Article pubs.acs.org/EF
Fermentative Hydrogen Production from Waste Sludge Solubilized by Low-Pressure Wet Oxidation Treatment Yanan Yin† and Jianlong Wang*,†,‡ †
Collaborative Innovation Center for Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology (INET), and ‡Beijing Key Laboratory of Radioactive Waste Treatment, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: The low-pressure wet oxidation was used to solubilize the waste-activated sludge for recovering the carbon source, and the treated sludge was applied for the fermentative hydrogen production. The experimental results showed that, after lowpressure oxidation treatment, the concentration of soluble chemical oxygen demand (SCOD), polysaccharides, and protein present in the liquid phase was increased by 2.0, 2.2, and 102.5 times, respectively. Bio-hydrogen production was successfully achieved using solubilized sludge as the substrate. Through comparison of the hydrogen production from glucose, the treated sludge, and the mixture of the sludge and glucose, it was found that the hydrogen yield of tests using different substrates showed a positive relation with the ratio of acetic acid/butyric acid in soluble metabolites and the ratio of polysaccharides/SCOD in substrates. It can be concluded that the waste-activated sludge treated by low-pressure wet oxidation can be used as low-cost substrate for bio-hydrogen production.
1. INTRODUCTION Hydrogen is one of the most ideal alternative fuels for its high energy content and clean combustion products. Besides, electricity can be easily generated from hydrogen using fuel cells.1 Hydrogen can be produced through numerous ways, among which biological hydrogen production is a most promising candidate.2 Different kinds of substrates have been studied for bio-hydrogen production, including cassava starch,3 sweet potatoes,4 lignocellulosic materials,5−8 microalgal biomass,9−11 industrial wastewater,12,13 municipal solid waste,14,15 and waste-activated sludge from various wastewater treatment plants.16 Dark fermentation of organic biomass, i.e., agricultural residues, agro-industrial wastes, and organic municipal waste, is a promising technology for producing renewable bio-hydrogen,17 because it owns several advantages, including mild reaction conditions, low-energy consumption, good environmental benefit, and extensive low-cost substrate resources.18 Hydrogen production from organic wastes provides dual environmental benefits in the direction of waste treatment along with bioenergy generation. Waste-activated sludge is produced during the biological wastewater treatment process. About 50% organic pollutants [in terms of chemical oxygen demand (COD)] in wastewater was transformed to sludge. The disposal of sludge costs a lot, and the public demand for the sustainable management of wastes is increasing.19 Sludge is mainly composed of microbial biomass, which is rich in polysaccharides and proteins; therefore, it is a potential substrate for producing hydrogen. Thus, hydrogen production from sludge not only addresses the issue of sludge disposal but also generates clean energy. However, most of the biodegradable organics in sludge are encapsulated within microbial cell membranes, while the extracellular polymeric substances are non-biodegradable. Furthermore, a high particulate content also resulted in a low hydrolysis rate. Therefore, pretreatments are necessary to increase the sludge biodegradability. © XXXX American Chemical Society
Different pretreatment methods have been studied to recover the carbon source from sludge, such as thermal/hydrothermal,20,21 acid,22 base,23 aeration,24 ultrasonication,25 microwave,26 enzymatic,27 ionizing irradiation,16,28−30 etc. Wet oxidation has been used for treating sludge for various purposes, such as enhancing the sludge degradation,19 improving the sludge dewaterability,18 and converting sludge to a carbonaceous product.31−34 However, the high temperature of traditional wet oxidation can lead to the excessive degradation of organic matter in sludge, reducing the value of sludge as an organic source for fermentative hydrogen production.20 Furthermore, studies have also shown that a higher temperature can lead to the formation of more recalcitrant compounds.36 Thus, low-pressure wet oxidation is preferred in recovering the valuable organic matter from sludge, such as phosphorus recovery, and reclaiming the carbon source for denitrification.19 However, wet oxidation has not been applied for solubilizing the sludge for fermentative hydrogen production. The objective of this study was to investigate the effects of low-pressure wet oxidation on sludge solubilization and examine the characteristics of hydrogen production from the solubilized sludge.
2. MATERIALS AND METHODS 2.1. Sludge. Waste-activated sludge was collected from a sewage treatment plant that adopts an anoxic−anaerobic−oxic (A2/O) process. The collected sludge settled about 2 h to measure the physicochemical characteristics, including pH, suspended solid (SS), volatile suspended solid (VSS), total chemical oxygen demand (TCOD), soluble chemical oxygen demand (SCOD), soluble total phosphorus (STP), soluble total nitrogen (STN), polysaccharides, Received: May 6, 2016 Revised: June 23, 2016
A
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comparison, 0.1 g of glucose, 80 mL of pretreated sludge (SCOD = 10 914.2 ± 480 mg/L), and a mixture of 0.1 g of glucose and 80 mL of pretreated sludge were used as the substrate, respectively. A total of 10 mL of seeding sludge and 10 mL of nutrient solution mentioned above were added to each batch, and all of the groups were filled with deionized water to a total volume of 100 mL. Before the incubation process, the initial pH of the medium was adjusted to 7.0 and an oxygen-free environment was provided in each batch, as indicated in refs 16 and 30. During the fermentation, bottles were placed in a reciprocal shaker and the rotation speed and temperature were controlled at 100 rpm and 36 °C, respectively. Each batch test was conducted in three replicates. The modified Gompertz equation was employed in this study to describe the cumulative hydrogen production in the batch tests
Table 1. Physicochemical Characteristics of the WasteActivated Sludge item pH SS (mg/L) VSS (mg/L) TCOD (mg/L) SCOD (mg/L) protein (mg/L) polysaccharides (mg/L) STN (mg/L) STP (mg/L) formic acid (mg/L) acetic acid (mg/L) propionic acid (mg/L) butyric acid (mg/L)
value 6.7 16426 11342 18638 3579.9 32.2 315.5 5.0 19.4 199.6
± ± ± ± ± ± ± ± ± ±
0.2 640 480 670 154 1.7 13.8 0.2 0.8 9.7
H = P exp{− exp[R me(λ − t )/P + 1]} where P is the cumulative hydrogen production potential (mL), Rm is the maximum hydrogen production rate, λ is the lag time (h), t is the cultivation time (h), and e = 2.718 28. 2.5. Analytical Methods. The volume of biogas produced was determined by the water displacement method at room temperature (25 °C). A gas chromatograph (model 112A, Shanghai, China) was used to determine the fraction of H2 in the biogas. The gas chromatograph was equipped with a thermal conductivity detector (TCD) and a packed column (model TDX-01, 3 m in length and 3 mm in diameter). The temperatures of the column, detector, and injector were 160, 110, and 180 °C, respectively. Argon was used as the carrier gas, and the pre-column pressure was 0.2 MPa. The physicochemical characteristics of sludge at different statuses were measured by standard methods,37 including SS, VSS, TCOD, SCOD, total phosphorus (TP), and total nitrogen (TN). The protein content was measured by the modified Lowry method using bovine serum albumin as the standard.38 The polysaccharide content was measured by the phenol sulfuric acid method using glucose as the standard.39 The pH value was measured by a pH meter (model 526, Germany). The VFAs were analyzed using an ion chromatograph (Dionex model ICS 2100) equipped with a dual-piston pump, a Dionex IonPac AS11-HC analytical column (4 × 250 mm), an IonPac AG11-HC guard column (4 × 50 mm), and a DS6 conductivity detector.
168.7 ± 7.4
2.2. Sludge Pretreatment. Low-pressure wet oxidation was applied as a pretreatment method to disintegrate waste-activated sludge. The pretreatment process not only released organic matter encapsulated in cells but also inactivated microorganisms present in the sludge to prevent their consumption of nutrients and hydrogen. The low-pressure wet oxidation was performed in a 500 mL stainless autoclave with a magnetic stirrer (Figure 1). An external
3. RESULTS AND DISCUSSION 3.1. Sludge Dissolution. Waste-activated sludge was treated at 175 °C under a low pressure for 30 min, and the pretreated sludge showed a darker brown color than the raw sludge. Figure 2 shows the general physical and chemical properties of the raw sludge and sludge after wet oxidation. It can be seen from Figure 2A that, after wet oxidation treatment, around 5% reduction of SS, VSS, and TCOD was observed, indicating that some of the suspended matter were dissolved into the solution and some organic components were transformed into gas at a high temperature. Similar results were obtained by Strong et al.40 and Shanableh,21 who used a similar temperature to recover useful organic matter from waste-activated sludge. A low percentage of reduction also proved that low-pressure wet oxidation can preserve most organic matter in waste-activated sludge for fermentative hydrogen production. Besides, SCOD showed a significant increase after treatment, which was 2 times higher than in raw sludge, indicating that low-pressure wet oxidation can disrupt the sludge floc structure and release intracellular compounds into the soluble phase effectively.21 Wet oxidation was used as an advanced and sustainable technology for sludge treatment and management, and techno-economic and environmental assessment of sewage sludge wet oxidation was studied.41,42
Figure 1. Experimental setup used for sludge solubilization. electrical furnace was used to heat the autoclave, and the temperature was measured by a thermocouple. In a typical run, a certain amount of biomass, i.e., 400 mL (VSS = 11 342 ± 480 mg/L) of raw sludge, was fed into the reactor and sealed. Then, the autoclave was heated to 175 ± 2 °C (0.89 MPa), which usually takes about 40 min, and then maintained for 30 min before the electric furnace was removed. Then, the autoclave was cooled to room temperature. 2.3. Inoculum. The seed sludge was taken from an anaerobic digester of a sewage treatment plant located in Beijing, China. The VSS of the sludge was 2.42 ± 0.21 g/L. Prior to inoculation, 5 kGy γ irradiation was applied to the seed sludge to inactivate the methanogenic bacteria and a pre-culture process was used to cultivate the hydrogen producers.35 A pre-culture medium and nutrient solution composition were applied as described in ref 30. 2.4. Bio-hydrogen Production. To examine the possibility of hydrogen production from low-pressure wet oxidized sludge, batch experiments were conducted in 150 mL Erlenmeyer flasks. To make a B
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batch tests. According to the polysaccharide concentration in the treated sludge, 1 g/L glucose was used as a substrate alone to make a comparison. Furthermore, the mixture of the treated sludge and 1 g/L glucose was also used as substrate to determine the effect of an additional carbon source on hydrogen production from treated sludge. As shown in Figure 3, the hydrogen production process of all three groups finished within 20 h. A mixture of the treated
Figure 3. Cumulative hydrogen production from different substrates.
sludge and glucose showed the highest cumulative hydrogen production, followed by wet oxidized sludge and glucose. On the basis of the data shown in Figure 3, the kinetics of the fermentative hydrogen production process was successfully modeled by the modified Gompertz equation, and the kinetic parameters obtained were shown in Table 2.
Figure 2. Sludge dissolution by low-pressure wet oxidation.
The main organic components used in fermentative hydrogen production include polysaccharides and protein. Thus, the disintegration of waste-activated sludge leading to the nutrient recovery can be quantified by parameters such as protein, polysaccharides, nitrogen, and phosphorus. It can be seen from Figure 2B that, after wet oxidation treatment, concentrations of protein and polysaccharides were significantly improved by 102.5 and 2.2 times, respectively. Studies have shown that protein can be hardly decomposed in heat treatment;43,44 thus, protein was highly accumulated during the treatment process. Otherwise, as the necessary elements for microbial growth, nitrogen and phosphorus were also released to the liquid phase, leading to the increase of STN and STP contents. In general, low-pressure wet oxidation exhibited a significant effect on nutrients recovering from waste-activated sludge. The pH increased slightly from 6.7 of raw sludge to 7.0 of treated sludge. A similar phenomenon was also observed by Yin et al.44 Studies have identified that acetic acid was highly recalcitrant within wet oxidation,40 and it can be produced as the primary VFA during hydrothermal treatment of organic wastes.45 However, in this study, the concentration of acetic acid in both raw sludge and treated sludge was below the detection limit. Because studies have shown that the increase of acetate was usually accompanied by carbohydrate reduction in thermal treatment,44 no detection of acetic acid suggested that the low-pressure wet oxidation can prevent the transformation of polysaccharides to acetic acid. 3.2. Kinetic Analysis of Hydrogen Production. Hydrogen production potential of the waste-activated sludge pretreated by low-pressure wet oxidation was assessed using
Table 2. Parameters Estimated by the Modified Gompertz Model sample
glucose
sludge
treated sludge + glucose
P (mL) λ (h) R (mL/h) R2 VHPR (mL h−1 L−1)
14.8 6.5 2.0 0.990 8.2
18.7 4.9 2.2 0.989 13.4
34.5 5.0 5.9 0.992 24.6
It can be seen from Table 2 that the cumulative hydrogen production potential of the mixture was slightly higher than the sum of cumulative hydrogen production from glucose and the wet oxidized sludge separately. A similar phenomenon was observed in our previous study applying γ-irradiated sludge as the substrate.16 A possible reason is that the addition of glucose enhanced the C/N ratio of treated sludge from 0.3 to 0.6, leading to the promotion of fermentative hydrogen production. Chen et al.46 studied the effect of the carbohydrate/protein ratio on hydrogen production using waste-activated sludge as the substrate, and they found that hydrogen production was improved with the increasing carbohydrate/protein ratio from 0.2 to 5.0. The higher C/N ratio can result in higher hydrogen production.47,48 Lag times of 6.9, 4.9, and 5.0 h were observed in groups with glucose, the treated sludge, and the mixture of the treated sludge and glucose, respectively. The lag time was longer than our previous studies using γ-irradiation-treated sludge as the C
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Energy & Fuels substrate16,30 but shorter than studies using alkaline-, microwave-, and sterilization-treated sludge,23,49 indicating that lowpressure wet oxidation had an advantage over alkaline, microwave, and sterilization in enhancing hydrogen production from waste-activated sludge. Groups using sludge as the substrate all showed a lower lag time (λ) than that using pure glucose as the substrate, indicating that hydrogen producers can better adapt to the environment with treated sludge as the substrate. A possible reason is that the nutrient elements released from sludge can improve the microbial activity. A maximum hydrogen production rate (R) of 2.2 mL/h was obtained from test using the treated sludge as the substrate, which was lower than 5.9 mL/h achieved from the test using a mixture of glucose and sludge as the substrate, suggesting that the insufficient carbon source from treated sludge restricted the hydrogen production rate. The volumetric hydrogen production rate (VHPR) was calculated as the production rate divided by the volume of the reactor. Hydrogen production from treated sludge in this study obtained VHPR of 13.4−24.6 mL h−1 L−1, which was higher than VHPR of 0.18−6.59 mL h−1 L−1 obtained in the study using alkaline-treated sludge as the substrate.23 Various studies have found the positive effect of nutrients, such as minerals, vitamins, and trace elements, on fermentative hydrogen production,50−54 concluding that the waste-activated sludge was rich in various nutrients, which can be helpful for fermentative hydrogen production.55 This phenomenon indicated that treated sludge can be a good source of nutrients for fermentative hydrogen production, which can be helpful to shorten the reaction time and enhance the cumulative hydrogen production. 3.3. Substrate Consumption. Substrate degradation was accompanied by fermentative hydrogen production, and the changes of SCOD, polysaccharide, and protein concentrations in all three fermentation groups were depicted in Figure 4. It can be seen that SCOD of all groups decreased. For the group using glucose as the substrate, glucose was consumed as the sole carbon source for fermentative hydrogen production and the degradation rate of 94.1% was achieved. Besides the utilization of glucose, a small increase of protein was detected, which may be due to the metabolism and growth of hydrogen producers during the fermentation process. A similar phenomenon was also observed.49 With regard to the group using wet oxidized sludge as the substrate, both polysaccharides and protein were decreased significantly with a degradation rate of 55.7 and 36.0%, respectively. This phenomenon was different with our previous study using γ-irradiated sludge as the substrate, in which little change was observed in the polysaccharide concentration after fermentation.16 A possible reason may be that γ irradiation treatment could cause excessive degradation of degradable polysaccharides, indicating that lowpressure wet oxidation was more preferable for degradable polysaccharide reservation during the sludge dissolution. When it came to the group using the mixture of the treated sludge and glucose, polysaccharides were degraded by 24.9% and higher protein degradation of 37.9% was obtained in comparison to the test using treated sludge as the substrate, showing that the addition of glucose can promote the utilization of protein. This may be because the more abundant degradable carbohydrate stimulated the growth of hydrogen producers, leading to both more hydrogen production and further substrate utilization. Furthermore, both test groups using treated sludge as the substrate showed more protein utilization than polysaccharides,
Figure 4. Substrate degradation in different test groups.
indicating that protein acted as the main carbon source for fermentative hydrogen production from waste-activated sludge. 3.4. VFA Formation. Substrates consumed during the fermentation system were turned into biogas, microorganisms, and soluble metabolites, among which the concentrations of soluble metabolites are useful indicators for monitoring the biological hydrogen production process.56 The major soluble metabolites formed during the fermentation process are VFAs, and the detected VFAs in this study are shown in Figure 5. It can be seen from Figure 5 that acetic acid, butyric acid, and propionic acid were detected in this study. For both tests using glucose and sludge as the substrate, only acetic acid and butyric acid were detected, among which acetic acid occupied 83.4 and 65.4% of total VFAs formed in two groups, indicating that the fermentation type of these two groups was dominated by acetate-type fermentation. With regard to the test using the D
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Figure 5. VFA formation during fermentation.
mixture of sludge and glucose as the substrate, acetic acid, butyric acid, and propionic acid were all formed, suggesting that it belonged to mixed-acid-type fermentation. It can be seen from this phenomenon that the metabolic pathway of microorganisms was significantly affected by the composition of the substrate, leading to varied fermentation types. A similar conclusion was also obtained by many other studies.16,57−59 Accompanied with the accumulation of VFA, pH of tests with glucose, the treated sludge, and the mixture of the treated sludge and glucose decreased from 7.0 to 5.2, 4.9, and 4.6, respectively. It has been reported that more acetic acid was formed when liquid pH was above 5.0, and the fermentation was dominated by acetate-type fermentation. When liquid pH was lower than 5.0, the metabolic pathway was dominated by mixed acid fermentation.58 3.5. Hydrogen Yield of Dark Fermentation. The hydrogen yield was estimated by dividing the amount of cumulative hydrogen production by the amount of SCOD consumed. For the test groups using the treated sludge, glucose, and the mixture of the treated sludge and glucose as the substrate, the hydrogen yield was 55.4, 90.7, and 81.9 mL/g of SCODconsumed, respectively. A literature review showed that the hydrogen yield of 15.0, 11.4, and 4.7 mL/g of COD was achieved from sterilization-, microwave-, and ultrasound-treated waste-activated sludge, respectively,55 and the hydrogen yield of 1.65−10.1 and 4.5−15.9 mL/g of COD was obtained from alkaline-treated and boiled sludge, respectively.23,60 A higher hydrogen yield of 40.3 mL/g of COD was obtained when the combination of freezing and thawing and sterilization treatment was used,60 indicating that the hydrogen yield varied with different pretreatment methods applied to waste-activated sludge. Thus, the higher hydrogen yield obtained in this study showed the advantage of low-pressure wet oxidation in treating waste-activated sludge for fermentative hydrogen production. The theoretical hydrogen production through acetate-type fermentation and butyric-type fermentation is 4 and 2 mol of H2/mol of glucose, respectively.56 Thus, the acetic acid/butyric acid ratio can be a good indicator for the hydrogen yield. Accordingly, we examined the correlation between the hydrogen yield with the ratio of acetic acid/butyric acid (Figure 6A). It can be seen that the hydrogen yield increased linearly with the increase of the acetic acid/butyric acid ratio, indicating that microbial metabolism was affected by substrate composition.16,56,61
Figure 6. Relationship between the hydrogen yield and the ratio of (A) acetate/butyrate and (B) polysaccharides/SCOD.
Because it is known that polysaccharides can be more easily used by microorganisms, we suspect that a higher hydrogen yield can be obtained from polysaccharide degradation. Thus, the polysaccharides/SCOD ratio of different substrates was examined in this study, and the relationship between the hydrogen yield and polysaccharides/SCOD ratio was depicted in Figure 6B. It can be seen that the highest hydrogen yield was obtained for the test using glucose as the substrate, while the lowest hydrogen yield was achieved for the test using the treated sludge as the substrate. The hydrogen yield was highly related with the polysaccharides/SCOD ratio of the substrate. Although studies have also shown the positive correlation between the hydrogen yield and the polysaccharides/SCOD ratio of the substrate,46 the linear relation has not been reported before. Because the number of samples in this study was too small, the linear relation between the hydrogen yield and the polysaccharides/SCOD ratio needs to be further verified.
4. CONCLUSION Low-pressure wet oxidation can be used for recovering the carbon source from waste-activated sludge for fermentative hydrogen production. After the treatment, increases of SCOD, polysaccharide, and protein concentrations present in the liquid phase by 2.0, 2.2, and 102.5 times were obtained, respectively. E
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Bio-hydrogen can be produced efficiently from the treated sludge, and the cumulative hydrogen production can be promoted through enhancing the C/N ratio in the substrate. Through a comparison of the hydrogen production using glucose, the treated sludge, and the mixture of glucose and the sludge as the substrate, the hydrogen yield showed a linear relation with the acetic acid/butyric acid ratio in soluble metabolites. Besides, the hydrogen yield can be promoted through enhancing the polysaccharides/SCOD ratio in the substrate.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research was supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT13026). The authors also thank the financial support provided by the National Natural Science Foundation of China (51338005).
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DOI: 10.1021/acs.energyfuels.6b01034 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.6b01034 Energy Fuels XXXX, XXX, XXX−XXX
