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Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors
Bioresource Technology xxx (2013) xxx–xxx
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Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors Won-Ho Choi, Chang-Ha Shin, Sung-Min Son, Praveen A. Ghorpade, Jeong-Joo Kim, Joo-Yang Park ⇑ Department of Civil and Environmental Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
h i g h l i g h t s " The combined sequential anaerobic reactors were developed for POME treatment. 3
" Average COD removal of 93.5% was obtained with the OLR of 8.5–23 kg [COD]/m /d. 3
" Maximum COD removal of 95.6% was achieved at 13 kg [COD]/m /d OLR in primary AHR. " The secondary reactors helped to improve the safety and performance of the system. " Average methane yield of 0.171–0.269 l [CH4]/g [COD removed] was obtained.
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Article history: Available online xxxx Keywords: Palm oil mill effluent (POME) Anaerobic hybrid reactor Anaerobic filter reactor Organic loading rate (OLR) Organic removal rate
a b s t r a c t Combined system of high-rate anaerobic reactors for treating palm oil mill effluent (POME) was developed and investigated in this study. The system composed of one common primary hybrid reactor which was shared by two different secondary filter reactors. An overall COD removal efficiency of 93.5% was achieved in both systems. The secondary reactors contributed not only in enhancing the COD removal efficiency, but also ensured the performance stability of the entire system. Biomass remained intact in the secondary reactor in contrast to the primary reactor in which occasional washout of biomass was observed. The pH of POME was adjusted at the beginning of the operation, as the process continued POME did not require the external pH adjustment as the pH was maintained in desired range. The biogas was produced up to 110 l/d with the yield of 0.171–0.269 l [CH4]/g [COD removed] and 59.5–78.2% content of methane. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Palm oil is one of the most widely produced vegetable oils in the world and currently its production is being boosted with extending their use in making biodiesel (Lim and Teong, 2010). However, the unsustainability of palm oil production has been constantly criticized, because the large quantities of biomass residues (almost 5 times the weight of oil production) are a serious threat to the environment (Ahmad et al., 2003). In particular, palm oil mill effluent (POME) causes a greater impact than the other by-products of palm oil production, of which estimated amount is 3 times more than that of crude palm oil (Wu et al., 2010; Yeoh et al., 2011). POME is a viscous brown liquid containing high concentrations of organic acids with a COD level higher than 20,000 mg/l (Lam and Lee, 2011; Najafpour et al., 2006). Aerobic treatment is not suitable for POME which has insufficient amount of nutrients (Chin et al., 1996). On the other hand anaerobic treatment is favorable for ⇑ Corresponding author. Tel.: +82 2 2220 0411; fax: +82 2 2220 1945. E-mail address: [email protected] (J.-Y. Park).
POME treatment as it can remove much more organics even with limited available nutrients. Therefore, anaerobic treatment processes have primarily been adopted for POME in the field (Poh and Chong, 2009). Facultative ponds and open digesting tanks are the most commonly used anaerobic processes for the treatment of POME (Yacob et al., 2005). Although these conventional processes require relatively little energy to operate, they demand extensive land area and long retention time (Lam and Lee, 2011; Wu et al., 2010). Besides, a large quantity of greenhouse gases including methane and carbon dioxide is produced from open ponds and tanks, and these gases are emitted directly into the atmosphere, contributing to a serious global warming problem. Since methane has 21 times more global warming potential than carbon dioxide, the total emission of the greenhouse gases can be substantially reduced if the methane is completely recovered in sealed anaerobic reactors and simply burnt to convert it into carbon dioxide. Furthermore, the biogas containing methane from the anaerobic digestion of POME is a very promising source of renewable energy (Fang et al., 2011; Yacob et al., 2006).
0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.02.055
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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The upflow anaerobic sludge blanket (UASB) system, in which settleable anaerobic biomass is aggregated and immobilized in the granules of a dense sludge bed, is a high-rate anaerobic process that has been successfully applied for various types of wastewater (Fang et al., 2011; Poh and Chong, 2009). The potential of UASB for treating POME has been identified (Borja and Banks, 1994; Borja et al., 1996; Wu et al., 2010). High contents of suspended and colloidal solids as well as residual oil in POME at high organic loading rates (OLR) can cause adverse effects on the performance of UASB reactor including the deterioration of microbial activities, the washout of sludge granules, and hindered growth of granules (Poh and Chong, 2009). The sequential two-stage anaerobic treatment system composed of a first stage UASB and a second-stage downflow anaerobic filter (DFAF) has been investigated to treat confectionary wastewater (Beal and Raman, 2000). This combination system was able to achieve an overall COD removal efficiency of 98% at the overall OLR of 12.5 kg [COD]/m3/d. The secondary reactor contributed not only to the greater removal of organic carbon, but it also helped to buffer the fluctuations in the effluent of the primary reactor. These properties can enhance the applicability of UASB in many cases. However, it has been suggested that the combination system containing UASB is inappropriate for the treatment of POME, because the deficiencies of UASB that are mentioned above still exist. The upflow anaerobic sludge fixed-film (UASFF) reactor, a hybrid bioreactor with UASB and anaerobic filters, shows greater stability than the UASB when operated under high OLR. Since the filter media packed at the upper part can overcome the well-known deficiencies of UASB, the UASFF reactor has been successfully applied for the treatment of various types of wastewater as well as POME (Wu et al., 2010). In the treatment of POME (>40,000 mg [COD]/l) using 5 l UASFF reactor, the COD removal efficiencies of more than approximately 90% were achieved with a hydraulic retention time (HRT) in the range of 1.5–3 days (Najafpour et al., 2006; Zinatizadeh et al., 2006). According to these studies, internal packing effectively contributed to the maintenance of the performance of UASFF reactor by capturing the solids that floated from sludge bed. This result allowed for a high ratio of effluent recycle, which was required for the treatment of high concentrations of organic acids in POME. However, it was indicated that a higher HRT was needed in order to obtain more complete digestion, which is usually not available in the field where a large amount of POME is discharged. In this study, two high-rate anaerobic processes were designed and investigated for the treatment of POME, which aimed to utilize the advantages of a hybrid reactor and combination system. The developed processes were composed of one hybrid reactor and two different filter reactors, which share the hybrid reactor as the primary reactor. The primary reactor, an anaerobic hybrid reactor (AHR), was composed of sludge bed for the lower zone like a conventional UASB and filter media for the upper zone. In an AHR, the wastewater is first treated by the anaerobic microorganisms of the granular sludge in the lower sludge bed zone and then it is also treated by the biofilm on the surface of the filters in the upper filter media zone. The media in the upper zone prevents the washout of granules from the lower zone, which occurs frequently under high flow rates. The secondary reactors were an anaerobic baffled filter reactor (ABF) and an anaerobic downflow filter reactor (ADF). The ABF used the same filter media in the AHR and had a baffle installed vertically in the middle of the reactor to obtain more plug flow condition with up-and-down flow path. The ADF was packed with baked clay globules which have a high adhesion capacity for bacteria. The objectives of the current study were: (1) to evaluate the feasibility of combination systems (AHR + ABF and AHR + ADF) for POME treatment and (2) to investigate the performance of each
reactor in terms of COD removal, OLR, suspended solids (SS), pH, biogas production, and methane yield. For this purpose, lab pilots of the combination systems were operated near a palm oil mill in Indonesia. 2. Methods 2.1. Characteristics of POME The raw POME was collected by onsite sampling in Indonesia. Generally, the characteristics of POME depend on the features of the manufacturing process in the palm oil mill plant and seasonal changes in the crops (Poh and Chong, 2009). The efficiency of anaerobic treatment of POME is significantly affected by these characteristics. High OLR during the anaerobic treatment process may cause a decrease in the removal efficiency by producing scum or clogging the system (Latif et al., 2011). In order to avoid the scum and clogging problems caused by high concentrations of SS and residual oil in POME in the current study, the raw POME was pretreated to remove excess SS and oil using a 3-phase screw decanter. The characteristics of the pretreated POME (after the 3phase screw decanter) were as follows: sCODcr 19,700 mg/l, TN 154 mg/l, TP 212 mg/l, TSS 3,115 mg/l and pH 3.9. The recommended ratio of COD:N:P in an anaerobic bioreactor is 300:5:1 during the start-up period and 600:5:1 in the steady state (Annachhatre, 1996). The samples satisfied this ratio and, therefore, additional nutrients were not added. 2.2. Reactor design and configuration The schematic diagram of the experimental set-up for the combination of the AHR followed by ABF and ADF is shown in Fig. 1. The AHR was made of acryl resin with a cylindrical shape. The total volume was designed as 20 l (H 65 cm I.D. 20 cm). The upper part of the AHR was packed with pall rings up to a depth of 15 cm (polyethylene, diameter: 5/8 in., specific area: 362 m2/m3, porosity: 92%, Solmaro Trading & Engineering Company, South Korea). This packing served as a gas–solid liquid separator as well as filter. The AHR was designed as a completely mixed reactor with a sludge blanket. A surge tank was placed immediately after the AHR in order to recycle the effluent. The recycle ratio (the ratio of flow rates of recirculation to influent) was maintained at 30:1 to create a close complete mix condition in the AHR taking into consideration the high concentrations of organic acids in POME. The reactors that followed the primary reactor (10 l, H 61 cm I.D. 15 cm) were made to have half of the volume of the AHR. The ABF was divided into two chambers by a baffle made of acrylic resin (T 0.5 cm W 15 cm H 55 cm). The ABF was packed with the same pall rings that were in the AHR from the surface to a depth of 45 cm (70%, H/H). The ADF reactor was packed with clay ball media (diameter: 1/2 in.) up to 70% of the effective volume in the reactor. Both the ADF and ABF were connected to the AHR reactor by a silicon tube (I.D. 8 mm, MasterflexÒ) with a gradient of 0.2 to allow for gravity flow. The flow rate was controlled by using peristaltic pumps. Biogas was measured with a rotary gas meter (Ritter, TG 05/5). A water jacket was installed to maintain the temperature of all of the reactors by circulating heated water. 2.3. Experimental method For start-up, 2 l of granular biomass from a lab-scale UASB reactor was seeded into the AHR. The reactors that followed the primary reactor were unseeded. The combination systems were operated under mesophilic conditions (36 ± 1 °C). The pH in the AHR was adjusted to 7 ± 0.5 using NaHCO3 during the initial stage.
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
W.-H. Choi et al. / Bioresource Technology xxx (2013) xxx–xxx
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Fig. 1. Schematic diagram of the developed systems containing combined high-rate anaerobic reactors (AHR + ABF and AHR + ADF).
The overall HRT ranged from 0.7 to 2.4 days. Samples were taken every 6 h. Except for the period of the initial start-up stages, POME was fed into the reactors for 5 weekdays only. The OLR was increased stepwise from 0.91 to 23 kg [COD]/m3/day taking into consideration the criteria for a steady state proposed in the previous study (Wu et al., 2000). 2.4. Analytical method The COD values were measured using the reactor digestion method (HACH, method 8000, range: 20–1500 mg/l) after pretreating the samples with a centrifugal separator for 15 min at 3000 rpm. The T N was measured using the persulfate digestion method (range: 2–150 mg/l) and the T P was measured using the molybdovanadate method (range: 1–100 mg/l) with a spectrophotometer (DR-2800, HACH). The SS were measured as Total SS (TSS) using the standard method (20ed, 2540). The total volume of biogas was measured using rotary gas meter (Ritter, TG-05/5). It was assumed that CH4, CO2, and H2S were the primary components of the biogas produced from the systems, but the H2S was removed entirely using a desulfurization column (H 8 cm I.D. 12 cm), which was packed with Fe2O3 (94%, BayferroxÒ). The CO2 contents were measured using a gas detection tube (Gastech, 2HT, range: 10–100%), then the methane contents were determined assuming that the remaining volume of the biogas was methane. 3. Results and discussion 3.1. Effect of different operational conditions The experiment was carried out continuously for approximately 25 weeks and the overall performance of the systems in terms of COD removal and OLR are described in Fig. 2. The AHR reactor was operated throughout the experiment. The ABF reactor was also installed and operated together with the AHR from the beginning of experiment, but the ADF reactor was not installed and operated until the 9th week. The effluent from the primary reactor was evenly distributed by V-notched weirs to the secondary reactors, which had half the volumes of the AHR.
The influent OLR was maintained within the range of 0.91– 23.0 kg [COD]/m3/d by different dilutions with POME and/or by adjustment of the flow rate. Initially the OLR was gradually increased from 0.91 to 13.73 kg [COD]/m3/d during the first 2 weeks. However, the COD removal efficiency in the reactors rapidly decreased and stable operation could not be maintained due to the shock overloading by the rapid increase of the OLR (period A in Fig. 2). Therefore, the OLR was moderated to the level of approximately 10 kg [COD]/m3/d in order to recover the reactor performance during the 3rd week. From 4th to 22nd weeks (period B in Fig. 2), the reactor was operated with a stepwise increase in the OLR from approximately 10–23 kg [COD]/m3/d. Meanwhile, a sudden decrease from 93.3% to 75.8% in the COD removal was observed in the AHR reactor during the 9th week (indicated by arrow in Fig. 2). High levels of SS concentration in the influent could be responsible for this abruption. Therefore, the process to remove the SS from POME was also performed through the settling of the influent solids for 24 h before being allowed to flow into the system from the 9th week on. During the last 2 weeks (period C in Fig. 2), the OLR was changed to approximately 15 kg [COD]/m3/d in order to investigate the effects of the SS on the COD removal efficiency that will be discussed later in this article. 3.2. Steady-state performance of the process in terms of COD removal The stable process performance was not achieved during the period A, because of the operational failure by rapid increase of the influent OLR (as mentioned in Section 3.1). The process performance was restored by adjusting the OLR in 3rd week, however the reactor was stabilized in terms of COD removal after 4th week. Once the steady-state condition was achieved, the process performance of the reactor in terms of COD removal was evaluated throughout the period B. During the experimental period B in Fig. 2, the COD removal efficiencies of the combined reactors were almost always maintained at high levels even though the initial OLR of 10 kg [COD]/ m3/d was increased more than double to 23 kg [COD]/m3/d. A reasonably stable COD removal of more than 88% was achieved and the maximum COD removal efficiency was found to be 95.6% at the OLR of approximately 13 kg [COD]/m3/d in the AHR reactor. This performance was similar and/or higher than the hybrid reac-
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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Fig. 2. The COD removal efficiencies with a change in OLR in the AHR reactor and the combination system during the entire experiment. The letters A, B, and C in the figure indicate the periods distinguished by different operational conditions. The arrow indicates the sudden decrease in the COD removal by high levels of SS concentration in the AHR.
Fig. 3a. The behavior of the COD removal efficiency and organic removal rate with an increase in the OLR in the AHR reactor during the 4th–22nd weeks.
Fig. 3b. The COD removal efficiencies with an increase in the OLR in the ABF and ADF reactors during the 4th–22nd weeks.
Fig. 3c. The organic removal rates with an increase in the OLR in the ABF and ADF reactors during the 4th–22nd weeks.
tors for POME treatment reported in the previous studies (Zinatizadeh et al., 2006; Habeeb et al., 2011). The COD removal efficiency in the AHR was relatively similar with the UASFF reactor where the COD removal efficiencies of approximately 90% at an OLR of 14.93 g COD/l/d was observed in stable condition (Zinatizadeh et al., 2006). But the performance of the AHR was far more stable even with higher OLR (23 kg [COD]/m3/d) and longer period (19 weeks) when compared to that of the UASFF hybrid reactor (25 days). Besides, as compared to the HUASB hybrid reactor where the maximum COD removal efficiency was 91% at the OLR of 9.37 kg/m3/d at 46 °C (Habeeb et al., 2011), the AHR also showed a better performance even with the higher OLR and lower temperature. Fig. 3a shows the changes in the COD removal efficiency and the organic removal rate along with the different OLR values in the AHR reactor. The COD removal efficiency of the AHR reactor somewhat fluctuated, but the range of fluctuation was not significant throughout the entire experiment. The performance of the COD removal was maintained almost up to an OLR of 23 kg [COD]/m3/d, which was the maximum level reached in the experiment. The changes in the organic removal rate reasonably reflect the
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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above-mentioned behaviors in the AHR reactor. The organic removal rate increased from 7.77 to 20.75 kg [COD]/m3/d with an increase in the OLR from 8.53 to 23 kg [COD]/m3/d. A linear relationship was observed when the organic removal rate was plotted as a function of the OLR. The best-fit linear regression line of the removal rates to the loading rates shows a fairly high correlation with the coefficient of determination R2 0.98, and the slope of regression line indicates an average COD removal efficiency of 88.9% in the AHR. This result implies that the performance stability of the AHR reactor could be maintained constantly under substantial changes in the OLR. Consequently, the AHR reactor can be regarded as a highly efficient anaerobic reactor that achieves sufficient COD removal even if it will be the only reactor used. The effluent from the primary reactor was additionally treated by the secondary reactors, thus the average COD removal efficiencies of the combination systems (AHR + ABF and AHR + ADF) were observed to be approximately 93.5%. The OLRs of the secondary reactors were very much less than those of the primary reactor, ranging from approximately 0.58 to 3.82 kg [COD]/m3/d during period B, because the influent into the secondary reactors was previously treated through AHR reactor and its flow rate was reduced to half of the original rate by the V-notched distributor. These lower OLR values were introduced to the different performance behaviors of the secondary reactors as shown in Figs. 3b and 3c. In contrast with the results of the primary reactor, the COD removal efficiencies with an increase of OLR were not maintained at a certain level in the secondary reactors (Fig. 3b). The plotted data were scattered so that the clear relationship was not seen, but the COD removal efficiencies of the secondary reactors showed a reasonable trend toward increasing along with an increase in the OLR though their values were relatively less than those of the primary reactor. This result suggests that the secondary reactors can be operated while ensuring the stable removal of COD even under much higher OLR conditions than in the primary reactor. Therefore, the capacity reserved in the secondary reactors was high enough to help improve the safety and performance of the whole process of the combination systems. Fig. 3c shows that the organic removal rate of the secondary reactors increased with an increase in the OLR, which was similar to the results of the AHR reactor. Linear relationships between the organic loading and removal rates were observed, but their correlations were a little lower due to their operation under much lower OLR values. From the slopes of the regression lines, the average organic removal rates of the ABF and ADF reactors were found to be 47.9% and 47.1%, respectively. The influent OLR was increased at the beginning of every week, during this time the fluctuation in the COD removal was observed (Fig. 2). However, the COD removal efficiency of secondary reactors was unaffected with a minor fluctuation. This shows that the combined reactors in present system are capable of maintaining the stable performance even in situations, such as shock loading and unexpected operational changes. 3.3. Other parameters 3.3.1. Effect of suspended solids on COD removal As mentioned in Section 3.1 and can be seen in Fig. 2 (indicated by arrow), there was a sudden decline in the COD removal in the AHR reactor at approximately 4000 mg/l of SS during the 9th week. Therefore, the SS in POME were treated in order to obtain a value below 3000 mg/l before they were allowed to flow into AHR reactor. An additional experiment was carried out to investigate the effects of the SS on the COD removal during period C. The COD removal efficiency of the AHR reactor increased when the OLR was adjusted to values similar to those in period A. This result suggests that a specific SS concentration may exist that can influence the performance of the AHR reactor. The fluctuations in the COD
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Fig. 4a. The COD removal efficiency and SS concentration with an increase in the SS loading rate in the AHR reactor.
removal efficiency due to changes in the SS concentration can be seen in Fig. 4a, which shows the changes in the COD removal efficiency based on the SS loading rate. Although the relationship between the COD removal efficiency and the SS loading rate was not clear, the highest COD removal efficiency of approximately 93% was observed at a SS loading rate of approximately 2.6 kg/m3/d where the level of SS concentrations around 3000 mg/l. This could be because of the presence of digestible organic solids suspended in POME. It was observed that the COD removal efficiencies were slightly lower after the SS loading rate of 2.6 kg/m3/d, because relatively high levels of SS probably disturbed the hydrolysis of the anaerobic digestion (Latif et al., 2011). Therefore, when the SS loading rate with respect to the COD removal was taken into consideration, the pretreatment of POME to achieve a SS value below 3000 mg/l was required to ensure the performance and stability of the AHR reactor. Analysis of the SS removal in the effluents of each of the reactors was conducted during this experiment, the resulting SS removal efficiencies are described in Figs. 4b and 4c. For better understanding of the relationship between the SS removal and OLR in the AHR, the average values of measured data (except for negative levels) were plotted with the error bars that reflect the 95% confidence limit as shown in Fig. 4b. A decreasing trend in the SS removal efficiency with an increase in the OLR was observed mostly in the AHR. Occasionally some negative levels were observed during operation within the OLR of 17–21 kg [COD]/m3/d. The negative SS removal efficiencies indicate that the fractions of the biomass were washed out of the reactor. The SS removal efficiency at the OLR of 20 kg [COD]/m3/d was relatively higher which deviated slightly from the trend. This behavior resulted because larger extent and more frequent washout of biomass occurred at the aforementioned level of OLR (20 kg [COD]/m3/d), as represented by three negative levels in the Fig. 4b. Several reasons could attribute to the sludge washout, such as the hydrodynamic turbulence generated by higher flow rates of influent. Additionally the increase in biogas production under higher OLR could loosen the fraction of organic matter from the granular sludge (Chan et al., 2012; Khemkhao et al., 2011, 2012;). In contrast, the washout phenomenon was not observed in the secondary reactors and the SS removal efficiencies of both of the reactors were almost same (Fig. 4c). From these results, it was ascertained that the secondary reactors played an important role as a filter to prevent the washout of sludge from the primary reactor and, therefore, increased the amount of COD removal.
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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Fig. 4b. the SS removal efficiencies and flow rate with an increase in OLR in the AHR reactor (the error bars represent the 95% confidence interval).
Fig. 5. The effluent pHs of the AHR, ABF, and ADF reactors during the entire experiment. The A, B, and C indicate the periods for different injection amounts of NaHCO3.
tively high pH conditions, but also by the dissociation of the carbonic acid produced from the dissolved CO2 (Chui et al., 1994; Razo-Flores et al., 1997). In addition, it was suggested that the high recycling ratio of 30:1 supported to provide alkalinity and to maintain the pH level in the AHR reactor (Zinatizadeh et al., 2006). Although further research is required to determine the presence and/or concentration of the substances with respect to alkalinity generation in the AHR, such bicarbonate species may possess a high portion of alkalinity as was determined in previous studies. Meanwhile, the effluent pHs of the secondary reactors were determined to be greater than 7 and they increased gradually up to a pH of approximately 8 during the experiment. It was proposed that the pH in the secondary reactors was elevated due to the fact that methanogens are more predominant in the secondary reactors than in the primary reactor in which the acidogens are more abundant from the inflow of the high OLR. Fig. 4c. The SS removal efficiencies with an increase in the OLR in the ABF and ADF reactors.
3.4. Biogas production
3.3.2. pH and buffer Adjusting the pH within the optimum range is very important to achieve performance stability of an anaerobic reactor. Therefore, artificial alkalinity is often added to raw wastewater that contains insufficient amounts of alkaline substances. The POME in this study was acidic with a pH around 4. Thus adjustment of the pH to within the range of 6.8–7.2, which is known to be the optimum range for anaerobic microbial growth (Gerardi, 2006), was taken into consideration in this experiment. During the early stage of this experiment (period A in Fig. 5), 10 g/l of sodium bicarbonate was added to the influent before it was fed into the AHR reactor. The effluent pH increased to approximately 7.1 from an initial level of approximately 3.9. Since stabilized system operation was available from the 4th week on, the amount of sodium bicarbonate was decreased to 5 and 2.5 g/l on the 32nd and 42nd days, respectively. With a decrease in the amount of buffer added, the effluent pH of the AHR reactor was decreased slightly, but not significantly (period B and C in Fig. 5). Sodium bicarbonate was not added anymore after the 7th week. After the 7th week, the effluent pH of the AHR reactor was decreased gradually during the following 2 weeks, but it was constantly maintained at a level of approximately 6.6. This result implies the possibility that alkaline substances were generated during anaerobic digestion in the AHR reactor. Bicarbonate can be generated not only by the conversion of CO2 under rela-
The analysis of biogas production was performed by gathering all of the gases produced from each reactor during period B in Fig. 2. Assuming that the biogas was composed of methane and carbon dioxide, the methane content of the biogas was estimated by measuring the carbon dioxide production with a detector tube. Fig. 6 shows the results of the analysis of the biogas production, methane yield, and carbon dioxide content. The total biogas production increased with an increase in the OLR at 15.0 kg [COD]/ m3/d. The production somewhat elevated at an OLR of 15.4 kg [COD]/m3/d, because of the addition of the ADF reactor to the system. However, the production decreased when the OLR was between 15.4 and 16.9 kg [COD]/m3/d, which indicates that the performance of the AHR reactor was not stabilized during this period. The total biogas production began to increase at an OLR of 18 kg [COD]/m3/d and its maximum level was approximately 110 l/d at an OLR of 18.9 kg [COD]/m3/d. After that, the total biogas production gradually decreased again except for a sudden increase at an OLR of 22.5 kg [COD]/m3/d. Such changes in the total biogas production seemed to be affected by the methane yield. The methane yield showed a similar tendency with the total biogas production and it reached the maximum level of 65.7 l/d at the same OLR as noted above. The methane yield decreased when the OLR increased from 19.9 to 21.9 kg [COD]/m3/d, probably because methanogens became less active with a high OLR in the AHR reactor. The average methane production rate was within the range of 0.171–
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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Fig. 6. The amount of biogas production, methane yield and CO2 content with respect to an increase in the OLR in the combination system during the 4th–22nd weeks.
0.269 l [CH4]/g [COD removed]. This was similar to the previously reported rate reported in research on two-stage reactor (0.26 l [CH4]/g [COD removed]), but lower than that of other research on up-flow anaerobic sludge fixed film (0.278–0.269 l [CH4]/g [COD removed]). The carbon dioxide content of the produced biogas was in the range of 21.8–40.5%. The average carbon dioxide content was approximately 35.4%. This result was in agreement with the values from other studies that were conducted on the generic anaerobic digestion process (Poh and Chong, 2009). Considering the results with respect to biogas production, it could be concluded that the combined reactors for POME treatment were operated properly. 4. Conclusions The combination systems investigated in this study were found to be a successful anaerobic process for the treatment of POME. The AHR reactor was highly efficient and stable even under high OLR conditions and the secondary ABF and ADF reactors helped to improve the safety and performance of the entire process. The systems showed many advantages in terms of the COD removal, performance stability, and pH maintenance. Consequently, the combination of sequential high-rate anaerobic reactors can be regarded as a highly applicable process for POME treatment in the field where a large amount of POME with unstable characteristic is discharged. Acknowledgements This research (No. 10-01-021) was financially supported by the ‘‘Center for Waste Eco-Energy and Non-CO2 Greenhouse Gases (CWEG)’’ as Eco-STAR Project by Ministry of Environment, Korea. References Ahmad, A.L., Ismail, S., Bhatia, S., 2003. Water recycling from palm oil mill effluent (POME) using membrane technology. Desalination 157, 87–95. Annachhatre, A.P., 1996. Anaerobic treatment of industrial wastewaters. Resour. Conserv. Recy. 16 (1–4), 161–166. Beal, L.J., Raman, D.R., 2000. Sequential two-stage anaerobic treatment of confectionery wastewater. J. Agric. Eng. Res. 76 (2), 211–217. Borja, R., Banks, C.J., 1994. Anaerobic digestion of palm oil mill effluent using an upflow anaerobic sludge blanket reactor. Biomass Bioenergy 6 (5), 381–389. Borja, R., Banks, C.J., Sánchez, E., 1996. Anaerobic treatment of palm oil mill effluent in a two-stage up-flow anaerobic sludge blanket (UASB) system. J. Biotechnol. 45 (2), 125–135.
Chan, Y.J., Chong, M.F., Law, C.L., 2012. Start-up, steady state performance and kinetic evaluation of a thermophilic integrated anaerobic-aerobic bioreactor (IAAB). Bioresour. Technol. 125, 145–157. Chin, K.K., Lee, S.W., Mohammad, H.H., 1996. A study of palm oil mill effluent treatment using a pond system. Water Sci. Technol. 34 (11), 119–123. Chui, H., Fang, H., Li, Y., 1994. Removal of formate from wastewater by anaerobic process. J. Environ. Eng. ASCE 120 (5), 1308–1320. Fang, C., Thong, O-, Boe, S, Angelidaki, I, 2011. Comparison of UASB and EGSB reactors performance, for treatment of raw and deoiled palm oil mill effluent (POME). J. Hazard. Mater. 189 (1–2), 229–234. Gerardi, M.H., 2006. Wastewater Bacteria. John Wiley and Sons, Inc., Hoboken, New Jersey, 19–31. Habeeb, S.A., Lariff, A.A.A., Daud, Z., Ahmad, Z., 2011. The start-up of hybrid, anaerobic up-flow sludge blanket (HUASB) under a range of mesophilic and thermophilic temperatures. Environ. Asia 4 (2), 63–68. Khemkhao, M., Nuntakumjorn, B., Techkarnjanaruk, S., Phalakornkule, C., 2011. Effect of chitosan on UASB treating POME during a transition from mesophilic to thermophilic conditions. Bioresour. Technol. 102 (7), 4674–4681. Khemkhao, M., Nuntakumjorn, B., Techkarnjanaruk, S., Phalakornkule, C., 2012. UASB performance and microbial adaptation during a transition from mesophilic to thermophilic treatment of palm oil mill effluent. J. Environ. Manage. 103, 74–82. Lam, M.K., Lee, K.T., 2011. Renewable and sustainable bioenergies production from palm oil mill effluent (POME): Win–win strategies toward better environmental protection. Biotechnol. Adv. 29 (1), 124–141. Latif, M.A., Ghufran, R., Wahid, Z.A., Ahmad, A., 2011. Integrated application of upflow anaerobic sludge blanket reactor for the treatment of wastewaters. Water Res. 45, 4683–4699. Lim, S., Teong, L.K., 2010. Recent trends, opportunities and challenges of biodiesel in Malaysia: an overview. Renew. Sust. Energ. Rev. 14 (3), 938–954. Najafpour, G.D., Zinatizadeh, A.A.L., Mohamed, A.R., Hasnain Isa, M., Nasrollahzadeh, H., 2006. High-rate anaerobic digestion of palm oil mill effluent in an upflow anaerobic sludge-fixed film bioreactor. Process Biochem. 41 (2), 370–379. Poh, P.E., Chong, M.F., 2009. Development of anaerobic digestion methods for palm oil mill effluent (POME) treatment. Bioresour. Technol. 100 (1), 1–9. Razo-Flores, E, Luijten, M, Donlon, B, G, Lettinga, Field, J, 1997. Biodegradation of selected azo dyes under methanogenic conditions. Water Sci. Technol. 36 (6–7), 65–72. Wu, M., Wilson, F., Tay, J.H., 2000. Influence of media-packing ratio on performance of anaerobic hybrid reactors. Bioresour. Technol. 71 (2), 151–157. Wu, T.Y., Mohammad, A.W., Jahim, J.M., Anuar, N., 2010. Pollution control technologies for the treatment of palm oil mill effluent (POME) through endof-pipe processes. J. Environ. Manage. 91 (7), 1467–1490. Yacob, S., Hassan, M.A., Shirai, Y., Wakisaka, M., Subash, S., 2005. Baseline study of methane emission from open digesting tanks of palm oil mill effluent treatment. Chemosphere 59 (11), 1575–1581. Yacob, S., Hassan, M.A., Shirai, Y., Wakisaka, M., Subash, S., 2006. Baseline study of methane emission from anaerobic ponds of palm oil mill effluent treatment. Sci. Total Environ. 366 (1), 187–196. Yeoh, C.Y., Chin, N.L., Tan, C.S., 2011. Co-composting of palm oil mill wastes. J. Food Agric. Environ. 9 (3–4), 880–885. Zinatizadeh, A.A.L., Mohamed, A.R., Abdullah, A.Z., Mashitah, M.D., Hasnain Isa, M., Najafpour, G.D., 2006. Process modeling and analysis of palm oil mill effluent treatment in an up-flow anaerobic sludge fixed film bioreactor using response surface methodology (RSM). Water Res. 40, 3193–3208.
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Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors Won-Ho Choi, Chang-Ha Shin, Sung-Min Son, Praveen A. Ghorpade, Jeong-Joo Kim, Joo-Yang Park ⇑ Department of Civil and Environmental Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
h i g h l i g h t s " The combined sequential anaerobic reactors were developed for POME treatment. 3
" Average COD removal of 93.5% was obtained with the OLR of 8.5–23 kg [COD]/m /d. 3
" Maximum COD removal of 95.6% was achieved at 13 kg [COD]/m /d OLR in primary AHR. " The secondary reactors helped to improve the safety and performance of the system. " Average methane yield of 0.171–0.269 l [CH4]/g [COD removed] was obtained.
a r t i c l e
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Article history: Available online xxxx Keywords: Palm oil mill effluent (POME) Anaerobic hybrid reactor Anaerobic filter reactor Organic loading rate (OLR) Organic removal rate
a b s t r a c t Combined system of high-rate anaerobic reactors for treating palm oil mill effluent (POME) was developed and investigated in this study. The system composed of one common primary hybrid reactor which was shared by two different secondary filter reactors. An overall COD removal efficiency of 93.5% was achieved in both systems. The secondary reactors contributed not only in enhancing the COD removal efficiency, but also ensured the performance stability of the entire system. Biomass remained intact in the secondary reactor in contrast to the primary reactor in which occasional washout of biomass was observed. The pH of POME was adjusted at the beginning of the operation, as the process continued POME did not require the external pH adjustment as the pH was maintained in desired range. The biogas was produced up to 110 l/d with the yield of 0.171–0.269 l [CH4]/g [COD removed] and 59.5–78.2% content of methane. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Palm oil is one of the most widely produced vegetable oils in the world and currently its production is being boosted with extending their use in making biodiesel (Lim and Teong, 2010). However, the unsustainability of palm oil production has been constantly criticized, because the large quantities of biomass residues (almost 5 times the weight of oil production) are a serious threat to the environment (Ahmad et al., 2003). In particular, palm oil mill effluent (POME) causes a greater impact than the other by-products of palm oil production, of which estimated amount is 3 times more than that of crude palm oil (Wu et al., 2010; Yeoh et al., 2011). POME is a viscous brown liquid containing high concentrations of organic acids with a COD level higher than 20,000 mg/l (Lam and Lee, 2011; Najafpour et al., 2006). Aerobic treatment is not suitable for POME which has insufficient amount of nutrients (Chin et al., 1996). On the other hand anaerobic treatment is favorable for ⇑ Corresponding author. Tel.: +82 2 2220 0411; fax: +82 2 2220 1945. E-mail address: [email protected] (J.-Y. Park).
POME treatment as it can remove much more organics even with limited available nutrients. Therefore, anaerobic treatment processes have primarily been adopted for POME in the field (Poh and Chong, 2009). Facultative ponds and open digesting tanks are the most commonly used anaerobic processes for the treatment of POME (Yacob et al., 2005). Although these conventional processes require relatively little energy to operate, they demand extensive land area and long retention time (Lam and Lee, 2011; Wu et al., 2010). Besides, a large quantity of greenhouse gases including methane and carbon dioxide is produced from open ponds and tanks, and these gases are emitted directly into the atmosphere, contributing to a serious global warming problem. Since methane has 21 times more global warming potential than carbon dioxide, the total emission of the greenhouse gases can be substantially reduced if the methane is completely recovered in sealed anaerobic reactors and simply burnt to convert it into carbon dioxide. Furthermore, the biogas containing methane from the anaerobic digestion of POME is a very promising source of renewable energy (Fang et al., 2011; Yacob et al., 2006).
0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.02.055
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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The upflow anaerobic sludge blanket (UASB) system, in which settleable anaerobic biomass is aggregated and immobilized in the granules of a dense sludge bed, is a high-rate anaerobic process that has been successfully applied for various types of wastewater (Fang et al., 2011; Poh and Chong, 2009). The potential of UASB for treating POME has been identified (Borja and Banks, 1994; Borja et al., 1996; Wu et al., 2010). High contents of suspended and colloidal solids as well as residual oil in POME at high organic loading rates (OLR) can cause adverse effects on the performance of UASB reactor including the deterioration of microbial activities, the washout of sludge granules, and hindered growth of granules (Poh and Chong, 2009). The sequential two-stage anaerobic treatment system composed of a first stage UASB and a second-stage downflow anaerobic filter (DFAF) has been investigated to treat confectionary wastewater (Beal and Raman, 2000). This combination system was able to achieve an overall COD removal efficiency of 98% at the overall OLR of 12.5 kg [COD]/m3/d. The secondary reactor contributed not only to the greater removal of organic carbon, but it also helped to buffer the fluctuations in the effluent of the primary reactor. These properties can enhance the applicability of UASB in many cases. However, it has been suggested that the combination system containing UASB is inappropriate for the treatment of POME, because the deficiencies of UASB that are mentioned above still exist. The upflow anaerobic sludge fixed-film (UASFF) reactor, a hybrid bioreactor with UASB and anaerobic filters, shows greater stability than the UASB when operated under high OLR. Since the filter media packed at the upper part can overcome the well-known deficiencies of UASB, the UASFF reactor has been successfully applied for the treatment of various types of wastewater as well as POME (Wu et al., 2010). In the treatment of POME (>40,000 mg [COD]/l) using 5 l UASFF reactor, the COD removal efficiencies of more than approximately 90% were achieved with a hydraulic retention time (HRT) in the range of 1.5–3 days (Najafpour et al., 2006; Zinatizadeh et al., 2006). According to these studies, internal packing effectively contributed to the maintenance of the performance of UASFF reactor by capturing the solids that floated from sludge bed. This result allowed for a high ratio of effluent recycle, which was required for the treatment of high concentrations of organic acids in POME. However, it was indicated that a higher HRT was needed in order to obtain more complete digestion, which is usually not available in the field where a large amount of POME is discharged. In this study, two high-rate anaerobic processes were designed and investigated for the treatment of POME, which aimed to utilize the advantages of a hybrid reactor and combination system. The developed processes were composed of one hybrid reactor and two different filter reactors, which share the hybrid reactor as the primary reactor. The primary reactor, an anaerobic hybrid reactor (AHR), was composed of sludge bed for the lower zone like a conventional UASB and filter media for the upper zone. In an AHR, the wastewater is first treated by the anaerobic microorganisms of the granular sludge in the lower sludge bed zone and then it is also treated by the biofilm on the surface of the filters in the upper filter media zone. The media in the upper zone prevents the washout of granules from the lower zone, which occurs frequently under high flow rates. The secondary reactors were an anaerobic baffled filter reactor (ABF) and an anaerobic downflow filter reactor (ADF). The ABF used the same filter media in the AHR and had a baffle installed vertically in the middle of the reactor to obtain more plug flow condition with up-and-down flow path. The ADF was packed with baked clay globules which have a high adhesion capacity for bacteria. The objectives of the current study were: (1) to evaluate the feasibility of combination systems (AHR + ABF and AHR + ADF) for POME treatment and (2) to investigate the performance of each
reactor in terms of COD removal, OLR, suspended solids (SS), pH, biogas production, and methane yield. For this purpose, lab pilots of the combination systems were operated near a palm oil mill in Indonesia. 2. Methods 2.1. Characteristics of POME The raw POME was collected by onsite sampling in Indonesia. Generally, the characteristics of POME depend on the features of the manufacturing process in the palm oil mill plant and seasonal changes in the crops (Poh and Chong, 2009). The efficiency of anaerobic treatment of POME is significantly affected by these characteristics. High OLR during the anaerobic treatment process may cause a decrease in the removal efficiency by producing scum or clogging the system (Latif et al., 2011). In order to avoid the scum and clogging problems caused by high concentrations of SS and residual oil in POME in the current study, the raw POME was pretreated to remove excess SS and oil using a 3-phase screw decanter. The characteristics of the pretreated POME (after the 3phase screw decanter) were as follows: sCODcr 19,700 mg/l, TN 154 mg/l, TP 212 mg/l, TSS 3,115 mg/l and pH 3.9. The recommended ratio of COD:N:P in an anaerobic bioreactor is 300:5:1 during the start-up period and 600:5:1 in the steady state (Annachhatre, 1996). The samples satisfied this ratio and, therefore, additional nutrients were not added. 2.2. Reactor design and configuration The schematic diagram of the experimental set-up for the combination of the AHR followed by ABF and ADF is shown in Fig. 1. The AHR was made of acryl resin with a cylindrical shape. The total volume was designed as 20 l (H 65 cm I.D. 20 cm). The upper part of the AHR was packed with pall rings up to a depth of 15 cm (polyethylene, diameter: 5/8 in., specific area: 362 m2/m3, porosity: 92%, Solmaro Trading & Engineering Company, South Korea). This packing served as a gas–solid liquid separator as well as filter. The AHR was designed as a completely mixed reactor with a sludge blanket. A surge tank was placed immediately after the AHR in order to recycle the effluent. The recycle ratio (the ratio of flow rates of recirculation to influent) was maintained at 30:1 to create a close complete mix condition in the AHR taking into consideration the high concentrations of organic acids in POME. The reactors that followed the primary reactor (10 l, H 61 cm I.D. 15 cm) were made to have half of the volume of the AHR. The ABF was divided into two chambers by a baffle made of acrylic resin (T 0.5 cm W 15 cm H 55 cm). The ABF was packed with the same pall rings that were in the AHR from the surface to a depth of 45 cm (70%, H/H). The ADF reactor was packed with clay ball media (diameter: 1/2 in.) up to 70% of the effective volume in the reactor. Both the ADF and ABF were connected to the AHR reactor by a silicon tube (I.D. 8 mm, MasterflexÒ) with a gradient of 0.2 to allow for gravity flow. The flow rate was controlled by using peristaltic pumps. Biogas was measured with a rotary gas meter (Ritter, TG 05/5). A water jacket was installed to maintain the temperature of all of the reactors by circulating heated water. 2.3. Experimental method For start-up, 2 l of granular biomass from a lab-scale UASB reactor was seeded into the AHR. The reactors that followed the primary reactor were unseeded. The combination systems were operated under mesophilic conditions (36 ± 1 °C). The pH in the AHR was adjusted to 7 ± 0.5 using NaHCO3 during the initial stage.
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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Fig. 1. Schematic diagram of the developed systems containing combined high-rate anaerobic reactors (AHR + ABF and AHR + ADF).
The overall HRT ranged from 0.7 to 2.4 days. Samples were taken every 6 h. Except for the period of the initial start-up stages, POME was fed into the reactors for 5 weekdays only. The OLR was increased stepwise from 0.91 to 23 kg [COD]/m3/day taking into consideration the criteria for a steady state proposed in the previous study (Wu et al., 2000). 2.4. Analytical method The COD values were measured using the reactor digestion method (HACH, method 8000, range: 20–1500 mg/l) after pretreating the samples with a centrifugal separator for 15 min at 3000 rpm. The T N was measured using the persulfate digestion method (range: 2–150 mg/l) and the T P was measured using the molybdovanadate method (range: 1–100 mg/l) with a spectrophotometer (DR-2800, HACH). The SS were measured as Total SS (TSS) using the standard method (20ed, 2540). The total volume of biogas was measured using rotary gas meter (Ritter, TG-05/5). It was assumed that CH4, CO2, and H2S were the primary components of the biogas produced from the systems, but the H2S was removed entirely using a desulfurization column (H 8 cm I.D. 12 cm), which was packed with Fe2O3 (94%, BayferroxÒ). The CO2 contents were measured using a gas detection tube (Gastech, 2HT, range: 10–100%), then the methane contents were determined assuming that the remaining volume of the biogas was methane. 3. Results and discussion 3.1. Effect of different operational conditions The experiment was carried out continuously for approximately 25 weeks and the overall performance of the systems in terms of COD removal and OLR are described in Fig. 2. The AHR reactor was operated throughout the experiment. The ABF reactor was also installed and operated together with the AHR from the beginning of experiment, but the ADF reactor was not installed and operated until the 9th week. The effluent from the primary reactor was evenly distributed by V-notched weirs to the secondary reactors, which had half the volumes of the AHR.
The influent OLR was maintained within the range of 0.91– 23.0 kg [COD]/m3/d by different dilutions with POME and/or by adjustment of the flow rate. Initially the OLR was gradually increased from 0.91 to 13.73 kg [COD]/m3/d during the first 2 weeks. However, the COD removal efficiency in the reactors rapidly decreased and stable operation could not be maintained due to the shock overloading by the rapid increase of the OLR (period A in Fig. 2). Therefore, the OLR was moderated to the level of approximately 10 kg [COD]/m3/d in order to recover the reactor performance during the 3rd week. From 4th to 22nd weeks (period B in Fig. 2), the reactor was operated with a stepwise increase in the OLR from approximately 10–23 kg [COD]/m3/d. Meanwhile, a sudden decrease from 93.3% to 75.8% in the COD removal was observed in the AHR reactor during the 9th week (indicated by arrow in Fig. 2). High levels of SS concentration in the influent could be responsible for this abruption. Therefore, the process to remove the SS from POME was also performed through the settling of the influent solids for 24 h before being allowed to flow into the system from the 9th week on. During the last 2 weeks (period C in Fig. 2), the OLR was changed to approximately 15 kg [COD]/m3/d in order to investigate the effects of the SS on the COD removal efficiency that will be discussed later in this article. 3.2. Steady-state performance of the process in terms of COD removal The stable process performance was not achieved during the period A, because of the operational failure by rapid increase of the influent OLR (as mentioned in Section 3.1). The process performance was restored by adjusting the OLR in 3rd week, however the reactor was stabilized in terms of COD removal after 4th week. Once the steady-state condition was achieved, the process performance of the reactor in terms of COD removal was evaluated throughout the period B. During the experimental period B in Fig. 2, the COD removal efficiencies of the combined reactors were almost always maintained at high levels even though the initial OLR of 10 kg [COD]/ m3/d was increased more than double to 23 kg [COD]/m3/d. A reasonably stable COD removal of more than 88% was achieved and the maximum COD removal efficiency was found to be 95.6% at the OLR of approximately 13 kg [COD]/m3/d in the AHR reactor. This performance was similar and/or higher than the hybrid reac-
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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Fig. 2. The COD removal efficiencies with a change in OLR in the AHR reactor and the combination system during the entire experiment. The letters A, B, and C in the figure indicate the periods distinguished by different operational conditions. The arrow indicates the sudden decrease in the COD removal by high levels of SS concentration in the AHR.
Fig. 3a. The behavior of the COD removal efficiency and organic removal rate with an increase in the OLR in the AHR reactor during the 4th–22nd weeks.
Fig. 3b. The COD removal efficiencies with an increase in the OLR in the ABF and ADF reactors during the 4th–22nd weeks.
Fig. 3c. The organic removal rates with an increase in the OLR in the ABF and ADF reactors during the 4th–22nd weeks.
tors for POME treatment reported in the previous studies (Zinatizadeh et al., 2006; Habeeb et al., 2011). The COD removal efficiency in the AHR was relatively similar with the UASFF reactor where the COD removal efficiencies of approximately 90% at an OLR of 14.93 g COD/l/d was observed in stable condition (Zinatizadeh et al., 2006). But the performance of the AHR was far more stable even with higher OLR (23 kg [COD]/m3/d) and longer period (19 weeks) when compared to that of the UASFF hybrid reactor (25 days). Besides, as compared to the HUASB hybrid reactor where the maximum COD removal efficiency was 91% at the OLR of 9.37 kg/m3/d at 46 °C (Habeeb et al., 2011), the AHR also showed a better performance even with the higher OLR and lower temperature. Fig. 3a shows the changes in the COD removal efficiency and the organic removal rate along with the different OLR values in the AHR reactor. The COD removal efficiency of the AHR reactor somewhat fluctuated, but the range of fluctuation was not significant throughout the entire experiment. The performance of the COD removal was maintained almost up to an OLR of 23 kg [COD]/m3/d, which was the maximum level reached in the experiment. The changes in the organic removal rate reasonably reflect the
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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above-mentioned behaviors in the AHR reactor. The organic removal rate increased from 7.77 to 20.75 kg [COD]/m3/d with an increase in the OLR from 8.53 to 23 kg [COD]/m3/d. A linear relationship was observed when the organic removal rate was plotted as a function of the OLR. The best-fit linear regression line of the removal rates to the loading rates shows a fairly high correlation with the coefficient of determination R2 0.98, and the slope of regression line indicates an average COD removal efficiency of 88.9% in the AHR. This result implies that the performance stability of the AHR reactor could be maintained constantly under substantial changes in the OLR. Consequently, the AHR reactor can be regarded as a highly efficient anaerobic reactor that achieves sufficient COD removal even if it will be the only reactor used. The effluent from the primary reactor was additionally treated by the secondary reactors, thus the average COD removal efficiencies of the combination systems (AHR + ABF and AHR + ADF) were observed to be approximately 93.5%. The OLRs of the secondary reactors were very much less than those of the primary reactor, ranging from approximately 0.58 to 3.82 kg [COD]/m3/d during period B, because the influent into the secondary reactors was previously treated through AHR reactor and its flow rate was reduced to half of the original rate by the V-notched distributor. These lower OLR values were introduced to the different performance behaviors of the secondary reactors as shown in Figs. 3b and 3c. In contrast with the results of the primary reactor, the COD removal efficiencies with an increase of OLR were not maintained at a certain level in the secondary reactors (Fig. 3b). The plotted data were scattered so that the clear relationship was not seen, but the COD removal efficiencies of the secondary reactors showed a reasonable trend toward increasing along with an increase in the OLR though their values were relatively less than those of the primary reactor. This result suggests that the secondary reactors can be operated while ensuring the stable removal of COD even under much higher OLR conditions than in the primary reactor. Therefore, the capacity reserved in the secondary reactors was high enough to help improve the safety and performance of the whole process of the combination systems. Fig. 3c shows that the organic removal rate of the secondary reactors increased with an increase in the OLR, which was similar to the results of the AHR reactor. Linear relationships between the organic loading and removal rates were observed, but their correlations were a little lower due to their operation under much lower OLR values. From the slopes of the regression lines, the average organic removal rates of the ABF and ADF reactors were found to be 47.9% and 47.1%, respectively. The influent OLR was increased at the beginning of every week, during this time the fluctuation in the COD removal was observed (Fig. 2). However, the COD removal efficiency of secondary reactors was unaffected with a minor fluctuation. This shows that the combined reactors in present system are capable of maintaining the stable performance even in situations, such as shock loading and unexpected operational changes. 3.3. Other parameters 3.3.1. Effect of suspended solids on COD removal As mentioned in Section 3.1 and can be seen in Fig. 2 (indicated by arrow), there was a sudden decline in the COD removal in the AHR reactor at approximately 4000 mg/l of SS during the 9th week. Therefore, the SS in POME were treated in order to obtain a value below 3000 mg/l before they were allowed to flow into AHR reactor. An additional experiment was carried out to investigate the effects of the SS on the COD removal during period C. The COD removal efficiency of the AHR reactor increased when the OLR was adjusted to values similar to those in period A. This result suggests that a specific SS concentration may exist that can influence the performance of the AHR reactor. The fluctuations in the COD
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Fig. 4a. The COD removal efficiency and SS concentration with an increase in the SS loading rate in the AHR reactor.
removal efficiency due to changes in the SS concentration can be seen in Fig. 4a, which shows the changes in the COD removal efficiency based on the SS loading rate. Although the relationship between the COD removal efficiency and the SS loading rate was not clear, the highest COD removal efficiency of approximately 93% was observed at a SS loading rate of approximately 2.6 kg/m3/d where the level of SS concentrations around 3000 mg/l. This could be because of the presence of digestible organic solids suspended in POME. It was observed that the COD removal efficiencies were slightly lower after the SS loading rate of 2.6 kg/m3/d, because relatively high levels of SS probably disturbed the hydrolysis of the anaerobic digestion (Latif et al., 2011). Therefore, when the SS loading rate with respect to the COD removal was taken into consideration, the pretreatment of POME to achieve a SS value below 3000 mg/l was required to ensure the performance and stability of the AHR reactor. Analysis of the SS removal in the effluents of each of the reactors was conducted during this experiment, the resulting SS removal efficiencies are described in Figs. 4b and 4c. For better understanding of the relationship between the SS removal and OLR in the AHR, the average values of measured data (except for negative levels) were plotted with the error bars that reflect the 95% confidence limit as shown in Fig. 4b. A decreasing trend in the SS removal efficiency with an increase in the OLR was observed mostly in the AHR. Occasionally some negative levels were observed during operation within the OLR of 17–21 kg [COD]/m3/d. The negative SS removal efficiencies indicate that the fractions of the biomass were washed out of the reactor. The SS removal efficiency at the OLR of 20 kg [COD]/m3/d was relatively higher which deviated slightly from the trend. This behavior resulted because larger extent and more frequent washout of biomass occurred at the aforementioned level of OLR (20 kg [COD]/m3/d), as represented by three negative levels in the Fig. 4b. Several reasons could attribute to the sludge washout, such as the hydrodynamic turbulence generated by higher flow rates of influent. Additionally the increase in biogas production under higher OLR could loosen the fraction of organic matter from the granular sludge (Chan et al., 2012; Khemkhao et al., 2011, 2012;). In contrast, the washout phenomenon was not observed in the secondary reactors and the SS removal efficiencies of both of the reactors were almost same (Fig. 4c). From these results, it was ascertained that the secondary reactors played an important role as a filter to prevent the washout of sludge from the primary reactor and, therefore, increased the amount of COD removal.
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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Fig. 4b. the SS removal efficiencies and flow rate with an increase in OLR in the AHR reactor (the error bars represent the 95% confidence interval).
Fig. 5. The effluent pHs of the AHR, ABF, and ADF reactors during the entire experiment. The A, B, and C indicate the periods for different injection amounts of NaHCO3.
tively high pH conditions, but also by the dissociation of the carbonic acid produced from the dissolved CO2 (Chui et al., 1994; Razo-Flores et al., 1997). In addition, it was suggested that the high recycling ratio of 30:1 supported to provide alkalinity and to maintain the pH level in the AHR reactor (Zinatizadeh et al., 2006). Although further research is required to determine the presence and/or concentration of the substances with respect to alkalinity generation in the AHR, such bicarbonate species may possess a high portion of alkalinity as was determined in previous studies. Meanwhile, the effluent pHs of the secondary reactors were determined to be greater than 7 and they increased gradually up to a pH of approximately 8 during the experiment. It was proposed that the pH in the secondary reactors was elevated due to the fact that methanogens are more predominant in the secondary reactors than in the primary reactor in which the acidogens are more abundant from the inflow of the high OLR. Fig. 4c. The SS removal efficiencies with an increase in the OLR in the ABF and ADF reactors.
3.4. Biogas production
3.3.2. pH and buffer Adjusting the pH within the optimum range is very important to achieve performance stability of an anaerobic reactor. Therefore, artificial alkalinity is often added to raw wastewater that contains insufficient amounts of alkaline substances. The POME in this study was acidic with a pH around 4. Thus adjustment of the pH to within the range of 6.8–7.2, which is known to be the optimum range for anaerobic microbial growth (Gerardi, 2006), was taken into consideration in this experiment. During the early stage of this experiment (period A in Fig. 5), 10 g/l of sodium bicarbonate was added to the influent before it was fed into the AHR reactor. The effluent pH increased to approximately 7.1 from an initial level of approximately 3.9. Since stabilized system operation was available from the 4th week on, the amount of sodium bicarbonate was decreased to 5 and 2.5 g/l on the 32nd and 42nd days, respectively. With a decrease in the amount of buffer added, the effluent pH of the AHR reactor was decreased slightly, but not significantly (period B and C in Fig. 5). Sodium bicarbonate was not added anymore after the 7th week. After the 7th week, the effluent pH of the AHR reactor was decreased gradually during the following 2 weeks, but it was constantly maintained at a level of approximately 6.6. This result implies the possibility that alkaline substances were generated during anaerobic digestion in the AHR reactor. Bicarbonate can be generated not only by the conversion of CO2 under rela-
The analysis of biogas production was performed by gathering all of the gases produced from each reactor during period B in Fig. 2. Assuming that the biogas was composed of methane and carbon dioxide, the methane content of the biogas was estimated by measuring the carbon dioxide production with a detector tube. Fig. 6 shows the results of the analysis of the biogas production, methane yield, and carbon dioxide content. The total biogas production increased with an increase in the OLR at 15.0 kg [COD]/ m3/d. The production somewhat elevated at an OLR of 15.4 kg [COD]/m3/d, because of the addition of the ADF reactor to the system. However, the production decreased when the OLR was between 15.4 and 16.9 kg [COD]/m3/d, which indicates that the performance of the AHR reactor was not stabilized during this period. The total biogas production began to increase at an OLR of 18 kg [COD]/m3/d and its maximum level was approximately 110 l/d at an OLR of 18.9 kg [COD]/m3/d. After that, the total biogas production gradually decreased again except for a sudden increase at an OLR of 22.5 kg [COD]/m3/d. Such changes in the total biogas production seemed to be affected by the methane yield. The methane yield showed a similar tendency with the total biogas production and it reached the maximum level of 65.7 l/d at the same OLR as noted above. The methane yield decreased when the OLR increased from 19.9 to 21.9 kg [COD]/m3/d, probably because methanogens became less active with a high OLR in the AHR reactor. The average methane production rate was within the range of 0.171–
Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055
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Fig. 6. The amount of biogas production, methane yield and CO2 content with respect to an increase in the OLR in the combination system during the 4th–22nd weeks.
0.269 l [CH4]/g [COD removed]. This was similar to the previously reported rate reported in research on two-stage reactor (0.26 l [CH4]/g [COD removed]), but lower than that of other research on up-flow anaerobic sludge fixed film (0.278–0.269 l [CH4]/g [COD removed]). The carbon dioxide content of the produced biogas was in the range of 21.8–40.5%. The average carbon dioxide content was approximately 35.4%. This result was in agreement with the values from other studies that were conducted on the generic anaerobic digestion process (Poh and Chong, 2009). Considering the results with respect to biogas production, it could be concluded that the combined reactors for POME treatment were operated properly. 4. Conclusions The combination systems investigated in this study were found to be a successful anaerobic process for the treatment of POME. The AHR reactor was highly efficient and stable even under high OLR conditions and the secondary ABF and ADF reactors helped to improve the safety and performance of the entire process. The systems showed many advantages in terms of the COD removal, performance stability, and pH maintenance. Consequently, the combination of sequential high-rate anaerobic reactors can be regarded as a highly applicable process for POME treatment in the field where a large amount of POME with unstable characteristic is discharged. Acknowledgements This research (No. 10-01-021) was financially supported by the ‘‘Center for Waste Eco-Energy and Non-CO2 Greenhouse Gases (CWEG)’’ as Eco-STAR Project by Ministry of Environment, Korea. References Ahmad, A.L., Ismail, S., Bhatia, S., 2003. Water recycling from palm oil mill effluent (POME) using membrane technology. Desalination 157, 87–95. Annachhatre, A.P., 1996. Anaerobic treatment of industrial wastewaters. Resour. Conserv. Recy. 16 (1–4), 161–166. Beal, L.J., Raman, D.R., 2000. Sequential two-stage anaerobic treatment of confectionery wastewater. J. Agric. Eng. Res. 76 (2), 211–217. Borja, R., Banks, C.J., 1994. Anaerobic digestion of palm oil mill effluent using an upflow anaerobic sludge blanket reactor. Biomass Bioenergy 6 (5), 381–389. Borja, R., Banks, C.J., Sánchez, E., 1996. Anaerobic treatment of palm oil mill effluent in a two-stage up-flow anaerobic sludge blanket (UASB) system. J. Biotechnol. 45 (2), 125–135.
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Please cite this article in press as: Choi, W.-H., et al. Anaerobic treatment of palm oil mill effluent using combined high-rate anaerobic reactors. Bioresour. Technol. (2013), http://dx.doi.org/10.1016/j.biortech.2013.02.055