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Energy & Fuels 2008, 22, 108–112
Effect of Volatile Fatty Acid Composition on Upflow Anaerobic Sludge Blanket (UASB) Performance† Biing-Teo Wong,‡ Kuan-Yeow Show,§ Ay Su,| Rui-jyun Wong,‡ and Duu-Jong Lee*,‡ Department of Chemical Engineering, National Taiwan UniVersity, Taipei, 10617 Taiwan, Faculty of Engineering and Science, UniVersity of Tunku Abdul Rahman, Kuala Lumpur, Malaysia, and Fuel Cell Research Center, Department of Mechanical Engineering, Yuan Ze UniVersity, Tao Yuan, 32003 Taiwan ReceiVed May 27, 2007. ReVised Manuscript ReceiVed July 11, 2007
The effects of organic loading rates (OLRs) and volatile fatty acid (VFA) composition on upflow anaerobic sludge blanket (UASB) reactors were investigated. Four 2.5 L UASB reactors were fed by mixed VFA of 20 g of chemical oxygen demand (COD) L-1 initially and subsequently fed with different VFA compositions. In the initial stage of the experiment where the reactors were operated at an OLR between 0.82 and 10.4 g of COD L-1 day-1 with the substrate composed of a mixed acetate/propionate/butyrate composition, the COD removal efficiencies were in the range of 83 and 99% and the calculated methane yield ranged between 112 L of CH4 and 322 L of CH4 per g of COD removed. The result indicated that a higher methane yield corresponded with the increase of the OLR in the UASB treatment of the mixed VFA substrate. Mixed VFAfed sludge showed a rather high activity in pure acetate and butyrate than pure propionate in a specific methanogenic activity (SMA) test. As the substrate composition changed from the mixed VFA (32:28:40 acetate/ propionate/butyrate) to pure acetate, propionate, or butyrate at the OLR of 10.4 g of COD L-1 day-1, respectively, in the UASB reactors, the acetate- and butyrate-fed reactors showed higher gas production and COD removal than the pure propionate-fed reactor as expected. The result reviewed that the composition of VFAs has a significant influence on the reactor performance. The propionate substrate exhibited severe inhibition, leading to system failure at a feed concentration of 20 g of COD L-1, while the butyrate substrate can be efficiently degraded compared to the acetate substrate because of its high energy gain via degradation.
Introduction Anaerobic digestion involves a series of biochemical reactions in the degradation of complex organic matter into methane and carbon dioxide. These reactions are often classified into hydrolysis, acidogenesis, acetogenis, and methanogenesis. Volatile fatty acids (VFAs), which act as the most important intermediate products in the acidogenesis and acetogenesis steps, play a key role in the overall process. High levels of VFAs may cause inhibition of methanogenesis and even reactor failure. Propionate and butyrate are C3 and C4 VFAs, which are to be converted into acetate and hydrogen in the acetogenesis stage before the final conversion into methane. Acetate would be degraded further to methane and carbon dioxide in the terminal stage of methanogenesis. Accordingly, more than 75% of the methane production is derived from acetate.1 Because the slow growing methanogens lag behind the fast growing acidogens, the stability of the reactor greatly depends upon the degradation of VFA. This study aims to investigate the effect of VFA composition on upflow anaerobic sludge blanket (UASB) performance. The reactors were started with a mixed VFA substrate and, subsequently, fed with a pure substrate of acetate, propionate, and butyrate in the respective reactors. The reactor performance in † Presented at the International Conference on Bioenergy Outlook 2007, Singapore, April 26–27, 2007. * To whom correspondence should be addressed. Telephone: +886-223625632. Fax: +886-2-23623040. E-mail:
[email protected]. ‡ National Taiwan University. § University of Tunku Abdul Rahman. | Yuan Ze University. (1) Vanlier, J. B.; Grolle, K. C. F.; Frijters, C. T. M. J.; Stams, A. J. M.; Lettinga, G. Appl. EnViron. Microbiol. 1993, 59 (4), 1003–1011.
terms of chemical oxygen demand (COD) removal, biogas production, and methane yield are presented. Experimental Section Experimental Setup. Four UASB reactors with a working volume of 2.5 L were maintained at 35 °C, while the feed substrate was stored at about 4 °C in the refrigerator. The detailed configuration is shown in Figure 1. The synthetic substrate was fed to the bottom of the reactor from the refrigerator. Five sampling ports were built along both sides of the reactor, with port 5 connected to port 1 with a recycling pump to enhance biomass mixing. The biogas produced by the reactor was passed through a water trap and was collected by the water-displacement method. Feed Composition. A high-strength synthetic VFA-based wastewater containing acetate, propionate, and butyrate in the COD ratio of 32:28:40, to a total of 20 g of COD L-1, was used as an influent feed. The composition of the feed is shown in Table 1. Inoculum. Aerobic sludge was obtained from the wastewater treatment plant of the Uni-President Oven Bakery Corp., Taoyuan, Taiwan. The sludge was stored for 1 month under an anaerobic condition before it was seeded in the anaerobic reactor. About 1 L of the sludge was inoculated in the reactors and acclimatized for 12 h at 35 °C. During the acclimatization period before the feeding substrate, methane composition was detected in the gas phase, while some sludge was floating with the biogas produced. The initial volatile suspension solid (VSS) concentration and VSS percentage of the sludge were 4.4 g L-1 and 59%. Analytical Method. Samples of bioreactor effluent were routinely taken for COD determination according to the standard methods [American Public Health Association (APHA), 1998] section 5220 D. Gas production was recorded daily by the waterdisplacement method. The biogas composition was determined by
10.1021/ef700282r CCC: $40.75 2008 American Chemical Society Published on Web 09/12/2007
VFA Composition on UASB Performance
Energy & Fuels, Vol. 22, No. 1, 2008 109 Table 2. Operating and Loading Conditions of Reactors at Different OLRs OLR (g of COD L-1 day-1)
Figure 1. Schematic diagram of the experimental system. Table 1. Synthetic Feed Composition major components
concentration (mg L-1)
trace components
concentration (mg L-1)
COD acetate propionate butyrate CaCl2 MgCl4 · 6H2O FeCl2 · 4H2O NH4Cl KH2PO4 Na2S L-cysteine
20 000 6000 3700 4400 200 200 50 1000 500 250 250
ZnCl2 CuCl2 NiCl2 · 6H2O MnSO4 · H2O H3BO3 (NH4)6Mo7 · 4H2O CoCl2 · 6H2O AlCl3 Na2SeO3 · 5H2O EDTA resazurin
0.5 0.5 1 1 0.5 0.5 0.5 0.5 0.5 4 0.5
the gas chromatography (Shimadzu, GC-9A) equipped with a thermal conductivity detector and column-packed with molecular sieve 5A 80/100 mesh. The oven temperature was set at 70 °C, and helium was used as a carrier gas at a flow rate of 30 mL min-1. Suspended solid concentrations were determined for both total and volatile fractions according to procedures described in standard methods (APHA, 1998) sections 2540D and 2540E, respectively. The specific methanogenic activity (SMA) test was carried out in 120 mL serum bottles. The VSS concentration of the sludge was analyzed before the test started. A prescribed amount of sludge was added to the bottles and diluted using mineral stock solution2 (pH 7), to a total volume of 60 mL. After acclimatization for 5 h until no more biogas was produced, each VFA substrate at a total of 200 mg of COD was added subsequently. Mixing was maintained at a speed of 150 rpm, and the temperature was set at 35 °C throughout the testing period. The supernatant was taken for the measurement of the remained VFA by a high-performance liquid chromatograph system (Ecom LCP 4100 Pump, LCD 2083 detector) with an autosampler (hta HT3000L). The methane production was recorded to calculate the SMA [g of CH4 (g of COD)-1 (d of VSS)-1]. Experimental Design. Two experiments were set up to study the biogas production and organics removal of VFAs in UASB reactors. In experiment 1, four different organic loading rates (OLRs) were investigated in the reactors. Table 2 lists the operating and loading conditions of the reactors in nearly 4 months. The feed substrate was composed of mixed VFAs as previously described. The pH of feed was between 4.9 and 5.2. The reactors were operated until a steady-state performance was reached, as marked by consistent gas production and an effluent COD concentration. After seeding and startup, all reactors were operated simultaneously at an OLR of 0.82 g of COD L-1 day-1, corresponding to a COD concentration of 20 g of COD L-1 and a hydraulic retention time (HRT) of 24.4 days. After operation for 36 days, OLRs of R1, R2, and R4 were step-increased to 3.84 g of COD L-1 day-1. At day (2) Ince, O.; Ince, B. K.; Yenigun, O. J. Chem. Technol. Biotechnol. 2001, 76 (6), 573–578.
0.82
3.84
5.2
10.4
COD concentration (g of COD L-1) operating reactor
20
20
20
20
∼R1–R4 24.4 36
R1, R3, and R4 3.8 39
∼R1–R4
HRT (day) operating duration (day)
R1, R2, and R4 5.2 30
1.9 26
67, the OLRs of R1, R3, and R4 increased further to 5.2 g of COD L-1 day-1. Finally, all reactors reached an OLR of 10.4 g of COD L-1 day-1 at day 105. In experiment 2, a different VFA composition was studied at a constant OLR of 10.4 g of COD L-1 day-1 and COD concentration of 20 g of COD L-1. The composition of the feed substrate changed from 32:28:40 acetate/propionate/butyrate (in a COD ratio) to pure acetate, propionate, and butyrate separately in R1, R3, and R4, while R2 served as a control reactor. Moreover, the SMA test was carried out in different VFA substrates to predict the UASB performance by using mixed VFA-fed sludge. The pH of each feed was kept constant at 6.5. Gas production and composition were recorded at intervals of 5 h. Samples from port 1 to port 3 were analyzed for remaining COD.
Results and Discussions Effect of Loading Rates on UASB Performance at Steady State. All reactors were operated for more than 1 month at corresponding OLRs. A summary of the results for the performance of the reactors at steady state is reported in Table 3. At OLR of 0.82 g of COD L-1 day-1, the biogas production of all reactors was between 281 and 391 mL day-1. As the OLR increased by 4 times to 3.84 g of COD L-1 day-1, the gas production increased by nearly 8 times. As the OLR increased further 1.35 times to 5.2 g of COD L-1 day-1, gas production increased by 1.8 times. At the highest OLR of 10.4 g of COD L-1 day-1, which increased by 2 times from 5.2 g of COD L-1 day-1, the gas production increased 1.8 times. COD removal efficiencies decreased from 99 to around 83% after the OLR was increased from 0.82 to 10.4 g of COD L-1 day-1. The calculated methane yield increased from 112 to 322 L of CH4 per g of COD removed once OLR increased from 0.82 to 10.4 g of COD L-1 day-1. More COD were converted to methane at higher loading rates. Anabolism may be the main COD removal pathway at low OLR in the initial startup stage, thereby yielding a low methane yield.3,4 SMA of Granules/Sludges with Fatty Acids as Substrates. Table 4shows the SMA tests with mixed VFA-fed sludge using individual fatty acids or mixed VFA as organic substrates. The corresponding SMA data for other studies are listed for a comparison. The SMA in the experiments using the VFA mixture as the substrate was 2.3 g of CH4 (g of COD)-1 (d of VSS)-1, which was comparably higher than those using acetate [1.6 g of CH4 (g of COD)-1 (d of VSS)-1] and butyrate [1.8 g of CH4 (g of COD)-1 (d of VSS)-1] as the substrate. On the other hand, the propionate substrate showed rather poor activity [0.5 g of CH4 (g of COD)-1 (d of VSS)-1]. (3) Michaud, S.; Bernet, N.; Buffiere, P.; Roustan, M.; Moletta, R. Water Res. 2002, 36 (5), 1385–1391. (4) Rincon, B.; Raposo, F.; Borja, R.; Gonzalez, J. M.; Portillo, M. C.; Saiz-Jimenez, C. J. Biotechnol. 2006, 121 (4), 534–543. (5) Schmidt, J. E.; Ahring, B. K. Biotechnol. Bioeng. 1997, 53 (4), 442– 442. (6) Dolfing, J.; Mulder, J. W. Appl. EnViron. Microbiol. 1985, 49 (5), 1142–1145. (7) Visser, A.; Nozhevnikova, A. N.; Lettinga, G. J. Chem. Technol. Biotechnol. 1993, 57 (1), 9–13.
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Table 3. Summary of the Performance of the Reactors OLR reactor COD removal (%) effluent COD (mg of COD L-1) gas production (mL day-1) methane composition (%) carbon dioxide (%) effluent pH methane yield (L of CH4/kg of COD removed)
0.82 g of COD L-1 day-1
3.84 g of COD L-1 day-1
5.2 g of COD L-1 day-1
10.4 g of COD L-1 day-1
R1 98 311
R2 99 295
R3 99 123
R4 98 313
R1 99 286
R2 99 187
R4 99 138
R1 98 342
391
349
334
281
2731
2773
2419
62 14 8.8 158
61 16 8.9 139
58 16 8.9 131
58 12 8.7 112
64 21 8.5 210
63 21 8.5 214
60 18 8.5 188
Figure 2 shows the results of SMA tests. The mixed VFAfed sludge significantly degraded pure acetate, with 50 mg of COD left following 22 h of degradation (Figure 2a). Butyrate was degraded rather rapidly (Figure 2c). Pure propionate substrate was kept at 180–200 mg of COD range throughout the experiment (Figure 2b). Mixed VFA-fed sludge showed its inability to degrade pure propionate. However, propionate degradation was feasible while accompanied with the degradation of acetate and butyrate in the mixed VFA substrate. Energy
R3
R2
R3 96 826
R4 98 461
R1 83 3443
R2 90 1917
R3 86 2723
R4 88 2376
4434
4977
4842
7812
9319
8702
9145
69 27 8.5 251
65 26 8.5 280
65 26 8.4 274
82 15 8.2 299
72 22 8.4 287
82 15 7.9 311
81 15 8.1 322
Table 4. Specific Methanogenic Activities of Granules/Sludges Using Fatty Acids as Substrates substrate SMA [g of CH4 (g of COD)-1 (d of VSS)-1] wastewater
acetate
VFA mixture (this study) 1.6 VFA mixture5 1.4–2.0 acetate6–8 1.2–2.4 butyrate9 1.5 0.8–1.9 propionate10,11
propionate butyrate VFA mixture 0.5 0.6
1.8 1.6–2.1
0.6–1.8
1.3 1.3
2.3
gain from butyrate and acetate degradation is helpful to overcome the energy barrier of propionate degradation.12 Effect of VFA Composition on UASB Performance. After UASB reactors were operated at an OLR of 10.4 g of COD L-1 day-1 for 3 months, the feed composition of R1, R3, and R4 was changed to pure acetate, propionate, and butyrate, respectively, at the same COD concentration, while R2 served as the control reactor. The result of the pure acetate substrate is shown in Figure 3. Feeding of pure acetate was started from
Figure 2. SMA test of mixed VFA-fed sludge treating (a) acetate, (b) propionate, (c) butyrate, and (d) mixed VFAs.
Figure 3. Effect of the composition changed from mixed acid to R1 (acetate).
VFA Composition on UASB Performance
Energy & Fuels, Vol. 22, No. 1, 2008 111
Figure 4. Effect of composition changed from mixed acid to R4 (butyrate).
Figure 5. Effect of composition changed from mixed acid to R3 (propionate).
50 h. As illustrated in Figure 3, the gas production dropped drastically in the first 200 h, indicating that VFA inhibition may occur when pure acetate was fed at a concentration of 20 g of COD L-1 in place of mixed VFA. After 200 h of operation, the gas production recovered to about 2 L per 5 h. On the other hand, the biogas produced showed a high methane composition, which increased from 80 to 90%, with the remaining 10% being carbon dioxide. The samples taken from port 1 (bottom), port 2 (middle), and port 3 (upper) of the reactor were also tested for COD. COD removal efficiency of only 60% was determined in the first 350 h. After 350 h of operation, the COD removal efficiency recovered to 75%. In comparison to the previous mixed VFA substrate, the COD efficiency of acetate dropped from 90 to 75%. R1 performed similarly in biogas production but at lower COD removal after the feed was changed from mixed VFA to pure acetate. Figure 4 shows the degradation of the pure butyrate substrate in R4. In comparison to R1 fed with pure acetate, gas production of R4 decreased slightly in the first 100 h. The gas production, however, was rather inconsistent. This is probably due to the fact that degradation of butyrate involves two different stages: conversion of butyrate into acetate in the acidogenesis stage and conversion of the acetate into methane and carbon dioxide in the methanogenesis stage. The methane composition decreased initially to 72% at the first 100 h of operation; it then increased gradually to 80% after 500 h of operation. In comparison to R1 with pure acetate feed, the biogas produced in R4 showed relatively high carbon dioxide content. The COD result showed that more than 95% of COD removal efficiency was attained after operating for 400 h. This corresponded to a remarkable increase of COD removal efficiency from 70 to 95% after the feed had changed from the mixed VFA to pure butyrate.
Figure 5 shows the degradation of propionate by reactor R3. Unlike pure acetate (R1) and butyrate (R4), a feed of 20 g of COD L-1 of pure propionate, as expected from the SMA test, showed a complete inhibition phenomenon. No biogas was produced after 200 h of operation, with almost all of the influent undegraded after operating for 250 h. In comparison to the pure substrates of acetate in R1 and butyrate in R4, the inhibition in R3 could be due to the finding that had been reported by Aguilar and Casas,13 in which VFAs of an even number of carbon atoms are easier to degrade than VFAs of an odd number of carbon atoms. It could also be due to the fact that propionate is easily accumulated in the reactor when the reactor is overloaded.13 It had also been reported that inhibition caused by propionate was greater than other VFAs, and this could occur even at a low concentration of propionate.14 Moreover, degradation of propionate to acetate is not feasible thermodynamically unless the byproduct hydrogen is removed by the hydrogen-consuming methanogens.15 (8) Rinzema, A.; Vanlier, J.; Lettinga, G. Enzyme Microb. Technol. 1988, 10 (1), 24–32. (9) Fang, H. H. P.; Chui, H. K.; Li, Y. Y. Bioresour. Technol. 1995, 51 (1), 75–81. (10) Grotenhuis, J. T. C.; Smit, M.; Plugge, C. M.; Xu, Y. S.; Vanlammeren, A. A. M.; Stams, A. J. M.; Zehnder, A. J. B. Appl. EnViron. Microbiol. 1991, 57 (7), 1942–1949. (11) Grotenhuis, J. T. C.; Kissel, J. C.; Plugge, C. M.; Stams, A. J. M.; Zehnder, A. J. B. Water Res. 1991, 25 (1), 21–27. (12) Wu, S. T.; Huang, C. C.; Yu, S. T.; Too, J. R. J. Chin. Inst. Chem. Eng. 2006, 37, 501–508. (13) Aguilar, A.; Casas, C.; Lema, J. M. Water Res. 1995, 29 (2), 505– 509. (14) Mosche, M.; Jordening, H. J. Appl. Microbiol. Biotechnol. 1998, 49 (6), 793–799. (15) Boone, D. R.; Xun, L. Y. Appl. EnViron. Microbiol. 1987, 53 (7), 1589–1592.
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Figure 7. Methane yields of reactors fed with different substrates.
Reactor R3 experienced a severe inhibition effect, leading to reactor failure, and no biogas was produced. Pure propionate was found as a strong inhibitor among tested VFAs, regardless of whether in a SMA test or a UASB reactor experiment. It was reported that the anaerobic digester was inhibited by the propionate concentration over 3000 mg L-1.19 In this study, pure propionate fed with a concentration of 20 g of COD L-1, as expected, directly lead to digester failure. Theoretically, degradation of propionate is only feasible at the hydrogen partial pressure of 10-5 to 10-6 atm.20,21 Figure 7 showed the methane yields of acetate-fed (R1), mixed VFA-fed (R2), and butyrate-fed (R4), namely, 440, 308, and 310 mL of CH4 per g of COD removed, respectively. In comparison to the butyrate and mixed VFA substrates, the acetate substrate showed a higher methane yield. This could explain that the one-step degradation of acetate (methanogenesis) is more efficient than the two-step degradation of butyrate (acetogenesis and methanogenesis). The VSS of butyrate-fed sludge (42.3 g of VSS L-1) was higher than the acetate-fed sludge (37.0 g of VSS L-1) after a 2 month operation at steady state. Granulation of butyrate-fed sludge was observed.
Figure 6. Comparison of the overall performance of reactors in different substrate compositions.
The overall performance of UASB reactors in experiment 2 is shown in Figure 6. Figure 6a indicates that the biogas production of R1 with the pure acetate substrate is more or less similar to that of R4 fed with the pure butyrate substrate. Reactor R4, which was fed by pure butyrate, showed a better performance of COD removal of 95% than acetate of 75%. Butyrate feed can be degraded more efficiently than the others because of its high energy gain via degradation.16 Energetically, acetate is a poor substrate. At low concentrations, the energy required for acetate uptake exceeded the energy gain in acetate degradation, thereby limiting the acetate removal in the overall digester performance.17,18 Thus, Acetate degradation was always found to be the rate-limiting step either in butyrate or acetate degradation.18 (16) McCarty, P. L.; Smith, D. P. EnViron. Sci. Technol. 1986, 20 (12), 1200–1206.
Conclusions The effects of the compositionof OLRs and VFAs in UASB were investigated. High OLRs would result in a higher methane yield in the UASB treatment of the mixed VFA substrate. The SMA test showed that degradation of propionate in the mixed VFA substrate is only feasible while combined with acetate and butyrate degradation. The composition of VFAs has a significant influence on the reactor performance. The propionate substrate, as expected by the SMA test, exhibited more severe inhibition than butyrate and acetate at a feed concentration of 20 g of COD L-1. Butyrate feed can be degraded more efficiently than the others because of its high energy gain via degradation. In contrast, acetate can be regarded as an efficient substrate to exhibit a high methane yield because of its one-step degradation. EF700282R (17) Zehnder, A. J. B.; Ingvorsen, K.; Marti, T. In Anaerobic Digestion 1981; Hughes, D. E., et al., Eds; Elsevier Biomedical Press: Amsterdam, The Netherlands, 1982; pp 45–68. (18) Ahring, B. K.; Westermann, P. Appl. EnViron. Microbiol. 1987, 53 (2), 434–439. (19) Iannotti, E. L.; Fischer, J. R. DeV. Ind. Microbiol. 1984, 25, 741– 747. (20) McInerney, M. J.; Bryant, M. P. In Anaerobes and Anaerobic Infections; Gottschalk, G., Ed.; Gustav Fisher Verlag: Stuttgart, Germany, 1980; pp 117–126. (21) Zabranska, J.; Stepova, J.; Wachtl, R.; Jenicek, P.; Dohanyos, M. Water Sci. Technol. 2000, 42 (9), 49–56.
