- Email: [email protected]
Start-up Strategy of a Thermophilic Upflow Anaerobic Filter for Treating Palm Oil Mill Effluent
0957–5820/03/$23.50+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 81, Part B, July 2003
www.ingentaselect.com=titles=09575820.htm
START-UP STRATEGY OF A THERMOPHILIC UPFLOW ANAEROBIC FILTER FOR TREATING PALM OIL MILL EFFLUENT S. MUSTAPHA1 , B. ASHHUBY 2 , M. RASHID 3 and I. AZNI1 1
Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor Darul Ehsan, Malaysia 2 Department of Environmental Studies, Faculty of Engineering and Technology, University of Sheba, Brack-Alshati, Libya 3 Department of Chemical Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia.
A
thermophilic up ow anaerobic lter treating palm oil mill ef uent was started up in a dm3 laboratory-scale reactor. The reactor was operated continuously at a thermophilic temperature 55 C for over 10 weeks with increased loadings of chemical oxygen demand (COD) of 0.1–0.5 kg COD m 3 day 1. The start-up strategy was accomplished over sequential increase of temperature of 0.5–1 C per day in which the mesophilic bacterial seed was gradually acclimatized to the thermophilic conditions. At these conditions, the reactor showed satisfactory results in organic removal ef ciency (up to 97 and 94% for biological oxygen demand (BOD) and COD, respectively) and biogas production rates of 26.8– 116.1 m 3 day 1 for organic loading rates varied from an initial value of 5.8 to 10.9 kg COD m 3 day 1. The thermophilic anaerobic lter was stable in terms of acidity and alkalinity. In this study, the start-up process of the thermophilic reactor was accomplished in a relatively short time and was stable under mesophilic conditions. Keywords: palm oil mill ef uent; thermophilic digestion; up ow anaerobic lter; start-up; stability; biogas.
mills, alcohol distillery, canning factory), and pulp and paper factories (Ahring, 1994; Borja and Banks, 1993). This may offer advantages for POME treatment since the temperature of POME leaving palm oil mills varies between 50 and 70¯ C, thus eliminating the requirement for cooling prior to biological treatment (Mendez et al., 1995; Dinsdale et al., 1996). In addition, at high temperatures reaction rates proceed much faster which, in turn, results in much higher potentials of thermophilic reactors in comparison to mesophilic reactors (Ahring, 1994; Lettinga, 1995). A previous study reported that the thermophilic POME sample gave higher yields than the mesophilic POME sample (Quarmby and Foster, 1995). However, start-up of the thermophilic anaerobic reactors and the high process sensitivity to environmental changes, e.g. temperature during the start-up stage, are drawbacks (Lettinga, 1995). This paper presents the results of study using a modi ed start-up strategy for a thermophilic up- ow anaerobic lter treating palm oil mill ef uent.
INTRODUCTION The production of palm oil from the fruit Elaeis guineensis is a major industry in Southeast Asia, notably in Malaysia (Ma, l998a). However, the ef uent resulting from the processing operations contains high concentrations of free and dissolved oil and fatty acids, glycerine, crude oil solids, starches, proteins and plant tissues (Cheah et al., 1998). In practice the ef uents from different sources are combined together in one stream commonly known as palm oil mill ef uent (POME). POME typically has a biological oxygen demand (BOD) of 25–40 g l¡1; chemical oxygen demand (COD) of 40–70 g l¡1; suspended solids, 20–40 g l¡1; oil and grease, 4–12 g l¡1 and pH 4.5–5.0 (Ashhuby et al., 1996). The use of conventional digesters (such as close tank digester) to treat POME is characterized by long residence times, often excess of 20 days and poor performance (Ma, 1998a). Some research on POME has been reported regarding the application of modern anaerobic sludge blanket system (Borja and Banks, 1994). Up ow anaerobic lters (UAF) can be operated at either mesophilic or thermophilic temperature ranges. Thermophilic anaerobic lters offer an attractive alternative for the treatment of medium and high strength wastewaters, especially for those wastewaters which are discharged at high temperatures like wastewaters from many food processing industries (palm oil
MATERIALS AND METHODS Experimental Rig Description Figure 1 illustrates the process ow diagram of the experimental set-up of the up ow anaerobic lter. The lter was 262
THERMOPHILIC UPFLOW ANAEROBIC FILTER
263
Figure 1. Schematic diagram of thermophilic up ow anaerobic lter.
constructed of Perspex column with an internal diameter of 19.6 cm. An inlet section 10 cm long at the bottom of the lter and a partitioning chamber of 15 cm at the top were provided (Figure 1). The column was hand packed with a helical plastic rings commercially named ‘Tollerette’ as a support surface for bacteria. The plastic packing medium has dimensions of 4.2–4.5 cm in diameter and 1.8–1.9 cm height and has a high porosity of 93%, so a smaller fraction of the reactor volume is occupied, giving an effective reactor volume of 23.8 l. The total volume of the media was 22.85 l, which represents 89% of the total reactor volume. The lter was operated in the thermophilic temperature range (e.g. 54–56¯ C). The operating temperature in the lter was maintained by continuously circulating hot water from a thermostatic water bath through a water jacket around the column. A thermometer was inserted at the top of the column to monitor the operating temperature of the lter. Three sampling ports were provided at 35, 60 and 85 cm intervals from the bottom of the lter for the purpose of sampling collection at various depths of the lter. Anaerobic conditions in the lter were maintained by introducing the feed (POME) at the bottom of the lter via a variable speed peristaltic pump. The feed was stored at 4¯ C and was daily fed to the lter at room temperature. The treated ef uent was withdrawn through a gas–liquid closed separator, in which the gas bubbles were separated from the liquid ef uent. The volume of the evolved biogas was measure using a wet-test gas meter (Model OSKI, Japan).
water surface during the sampling time. The anaerobic sludge was anaerobic sampled from at least 50 cm deep under the water surface to ensure that the sample was undoubtedly taken from the zone of anaerobic bacteria and not from the facultative one (Ahring, 1994). The sludge was pumped from previously conditioned containers using nitrogen gas to purge the air space above the sludge layer. The physical and chemical compositions of this sludge is summarized in Table 1. POME POME samples were collected two to three times a month from Pengilli Palm Oil Mill located in the state of Johor. The samples were taken from about 20 m from the discharging point. The POME samples were stored in PVC container at 4¯ C for daily use. The characteristics of the POME used are shown in Table 2. The values shown represent the average values from duplicate samples carried out in the study. Anaerobic Filter Conditioni ng and Seeding After lling the anaerobic lter with the packing material, a water heater was set to achieve mesophilic temperature (i.e. 34¯ C). Then an oxygen-free nitrogen gas was purged at the bottom of the lter to replace the air inside the lter prior Table 1. Characterisitic of the anaerobic sludge seed.
POME and Anaerobic Sludge Samples Inoculum (anaerobic sludge) The anaerobic lter was inoculated using anaerobic sludge taken from the anaerobic pond at Elmina Palm Oil Mill located at Sungai Buloh in the State of Selangor, Malaysia. The sludge was methanogenically active as the biogas bubbles were apparently observed stripping at the Trans IChemE, Vol 81, Part B, July 2003
Parameter
Unit
Value
Temperature pH BOD COD TSS VSS
C — mg l mg l mg l mg l
34 7.2 4800 11,150 13,250 10,600
1 1 1 1
MUSTAPHA et al.
264 Table 2. Composition and features of POME samples. Parameter
Unit
Temperature PH COD BOD TSS VSS TS TKN PO4 NO3 Cl Mg Cu Zn Mn Pb
C mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Value measureda
Ma (1988b) data
73 4.6 58,000 26,000 23,200 20,800 45,000 920 775 62.5 900 43.4 1.8 4.7 6.9 12.4
— — 50,000 25,000 — — 40,000 — — — — 615 0.89 2.3 2.0 —
a
Result in average of triplicate test.
to seeding the bacterial inoculum (anaerobic sludge). The nitrogen gas was introduced at a ow rate of 15 l min¡1 at 20 psi for 15 min. Inoculation of the lter was conducted by anaerobic lling into the column. The operation of the reactor was started by introducing the feed (diluted POME: 10 g l¡1 COD) at very slow ow rates (2.38 l day¡1), which is equivalent to a hydraulic retention time of 10 days. Analytical Analyses The following parameters: COD, BOD, total suspended solids (TSS), volatile suspended solid (VSS), volatile fatty acid (VFA), pH and biogas production rate were analyzed daily during the start-up stage. These analyses were carried out according to the standard procedures (APRA et al., 1989). The elemental composition of POME and sludge was analysed using an atomic absorption spectrophotometer. A gas chromatograph (GC) equipped with a 10 m £ 0.53 mm £ 0.1 mm HP-FFAP fused-silica capillary column, ame ionization detector (FID) and a mass spectroscopy detector (GC-MS) was used to measure the composition once a week. Detailed analysis procedures are presented in the previous study (Ashhuby et al., 1997). Start-up Strategy Initially, the reactor was operated at the mesophilic temperature (i.e. 37¯ C) at 10 days hydraulic retention time (HRT).
In the rst four runs, the concentration of the feed to the reactor was increased gradually by 20, 40 and 80% of the POME original concentration. In run 5, the operating parameters (original loading rate (OLR) and HRT) in the reactor were kept constant while the temperature of the reactor was promoted gradually (i.e. 1¯ C day¡1), until reaching the target thermophilic temperature (i.e. 55¯ C), as shown in Table 3. At that condition, the reactor was operated for few days until the stability and performance ef ciency were reached, since then, the volumetric loading rate was gradually increased by lowering the HRT in the lter from 10 to 5 days. RESULT AND DISCUSSION The original heterogeneous biomass population in a bioreactor had to undergo biochemical acclimation and selection of the species best able to grow on the carbon sources available in order to ensure successful and sustainable operation. Typically bioreactor acclimation is a timedependent process and it can be in uenced by the seed used, the characteristics of the feed, the start-up strategy followed, and the chosen operational and environment conditions (Tay and Yan, 1996). In this case a slow start-up acclimatization was adopted. Volatile Acidity Alkalinity Relationships duri ng Start-up Stage During the rst few weeks, when the biomass population was small, the methanogenic activity was minimal, resulting in volatile fatty acids accumulation in the lower part of the reactor. These results are typical of those expected during the start-up, where the response of the acid-producing fraction of the microbial consortium is always more rapid than that of the methanogens to step-wise increases in substrate loading, (Borja and Banks, 1995). Thermophilic operation of the anaerobic lter during start-up was generally stable. Since the reactor was initially operated in a mesophilic temperature range, followed by thermophilic operation (i.e. 55¯ C), process stability in the reactor operation uctuated twice. Firstly, during the third week of operation, the reactor stability was interrupted due to the excessive accumulation of the VFAs as shown Figure 1. The VFA concentration increased two-fold, corresponding to only half of the alkalinity in the reactor. However, in the fourth week the reactor recovered its stability again, and the biomass resumed its methanogenic activity, which was indicated by the decrease in the ef uent
Table 3. Start-up strategy for thermophilic up ow anerobic lter.
Run
Duration (days)
1 2 3 4 5
3 8 8 6 26
6 7 8
12 11 6
Temperature ( C) ( 1.5%)
Flow rate (ml day 1) ( 10%)
In uent COD (mg l 1) ( 10%)
34 C 35 C 36 C 37 C (38–55 C) (0.5–1 C day 1) 55 C 55 C 55 C
2380 2380 2380 2380
10,000 20,000 40,000 55,000
1.000 2.000 4.000 5.500
10 10 10 10
2380 2975 3976 4760
55,000 55,000 55,000 55,000
5.500 6.875 9.189 11.000
10 8 6 5
OLR (g-COD l ( 10%)
1
day 1)
HRT (days) ( 1.5%)
Trans IChemE, Vol 81, Part B, July 2003
THERMOPHILIC UPFLOW ANAEROBIC FILTER
265
Figure 4. OLR vs biogas production during start-up stage. Figure 2. Alkalinity vs VFA during start-up stage.
organic content (e.g. BOD 5 and COD) and the increase in the biogas production rate. After the lter operation reached stability, the operating temperature of the reactor was gradually increased to reach a thermophilic temperature range. During this thermal transition stage, a drastic instability in the reactor operation occurred. In this critical period, the VFAs concentration increased tremendously to ve- to six-fold (Figure 2). However, the lter alkalinity dropped. The pH also sharply dropped to 5.8 (Figure 3) so that it was thought the reactor operation had completely failed. However, the biomass generation rate increased, as well as the lter performance ef ciency, which gradually improved accordingly. Anaerobic Filter Performance During Start-up Stage Anaerobic lter operation at the thermophilic temperature is characterized by higher sensitivity to operating and environmental conditions and performance instability, especially during the start-up stage, as well as higher organic removal ef ciency (Ahring, 1994; Lettinga, 1995; Mendez et al., 1995). In this study, the operation and performance of the anaerobic lter at thermophilic temperature was closely investigated during the start-up stage. The assessment of the lter performance was evaluated by biogas conversion rate (biogas production), and the organic removal ef ciency (COD and BOD5) and solids removal ef ciency. During the start-up period, the lter performance retarded twice. Firstly, on the second week when VFAs accumulated in the reactor and secondly in the fourth week when the operation of reactors was shifted from mesophilic to thermophilic operation. The instant response of the reactor to the temperature change was clearly indicated by comparing the volumetric biogas yield with the lter performance
Figure 3. pH vs temperature change during start-up stage.
Trans IChemE, Vol 81, Part B, July 2003
(e.g. TSS, BOD5 and COD removal ef ciencies; Figures 4–6). During the thermal change trial, the hydraulic retention time was maintained at 10 days in order to maintain as much as possible of biomass concentration in the reactor during that critical period. The COD removal ef ciency was remarkably decreased from 94 to 60% and so was the BOD5 removal ef ciency (from 97 to 65%). However, the removal ef ciency of TSS did not show a signi cant effect during this failure. This might be explained by the reactor acting as a physical clari er during the period. The lter resumed its performance and stability as the operating temperature exceeded 50¯ C. At that temperature range, the thermophilic methanogens started to acclimatize with the new environment showing fast growth and activity after only a few days. In addition, the performance and stability of the reactor were observed to recover after only one week. The steady-state condition as de ned by %COD removal was accomplished after 8 weeks in the thermophilic reactor. In addition, the duration of the start-up operation was relatively shorter compared with similar studies by Borja and Banks (1994) (3–6 months) and Mendez et al. (1995) (6–8 months).
Figure 5. OLR vs TSS removal ef ciency during start-up stage.
Figure 6. OLR vs BOD removal ef ciency during start-up.
MUSTAPHA et al.
266
necessary for pH adjustment. The lter also showed a satisfactory performance in TSS, COD and BOD removals whereby high organic convention (up to 88 and 94% for COD and BOD removal) were obtained. The lter also achieved biogas production rate of 1.16 l day¡1 with a biogas yield of 0.822 l g¡1 COD. Steady-state conditions were achieved after 8 weeks of lter operation.
REFERENCES Figure 7. OLR vs COD removal ef ciency during start-up.
Biomass Acclimation and Growth in Thermophil ic Anaerobic Filter As mentioned earlier, the reactor was started up at a mesophilic temperature range (i.e. 34–37¯ C), followed by a gradual rise in the temperature, with an increment of (0.5–1¯ C day¡1). This practice was carried out to allow the biomass in the reactor to adapt and acclimatize with the new temperature range. The biomass started to grow as the VSS concentration steadily increased in the reactor (1.8 and 4.9 g l¡1 for weeks 6 and 8, respectively). The green colour of the biomass in the reactor gradually turned to grey and brownish-black and the bigger granules started to settle to the bottom of the reactor. Similarly, the colour of the bio- lm on the surface of the packing material became yellowish, as was the liquid inside the reactor. Furthermore, the methanogenic activity of the biomass started to decrease as the biogas production rate signi cantly decreased (from 1.7 to 0.5 l day¡1), as depicted in Figure 4. Once the operation of the reactor exceeded the thermal transition zone (42–52¯ C), the biomass started to grow as the VSS concentration increased steadily, especially in the upper part of the reactor. The methanogenic activity of the biomass also recovered, which was indicated by the increase in the biogas production rate and the reactor performance (Qgas from 0.5 to 1.4 l day¡1, COD removal from 60 to 85% and BOD 5 from 70 to 76%; Figures 4, 6 and 7, respectively). CONCLUSION The feasibility of palm oil mill ef uent treatment by thermophilic up ow anaerobic ltration was demonstrated in a laboratory-scale UAF reactor. The start-up process of the thermophilic lter was possible using a mesophilic bacterial seed which was successfully acclimatized to the thermophilic conditions. The VSS concentration in the reactor increased steadily from week 5. It varied from 1.8 to 6.1 g l¡1 during the start-up stage (10 weeks). During the start-up period, the lter pH was within an acceptable range and no addition of alkalinity solutions was
Ahring, B.K., 1994, Status on science and application of thermophilic anaerobic digestion, Water Sci Tech, 30(12): 241–249. APRA, AWWA and WPCF, 1989, Standard Methods for the Examination of Water and Wastewater, 17th edition (American Public Health Association, Washington, DC, USA). Ashhuby, B.A., Idrees, A. and Mustapha, S., 1996, Up ow anaerobic ltration of palm oil mill ef uent, in Symposium of Malaysian Chemical Engineers, Vol 1, pp 223–229. Ashhuby, B.A., Idrees, M. and Mustapha, S., 1997, Start-up an up ow anaerobic lter treating palm oil mill ef uent, in Regional Symposium on Chemical Engineering, 13–15 October, Johor Bahru, Malaysia. Borja, R. and Banks, C.J., l993, Thermophilic semicontinuos anaerobic treatment of palm oil mill ef uent, Biotechnol Lett, 15(7): 761–766. Borja, R. and Banks, C.J., 1994, Treatment of palm oil mill ef uent by up ow anaerobic ltration, J Chem Tech Biotech, 61: 103–109. Borja, R. and Banks, C.J., 1995, Comparison of an anaerobic lter and an anaerobic uidised bed reactor treating palm oil mill ef uent, Process Biochem, 30(6): 511–521. Cheah, S.C., Ma, A.N., Ooi, L.C.L. and Ong, A.S.H., 1998, Biotechnological applications for the utilisation of wastes from palm oil mills, Fat Sci Technol, 90: 536–540. Dinsdale, R.M., Hawkes, F.R. and Hawkes, D.L., 1996, The mesophilic and thermophilic anaerobic digestion of coffee waste containing coffee grounds, Wat Res, 18: 767–773. Lettinga, G., 1995, Anaerobic digestion and wastewater treatment systems, J Gen Mol Microbiol, 67(1): 3–28. Ma, A.N., 1998a, Environmental management for the palm oil industry, Palm Oil Dev, 30: 1–10. Ma, A.N., 1998b, Environmental management for the palm oil industry, in IKM Sarawak Seminar ‘Industrial Development—Safety, Quality Assurance and Environmental Management’, 24–25 March, Bintulu Sarawak. Mendez, R., Lama, J.M. and Soto, M., 1995, Treatment of seafoodprocessing wastewater in mesophilic and thermophilic anaerobic lters, Water Environ Res, 67(1): 33–45. Quarmby, J. and Foster, C.F., 1995, A comparative study of the structure of thermophilic and mesophilic anaerobic granules, Enzyme Microbial Technol, 17: 493–498. Tay, J. and Yan, Y., 1996, In uence of substrate concentration on microbial selection and granulation during start-up of up ow anaerobic sludge blanket reactors, Water Environ Res, 68(7): 1140–1150.
ADDRESS Correspondence concerning this paper should be addressed to Dr. S. Mustapha, Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia. E-mail: [email protected] The manuscript was received 2 July 2002 and accepted for publication after revision 27 May 2003.
Trans IChemE, Vol 81, Part B, July 2003
www.ingentaselect.com=titles=09575820.htm
START-UP STRATEGY OF A THERMOPHILIC UPFLOW ANAEROBIC FILTER FOR TREATING PALM OIL MILL EFFLUENT S. MUSTAPHA1 , B. ASHHUBY 2 , M. RASHID 3 and I. AZNI1 1
Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor Darul Ehsan, Malaysia 2 Department of Environmental Studies, Faculty of Engineering and Technology, University of Sheba, Brack-Alshati, Libya 3 Department of Chemical Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia.
A
thermophilic up ow anaerobic lter treating palm oil mill ef uent was started up in a dm3 laboratory-scale reactor. The reactor was operated continuously at a thermophilic temperature 55 C for over 10 weeks with increased loadings of chemical oxygen demand (COD) of 0.1–0.5 kg COD m 3 day 1. The start-up strategy was accomplished over sequential increase of temperature of 0.5–1 C per day in which the mesophilic bacterial seed was gradually acclimatized to the thermophilic conditions. At these conditions, the reactor showed satisfactory results in organic removal ef ciency (up to 97 and 94% for biological oxygen demand (BOD) and COD, respectively) and biogas production rates of 26.8– 116.1 m 3 day 1 for organic loading rates varied from an initial value of 5.8 to 10.9 kg COD m 3 day 1. The thermophilic anaerobic lter was stable in terms of acidity and alkalinity. In this study, the start-up process of the thermophilic reactor was accomplished in a relatively short time and was stable under mesophilic conditions. Keywords: palm oil mill ef uent; thermophilic digestion; up ow anaerobic lter; start-up; stability; biogas.
mills, alcohol distillery, canning factory), and pulp and paper factories (Ahring, 1994; Borja and Banks, 1993). This may offer advantages for POME treatment since the temperature of POME leaving palm oil mills varies between 50 and 70¯ C, thus eliminating the requirement for cooling prior to biological treatment (Mendez et al., 1995; Dinsdale et al., 1996). In addition, at high temperatures reaction rates proceed much faster which, in turn, results in much higher potentials of thermophilic reactors in comparison to mesophilic reactors (Ahring, 1994; Lettinga, 1995). A previous study reported that the thermophilic POME sample gave higher yields than the mesophilic POME sample (Quarmby and Foster, 1995). However, start-up of the thermophilic anaerobic reactors and the high process sensitivity to environmental changes, e.g. temperature during the start-up stage, are drawbacks (Lettinga, 1995). This paper presents the results of study using a modi ed start-up strategy for a thermophilic up- ow anaerobic lter treating palm oil mill ef uent.
INTRODUCTION The production of palm oil from the fruit Elaeis guineensis is a major industry in Southeast Asia, notably in Malaysia (Ma, l998a). However, the ef uent resulting from the processing operations contains high concentrations of free and dissolved oil and fatty acids, glycerine, crude oil solids, starches, proteins and plant tissues (Cheah et al., 1998). In practice the ef uents from different sources are combined together in one stream commonly known as palm oil mill ef uent (POME). POME typically has a biological oxygen demand (BOD) of 25–40 g l¡1; chemical oxygen demand (COD) of 40–70 g l¡1; suspended solids, 20–40 g l¡1; oil and grease, 4–12 g l¡1 and pH 4.5–5.0 (Ashhuby et al., 1996). The use of conventional digesters (such as close tank digester) to treat POME is characterized by long residence times, often excess of 20 days and poor performance (Ma, 1998a). Some research on POME has been reported regarding the application of modern anaerobic sludge blanket system (Borja and Banks, 1994). Up ow anaerobic lters (UAF) can be operated at either mesophilic or thermophilic temperature ranges. Thermophilic anaerobic lters offer an attractive alternative for the treatment of medium and high strength wastewaters, especially for those wastewaters which are discharged at high temperatures like wastewaters from many food processing industries (palm oil
MATERIALS AND METHODS Experimental Rig Description Figure 1 illustrates the process ow diagram of the experimental set-up of the up ow anaerobic lter. The lter was 262
THERMOPHILIC UPFLOW ANAEROBIC FILTER
263
Figure 1. Schematic diagram of thermophilic up ow anaerobic lter.
constructed of Perspex column with an internal diameter of 19.6 cm. An inlet section 10 cm long at the bottom of the lter and a partitioning chamber of 15 cm at the top were provided (Figure 1). The column was hand packed with a helical plastic rings commercially named ‘Tollerette’ as a support surface for bacteria. The plastic packing medium has dimensions of 4.2–4.5 cm in diameter and 1.8–1.9 cm height and has a high porosity of 93%, so a smaller fraction of the reactor volume is occupied, giving an effective reactor volume of 23.8 l. The total volume of the media was 22.85 l, which represents 89% of the total reactor volume. The lter was operated in the thermophilic temperature range (e.g. 54–56¯ C). The operating temperature in the lter was maintained by continuously circulating hot water from a thermostatic water bath through a water jacket around the column. A thermometer was inserted at the top of the column to monitor the operating temperature of the lter. Three sampling ports were provided at 35, 60 and 85 cm intervals from the bottom of the lter for the purpose of sampling collection at various depths of the lter. Anaerobic conditions in the lter were maintained by introducing the feed (POME) at the bottom of the lter via a variable speed peristaltic pump. The feed was stored at 4¯ C and was daily fed to the lter at room temperature. The treated ef uent was withdrawn through a gas–liquid closed separator, in which the gas bubbles were separated from the liquid ef uent. The volume of the evolved biogas was measure using a wet-test gas meter (Model OSKI, Japan).
water surface during the sampling time. The anaerobic sludge was anaerobic sampled from at least 50 cm deep under the water surface to ensure that the sample was undoubtedly taken from the zone of anaerobic bacteria and not from the facultative one (Ahring, 1994). The sludge was pumped from previously conditioned containers using nitrogen gas to purge the air space above the sludge layer. The physical and chemical compositions of this sludge is summarized in Table 1. POME POME samples were collected two to three times a month from Pengilli Palm Oil Mill located in the state of Johor. The samples were taken from about 20 m from the discharging point. The POME samples were stored in PVC container at 4¯ C for daily use. The characteristics of the POME used are shown in Table 2. The values shown represent the average values from duplicate samples carried out in the study. Anaerobic Filter Conditioni ng and Seeding After lling the anaerobic lter with the packing material, a water heater was set to achieve mesophilic temperature (i.e. 34¯ C). Then an oxygen-free nitrogen gas was purged at the bottom of the lter to replace the air inside the lter prior Table 1. Characterisitic of the anaerobic sludge seed.
POME and Anaerobic Sludge Samples Inoculum (anaerobic sludge) The anaerobic lter was inoculated using anaerobic sludge taken from the anaerobic pond at Elmina Palm Oil Mill located at Sungai Buloh in the State of Selangor, Malaysia. The sludge was methanogenically active as the biogas bubbles were apparently observed stripping at the Trans IChemE, Vol 81, Part B, July 2003
Parameter
Unit
Value
Temperature pH BOD COD TSS VSS
C — mg l mg l mg l mg l
34 7.2 4800 11,150 13,250 10,600
1 1 1 1
MUSTAPHA et al.
264 Table 2. Composition and features of POME samples. Parameter
Unit
Temperature PH COD BOD TSS VSS TS TKN PO4 NO3 Cl Mg Cu Zn Mn Pb
C mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l mg l
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Value measureda
Ma (1988b) data
73 4.6 58,000 26,000 23,200 20,800 45,000 920 775 62.5 900 43.4 1.8 4.7 6.9 12.4
— — 50,000 25,000 — — 40,000 — — — — 615 0.89 2.3 2.0 —
a
Result in average of triplicate test.
to seeding the bacterial inoculum (anaerobic sludge). The nitrogen gas was introduced at a ow rate of 15 l min¡1 at 20 psi for 15 min. Inoculation of the lter was conducted by anaerobic lling into the column. The operation of the reactor was started by introducing the feed (diluted POME: 10 g l¡1 COD) at very slow ow rates (2.38 l day¡1), which is equivalent to a hydraulic retention time of 10 days. Analytical Analyses The following parameters: COD, BOD, total suspended solids (TSS), volatile suspended solid (VSS), volatile fatty acid (VFA), pH and biogas production rate were analyzed daily during the start-up stage. These analyses were carried out according to the standard procedures (APRA et al., 1989). The elemental composition of POME and sludge was analysed using an atomic absorption spectrophotometer. A gas chromatograph (GC) equipped with a 10 m £ 0.53 mm £ 0.1 mm HP-FFAP fused-silica capillary column, ame ionization detector (FID) and a mass spectroscopy detector (GC-MS) was used to measure the composition once a week. Detailed analysis procedures are presented in the previous study (Ashhuby et al., 1997). Start-up Strategy Initially, the reactor was operated at the mesophilic temperature (i.e. 37¯ C) at 10 days hydraulic retention time (HRT).
In the rst four runs, the concentration of the feed to the reactor was increased gradually by 20, 40 and 80% of the POME original concentration. In run 5, the operating parameters (original loading rate (OLR) and HRT) in the reactor were kept constant while the temperature of the reactor was promoted gradually (i.e. 1¯ C day¡1), until reaching the target thermophilic temperature (i.e. 55¯ C), as shown in Table 3. At that condition, the reactor was operated for few days until the stability and performance ef ciency were reached, since then, the volumetric loading rate was gradually increased by lowering the HRT in the lter from 10 to 5 days. RESULT AND DISCUSSION The original heterogeneous biomass population in a bioreactor had to undergo biochemical acclimation and selection of the species best able to grow on the carbon sources available in order to ensure successful and sustainable operation. Typically bioreactor acclimation is a timedependent process and it can be in uenced by the seed used, the characteristics of the feed, the start-up strategy followed, and the chosen operational and environment conditions (Tay and Yan, 1996). In this case a slow start-up acclimatization was adopted. Volatile Acidity Alkalinity Relationships duri ng Start-up Stage During the rst few weeks, when the biomass population was small, the methanogenic activity was minimal, resulting in volatile fatty acids accumulation in the lower part of the reactor. These results are typical of those expected during the start-up, where the response of the acid-producing fraction of the microbial consortium is always more rapid than that of the methanogens to step-wise increases in substrate loading, (Borja and Banks, 1995). Thermophilic operation of the anaerobic lter during start-up was generally stable. Since the reactor was initially operated in a mesophilic temperature range, followed by thermophilic operation (i.e. 55¯ C), process stability in the reactor operation uctuated twice. Firstly, during the third week of operation, the reactor stability was interrupted due to the excessive accumulation of the VFAs as shown Figure 1. The VFA concentration increased two-fold, corresponding to only half of the alkalinity in the reactor. However, in the fourth week the reactor recovered its stability again, and the biomass resumed its methanogenic activity, which was indicated by the decrease in the ef uent
Table 3. Start-up strategy for thermophilic up ow anerobic lter.
Run
Duration (days)
1 2 3 4 5
3 8 8 6 26
6 7 8
12 11 6
Temperature ( C) ( 1.5%)
Flow rate (ml day 1) ( 10%)
In uent COD (mg l 1) ( 10%)
34 C 35 C 36 C 37 C (38–55 C) (0.5–1 C day 1) 55 C 55 C 55 C
2380 2380 2380 2380
10,000 20,000 40,000 55,000
1.000 2.000 4.000 5.500
10 10 10 10
2380 2975 3976 4760
55,000 55,000 55,000 55,000
5.500 6.875 9.189 11.000
10 8 6 5
OLR (g-COD l ( 10%)
1
day 1)
HRT (days) ( 1.5%)
Trans IChemE, Vol 81, Part B, July 2003
THERMOPHILIC UPFLOW ANAEROBIC FILTER
265
Figure 4. OLR vs biogas production during start-up stage. Figure 2. Alkalinity vs VFA during start-up stage.
organic content (e.g. BOD 5 and COD) and the increase in the biogas production rate. After the lter operation reached stability, the operating temperature of the reactor was gradually increased to reach a thermophilic temperature range. During this thermal transition stage, a drastic instability in the reactor operation occurred. In this critical period, the VFAs concentration increased tremendously to ve- to six-fold (Figure 2). However, the lter alkalinity dropped. The pH also sharply dropped to 5.8 (Figure 3) so that it was thought the reactor operation had completely failed. However, the biomass generation rate increased, as well as the lter performance ef ciency, which gradually improved accordingly. Anaerobic Filter Performance During Start-up Stage Anaerobic lter operation at the thermophilic temperature is characterized by higher sensitivity to operating and environmental conditions and performance instability, especially during the start-up stage, as well as higher organic removal ef ciency (Ahring, 1994; Lettinga, 1995; Mendez et al., 1995). In this study, the operation and performance of the anaerobic lter at thermophilic temperature was closely investigated during the start-up stage. The assessment of the lter performance was evaluated by biogas conversion rate (biogas production), and the organic removal ef ciency (COD and BOD5) and solids removal ef ciency. During the start-up period, the lter performance retarded twice. Firstly, on the second week when VFAs accumulated in the reactor and secondly in the fourth week when the operation of reactors was shifted from mesophilic to thermophilic operation. The instant response of the reactor to the temperature change was clearly indicated by comparing the volumetric biogas yield with the lter performance
Figure 3. pH vs temperature change during start-up stage.
Trans IChemE, Vol 81, Part B, July 2003
(e.g. TSS, BOD5 and COD removal ef ciencies; Figures 4–6). During the thermal change trial, the hydraulic retention time was maintained at 10 days in order to maintain as much as possible of biomass concentration in the reactor during that critical period. The COD removal ef ciency was remarkably decreased from 94 to 60% and so was the BOD5 removal ef ciency (from 97 to 65%). However, the removal ef ciency of TSS did not show a signi cant effect during this failure. This might be explained by the reactor acting as a physical clari er during the period. The lter resumed its performance and stability as the operating temperature exceeded 50¯ C. At that temperature range, the thermophilic methanogens started to acclimatize with the new environment showing fast growth and activity after only a few days. In addition, the performance and stability of the reactor were observed to recover after only one week. The steady-state condition as de ned by %COD removal was accomplished after 8 weeks in the thermophilic reactor. In addition, the duration of the start-up operation was relatively shorter compared with similar studies by Borja and Banks (1994) (3–6 months) and Mendez et al. (1995) (6–8 months).
Figure 5. OLR vs TSS removal ef ciency during start-up stage.
Figure 6. OLR vs BOD removal ef ciency during start-up.
MUSTAPHA et al.
266
necessary for pH adjustment. The lter also showed a satisfactory performance in TSS, COD and BOD removals whereby high organic convention (up to 88 and 94% for COD and BOD removal) were obtained. The lter also achieved biogas production rate of 1.16 l day¡1 with a biogas yield of 0.822 l g¡1 COD. Steady-state conditions were achieved after 8 weeks of lter operation.
REFERENCES Figure 7. OLR vs COD removal ef ciency during start-up.
Biomass Acclimation and Growth in Thermophil ic Anaerobic Filter As mentioned earlier, the reactor was started up at a mesophilic temperature range (i.e. 34–37¯ C), followed by a gradual rise in the temperature, with an increment of (0.5–1¯ C day¡1). This practice was carried out to allow the biomass in the reactor to adapt and acclimatize with the new temperature range. The biomass started to grow as the VSS concentration steadily increased in the reactor (1.8 and 4.9 g l¡1 for weeks 6 and 8, respectively). The green colour of the biomass in the reactor gradually turned to grey and brownish-black and the bigger granules started to settle to the bottom of the reactor. Similarly, the colour of the bio- lm on the surface of the packing material became yellowish, as was the liquid inside the reactor. Furthermore, the methanogenic activity of the biomass started to decrease as the biogas production rate signi cantly decreased (from 1.7 to 0.5 l day¡1), as depicted in Figure 4. Once the operation of the reactor exceeded the thermal transition zone (42–52¯ C), the biomass started to grow as the VSS concentration increased steadily, especially in the upper part of the reactor. The methanogenic activity of the biomass also recovered, which was indicated by the increase in the biogas production rate and the reactor performance (Qgas from 0.5 to 1.4 l day¡1, COD removal from 60 to 85% and BOD 5 from 70 to 76%; Figures 4, 6 and 7, respectively). CONCLUSION The feasibility of palm oil mill ef uent treatment by thermophilic up ow anaerobic ltration was demonstrated in a laboratory-scale UAF reactor. The start-up process of the thermophilic lter was possible using a mesophilic bacterial seed which was successfully acclimatized to the thermophilic conditions. The VSS concentration in the reactor increased steadily from week 5. It varied from 1.8 to 6.1 g l¡1 during the start-up stage (10 weeks). During the start-up period, the lter pH was within an acceptable range and no addition of alkalinity solutions was
Ahring, B.K., 1994, Status on science and application of thermophilic anaerobic digestion, Water Sci Tech, 30(12): 241–249. APRA, AWWA and WPCF, 1989, Standard Methods for the Examination of Water and Wastewater, 17th edition (American Public Health Association, Washington, DC, USA). Ashhuby, B.A., Idrees, A. and Mustapha, S., 1996, Up ow anaerobic ltration of palm oil mill ef uent, in Symposium of Malaysian Chemical Engineers, Vol 1, pp 223–229. Ashhuby, B.A., Idrees, M. and Mustapha, S., 1997, Start-up an up ow anaerobic lter treating palm oil mill ef uent, in Regional Symposium on Chemical Engineering, 13–15 October, Johor Bahru, Malaysia. Borja, R. and Banks, C.J., l993, Thermophilic semicontinuos anaerobic treatment of palm oil mill ef uent, Biotechnol Lett, 15(7): 761–766. Borja, R. and Banks, C.J., 1994, Treatment of palm oil mill ef uent by up ow anaerobic ltration, J Chem Tech Biotech, 61: 103–109. Borja, R. and Banks, C.J., 1995, Comparison of an anaerobic lter and an anaerobic uidised bed reactor treating palm oil mill ef uent, Process Biochem, 30(6): 511–521. Cheah, S.C., Ma, A.N., Ooi, L.C.L. and Ong, A.S.H., 1998, Biotechnological applications for the utilisation of wastes from palm oil mills, Fat Sci Technol, 90: 536–540. Dinsdale, R.M., Hawkes, F.R. and Hawkes, D.L., 1996, The mesophilic and thermophilic anaerobic digestion of coffee waste containing coffee grounds, Wat Res, 18: 767–773. Lettinga, G., 1995, Anaerobic digestion and wastewater treatment systems, J Gen Mol Microbiol, 67(1): 3–28. Ma, A.N., 1998a, Environmental management for the palm oil industry, Palm Oil Dev, 30: 1–10. Ma, A.N., 1998b, Environmental management for the palm oil industry, in IKM Sarawak Seminar ‘Industrial Development—Safety, Quality Assurance and Environmental Management’, 24–25 March, Bintulu Sarawak. Mendez, R., Lama, J.M. and Soto, M., 1995, Treatment of seafoodprocessing wastewater in mesophilic and thermophilic anaerobic lters, Water Environ Res, 67(1): 33–45. Quarmby, J. and Foster, C.F., 1995, A comparative study of the structure of thermophilic and mesophilic anaerobic granules, Enzyme Microbial Technol, 17: 493–498. Tay, J. and Yan, Y., 1996, In uence of substrate concentration on microbial selection and granulation during start-up of up ow anaerobic sludge blanket reactors, Water Environ Res, 68(7): 1140–1150.
ADDRESS Correspondence concerning this paper should be addressed to Dr. S. Mustapha, Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia. E-mail: [email protected] The manuscript was received 2 July 2002 and accepted for publication after revision 27 May 2003.
Trans IChemE, Vol 81, Part B, July 2003