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Anaerobic acidification of sugar-beet processing wastes: Effect of operational parameters
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 2 e3 9
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Anaerobic acidification of sugar-beet processing wastes: Effect of operational parameters Emrah Alkaya, Go¨ksel N. Demirer* Department of Environmental Engineering, Middle East Technical University, Inonu Bulvari, 06531 Ankara, Turkey
article info
abstract
Article history:
The objective of this study was to maximize the hydrolysis and acidification of sugar-beet
Received 20 January 2010
processing wastewater and beet pulp for volatile fatty acid (VFA) production through
Received in revised form
acidogenic anaerobic metabolism. Experiments were conducted to determine the optimum
2 August 2010
operational conditions (HRT, waste-mixing ratio and pH) for effective acidification in daily-
Accepted 2 August 2010
fed, continuously mixed anaerobic reactors. For this purpose, reactors were operated at
Available online 1 September 2010
35 1 C with different combinations of HRT (2e4 days), wastewater-pulp mixing ratios (1:0e1:1, in terms of COD) and pH ranges (5.7e7.5). Increased OLRs, resulting from pulp
Keywords:
addition, increased the amount of acidification products (VFAs) which led to relatively low
Anaerobic digestion
operational pH values (5.7e6.8). In this pH range, methanogenic activity was successfully
Sugar industry
inhibited and the lowest methane percentages (5.6e16.3%) were observed in the produced
Acidification
biogas. The optimum operational conditions were determined to be 2-day HRT and 1:1
Volatile fatty acids
waste mixing ratio (in terms of COD) without external alkalinity addition. These operational conditions led to the highest tVFA concentration (3635 209 mg/L as H-Ac) with the acidification degree of 46.9 2.1%. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Anaerobic digestion is an established technology, used for the treatment of wide variety of organic wastes throughout the decades. It is one of the several biological processing strategies which produce bioenergy and/or biochemicals while treating industrial and agricultural wastes [1]. Anaerobic biodegradation can be separated into two phases in order to enhance treatment efficiencies and/or produce bio-products. Two-phase anaerobic systems have been extensively studied and numerous advantages of phase separation over conventional anaerobic digestion have been described/ demonstrated in numerous studies [2e8]. Some of these advantages include, increased process stability and control, smaller reactor volumes and high tolerance to toxicity and shock loads. These advantages enable the two-phase anaerobic
systems to treat many kinds of solid, industrial and agroindustrial wastes such as distillery, landfill leachate, coffee, cheese whey, dairy, starch, fruit, vegetable solid, food, pulp and paper, olive mill, abattoir, dye wastewaters, primary and activated sludge [9]. Along with its applications as the first step of a phaseseparated anaerobic waste treatment configuration, anaerobic acidification can be exploited separately for bio-product formation. For example, Parawira et al. [10] stated that anaerobic acidification could be useful for the production of organic acids (e.g. VFA) which have variety of industrial uses. VFAs are utilized for the manufacture of various organic compounds and some plastics [11]. Moreover, these organic acids play an instrumental role as a carbon source in the removal of nutrients from wastewaters [12]. Recently, acidphase anaerobic digestion received considerable attention
* Corresponding author. Tel.: þ90 312 210 58 67; fax: þ90 312 210 26 46. E-mail address: [email protected] (G.N. Demirer). 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.08.002
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Notations COD H-Ac H-Bu H-Pr HRT OLR ORP
chemical oxygen demand acetic acid butyric acid propionic acid hydraulic retention time organic loading rate oxidation-reduction potential
and relevant studies covered a wide range of operational parameters including; pH [13,14] waste type [15], HRT [2,6,12], temperature [12,16], mixing of wastes [3]and OLR [17]. Today’s excessive resource use and devastating change in global environment led to the increased pressure on polluting industries. Sugar-beet processing industry which falls into this category, devotes increasing attention to environmental problems associated with high energy consumption and production of large amounts of wastes. Recently, sugar plants take measures to reduce energy consumption, recycle materials and energy and optimize the operation of sugar process in order to achieve waste minimization and sustainable production [18]. As a viable option for waste management in sugar-beet processing facilities, anaerobic digestion was broadly evaluated. Most of the studies focused on the anaerobic treatment and biogasification of wastewater and beet-pulp which are two major waste streams of beet processing plants. Wastewater from the beet-sugar mill contains high concentrations of hydrocarbons which makes it suitable for anaerobic biological degradation [19]. Consequently, there have been several studies on anaerobic digestion of beet processing wastewater [19e22]. Contrary to wastewater, beet-pulp is generally considered as a by-product and utilized for further use instead of a waste to be treated and disposed. It is mostly used as animal feed in cattle-raising industry [23]. For this purpose, beet-pulp may be used directly or could be dried alone or in combination with molasses [18]. Koppar et al. [24] advocated that the cost of the beet-pulp drying is increasing due to the increase in fuel prices and more profitable applications need to be developed. In some studies, anaerobic digestion is proven to be a more feasible alternative for the utilization of beet-pulp [24e26]. Moreover, studies which cover two-phase anaerobic digestion of beet-pulp were conducted and merits of phase separation were indicated for biogasification and methane production [23,27,28]. Most of the studies regarding the anaerobic digestion of sugar-beet processing wastes targeted to the effective biogasification and waste stabilization of each individual waste. However, acid-phase anaerobic co-digestion of these wastes together can be used as a practical and economically feasible waste management option together with the effective recovery methods of the produced valuable organic acids. Therefore, the objective of the study was set as maximizing the hydrolysis and acidification of sugar-beet processing wastewater and beet-pulp simultaneously in an acidogenic anaerobic reactor for VFA production. The effect of HRT, waste-mixing ratio and pH on acidification was investigated to determine optimum operational conditions at mesophilic
TS TSS TVS TKN VSS VFA
total solids total suspended solids total volatile solids total Kjeldahl nitrogen volatile suspended solids volatile fatty acids
Subscripts t, s total, soluble
temperatures. The performance of the process was mainly evaluated in terms of VFA accumulation and the degree of acidification.
2.
Materials and methods
2.1.
Waste characteristics
Sugar industry wastes (wastewater and pressed beet-pulp) were obtained from a private beet-sugar factory located near Amasya, Turkey during the beet-campaign period. Characterizations of the wastes were carried out and the results were tabulated in Tables 1 and 2. After characterization, wastes were kept frozen at 20 C in order to prevent biological activity prior to the use in the experimental studies. Before the characterization and the use in the study, wastewater was settled for 1-h to remove the inorganic suspended materials which are very common for sugar-beet processing wastewater. The time period of 1-h was chosen to represent the typical hydraulic retention time of primary sedimentation before secondary treatment systems. In order to achieve physical homogeneity, beet-pulp was processed as follows: First the frozen beet-pulp was thawed at room temperature and further dried at 105 C for 24 h. Then, the dried pulp particles were grinded by the help of a pestle. So, the homogenized powdered pulp was used for reactor feeding.
2.2.
Inoculum
The mixed anaerobic cultures that were used as the inoculum (seed) source were obtained from the anaerobic sludge
Table 1 e Wastewater characteristics (1 h settled). Parameters tCOD sCOD TS TSS VSS pH Alkalinity TKN PTotal tVFA H-Ac H-Pr H-Bu
Values (mg/L) 6621 113.2 6165 517.1 6062 53.0 665 21.2 335 7.1 6.82 1760 (as CaCO3) 10 2.7 1115 20 (as H-Ac) 394 5 610 12 46 1
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Table 2 e Pressed beet-pulp characteristics. Parameter
Value (mg/L)
Moisture (%) TS (%) TVS (% TS) COD (g/g dry weight) TKN (% TS) PTotal (% TS)
85 15 94 1.22 7.28 1.0
0.1 0.1 0.01 0.15 0.28
digesters of the municipal wastewater treatment plant of Ankara. Prior to the use as inoculum, it was concentrated by gravity settling for 24 h. By this way, the VSS concentration was increased up to 41,212 2324 mg/L and then used in the experiments.
2.3.
Basal medium
In order to supply adequate micronutrients for an optimum microbial growth, reactors were daily fed by basal medium (BM) which contains the following constituents (concentrations are given in parentheses as mg/L): NH4Cl (1200), MgSO4 7H2O (400), KCl (400), Na2S 9H2O (300), CaCl2 2H2O (50), (NH4)2 HPO4 (80), FeCl2 4H2O (40), CoCl2 6H2O (10), KI (10), MnCl2 4H2O (0.5), CuCl2 2H2O (0.5), ZnCl2 (0.5), AlCl3 6H2O (0.5), NaMoO4 2H2O (0.5), H3BO3 (0.5), NiCl2 6H2O (0.5), NaWO4 2H2O (0.5), Cysteine (10) [29].
2.4.
Analytical methods
Solids, tCOD (for beet-pulp), TKN, PTotal and alkalinity determinations were carried out as described in Standard Methods [30] pH values were measured with a pH meter (HI 8314, Hanna Instruments) and a pH probe (HI 1230, Hanna Instruments). tCOD (for wastewater) and sCOD (0.45 mm pore-sized filters were used) determinations were performed by using a spectrophotometer (SN 05827, PC Multidirect) and vials specifically prepared for the determination range of 0e1500 mg/L. ORP values of the reactors were measured with a pH meter (PH 510, Eutech) equipped with an ORP electrode (Recorder S-500 C, Sensorex). Daily biogas productions were measured with a water replacement device and the compositions were periodically determined with a gas chromatograph (Thermo Electron Co.) equipped with a thermal conductivity detector (TCD). Produced biogases were separated as hydrogen (H2), carbon dioxide (CO2), oxygen (O2), methane (CH4) and nitrogen (N2) by using serially connected columns (CP-Moliseve 5A and CPPorabond Q) at a fixed oven temperature of 45 C. Helium was used as carrier gas at 100 kPa constant pressure. The inlet and detector temperatures were set to 50 C and 80 C respectively. Prior to the gas chromatography injections, series of pretreatments were conducted for VFA measurements: First, samples were filtered through 0.22 mm pore-sized filters. Then the samples were diluted with deionized water to assure the VFA concentration of the sample to be in the range of pure VFA calibration of gas chromatograph. After filtering and dilution, the samples were acidified with 98% formic acid to a pH less than 2.5, in order to convert the fatty acids to their undissociated forms (i.e. acid forms).
The gas chromatograph (Thermo Electron Co.), used for biogas composition determinations, was also used for the periodical VFA measurements. However the column, the detector and the operational conditions were different: Nukol column (Model 25,326, 15 m 0.53 mm) was used to separate VFAs (acetic, propionic, butyric, iso-butyric, valeric, iso-valeric, caproic, iso-caproic and heptanoic acids). Flame ionization detector (FID) was used for this purpose which was adjusted to 280 C as operating temperature. Helium was used as carrier gas with a constant flow rate of 6 mL/min and the inlet temperature was kept at 250 C. Oven temperature was initially set to 100 C with 2 min holding time and then increased to 200 C with 8 C/min ramping. One of the main interests of this study was to compare the proportions of the substrates converted to VFAs. For this purpose, “acidification degree” was calculated for each of the reactor in order to express the acidification efficiency (Fig. 3). So, the organic content of initial substrate and VFAs were based on COD concentrations. By this way, it was possible to present the related conversion efficiencies for all operated reactors as: Degree of acidification ð%Þ ¼ Sf =Si 100
(1)
Where; Si: Initial substrate concentration, measured in COD as mg/L, *Sf: Produced net VFAs (finaleinitial), expressed as theoretical equivalents of COD concentrations as mg/L. *The COD equivalents of each VFA: Acetic acid, 1.066; Propionic acid, 1.512; Butyric acid, 1.816; Valeric, 2.036; Caproic acid, 2.204. [7]. When calculating acidification degrees obtained in the reactors, initial tVFA concentration (1115 20 mg/L as H-Ac) of the wastewater was taken into consideration. In other words, reactors having wastewater as the feed had already VFAs mainly in the form of acetic, butyric and propionic acids. In order to determine actual conversion of COD into VFA, initial tVFA concentration was subtracted from the final tVFA values giving net tVFA production.
2.5.
Experimental set-up and procedures
Six reactors were operated with different combinations of the operational parameters (HRT, waste mixing ratio and pH) (Table 3). Reactors were run as duplicates to test statistical reliability and the presentation of the data (tables and figures) involves the averaged values of duplicate reactors. 250 mL reactors with effective volume of 150 mL were operated by daily fed-batch feeding strategy as continuously mixed acidogenic reactors. Reactors were kept continuously mixing
Table 3 e Operational parameters of the reactors. Reactor R1 R2 R3 R4 R5 R6
Feed Stock Stock Stock Stock Stock Stock
#1 #2 #3 #1 #2 #3
HRT (days)
Waste mixing ratio (WW:P)
OLR (g COD/L-day)
2 2 2 4 4 4
1:0 1:0.5 1:1 1:0 1:0.5 1:1
2.7 4 5.4 1.35 2 2.7
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 2 e3 9
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at 175 rpm by using a mechanical shaker in a temperature controlled room (35 1 C) for 41 days. In order to prevent uncontrolled pH drops and to investigate the effect of higher operational pH (6.9e7.5) on the acidification, first 20 days of operation was carried out by adding external alkalinity in the form of NaHCO3. In this period, reactors were supplied with NaHCO3 to set daily initial concentrations to 6000 mg/L by daily additions along with BM. Then, the reactors were operated 21 more days without adding any external alkalinity to observe the natural pH drop, as a result of acidification, and to investigate the effect of lower operational pH (5.7e7.4) on acidification. For the purpose of reactor feeding, three different stock solutions were prepared (Table 4). In order to investigate the effect of waste mixing ratio on acidification, prepared stock solutions involved same concentrations of BM but different mixing ratio of wastewater (WW) and pulp (P) in terms of tCOD. As the control parameters of reactor operations, pH, ORP, biogas production, biogas composition, VFA and sCOD concentrations were measured. Among these parameters, pH, ORP and biogas production were daily, VFA, sCOD, and biogas compositions were periodically measured.
3.
Results and discussions
3.1.
pH profiles of the reactors
pH drops were observed with varying rates and extents for all reactors (Fig. 1a). Organic acid productions were initiated from the onset of the reactor operations which resulted in these expected pH drops (Fig. 2). For the same substrate (stock solution), 2 days of HRT led to lower pH values than 4 days. The noticeable differences between the operational pHs were mainly due to the differences of the methanogenic activities in the reactors. Since, growth rates of methanogenic microorganisms are lower than the other members of anaerobic consortia, they require more time to survive in anaerobic systems [31]. So, in this study, 4 days of HRT served as a better growing condition for methanogens than 2 days. As a result, methanogenic microorganisms led to VFA consumption and corresponding buffering effect that prevented further pH drops at 4-day HRT (R4, R5, R6). This situation was the reason of higher pH values, observed in these reactors. Indeed, VFA concentration profiles (Fig. 2) and biogas compositions (Table 5) of the reactors supported this speculation. After external alkalinity addition was ceased, pH values were further decreased and stabilized at lower levels
Table 4 e Compositions of stock solutions. Composition
Stock #1
Stock #2
Stock #3
Wastewater (mg/L as tCOD) Pulp (mg/L as tCOD) tCOD (mg/L) sCOD (mg/L) pH
5300 e 5300 4932 72 8.02
5300 2650 7950 5211 421 7.99
5300 5300 10,600 5318 288 8.08
Fig. 1 e Temporal changes of reactor conditions: (a) pH; (b) chemical oxygen demand (soluble); (c) oxidatione reduction potential; (d) daily gas production.
(Fig. 1a). The observed pH decreases were steeper (down to 5.7e7.1) at 2-day HRT since the VFA accumulations (Fig. 2) were too high for the inherent alkalinity of wastewater to buffer. In this period, steady-state pH values were higher at 4-day HRT (6.6e7.4). Additionally, pH patterns of the reactors differed from each other when they are compared in terms of waste mixing ratios. For the same HRT, increased OLRs, resulting from pulp addition, increased the amount of acidification products (VFAs) which lead to low operational pHs (5.7e6.8). It was reported that high OLRs result in higher VFA accumulations and related pH decreases [32]. In this study, the reactors R2 and R3 which received pulp as well as wastewater were operated in the pH range of 5.6e6.2 without external alkalinity addition at HRT of 2 days. Since the pH range of 4e6.5 is accepted as optimum for the growth of anaerobic acidogenic microorganisms [31], proper pH conditions were established in R2 and R3.
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80 External Alkalinity No External Alkalinity
Acidification Degree (%)
70 60 50 40 30 20 10 0 R1
R2
R3
R4
R5
R6
Reactor Fig. 3 e Observed acidification degrees in the reactors.
Fig. 2 e VFA concentration profiles of the reactors: (a) volatile fatty acids (total); (b) acetic acid; (c) propionic acid; (d) butyric acid.
3.2.
Volatile fatty acid productions
Fig. 2 illustrates the inverse relationship between VFA accumulation and operational HRT of the reactors. 2 days of HRT yielded higher tVFA concentrations (2159e3635 mg/L as H-Ac), which is the indication of higher acidogenic activity and/or lower methanogenic activity. Since the acidogenic microorganisms grow much faster than the methanogens, they could successively survive in HRTs as low as 2 days. This also indicates that at 4-day HRT (R4, R5, R6) methanogens could be retained long enough to metabolize VFAs for methane production, causing to lower VFA concentrations (1814e2640 mg/L as H-Ac). The composition of the produced biogas which indicates higher methane percentages at 4-day HRT (Table 5) was a strong evidence of this claim. Lowest VFA concentrations (1814e2244 mg/L as H-Ac) were recorded in the reactors, which were fed only by wastewater (R1 and R4). As expected, VFA concentrations were proportionally
increased with the increase in amount of pulp added and the highest concentrations were observed in 2-day-HRT reactor (R3) which was fed by wastewater and pulp in equal amounts in terms of COD (1:1). These results indicated that pulp was successfully acidified together with wastewater, which led to the increase of the VFA concentrations. After the cessation of alkalinity addition, VFA concentrations were not changed noticeably. This observation indicated that, the change in narrow operating pH values (Fig. 1a) did not result in a major change in the acidification trend of sugar industry wastes, even in R3 which indicated the steepest decrease from 6.9 to 5.7. Dinopoulou et al. [33] stated that acidification degree can be determined by calculating the proportion of the initial substrate which is converted to VFAs as end products. In this study, the highest acidification degrees (60.3e64.2%) were observed in R1, which was only fed by wastewater and operated at 2 days of HRT (Fig. 3). The increase in pulp addition caused a decrease in acidification degree which was accepted as the direct result of increasing OLR. Similarly, in a complex wastewater acidification study, the degree of acidification was found to diminish with increasing OLR [33]. Still, the reactors which were fed by wastewater and pulp with HRT of 2 days (R2, R3) denoted substantial degrees of acidification (44.1e53.5%) when compared with other studies: 15e60% for complex wastewater [33]; 56% for dairy wastewater [34]; 10.3e43.4% forfish meal processing wastewater [35]. In all of the reactors, the main acidification products were H-Ac (40.3e49.2% w/w of tVFA), H-Pr (36.3e42.6% w/w of tVFA) and H-Bu (3.6e7.5% w/w of tVFA) comprising 88.3e96.2% of tVFAs. The higher molecular weight VFAs (valeric, caproic, heptanoic etc.) were produced in insignificant amounts. On the other hand, alcohols (e.g. ethanol, methanol), other major metabolites of anaerobic acidification, were not detected at all. It is a well-known fact that substrate characteristics and operational conditions play a major role on product distribution in an acidification reactor [13,14,33]. In this study, the dominance of H-Ac, H-pr and H-Bu can be associated with the carbohydrate degradation, since both wastewater [19] and pulp [23] contains high amounts of sugars. This is relevant with the literature in which the short-chain fatty acids were
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Table 5 e Steady-state biogas compositions of the reactors. Reactor
R1 R2 R3 R4 R5 R6
External alkalinity
No external alkalinity
CH4 (%)
CO2 (%)
CH4 (%)
CO2 (%)
25.3 1.7 16.3 2.1 6.4 1.5 53.4 0.4 51.6 3.3 44.5 2.7
74.7 83.7 93.7 46.6 48.4 55.5
5.9 3.3 0.5 2.1 2.6 2.2
76.8 5.9 83.3 3.3 94.4 0.5 50.2 2.1 54.9 2.6 60.5 2.2
1.7 2.1 1.5 0.4 3.3 2.7
23.2 16.7 5.6 49.8 45.1 39.5
found to be dominant in acidogenic reactors [14,33]. Parawira et al. [10], stated that higher molecular weight VFAs are generally found in protein fermentation.
3.3.
Soluble chemical oxygen demand concentrations
sCOD concentration distributions of the reactors were depicted in Fig. 1b. The highest sCOD concentrations (5422e5826 mg/L) were observed in R3 which was operated at an HRT of 2 days. 4-day HRT led lo lower sCOD concentrations resulting from the consumption of VFAs which contribute to sCOD. It can be seen from Fig. 1 that, there is a direct relationship between VFA and sCOD concentrations. Since, hydrolysis of particulate organic matter occurs simultaneously during the acidification of soluble organics, sCOD concentrations were higher (3490e5826 mg/L) in the reactors which have pulp addition together with wastewater. The effect of pH values on sCOD concentrations was negligible when the alkalinity-added period was compared with the period without alkalinity addition. This result indicated that the trend of hydrolysis was not significantly altered with the decreases in operational pH values.
3.4.
Oxidation-reduction potentials
ORP values of anaerobic reactors were inspected to detect the relative amounts of oxidized materials such as nitrate ions 2 (NO 3 ) and sulfate ions (SO4 ), and reduced materials, such as þ ammonium ions (NH4 ). At ORP values between 100 mV and 300 mV, degradation of organic compounds proceeds primarily as fermentation and acid formation. On the other hand, ORP values lower than 300 mV indicates considerable methanogenic activity in anaerobic conditions [36]. Fig. 1c depicts that, ORPs of reactors were diminished to steady-state values after the operation of 3e5 days. ORP values of the reactors were within 226 mV and 350 mV in the period of alkalinity addition. After alkalinity addition was stopped, ORP values started to increase and attained steady-state between 162 mV and 300 mV. This observation indicates that, the pH drops resulting from the cessation of alkalinity addition slightly altered the circumstances in favor of fermentation and mixedacid formation, inhibiting the methanogenic activities. This idea was supported by the results of biogas compositions (Table 5) which indicate that methane percentages were slightly decreased along with decreasing pH values. Moreover, 2-day HRT yielded higher ORP values (183 mV to 234 mV) than 4-day HRT especially in the period without alkalinity addition. In 4-day-HRT reactors, the observed ORP
37
values were between 273 mV and 318 mV which indicated higher methanogenic activities than 2-day-HRT reactors.
3.5.
Biogas productions and compositions
The direct relationship between the operational HRT and the amount of daily produced biogas can be observed in Fig. 1d. Reactors, which were operated with an HRT of 4 days (R4, R5, R6) produced higher amounts of biogas (27e72 mL/day) compared to those operated with an HRT of 2 days (19e25 mL/ day). When the biogas productions are interpreted in combination with their composition (Table 5), it can be postulated that, higher methanogenic activity was responsible for the higher biogas productions in 4-day-HRT reactors. Since, the daily biogas productions were primarily affected by methane productions, conditions, influencing methanogenic activity, also influenced the total biogas productions. So, this explains why the lowest biogas productions (19e21 mL/day) were observed in R2 and R3, in which pH values were low enough (5.7e6.2) to inhibit methane productions to some extent. Thus, pulp addition increased the substrate concentrations and highest biogas productions (31e72 mL/day) were recorded at R5 and R6 which were operated with 4 days of HRT. In acidogenic anaerobic reactors, methanogenic activity must be restrained to be able to reach high VFA accumulations. Methanogenic activities and related methane productions could be suppressed to some extent with 2 days of HRT (R1, R2, R3). As a result, lower methane percentages (5.6e25.3%) were detected within the produced biogases of these reactors when compared with the percentages appeared in the 4-day-HRT reactors (39.5e53.4%). Furthermore, Table 5 indicates the inverse relationship between substrate concentrations and methane productions. It can be observed that, increased substrate concentrations resulting from pulp additions caused significant decreases in methane productions; especially at 2-day HRT. The main reason was the increased VFA accumulation in corresponding reactors, resulting in the lower operational pHs (5.7e6.9). So, it was evident that, pulp addition, causing to VFA accumulation, suppressed the methane production. When the biogas compositions of 4-day-HRT reactors were inspected, it was claimed that neither HRTs nor the operational pH values (6.6e7.5) were acceptable for the targeted inhibition of the methanogenic activity. In fact, Yu and Fang [14] stated that the pH values of acidogenic reactors must be kept below 5.5 to inhibit methanogenic microorganisms after observing high methane concentrations (31%) even at pH values as low as 6.5. After cessation of alkalinity addition, methane percentages of the reactors were slightly decreased following the decreases in pH values. The lowest methane percentage (5.6%) was observed at R3 in the period without alkalinity addition. This result indicates that, the optimum conditions for inhibition of methanogenic activity were 2 days of HRT and 5.7e6.2 range of pH in this study.
4.
Conclusions
From the obtained results, the following conclusions can be drawn:
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Sugar industry wastewater and beet-pulp can simultaneously be converted to VFAs in acidogenic anaerobic reactors with considerable acidification degrees (43.8e52.9%). In this study, wastewater was used for dilution of beet-pulp instead of fresh water, which serves for resource conservation. By this way, external alkalinity addition, a common application, could also be avoided due to the inherent alkalinity of the wastewater. Increased OLRs, resulting from pulp addition, increased the amount of acidification products (VFAs) which lead to relatively low operational pH values (5.7e6.8). In this pH range, methanogenic activity was inhibited and lowest methane percentages (5.6e16.3%) were observed in biogas compositions. The optimum operational conditions were selected as 2-day HRT and 1:1 waste mixing ratio (in terms of COD) without external alkalinity addition. These operational conditions led to the highest tVFA concentrations (3635 209 mg/L as H-Ac) with an acidification degree of 46.9% at the highest OLR of 5.4 g COD/L-d.
Acknowledgments This study was funded by The Scientific and Technological Research Council of Turkey through Grant Number 104I127.
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[10] Parawira W, Murto M, Read JS, Mattiasson B. Volatile fatty acid production during anaerobic mesophilic digestion of solid potato waste. J Chem Technol Biotechnol 2004;79:673e7. [11] Compere AL, Griffith WL. Fermentation of waste materials to produce industrial intermediates. DevInd Microbiol 1975;17: 247e52. [12] Maharaj I, Elefsiniotis P. The role of HRT and low temperature on the acid-phase anaerobic digestion of municipal and industrial wastewaters. Bioresour Technol 2001;76:191e7. [13] Hwang JH, Choi JA, Abou-Shanab RAI, Bhatnagar A, Min B, Song H. Effect of pH and sulfate concentration on hydrogen production using anaerobic mixed microflora. Int J Hydrogen Energ 2009;34(24):9702e10. [14] Yu HQ, Fang HHP. Acidogenesis of dairy wastewater at various pH levels. Water Sci Technol 2002;45(10):201e6. [15] Dareioti MA, Dokianakis SN, Stamatelatou K, Zafiri C, Kornaros M. Biogas production from anaerobic co-digestion of agroindustrial wastewaters under mesophilic conditions in a two-stage process. Desalination 2009;248(1e3):891e906. [16] Komemoto K, Lim YG, Nagao N, Onoue Y, Niwa C, Toda T. Effect of temperature on VFA’s and biogas production in anaerobic solubilization of food waste. Waste Manage 2009; 29(12):2950e5. [17] Ding HB, Wang JY. Responses of the methanogenic reactor to different effluent fractions of fermentative hydrogen production in a phase-separated anaerobic digestion system. Int J Hydrogen Energ 2008;33:6993e7005. [18] Krajnc D, Mele M, Glavic P. Improving the economic and environmental performances of the beet sugar industry in Slovenia: increasing fuel efficiency and using by-products for ethanol. J Clean Prod 2007;15:1240e52. [19] Wang BZ, Sui J, Liu RF, Yang G, Qi PS. Anaerobic reactors treating beet sugar effluents. Effluent Water Treat 1986;26(5): 150e62. [20] Shore M, Broughton NW, Bumstead N. Anaerobic treatment of waste waters in the beet sugar industry. Water Pollut Control 1984;83(4):499e506. [21] Iza J, Palencia JI, Fdz-Polanco F. Waste water management in a sugar beet factory: a case study of comparison between anaerobic technologies. Water Sci Technol 1990;22(9): 123e30. [22] Farhadian M, Borghei M, Umrania VV. Treatment of beet sugar wastewater by UAFB bioprocess. Bioresour Technol 2007;98:3080e3. [23] Hutnan M, Drtil M, Mrafkova L. Anaerobic biodegradation of sugar beet pulp. Biodegradation 2000;11:203e11. [24] Koppar A, Pullammanappallil P. Single-stage, batch, leachbed, thermophilic anaerobic digestion of spent sugar beet pulp. Bioresour Technol 2008;99:2831e9. [25] Lane AG. Laboratory scale anaerobic digestion of fruit and vegetable solid waste. Biomass 1984;5:245e59. [26] Weiland P. One- and two-step anaerobic digestion of solid agroindustrial residues. Water Sci Technol 1993;27:145e51. [27] Stoppok E, Buchholz K. Continuous anaerobic conversion of sugar beet pulp to biogas. Biotechnol Lett 1985;7(2):119e24. [28] Hutnan M, Drtil M, Derco J, Mrafkova L, Hornak M, Mico S. Two-step pilot-scale anaerobic treatment of sugar beet pulp. Polish J Environ Stud 2001;10(4):237e43. [29] Gungor-Demirci G, Demirer GN. Effect of initial COD concentration, nutrient addition, temperature and microbial acclimation on anaerobic treatability of broiler and cattle manure. Bioresour Technol 2004;93:109e17. [30] APHA. Standard methods for the examination of water and wastewater. 21st ed. Washington, DC, USA: American Public Health Association; 2005. [31] Speece RE. Anaerobic biotechnology for industrial wastewaters. Nashville, USA: Arachae Press; 1996.
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[32] Ghosh S. Improved sludge gasification by two-phase anaerobic digestion. J Environ Eng 1987;113:1265e84. [33] Dinopoulou G, Rudd T, Lester JN. Anaerobic acidogenesis of a complex wastewater: 1. the influence of operational parameters on reactor performance. Biotechnol Bioeng 1988; 31:958e68. [34] Demirel B, Yenigun O. Anaerobic acidogenesis of dairy wastewater: the effects of variations in hydraulic retention
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time with no pH control. J Chem Technol Biotechnol 2004;79: 755e60. [35] Guerrero L, Omil F, Mendez R, Lema JM. Anaerobic hydrolysis and acidogenesis of wastewaters from food industries with high content of organic solids and protein. Water Res 1999;33 (10):3281e90. [36] Gerardi MH. The microbiology of anaerobic digesters. Hoboken, New Jersey: John Wiley & Sons, Inc.; 2003.
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Anaerobic acidification of sugar-beet processing wastes: Effect of operational parameters Emrah Alkaya, Go¨ksel N. Demirer* Department of Environmental Engineering, Middle East Technical University, Inonu Bulvari, 06531 Ankara, Turkey
article info
abstract
Article history:
The objective of this study was to maximize the hydrolysis and acidification of sugar-beet
Received 20 January 2010
processing wastewater and beet pulp for volatile fatty acid (VFA) production through
Received in revised form
acidogenic anaerobic metabolism. Experiments were conducted to determine the optimum
2 August 2010
operational conditions (HRT, waste-mixing ratio and pH) for effective acidification in daily-
Accepted 2 August 2010
fed, continuously mixed anaerobic reactors. For this purpose, reactors were operated at
Available online 1 September 2010
35 1 C with different combinations of HRT (2e4 days), wastewater-pulp mixing ratios (1:0e1:1, in terms of COD) and pH ranges (5.7e7.5). Increased OLRs, resulting from pulp
Keywords:
addition, increased the amount of acidification products (VFAs) which led to relatively low
Anaerobic digestion
operational pH values (5.7e6.8). In this pH range, methanogenic activity was successfully
Sugar industry
inhibited and the lowest methane percentages (5.6e16.3%) were observed in the produced
Acidification
biogas. The optimum operational conditions were determined to be 2-day HRT and 1:1
Volatile fatty acids
waste mixing ratio (in terms of COD) without external alkalinity addition. These operational conditions led to the highest tVFA concentration (3635 209 mg/L as H-Ac) with the acidification degree of 46.9 2.1%. ª 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Anaerobic digestion is an established technology, used for the treatment of wide variety of organic wastes throughout the decades. It is one of the several biological processing strategies which produce bioenergy and/or biochemicals while treating industrial and agricultural wastes [1]. Anaerobic biodegradation can be separated into two phases in order to enhance treatment efficiencies and/or produce bio-products. Two-phase anaerobic systems have been extensively studied and numerous advantages of phase separation over conventional anaerobic digestion have been described/ demonstrated in numerous studies [2e8]. Some of these advantages include, increased process stability and control, smaller reactor volumes and high tolerance to toxicity and shock loads. These advantages enable the two-phase anaerobic
systems to treat many kinds of solid, industrial and agroindustrial wastes such as distillery, landfill leachate, coffee, cheese whey, dairy, starch, fruit, vegetable solid, food, pulp and paper, olive mill, abattoir, dye wastewaters, primary and activated sludge [9]. Along with its applications as the first step of a phaseseparated anaerobic waste treatment configuration, anaerobic acidification can be exploited separately for bio-product formation. For example, Parawira et al. [10] stated that anaerobic acidification could be useful for the production of organic acids (e.g. VFA) which have variety of industrial uses. VFAs are utilized for the manufacture of various organic compounds and some plastics [11]. Moreover, these organic acids play an instrumental role as a carbon source in the removal of nutrients from wastewaters [12]. Recently, acidphase anaerobic digestion received considerable attention
* Corresponding author. Tel.: þ90 312 210 58 67; fax: þ90 312 210 26 46. E-mail address: [email protected] (G.N. Demirer). 0961-9534/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2010.08.002
33
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Notations COD H-Ac H-Bu H-Pr HRT OLR ORP
chemical oxygen demand acetic acid butyric acid propionic acid hydraulic retention time organic loading rate oxidation-reduction potential
and relevant studies covered a wide range of operational parameters including; pH [13,14] waste type [15], HRT [2,6,12], temperature [12,16], mixing of wastes [3]and OLR [17]. Today’s excessive resource use and devastating change in global environment led to the increased pressure on polluting industries. Sugar-beet processing industry which falls into this category, devotes increasing attention to environmental problems associated with high energy consumption and production of large amounts of wastes. Recently, sugar plants take measures to reduce energy consumption, recycle materials and energy and optimize the operation of sugar process in order to achieve waste minimization and sustainable production [18]. As a viable option for waste management in sugar-beet processing facilities, anaerobic digestion was broadly evaluated. Most of the studies focused on the anaerobic treatment and biogasification of wastewater and beet-pulp which are two major waste streams of beet processing plants. Wastewater from the beet-sugar mill contains high concentrations of hydrocarbons which makes it suitable for anaerobic biological degradation [19]. Consequently, there have been several studies on anaerobic digestion of beet processing wastewater [19e22]. Contrary to wastewater, beet-pulp is generally considered as a by-product and utilized for further use instead of a waste to be treated and disposed. It is mostly used as animal feed in cattle-raising industry [23]. For this purpose, beet-pulp may be used directly or could be dried alone or in combination with molasses [18]. Koppar et al. [24] advocated that the cost of the beet-pulp drying is increasing due to the increase in fuel prices and more profitable applications need to be developed. In some studies, anaerobic digestion is proven to be a more feasible alternative for the utilization of beet-pulp [24e26]. Moreover, studies which cover two-phase anaerobic digestion of beet-pulp were conducted and merits of phase separation were indicated for biogasification and methane production [23,27,28]. Most of the studies regarding the anaerobic digestion of sugar-beet processing wastes targeted to the effective biogasification and waste stabilization of each individual waste. However, acid-phase anaerobic co-digestion of these wastes together can be used as a practical and economically feasible waste management option together with the effective recovery methods of the produced valuable organic acids. Therefore, the objective of the study was set as maximizing the hydrolysis and acidification of sugar-beet processing wastewater and beet-pulp simultaneously in an acidogenic anaerobic reactor for VFA production. The effect of HRT, waste-mixing ratio and pH on acidification was investigated to determine optimum operational conditions at mesophilic
TS TSS TVS TKN VSS VFA
total solids total suspended solids total volatile solids total Kjeldahl nitrogen volatile suspended solids volatile fatty acids
Subscripts t, s total, soluble
temperatures. The performance of the process was mainly evaluated in terms of VFA accumulation and the degree of acidification.
2.
Materials and methods
2.1.
Waste characteristics
Sugar industry wastes (wastewater and pressed beet-pulp) were obtained from a private beet-sugar factory located near Amasya, Turkey during the beet-campaign period. Characterizations of the wastes were carried out and the results were tabulated in Tables 1 and 2. After characterization, wastes were kept frozen at 20 C in order to prevent biological activity prior to the use in the experimental studies. Before the characterization and the use in the study, wastewater was settled for 1-h to remove the inorganic suspended materials which are very common for sugar-beet processing wastewater. The time period of 1-h was chosen to represent the typical hydraulic retention time of primary sedimentation before secondary treatment systems. In order to achieve physical homogeneity, beet-pulp was processed as follows: First the frozen beet-pulp was thawed at room temperature and further dried at 105 C for 24 h. Then, the dried pulp particles were grinded by the help of a pestle. So, the homogenized powdered pulp was used for reactor feeding.
2.2.
Inoculum
The mixed anaerobic cultures that were used as the inoculum (seed) source were obtained from the anaerobic sludge
Table 1 e Wastewater characteristics (1 h settled). Parameters tCOD sCOD TS TSS VSS pH Alkalinity TKN PTotal tVFA H-Ac H-Pr H-Bu
Values (mg/L) 6621 113.2 6165 517.1 6062 53.0 665 21.2 335 7.1 6.82 1760 (as CaCO3) 10 2.7 1115 20 (as H-Ac) 394 5 610 12 46 1
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Table 2 e Pressed beet-pulp characteristics. Parameter
Value (mg/L)
Moisture (%) TS (%) TVS (% TS) COD (g/g dry weight) TKN (% TS) PTotal (% TS)
85 15 94 1.22 7.28 1.0
0.1 0.1 0.01 0.15 0.28
digesters of the municipal wastewater treatment plant of Ankara. Prior to the use as inoculum, it was concentrated by gravity settling for 24 h. By this way, the VSS concentration was increased up to 41,212 2324 mg/L and then used in the experiments.
2.3.
Basal medium
In order to supply adequate micronutrients for an optimum microbial growth, reactors were daily fed by basal medium (BM) which contains the following constituents (concentrations are given in parentheses as mg/L): NH4Cl (1200), MgSO4 7H2O (400), KCl (400), Na2S 9H2O (300), CaCl2 2H2O (50), (NH4)2 HPO4 (80), FeCl2 4H2O (40), CoCl2 6H2O (10), KI (10), MnCl2 4H2O (0.5), CuCl2 2H2O (0.5), ZnCl2 (0.5), AlCl3 6H2O (0.5), NaMoO4 2H2O (0.5), H3BO3 (0.5), NiCl2 6H2O (0.5), NaWO4 2H2O (0.5), Cysteine (10) [29].
2.4.
Analytical methods
Solids, tCOD (for beet-pulp), TKN, PTotal and alkalinity determinations were carried out as described in Standard Methods [30] pH values were measured with a pH meter (HI 8314, Hanna Instruments) and a pH probe (HI 1230, Hanna Instruments). tCOD (for wastewater) and sCOD (0.45 mm pore-sized filters were used) determinations were performed by using a spectrophotometer (SN 05827, PC Multidirect) and vials specifically prepared for the determination range of 0e1500 mg/L. ORP values of the reactors were measured with a pH meter (PH 510, Eutech) equipped with an ORP electrode (Recorder S-500 C, Sensorex). Daily biogas productions were measured with a water replacement device and the compositions were periodically determined with a gas chromatograph (Thermo Electron Co.) equipped with a thermal conductivity detector (TCD). Produced biogases were separated as hydrogen (H2), carbon dioxide (CO2), oxygen (O2), methane (CH4) and nitrogen (N2) by using serially connected columns (CP-Moliseve 5A and CPPorabond Q) at a fixed oven temperature of 45 C. Helium was used as carrier gas at 100 kPa constant pressure. The inlet and detector temperatures were set to 50 C and 80 C respectively. Prior to the gas chromatography injections, series of pretreatments were conducted for VFA measurements: First, samples were filtered through 0.22 mm pore-sized filters. Then the samples were diluted with deionized water to assure the VFA concentration of the sample to be in the range of pure VFA calibration of gas chromatograph. After filtering and dilution, the samples were acidified with 98% formic acid to a pH less than 2.5, in order to convert the fatty acids to their undissociated forms (i.e. acid forms).
The gas chromatograph (Thermo Electron Co.), used for biogas composition determinations, was also used for the periodical VFA measurements. However the column, the detector and the operational conditions were different: Nukol column (Model 25,326, 15 m 0.53 mm) was used to separate VFAs (acetic, propionic, butyric, iso-butyric, valeric, iso-valeric, caproic, iso-caproic and heptanoic acids). Flame ionization detector (FID) was used for this purpose which was adjusted to 280 C as operating temperature. Helium was used as carrier gas with a constant flow rate of 6 mL/min and the inlet temperature was kept at 250 C. Oven temperature was initially set to 100 C with 2 min holding time and then increased to 200 C with 8 C/min ramping. One of the main interests of this study was to compare the proportions of the substrates converted to VFAs. For this purpose, “acidification degree” was calculated for each of the reactor in order to express the acidification efficiency (Fig. 3). So, the organic content of initial substrate and VFAs were based on COD concentrations. By this way, it was possible to present the related conversion efficiencies for all operated reactors as: Degree of acidification ð%Þ ¼ Sf =Si 100
(1)
Where; Si: Initial substrate concentration, measured in COD as mg/L, *Sf: Produced net VFAs (finaleinitial), expressed as theoretical equivalents of COD concentrations as mg/L. *The COD equivalents of each VFA: Acetic acid, 1.066; Propionic acid, 1.512; Butyric acid, 1.816; Valeric, 2.036; Caproic acid, 2.204. [7]. When calculating acidification degrees obtained in the reactors, initial tVFA concentration (1115 20 mg/L as H-Ac) of the wastewater was taken into consideration. In other words, reactors having wastewater as the feed had already VFAs mainly in the form of acetic, butyric and propionic acids. In order to determine actual conversion of COD into VFA, initial tVFA concentration was subtracted from the final tVFA values giving net tVFA production.
2.5.
Experimental set-up and procedures
Six reactors were operated with different combinations of the operational parameters (HRT, waste mixing ratio and pH) (Table 3). Reactors were run as duplicates to test statistical reliability and the presentation of the data (tables and figures) involves the averaged values of duplicate reactors. 250 mL reactors with effective volume of 150 mL were operated by daily fed-batch feeding strategy as continuously mixed acidogenic reactors. Reactors were kept continuously mixing
Table 3 e Operational parameters of the reactors. Reactor R1 R2 R3 R4 R5 R6
Feed Stock Stock Stock Stock Stock Stock
#1 #2 #3 #1 #2 #3
HRT (days)
Waste mixing ratio (WW:P)
OLR (g COD/L-day)
2 2 2 4 4 4
1:0 1:0.5 1:1 1:0 1:0.5 1:1
2.7 4 5.4 1.35 2 2.7
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35
at 175 rpm by using a mechanical shaker in a temperature controlled room (35 1 C) for 41 days. In order to prevent uncontrolled pH drops and to investigate the effect of higher operational pH (6.9e7.5) on the acidification, first 20 days of operation was carried out by adding external alkalinity in the form of NaHCO3. In this period, reactors were supplied with NaHCO3 to set daily initial concentrations to 6000 mg/L by daily additions along with BM. Then, the reactors were operated 21 more days without adding any external alkalinity to observe the natural pH drop, as a result of acidification, and to investigate the effect of lower operational pH (5.7e7.4) on acidification. For the purpose of reactor feeding, three different stock solutions were prepared (Table 4). In order to investigate the effect of waste mixing ratio on acidification, prepared stock solutions involved same concentrations of BM but different mixing ratio of wastewater (WW) and pulp (P) in terms of tCOD. As the control parameters of reactor operations, pH, ORP, biogas production, biogas composition, VFA and sCOD concentrations were measured. Among these parameters, pH, ORP and biogas production were daily, VFA, sCOD, and biogas compositions were periodically measured.
3.
Results and discussions
3.1.
pH profiles of the reactors
pH drops were observed with varying rates and extents for all reactors (Fig. 1a). Organic acid productions were initiated from the onset of the reactor operations which resulted in these expected pH drops (Fig. 2). For the same substrate (stock solution), 2 days of HRT led to lower pH values than 4 days. The noticeable differences between the operational pHs were mainly due to the differences of the methanogenic activities in the reactors. Since, growth rates of methanogenic microorganisms are lower than the other members of anaerobic consortia, they require more time to survive in anaerobic systems [31]. So, in this study, 4 days of HRT served as a better growing condition for methanogens than 2 days. As a result, methanogenic microorganisms led to VFA consumption and corresponding buffering effect that prevented further pH drops at 4-day HRT (R4, R5, R6). This situation was the reason of higher pH values, observed in these reactors. Indeed, VFA concentration profiles (Fig. 2) and biogas compositions (Table 5) of the reactors supported this speculation. After external alkalinity addition was ceased, pH values were further decreased and stabilized at lower levels
Table 4 e Compositions of stock solutions. Composition
Stock #1
Stock #2
Stock #3
Wastewater (mg/L as tCOD) Pulp (mg/L as tCOD) tCOD (mg/L) sCOD (mg/L) pH
5300 e 5300 4932 72 8.02
5300 2650 7950 5211 421 7.99
5300 5300 10,600 5318 288 8.08
Fig. 1 e Temporal changes of reactor conditions: (a) pH; (b) chemical oxygen demand (soluble); (c) oxidatione reduction potential; (d) daily gas production.
(Fig. 1a). The observed pH decreases were steeper (down to 5.7e7.1) at 2-day HRT since the VFA accumulations (Fig. 2) were too high for the inherent alkalinity of wastewater to buffer. In this period, steady-state pH values were higher at 4-day HRT (6.6e7.4). Additionally, pH patterns of the reactors differed from each other when they are compared in terms of waste mixing ratios. For the same HRT, increased OLRs, resulting from pulp addition, increased the amount of acidification products (VFAs) which lead to low operational pHs (5.7e6.8). It was reported that high OLRs result in higher VFA accumulations and related pH decreases [32]. In this study, the reactors R2 and R3 which received pulp as well as wastewater were operated in the pH range of 5.6e6.2 without external alkalinity addition at HRT of 2 days. Since the pH range of 4e6.5 is accepted as optimum for the growth of anaerobic acidogenic microorganisms [31], proper pH conditions were established in R2 and R3.
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80 External Alkalinity No External Alkalinity
Acidification Degree (%)
70 60 50 40 30 20 10 0 R1
R2
R3
R4
R5
R6
Reactor Fig. 3 e Observed acidification degrees in the reactors.
Fig. 2 e VFA concentration profiles of the reactors: (a) volatile fatty acids (total); (b) acetic acid; (c) propionic acid; (d) butyric acid.
3.2.
Volatile fatty acid productions
Fig. 2 illustrates the inverse relationship between VFA accumulation and operational HRT of the reactors. 2 days of HRT yielded higher tVFA concentrations (2159e3635 mg/L as H-Ac), which is the indication of higher acidogenic activity and/or lower methanogenic activity. Since the acidogenic microorganisms grow much faster than the methanogens, they could successively survive in HRTs as low as 2 days. This also indicates that at 4-day HRT (R4, R5, R6) methanogens could be retained long enough to metabolize VFAs for methane production, causing to lower VFA concentrations (1814e2640 mg/L as H-Ac). The composition of the produced biogas which indicates higher methane percentages at 4-day HRT (Table 5) was a strong evidence of this claim. Lowest VFA concentrations (1814e2244 mg/L as H-Ac) were recorded in the reactors, which were fed only by wastewater (R1 and R4). As expected, VFA concentrations were proportionally
increased with the increase in amount of pulp added and the highest concentrations were observed in 2-day-HRT reactor (R3) which was fed by wastewater and pulp in equal amounts in terms of COD (1:1). These results indicated that pulp was successfully acidified together with wastewater, which led to the increase of the VFA concentrations. After the cessation of alkalinity addition, VFA concentrations were not changed noticeably. This observation indicated that, the change in narrow operating pH values (Fig. 1a) did not result in a major change in the acidification trend of sugar industry wastes, even in R3 which indicated the steepest decrease from 6.9 to 5.7. Dinopoulou et al. [33] stated that acidification degree can be determined by calculating the proportion of the initial substrate which is converted to VFAs as end products. In this study, the highest acidification degrees (60.3e64.2%) were observed in R1, which was only fed by wastewater and operated at 2 days of HRT (Fig. 3). The increase in pulp addition caused a decrease in acidification degree which was accepted as the direct result of increasing OLR. Similarly, in a complex wastewater acidification study, the degree of acidification was found to diminish with increasing OLR [33]. Still, the reactors which were fed by wastewater and pulp with HRT of 2 days (R2, R3) denoted substantial degrees of acidification (44.1e53.5%) when compared with other studies: 15e60% for complex wastewater [33]; 56% for dairy wastewater [34]; 10.3e43.4% forfish meal processing wastewater [35]. In all of the reactors, the main acidification products were H-Ac (40.3e49.2% w/w of tVFA), H-Pr (36.3e42.6% w/w of tVFA) and H-Bu (3.6e7.5% w/w of tVFA) comprising 88.3e96.2% of tVFAs. The higher molecular weight VFAs (valeric, caproic, heptanoic etc.) were produced in insignificant amounts. On the other hand, alcohols (e.g. ethanol, methanol), other major metabolites of anaerobic acidification, were not detected at all. It is a well-known fact that substrate characteristics and operational conditions play a major role on product distribution in an acidification reactor [13,14,33]. In this study, the dominance of H-Ac, H-pr and H-Bu can be associated with the carbohydrate degradation, since both wastewater [19] and pulp [23] contains high amounts of sugars. This is relevant with the literature in which the short-chain fatty acids were
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Table 5 e Steady-state biogas compositions of the reactors. Reactor
R1 R2 R3 R4 R5 R6
External alkalinity
No external alkalinity
CH4 (%)
CO2 (%)
CH4 (%)
CO2 (%)
25.3 1.7 16.3 2.1 6.4 1.5 53.4 0.4 51.6 3.3 44.5 2.7
74.7 83.7 93.7 46.6 48.4 55.5
5.9 3.3 0.5 2.1 2.6 2.2
76.8 5.9 83.3 3.3 94.4 0.5 50.2 2.1 54.9 2.6 60.5 2.2
1.7 2.1 1.5 0.4 3.3 2.7
23.2 16.7 5.6 49.8 45.1 39.5
found to be dominant in acidogenic reactors [14,33]. Parawira et al. [10], stated that higher molecular weight VFAs are generally found in protein fermentation.
3.3.
Soluble chemical oxygen demand concentrations
sCOD concentration distributions of the reactors were depicted in Fig. 1b. The highest sCOD concentrations (5422e5826 mg/L) were observed in R3 which was operated at an HRT of 2 days. 4-day HRT led lo lower sCOD concentrations resulting from the consumption of VFAs which contribute to sCOD. It can be seen from Fig. 1 that, there is a direct relationship between VFA and sCOD concentrations. Since, hydrolysis of particulate organic matter occurs simultaneously during the acidification of soluble organics, sCOD concentrations were higher (3490e5826 mg/L) in the reactors which have pulp addition together with wastewater. The effect of pH values on sCOD concentrations was negligible when the alkalinity-added period was compared with the period without alkalinity addition. This result indicated that the trend of hydrolysis was not significantly altered with the decreases in operational pH values.
3.4.
Oxidation-reduction potentials
ORP values of anaerobic reactors were inspected to detect the relative amounts of oxidized materials such as nitrate ions 2 (NO 3 ) and sulfate ions (SO4 ), and reduced materials, such as þ ammonium ions (NH4 ). At ORP values between 100 mV and 300 mV, degradation of organic compounds proceeds primarily as fermentation and acid formation. On the other hand, ORP values lower than 300 mV indicates considerable methanogenic activity in anaerobic conditions [36]. Fig. 1c depicts that, ORPs of reactors were diminished to steady-state values after the operation of 3e5 days. ORP values of the reactors were within 226 mV and 350 mV in the period of alkalinity addition. After alkalinity addition was stopped, ORP values started to increase and attained steady-state between 162 mV and 300 mV. This observation indicates that, the pH drops resulting from the cessation of alkalinity addition slightly altered the circumstances in favor of fermentation and mixedacid formation, inhibiting the methanogenic activities. This idea was supported by the results of biogas compositions (Table 5) which indicate that methane percentages were slightly decreased along with decreasing pH values. Moreover, 2-day HRT yielded higher ORP values (183 mV to 234 mV) than 4-day HRT especially in the period without alkalinity addition. In 4-day-HRT reactors, the observed ORP
37
values were between 273 mV and 318 mV which indicated higher methanogenic activities than 2-day-HRT reactors.
3.5.
Biogas productions and compositions
The direct relationship between the operational HRT and the amount of daily produced biogas can be observed in Fig. 1d. Reactors, which were operated with an HRT of 4 days (R4, R5, R6) produced higher amounts of biogas (27e72 mL/day) compared to those operated with an HRT of 2 days (19e25 mL/ day). When the biogas productions are interpreted in combination with their composition (Table 5), it can be postulated that, higher methanogenic activity was responsible for the higher biogas productions in 4-day-HRT reactors. Since, the daily biogas productions were primarily affected by methane productions, conditions, influencing methanogenic activity, also influenced the total biogas productions. So, this explains why the lowest biogas productions (19e21 mL/day) were observed in R2 and R3, in which pH values were low enough (5.7e6.2) to inhibit methane productions to some extent. Thus, pulp addition increased the substrate concentrations and highest biogas productions (31e72 mL/day) were recorded at R5 and R6 which were operated with 4 days of HRT. In acidogenic anaerobic reactors, methanogenic activity must be restrained to be able to reach high VFA accumulations. Methanogenic activities and related methane productions could be suppressed to some extent with 2 days of HRT (R1, R2, R3). As a result, lower methane percentages (5.6e25.3%) were detected within the produced biogases of these reactors when compared with the percentages appeared in the 4-day-HRT reactors (39.5e53.4%). Furthermore, Table 5 indicates the inverse relationship between substrate concentrations and methane productions. It can be observed that, increased substrate concentrations resulting from pulp additions caused significant decreases in methane productions; especially at 2-day HRT. The main reason was the increased VFA accumulation in corresponding reactors, resulting in the lower operational pHs (5.7e6.9). So, it was evident that, pulp addition, causing to VFA accumulation, suppressed the methane production. When the biogas compositions of 4-day-HRT reactors were inspected, it was claimed that neither HRTs nor the operational pH values (6.6e7.5) were acceptable for the targeted inhibition of the methanogenic activity. In fact, Yu and Fang [14] stated that the pH values of acidogenic reactors must be kept below 5.5 to inhibit methanogenic microorganisms after observing high methane concentrations (31%) even at pH values as low as 6.5. After cessation of alkalinity addition, methane percentages of the reactors were slightly decreased following the decreases in pH values. The lowest methane percentage (5.6%) was observed at R3 in the period without alkalinity addition. This result indicates that, the optimum conditions for inhibition of methanogenic activity were 2 days of HRT and 5.7e6.2 range of pH in this study.
4.
Conclusions
From the obtained results, the following conclusions can be drawn:
38
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 3 2 e3 9
Sugar industry wastewater and beet-pulp can simultaneously be converted to VFAs in acidogenic anaerobic reactors with considerable acidification degrees (43.8e52.9%). In this study, wastewater was used for dilution of beet-pulp instead of fresh water, which serves for resource conservation. By this way, external alkalinity addition, a common application, could also be avoided due to the inherent alkalinity of the wastewater. Increased OLRs, resulting from pulp addition, increased the amount of acidification products (VFAs) which lead to relatively low operational pH values (5.7e6.8). In this pH range, methanogenic activity was inhibited and lowest methane percentages (5.6e16.3%) were observed in biogas compositions. The optimum operational conditions were selected as 2-day HRT and 1:1 waste mixing ratio (in terms of COD) without external alkalinity addition. These operational conditions led to the highest tVFA concentrations (3635 209 mg/L as H-Ac) with an acidification degree of 46.9% at the highest OLR of 5.4 g COD/L-d.
Acknowledgments This study was funded by The Scientific and Technological Research Council of Turkey through Grant Number 104I127.
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