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Effect of hydraulic retention time on biohydrogen production from palm oil mill effluent in anaerobic sequencing batch reactor
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Effect of hydraulic retention time on biohydrogen production from palm oil mill effluent in anaerobic sequencing batch reactor Marzieh Badiei, Jamaliah Md Jahim*, Nurina Anuar, Siti Rozaimah Sheikh Abdullah Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
article info
abstract
Article history:
The feasibility of hydrogen generation from palm oil mill effluent (POME), a high strength
Received 8 December 2010
wastewater with high solid content, was evaluated in an anaerobic sequencing batch
Received in revised form
reactor (ASBR) using enriched mixed microflora, under mesophilic digestion process at
7 February 2011
37 C. Four different hydraulic retention times (HRT), ranging from 96 h to 36 h at constant
Accepted 10 February 2011
cycle length of 24 h and various organic loading rate (OLR) concentrations were tested to
Available online 2 April 2011
evaluate hydrogen productivity and operational stability of ASBR. The results showed higher system efficiency was achieved at HRT of 72 h with maximum hydrogen production
Keywords:
rate of 6.7 LH2/L/d and hydrogen yield of 0.34 LH2/g CODfeeding, while in longer and shorter
Biohydrogen
HRTs, hydrogen productivity decreased. Organic matter removal efficiency was affected by
Palm oil mill effluent (POME)
HRT; accordingly, total and soluble COD removal reached more than 37% and 50%,
Anaerobic sequencing batch reactor
respectively. Solid retention time (SRT) of 4e19 days was achieved at these wide ranges of
(ASBR)
HRTs. Butyrate was found to be the dominant metabolite in all HRTs. Low concentration of
Hydraulic retention time (HRT)
volatile fatty acid (VFA) confirmed the state of stability and efficiency of sequential batch
Solid retention time (SRT)
mode operation was achieved in ASBR. Results also suggest that ASBR has the potential to offer high digestion rate and good stability of operation for POME treatment. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Regarding the limited fossil energy resources, energy security and the global environment, a great attention is being paid to the usage of hydrogen as fuel to reduce the emission of CO2 to the atmosphere. Hydrogen as the future energy source has recyclability and non-polluting nature [1]. Biological hydrogen production is of great interest because it can be operated at ambient temperature and pressure and consequently is less energy intensive compared to traditional thermo-chemical and electro-chemical process. Production of hydrogen through biological process can be achieved by using anaerobic process.
Moreover, fermentative H2 production using high strength wastewater as substrate attracted considerable attention, since it is direct treatment of wastewater along with sustainable bioenergy generation [2]. However, most organic waste products are complex substrates and information on their conversion to hydrogen in continuous process, whether by defined cultures or mixed microflora, is very limited [3]. Palm oil mill effluent (POME) and its derivatives have been exploited as fermentation media to produce various products [4,5] and have shown high potential as a substrate for generation of hydrogen [6,7]. The possibility of reusing POME as fermentation media is largely due to the fact that POME contains
* Corresponding author. E-mail address: [email protected] (J.M. Jahim). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.02.054
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Nomenclature ASBR COD HPR HRT HY MLSS MLVSS OLR
anaerobic sequencing batch reactor chemical oxygen demand, g/L hydrogen production rate, L H2/L/d hydraulic retention time, hour hydrogen yield, L H2/g COD mixed liquor suspended solid, mg/L mixed liquor volatile suspended solid, mg/L organic loading rate, g COD/L/d
high concentration of carbohydrates, protein, nitrogenous compounds, lipids and minerals [8,9]. Many anaerobic fermentation studies used mixed culture of POME sludge to convert POME to hydrogen and their operating conditions and reactors configuration always affected on efficiency of microbial degradation of wastewaters by anaerobic fermentation. There are not plenty of reports about ASBR on production of hydrogen, however; they have been quite enough to introduce anaerobic sequencing batch reactor (ASBR) as a promising high-rate anaerobic reactor [10] and efficient technology for hydrogen production from high solid content wastewaters [11]. ASBR is known as high biomass retaining reactor which can decouple solid retention time (SRT) from HRT and maintain high biomass concentration [12]. It has a high degree of process flexibility and simple operation without requirement of separate clarifiers [13]. Besides, it provides feasibility of both batch and continuous operations. The operation of ASBR is based on the four phases including feed, react, settling and decants which are continuously repeated in each cycle. Wastewater is mixed completely during reaction phase with microflora which improves the digestibility of wastewater. Thereafter, internal solid separation happens in settling phase which keeps the solid and sludge in the reactor. Among different operational parameters such as organic loading rate [14], cyclic duration time [15], solid retention time [16] and hydraulic retention times [12] that have been investigated for hydrogen production in ASBR, hydraulic retention time regarded as the most important one because it determines the economics of hydrogen production process [17]. In addition, HRT is reported as one of the most important parameters that must be controlled for treating wastewaters [18]. This study was conducted with the aim to apply ASBR using mixed microflora originating from Palm Oil Mill industry as inoculum to produce hydrogen. POME was used as substrate which contains all necessary components for hydrogen production by anaerobic mixed microflora. Then, the effect of HRT by using raw POME on hydrogen productivity and stability of ASBR was investigated. Accordingly, major parameters of hydrogen production rate, hydrogen production yield, COD removal efficiency and soluble metabolite products were measured at different HRTs.
2.
Materials and methods
2.1.
Reactor setup and operation
A laboratory scale ASBR with a total volume of 5 L and working volume of 3 L was used in this study. Schematic description of
POME SDR SMP SRT TKN TS TVS VFAs
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palm oil mill effluent substrate degradation rate, g COD/L/d soluble metabolite products, mg/L solid retention time, day total Kjeldahl nitrogen, mg/L total solid, mg/L total volatile solid, mg/L volatile fatty acids, mg/L
bioreactor setup for hydrogen production is shown in Fig. 1. One liter of acclimated mixed microflora (MLVSS 3.8 g/L) was added to 2 L of prepared raw POME (20 g COD/L) which was already saturated with N2 free oxygen gas to remove dissolved oxygen and make it completely anaerobic. The reactor was initially operated in batch mode for 24 h while prior acclimated microflora was completely mixed with POME. Then, the reactor was operated under ASBR mode, over a wide range of HRTs from 96 h to 36 h and a cycle time of 24 h at temperature of 37 C. Temperature was controlled by water circulation through a jacket in the reactor. Table 1 shows the operating conditions of ASBR. Each cycle consists of 30 min feeding which was followed by 22 h reaction period. After 60 min of settling period, the required volume of supernatant was decanted. The pH of raw POME, which was diluted by tap water, maintained at the range of 7.5e8.0 by adding 4 N NaOH, whereby the pH of fermentation culture broth was regulated about 6.5e7.0 (most preferably about 6.8) at the beginning of each cycle. This pH was chosen among varying tested pHs to determine the optimum initial pH for anaerobic mesophilic hydrogen production from POME using mixed culture. pH was not controlled during the fermentation in the ASBR. During the reaction phase, the reactor was intermittently mixed with liquid recirculation to provide better distribution of biomass and improve contact of microflora with wastewater. Experiments were conducted successively to determine the optimum HRT for maximum hydrogen productivity of mixed microflora. Regarding that, organic loading rate was increased with decreasing HRT. The ASBR operation was initiated with an HRT of 96 h and continued in progressive shortening mode to HRT 36 h without replacing new seed sludge. The ASBR system worked cycle after cycle to carry out the sequential operation for the ASBR system and producing biogas continuously. Evaluation of system performance for each HRT was carried out during pseudo steady-state conditions, when biogas production, hydrogen content and COD variations were less than 10%. The steady-state performance of the ASBR could be regarded as pseudo steady-state instead of a steady state because artificial solids retention time (SRT) control was not employed. The SRT of the ASBR was controlled only by the loss of solids in the decant phase. The amount and quantity of biogas produced was monitored daily. At the end of reaction time, samples of well-mixed culture were taken to analyze the soluble metabolites, solids concentrations and organic compounds level in the reactor. In addition to COD removal efficiency (%), substrate degradation rate (SDR, g COD/L/d) was also calculated to study the rate and pattern of COD removal during the cycle operation according to the following Equation:
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Fig. 1 e Schematic description of bioreactor setup for hydrogen production.
SDR ¼
ðCOD0 CODt Þ FR Vr
adding peptone and KH2PO4 to the diluted POME. The characteristics of raw POME and prepared substrate are shown in Table 2.
(1)
where, COD0 and CODt (g/L) represent COD at 0 and t times, respectively. FR represents feeding rate (L/d) and Vr is reactor volume (L) [14]. The data presented in this paper are results of daily measurement (n 7) at steady-state conditions.
2.2.
2.4.
Hydrogen gas production was calculated from the headspace measurement of gas compositions and the total volume of biogas produced at each time interval using:
Anaerobic mixed microflora and pretreatment
The hydrogen producing mixed microflora was collected from the bottom of the only anaerobic pond located at Seri Ulu Langat Palm Oil Industry, Dengkil, Malaysia. The culture sample was heat-treated at 85 C for 60 min to inhibit the bioactivity of hydrogen consumers and methanogens [19] before anaerobic fermentation being started. The heattreated sludge was acclimated with POME to develop a stable microbial community, for further fermentation. The acclimated inoculum concentration used for ASBR in terms of mixed liquor suspended solids (MLSS) and mixed liquor volatile solids (MLVSS), were about 5.7 g SS/L and 3.8 g VSS/L, respectively.
2.3.
Monitoring and analysis
VH;i ¼ VH;i1 þ CH;i VG;i VG;i1 þ VH CH;i CH;i1
(2)
where VH,i and VH,i1 are cumulative hydrogen gas volumes at the current (i) and previous (i 1) time intervals, VG,i and VG,i1 are the total biogas volumes in the respective current and previous time intervals, CH,i and CH,i1 are the fraction of hydrogen gas in the headspace of the reactor measured using gas chromatography in the current and previous intervals, and VH the total volume of headspace in the reactor [20]. The hydrogen volume was corrected to standard pressure and temperature at 37 C using the ideal gas law. The volume of biogas produced was measured using water displacement method and its hydrogen content was analyzed by a gas chromatograph (GC) (SRI instrument, Model 8610 C) equipped with TCD/HID detectors and a (10m 6 A) (inside diameter) stainless steel column packed with MS 13X. The injector, detector and column temperatures were kept at 100, 150 and 50 C, respectively. Nitrogen was used as a carrier gas at a flow rate of 30 ml min1. The alcohols and volatile fatty acid content of filtered samples (0.2 mm) were determined by another gas chromatograph (GC) (RESTEK/OSA instrument) equipped with Stabilwax-DA column of 30 m 0.25 mm 0.25 mm (length ID film) at 195 C, while the injector and FID detector, both
Palm oil mill effluent (POME)
The fresh raw POME used in this study was collected from the final discharge point of Palm Oil Mill, Seri Ulu Langat, Palm Oil Industry, Dengkil, Malaysia. POME was stored at 4 C to minimize the self-biodegradation and acidification with the time. The raw wastewater was diluted by a factor of 1:5 using tap water to provide the required concentration for feeding to reactor. To assist the growth of hydrogen producing bacteria, COD:N:P was maintained at an average ratio of 500:5:1 by
Table 1 e Operating condition for ASBR. HRT (h)
96 72 48 36
OLR (g COD/L/d)
5 6.6 10 13.3
Total cycle time (h)
24
Cycle time Feeding (min)
Reaction (h)
Settling (min)
Decanting (min)
30
22
60
30
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120
pH TS (mg/l) TVS (mg/l) MLSS (mg/l) MLVSS (mg/l) COD Total (mg/l) T-Carbohydrate (mg/l) TKN (mg N/l) Total P (mg P/l)
Values of original POME
Values of prepared POME as substrate
4.5 720,000 48,631 26,385 19,604 100,000 24,686 970 470
4.6 14,550 11,700 4650 3600 20,000 5473 428 139
were operated at 200 C. Chemical oxygen demand (COD) was determined by the dichromate method using a COD analyzer (DR 2800, HACH). Total phosphorous content of wastewater was determined according to molybdovanadate acid persulfate digestion method proposed by HACH Company. The carbohydrate concentration of unfiltered and filtered liquid samples (0.45 mm) was determined by the phenol-sulfuric acid method [21] using glucose as the standard. A DR 2800 spectrophotometer (Hach Company, Loveland, CO 80539) was used for colorimetric analysis. Total Kjeldahl nitrogen (TKN), mixed liquor suspended solid (MLSS) and volatile suspended solid (MLVSS), were measured according to the standard methods [22]. Average of three readings obtained during the pseudo steady-state conditions of each HRT has presented as the final result.
3.
Results and discussion
3.1.
Biohydrogen production
After 24 h of batch operation, the sequencing batch mode operation was started at a progressively decreasing HRT from 96 h to 36 h, using POME wastewater as feed to evaluate the hydrogen production capability in ASBR. The pH of influent was already adjusted more than 7.0 to keep a pH level of 6.8 in the reactor. The data for each HRT were evaluated during pseudo steady-state conditions in the last 7 days of each run when biogas production, hydrogen content and COD removal were consistent within 10% variation. The reactor was shifted to lower HRT after at least 8 days operation at pseudo steadystate conditions. The performance with respect to biogas production and H2 generation was found to be dependent on the operating HRT. The biogas produced consisted of H2 and CO2 and was free of CH4 which support the effectiveness of pretreatment process on seed sludge. The variation of hydrogen content of biogas along with time was plotted in Fig. 2. The average hydrogen content of HRT 96 h was 19% which was raised to 50% and then decreased to 34% and 14% at HRT 72 h, 48 h and 36 h, respectively, despite of higher biogas produced at HRT 96 h than that of HRT 48 h. The result showed that variation in hydrogen production rate was in the same trend to that of hydrogen yield. Variation of HPR and HY at different HRTs are shown in Fig. 3. At initial operating HRT of 96 h, hydrogen production
100 HRT (hours)
Parameters
60 HRT H2%
80
50 40
60
30
40
20
20
10
0
0
20
40 60 Time (day)
80
100
H2content , %
Table 2 e Characteristics of the raw palm oil mill effluent and the prepared POME as substrate.
0
Fig. 2 e Variation in hydrogen content (%) with time at different HRTs.
was observed from the first day, however; the operational stability with respect to H2 generation and biogas production attained within 20 days after start-up. The system showed maximum volumetric H2 production rate of 3.1 0.24 L H2/L/d (or 0.12 0.01 mol H2/L/d) which equals to a cumulative H2 yield of 0.15 0.01 L H2/g CODfeeding (or 0.32 0.05 L H2/g CODremoved). Hydrogen yield increased to 0.34 0.01 L H2/g
Fig. 3 e Variation of HPR, HY and COD removal efficiency at different HRTs.
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CODfeeding (or 0.94 0.04 L H2/g CODremoved) when the HRT decreased to 72 h. Maximum HPR of 6.7 0.15 L H2/L/d (or 0.26 0.01 mol H2/L/d) were obtained after 14 days when reactor showed stable performance operation and it remained more or less uniform after that. In the case of HRT 48 h, stabilized performance was attained after 15 days of operation. The volumetric H2 production rate and H2 yield decreased to 2.16 0.07 L H2/L/d (0.08 0.003 mol H2/L/d) and 0.11 0.004 L H2/g CODfeeding (0.47 0.03 L H2/g CODremoved), respectively. At HRT 96 h and OLR of 5 g COD/L/d, longer cyclic duration resulted in lower hydrogen yield as compared to 72 h and 48 h HRT, attributing to the low H2 content. One explanation for low hydrogen yield in HRT 96 h is that bacteria are inhibited by low substrate supplied which facilitate microbial population shift and growth of non-hydrogen producing bacteria in the reactor [15]. Moreover, Xiao et al. [23] reported that solid accumulation resulted from substrate content provide an opportunity for developing non-hydrogen producing microbial populations in ASBR. Other possible reason is generation of inhibitive by-products such as volatile fatty acids (VFAs) which was insufficiently washed out due to longer HRT. Accumulation of these types of products can cause imbalance in the reactor and prevent the culture from effectively utilizing the substrate [24]. Higher hydrogen production ability at HRT 72 h possibly correlates to existing appropriate condition to activate spore formed bacteria and recover hydrogen producing bacterial population which could utilize the carbohydrates more efficiently for H2 production. It was widely reported that the H2 yield increased with decreasing HRT [25], whereas the results from this study showed decrease in the hydrogen yield to 0.11 L H2/g CODfeeding (or 0.47 L H2/g CODremoved) when the HRT reduced to 48 h. The reason for low yield could be sudden washout of biomass and decrease in microbial population at this lower HRT [15], i.e., half of the reactor content was decanted and fed for each cycle at HRT 48 h. Despite of this, high hydrogen content in biogas draws a conclusion that higher organic loading rate in a shorter hydraulic retention time provided favorable condition for remaining H2-producing microorganisms. Moreover, the survival and maintenance of consortia suggests that the growth rate of organism might be higher than washout [26]. Further decreasing of HRT to 36 h in the final run, resulted in the process failure after 5 days of operation. It is presumably due to severe washout of sludge through effluent discharge;
accordingly, hydrogen productivity dropped to zero and system did not achieve stability. One liter of acclimated sludge (3.8 g VSS/L) was added in the reactor to recover the system activity, then it was operated at the previous condition, however; the culture failed to produce hydrogen after 3 days. During the first 5 days of operation in HRT 36 h, average hydrogen production rate of 0.45 0.42 L H2/L/d (or 0.02 0.01 mol H2/L/d) and average hydrogen yield of 0.02 0.02 L H2/g CODfeeding (or 0.24 0.17 L H2/g CODremoved) were observed. Average of hydrogen content was about 14%. Table 3 summarizes experimental data obtained under steady-state conditions from various dark fermentative hydrogen production ASBRs. It is worth mentioned that Poonsuk et al. [27] used POME as substrate at similar HRTs but at thermophilic temperature in ASBR. Their research resulted in hydrogen yield of 0.27 L H2/g COD and HPR of 4.7 L H2/L/d at OLR of 20 g COD/L/d and HRT 48 h. High productivity could be attributed to thermophilic temperature and higher influent concentration used. This indicates the importance of environmental variables which affect on hydrogen productivity in ASBR.
3.2.
Substrate degradation, solid content and SRT values
Variation of COD removal efficiency with hydraulic retention time is shown in Fig. 3. Referring to the results, HRT indirectly impacted on the performance of COD removal efficiency in ASBR. Despite of highest COD removal efficiency of 37% at HRT 96 h and OLR of 5 g COD/L/d, the hydrogen yield was low compared to that at HRT 72 h which possibly relates to microbial shift from hydrogen producing bacteria to nonhydrogen producing bacteria resulted from long retention time of substrate in the reactor. This amount of substrate degradation accounted for SDR of 1.9 g COD/L/d at steadystate condition. At HRT 72 h and higher OLR of 6.6 g COD/L/d, COD removal efficiency reached to more than 35% accounting for SDR of 2.4 g COD/L/d. COD removal efficiency decreased to 34% while OLR increased to 10 g COD/L/d accounting for SDR of 3.4 g COD/L/d. Decrease in hydrogen yield at HRT 48 h possibly results from shortage of microbial population which being washed out, although sufficient substrate was available and metabolized at this condition. Results suggest stability in treatment efficiency could be aligned with increasing organic loading rate. Moreover, hydrogen productivity appeared rather consistent along with substrate degradation. For all four HRT studied, carbohydrate utilization efficiency was over
Table 3 e Experimental data from fermentation of various types of organic wastes in ASBR. VFA/SMP H2% Reference Source of Microorganism Source of HRT OLR Cycle pH/Temp HPR organic waste microorganism (h) (g COD/L/d) length (h) (L H2/L/d) 48 6
20 80
24 3
5.5/60 5.3/35
4.7 4.12
NAa NA
60 63
[27] [12]
Mixed/anaerobic
POME sludge Municipal wastewater Cassava sludge
24
30
4
5.5/37
3.8
NA
40
[38]
Mixed/anaerobic Mixed/anaerobic
Dairy manure POME sludge
16 72
12.3 6.6
4 24
5.0/37 6.8/37
2.25 6.7
NA 0.87
43 50
[41] This study
POME Corn starch
Mixed/anaerobic Mixed/anaerobic
Cassava wastewater Swine manure POME
a NA: not available.
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90% which shows the need of microbial population for biodegradable carbohydrates. Microbial concentration in the ASBR cannot be explained by MLVSS, because MLVSS value of well-mixed liquor represents volatile solids of influent and microorganism concentration. This demonstrates the uncertainty of MLVSS measuring method to estimate the microbial population of the solids in the mixed liquor. Nevertheless, solid quantification was performed by sampling from mixed liquor in the reactor before settling period started and from effluent. The major part of these solids was from influent which contained considerable amount of suspended and volatile solids. Solid analysis revealed that there was considerable loss of volatile solids at HRT 48 h and 36 h which ultimately led to depletion of bacterial population at HRT 36 h. This easily elucidates the low performance of ASBR at HRT 36 h in terms of hydrogen production and organic removal. These factors affected the stability of system and ultimately collapsed the system. The changes of HRT may possibly provide an opportunity to select the more dominant hydrogen producing microbial populations in the reactor. It is reported that HRT affects the microbial community to a certain extent and in turn shows an impact on hydrogen yield [28,29]. Chen and Lin [30] also reported that possible reason for increase in hydrogen yield with shortened HRT is contributions of HRT-selected microbial population. The solid retention time (SRT) was calculated from the suspended solid content of reactor and the suspended solid lost in the effluent at the period of steady state at each HRT: MLVSS concentration in reactor HRT MLVSS concentration of effluent
SRT ¼
(3)
The calculated SRTs were approximately 19 days for HRT 96 h, 11 days for HRT 72 h and 5.5 days at HRT 48 h. In this study hydrogen production did not show consistent relationship to SRT and HRT. The longer HRT yielded lower solid removal and showed the longest SRT which is supposed to yield maximum hydrogen production in this system, however; it did not happen which could be attributed to prevalence of H2-consuming or non-hydrogen producing bacteria [31]. Appropriate operating condition at shorter HRT of 72 h with SRT of 11 days supported the retention of active hydrogen producing bacteria and restored the stability in system. Thereafter, at lower HRTs, SRT decreased which may have been related to biomass washout in the system.
3.3.
Variation of VFA and solvent concentration
Results of soluble metabolite composition and concentration at various HRTs, presented in Table 4 can describe the performance of reactor. VFA is known as intermediate and indicator in the anaerobic process [32]. In this study, lower efficiency in total soluble metabolite production was observed, although high carbohydrate was consumed by the mixed anaerobic bacteria. In all ranges of HRT, butyrate followed by acetate were the main acids throughout the fermentation in ASBR along with much lower amounts of propionic acid and iso-butyric acid with small amounts of heptanoic acid. It should be noted that solvent concentrations, which is unfavorable metabolite for hydrogen generation, was lower in comparison with acid concentrations throughout the tested HRTs range and butanol was the predominant species among solvents. The influent POME might be the main source of butanol which already contained 700 mg/L of this solvent, suggesting that high butanol concentration in the feed affected the one in the effluent. VFA/SMP ratio was essentially higher than 0.63 in this study. The abundance of VFAs in total SMP (VFA/SMP) suggests that H2 production was metabolically favorable since acidogenic pathway was predominant over solventogenesis [33,34]. Hydrogen production in acidogenesis comes mainly from oxidative decarboxylation of pyruvic acid to acetyl CoA,and is also linked to converting acetyl CoA to butyrate and ethanol, together with production of acetate [35]. Moreover, HBu/VFA ratio was higher than 0.7 in this study, and fraction of butyric acid in soluble metabolites increased by decreasing HRT. Simultaneous decrease in the acetate concentration and an increase in H2 production observed which also reported in previous studies in which mixed culture were used [3]. Butyrate and acetate have been reported as abundant liquid products in anaerobic hydrogen production from mixed culture [36,37], although different intermediates were produced in mixed fermentation culture. This is likely due to different types of fermentation pathway used by the mixed anaerobic microorganisms [38]. Propionic acid level was low and did not vary markedly in HRTs ranges of 96e36 h. Highest hydrogen yield at 72 h was found accompanied with high butyrate concentration and low propionate concentration. Cohen et al. [39] showed a linear inverse relationship between propionate and butyrate formation and a linear positive correlation between butyrate and H2 production. Relatively consistent but low percentage of ethanol was also obtained at the steady-state conditions.
Table 4 e Soluble metabolites obtained at steady-state conditions at different HRTs. HRT, h
Soluble metabolites a
96 72 48 36
a
a
a
a
Et , %
Bu , %
HAc , %
HPr , %
Iso-Bu , %
HBua, %
HHpa, %
SMP, mg/l
VFA, mg/l
VFA/SMP
HBu/VFA
2.9 0.12 0.49 0.77
38.0 14.0 17.9 20
13.8 12.45 9.49 29
0.16 0.34 0.22 0.96
1.24 0.75 1.13 0.96
43.2 72.0 70.5 89.3
0.66 0.15 0.24 0.30
2876 0.8 2653 0.6b 3889 0.45 1555 1
1698 0.2 2277 0.33 3172 0.4 1826 0.4
0.63 0.0001 0.87 0.001 0.82 0.07 1.25 0.04
0.73 0.007 0.96 0.002 0.97 0.002 0.7 0.007
a Et ¼ ethanol; Bu ¼ butanol; HAc ¼ acetic acid; HPr ¼ propionic acid; Iso-Bu ¼ iso-butyric acid; HBu ¼ butyric acid; and HHp ¼ heptanoic acid. b Mean value standard deviation.
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Distribution of soluble microbial products indicated the butyrateeacetate fermentation type in the ASBR, which is characterized by the production of butyrate and acetate, plus carbon dioxide and hydrogen in acidogenic fermentation pathway [40]. In this investigation, fermentation metabolites were generated regardless of variation of HRT which confirms little impact of HRT on these by-products, while previous studies claimed that HRT was an important environmental factor to affect the metabolic pathway shifts [41]. In our future works, further investigation is needed to elucidate the microbial mechanism of soluble metabolite production in the mesophilic mixed cultures study.
4.
Conclusions
In this study we investigated higher efficiency for hydrogen production in ASBR through treating organic wastewater by changing HRT and different organic loading condition. The study found that to achieve both satisfactory production rate and yield efficiency, stable performance must be established in ASBR which is possible by using stable microbial community and modifying the operational parameters. Experimental results revealed that HRT affects on many aspects of hydrogen production process. According to the results from this study, HRT of 72 h could be considered as the optimum HRT for treating diluted POME in ASBR system with respect to higher production rate and yield efficiency, while the longer HRT would provoke development of non-hydrogen producing bacteria and shorter HRTs lowers H2 yield. In addition, Low VFA concentration confirms the stability of operation at HRT 72 h. Based on the calculated SRT values, SRT showed decreasing trend as HRT decreased progressively and system did not maintain the solids. This happened due to the increase of suspended solids in the effluent because of decrease in HRT and leading to washout of active hydrogen producing bacteria from reactor. On overall, continuous shortening the HRT deteriorated the productivity by washing out the active bacteria from the system. This work has illustrated the feasibility of using POME as an inexpensive feedstock for production of hydrogen in ASBR, meanwhile, provides valuable information to design more effective system for palm oil mill waste treatment.
Acknowledgement The authors wish to thank the financial support from Universiti Kebangsaan Malaysia and the Ministry of Science, Technology and Innovation (MOSTI) Malaysia, under UKMAP-PI-13-2010 and the Science-fund Research program of UKM-MGI-NBD16-2007, for funding of this project.
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[2] Vijayaraghavan K, Ahmad D, Ibrahim MKB. Biohydrogen generation from jackfruit peel using anaerobic contact filter. Int J Hydrogen Energy 2005;31:569e79. [3] Hussy I, Hawkes FR, Dinsdale R, Hawkes DL. Continuous fermentative hydrogen production from a wheat starch coproduct by mixed microflora. Biotechnol Bioeng 2003;84:619e26. [4] Wu TY, Mohammad AW, Md Jahim J, Anuar N. Palm oil mill effluent (POME) treatment and bioresources recovery using ultrafiltration membrane: effect of pressure on membrane fouling. Biochem Eng J 2007;35:309e17. [5] Wu TY, Mohammad AW, Md Jahim J, Anuar N. Investigations on protease production by a wild-type Aspergillus terreus strain using diluted retentate of pre-filtered palm oil mill effluent (POME) as substrate. Enzyme Microb Technol 2006; 39:1223e9. [6] O-Thong Sompong, Prasertsan Poonsuk, Intrasungkha Nugul. Improvement of biohydrogen production and treatment efficiency on palm oil mill effluent with nutrient supplementation at thermophilic condition using an anaerobic sequencing batch reactor. Enzyme Microb Technol 2007;41:583e90. [7] Vijayaraghavan K, Ahmad D. Biohydrogen generation from palm oil mill effluent using anaerobic contact filter. Int J Hydrogen Energy 2006;31:1284e91. [8] Wu Ta Yeong, Mohammad Abdul Wahab, Md. Jahim Jamalaih, Anuar Nurina. A holistic approach to managing palm oil mill effluent (POME): biotechnological advances in the sustainable reuse of POME. Biotechnol Adv 2009;27:40e52. [9] Wu TY, Mohammad AW, Md Jahim J, Anuar N. Pollution control technologies for the treatment of palm oil mill effluent (POME) through end-of-pipe processes. J Environ Manage 2010;91:1467e90. [10] Chen Shing-Der, Lo Yung-Chung, Lee Kuo-Shing, Huang Tian-I, Chang Jo-Shu. Sequencing batch reactor enhances bacterial hydrolysis of starch promoting continuous bio-hydrogen production from starch feedstock. Int J Hydrogen Energy 2009;34:8549e57. [11] Saraphirom P, Reungsang A. Biological hydrogen production from sweet sorghum syrup by mixed cultures using an anaerobic sequencing batch reactor (ASBR). Int J Hydrogen Energy; 2010. doi:10.1016/j.ijhydene.2010.08.058. [12] Farhan MA, Han SK, Kim SH, Kim DH, Shin HS. Effect of HRT on ASBR converting starch into biological hydrogen. Int J Hydrogen Energy 2008;33:6509e14. [13] Ratusznei SM, Rodrigues JAD, Camargo EFM, Zaiat M, Borzani W. Feasibility of a stirred anaerobic sequencing batch reactor containing immobilized biomass for wastewater treatment. Bioresour Technol 2000;75:127e32. [14] Venkata Mohan S, Lalit Babu V, Sarma PN. Anaerobic biohydrogen production from dairy wastewater treatment in sequencing batch reactor (AnSBR): effect of organic loading rate. Enzyme Microb Technol 2007;41:506e15. [15] Chen Wen-Hsing, Sung Shihwu, Chen Shen-Yi. Biological hydrogen production in an anaerobic sequencing batch reactor: pH and cyclic duration effects. Int J Hydrogen Energy 2008;34:1e8. [16] Kim SH, Han SK, Shin HS. Optimization of continuous hydrogen fermentation of food waste as a function of solids retention time independent of hydraulic retention time. Process Biochem 2008;43:213e8. [17] Kim SH, Han SK, Shin HS. Effect of substrate concentration on hydrogen production and 16S rDNA-based analysis of the microbial community in a continuous fermenter. Process Biochem 2006;41:199e207. [18] Dabrock B, Bahl H, Gottschalk G. Parameters affecting solvent production by Clostridium pasteurianum. Appl Environ Microbiol 1992;58(4):1233e9.
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[19] Kim Dong-Hoon, Han Sun-Kee, Kim Sang-Hyoun, Shin HangSik. Effect of gas sparging on continuous fermentative hydrogen production. Int J Hydrogen Energy 2006;31: 2158e64. [20] Van Ginkel SW, Oh SE, Logan B. Biohydrogen gas production from food processing and domestic wastewaters. Int J Hydrogen Energy 2005;30:1535e42. [21] Dubois M, Gilles KA, Hamilton JK, Rebers PA, Smith F. Colorimetric method for determination of sugars and related substrates. Anal Chem 1956;28(3):350e6. [22] APHA AWWA WEF. Standard methods for examination of water and wastewater. 20th ed. Washington, DC, USA: APHA; 1998. [23] Xiao W, Zh Jun, Chunying D, Curt M, Yecong L, Liang W, et al. Continuous biohydrogen production from liquid swine manure supplemented with glucose using an anaerobic sequencing batch reactor. Int J Hydrogen Energy 2009;34: 6636e45. [24] Stamatelatou K, Vavilin V, Lyberatos G. performance of a glucose fed periodic anaerobic baffled reactor under increasing organic loading conditions: 1. Experimental results. Bioresour Technol 2003;88:131e6. [25] Shao X, Peng D, Teng Zh, Ju X. Treatment of brewery wastewater using anaerobic sequencing batch reactor (ASBR). Bioresour Technol 2008;99:3182e6. [26] Lin CY, Chou CH. Anaerobic hydrogen production from sucrose using an acid e enriched sewage sludge microflora. Eng Life Sci 2004;4(1):66e70. [27] Prasertsan P, O-Thong S, Birkeland NK. Optimization and microbial community analysis for production of biohydrogen from palm oil mill effluent by thermophilic fermentative process. Int J Hydrogen Energy 2009;34:7448e59. [28] Zhang Zhen-Peng, Show Kuan-Yeow, Tay Joo-Hwa, Liang David Tee, Lee Duu-Jong, Jiang Wen-Ju. Effect of hydraulic retention time on biohydrogen production and anaerobic microbial community. Proc Biochem 2006;41:2118e23. [29] Fan KS, Kan NR, Lay JJ. Effect of hydraulic retention time on anaerobic hydrogenesis in CSTR. Bioresour Technol 2006;97 (1):84e9. [30] Chen CC, Lin CY. Using sucrose as substrate in an anaerobic hydrogen-producing reactor. Adv Environ Res 2003;7(3):695e9.
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[31] Cohen A, Distel B, Vandeursen A, Breure AM, Vanandej JG. Role of anaerobic spore-forming bacteria in the acidogenesis of glucose-changes induced by discontinuous or low-rate feed supply. Anton Leeuw J Microbiol 1985;51:179e92. [32] Khanal SK, Li L, Sung S. Biological hydrogen production: effects of pH and intermediate products. Int J Hydrogen Energy 2004;29:1123e31. [33] Mohan SV, Vijaya BY, Murali KP. Biohydrogen production from chemical wastewater as substrate by selectively enriched anaerobic mixed consortia: influence of fermentation pH and substrate composition. Int J Hydrogen Energy 2007;32(13):2286e95. [34] Sreethawong T, Niyamapa T, Neramitsuk H. Hydrogen production from glucose-containing wastewater using an anaerobic sequencing batch reactor: effects of COD loading rate, nitrogen content and organic acid composition. Chem Eng J 2010;160:322e32. [35] Cheong DY, Hansen CL, Stevens DK. Production of biohydrogen by mesophilic anaerobic fermentation in an acidphase sequencing batch reactor. Biotechnol Bioeng 2007;96 (3):421e31. [36] Fang HHP, Liu H. Effect of pH on hydrogen production from glucose by a mixed culture. Bioresour Technol 2002;82:87e93. [37] Chen CC, Lin CY, Chang JS. Kinetics of hydrogen production with continuous anaerobic cultures utilizing sucrose as the limiting substrate. Appl Microbiol Biotechnol 2001;57:56e64. [38] Sreethawong T, Chatsiriwatana S, Rangsunvigit P, Chavadej S. Hydrogen production from cassava wastewater using an anaerobic sequencing batch reactor: effects of operational parameters, COD:N ratio and organic acid composition. Int J Hydrogen Energy 2010;35:4092e102. [39] Cohen A, Distel B, Van Deursen A, Breure AM, Van Andel JG. Role of anaerobic spore-forming bacteria in the acidogenesis of glucose: changes induced by discontinuous or low-rate feed supply. Antonie Van Leeuwenhock 1985;51(2):179e92. [40] Yu HO, Fang HHP. Thermophilic acidification of dairy wastewater. Appl Microbiol Biotechnol 2000;54:439e44. [41] Xiao W, Wanying Y, Zh Jun. Effect of pH on continuous biohydrogen production from liquid swine manure with glucose supplement using an anaerobic sequencing batch reactor. Int J Hydrogen Energy 2010;35:6592e9.
Available at www.sciencedirect.com
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Effect of hydraulic retention time on biohydrogen production from palm oil mill effluent in anaerobic sequencing batch reactor Marzieh Badiei, Jamaliah Md Jahim*, Nurina Anuar, Siti Rozaimah Sheikh Abdullah Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
article info
abstract
Article history:
The feasibility of hydrogen generation from palm oil mill effluent (POME), a high strength
Received 8 December 2010
wastewater with high solid content, was evaluated in an anaerobic sequencing batch
Received in revised form
reactor (ASBR) using enriched mixed microflora, under mesophilic digestion process at
7 February 2011
37 C. Four different hydraulic retention times (HRT), ranging from 96 h to 36 h at constant
Accepted 10 February 2011
cycle length of 24 h and various organic loading rate (OLR) concentrations were tested to
Available online 2 April 2011
evaluate hydrogen productivity and operational stability of ASBR. The results showed higher system efficiency was achieved at HRT of 72 h with maximum hydrogen production
Keywords:
rate of 6.7 LH2/L/d and hydrogen yield of 0.34 LH2/g CODfeeding, while in longer and shorter
Biohydrogen
HRTs, hydrogen productivity decreased. Organic matter removal efficiency was affected by
Palm oil mill effluent (POME)
HRT; accordingly, total and soluble COD removal reached more than 37% and 50%,
Anaerobic sequencing batch reactor
respectively. Solid retention time (SRT) of 4e19 days was achieved at these wide ranges of
(ASBR)
HRTs. Butyrate was found to be the dominant metabolite in all HRTs. Low concentration of
Hydraulic retention time (HRT)
volatile fatty acid (VFA) confirmed the state of stability and efficiency of sequential batch
Solid retention time (SRT)
mode operation was achieved in ASBR. Results also suggest that ASBR has the potential to offer high digestion rate and good stability of operation for POME treatment. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Regarding the limited fossil energy resources, energy security and the global environment, a great attention is being paid to the usage of hydrogen as fuel to reduce the emission of CO2 to the atmosphere. Hydrogen as the future energy source has recyclability and non-polluting nature [1]. Biological hydrogen production is of great interest because it can be operated at ambient temperature and pressure and consequently is less energy intensive compared to traditional thermo-chemical and electro-chemical process. Production of hydrogen through biological process can be achieved by using anaerobic process.
Moreover, fermentative H2 production using high strength wastewater as substrate attracted considerable attention, since it is direct treatment of wastewater along with sustainable bioenergy generation [2]. However, most organic waste products are complex substrates and information on their conversion to hydrogen in continuous process, whether by defined cultures or mixed microflora, is very limited [3]. Palm oil mill effluent (POME) and its derivatives have been exploited as fermentation media to produce various products [4,5] and have shown high potential as a substrate for generation of hydrogen [6,7]. The possibility of reusing POME as fermentation media is largely due to the fact that POME contains
* Corresponding author. E-mail address: [email protected] (J.M. Jahim). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.02.054
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Nomenclature ASBR COD HPR HRT HY MLSS MLVSS OLR
anaerobic sequencing batch reactor chemical oxygen demand, g/L hydrogen production rate, L H2/L/d hydraulic retention time, hour hydrogen yield, L H2/g COD mixed liquor suspended solid, mg/L mixed liquor volatile suspended solid, mg/L organic loading rate, g COD/L/d
high concentration of carbohydrates, protein, nitrogenous compounds, lipids and minerals [8,9]. Many anaerobic fermentation studies used mixed culture of POME sludge to convert POME to hydrogen and their operating conditions and reactors configuration always affected on efficiency of microbial degradation of wastewaters by anaerobic fermentation. There are not plenty of reports about ASBR on production of hydrogen, however; they have been quite enough to introduce anaerobic sequencing batch reactor (ASBR) as a promising high-rate anaerobic reactor [10] and efficient technology for hydrogen production from high solid content wastewaters [11]. ASBR is known as high biomass retaining reactor which can decouple solid retention time (SRT) from HRT and maintain high biomass concentration [12]. It has a high degree of process flexibility and simple operation without requirement of separate clarifiers [13]. Besides, it provides feasibility of both batch and continuous operations. The operation of ASBR is based on the four phases including feed, react, settling and decants which are continuously repeated in each cycle. Wastewater is mixed completely during reaction phase with microflora which improves the digestibility of wastewater. Thereafter, internal solid separation happens in settling phase which keeps the solid and sludge in the reactor. Among different operational parameters such as organic loading rate [14], cyclic duration time [15], solid retention time [16] and hydraulic retention times [12] that have been investigated for hydrogen production in ASBR, hydraulic retention time regarded as the most important one because it determines the economics of hydrogen production process [17]. In addition, HRT is reported as one of the most important parameters that must be controlled for treating wastewaters [18]. This study was conducted with the aim to apply ASBR using mixed microflora originating from Palm Oil Mill industry as inoculum to produce hydrogen. POME was used as substrate which contains all necessary components for hydrogen production by anaerobic mixed microflora. Then, the effect of HRT by using raw POME on hydrogen productivity and stability of ASBR was investigated. Accordingly, major parameters of hydrogen production rate, hydrogen production yield, COD removal efficiency and soluble metabolite products were measured at different HRTs.
2.
Materials and methods
2.1.
Reactor setup and operation
A laboratory scale ASBR with a total volume of 5 L and working volume of 3 L was used in this study. Schematic description of
POME SDR SMP SRT TKN TS TVS VFAs
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palm oil mill effluent substrate degradation rate, g COD/L/d soluble metabolite products, mg/L solid retention time, day total Kjeldahl nitrogen, mg/L total solid, mg/L total volatile solid, mg/L volatile fatty acids, mg/L
bioreactor setup for hydrogen production is shown in Fig. 1. One liter of acclimated mixed microflora (MLVSS 3.8 g/L) was added to 2 L of prepared raw POME (20 g COD/L) which was already saturated with N2 free oxygen gas to remove dissolved oxygen and make it completely anaerobic. The reactor was initially operated in batch mode for 24 h while prior acclimated microflora was completely mixed with POME. Then, the reactor was operated under ASBR mode, over a wide range of HRTs from 96 h to 36 h and a cycle time of 24 h at temperature of 37 C. Temperature was controlled by water circulation through a jacket in the reactor. Table 1 shows the operating conditions of ASBR. Each cycle consists of 30 min feeding which was followed by 22 h reaction period. After 60 min of settling period, the required volume of supernatant was decanted. The pH of raw POME, which was diluted by tap water, maintained at the range of 7.5e8.0 by adding 4 N NaOH, whereby the pH of fermentation culture broth was regulated about 6.5e7.0 (most preferably about 6.8) at the beginning of each cycle. This pH was chosen among varying tested pHs to determine the optimum initial pH for anaerobic mesophilic hydrogen production from POME using mixed culture. pH was not controlled during the fermentation in the ASBR. During the reaction phase, the reactor was intermittently mixed with liquid recirculation to provide better distribution of biomass and improve contact of microflora with wastewater. Experiments were conducted successively to determine the optimum HRT for maximum hydrogen productivity of mixed microflora. Regarding that, organic loading rate was increased with decreasing HRT. The ASBR operation was initiated with an HRT of 96 h and continued in progressive shortening mode to HRT 36 h without replacing new seed sludge. The ASBR system worked cycle after cycle to carry out the sequential operation for the ASBR system and producing biogas continuously. Evaluation of system performance for each HRT was carried out during pseudo steady-state conditions, when biogas production, hydrogen content and COD variations were less than 10%. The steady-state performance of the ASBR could be regarded as pseudo steady-state instead of a steady state because artificial solids retention time (SRT) control was not employed. The SRT of the ASBR was controlled only by the loss of solids in the decant phase. The amount and quantity of biogas produced was monitored daily. At the end of reaction time, samples of well-mixed culture were taken to analyze the soluble metabolites, solids concentrations and organic compounds level in the reactor. In addition to COD removal efficiency (%), substrate degradation rate (SDR, g COD/L/d) was also calculated to study the rate and pattern of COD removal during the cycle operation according to the following Equation:
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Fig. 1 e Schematic description of bioreactor setup for hydrogen production.
SDR ¼
ðCOD0 CODt Þ FR Vr
adding peptone and KH2PO4 to the diluted POME. The characteristics of raw POME and prepared substrate are shown in Table 2.
(1)
where, COD0 and CODt (g/L) represent COD at 0 and t times, respectively. FR represents feeding rate (L/d) and Vr is reactor volume (L) [14]. The data presented in this paper are results of daily measurement (n 7) at steady-state conditions.
2.2.
2.4.
Hydrogen gas production was calculated from the headspace measurement of gas compositions and the total volume of biogas produced at each time interval using:
Anaerobic mixed microflora and pretreatment
The hydrogen producing mixed microflora was collected from the bottom of the only anaerobic pond located at Seri Ulu Langat Palm Oil Industry, Dengkil, Malaysia. The culture sample was heat-treated at 85 C for 60 min to inhibit the bioactivity of hydrogen consumers and methanogens [19] before anaerobic fermentation being started. The heattreated sludge was acclimated with POME to develop a stable microbial community, for further fermentation. The acclimated inoculum concentration used for ASBR in terms of mixed liquor suspended solids (MLSS) and mixed liquor volatile solids (MLVSS), were about 5.7 g SS/L and 3.8 g VSS/L, respectively.
2.3.
Monitoring and analysis
VH;i ¼ VH;i1 þ CH;i VG;i VG;i1 þ VH CH;i CH;i1
(2)
where VH,i and VH,i1 are cumulative hydrogen gas volumes at the current (i) and previous (i 1) time intervals, VG,i and VG,i1 are the total biogas volumes in the respective current and previous time intervals, CH,i and CH,i1 are the fraction of hydrogen gas in the headspace of the reactor measured using gas chromatography in the current and previous intervals, and VH the total volume of headspace in the reactor [20]. The hydrogen volume was corrected to standard pressure and temperature at 37 C using the ideal gas law. The volume of biogas produced was measured using water displacement method and its hydrogen content was analyzed by a gas chromatograph (GC) (SRI instrument, Model 8610 C) equipped with TCD/HID detectors and a (10m 6 A) (inside diameter) stainless steel column packed with MS 13X. The injector, detector and column temperatures were kept at 100, 150 and 50 C, respectively. Nitrogen was used as a carrier gas at a flow rate of 30 ml min1. The alcohols and volatile fatty acid content of filtered samples (0.2 mm) were determined by another gas chromatograph (GC) (RESTEK/OSA instrument) equipped with Stabilwax-DA column of 30 m 0.25 mm 0.25 mm (length ID film) at 195 C, while the injector and FID detector, both
Palm oil mill effluent (POME)
The fresh raw POME used in this study was collected from the final discharge point of Palm Oil Mill, Seri Ulu Langat, Palm Oil Industry, Dengkil, Malaysia. POME was stored at 4 C to minimize the self-biodegradation and acidification with the time. The raw wastewater was diluted by a factor of 1:5 using tap water to provide the required concentration for feeding to reactor. To assist the growth of hydrogen producing bacteria, COD:N:P was maintained at an average ratio of 500:5:1 by
Table 1 e Operating condition for ASBR. HRT (h)
96 72 48 36
OLR (g COD/L/d)
5 6.6 10 13.3
Total cycle time (h)
24
Cycle time Feeding (min)
Reaction (h)
Settling (min)
Decanting (min)
30
22
60
30
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120
pH TS (mg/l) TVS (mg/l) MLSS (mg/l) MLVSS (mg/l) COD Total (mg/l) T-Carbohydrate (mg/l) TKN (mg N/l) Total P (mg P/l)
Values of original POME
Values of prepared POME as substrate
4.5 720,000 48,631 26,385 19,604 100,000 24,686 970 470
4.6 14,550 11,700 4650 3600 20,000 5473 428 139
were operated at 200 C. Chemical oxygen demand (COD) was determined by the dichromate method using a COD analyzer (DR 2800, HACH). Total phosphorous content of wastewater was determined according to molybdovanadate acid persulfate digestion method proposed by HACH Company. The carbohydrate concentration of unfiltered and filtered liquid samples (0.45 mm) was determined by the phenol-sulfuric acid method [21] using glucose as the standard. A DR 2800 spectrophotometer (Hach Company, Loveland, CO 80539) was used for colorimetric analysis. Total Kjeldahl nitrogen (TKN), mixed liquor suspended solid (MLSS) and volatile suspended solid (MLVSS), were measured according to the standard methods [22]. Average of three readings obtained during the pseudo steady-state conditions of each HRT has presented as the final result.
3.
Results and discussion
3.1.
Biohydrogen production
After 24 h of batch operation, the sequencing batch mode operation was started at a progressively decreasing HRT from 96 h to 36 h, using POME wastewater as feed to evaluate the hydrogen production capability in ASBR. The pH of influent was already adjusted more than 7.0 to keep a pH level of 6.8 in the reactor. The data for each HRT were evaluated during pseudo steady-state conditions in the last 7 days of each run when biogas production, hydrogen content and COD removal were consistent within 10% variation. The reactor was shifted to lower HRT after at least 8 days operation at pseudo steadystate conditions. The performance with respect to biogas production and H2 generation was found to be dependent on the operating HRT. The biogas produced consisted of H2 and CO2 and was free of CH4 which support the effectiveness of pretreatment process on seed sludge. The variation of hydrogen content of biogas along with time was plotted in Fig. 2. The average hydrogen content of HRT 96 h was 19% which was raised to 50% and then decreased to 34% and 14% at HRT 72 h, 48 h and 36 h, respectively, despite of higher biogas produced at HRT 96 h than that of HRT 48 h. The result showed that variation in hydrogen production rate was in the same trend to that of hydrogen yield. Variation of HPR and HY at different HRTs are shown in Fig. 3. At initial operating HRT of 96 h, hydrogen production
100 HRT (hours)
Parameters
60 HRT H2%
80
50 40
60
30
40
20
20
10
0
0
20
40 60 Time (day)
80
100
H2content , %
Table 2 e Characteristics of the raw palm oil mill effluent and the prepared POME as substrate.
0
Fig. 2 e Variation in hydrogen content (%) with time at different HRTs.
was observed from the first day, however; the operational stability with respect to H2 generation and biogas production attained within 20 days after start-up. The system showed maximum volumetric H2 production rate of 3.1 0.24 L H2/L/d (or 0.12 0.01 mol H2/L/d) which equals to a cumulative H2 yield of 0.15 0.01 L H2/g CODfeeding (or 0.32 0.05 L H2/g CODremoved). Hydrogen yield increased to 0.34 0.01 L H2/g
Fig. 3 e Variation of HPR, HY and COD removal efficiency at different HRTs.
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CODfeeding (or 0.94 0.04 L H2/g CODremoved) when the HRT decreased to 72 h. Maximum HPR of 6.7 0.15 L H2/L/d (or 0.26 0.01 mol H2/L/d) were obtained after 14 days when reactor showed stable performance operation and it remained more or less uniform after that. In the case of HRT 48 h, stabilized performance was attained after 15 days of operation. The volumetric H2 production rate and H2 yield decreased to 2.16 0.07 L H2/L/d (0.08 0.003 mol H2/L/d) and 0.11 0.004 L H2/g CODfeeding (0.47 0.03 L H2/g CODremoved), respectively. At HRT 96 h and OLR of 5 g COD/L/d, longer cyclic duration resulted in lower hydrogen yield as compared to 72 h and 48 h HRT, attributing to the low H2 content. One explanation for low hydrogen yield in HRT 96 h is that bacteria are inhibited by low substrate supplied which facilitate microbial population shift and growth of non-hydrogen producing bacteria in the reactor [15]. Moreover, Xiao et al. [23] reported that solid accumulation resulted from substrate content provide an opportunity for developing non-hydrogen producing microbial populations in ASBR. Other possible reason is generation of inhibitive by-products such as volatile fatty acids (VFAs) which was insufficiently washed out due to longer HRT. Accumulation of these types of products can cause imbalance in the reactor and prevent the culture from effectively utilizing the substrate [24]. Higher hydrogen production ability at HRT 72 h possibly correlates to existing appropriate condition to activate spore formed bacteria and recover hydrogen producing bacterial population which could utilize the carbohydrates more efficiently for H2 production. It was widely reported that the H2 yield increased with decreasing HRT [25], whereas the results from this study showed decrease in the hydrogen yield to 0.11 L H2/g CODfeeding (or 0.47 L H2/g CODremoved) when the HRT reduced to 48 h. The reason for low yield could be sudden washout of biomass and decrease in microbial population at this lower HRT [15], i.e., half of the reactor content was decanted and fed for each cycle at HRT 48 h. Despite of this, high hydrogen content in biogas draws a conclusion that higher organic loading rate in a shorter hydraulic retention time provided favorable condition for remaining H2-producing microorganisms. Moreover, the survival and maintenance of consortia suggests that the growth rate of organism might be higher than washout [26]. Further decreasing of HRT to 36 h in the final run, resulted in the process failure after 5 days of operation. It is presumably due to severe washout of sludge through effluent discharge;
accordingly, hydrogen productivity dropped to zero and system did not achieve stability. One liter of acclimated sludge (3.8 g VSS/L) was added in the reactor to recover the system activity, then it was operated at the previous condition, however; the culture failed to produce hydrogen after 3 days. During the first 5 days of operation in HRT 36 h, average hydrogen production rate of 0.45 0.42 L H2/L/d (or 0.02 0.01 mol H2/L/d) and average hydrogen yield of 0.02 0.02 L H2/g CODfeeding (or 0.24 0.17 L H2/g CODremoved) were observed. Average of hydrogen content was about 14%. Table 3 summarizes experimental data obtained under steady-state conditions from various dark fermentative hydrogen production ASBRs. It is worth mentioned that Poonsuk et al. [27] used POME as substrate at similar HRTs but at thermophilic temperature in ASBR. Their research resulted in hydrogen yield of 0.27 L H2/g COD and HPR of 4.7 L H2/L/d at OLR of 20 g COD/L/d and HRT 48 h. High productivity could be attributed to thermophilic temperature and higher influent concentration used. This indicates the importance of environmental variables which affect on hydrogen productivity in ASBR.
3.2.
Substrate degradation, solid content and SRT values
Variation of COD removal efficiency with hydraulic retention time is shown in Fig. 3. Referring to the results, HRT indirectly impacted on the performance of COD removal efficiency in ASBR. Despite of highest COD removal efficiency of 37% at HRT 96 h and OLR of 5 g COD/L/d, the hydrogen yield was low compared to that at HRT 72 h which possibly relates to microbial shift from hydrogen producing bacteria to nonhydrogen producing bacteria resulted from long retention time of substrate in the reactor. This amount of substrate degradation accounted for SDR of 1.9 g COD/L/d at steadystate condition. At HRT 72 h and higher OLR of 6.6 g COD/L/d, COD removal efficiency reached to more than 35% accounting for SDR of 2.4 g COD/L/d. COD removal efficiency decreased to 34% while OLR increased to 10 g COD/L/d accounting for SDR of 3.4 g COD/L/d. Decrease in hydrogen yield at HRT 48 h possibly results from shortage of microbial population which being washed out, although sufficient substrate was available and metabolized at this condition. Results suggest stability in treatment efficiency could be aligned with increasing organic loading rate. Moreover, hydrogen productivity appeared rather consistent along with substrate degradation. For all four HRT studied, carbohydrate utilization efficiency was over
Table 3 e Experimental data from fermentation of various types of organic wastes in ASBR. VFA/SMP H2% Reference Source of Microorganism Source of HRT OLR Cycle pH/Temp HPR organic waste microorganism (h) (g COD/L/d) length (h) (L H2/L/d) 48 6
20 80
24 3
5.5/60 5.3/35
4.7 4.12
NAa NA
60 63
[27] [12]
Mixed/anaerobic
POME sludge Municipal wastewater Cassava sludge
24
30
4
5.5/37
3.8
NA
40
[38]
Mixed/anaerobic Mixed/anaerobic
Dairy manure POME sludge
16 72
12.3 6.6
4 24
5.0/37 6.8/37
2.25 6.7
NA 0.87
43 50
[41] This study
POME Corn starch
Mixed/anaerobic Mixed/anaerobic
Cassava wastewater Swine manure POME
a NA: not available.
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90% which shows the need of microbial population for biodegradable carbohydrates. Microbial concentration in the ASBR cannot be explained by MLVSS, because MLVSS value of well-mixed liquor represents volatile solids of influent and microorganism concentration. This demonstrates the uncertainty of MLVSS measuring method to estimate the microbial population of the solids in the mixed liquor. Nevertheless, solid quantification was performed by sampling from mixed liquor in the reactor before settling period started and from effluent. The major part of these solids was from influent which contained considerable amount of suspended and volatile solids. Solid analysis revealed that there was considerable loss of volatile solids at HRT 48 h and 36 h which ultimately led to depletion of bacterial population at HRT 36 h. This easily elucidates the low performance of ASBR at HRT 36 h in terms of hydrogen production and organic removal. These factors affected the stability of system and ultimately collapsed the system. The changes of HRT may possibly provide an opportunity to select the more dominant hydrogen producing microbial populations in the reactor. It is reported that HRT affects the microbial community to a certain extent and in turn shows an impact on hydrogen yield [28,29]. Chen and Lin [30] also reported that possible reason for increase in hydrogen yield with shortened HRT is contributions of HRT-selected microbial population. The solid retention time (SRT) was calculated from the suspended solid content of reactor and the suspended solid lost in the effluent at the period of steady state at each HRT: MLVSS concentration in reactor HRT MLVSS concentration of effluent
SRT ¼
(3)
The calculated SRTs were approximately 19 days for HRT 96 h, 11 days for HRT 72 h and 5.5 days at HRT 48 h. In this study hydrogen production did not show consistent relationship to SRT and HRT. The longer HRT yielded lower solid removal and showed the longest SRT which is supposed to yield maximum hydrogen production in this system, however; it did not happen which could be attributed to prevalence of H2-consuming or non-hydrogen producing bacteria [31]. Appropriate operating condition at shorter HRT of 72 h with SRT of 11 days supported the retention of active hydrogen producing bacteria and restored the stability in system. Thereafter, at lower HRTs, SRT decreased which may have been related to biomass washout in the system.
3.3.
Variation of VFA and solvent concentration
Results of soluble metabolite composition and concentration at various HRTs, presented in Table 4 can describe the performance of reactor. VFA is known as intermediate and indicator in the anaerobic process [32]. In this study, lower efficiency in total soluble metabolite production was observed, although high carbohydrate was consumed by the mixed anaerobic bacteria. In all ranges of HRT, butyrate followed by acetate were the main acids throughout the fermentation in ASBR along with much lower amounts of propionic acid and iso-butyric acid with small amounts of heptanoic acid. It should be noted that solvent concentrations, which is unfavorable metabolite for hydrogen generation, was lower in comparison with acid concentrations throughout the tested HRTs range and butanol was the predominant species among solvents. The influent POME might be the main source of butanol which already contained 700 mg/L of this solvent, suggesting that high butanol concentration in the feed affected the one in the effluent. VFA/SMP ratio was essentially higher than 0.63 in this study. The abundance of VFAs in total SMP (VFA/SMP) suggests that H2 production was metabolically favorable since acidogenic pathway was predominant over solventogenesis [33,34]. Hydrogen production in acidogenesis comes mainly from oxidative decarboxylation of pyruvic acid to acetyl CoA,and is also linked to converting acetyl CoA to butyrate and ethanol, together with production of acetate [35]. Moreover, HBu/VFA ratio was higher than 0.7 in this study, and fraction of butyric acid in soluble metabolites increased by decreasing HRT. Simultaneous decrease in the acetate concentration and an increase in H2 production observed which also reported in previous studies in which mixed culture were used [3]. Butyrate and acetate have been reported as abundant liquid products in anaerobic hydrogen production from mixed culture [36,37], although different intermediates were produced in mixed fermentation culture. This is likely due to different types of fermentation pathway used by the mixed anaerobic microorganisms [38]. Propionic acid level was low and did not vary markedly in HRTs ranges of 96e36 h. Highest hydrogen yield at 72 h was found accompanied with high butyrate concentration and low propionate concentration. Cohen et al. [39] showed a linear inverse relationship between propionate and butyrate formation and a linear positive correlation between butyrate and H2 production. Relatively consistent but low percentage of ethanol was also obtained at the steady-state conditions.
Table 4 e Soluble metabolites obtained at steady-state conditions at different HRTs. HRT, h
Soluble metabolites a
96 72 48 36
a
a
a
a
Et , %
Bu , %
HAc , %
HPr , %
Iso-Bu , %
HBua, %
HHpa, %
SMP, mg/l
VFA, mg/l
VFA/SMP
HBu/VFA
2.9 0.12 0.49 0.77
38.0 14.0 17.9 20
13.8 12.45 9.49 29
0.16 0.34 0.22 0.96
1.24 0.75 1.13 0.96
43.2 72.0 70.5 89.3
0.66 0.15 0.24 0.30
2876 0.8 2653 0.6b 3889 0.45 1555 1
1698 0.2 2277 0.33 3172 0.4 1826 0.4
0.63 0.0001 0.87 0.001 0.82 0.07 1.25 0.04
0.73 0.007 0.96 0.002 0.97 0.002 0.7 0.007
a Et ¼ ethanol; Bu ¼ butanol; HAc ¼ acetic acid; HPr ¼ propionic acid; Iso-Bu ¼ iso-butyric acid; HBu ¼ butyric acid; and HHp ¼ heptanoic acid. b Mean value standard deviation.
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Distribution of soluble microbial products indicated the butyrateeacetate fermentation type in the ASBR, which is characterized by the production of butyrate and acetate, plus carbon dioxide and hydrogen in acidogenic fermentation pathway [40]. In this investigation, fermentation metabolites were generated regardless of variation of HRT which confirms little impact of HRT on these by-products, while previous studies claimed that HRT was an important environmental factor to affect the metabolic pathway shifts [41]. In our future works, further investigation is needed to elucidate the microbial mechanism of soluble metabolite production in the mesophilic mixed cultures study.
4.
Conclusions
In this study we investigated higher efficiency for hydrogen production in ASBR through treating organic wastewater by changing HRT and different organic loading condition. The study found that to achieve both satisfactory production rate and yield efficiency, stable performance must be established in ASBR which is possible by using stable microbial community and modifying the operational parameters. Experimental results revealed that HRT affects on many aspects of hydrogen production process. According to the results from this study, HRT of 72 h could be considered as the optimum HRT for treating diluted POME in ASBR system with respect to higher production rate and yield efficiency, while the longer HRT would provoke development of non-hydrogen producing bacteria and shorter HRTs lowers H2 yield. In addition, Low VFA concentration confirms the stability of operation at HRT 72 h. Based on the calculated SRT values, SRT showed decreasing trend as HRT decreased progressively and system did not maintain the solids. This happened due to the increase of suspended solids in the effluent because of decrease in HRT and leading to washout of active hydrogen producing bacteria from reactor. On overall, continuous shortening the HRT deteriorated the productivity by washing out the active bacteria from the system. This work has illustrated the feasibility of using POME as an inexpensive feedstock for production of hydrogen in ASBR, meanwhile, provides valuable information to design more effective system for palm oil mill waste treatment.
Acknowledgement The authors wish to thank the financial support from Universiti Kebangsaan Malaysia and the Ministry of Science, Technology and Innovation (MOSTI) Malaysia, under UKMAP-PI-13-2010 and the Science-fund Research program of UKM-MGI-NBD16-2007, for funding of this project.
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