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Abstract: A laboratory scale anaerobic treatment of POME being carried out through thermophilic upflow anaerobic sludge blanket (UASB) reactor at start-up strategy of the UASB reactor was monitored for 24 days. Chemical oxygen demand (COD) and total organic carbon (TOC) removal was 89.2% and 56% respectively. Biogas and methane production were 0.82 l/g-TOC removed and 0.58 l/g-TOC removed. Methane contents in biogas were 71%. The pH value 7±0.5 was maintained by continuous sampling of the reactor substrate. The pretreated POME was treated anaerobically in UASB reactor at 55°C (thermophilic condition). Stepwise increase in volumetric loading rate was done up to 2.4-3.55 g TOC/l. days. Thermophilic UASB reactor could be suitable for maximum COD removal contributing high methane contents in biogas.
Keywords: Thermophilic UASB, anaerobic treatment, palm oil mill effluent, COD, TOC, methane.
In 2008, the crude palm oil (CPO) production was 18 million tonnes which is 12.1% more than that of year 2007 (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 M tonnes in 2008 with 6% annual increase in the later one (MPOB, 2009). During palm oil extraction, generation of POME ranging from approximately 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 POME digested about 28.8 m3 of biogas can be generated but this is not a common practice at the moment. This biogas 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 1.4 106 t yr-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). So, the estimated CH4 gas emission might be 0.502 M t in the year of 2020.
POME is a thick brownish liquid, having average values of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) 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 is a viscous brown liquid with fine suspended solids at pH ranging between 4 and 5 (Najafpour et al., 2006). The COD removal efficiency achieved was 92% and 97% at low HRT levels of 3.5 and 3 d, respectively (So¨temann et al., 2005, Borja et al. 1996b and Najafpour et al. 2006). Due to lack of treatment of effluent, 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). Since methane is produced only during the anaerobic decay of organic matter, and not during aerobic decay, the diversion of organic waste from landfills to composting reduces methane production (Thompson et al., 2005).
POME is 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 sterilizer condensate, separator sludge and hydrocyclone wastewater (A. L. Ahmad et all, 2003). The raw or partially treated POME has an extremely high content of degradable organic matter, which is due in part to the presence of unrecovered palm oil. This highly polluting wastewater can therefore cause severe pollution of waterways due to oxygen depletion and other related effects. In order to regulate the discharge of effluent from the crude palm oil industry as well as to exercise other environmental controls, the Environmental Quality (Prescribed Premises) (Crude Palm Oil) Order, 1977, and the Environment Quality (Prescribed Premises) (Crude Palm Oil) Regulations, 1977, were promulgated under the Environmental Quality Act, 1974 (DOE, 1999). The POME is non toxic as no chemical is added in the oil extraction process. It contains appreciable amount of metals which are useful as plant nutrient. The POME characteristic and standard discharge limit is illustrated in Table 1.
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 a large quantity of organic material ,  so it is a precious biomass source for anaerobic digestion for energy production. Hydrolysis, acidogenesis and methanogenesis are the main step of anaerobic treatment , and involve continuous bacterial digestion reaction -.
There are three distinct stages in an anaerobic process . Firstly, the hydrolysis of long chain hydrocarbon into smaller chain hydrocarbon. The second phase is 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. The final stage of anaerobic process is the methanogenesis i.e. conversion of acetic acid 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 . It is ideal process for tropical climate due to highly fluctuation in daily temperature. The methanogenic variation is greater in reactors working at lower temperatures (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 most microbial growth is between 6.8 and 7.2 while pH lowers than 4 and higher than 9.5 are not tolerable .
It has been observed that during biodegradation of fermentative bacteria, the growth rate of VFA increases which reduces biogas production. This was due to the high growth rate of fermentative bacteria which inhibit the formation of acetogenic and methanogenic bacteria. Results showed that pH fall is due to the accumulation of fatty acids. It has been considered in earlier studies that hydrolysis took more time during POME digestion in the reactor (Eastman and Ferguson, 1981). The main return of thermophilic anaerobic digestion comprise a speed up of general reaction rate followed by the reduction of hydraulic retention time and reactor volumes which consecutively results low capital costs (Ferrer, 2008). On the other hand, long contact periods provide enough time for the degradation of the substrate and result in similar process performance in spite of process temperature. But long contact period of sludge within the reactor cause more accumulation of raw POME in settling tank.
The present work is carried out for the performance evaluation of thermophilic UASB reactor during POME treatment. POME has potential on methane production through thermophilic anaerobic digestion.
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 _m) in the ratio of 1:2. Later, the filtrate from the second filter bedwas subjected to simple surface filtration, under vacuum through a Whatman No. 41 filter paper (20-25_m) and finally a Whatman No. 40 filter paper (8_m) 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 of Palm Oil Mill effluent (POME)
2.2. Thermophilic UASB rector construction
The experiment was performed using thermophilic upflow anaerobic sludge blanket reactor (figure 2). 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 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 of approximately 1500 ml/d. 1 M NaOH was fed from separate NaOH solution tank and was being introduced in the reactor (figure 4). The reactor was continuously supplied with diluted liquid of the POME for 24 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 performance at start up
In the startup period, the reactor was operated for 35 days at 55 + 3 oC. The reactor was fed at lower COD concentrations (about 5-6 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 2-5.2 g COD/l/day and a hydraulic retention time was kept 2 days.
2.4. Experimental operation
Experimental operation was started after steady state condition which was achieved successfully at low influent COD concentrations and high temperatures. The influent for thermophilic UASB reactor was fed with 50 g/l with pH adjustment of 6.5. Organic loading rate increased stepwise from 2.7 to 18.0 g COD/l/day, and hydraulic retention time was kept 18.5 to 2.78 for this reactor. 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.
2.5. 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 bas chromatograph, HACH
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)
Organic loading rate can be changed by varying the influent COD concentration and flow rate. Hydraulic retention time (HRT) also affected by flow rate of the influent and the upflow velocity.
2.4. Effect of temperature on reactor performance
The reactor was operated initially at 35 oC to avoid aggressive behavior of digestion process at startup. Temperature was increased steadily to 55oC 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. Continuous monitoring of the temperature was done in order to check and improve the reactor performance.
2.6. Effect of pH on volatile fatty acids
2.5. Organic loading rate
2.7. Biogas monitoring
3. RESULTS AND DISCUSSION
3.1. Reactor Stability
3.2. Effect of pH and 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).
3.3. Effect of organic loading rate
3.2. Biogas and methane production
Figure 3 shows the change in biogas production. The gas yield gradually increased with increasing TOC loading rate, and the average biogas production was 0.71 and 0.94 l/g- TOC removed (figure 3) at loading rates of 2.88 and 3.40 g-TOC/l. d (figure 2), respectively. The methane yield was between 0.35 and 0.61 l/g-TOC removed (figure 3). The methane concentration decreased with increasing organic loading. Results show 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 with the reports , 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).
Figure 1. TOC concentrations before and after thermophilic anaerobic treatment of POME. - TOC before treatment, - TOC after treatment and - TOC removal efficiency.
Figure 2. Change in volumetric 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.
Acetic Acid (g/l)
Propionic Acid (g/l)
Butyric Acid (g/l)
Lactic Acid (g/l)
COD in (mg/l)
COD removal (%)
Figure 4: Experimental setup of Thermophilic UASB Reactor.
This study shows the high rate anaerobic digestion of POME under thermophilic conditions. Thermophilic UASB reactor is efficient for COD removal and high methane contents. A lab scale reactor was constructed to study the thermophilic anaerobic treatment of POME collected from university cafeteria and residential blocks. VFAs accumulation was low and methane production was comparatively high due to controlled temperature and pH value. Acidity of the substrate during digestion was low due to the continuous monitoring of pH valve in the reactor. Further studies can be done for full batch experiments to evaluate the proper performance of thermophilic UASB reactor. At high temperature (55oC), COD increased gradually from the start of the experiment, and did not increase thereafter. These results might suggest that the solid substrate decomposed physicochemically under the high temperature conditions. The high COD concentrations in the early experimental period at 55oC provide an adequate substrate supply for the subsequent acidogenesis and methanogenesis steps. Therefore, high temperature conditions would decrease the HRT of the solubilization process and make it possible to decrease the total volume of the reactor.