Biological hydrogen production by anaerobic digestion ... - Springer Link

Biodegradation (2013) 24:753–764 DOI 10.1007/s10532-013-9623-8

ORIGINAL PAPER

Biological hydrogen production by anaerobic digestion of food waste and sewage sludge treated using various pretreatment technologies Seungjin Kim • Kwangkeun Choi Jong-Oh Kim • Jinwook Chung

•

Received: 30 May 2012 / Accepted: 17 January 2013 / Published online: 7 February 2013 Ó Springer Science+Business Media Dordrecht 2013

Abstract The purpose of this study was to enhance the efficiency of anaerobic co-digestion with sewage sludge using pretreatment technologies and food waste. We studied the effects of various pretreatment methods (thermal, chemical, ultrasonic, and their combination) on hydrogen production and the characteristics of volatile fatty acids (VFAs) using sewage sludge alone and a mixture of sewage sludge and food waste. The pretreatment combination of alkalization and ultrasonication performed best, effecting a high solubilization rate and high hydrogen production (13.8 mL H2/g VSSconsumed). At a food waste:pretreated sewage sludge ratio of 2:1 in the mixture, the peak hydrogen production value was 5.0 L H2/L/d. As the production of hydrogen increased, propionate levels fell but butyrate concentrations rose gradually.

S. Kim J. Chung (&) R&D Center, Samsung Engineering Co. Ltd., 415-10 Woncheon-Dong, Youngting-Gu, Suwon, Gyeonggi-Do 443-823, Korea e-mail: [email protected] K. Choi 910 U-Tower Heungduk-Dong, Kiheung-Gu, Yongin, Gyeonggi-Do 446-982, Korea J.-O. Kim Department of Civil Engineering, Gangneung-Wonju National University, Gangneung-Daehangno 120, Gangneung, Gangwon-Do 210-702, Korea

Keywords Food waste Hydrogen Sewage sludge Solubilization Volatile fatty acids (VFAs)

Introduction Reports on sustainable treatment systems that minimize energy consumption have encouraged the use of anaerobic biological systems for intensive organic waste treatment due to their low operational cost (Letting et al. 1979; Jetten et al. 1997). Systems that are based on anaerobic biological processes have traditionally been adopted to stabilize primary and secondary waste sludge (Parkin and Owen 1986). Based on their advantages, such as high biogas production and waste stabilization, anaerobic systems are an appropriate organic waste management method. Further, biogas recovery from organic waste is the most important source of alternative energy (Piccinini 2008). However, anaerobic digestion of sewage sludge is inefficient, because the typical ratio of volatile solids (VS) to total solids (TS) is low (0.4–0.6). The biological hydrolysis of sludge is the rate-limiting step (Wang et al. 1999; Tiehm et al. 2001). To reduce the impact of this step, pretreatment of sludge is required, such as thermal (Li and Noike 1992; Tanaka et al. 1997; Sawayama et al. 1996), alkaline (Penaud et al. 1999; Ray et al. 1990; Lin et al. 1997), ultrasonic (Wang et al. 1999; Tiehm et al. 2001), microwave (Coelho et al. 2011; Eskicioglu et al. 2007), and mechanical disintegration (Nah et al. 2000).

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Another alternative is co-digestion, which is one of the most effective systems for the treatment of organic waste, generating various types of organic substrates as a homogeneous mixture that is input into the reactor (Braun 2002). Co-digestion of organic wastes improves the CH4 yield and process performance due to the synergy between substrates, which offsets the lack of nutrients (Del Borghi et al. 1999; Mata-Alvarez et al. 2000; Braun 2002; Viotti et al. 2004). However, anaerobic co-digestion processes require the proper conditions with regard to substrates. Among organic wastes, the anaerobic digestion of sewage sludge is ineffective due to its low VS content, and the anaerobic treatment of food waste is inefficient due to its extremely high biodegradability, resulting in the accumulation of volatile fatty acids (VFAs). Codigestion with the optimal dose of organic wastes using high-rate anaerobic digestion technologies is a promising method of improving these low digestion efficiencies. This study was performed to overcome the low efficiency of conventional anaerobic digestion of sewage sludge by co-digestion using sewage sludge with pretreatment and food waste as a co-substrate.

Biodegradation (2013) 24:753–764

Pretreatment of sewage sludge We measured the solubilization efficiency after applying the various pretreatment technologies to sewage sludge—thermal treatment, ultrasonication, alkalization, acidification, and a combination of alkalization and ultrasonication. For the thermal treatment, we applied heat for 30 min at 120 °C using a hightemperature and high-pressure wet sterilizer, after which we cooled the sample at room temperature and measured the solubilization efficiency. For alkalization and acidification, we performed the solubilization at pH 12 and pH 2 using 3 M NaOH and 3 M HCl, respectively. Solubilization with ultrasonic waves was conducted for 30 min using a digital ultrasonic homogenizer (20 kHz, Bandelin, Germany). For solubilization using a combination of alkalization and ultrasonication, we performed 2 experiments: ultrasonication after alkalization and alkalization after ultrasonication. Details on the solubilization treatments are shown in Table 2. As discussed, we input the data from the various solubilization technologies into Eq. (1) and obtained the solubilization efficiency: Solubilization efficiency ¼

Materials and methods Characteristics of substrate Seed sludge was taken from a full-scale thermophilic anaerobic digester at a local municipal wastewater treatment plant (WWTP) that was treating sewage sludge as substrate; the collected sludge was boiled at 90 °C for 15 min to eliminate non-spore formers and promote the germination of spores, after which the levels of active hydrogen producers (hydrogenotrophic bacteria) in the bioreactor increased, leading to greater potential hydrogen production (Cohen et al. 1985). The substrate was prepared individually to generate a mixture of sewage sludge and food waste. Sewage sludge from the thickener of a local municipal WWTP was filtered through a stainless steel sieve (pore size 2.0 mm). The food waste was sampled from a selfservice restaurant. The chief constituents of the food waste were classified as grain, vegetable, and meat. Samples were processed in an electric blender after dehydration in a drying oven at 105 °C. The characteristics of the substrate are summarized in Table 1.

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SCODf ; TCODf

ð1Þ

where, SCODf is the soluble chemical demand (SCOD) value after pretreatment (mg/L) and TCODf is the total chemical oxygen demand (TCOD) value after pretreatment (mg/L). Accordingly, the extent of solubilization of sewage sludge progresses as the solubilization efficiency increases; further, use of the resulting solubilized sewage sludge accelerate the degree of hydrolysis in the anaerobic digestion rate.

Table 1 Characteristics of sewage sludge and food waste as substrate Item

Unit

Sewage sludge

Alkalinity

mg/L

3,720 ± 650

VS

mg/L

10,550 ± 760

129,300 ± 8750

TS

mg/L

12,650 ± 1,890

183,530 ± 6,880

SCOD

mg/L

383 ± 73

TCOD

mg/L

10,050 ± 1870

pH

7.7 ± 0.2

Food waste 4.6 ± 0.2 0.3 ± 0.1

84,280 ± 11,230 164,670 ± 5,530


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Table 2 Experimental conditions for various pretreatment technologies Ultrasonication

pH control Ultrasonic (amplitude)

–

Alkalization

12

Acidification

Heating

2

Ultrasonication ? alkalization

Alkalization ? ultrasonication

–

12

12

120

120

120

–

–

–

Temperature (°C)

25

25

25

120

Treatment time (min)

30

30

30

30

Hydrogen production with various pretreatment technologies After the sewage sludge was treated using the various solubilization technologies, we mixed the prepared seed sludge and solubilized sludge in the reactor (5 L), as shown in Fig. 1, at at ratio of 1:1 (v/v) and examined the production of biological hydrogen after injecting a buffer solution of 1 M phosphate, pH 8. Hydrogen production with a mixture of sewage sludge and food waste In measuring hydrogen production by a mixture of food waste and sewage sludge that was pretreated with alkalization and ultrasonication, we used the same reactor as in the previous experiment with food waste:solubilized sewage sludge ratios of 1:1, 2:1, 3:1, and 1:3. The mixed substrate and prepared seed sludge were added to that reactor at a ratio of 1:1 (v/v).

25

25

30 (15 ? 15)

30 (15 ? 15).

The temperature and agitation speed were 37 ± 1 °C and 150 rpm, respectively. All experiments were batch-type and performed for 48 h at pH 5.5 ± 0.1. Hydrogen production at various pHs In measuring hydrogen production as a function of pH, we mixed food waste and sewage sludge that was solubilized by a combination of alkalization and ultrasonication at a ratio of 2:1 (v/v) and also mixed them with prepared seed sludge at a ratio of 1:1 (v/v) in an effective volume of 3 L. To meet the anaerobic conditions, we purged the mixture with N2 sufficiently. We conducted the batch-type experiment for 48 h at pH 4, 5, 6, 7, and 8 using 3 M KOH and 3 M HCl at 150 rpm and 37 ± 1 °C. Analysis The gas was measured immediately after its generation using a wet gas meter (Model W-NK-0.5,

Fig. 1 Schematic of batch-type experimental apparatus for biohydrogen production

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SHINAGAWA, Japan) to prevent the partial pressure of the hydrogen that was generated inside the reactor from affecting the efficiency of hydrogen production (Hawkes et al. 2002; Mizuno et al. 2000). We also analyzed total suspended solids (TSS), volatile suspended solids (VSS), TCOD, and SCOD at constant intervals per standard methods (APHA 1998). To analyze the characteristics of biological hydrogen production by solubilization conditions for sewage sludge and a mixture of food waste and sewage sludge, we measured hydrogen and VFAs simultaneously. The preparation of samples for the determination of VFAs by gas chromatography (GC) was based on Manni and Caron’s procedure (Manni and Caron 1995). Briefly, the samples were acidified to pH 2 using 65 % nitric acid. One-milliliter aliquots were shaken with 1 mL diethyl ether for *10 min, and the ether phases were transferred to 4-mL flasks, to which a small amount of anhydrous sodium sulfate was added. Five-hundred-microliter portions of the ether phase were transferred to new 4-mL flasks and 150 lL diazomethane was added. A series of VFA standards for the calibration curves were prepared as described above. Calibration curves were obtained using five aqueous acid solutions— acetic, propionic, butyric, valeric, and caproic—at 5–1,000 mg/mL. GC analyses were performed on an Agilent 7820A GC (Agilent Technologies, Korea), equipped with a flame ionization detector and a DB-23 capillary column (30 m, 0.25 mm I.D., 0.25-lm film thickness, Alltech, Poland). The injector and detector temperatures were both 170 °C. The carrier gas was argon. The analyses were performed using the following program: 5 min at 30 °C and a linear gradient from 30 to 130 °C at 10 °C per min. In each case, 2 lL of sample was injected (flow split 1:10).

Results and discussion Pretreatment of sewage sludge Solubilization efficiency by pretreatment technology To use the sewage sludge as substrates for biological hydrogen production, we applied various solubilization technologies to sewage sludge alone or in combination. As shown in Fig. 2, the value of the final SCOD increased for all methods, and the combined application

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of each solubilization technology effected greater efficiency than the individual methods. For application of ultrasonication (30 min at 20 kHz) and alkalization (30 min at pH 12 with NaOH) alone for each case, the efficiency of solubilization was approximately 0.2 and 0.3, respectively. For ultrasonication alone, a treatment time of 30 min was the most effective, because the solubilization did not improve beyond this point (data not shown). In addition, the efficiency of solubilization peaked when NaOH of 2 g/L-sludge was injected, because increased quantities of alkali result in high viscosity of sewage sludge and lack of agitation; thus, the injected alkali did not affect the flocs or microorganisms in sewage sludge (data not shown). Therefore, the injection of alkali (based on NaOH) at 2 g/L-sludge is the most cost-effective and efficient solubilization method. As shown in Fig. 2, the most extensive effects of solubilization occurred with an initial alkalization step for sewage sludge, followed by ultrasonication. Alkalization can decompose flocs of sewage sludge easily compared with ultrasonication, and after the collapse of flocs by alkalization, the effect of solubilization was maximized by the cavitation effects of ultrasonic waves. We hypothesize that at the initial stage of solubilization, the alkalization step caused the collapse in flocs, and subsequently, the cavitation effect by ultrasonic waves destroyed the cellular walls of each microorganism. Accordingly, ultrasonication after alkalization is the more effective order of this combination of methods. On the other hand, alkalization, combined with heat or acidification, was seldom considered because solubilization efficiencies by heat and acidification alone were significantly lower than those by ultrasonication alone. The comparative results on the combined application of alkalization and ultrasonication are shown in Fig. 3. Solubilization of sewage sludge was performed with 1 g-NaOH/L-sludge and ultrasonic waves (20 kHz) at the initial stages of the experiment and with 2 g-NaOH/ L-sludge when the solubilization efficiencies did not increase. When alkalization occurred after ultrasonication, the solubilization efficiency was *0.18 after 20 min versus 0.50 when ultrasonication was performed after alkalization. The solubilization efficiency of the combination of ultrasonication and alkalization with 1 g-NaOH/Lsludge did not differ compared with the individual methods, implying that more energy is needed to


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Fig. 2 Effect of various pretreatment technologies on solubilization of sewage sludge

deconstruct the cellular walls of microorganisms after the collapse of flocs, although ultrasonication and alkalization are effective in collapsing sludge flocs. Accordingly, when the effects of solubilization failed to change further, the addition of NaOH was increased to 2 g, and the solubilization efficiency improved to 0.6. This result implies that the deconstruction of cellular walls is accelerated by additional alkalization. In addition, with alkalization being performed after ultrasonication, VSS at the initial stages of solubilization decreased slightly, but when the solubilization did not increase any further, this decline ceased. With ultrasonication after alkalization, however, the reduction rate in VSS increased gradually. Thus, ultrasonication after alkalization is the most optimal technology for solubilization of sewage sludge. Hydrogen production by pretreatment technology Figure 4a shows biological hydrogen production with sewage sludge that has been pretreated with various solubilization technologies; at higher solubilization efficiencies, the efficiency of hydrogen production increased. Compared with the control (without pretreatment), however, the efficiency of hydrogen production after thermal treatment was higher, although the solubilization efficiencies were similar. This low

efficiency was attributed to the low amounts of VFAs that were produced in anaerobic digestion. Consequently, the hydrogen productivity peaked with ultrasonication after alkalization, followed by alkalization after ultrasonication. This result suggests that a combination of technologies for solubilization increases the production of hydrogen versus each individual method. Figure 4b shows the concentrations of the acetic acid, butyric acid, and propionic acid that were produced during hydrogen production. In all cases except for the control, large amounts of VFAs were produced, indicating that anaerobic digestion progressed sufficiently. In biological hydrogen production, the ratio of butyric acid to acetic acid (B/A ratio) and propionic acid concentration are indicators of hydrogen production; higher B/A ratios and lower concentrations of propionic acid reflect higher efficiency of biological hydrogen production (Han and Shin 2004; Chen et al. 2002; Payot et al. 1998), because thermal treatment of anaerobic sludge is predominated by spore-forming microorganisms, most of which are Clostridia species, which produce hydrogen during butyric acid production. Consequently, the concentration of butyric acid rises among VFAs (Lay et al. 1999). In addition, to maintain the high concentration of hydrogen in a fermentor, no methanogens or bacteria

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Fig. 3 Effect of combined treatment with alkalization and ultrasonication on solubilization and VSS reduction in sewage sludge

that produce propionic acid should exist, because the hydrogen that is produced by Clostridia species is consumed by methanogens and propionic acid-producing bacteria for the generation of methane or propionic acid (Sparling et al. 1997). Therefore, among VFAs, at lower concentrations of propionic acid and higher B/A ratios, the efficiency of hydrogen production increase. Consequently, as shown in Fig. 4a, b, we obtained the highest B/A ratio, lowest concentration of propionic acid, and highest efficiency of hydrogen production with ultrasonication after alkalization. In the control, the B/A ratio was higher than with the thermal treatment, but the efficiency of hydrogen production was lower, because the concentration of VFAs was greater and more butyric acid was produced in the thermal treatment. In addition, compared with acidification and thermal treatment, we predicted that the amounts of hydrogen that was produced would be low due to the lower efficiency of solubilization. However, the hydrogen that was produced with thermal treatment was higher than with acidification, implying a higher B/A ratio in the thermal treatment versus acidification and greater hydrogen production by Clostridia species.

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Mixture of food waste and pretreated sewage sludge Hydrogen production by pretreatment technology To improve the efficiency of hydrogen production, we mixed pretreated sewage sludge and food waste that contained high organic matter. After applying various solubilization technologies to sewage sludge, we mixed solubilized sewage sludge and food waste at the same ratio (1:1 (v/v)) and mixed them with cosubstrate and prepared sludge for seeding at a ratio of 1:1 (v/v) at pH 5.5 ± 0.1. As shown in Fig. 5a, when food waste and solubilized sewage sludge were mixed, the yield of hydrogen with all solubilization technologies was significantly higher than with solubilized sewage sludge alone. Also, the highest yield of hydrogen with ultrasonication after alkalization was 13.8 mL H2/g-VSSconsumed, 4 times greater than with solubilized sewage sludge alone. It is difficult to use sewage sludge alone as substrate for hydrogen production, because sewage sludge contains a low content of organic matter. However, food waste was added to sewage sludge, and the dissolved organic


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Fig. 4 Biological hydrogen production (a) and VFA production (b) with sewage sludge treated by various pretreatment technologies

content and alkalinity increased simultaneously due to the complementary reaction of the low alkalinity of food waste and low organic content of sewage sludge. Therefore, co-substrates can have antagonistic activities due to the characteristics of the two wastes, resulting in improved hydrogen yield. The production of hydrogen from organic matters during anaerobic digestion occurred primarily at the

stage of acidification, at which time organic acid was produced. With the combination treatment, the B/A ratio peaked, and most metabolism occurred in the path of hydrogen production. The combination treatment effected the lowest concentration of propionic acid, whereas propionic acid levels peaked in the thermal treatment, during which less hydrogen is produced. Accordingly, although the efficiency of

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Fig. 5 Biological hydrogen production (a) and VFA production (b) from mixed food waste and sewage sludge

solubilization increases with thermal treatment of sewage sludge, the production of hydrogen is inhibited by relatively high levels of propionic acid. As shown in Table 3, in this study the VFA yields of pretreated sewage sludge ranged from 0.25 to 1.43 g VFA-COD/g TOCD, which is 2–10 times higher than in earlier reports, but lower than those of mixture conditions 0.011–0.199 g VFA-COD/g TOCD. Compared with VFA yield by codigestion of food waste

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and sewage sludge, VFA yield by sewage sludge decreased but there was little difference in hydrogen production.

Hydrogen production at various pHs Because the organic acid that is produced significantly affects pH, pH is one of the environmental factors that

Biodegradation (2013) 24:753–764 Table 3 Comparison with VFA yields of previous research and this study

761

Feed source

g VFA COD/g TCOD

References

Sewage sludge

0.058–0.14

Yuan et al. (2009)

Sewage sludge

0.095–0.19

Ubay-Cokgor et al. (2005)

Sewage sludge

0.05–0.11

Banister and Pretorius (1998)

Sewage sludge

0.044–0.14

Ucisik and Henze (2008)

Primary sludge

0.19


Sewage sludge ? primary sludge

0.081


Sewage sludge

0.125

This study

Pretreated sewage sludge Food waste ? pretreated sewage sludge

0.250–1.43 0.011–0.199

This study This study

affect the production of hydrogen. As shown in Fig. 6a, the highest amount of hydrogen was 1.8 L H2/L/d at pH 5.5 (1.6 L H2/L/d at pH 5.0). Below pH 4 and above pH 7, hydrogen was hardly produced, similar to other studies (Fang and Liu 2002; Lay 2000; Fan et al. 2004). We examined the changes in organic acid that was produced during hydrogen production. As shown in Fig. 6b, the production of hydrogen began after 3 h, peaking at 16 h of operation and declining after 18 h. With regard to VFA composition, because the production rate of butyric acid increased, rather than acetate acid, it had more advantageous effects on the production of hydrogen. In addition, after 18 h, as the production rate of propionic acid increased, the metabolism shifted from the production of hydrogen toward its consumption. Hydrogen production of mixture Based on previous experiments, the efficiency of hydrogen production increased in a mixture of food waste and sewage sludge versus sewage sludge alone. Figure 7a shows the change in hydrogen production and B/A ratio at various ratios of food waste and solubilized sewage sludge. When food waste and sewage sludge were mixed at ratios of 1:1 and 2:1, 1.8 and 5.0 L H2/L/d was produced, respectively; the production rate of hydrogen increased as the concentration of dissolved organic matter increased. When the ratio rose to 3:1, little hydrogen was produced, possibly due to the adverse effects of hydrogen-producing bacteria and the high level of salinity or excessive production of organic acid (Mizuno et al. 2000; Kim 2005; van Ginkel et al. 2001). One of the reasons why a mixture of food waste and

sewage sludge was used as co-substrate was to dilute the concentration of salinity in food waste; the concentration of salinity is shown as percentages in parentheses, such as 1:1 (0.6 %), 2:1 (1.1 %), 3:1 (2.1 %), and 1:3 (0.3 %). Figure 7b shows the change in VFAs at the time of hydrogen production at various ratios of food waste and sewage sludge. VFA levels at ratios of 2:1 and 3:1 peaked at 48,675 and 47,901 mg-COD/L, respectively, indicating that the content of organic matter and production efficiency of VFAs is high when the rate of food waste is relatively high. However, the produced quantity of hydrogen was minimal when the ratio was 3:1, caused by high contents of propionic acid and a low B/A ratio. Therefore, when a mixture of food waste and sewage sludge is used as substrate at a ratio of 3:1, VFAs can be produced sufficiently, but this ratio is inappropriate as substrate for the production of hydrogen due to the low productivity of hydrogen. Thus, the optimal ratio of food waste and sewage sludge for the production of hydrogen was 2:1.

Conclusions In this study, various pretreatment technologies of sewage sludge and co-digestion with food waste were examined to improve anaerobic digestion efficiency and hydrogen production. The most important results are: 1)

Pretreatment, combined with alkalization and ultrasonication, is ideal for treating sewage sludge, effecting a solubilization rate and elution rate of organic material of 0.9 and 0.076/min, respectively.

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Fig. 6 Biological hydrogen production from mixture of food waste and sewage sludge at various pHs (a) and butyrate/acetate ratios in the range of pH 5.0–5.5 (b)

2)

3)

The highest hydrogen production values of sewage sludge alone and mixed food waste and sewage sludge were 4.3 mL H2/g VSSconsumed and 13.8 mL H2/g VSSconsumed with the combination pretreatment with alkalization and ultrasonication. The optimal pH for hydrogen production was 5.0–5.5, and the hydrogen production rate was

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4)

5)

1.6–1.8 L H2/L/d at a food waste:pretreated sewage sludge ratio of 1:1 (v/v). Hydrogen production peaked at 5.0 L H2/L/d when the ratio of food waste and pretreated sewage sludge was 2:1 (v/v). At higher hydrogen production rates, propionate concentration was relatively lower, but butyrate and acetate concentrations were significantly


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Fig. 7 Biological hydrogen production and butyrate/acetate ratio (a) and VFA distribution (b) at various ratios of food waste and sewage sludge

higher, indicating that the chief metabolic pathway relies on hydrogen production. References APHA (1998) Standard methods for examination of water and wastewater, 20th edn. American Public Health Association, Washington, DC

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