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Insight into mass transfer by convective diffusion in anaerobic granules to enhance biogas production
Accepted Manuscript Title: Insight into mass transfer by convective diffusion in anaerobic granules to enhance biogas production Authors: Zohaib Ur Rehman Afridi, Jing Wu, Zhi Ping Cao, Zhong Liang Zhang, Zhong Hua Li, Souhila Poncin, Huai Zhi Li PII: DOI: Reference:
S1369-703X(17)30196-1 http://dx.doi.org/doi:10.1016/j.bej.2017.07.012 BEJ 6756
To appear in:
Biochemical Engineering Journal
Received date: Revised date: Accepted date:
2-5-2017 11-7-2017 27-7-2017
Please cite this article as: Zohaib Ur Rehman Afridi, Jing Wu, Zhi Ping Cao, Zhong Liang Zhang, Zhong Hua Li, Souhila Poncin, Huai Zhi Li, Insight into mass transfer by convective diffusion in anaerobic granules to enhance biogas production, Biochemical Engineering Journalhttp://dx.doi.org/10.1016/j.bej.2017.07.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Insight into mass transfer by convective diffusion in anaerobic granules to enhance biogas production Zohaib Ur Rehman Afridia, Jing Wua*, Zhi Ping Caoa, Zhong Liang Zhanga,b, Zhong Hua Lia, Souhila Poncinc, Huai Zhi Lic
a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of
Environment, Tsinghua University, Beijing 100084 P.R. China b c
Jiansu Institute of Urban Planning and Design, Nanjing, Jiangsu 210036 P.R. China
Laboratory of Reactions and Process Engineering, Université de Lorraine, CNRS, 1, rue
Grandville, BP 20451, 54001 Nancy cedex, France *Corresponding author: [email protected] Phone: +86-10-62789121 Fax: +86-10-62785687 Graphical abstract
1
Highlights
Mass transfer was characterized as convective and molecular diffusion for large, medium and small granules.
The convective diffusion rates were 2-3 magnitudes greater than molecular diffusion rate.
Biogas production rate was enhanced up to 1.3-4 folds by convective diffusion.
Internal structure of anaerobic granule can significantly affect mass transfer process.
The study could facilitate understanding the mass transfer process within granules to improve biogas production.
Abstract Mass transfer in anaerobic granule is crucial for efficient biogas production, but so far the fundamental understanding remains poor. This study aims at gaining an insight into convective diffusion to enhance mass transfer within small (0.5-1 mm), medium (1.5-2 mm) and large (3-3.5 mm) granules. The convective diffusion rates (FCD) of granules at a superficial liquid velocity of 3 m/h were 2-3 magnitudes greater than molecular diffusion rates (FMD), and significantly enhanced the biogas production rate of small, medium and large granules by 4, 1.5 and 1.3 folds. The granules were permeable in nature as fluid collection efficiency (η) of small, medium and large granules of 0.97, 0.89 and 0.58 indicated, and were of cluster-cluster formation as fractal dimension (Df) revealed. The large granules possess best mass transfer condition and highest biogas production rate because they had highest FCD due to the biggest permeable area despite lowest η. This work for the first time elaborates the important roles of internal structures in terms 2
of permeability and fractal dimension on the convective mass transfer in anaerobic granules, and it also highlights the importance of molecular diffusion. The results could facilitate upgrading the understanding of mass transfer process in anaerobic granules to enhance biogas production. Keywords: Anaerobic granule; Biogas production; Convective diffusion; Molecular diffusion; Mass transfer.
1. Introduction Biogas is a renewable energy that can provide methane to mitigate the fossil energy crisis [1, 2]. Biogas production by anaerobic digestion is drawing significant attention as a progressive sustainable energy technology [3, 4]. At present, granule-based bioreactors are commonly used to produce biogas and treat a wide variety of high-strength organic wastewaters [5-7]. The overall efficiency of anaerobic reactors associates with the mass transfer within granules as the granule is the key engineering component of the reactor. Mass transfer within granule is attributed to play a major role to produce biogas [8-10]. However, up till now the mass transfer process within the granule remains unknown due to relatively small size (0.5-3.5 mm) and complex internal structure of the granule which is comprised of micro and nanoscale channels [11]. Nonetheless, in decades of studies, researchers have tried to improve the biogas production by optimizing several operational parameters of the anaerobic reactors. YingYu, et al. [12] found that in the UASB-AnMBRs, biogas yield increased from 0.062 to 0.12 L/g CODremoved when HRT was decreased from 10 h to 5.5 h. The enhanced gas production was attributed to mass transfer in biomass at a higher upflow velocity. Others correlated the increase in biogas production with parameters such as OLR, HRT, pH, temperature and upflow
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liquid velocity, etc. [13-17]. However, studies could not explain the fundamental mass transfer mechanism within granule which affected the biogas production. Mass transfer in granule could not be entirely explained due to smaller size, complex internal structure and most importantly the contradicting results about nature of granule. Most of the researchers thought granule was impermeable and considered molecular diffusion as the sole mass transfer mechanism within granules [18, 19]. Gonzalez-Gil, et al. [20] investigated the kinetics and mass transfer phenomena in anaerobic granules and concluded that mass transfer was based on molecular diffusion as there was no evidence of convective flow inside the anaerobic biofilms. Mass transfer in anaerobic biofilm is different than granular sludge. But Hulshoff Pol [21] and Li and Yu [22] inferred that mass transfer process would be convective in nature due to their porous nature Winkler [23] confirmed the permeability in granules using diffusion of salt. Chu and Lee [24] investigated the fluid flow and mass transport in pores and identified that intra-floc transport processes are carried by convective diffusion. Consequently, a knowledge gap exists about the fundamental understanding of mass transfer in anaerobic granules. Thus, it is of major importance to get a better understanding to achieve higher biogas production. The objective of this study was to investigate the mass transfer process within the granules especially the convective diffusion for different size granules. Furthermore, impacts of the internal structure of granules in terms of fractal dimension, fluid collection efficiency and permeable area on mass transfer within granules was studied. In order to understand the enhancement of convective diffusion on biogas production, molecular diffusion was also investigated in this study. To our best knowledge, it is the first time to investigate the effect of the internal structure of granules on mass transfer within granules. The results would facilitate understanding the anaerobic process and improving the performance of the granule-based reactors. 4
2. Materials and methods 2.1 Experimental setup and operating conditions
A transparent upflow micro-reactor was used in this study. It was made of polymethyl methacrylate (PMMA) with the inner working section of width × length× thickness 7.5 cm × 19.0 cm × 0.5 cm. The experimental setup was similar to our previous study [11] and an external peristaltic feed pump (Longer, BT100-2J, China) was installed to maintain a constant superficial liquid velocity of 0 m/h or 3.3 m/h as shown in Fig. 1. As under 0 m/h, molecular mass transfer is the sole mass transfer mechanism in the granule while at 3.3m/h both molecular and convective diffusion will play a role in mass transfer. In our previous study, Zhang, et al. [25] investigated the influence of different superficial liquid velocities in the upflow reactor and found that the optimum superficial liquid velocity for upflow reactor was 3.3 m/h which resulted in highest biogas production and bed expansion without sludge washout. Therefore, superficial liquid velocity was set at 3.3 m/h for this study. The feed was stored in a tightly sealed steel tank with a volume of 25 liters. The composition of synthetic feed was as same as that used in the previous work [8] and Chemical Oxygen Demand (COD) remained relatively stable around 6000 mg/L as listed in table 1. For both 0 m/h and 3.3 m/h experiments, the synthetic feed solution was changed after every 12 hours to ensure that the concentration remained 6000 mg/L in the feed storage tank during the experiment. The pH of the feed was set between 7.2± 0.2 at the start of each experiment using NaHCO3 as alkalinity buffer during the test. The temperature of the reactor was controlled at 37± 1 °C by a thermal heater with a digital controller. Fig.1
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The anaerobic granular sludge was from a full-scale internal circulation anaerobic reactor treating starch wastewater in northern China. The granules were divided into three groups of large, medium and small sizes with a diameter of 3-3.5 mm, 1.5-2 mm and 0.5-1 mm using a microscope (Motic Group, China) equipped with a digital camera. The granule diameter was measured with a quantitative image analysis program (Motic Images Advanced 3.2, China). The corresponding average diameter of large, medium and small groups was 3.11± 0.23, 1.90±0.26 and 0.77± 0.17 mm. The granules were divided into three discreet groups because in our previous study [25] we found that granular sludge with different size had different internal structure, bioactivity and physical features. Before the test, anaerobic granules were preserved in glass bottle containing oxygen-free water with low COD concentration (200 mg/L) at 37 °C in a thermal bath. For each experiment, the micro-reactor was filled up to 30% (v/v) with 3-3.5 mm, 1.5-2 mm, and 0.5-1 mm granules, respectively. For experiments at 0 m/h, the granules were cultured at COD concentration of 6000 mg/L and at 37℃ for several days to achieve the steady state. Under the steady state, the biogas production rate was maximum and stable. For experiments at 3.3 m/h, the preserved granules were filled into the reactor and continuously fed at superficial upflow velocity of 3.3 m/h with a synthetic solution of COD 6000 mg/L (20-25 days approximately) until its performance achieved steady state. All experiments were run in triplicate. At the end of each experiment, granules were observed under the microscope and no sign of aggregation was recorded as granules were already matured in the full scale upflow reactor. A high-speed digital camera (Cam Record CL600 Optronis GmbH, Germany) equipped with a high magnification lens was utilized to record the produced biogas bubbles in the micro-reactor. In this work, a frame rate of 500 fps and an exposure time of 1/500 s were used. The camera resolution was 1280 x 1024 pixels under the monochrome condition. The necessary illumination 6
was provided by a halogen light whose intensity was adjusted to 800 W. The image sequences obtained were then used to measure the diameter and the number of biogas bubbles.
2.2 Experimental analysis
2.2.1 Biogas production
The biogas production rate was calculated using a high-speed camera. The recordings were made during a period of time (typically 10 min) after every two hours during a period of 24 h for both 0 m/h and 3.3 m/h experiments. The size of biogas bubbles was determined using a quantitative image analysis software (Motic Images Advanced 3.2, China). The volume of bubbles was estimated assuming that the bubbles were spherical because the bubble shape was quite circular due to small size. Then, the biogas production rate at that moment was calculated according to the volume of all bubbles recorded during that period.
2.2.2 Fractal dimension of granule
Fractal dimension (Df) is an effective tool to characterize the sedimentation and structural properties of biological aggregates [26]. The flocs and granules are of the multi-layered structure, and these aggregates usually follow a fractal scaling relationship of Wd ∼ dDf, where Wd is the dry weight of aggregate (µg), and d is the diameter of the aggregate (mm) [27]. Df is the fractal dimension which can be calculated from the slope of a log-log plot of the dry mass and size of the aggregates. In this study, the granules were dried at 30 oC for 24 hours on a pre-weighted membrane filter (pore size 0.4µm) [28]. The dry weight of individual granules was measured using a microbalance (Metler Toledo, USA).
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2.2.3 Permeability of granules The permeability of granular sludge in terms of fluid collection efficiency (η) and permeability area (k) is important factor for the mass transfer in granules. η is defined as the ratio of the interior flow passing through the aggregate to the flow approaching it. A series of mathematical formulations have been described for calculating the permeability area of an aggregate (k) [29] as described in Equations 1-4 below.
d 2 3 - 4.5 4.5 5 - 3 6 k 18 3 (3 2 5
where 3 1-
(1)
(2)
The correlation (δ) as proposed by Happel provides a more realistic estimate of the intra-aggregate permeability for immobilized biological flocs. The internal permeation (ξ) of these granules can thus be estimated from the k-relationship via the correlation [30].
d 2 k
(3)
From the value, the fluid collection efficiency of the granule (η) can be calculated as follows [31]:
9 tanh
2 3 tanh 3
(4)
2.2.4 Mass transfer in anaerobic granules
Both the molecular and convective diffusion contribute to the mass transfer in the granule [11, 32]. In the absence of external liquid flow, molecular diffusion is the predominant mechanism. 8
However, the convective diffusion always occurs because of the constant liquid flow in an anaerobic reactor [33]. In order to study the convective diffusion, the molecular diffusion was calculated at a superficial liquid velocity of 0 m/h as control. The convective diffusion at superficial liquid velocity of 3.3 m/h was studied. The mathematical model for internal mass transfer was based on certain assumptions. 1. The large, medium and small granules are spherical in shape and have a homogeneous biofilm of uniform thickness. 2. The concentration of the reactor bottom is almost the same as the influent. 3. The mass transfer is defined as the diffusion within the granules. Diffusion within the granules will be the rate-limiting step rather than external mass transfer i.e. Substrate transfer from the bulk solution into the granule. 4. The wall effect is negligible in the micro-reactor. In our previous study on hydrodynamic
conditions in the micro-reactor, it was concluded that shear generated around the granule was very low at various upflow velocities. Therefore, in the micro-reactor it can be ignored. The molecular diffusion rate (FMD) within a granule was calculated using the modified Fick’s law as follows [11]: FMD 4 r 2 DM C
(5)
where r is the radius of the granule (cm), DM is the diffusivity of the substrates in water (cm2/s), C is the substrate concentration (mg/L), and ε is dimensionless porosity. DM of glucose is 0.94 x 105
cm2/s [34]. The porosity (ε) of each group of granules was obtained using Hg- porosimeter
(AutoPore IV 9500, USA) and the granule samples were prepared by following [11]. Φ is the assumed diffusion layer from the surface into the granule (cm). 9
The mathematical equation for the convective diffusion rate (FCD) is defined [33] as: FCD =2πr 2 ηCU
(6)
where 𝜂 is the granule fluid collection efficiency (dimensionless) and U is superficial liquid velocity in the reactor (cm/s). 2.2.5 Other analytical methods Hg- Porosimeter (AutoPore IV 9500, USA) was used to measure the porosity and density of granules. COD was measured with the COD meter (Hebei Huatong Co., YL-1A, China. Volatile suspended solid (VSS) was measured with the standard weight methods [35].
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Results and discussion
3.1 Specific biogas production rate under superficial liquid velocity at 0 and 3.3 m/h
The biogas production rate corresponding to liquid velocities at 0 m/h and 3.3m/h can indicate the mass transfer inside the granule. Fig.2 illustrates a comparison of biogas production for granules of different size under the superficial liquid velocity of 0 m/h and constant superficial liquid velocity of 3.3 m/h. In both cases, the specific biogas production rate exhibited a positive relationship with the granule size. At a superficial liquid velocity of 0 m/h, a maximum specific biogas rate, i.e., 0.031 m3/kgVSS/d was observed for large granules while the production rate of the medium and small granules was 0.016 and 0.006 m3/kgVSS/d respectively. On the other hand, the biogas production rates for large, medium and small granules were 0.070, 0.040 and 0.030 m3/kgVSS/d under constant superficial liquid velocity of 3.3 m/h, being at least 1.3-4.0 folds higher than those of 0 m/h. The higher biogas production under constant superficial liquid velocity 10
of 3.3 m/h indicates faster mass transfer. At 0 m/h, molecular diffusion was dominant mass transfer mechanism which is much slower than convective diffusion. Thus a lower biogas production rate can be observed for different granules.
Fig. 2 3.2 Permeability of anaerobic granules
Due to the size variation and complex internal structure, little literature is available about the role of permeability on mass transfer process in anaerobic granule [36]. In our previous study, granules were found to be porous in nature [11] and the porosity increased with the size of the granule ranging from 18.7 to 40.1%. This indicates that granules would be permeable. In this study, the permeability of anaerobic granules was verified by both experimental and mathematical approaches. The fractal dimension (Df) can provide information about the internal structure of granules, whereas the fluid collection efficiency (η) is considered as a mathematical approach to verify the permeability of granules. Furthermore, permeable area (k) is studied. 3.2.1 Fractal dimension of anaerobic granules
Totally 150 anaerobic granules were analyzed for the fractal dimension (Df) analysis. The dry weight of individual granule of the small, medium and large granules varied between 35-107 µg, 133-467 µg, and 643-1692 µg respectively. The fractal dimensions were obtained based on the slopes of the regression lines of the logarithmic relationship between the dry mass and size. The Df of small, medium and large granules were 1.96, 2.80 and 2.88 as shown in Fig. 3.
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Fig.3 Table 2 lists the Df of granular sludge in different studies. The Dfs of granules in our study were close to previous studies.
Gmachowski [38] reported that the internal geometry of granules has a great influence on the mass transfer process. The fractal dimension of aggregates can describe the internal geometry of granules which can indicate the compactness of granule and influence the mass transfer process of granules [39]. Li and Logan [29] and Logan [40] reported that aggregates with Df values between 1-3 were compact and formed via cluster–cluster formation. Hence, anaerobic granules of this study were of compact and dense fractal features with a cluster-cluster formation scheme. This type of structure is of permeable nature and furthermore, it is good for interior mass transfer as it can provide vacancy to allow the fluid flow through the granules. 3.2.2 Permeability of granules The fluid collection efficiency (η) is a key parameter to indicate the permeability. η of all granule groups were calculated by Eq. 1-4. The calculated collection efficiencies were inversely related to the size of the granules ranging from 0.99-0.97, 0.98-0.91 and 0.78-0.62 for small, medium and large granules respectively (Fig. 4). The collection efficiency of small granules was highest as compared to the medium and large granules with an average value of 0.97±0.007, 0.89±0.019 and 0.58±0.112 respectively.
Fig. 4 12
Anaerobic granules in this study had higher permeability as compared with the previously reported granules. Previous studies indicate that anaerobic granules have a wide range of fluid collection efficiency ranging from 0.08 to 0.83 as summarized in Fig. 4. Li and Logan [29] confirmed the permeability of granules with η of 0.65. Zhang, Li, Oh and Logan [30] reported very low fluid collection efficiency of 0.083 for latex microspheres granules of size between 0.5-2 mm. Mu, et al. [41] and Mu, Yu and Wang [41] reported that η of the anaerobic granular sludge of 0.4-3 mm varied between 0.01-0.4. In this study, the fluid collection efficiency of granules ranging from 0.58-0.97 indicated the permeable nature of anaerobic granules. This nature allows the substrate to pass through the granule to enhance the mass transfer. 3.3
Molecular and convective diffusion rates in anaerobic granules
Mass transfer occurring in anaerobic granules is a complex phenomenon. The difference in biogas production rate under static and constant upflow velocity condition highlighted the existence of convective diffusion in the granule. Fig. 5 shows the molecular and convective diffusion rates for different size granules. The convective diffusion rate were 2-3 magnitude greater than molecular diffusion rates. The former ranged for small, medium and large granules between 2x10-3–1x10-2, 2x10-2–5x10-2 and 3.5x10-2 –7x10-2 mg/s, while the latter was 6.5x10-6 – 1.45 x10-5, 2 x10-5 – 3.3 x10-5 and 3.7 x10-5 – 4.87 x10-5 mg/s respectively. The results indicate a positive relationship between the diffusion rate (of both molecular and convective) and the granule size for all three granule groups. Overall, large granules (3-3.5 mm) had highest mass transfer rates whereas small granules (0.5-1) had least mass transfer rates.
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Fig. 5 The permeability of granule can influence the convective mass transfer rates thus to increase or limit the biogas production rate. In this study, Df of small granules was 1.98 which was comparatively less compact than large granules which had Df of 2.88. This indicates that in small granules the vacancy between clusters would be greater, therefore, for same permeability area more substrate will flow into the granules. The highest η of small granules affirmed that highest ratio of the substrate on granular surface could pass through the small granules thus biogas production rate increased most (by 4 folds). On the other hand, large granules being more compact had lowest η and biogas production was increased by 1.3 folds. Snidaro, et al. [42] reported that the permeability of flocs decreased when the fractal dimension was larger than 2, and our results are in agreement with the reported results. The η of granules decreases as the size of the granule increases. This possibly derived from the increase in density of granules along with size. The density of small, medium and large granules were 1.15, 1.20 and 1.90 g/mL, respectively. The large granules are located at the bottom of the upflow reactor where nutrient condition is quite good. For this reason, large granules have better growth condition and become dense in nature. Secondly, granules also get internally compressed during the process of biogas production. In our previous study we found that there are two channel systems inside the granule i.e, small (nano level) channels which merge into big (micro level) channels. During the biogas production process, internal pressure is transferred from nano to micro channels which compresses the internal layers and granules become dense in nature [11]. It is worth noting that large granules possess best mass transfer condition and highest biogas production rate because they had highest FCD despite lowest η. This was possibly due to the fact that large granules had the largest permeable area (k) of 6.17 mm2, while the small ones were of 14
only 0.02 mm2 as shown in Fig. 6. The results indicate that k increased along the granule size thus more area was available for substrate diffusion in large granules as compared to medium and small granules. k affects the overall mass transfer rates in the granule. Thus for the large granules, the substrate on granular surface going into the granules by convective flow was most the although the ratio of substrate going into the granules was lowest (η was lowest). This highest uptake of substrate in large granule leads to highest biogas production.
Fig. 6 Interestingly, the results also show that the molecular mass transfer cannot be neglected although it is a slow process. The importance of molecular diffusion was highlighted as even though the convective diffusion rate was 2-3 magnitudes faster than the molecular diffusion but the biogas production rate increased only 1.3-4 folds under constant superficial liquid velocity of 3.3m/h (Fig 2). This indicates that convective diffusion is not the sole mass transfer mechanism in the granules under liquid flow condition whereas molecular and convective diffusion co-exist in the granules. Overall, mass transfer by convective diffusion enhanced the biogas production in the anaerobic granule.
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Conclusion
In this study, convective mass transfer within different sized granules was investigated. It increased the biogas production rate by 1.3-4 folds. It closely related with the internal structure of granule. Overall, the anaerobic granules were permeable in nature and allowed fluid flow into the granule. Among all granules, the large granules exhibited well-developed mass transfer conditions for fastest diffusion rate due to largest permeable area for highest substrate uptake. This study 15
represents a step forward toward understanding the mass transfer phenomena of anaerobic granular sludge, and suggests useful information for the design and operation of anaerobic bioreactors, such as the intermittent high upflow velocity by recirculation and increase the turbulence degree by optimizing reactor inner structures rate in anaerobic granular sludge reactor.
Acknowledgments The financial support provided by the Natural Science Foundation of China (91334112, 51678338) and National Major Science and Technology Project about Water Pollution Control (2014ZX07114001) are gratefully acknowledged by Chinese authors. French authors acknowledge the assistance for this study from the French Agence Nationale de la Recherche (PRC CNRSNSFC).
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List of Figures Fig.1: Schematic view of experimental setup Fig.2: Biogas production under superficial liquid velocities of 0 and 3.3 m/h Fig.3: Fractal dimension of (a) small, (b) medium and (c) large granules. Fig.4: Fluid collection efficiency of granules Fig.5: Molecular diffusion rates and convective diffusion rates at liquid velocity of 3.3 m/h of granules. Fig. 6: Permeable area of anaerobic granules
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Fig. 1
23
Fig. 2
24
Fig. 3
25
Fig. 4
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Fig. 5
27
Fig. 6
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Table 1: Synthetic substrate composition Component
mg·L−1
Macronutrient glucose
6500
urea
170
K2HPO4
72
yeast
0.128 mL (1:1 with water)
Micronutrients MgCl2
156.44
FeSO4
27.5
NiSO4
9.32
CoCl2
0.94
NH4MoO3
0.34
ZnSO4
0.82
MnCl2
0.22
CuSO4
0.12
Alkalinity buffer NaHCO3
29
Table 2: Fractal dimension of various granules Sludge
Average Size (mm)
Df
Reference
Granular sludge
0.7-3.1
1.96 - 2.88
This study
Granular sludge from UASB
1-3.5
2.79 ± 0.3
[37]
Aerobic granular sludge
1.8-3.8
1.80
[28]
30
S1369-703X(17)30196-1 http://dx.doi.org/doi:10.1016/j.bej.2017.07.012 BEJ 6756
To appear in:
Biochemical Engineering Journal
Received date: Revised date: Accepted date:
2-5-2017 11-7-2017 27-7-2017
Please cite this article as: Zohaib Ur Rehman Afridi, Jing Wu, Zhi Ping Cao, Zhong Liang Zhang, Zhong Hua Li, Souhila Poncin, Huai Zhi Li, Insight into mass transfer by convective diffusion in anaerobic granules to enhance biogas production, Biochemical Engineering Journalhttp://dx.doi.org/10.1016/j.bej.2017.07.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Insight into mass transfer by convective diffusion in anaerobic granules to enhance biogas production Zohaib Ur Rehman Afridia, Jing Wua*, Zhi Ping Caoa, Zhong Liang Zhanga,b, Zhong Hua Lia, Souhila Poncinc, Huai Zhi Lic
a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of
Environment, Tsinghua University, Beijing 100084 P.R. China b c
Jiansu Institute of Urban Planning and Design, Nanjing, Jiangsu 210036 P.R. China
Laboratory of Reactions and Process Engineering, Université de Lorraine, CNRS, 1, rue
Grandville, BP 20451, 54001 Nancy cedex, France *Corresponding author: [email protected] Phone: +86-10-62789121 Fax: +86-10-62785687 Graphical abstract
1
Highlights
Mass transfer was characterized as convective and molecular diffusion for large, medium and small granules.
The convective diffusion rates were 2-3 magnitudes greater than molecular diffusion rate.
Biogas production rate was enhanced up to 1.3-4 folds by convective diffusion.
Internal structure of anaerobic granule can significantly affect mass transfer process.
The study could facilitate understanding the mass transfer process within granules to improve biogas production.
Abstract Mass transfer in anaerobic granule is crucial for efficient biogas production, but so far the fundamental understanding remains poor. This study aims at gaining an insight into convective diffusion to enhance mass transfer within small (0.5-1 mm), medium (1.5-2 mm) and large (3-3.5 mm) granules. The convective diffusion rates (FCD) of granules at a superficial liquid velocity of 3 m/h were 2-3 magnitudes greater than molecular diffusion rates (FMD), and significantly enhanced the biogas production rate of small, medium and large granules by 4, 1.5 and 1.3 folds. The granules were permeable in nature as fluid collection efficiency (η) of small, medium and large granules of 0.97, 0.89 and 0.58 indicated, and were of cluster-cluster formation as fractal dimension (Df) revealed. The large granules possess best mass transfer condition and highest biogas production rate because they had highest FCD due to the biggest permeable area despite lowest η. This work for the first time elaborates the important roles of internal structures in terms 2
of permeability and fractal dimension on the convective mass transfer in anaerobic granules, and it also highlights the importance of molecular diffusion. The results could facilitate upgrading the understanding of mass transfer process in anaerobic granules to enhance biogas production. Keywords: Anaerobic granule; Biogas production; Convective diffusion; Molecular diffusion; Mass transfer.
1. Introduction Biogas is a renewable energy that can provide methane to mitigate the fossil energy crisis [1, 2]. Biogas production by anaerobic digestion is drawing significant attention as a progressive sustainable energy technology [3, 4]. At present, granule-based bioreactors are commonly used to produce biogas and treat a wide variety of high-strength organic wastewaters [5-7]. The overall efficiency of anaerobic reactors associates with the mass transfer within granules as the granule is the key engineering component of the reactor. Mass transfer within granule is attributed to play a major role to produce biogas [8-10]. However, up till now the mass transfer process within the granule remains unknown due to relatively small size (0.5-3.5 mm) and complex internal structure of the granule which is comprised of micro and nanoscale channels [11]. Nonetheless, in decades of studies, researchers have tried to improve the biogas production by optimizing several operational parameters of the anaerobic reactors. YingYu, et al. [12] found that in the UASB-AnMBRs, biogas yield increased from 0.062 to 0.12 L/g CODremoved when HRT was decreased from 10 h to 5.5 h. The enhanced gas production was attributed to mass transfer in biomass at a higher upflow velocity. Others correlated the increase in biogas production with parameters such as OLR, HRT, pH, temperature and upflow
3
liquid velocity, etc. [13-17]. However, studies could not explain the fundamental mass transfer mechanism within granule which affected the biogas production. Mass transfer in granule could not be entirely explained due to smaller size, complex internal structure and most importantly the contradicting results about nature of granule. Most of the researchers thought granule was impermeable and considered molecular diffusion as the sole mass transfer mechanism within granules [18, 19]. Gonzalez-Gil, et al. [20] investigated the kinetics and mass transfer phenomena in anaerobic granules and concluded that mass transfer was based on molecular diffusion as there was no evidence of convective flow inside the anaerobic biofilms. Mass transfer in anaerobic biofilm is different than granular sludge. But Hulshoff Pol [21] and Li and Yu [22] inferred that mass transfer process would be convective in nature due to their porous nature Winkler [23] confirmed the permeability in granules using diffusion of salt. Chu and Lee [24] investigated the fluid flow and mass transport in pores and identified that intra-floc transport processes are carried by convective diffusion. Consequently, a knowledge gap exists about the fundamental understanding of mass transfer in anaerobic granules. Thus, it is of major importance to get a better understanding to achieve higher biogas production. The objective of this study was to investigate the mass transfer process within the granules especially the convective diffusion for different size granules. Furthermore, impacts of the internal structure of granules in terms of fractal dimension, fluid collection efficiency and permeable area on mass transfer within granules was studied. In order to understand the enhancement of convective diffusion on biogas production, molecular diffusion was also investigated in this study. To our best knowledge, it is the first time to investigate the effect of the internal structure of granules on mass transfer within granules. The results would facilitate understanding the anaerobic process and improving the performance of the granule-based reactors. 4
2. Materials and methods 2.1 Experimental setup and operating conditions
A transparent upflow micro-reactor was used in this study. It was made of polymethyl methacrylate (PMMA) with the inner working section of width × length× thickness 7.5 cm × 19.0 cm × 0.5 cm. The experimental setup was similar to our previous study [11] and an external peristaltic feed pump (Longer, BT100-2J, China) was installed to maintain a constant superficial liquid velocity of 0 m/h or 3.3 m/h as shown in Fig. 1. As under 0 m/h, molecular mass transfer is the sole mass transfer mechanism in the granule while at 3.3m/h both molecular and convective diffusion will play a role in mass transfer. In our previous study, Zhang, et al. [25] investigated the influence of different superficial liquid velocities in the upflow reactor and found that the optimum superficial liquid velocity for upflow reactor was 3.3 m/h which resulted in highest biogas production and bed expansion without sludge washout. Therefore, superficial liquid velocity was set at 3.3 m/h for this study. The feed was stored in a tightly sealed steel tank with a volume of 25 liters. The composition of synthetic feed was as same as that used in the previous work [8] and Chemical Oxygen Demand (COD) remained relatively stable around 6000 mg/L as listed in table 1. For both 0 m/h and 3.3 m/h experiments, the synthetic feed solution was changed after every 12 hours to ensure that the concentration remained 6000 mg/L in the feed storage tank during the experiment. The pH of the feed was set between 7.2± 0.2 at the start of each experiment using NaHCO3 as alkalinity buffer during the test. The temperature of the reactor was controlled at 37± 1 °C by a thermal heater with a digital controller. Fig.1
5
The anaerobic granular sludge was from a full-scale internal circulation anaerobic reactor treating starch wastewater in northern China. The granules were divided into three groups of large, medium and small sizes with a diameter of 3-3.5 mm, 1.5-2 mm and 0.5-1 mm using a microscope (Motic Group, China) equipped with a digital camera. The granule diameter was measured with a quantitative image analysis program (Motic Images Advanced 3.2, China). The corresponding average diameter of large, medium and small groups was 3.11± 0.23, 1.90±0.26 and 0.77± 0.17 mm. The granules were divided into three discreet groups because in our previous study [25] we found that granular sludge with different size had different internal structure, bioactivity and physical features. Before the test, anaerobic granules were preserved in glass bottle containing oxygen-free water with low COD concentration (200 mg/L) at 37 °C in a thermal bath. For each experiment, the micro-reactor was filled up to 30% (v/v) with 3-3.5 mm, 1.5-2 mm, and 0.5-1 mm granules, respectively. For experiments at 0 m/h, the granules were cultured at COD concentration of 6000 mg/L and at 37℃ for several days to achieve the steady state. Under the steady state, the biogas production rate was maximum and stable. For experiments at 3.3 m/h, the preserved granules were filled into the reactor and continuously fed at superficial upflow velocity of 3.3 m/h with a synthetic solution of COD 6000 mg/L (20-25 days approximately) until its performance achieved steady state. All experiments were run in triplicate. At the end of each experiment, granules were observed under the microscope and no sign of aggregation was recorded as granules were already matured in the full scale upflow reactor. A high-speed digital camera (Cam Record CL600 Optronis GmbH, Germany) equipped with a high magnification lens was utilized to record the produced biogas bubbles in the micro-reactor. In this work, a frame rate of 500 fps and an exposure time of 1/500 s were used. The camera resolution was 1280 x 1024 pixels under the monochrome condition. The necessary illumination 6
was provided by a halogen light whose intensity was adjusted to 800 W. The image sequences obtained were then used to measure the diameter and the number of biogas bubbles.
2.2 Experimental analysis
2.2.1 Biogas production
The biogas production rate was calculated using a high-speed camera. The recordings were made during a period of time (typically 10 min) after every two hours during a period of 24 h for both 0 m/h and 3.3 m/h experiments. The size of biogas bubbles was determined using a quantitative image analysis software (Motic Images Advanced 3.2, China). The volume of bubbles was estimated assuming that the bubbles were spherical because the bubble shape was quite circular due to small size. Then, the biogas production rate at that moment was calculated according to the volume of all bubbles recorded during that period.
2.2.2 Fractal dimension of granule
Fractal dimension (Df) is an effective tool to characterize the sedimentation and structural properties of biological aggregates [26]. The flocs and granules are of the multi-layered structure, and these aggregates usually follow a fractal scaling relationship of Wd ∼ dDf, where Wd is the dry weight of aggregate (µg), and d is the diameter of the aggregate (mm) [27]. Df is the fractal dimension which can be calculated from the slope of a log-log plot of the dry mass and size of the aggregates. In this study, the granules were dried at 30 oC for 24 hours on a pre-weighted membrane filter (pore size 0.4µm) [28]. The dry weight of individual granules was measured using a microbalance (Metler Toledo, USA).
7
2.2.3 Permeability of granules The permeability of granular sludge in terms of fluid collection efficiency (η) and permeability area (k) is important factor for the mass transfer in granules. η is defined as the ratio of the interior flow passing through the aggregate to the flow approaching it. A series of mathematical formulations have been described for calculating the permeability area of an aggregate (k) [29] as described in Equations 1-4 below.
d 2 3 - 4.5 4.5 5 - 3 6 k 18 3 (3 2 5
where 3 1-
(1)
(2)
The correlation (δ) as proposed by Happel provides a more realistic estimate of the intra-aggregate permeability for immobilized biological flocs. The internal permeation (ξ) of these granules can thus be estimated from the k-relationship via the correlation [30].
d 2 k
(3)
From the value, the fluid collection efficiency of the granule (η) can be calculated as follows [31]:
9 tanh
2 3 tanh 3
(4)
2.2.4 Mass transfer in anaerobic granules
Both the molecular and convective diffusion contribute to the mass transfer in the granule [11, 32]. In the absence of external liquid flow, molecular diffusion is the predominant mechanism. 8
However, the convective diffusion always occurs because of the constant liquid flow in an anaerobic reactor [33]. In order to study the convective diffusion, the molecular diffusion was calculated at a superficial liquid velocity of 0 m/h as control. The convective diffusion at superficial liquid velocity of 3.3 m/h was studied. The mathematical model for internal mass transfer was based on certain assumptions. 1. The large, medium and small granules are spherical in shape and have a homogeneous biofilm of uniform thickness. 2. The concentration of the reactor bottom is almost the same as the influent. 3. The mass transfer is defined as the diffusion within the granules. Diffusion within the granules will be the rate-limiting step rather than external mass transfer i.e. Substrate transfer from the bulk solution into the granule. 4. The wall effect is negligible in the micro-reactor. In our previous study on hydrodynamic
conditions in the micro-reactor, it was concluded that shear generated around the granule was very low at various upflow velocities. Therefore, in the micro-reactor it can be ignored. The molecular diffusion rate (FMD) within a granule was calculated using the modified Fick’s law as follows [11]: FMD 4 r 2 DM C
(5)
where r is the radius of the granule (cm), DM is the diffusivity of the substrates in water (cm2/s), C is the substrate concentration (mg/L), and ε is dimensionless porosity. DM of glucose is 0.94 x 105
cm2/s [34]. The porosity (ε) of each group of granules was obtained using Hg- porosimeter
(AutoPore IV 9500, USA) and the granule samples were prepared by following [11]. Φ is the assumed diffusion layer from the surface into the granule (cm). 9
The mathematical equation for the convective diffusion rate (FCD) is defined [33] as: FCD =2πr 2 ηCU
(6)
where 𝜂 is the granule fluid collection efficiency (dimensionless) and U is superficial liquid velocity in the reactor (cm/s). 2.2.5 Other analytical methods Hg- Porosimeter (AutoPore IV 9500, USA) was used to measure the porosity and density of granules. COD was measured with the COD meter (Hebei Huatong Co., YL-1A, China. Volatile suspended solid (VSS) was measured with the standard weight methods [35].
3
Results and discussion
3.1 Specific biogas production rate under superficial liquid velocity at 0 and 3.3 m/h
The biogas production rate corresponding to liquid velocities at 0 m/h and 3.3m/h can indicate the mass transfer inside the granule. Fig.2 illustrates a comparison of biogas production for granules of different size under the superficial liquid velocity of 0 m/h and constant superficial liquid velocity of 3.3 m/h. In both cases, the specific biogas production rate exhibited a positive relationship with the granule size. At a superficial liquid velocity of 0 m/h, a maximum specific biogas rate, i.e., 0.031 m3/kgVSS/d was observed for large granules while the production rate of the medium and small granules was 0.016 and 0.006 m3/kgVSS/d respectively. On the other hand, the biogas production rates for large, medium and small granules were 0.070, 0.040 and 0.030 m3/kgVSS/d under constant superficial liquid velocity of 3.3 m/h, being at least 1.3-4.0 folds higher than those of 0 m/h. The higher biogas production under constant superficial liquid velocity 10
of 3.3 m/h indicates faster mass transfer. At 0 m/h, molecular diffusion was dominant mass transfer mechanism which is much slower than convective diffusion. Thus a lower biogas production rate can be observed for different granules.
Fig. 2 3.2 Permeability of anaerobic granules
Due to the size variation and complex internal structure, little literature is available about the role of permeability on mass transfer process in anaerobic granule [36]. In our previous study, granules were found to be porous in nature [11] and the porosity increased with the size of the granule ranging from 18.7 to 40.1%. This indicates that granules would be permeable. In this study, the permeability of anaerobic granules was verified by both experimental and mathematical approaches. The fractal dimension (Df) can provide information about the internal structure of granules, whereas the fluid collection efficiency (η) is considered as a mathematical approach to verify the permeability of granules. Furthermore, permeable area (k) is studied. 3.2.1 Fractal dimension of anaerobic granules
Totally 150 anaerobic granules were analyzed for the fractal dimension (Df) analysis. The dry weight of individual granule of the small, medium and large granules varied between 35-107 µg, 133-467 µg, and 643-1692 µg respectively. The fractal dimensions were obtained based on the slopes of the regression lines of the logarithmic relationship between the dry mass and size. The Df of small, medium and large granules were 1.96, 2.80 and 2.88 as shown in Fig. 3.
11
Fig.3 Table 2 lists the Df of granular sludge in different studies. The Dfs of granules in our study were close to previous studies.
Gmachowski [38] reported that the internal geometry of granules has a great influence on the mass transfer process. The fractal dimension of aggregates can describe the internal geometry of granules which can indicate the compactness of granule and influence the mass transfer process of granules [39]. Li and Logan [29] and Logan [40] reported that aggregates with Df values between 1-3 were compact and formed via cluster–cluster formation. Hence, anaerobic granules of this study were of compact and dense fractal features with a cluster-cluster formation scheme. This type of structure is of permeable nature and furthermore, it is good for interior mass transfer as it can provide vacancy to allow the fluid flow through the granules. 3.2.2 Permeability of granules The fluid collection efficiency (η) is a key parameter to indicate the permeability. η of all granule groups were calculated by Eq. 1-4. The calculated collection efficiencies were inversely related to the size of the granules ranging from 0.99-0.97, 0.98-0.91 and 0.78-0.62 for small, medium and large granules respectively (Fig. 4). The collection efficiency of small granules was highest as compared to the medium and large granules with an average value of 0.97±0.007, 0.89±0.019 and 0.58±0.112 respectively.
Fig. 4 12
Anaerobic granules in this study had higher permeability as compared with the previously reported granules. Previous studies indicate that anaerobic granules have a wide range of fluid collection efficiency ranging from 0.08 to 0.83 as summarized in Fig. 4. Li and Logan [29] confirmed the permeability of granules with η of 0.65. Zhang, Li, Oh and Logan [30] reported very low fluid collection efficiency of 0.083 for latex microspheres granules of size between 0.5-2 mm. Mu, et al. [41] and Mu, Yu and Wang [41] reported that η of the anaerobic granular sludge of 0.4-3 mm varied between 0.01-0.4. In this study, the fluid collection efficiency of granules ranging from 0.58-0.97 indicated the permeable nature of anaerobic granules. This nature allows the substrate to pass through the granule to enhance the mass transfer. 3.3
Molecular and convective diffusion rates in anaerobic granules
Mass transfer occurring in anaerobic granules is a complex phenomenon. The difference in biogas production rate under static and constant upflow velocity condition highlighted the existence of convective diffusion in the granule. Fig. 5 shows the molecular and convective diffusion rates for different size granules. The convective diffusion rate were 2-3 magnitude greater than molecular diffusion rates. The former ranged for small, medium and large granules between 2x10-3–1x10-2, 2x10-2–5x10-2 and 3.5x10-2 –7x10-2 mg/s, while the latter was 6.5x10-6 – 1.45 x10-5, 2 x10-5 – 3.3 x10-5 and 3.7 x10-5 – 4.87 x10-5 mg/s respectively. The results indicate a positive relationship between the diffusion rate (of both molecular and convective) and the granule size for all three granule groups. Overall, large granules (3-3.5 mm) had highest mass transfer rates whereas small granules (0.5-1) had least mass transfer rates.
13
Fig. 5 The permeability of granule can influence the convective mass transfer rates thus to increase or limit the biogas production rate. In this study, Df of small granules was 1.98 which was comparatively less compact than large granules which had Df of 2.88. This indicates that in small granules the vacancy between clusters would be greater, therefore, for same permeability area more substrate will flow into the granules. The highest η of small granules affirmed that highest ratio of the substrate on granular surface could pass through the small granules thus biogas production rate increased most (by 4 folds). On the other hand, large granules being more compact had lowest η and biogas production was increased by 1.3 folds. Snidaro, et al. [42] reported that the permeability of flocs decreased when the fractal dimension was larger than 2, and our results are in agreement with the reported results. The η of granules decreases as the size of the granule increases. This possibly derived from the increase in density of granules along with size. The density of small, medium and large granules were 1.15, 1.20 and 1.90 g/mL, respectively. The large granules are located at the bottom of the upflow reactor where nutrient condition is quite good. For this reason, large granules have better growth condition and become dense in nature. Secondly, granules also get internally compressed during the process of biogas production. In our previous study we found that there are two channel systems inside the granule i.e, small (nano level) channels which merge into big (micro level) channels. During the biogas production process, internal pressure is transferred from nano to micro channels which compresses the internal layers and granules become dense in nature [11]. It is worth noting that large granules possess best mass transfer condition and highest biogas production rate because they had highest FCD despite lowest η. This was possibly due to the fact that large granules had the largest permeable area (k) of 6.17 mm2, while the small ones were of 14
only 0.02 mm2 as shown in Fig. 6. The results indicate that k increased along the granule size thus more area was available for substrate diffusion in large granules as compared to medium and small granules. k affects the overall mass transfer rates in the granule. Thus for the large granules, the substrate on granular surface going into the granules by convective flow was most the although the ratio of substrate going into the granules was lowest (η was lowest). This highest uptake of substrate in large granule leads to highest biogas production.
Fig. 6 Interestingly, the results also show that the molecular mass transfer cannot be neglected although it is a slow process. The importance of molecular diffusion was highlighted as even though the convective diffusion rate was 2-3 magnitudes faster than the molecular diffusion but the biogas production rate increased only 1.3-4 folds under constant superficial liquid velocity of 3.3m/h (Fig 2). This indicates that convective diffusion is not the sole mass transfer mechanism in the granules under liquid flow condition whereas molecular and convective diffusion co-exist in the granules. Overall, mass transfer by convective diffusion enhanced the biogas production in the anaerobic granule.
4
Conclusion
In this study, convective mass transfer within different sized granules was investigated. It increased the biogas production rate by 1.3-4 folds. It closely related with the internal structure of granule. Overall, the anaerobic granules were permeable in nature and allowed fluid flow into the granule. Among all granules, the large granules exhibited well-developed mass transfer conditions for fastest diffusion rate due to largest permeable area for highest substrate uptake. This study 15
represents a step forward toward understanding the mass transfer phenomena of anaerobic granular sludge, and suggests useful information for the design and operation of anaerobic bioreactors, such as the intermittent high upflow velocity by recirculation and increase the turbulence degree by optimizing reactor inner structures rate in anaerobic granular sludge reactor.
Acknowledgments The financial support provided by the Natural Science Foundation of China (91334112, 51678338) and National Major Science and Technology Project about Water Pollution Control (2014ZX07114001) are gratefully acknowledged by Chinese authors. French authors acknowledge the assistance for this study from the French Agence Nationale de la Recherche (PRC CNRSNSFC).
16
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[42] Snidaro, et al., Characterization of activated sludge flocs structure, International Water Association, London, ROYAUME-UNI, 1997.
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List of Figures Fig.1: Schematic view of experimental setup Fig.2: Biogas production under superficial liquid velocities of 0 and 3.3 m/h Fig.3: Fractal dimension of (a) small, (b) medium and (c) large granules. Fig.4: Fluid collection efficiency of granules Fig.5: Molecular diffusion rates and convective diffusion rates at liquid velocity of 3.3 m/h of granules. Fig. 6: Permeable area of anaerobic granules
22
Fig. 1
23
Fig. 2
24
Fig. 3
25
Fig. 4
26
Fig. 5
27
Fig. 6
28
Table 1: Synthetic substrate composition Component
mg·L−1
Macronutrient glucose
6500
urea
170
K2HPO4
72
yeast
0.128 mL (1:1 with water)
Micronutrients MgCl2
156.44
FeSO4
27.5
NiSO4
9.32
CoCl2
0.94
NH4MoO3
0.34
ZnSO4
0.82
MnCl2
0.22
CuSO4
0.12
Alkalinity buffer NaHCO3
29
Table 2: Fractal dimension of various granules Sludge
Average Size (mm)
Df
Reference
Granular sludge
0.7-3.1
1.96 - 2.88
This study
Granular sludge from UASB
1-3.5
2.79 ± 0.3
[37]
Aerobic granular sludge
1.8-3.8
1.80
[28]
30