrease from 1102.325 to around 192.5 kg CO₂eq. In the palm oil process, an 80% reduction in GHGs was obtained when biogas capturing technology was employed. Despite the obvious benefits of capturing biogas, it has not been seriously considered by POMs as a source of energy due to the high cost of infrastructure for biogas purification, easy access to the national grid, lack of information and interest, the low cost of electricity, and the lack of rules regarding the observance of environmental issues. Two sources of electricity are usually used in POMs. One source is the national grid and another source is steam turbines fueled by diesel or palm solid residue (PSR). Using electricity from the grid to start up the milling process can decrease GHG emissions compared with steam turbines. PSR such as EFB and fibers are usually used as a fuel for the boilers. The operation of these boilers requires a strategy to increase the rate of efficiency in order to conserve fibers so that any surplus can be sold to power plants. Improving the efficiency of boilers and emissions mitigation can be obtained through techniques such as process optimization and increasing the calorific value of the PSR by drying and heat lost recovery. If the oil lost to the cake, wastewater, and PSR is controlled during the milling process, then CPO production can be increased. Also, the loss of PK to PSR during separation process should be minimized. In order to increase OER and eventually the efficiency of the POM, all equipment should be used to its maximum capacity. Permanent condition monitoring (CM), preventive maintenance (PM), and improved equipment operation are factors crucial for obtaining high efficiency.

Strategies for GHG Reduction in Palm Oil Industry

Plantation and transportation

The use of nitrogen fertilizer on oil palm plantations is a major source of GHG emissions associated with the palm oil industry (Kaewmai et al., 2013). In order to avoid using excessive amounts of fertilizer, only the required amount should be fed to the plants. Direct injection of the fertilizer into the soil near the roots should be considered instead of the traditional application of nitrogen fertilizer on the surface (Paustian et al., 2006). Reducing nitrogen lost to surface run-off, denitrification, volatilization, and leaching are effective ways to reduce GHG emissions related to the use of nitrogen fertilizers (Schlesinger, 1999; Smith et al., 2008). GHG emissions can also be decreased by using slowly soluble osmocote fertilizer to cut down on N₂O emissions by controlling microbial transformation and by considering the ripeness of the FFB, which plays a role in the amount of oil yielded by the crude palm, as less oil can be extracted from unripe FFB. Selecting the best palm seeds for a plantation can ensure a high quality of FFB, high amounts of OER, and low GHG emissions (Corley and Tinker, 2008). Therefore, POMs should provide incentives to encourage cultivators to supply high-quality FFB. Since transportation is responsible for 1.3% of the GHGs associated with the production of palm oil, strategies should be developed for transporting FFB. For example, using pickup trucks for FFB transportation generates higher levels of GHGs, especially when the distance between the plantation to the POMs increases. Transportation-related GHGs could be decreased by more than 60% using collection points for FFB and full loads carried by 10-wheel trucks.

Ponding system

Ponding systems are the most common treatment methods for POME due to their low cost. The energy requirements for these systems are low because the control, monitoring, and mixing systems are not complicated. The main components of a ponding system are algae aerobic ponds, anaerobic areas, and facultative sections (Elango et al., 2007). Aerobic ponds and facultative sections play a crucial role in decreasing organic materials in the POME before they enter the water system. The results show that the rate of CH₄ formation increases in the static conditions of effluent inside the ponds. In the milling process, the application of anaerobic digestion in open ponds decreases the COD of raw water from 100,000 to 78,000 mg/L. This indicates that 225 kg CO₂eq/MT CPO of GHGs are generated by organic loss. Application of a unit for wastewater dispersal would effectively minimize GHG emissions in this step. Typically, POME is dispersed by pumps to the top of a unit with a large surface area for heat exchange. Ideally, a 2-kW pump operates for 300 working days to conduct POME to the dispersal unit that is 18 m high. The electricity demand of this process generates about 0.25 kg CO₂eq/MT CPO of GHGs, which is negligible compared with the GHGs generated from organic loss. Installation of air stripper vessels can decrease the rate of GHG emission from open ponds. An oil trap tank is usually installed to remove grease and oil from POME before moving it to the air stripper tower because the performance of the air stripper vessel decreases drastically in presence of grease and oil.

Open and closed digestion tanks

Open and closed digestion steel tanks are usually 600–3600 m³ in size. The hydraulic retention time (HRT) for open digestion tank is 20–25 days. However, in ponding systems HRT increases to 30–45 day. Today, closed anaerobic digestion tanks have attracted attention due to the clean development mechanism (CDM) strategy. Open and closed digestion tank designs are similar; however, closed digestion tanks are equipped with safety valves, covers, and control systems. POME biogas can be easily captured in closed digestion tanks allowing carbon emission reduction (CER) strategies to be achieved (Sulaiman et al., 2009). More CH₄ is trapped by closed digestion tanks than by open digestion tanks and open ponds. The treatment of POME can be completed at both thermophilic and mesophilic temperatures in closed digestion tanks, while allowing for the generation of biogases. POME treatment using thermophilic temperatures is quicker (Dolan et al., 2011). The amount of GHG emissions from open ponds (about 225 kg CO₂eq/MT CPO) could be decreased to 25 kg CO₂eq/MT CPO by covering the ponds. Installation of biogas capturing system can decrease the average of COD to about 17,100 mg/L. Furthermore, stabilizing ponds in the final step could decrease COD to 5220 mg/L. Using a biogas capturing system allows for the reduction of COD by 80%. Simultaneously using a biogas capturing system and by stabilizing ponds can mitigate COD by 96%. In these circumstances, 0.3 m³ of biogas per kg BOD could be captured. The average COD/BOD ratio of the POME was 1.67. This indicated that the rate of COD removed from open ponds (22,000 mg/L) can be converted to BOD removal of 13,174 mg/L. Since the average POME generated from POMs investigated in this study was 124,000 m³, the biogas generated from BOD removal was about 490,000 m³ per year. Captured biogas is usually used in boilers as a clean fuel. Approximately 2.62 kWhr electricity can be generated from 1 m³ POME biogas. Figure 5 illustrates a typical closed digestion tank in Malaysian POMs.

Figure 5. The schematic of closed digestion tank.

Biogas refinery and storage in palm oil mills are strategies that will ensure that environment protection goals are met. The capability of biogas in industrial and transportation systems highlights the economic importance of biogas production from POME. Moreover, the sustainability and environmentally friendly characteristics of POMs are enhanced by biogas refinery systems. Captured biogas should be modified and considered for use as a compressed natural gas (CNG) in transportation vehicles. For this application, the rate of CH₄ and CO₂ emissions should be around 97% and 3%, respectively (Najafpour et al., 2006). Biogas can be upgraded by using water scrubbing, polyethylene glycol scrubber systems, carbon molecular screens, chemical processes, and membranes. The amount of biogas escaping from these processes should be taken into account during their operation. The application of raw biogas for flameless combustion technologies is an easy and clean method for using biogas (Hosseini et al., 2012, 2013; Hosseini and Wahid, 2013b; Hosseini and Wahid et al., 2013a).

Summary

Scenarios that create pollution and environmental bottlenecks faced by palm oil production process have been investigated by looking at the data from palm oil mills. POMs without suitable POME treatments can threaten the environment because they can emit toxic gases and jeopardize water systems. Therefore, more stringent laws should be created and POM managers should be trained in environmental conservation. Biogas released by POME is an excellent renewable and sustainable energy source that can be captured by using the appropriate equipment. The majority of GHG emissions from palm oil production can be attributed to the lack of appropriate infrastructures for POME treatment. The rate of GHG emissions from POMs can be reduced by more than 80% by installing POME biogas capturing equipment. Also, increasing boilers efficiency and raising the rate of palm oil process performance could be helpful in GHG reduction purposes. To increase OER and eventually the efficiency of POMs, all equipment must be used to its maximum capacity. In order to increase the performance of boilers, heat must be recovered and the calorific value of PSR improved by drying. The real environmentally friendly characteristics of palm oil will be obvious when any air and water pollution caused when it is produced is controlled.

Additional information

Notes on contributors

Seyed Ehsan Hosseini

Seyed Ehsan Hosseini works as a research assistant and Mazlan Abdul Wahid is the head of High-Speed Reacting Flow Laboratory, Faculty of Mechanical Engineering, University Teknologi Malaysia, Skudai, Johor, Malaysia.

Mazlan Abdul Wahid

Seyed Ehsan Hosseini works as a research assistant and Mazlan Abdul Wahid is the head of High-Speed Reacting Flow Laboratory, Faculty of Mechanical Engineering, University Teknologi Malaysia, Skudai, Johor, Malaysia.