Article Cite This: Energy Fuels 2018, 32, 574−580
pubs.acs.org/EF
Fermentative Hydrogen Production Using Disintegrated WasteActivated Sludge by Low-Frequency Ultrasound Pretreatment Yanan Yin,† Guang Yang,† 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 Wastes Treatment, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: Fermentative hydrogen production was performed using waste-activated sludge pretreated by a low-frequency ultrasound. The experimental results showed that a low-frequency ultrasound can effectively solubilize the waste-activated sludge and release the organic matter for hydrogen production. The ratio of soluble chemical oxygen demand (SCOD)/total chemical oxygen demand (TCOD) achieved 20.2% after ultrasound pretreatment, and the increased SCOD was mainly comprised of dissolved polysaccharides. A hydrogen yield of 68.9 mL/g of SCOD was obtained when ultrasound-treated sludge was used as the substrate, and the linear relationship between the hydrogen yield and polysaccharide/SCOD ratio of the substrate was observed. Hydrogen production from ultrasound-treated sludge followed acetate-type fermentation.
1. INTRODUCTION Additionally, hydrogen can be easily converted to energy through combustion or fuel cells. Because the traditional hydrogen production methods are mostly high-energyconsuming, fermentative biohydrogen production owns more environmental benefits, especially when organic wastes are used as the substrate.1 Taking waste-activated sludge as the substrate for hydrogen production can achieve the dual benefits of waste management and clean energy generation. Aerobic biological treatment is the most widely used method for wastewater treatment. However, during the treatment process, only part of the organic pollutants in wastewater is mineralized into CO2 and H2O, and the rest is used to synthesize the microorganisms, which is also known as the waste-activated sludge.2 The escalation of wastewater all around the world caused increasing generation of wasteactivated sludge, which is becoming a serious environmental problem. At present, waste-activated sludge is mainly disposed of through landfill, incineration, and soil utilization. However, all of these methods have many limitations, such as high cost, source waste, second pollution, and the like. Furthermore, with the increasing requirement for the sustainable management, these traditional disposal options are becoming more and more inacceptable.3 Thus, the simultaneous minimization of the sludge amount and generation of valuable products through biological digestion attract attention all over the world.4−7 Waste-activated sludge is mainly composed of microbes, which can be a good organic source. Quite a few studies have explored energy as well as valuable chemical recovery from waste-activated sludge, such as biogas production,4−9 alcohol generation, volatile fatty acid derivation,10,11 and carbon source recovery for biological denitrification.12,13 Considering the environmental benefits of hydrogen, waste-activated sludge is also used as the substrate for fermentative hydrogen production.14 However, organic matter in waste-activated sludge is mostly encapsulated in microbial cells, which can hardly be used by hydrogen-producing microbes. Thus, it is necessary to take © 2017 American Chemical Society
some measurements to rupture the cell wall and release the nutrients into liquid solution. The treatment methods explored can be classified as mechanical, thermal, chemical, biological, and combined treatments.15−18 Commonly used methods include heat treatment, acid/base treatment, and the addition of enzymes.19 However, high temperature can usually lead to the formation of inhibitory compounds, which can constrain the microbial activity. The pH adjustment required after the acid/base treatment is neither economic nor environmentally friendly, which creates more work for the subsequent processing. Treatments with low energy input and little essential subsequent processes are preferred. Ultrasound is a sound wave with a frequency over 20 kHz. Ultrasound treatment is a kind of physical treatment without chemical addition and mild in reaction conditions; it owns both benefits of physical and chemical treatment methods.20 Microbubbles are formed when an ultrasound wave propagates in a liquid phase, and with the collapse of the bubbles, high localized pressure (180 MPa) and temperature (5000 K) as well as highly reactive radicals are generated. When a huge number of microbubbles collapse simultaneously, extreme shear forces are formed, which can disrupt the cell walls and membranes present in waste-activated sludge. As a physical process, the shear force disrupts the cell structure without forming secondary toxic compounds, the localized high temperature and pressure are helpful to break down the cell membrane, and the highly reactive radicals are capable of degrading the recalcitrant compounds. Studies have also found that shear forces were dominant when a lowfrequency ultrasound was applied, while oxidizing ability acted primarily when a high-frequency ultrasound was adopted.20 Tiehm et al.21 and Wang et al.22 found that the dissolution of waste-activated sludge was mainly due to the shear forces Received: October 24, 2017 Revised: December 12, 2017 Published: December 13, 2017 574
DOI: 10.1021/acs.energyfuels.7b03263 Energy Fuels 2018, 32, 574−580
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Energy & Fuels
at 36 °C. All batches were conducted in three replicates. The modified Gompertz equation was employed to describe the cumulative hydrogen production in batch tests30,31
generated during the ultrasound process. Le et al.23 found that 12 kHz always performs better than 20 kHz in solubilizing waste-activated sludge. Thus, a low-frequency ultrasound is preferred in sludge disintegration. Furthermore, less oxidative effects can help to protect the easily oxidized substances, such as polysaccharides, which is pretty useful for hydrogen production. Low-frequency ultrasound treatment has been widely used in enhancing the sludge dewaterability,24 removing hazardous materials,25 and releasing valuable nutrients.26 Zhu et al. found that soluble polysaccharides, soluble protein, and water removal efficiency of waste-activated sludge were enhanced after a low-frequency ultrasound treatment.27 Chowdhury et al. applied low-frequency ultrasonication in solubilizing waste-activated sludge, thus enhancing the anaerobic digestion process.28 However, few studies have applied a low-frequency ultrasound alone in recovering the carbon source from waste-activated sludge for fermentative hydrogen production. In this study, a low-frequency ultrasound was used to disintegrate the waste-activated sludge for fermentative hydrogen production. To make a comparison, 1 g/L glucose was added to enhance the polysaccharide/protein ratio and raw sludge was used as the control test.
H = P exp{− exp[R me(λ − t )/P + 1]} where P represents 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 is 2.71828. The values of P, Rm, and λ for each batch were fitted using MATLAB 7.0. 2.5. Analytical Methods. The volume of biogas produced was measured using the water displacement method at room temperature (25 °C). The fraction of H2 in biogas was determined using a gas chromatograph (model 112A, Shanghai, China), which 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. The carrier gas was pure argon, and the precolumn pressure was 0.2 MPa. The physicochemical characteristics of sludge measured in this study that include SS, VSS, TCOD, SCOD, soluble total phosphorus, and total nitrogen were all determined by standard methods.32 The pH value was measured by a pH meter (model 526, Germany). The protein content was measured by the modified Lowry method using bovine serum albumin as the standard. Polysaccharides were determined by the phenol sulfuric acid method using glucose as the standard. The volatile fatty acids (VFAs) were analyzed using an ion chromatograph (Dionex model ICS 2100) equipped with a dualpiston 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.33
2. MATERIALS AND METHODS 2.1. Inoculum Preparation. The seed sludge was obtained from a primary anaerobic digester of a municipal wastewater treatment plant (MWTP) located in Beijing, China. Volatile suspended solids (VSS) of the sludge were 2.42 g/L. To inactivate the non-hydrogen producers, seed sludge was pretreated by 5 kGy γ irradiation at a dose rate of 286 Gy/min at ambient temperature (around 25 °C), 60Co was used as the radiation source. Then, the pretreated seed sludge was pre-cultured at 35 °C for 48 h to enrich the hydrogen producers that survived after the radiation process.29 Composition of pre-culture medium contains 20 g/L glucose, 0.5 g/L yeast extract, 10 g/L peptone, and 10 mL/100 mL nutrient solution. Each liter of nutrient solution contains 40 g of NaHCO3, 5 g of NH4Cl, 5 g of NaH2PO4· 2H2O, 5 g of K2HPO4·3H2O, 0.25 g of FeSO4·7H2O, 0.085 g of MgCl2·6H2O, and 0.004 g of NiCl2·6H2O. 2.2. Feedstock. The feedstock was waste-activated sludge obtained from the secondary sedimentation tank of a MWTP located in Beijing, China. The collected sludge was settled about 2 h to yield solids content of 16.43 g/L suspended solids (SS) and 11.34 g/L VSS. The values of pH, total chemical oxygen demand (TCOD), and soluble chemical oxygen demand (SCOD) were 7.7, 18 638.2 mg/L, and 357.5 mg/L, respectively. The sludge was preserved in a 4 °C refrigerator until used. 2.3. Treatment of Waste-Activated Sludge. An ultrasonic cleaning bath (Shengda, Beijing, China) was used to treat the wasteactivated sludge. The bath has a fixed frequency of 40 kHz, power of 20 W/L, and temperature range of 25−100 °C. A total of 1 L of waste-activated sludge was put in a serum bottle and fixed in the center of the ultrasonic bath, with treatment lasting 60 min under 36 °C, and the treatment was in triplicate. 2.4. Biohydrogen Production. Batch experiments were conducted to examine the effects of a low-frequency ultrasound on hydrogen production from waste-activated sludge. Raw sludge was used as the control test, and to make a comparison, 1 g/L glucose was added to the pretreated sludge to adjust the polysaccharide/protein ratio. The 150 mL Erlenmeyer flasks with 100 mL working volume were used for the fermentations, which included 80 mL of sludge samples, 10 mL of seeding sludge, and 10 mL of nutrient solution mentioned above. The initial pH of the medium was adjusted to 7.0 with 5 mol/L HCl and 5 mol/L NaOH. Argon gas was passed through the sludge samples for 3 min to drive away dissolved oxygen and oxygen present in bottles. For the fermentation process, batches were placed in a reciprocal shaker with an agitation speed of 100 rpm
3. RESULTS AND DISCUSSION 3.1. Sludge Dissolution by Low-Frequency Ultrasound Treatment. Waste-activated sludge was disintegrated with the ultrasonic density of 73 kJ/g of total solids (TS). After treatment, TCOD was slightly decreased from 18 638.2 to 18 188.3 mg/L, which may be due to the oxidative and thermal effects of sonication.34 Some other studies have also observed a small reduction in TCOD of waste-activated sludge after the treatment process.35−37 The dissolved chemical parameters of raw sludge and sludge after ultrasound treatment are presented in Figure 1. It can be seen that SCOD of raw sludge was 375.5 mg/L, occupying only 1.9% of TCOD. After the ultrasound treatment, SCOD was increased significantly to 3672.9 mg/L, achieving the sludge solubilization (SCOD/TCOD) of 20.2%. The results
Figure 1. Sludge dissolution by low-frequency ultrasound treatment. 575
DOI: 10.1021/acs.energyfuels.7b03263 Energy Fuels 2018, 32, 574−580
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Energy & Fuels indicate that most nutrients of waste-activated sludge were insoluble before the pretreatment, which mainly include the organic matter encapsulated in microbial cells and the extracellular polymeric substances (EPS) attached to the cell surface. Low-frequency ultrasound treatment can disrupt the sludge floc structure and release the organic material to the liquid phase efficiently. Polysaccharides and protein are two major carbon sources for fermentative hydrogen production, and they were increased from 315.5 to 2094.4 mg/L and from 32.2 to 1199.4 mg/L, respectively. It is worth mentioning that more polysaccharides were released and accumulated than protein. A similar phenomenon was also observed by Guo et al.; they obtained 1.4 times more polysaccharides than protein after ultrasound treatment.38 However, many studies have shown more protein accumulation other than polysaccharides when other treatment methods are used. For example, in our previous study, the soluble protein concentration was 12.0, 9.6, 3.3, and 4.8 times that of polysaccharides when acid, alkaline, γ radiation, and hydrothermal treatments were used, respectively.35,39 Guo et al. also achieved higher protein solubilization with 2.0 and 1.3 times that of polysaccharides by sterilization and microwave treatment.38 This may be because the formed polysaccharides are more easily decomposed than protein in severe conditions, leading to a higher accumulation of protein than polysaccharides during the treatment process.40,41 It is known that polysaccharides can be more easily used by microbes and a higher hydrogen yield can be obtained from a higher polysaccharide/protein ratio.42 Thus, a higher accumulation of polysaccharides is preferred for hydrogen production, indicating the advantage of low-frequency ultrasonication in treating waste-activated sludge for hydrogen production. Besides the carbon sources, nitrogen and phosphorus were also released to the liquid phase, which are necessary elements for microbial metabolism.43 After sonication, nitrogen was increased from 5.0 to 33.8 mg/L and phosphorus was increased from 22.1 to 44.6 mg/L. A little decrease in pH was observed from 7.7 to 7.2 after the treatment, which may be due to the VFA formation accompanied by carbohydrate reduction in the treatment process.41 A similar phenomenon was also observed by Guo et al.38 In general, low-frequency ultrasound treatment can be a good treatment method in recovering the carbon source from waste-activated sludge for fermentative hydrogen production. 3.2. Biohydrogen Production. Hydrogen production processes were conducted in batch tests. To make a comparison, 1 g/L glucose, raw sludge, ultrasound-treated sludge, and a mixture of 1 g/L glucose and treated sludge were used as the substrate. Figure 2 shows the cumulative hydrogen production from different substrates. It can be seen that the hydrogen production process of all three groups was terminated in 18 h. The highest cumulative hydrogen production of 24 mL/100 mL was obtained by a mixture of treated sludge and glucose, followed by ultrasound-treated sludge with 20 mL/100 mL, glucose with 15 mL/100 mL, and raw sludge with 10 mL/100 mL. It can be seen that ultrasound treatment enhanced the hydrogen production from waste-activated sludge 2 times, indicating that ultrasound treatment was efficient in increasing the degradability of waste-activated sludge. However, the addition of glucose showed no increase on cumulative hydrogen production; hydrogen produced by the mixture (24 mL/100 mL) was much less than the sum of hydrogen
Figure 2. Cumulative hydrogen production with different substrates.
produced by treated sludge (20 mL/100 mL) and glucose (15 mL/100 mL). This phenomenon was different from our previous study that used γ irradiation as the treatment method. In our previous study, a mixture of glucose and treated sludge showed a higher cumulative hydrogen production over the sum of hydrogen production separately.38 Possible reasons are as follows: (1) A high polysaccharide/protein ratio was achieved by ultrasonication-treated sludge; thus, little promotion in hydrogen production was obtained by the addition of glucose. Chen et al. found that hydrogen production was improved with the increasing polysaccharide/protein ratio in the range of 0.2−5.0 and then decreased with a further increase of the polysaccharide/protein ratio.44 (2) Ultrasound treatment was unable to kill or inhibit the non-hydrogen producers present in waste-activated sludge, which may consume the carbohydrates and lead to the production of byproducts, such as VFAs and CO2, other than hydrogen. On the basis of the data shown in Figure 2, the kinetic parameters of the fermentative hydrogen production process were assessed by the modified Gompertz equation (Table 1). Groups using treated sludge as the substrate all showed a shorter lag time (λ) and higher maximum hydrogen production rate (R) than the test with pure glucose as the substrate. A possible reason was the various nutrient elements present in waste-activated sludge, which can stimulate microbial growth and metabolism, leading to a faster hydrogen production process.45 Many studies have tried to optimize the hydrogen production process through the addition of nutrients, such as minerals, vitamins, and trace elements. For example, minerals, such as ion and nickel, are vital elements in hydrogenase. Wang et al. and Taherdanak et al. explored the effects of ion and nickel on hydrogen production and showed close correction between the mineral concentration and hydrogen yield.46−48 Mudhoo et al. concluded that the heavy metals had both stimulatory and inhibitory effects on the fermentation process.49 Kim et al. found that sewage sludge contained abundant Fe, Ca, N, and P, which were essential elements for microbial growth and hydrogen production,50 which may be the reason for the shorter lag time shown in sludge-contained tests. Otherwise, hydrogen production from raw sludge showed the lowest maximum hydrogen production rate, which may be because the insufficient carbohydrates present in the liquid phase inhibited microbial metabolism. 576
DOI: 10.1021/acs.energyfuels.7b03263 Energy Fuels 2018, 32, 574−580
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Energy & Fuels Table 1. Kinetic Parameters Estimated by the Modified Gompertz Model sample
glucose
raw sludge
ultrasound-pretreated sludge
ultrasound-pretreated sludge + glucose
P (mL) λ (h) R (mL/h) R2
15.58 5.37 1.38 0.945
10.16 0 1.07 0.957
20.13 2.11 2.878 0.997
22.25 3.126 2.93 0.998
3.3. Substrate Degradation. Figure 3 depicts the changes of SCOD, polysaccharide, and protein concentrations of all
formed during the fermentation process. The concentration of soluble protein in the group with raw sludge as the substrate also showed a slight increase. This was caused by the hydrolysis of raw sludge accompanied by microbial metabolism. Cai et al. observed the increase in both protein and carbohydrates in the hydrogen production from raw sludge,51 indicating that the sludge digestion was predominant during the fermentation process, with raw sludge as the substrate. Besides the raw sludge, a similar phenomenon was also observed in tests with some treated sludge; Guo et al. detected the protein increase in fermentative hydrogen production from sterilization and ultrasound-treated sludge.51 However, for the test with sludge treated by alkaline, chlorine dioxide, homogenization, ozone, and ultrasound, no increase in soluble organic matter was observed,13,51,52 indicating that the hydrolysis effect depends upon the sludge treatment process. The efficient treatment methods can make the hydrolysis effects during the fermentation process undetectable. For the tests with ultrasound-treated sludge as the substrate, both polysaccharides and protein showed a significant decrease after fermentation, indicating that the low-frequency ultrasound efficiently disintegrated sludge in this study. It can be seen that the addition of glucose reduced polysaccharides with the SCOD degradation rate, which was different with our previous study. Many studies have observed the increase of sludge degradation through carbohydrate substrate addition. Our previous study with γ-irradiated and hydrothermal-treated sludge as the substrate showed that the addition of glucose significantly enhanced both the sludge degradation rate and protein utilization.39 Chen et al. conducted hydrogen production from the co-digestion of waste-activated sludge and starch and found that the substrate degradation rate increased with the increase of the carbohydrate/protein ratio when the ratio was less than 5, but a further increase of the carbohydrate/protein ratio resulted in the decrease of the degradation ratio.53 Thus, the decrease of the SCOD degradation rate in this study may due to the high carbohydrate/protein ratio of ultrasound-treated sludge, indicating that ultrasound-treated sludge can be used directly as a substrate for hydrogen production, sparing the substrate adjustment process. The hydrogen yield was estimated by dividing the cumulative hydrogen production by consumed SCOD. For the test groups with glucose, raw sludge, treated sludge, and a mixture of treated sludge and glucose as the substrate, the hydrogen yields of 159.4, 41.2, 68.9, and 101.1 mL/g of SCOD were obtained, respectively. Because some studies have shown the effect of polysaccharide/protein on hydrogen production,53 we examined the relationship between the hydrogen yield and polysaccharide/SCOD of different test groups. As shown in Figure 4, the hydrogen yield showed a linear relationship with the polysaccharide/SCOD ratio of the substrate. The positive correlation between the hydrogen yield and polysaccharide ratio of the substrate has been observed by many studies,44 but the linear relationship between the hydrogen yield and
Figure 3. Substrate degradation in different test groups.
four fermentation groups. For all of the test groups, hydrogen was produced and SCOD was decreased, indicating that soluble organic matter was used for hydrogen production. For the test with glucose as the substrate, over 94% of glucose was consumed. Besides, a little increase of protein was observed, which may be due to the extracellular polymers 577
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ozone and ultrasound-treated waste-activated sludge as the substrate, and the concentration of butyric acid showed 3−8 times that over acetic acid.52 However, studies found that the fermentation type can be changed through adjusting the C/N ratio. Wan et al. observed acetate-type fermentation when the hydrolysate of waste-activated sludge was used as the substrate.55 Chairattanamanokorn et al. obtained the acetatetype fermentation when the C/N ratio was adjusted to 40.56 In our previous study, the change of fermentation type from butyrate to acetate was observed when glucose was added to γirradiation-treated sludge.35 Thus, it can be seen that acetatetype fermentation was more likely to happen when polysaccharides were used as the main carbon source for hydrogen production.
4. CONCLUSION A low-frequency ultrasound is rarely used in energy recovery from waste-activated sludge for hydrogen production. In this study, fermentative hydrogen production was successfully conducted using low-frequency-ultrasonication-treated sludge as the substrate and a hydrogen yield of 68.9 mL/g of SCOD was obtained. The results showed that the hydrogen yield can be enhanced by increasing the polysaccharide/SCOD ratio of the substrate. Follow-up studies can explore the combination of low-frequency ultrasonication with other treatment methods, such as enzyme and heat treatments in solubilizing waste-activated sludge; operational conditions for fermentative hydrogen production can be further optimized to enhance the hydrogen production efficiency.
Figure 4. Correlation between the polysaccharide/SCOD ratio and hydrogen yield.
polysaccharide ratio has not been widely reported. In our previous study that explored the sludge solubilized by lowpressure wet oxidation, a linear relationship between the hydrogen yield and polysaccharide/SCOD ratio was also observed.39 3.4. VFA Formation. Substrate degradation and VFA formation were accompanied by hydrogen production during the fermentation process. Because different metabolism pathways lead to different kinds of VFA formation, the components of VFAs present in the terminal liquid phase are considered as useful indicators for monitoring the biological hydrogen production process.54 The major VFAs formed in this study were acetic and butyric acids, and their concentrations were shown in Figure 5.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-10-62784843. Fax: +86-10-62771150. Email:
[email protected]. ORCID
Jianlong Wang: 0000-0001-9572-851X 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 (51178241 and 51338005).
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REFERENCES
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Figure 5. VFA formation during fermentation.
It can be seen that acetic acid occupied 83.4, 100, 77.6, and 66.9% for the test groups with glucose, raw sludge, ultrasoundtreated sludge, and a mixture of treated sludge and glucose, respectively. Acetic acid was predominant in all test groups, indicating that the hydrogen production processes were dominated by acetate-type fermentation. However, butyratetype fermentation has been widely observed in studies using treated waste-activated sludge as the substrate. Yang et al. used 578
DOI: 10.1021/acs.energyfuels.7b03263 Energy Fuels 2018, 32, 574−580
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