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In Malaysia, the crude palm oil production was 18 million tonnes in 2008, which is 12.1 more than the previous year MPOB, 2009. The total oil palm planted area in the country increased by 4.3% to 4.48 million hectares, followed by the fresh fruit bunch production of 90.5 million tonnes in 2008 with 6% annual increase (MPOB, 2009). During palm oil extraction, generation of POME ranging from 0.6-1.5 tonne per tonne of fresh fruit bunch (FFB) processed by the mill (M.B. Wahid et al., 2006, Ahmad et al., 2003). For every tonne of palm oil mill effluent (POME), about 28.8 m3 of biogas can be generated, but this is not a common practice at the moment. Biogas produced is composed of; 65% CH4 and 35% CO2 (Shirai et al., 2003 and Yacob et al., 2005 M.B. Wahid et al., 2006). Atmospheric methane concentrations amazingly increased by 30% in the last 25 years (IPCC, 2007). Net carbon emission from POME is approximately 1.41 tone y-1 (Henson, 2009) and assuming mean annual increase of 29% as experienced from 1990-2004 (M.B. Wahid et al., 2006, Ahmad et al., 2003). The estimated CH4 emission might be 106.97 tones in the year of 2020.
POME is thick brownish liquid having a colloidal suspension of 95-96% water, 0.6-0.7% oil and 4-5% total solids including 2-4% suspended solids originating from the mixture of a sterilized condensate, separator sludge and hydrocyclone wastewater (A. L. Ahmad et all, 2003). Average values of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are 50,000 and 25,000 mg/l respectively. It is discharged at a temperature of 80-90oC (Ma and Halim, 1988; Polprasert, 1989; Singh et al., 1999). POME, consist of fine suspended solids at pH 4-5 (Najafpour et al., 2006). Due to the lack of treatment systems, 70% of POME is recycled (M.B. Wahid et al., 2006). Normally, palm oil mills are complied to maintain 100ppm of BOD in POME discharged. However, 20ppm requirements are imposed by the Department of Environment (MPOB, 2009).
At present, anaerobic digestion has a meticulous attraction for treatment methods of organic waste due to the economic advantages on energy production --. Due to high organic and moisture contents, biodegradation using anaerobes is a potential treatment method. Anaerobic treatment presents the potential of producing biogas, which can be used for cooking, heating and electricity generation. As POME contains large quantity of organic material -, so it is a valuable biomass for anaerobic digestion for energy production.
Anaerobic digestion is the most widely used method of wastewater due to its high performance in volume reduction and stabilization and the production of biogas that makes the process profitable. However, biological hydrolysis, which is the rate-limiting step for the anaerobic degradation  has to be improved to enhance the overall process performance and to reduce the associated cost. Several mechanical, thermal, chemical, or biological pretreatment methods have been considered to improve hydrolysis and anaerobic digestion performance. These pretreatments result in the lyses or breakdown of cells [2, 3] and release of intracellular matter that becomes more easy to get to anaerobic microorganisms , thus improving anaerobic digestion . Anaerobic fermentation significantly reduces the total mass of wastes, generates solid or liquid fertilizer and yields energy. It can be maintained at psychrophilic (12-16 °C, e.g. in landfills, swamps or sediments), mesophilic (35-37 °C, e.g. in the rumen and in anaerobic digester) and thermophilic conditions (55-60 °C; e.g. in anaerobic digesters or geothermally heated ecosystems). Disadvantages of thermophilic anaerobic fermentation are the reduced process stability and reduced dewatering properties of the fermented sludge and the requirement for large amounts of energy for heating, whereas the thermal destruction of pathogenic bacteria at elevated temperatures is considered a big advantage . The slightly higher rates of hydrolysis and fermentation under thermophilic conditions have not led to a higher methane yield. [6, 7] reported no significant change in the total methane yield from organic matter for fermentation temperatures ranging from 30 °C to 60 °C. Compared to mesophilic fermentation conditions, at higher temperatures the pH increased through a reduced solubility of carbon dioxide, leading to a higher proportion of free ammonia. Ammonia is generated during anaerobic degradation of urea or proteins.
The degradation of volatile suspended solids in the conventional mesophilic anaerobic process is about 40% at retention times between 30 an 40 days . For achieving successful sludge digestion several physical and chemical factors must be considered. The most important physical factor is temperature. In anaerobic digestion there are generally two temperature ranges (37oC and 55oC). Anaerobic sludge digestion can occur in the mesophilic range (35 °C), which is more usual, or in the thermophilic range (55 °C), which is less common. It is important that the temperature remains constant. Specific methane forming bacterium has an optimum for growth. For instance, the mesophilic temperature range is optimal for a large number of methane forming microorganisms. For other groups of microorganisms, optimal temperatures are in the thermophilic range. If the temperature fluctuation is high then no methane formers can achieve a high stable population. A smaller microorganism population means reduced stabilization and reduced methane formation. The range between the mesophilic and thermophilic range has not yet been entirely researched. However, biogas production in dependence of temperature  clearly in two ranges, first peak is in mesophilic temperature range, second in thermophilic range. These two peaks shown as biogas production actually reflect methane forming bacteria activity. The COD removal efficiency achieved was 92% and 97% at low HRT of 3.5 and 3 days, respectively (Sotemann et al., 2005, Borja et al. 1996b and Najafpour et al. 2006). In this study, we compare the fermentation of the palm oil mill effluent in the laboratory scale reactors under thermophilic conditions, concentrating on the biogas production and biogas composition in these conditions.
There are three distinct stages in an anaerobic process . Firstly, the hydrolysis of long chain hydrocarbon into smaller chain hydrocarbon. It has been observed that hydrolysis acquire more time during POME digestion in the reactor (Eastman and Ferguson, 1981) Secondly, the conversion of smaller chain hydrocarbon organic matter to acetic acid, fatty acid and hydrogen by acetogenic bacteria. At this stage, the pH will drop due formation of acid. Finally, methanogenesis; the acetic acid is converted into methane and carbon dioxide.
Factors that influence the anaerobic process are pH, temperature and nutrients. The temperature should ranges from 30 oC to 60 oC  during anaerobic treatment. It is ideal process for tropical climate due to highly fluctuation in daily temperature. The methanogenic variation is higher in reactors working at lower temperature (Dimiter et al., 2004). Methane producing bacteria require a neutral to slightly alkaline environment in order to produce maximum methane from POME. Optimum pH for the most of microbial growth is between 6.8 and 7.2, while values lower than 4 and higher than 9.5 are not tolerable .
2. MATERIALS AND METHODS
2.1. Sample collection and pretreatment
Raw POME was collected from a local oil palm mill working under Felda Oil Palm Industries (CPSC Oil Palm Plantation, Tun Razzaq Jalan Gambang Kuantan). Physio-chemical properties of raw POME are shown in table 1. The effluent was pre-filtered by means of simple depth filtration to remove the coarse solids found in the suspension. The raw POME was initially passed through a filter bed, which was consisted of minor stones with average size of 0.6 cm. Then, the collected filtrate was passed through another filter bed that was consisted of mixture of minor stones and sand (average diameter size of 300-600 µm) in the ratio of 1:2. Later, the filtrate from the second filter bed was subjected to simple surface filtration, under vacuum through a Whatman No. 41 filter paper (20-25µm) and finally a Whatman No. 40 filter paper (8µm) before proceeding to thermophilic UASB process. The filtrate after surface filtration was named as pretreated POME. The total filtrate collected from each step of the pretreatment processes would be analyzed accordingly for total suspended solids (TSS), turbidity, total dissolved solids (TDS), chemical oxygen demand (COD), concentration of protein and carbohydrate.
Table 1: Physio-chemical characteristics (POME)
Oil and Grease
All values are in mg/l except pH
2.2. Thermophilic UASB rector construction
The experiment was performed using thermophilic upflow anaerobic sludge blanket reactor. A stainless steel laboratory-scale thermophilic UASB reactor (2160 cm3) with 2 l effective volume was used in this study. The reactor was operated at approximately 55°C. Heating is done by electric heater installed in the reactor. The settled granular sludge bed volume was 795 cm3 (app. 37% portion of total reactor). The feed was introduced into the bottom of the reactor using a peristaltic pump having maximum flow rate capacity of 1500 ml/d. 1 M NaOH was fed from separate NaOH solution tank and was being introduced in the reactor (figure 1). This feed was done from a specific height of 30cm above the bottom of reactor. The reactor was designed having a liquid-gas-solid separator (baffle) at head of column and an influent liquid distributor at the base. Incremental changes in organic loading rate were made when the effluent COD fluctuation was less in early days of experiment. The reactor was monitored daily for flow rate, COD, VFAs, biogas and methane yield while temperature and pH were monitored quarterly in a day. The diluted liquid phase of POME was continuously fed into the reactor with stepwise increases in COD concentration.
2.3. Reactor operation
In the startup period, the reactor was operated for 40 days at 55 + 3 oC. The reactor was fed with diluted POME (about 20-50 g COD/l). 1M sodium hydroxide was used for pH adjustment to 7.0 + 5 (refe). The total alkalinity of the reactor was controlled in the range of 3.0-5.0 g NaOH/l. The influent COD concentration was increased gradually but in limiting extent. Organic loading rate increased from 2.0 to 5.2 g COD/l/day and a hydraulic retention time was kept 2 days. The samples were collected from effluent after every 3 days for VFA and COD analysis. Temperature and pH were monitored and adjusted twice a day. Also, biogas samples were analysed daily for methane contents.
Figure 1: Experimental setup of Thermophilic UASB Reactor.
2.4. Analytical methods
A 100 ml sample was homogenized for 30 seconds in a blender depending upon the amount of solids in the liquid and gently stirred in a beaker by magnetic stirrer. The samples were digested by COD digestion reactor and results calculated by HACH DR/2400 @ DR/2800. HACH programs, 430 COD LR (low range) and 435 COD HR (high range/high plus) was used for COD analysis. Volatile fatty acid were analysed by using HACH apparatus.
Biogas production was determined by gas flow meter installed in the way of gas outlet. The gas samples were taken from the top of the reactor using a precision analytical syringe (VICI, LA USA) to determine biogas composition by Multi-gas analyzer Model MX 2200 (OLDHAM; France).
2.6. Mathematical methods
2.6.1. Organic loading rate (OLR)
Organic loading rate (HRT) is directly proportional to influent COD and flow rate whereas inversely proportional to the reactor volume (equation 1). So, changing the flow rate will imply the hydraulic retention time and upflow velocity. (Mahmoud et al., 2003)
Q is the influent flow rate (ml/day),
V is volume of the reactor (l),
COD influent is the total influent COD (g/l)
The main part of the anaerobic process is the hydrolysis which takes more time for converting into next stage. This is due to reason that long chain polymers first converted into smaller chain monomers. Hydrolysis indicates the total degradation, acidogenesis and methanogenesis which can be determined by the following equations. (El-Mashad et al., 2004; Halalsheh et al., 2005)
CODdiseffluent and CODdisinfluent are the dissolved CODs (g/l) of effluent and influent respectively,
CODinfluent is the influent COD of POME (g/l); and CODCH4 is the COD of methane production.
Acidification is described in terms of volatile fatty acid as this the next step after hydrolysis in which volatile fatty acid formation started and the process becomes more complex. Acidification percentage is calculated in terms of volatile fatty acids and COD of methane production.
CODVFAeffluent and CODVFAinfluent are the CODs of effluent and influent VFAs respectively.
This is the important part of the digestion process where, acetates have to convert into methane. Digestion process slows down at this stage due to accumulation of volatile fatty acids. It can be expressed in terms of COD of methane production and influent COD.
2.7. Statistical Analysis
All date were analyzed by using data analysis toolbox in EXCEL 2007 version software. The statistical analysis was ANOVA single factor with replicates. To compare the mean values, least significant difference (Fisher's LSD) at t = 0.05 was used.
3.1. Reactor performance
The thermophilic UASB performance regarding COD removal is shown in figure 3. Stepwise increase in organic loading rate was done from 2.75 to 20.0 g/l/day. A decline in the effluent COD has been observed after 40 days (figure 2). During the start up period, the VFA concentration and COD removal in the thermophilic UASB reactor tended to increase with increasing organic loading rate from 2.0 to 5.2 g COD/l/day at a hydraulic retention time of 3 days. The maximum VFA of 5.0 g/l at OLR of 5.2 g COD/l/day was found at day 35. After that volatile fatty acids concentration decreased to 1.48 g/l (figure 2). Moreover, COD removal efficiency was almost 90% after the startup (figure 3). The biogas production resulted due to the decrease in volatile fatty acids concentration.
The initial organic loading was adjusted at 2.75 g COD/l/day and a hydraulic retention time of 18 days. Organic loading rate was increased step wise to 5.0, 8, 9.8, 13.2, 16.0 and 18 g COD/l/day by reducing the hydraulic retention time to 20, 9, 5.10, 3.80, 3.12 and 2.77 days, correspondingly.
3.2. Effect of temperature on reactor performance
The reactor was operated initially at lower temperature (mesophilic) ranges to avoid aggressive behavior of digestion process at startup. The temperature was increased steadily up to 55 oC during next five days. It has been observed that temperature has a great influence on hydrolysis which took place earlier as compared to working at lower temperatures. Moreover, treatment at high temperatures causes less COD removal.
3.3. Effect of Organic loading rate
It has been found that the volatile fatty acids concentration increased with the increasing organic loading rate. The maximum concentration of volatile fatty acids found was 4.6 g/l at and organic loading rate of 18.0 g COD/l/day. In addition, the concentration of volatile fatty acids significantly decreased washout was observed (figure 4). Furthermore, total biogas and methane production increased with increasing organic loading rate.
However, the methanogenesis percentage reduced quickly when organic loading rate increased from 16.0 to 18.0 g COD/l/day. On the other hand, acidification percentage showed no momentous change. Moreover, the conversion of organic matter in POME to volatile fatty acids was not different, but the conversion of volatile fatty acids to methane decreased with increasing organic loading rate. The COD removal efficiency was more than 90% at organic loading rate from 3.0-16 g COD/l/day (figure 7). Furthermore, when organic loading rate was increased from 16.0 to 18.0 g COD/l/day, COD removal efficiency rapidly decreased. However, 80% COD removal efficiency was achieved.
3.4. Effect of VFAs concentration
The production of VFAs under thermophilic conditions varied day by day (table 2). Lactic acid, which had the highest concentration of all the VFAs, was produced from the start of the experiment and acetic acid seemed to be produced from the early experimental period onward. In contrast, butyric acid was produced later in the experiment, and propionic acid was observed at concentrations less than 1000 mg/L throughout. Under thermophilic conditions, lactic acid reached its maximum concentration of 11,000-25,000 mg/l between 7 and 18 days after the start of the experiment (table 2).
3.2. Biogas and methane production
The maximum biogas and methane yield were 30.0 and 8.3 l/day, respectively at organic loading rate of 16.3 g COD/l/day (figure 5). The maximum methane production rate of 0.89 l CH4/l/day was also found at organic loading rate of 16 g COD/l/day (figure 6).
Figure 3 shows the change in biogas production. The gas yield gradually increased with increasing organic loading rate, and the average biogas production was 0.71 and 0.94 l/day (figure 3) at loading rates of 2.88 and 3.40 g COD/l/day (figure 2), respectively. The methane yield was between 0.35 and 0.61 l/day (figure 3). The methane concentration decreased with increasing organic loading above 16 g COD/l/day. Results showed that the microorganisms consumed a small portion of organic carbon in the reactor throughout the experiment due to the need of microorganism inoculation. Thus, an effective pretreatment that produced the soluble organic materials for consumption in the post-methane fermentation process was realized in the experiment.
The phenomenon of high lactic acid production relative to VFAs in the early experimental phase is consistent, that lactic acid bacterium has the ability to adapt to a wide range of temperatures. Although the increasing trends were similar under all temperature conditions, declining trends after reaching the maximum differed among temperature conditions. Acetic acid concentration was high in early stage of experiment under thermophilic conditions (55oC), and decreased rapidly after 4-6 days (table 2). Butyric acid concentration was low at startup but showed a moderately increasing trend from day 12 to end of the experiment (2600-3200 mg/l).
3.1. POME Characteristic
The physio-chemical properties of POME used in this study (table 1) were different from those reported previously (Ahmad et al., 2003; Najafpour et al., 2006). This can be due to the changing mill operations. If the mill uses smaller amount of water to remove majority of suspended solids from the lipids, which allowed considerable reduction in the volume of wastewater generated then, a higher content of organic and inorganic matter is produced. Biodegradability of organic matter will be good if COD/BOD ratio in POME is about 1.56 (Raj and Anjaneyulu, 2005). The main unruly organic material found in POME was lignocelluloses (Oswal et al., 2002). The higher amount of identified biodegradable compounds was oil and grease, which can be hydrolyzed by microorganisms to fatty acids. A few of these fatty acids are credible substrate for methane production which allows a favorable economic outcome (Angelidaki et al., 1990). Conversely, the lipid-rich waste contains long chain fatty acids, especially n-hexadecanoate (palmitate) and oleate, that were hydrolysis products of fat and oil and these have been reported to inhibit bacterial growth and methane formation (Cirne et al., 2007).
The high amounts of total and suspended solids in the POME come from insoluble substances being wash out during the digestion process. It has to be explained that the UASB process appears to partially sensitive to the loading of solids. Thus Borja et la. (1996) used a two stage UASB reactor for POME treatment. In this experiment, pretreatment was done in order to reduce solids from POME resulting in high COD removal and methane production.
Reactor performance usually evaluated in terms o stability and efficiency of the process through the measurement of pH, VFA and alkalinity, COD removal, gas production and methane production.
3.1. Effect of organic loading rate and pH
Reactor performance is usually evaluated in terms of stability and efficiency of the treatment process. Moreover, the treatment efficiency is predicted by pH, VFA, COD removal, gas and methane production. Zinatizadeh et al. (2006) proved that POME treatment in an up flow anaerobic sludge fixed film (UASFF) reactor at 38 oC, with organic loading rates of 14.49, 21,31, 26.21 and 34.73 g COD/l/day with hydraulic retention time of 1 day, the VFA concentration increased to 93.5, 165.1, 365.2 and 843.2 mg/l respectively. This caused an increasing instability between acid formation and methane production. However, under the conditions of this experiment, the effluent pH was in the optimal range (6.5-7.5) for anaerobic digestion, far from a pH of 5.3, known to decrease the methane concentration by about 59% (Bjornsson et al., 2000). Moreover, Song et al. (2004) reported that the buffering capacity was sufficient when TVFA/alkalinity was maintained below 0.4.
The pH values in the reactor working at thermophilic conditions (55oC) were higher than those of working at mesophilic conditions (37oC) at all organic loading rates (table 2). However, the pH values were all within the optimal ranges for methane production (Wheatley, 1990). At a hydraulic retention time of 7 days and organic loading rate increasing from 9.62 to 12.15 g/l/day, the level of total VFAs in the thermophilic reactor increased from 270.14 to 537.14 mg/l.
The average COD removal efficiency was 75.5% dd
Figure 1. COD concentrations before and after thermophilic anaerobic treatment of POME. - COD before treatment, - COD after treatment and - TOC removal efficiency.
Figure 2. Change in organic loading rate during thermophilic anaerobic treatment of POME.
Figure 3. Biogas and methane production along with methane content percentage in biogas during thermophilic anaerobic treatment of POME. - Biogas yield, - Methane yield and - Methane contents in biogas.
Table 2: Concentrations of VFA, acetic acid, propionic acid, n-butyric acid and lactic acid, influent soluble COD and COD removal efficiency.