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Characteristics of the biodiatomite dynamic membrane (cake layer) for municipal wastewater treatment
Desalination 250 (2010) 544–547
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Characteristics of the biodiatomite dynamic membrane (cake layer) for municipal wastewater treatment☆ Da-Wen Cao a, Hua-Qiang Chu b, Wei Jin b, Bing-Zhi Dong c,⁎ a b c
National Engineering Research Centre for Urban Pollution Control, Shanghai 200092, China School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Available online 7 October 2009 Keywords: Biodiatomite dynamic membrane Morphology Dynamic semicompressibility
a b s t r a c t According to the morphology investigation of the biodiatomite dynamic membrane (BDDM), the deposits on its surface was fine, homogeneous and of high porosity. The ratio of volatile suspended solids (VSS) to suspended solids (SS) of the BDDM was less than that in the biodiatomite mixed liquor. The research results showed that the hydraulic scour of the precoating stage caused microorganisms in the biodiatomite at the side of the cake layer near the support mesh to slough off, and therefore enhanced the porosity and augment the asymmetric pore structure of the cake layer. Diatomite, the core of biodiatomite, is the key factor of the dynamic semicompressibility of the BDDM. Thus, the BDDM has characteristics of good retention efficiency, high filtration flux and easy backwash. © 2009 Elsevier B.V. All rights reserved.
1. Introduction When filtering a solution containing fine particles, a dynamic membrane is formed on the underlying support mesh, which improved intrinsic separation efficiency of the underlying support mesh [1]. Dynamic membrane bioreactors are advantageous in low costs of investment for membrane modules, high filtration flux, low operation pressure, and easy backwash as well as with the advantages of membrane bioreactors for municipal wastewater treatment [2,3]. The characteristics of the dynamic membrane, such as morphology and components, can affect its performances in wastewater treatment process. Meng et al. [4] found that the activated sludge cake layer seemed to be dense and nonporous, and was easily compressed. Its pore distribution was strongly influenced with cake compressibility by the high transmembrane pressure. Diatomite, which consists primarily of amorphous SiO2, is the sedimentary fossils of ancient diatom and has the characteristics of light weight, high porosity and high chemical stability. The diatomite filtration process has been used for water supply in a long history. The diatomite dynamic membrane could not be compressed easily, and could achieve high flux, good effluent quality and easy backwash [5,6]. Zhao et al. [7] and Jin et al. [8] used the diatomite as the carriers for miscroorganisms to form biodiatomite. The biodiatomite combined with anoxic and/or aerobic processes was called biodiatomite reactor whose effluent quality was very good.
Combined with the dynamic membrane technology and the biodiatomite reactor, a biodiatomite dynamic membrane (BDDM) reactor for municipal wastewater treatment was investigated [9]. The results showed that the BDDM had the advantages of high filtration flux (50 L/m2 h–130 L/m2 h) and good retention efficiency of suspended solid with effluent turbidity less than 1 NTU. The good performance of the BDDM was determined by its own characteristics. Therefore, this work was focused on the investigation of the BDDM characteristics including cake layer morphology, components structure and filtration resistance variation, which could shed light on mechanisms of the BDDM for wastewater treatment. 2. Materials and methods 2.1. BDDM reactor As shown in Fig. 1-1, the BDDM reactor consists of three parts: an anoxic tank, an aerobic tank and a dynamic membrane filter (DMF). Each part has the same effective volume of 10 L. The BDDM support module (Fig. 1-2) seems like that of flat sheet membranes, which was made of 74 μm stainless steel mesh. It was fixed in submerged mode with a double-sided effective filtration area of 0.084 m2. Raw municipal wastewater (Table 1) for this experiment was derived from a local municipal wastewater treatment plant in Shanghai. 2.2. Typical experimental procedure
☆ Presented at the Conference on Membranes in Drinking and Industrial Water Production, 20–24 October 2008, Toulouse, France. ⁎ Corresponding author. Tel.: +86 21 65982691; fax: +86 21 65982313. E-mail addresses: [email protected] (H.-Q. Chu), [email protected] (B.-Z. Dong). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.020
2.2.1. Bioreactor operation The BDDM process consists of three basic stages: precoating, filtration, and backwash. In the precoating stage, the biodiatomite mixed liquor was recirculated to form the dynamic membrane. In the
D.-W. Cao et al. / Desalination 250 (2010) 544–547 Table 1 Quality of raw municipal wastewater.
Nomenclature R ΔP μ J Rt Rc Rm
545
filtration resistance (m− 1) transmembrane pressure (Pa) filtrate viscosity (Pa s) filtration flux of BDDM (m/s) total filtration resistance of BDDM (m− 1) cake layer resistance (m− 1) clean stainless steel mesh resistance (m− 1)
Items
Concentration
COD (mg/L) NH4–N (mg/L) SS (mg/L) Water temperature (°C) pH
260–600 7.6–11.2 30–360 23–33 6.5–7.4
Table 2 Operation parameters of the BDDM reactor.
filtration stage, a constant flux was applied and the operation pressure of the BDDM showed a continuous increase. A high precision vacuum pressure gauge was connected to the effluent pipe to measure the operation pressure which was used to calculate the transmembrane pressure and the filtration resistance of the BDDM. The filtration process was stopped once the operation pressure reached 40 kPa, and then backwash stage was started with air backwash mode. The operation parameters of the BDDM reactor are shown in Table 2.
Operation parameters
Value
MLSS (mg/L) Design flux (L/m2 h) Sludge retention time (d) Hydraulic retention time (h) Water temperature (°C) Aerobic tank DO (mg/L) Anoxic tank DO (mg/L)
11000 50–130 70 2.8–7.1 26–32 3–4 <0.15
2.2.2. Morphology analysis of the BDDM The cake layer on the support mesh was sampled after the BDDM module taken out of the reactor at the end of filtration stage. The cake layer was cut from the support mesh, and the disturbance to it was avoided to the most extent to keep the original morphology and structure of the cake layer in the sampling process. The cake surface and cross section structure was observed with the help of an environmental scanning electron microscope (ESEM) (XL-30ESEM, Philips, Holland).
where R=filtration resistance (m− 1), ΔP=transmembrane pressure (Pa), μ=filtrate viscosity (Pa s), and J=filtration flux of the BDDM (m/s). The specific resistance of the biodiatomite was measured with the vacuum filtration method [11].
2.2.3. Components analysis of the biodiatomite mixed liquor and the BDDM Suspended solid (SS) and volatile suspended solid (VSS) in the biodiatomite reactor and in the BDDM were analyzed according to the standard methods from Chinese NEPA Standard Methods [10]. The VSS/SS ratio variation of the BDDM was studied with the filtration flux 50 L/m2 h and the samples were taken at different operation pressure value.
The ESEM pictures of the surface near the mixed liquor, the back surface (binding surface of support mesh) and the cross section of the BDDM are shown in Fig. 2. As can be seen from Figs. 2-1 and 2-2, the deposits on the surface of the cake layer was fine, homogeneous and of high porosity. These properties are beneficial for improving filtration flux. It can also be observed that the cake layer surface was a little roughness, but this did not affect its SS retention capability. The SS BDDM effluent was zero at the most time during the experiment. The stainless steel mesh lines embedded deeply into the BDDM and left impressions on its back surface (Fig. 2-4). The support mesh intercepts the biodiatomite particles at the beginning of the dynamic membrane formation. Meanwhile, the biodiatomite particles depositing on the surface of the support mesh connect through bridging action. Though the size of the diatomite particles is smaller than the aperture of stainless steel mesh, microorganisms and extracellular polymer substances which adhere to the surface of diatomite (Fig. 2-5) make the biodiatomite particles bind tightly. Once the BDDM is formed, the support mesh does little contribution to the dynamic membrane retention capacity, and it only plays the role of supporting the dynamic membrane and improving the dynamic membrane strength.
2.2.4. Filtration resistance analysis of the BDDM Filtration resistance of the clean stainless steel mesh module submerged into clean water and of the BDDM were calculated by the Darcy's law as follows: R=
ΔP μJ
ð1Þ
3. Results and discussions 3.1. Morphology of the BDDM
3.2. Components variation of the BDDM and the biodiatomite mixed liquor
Fig. 1. Diagrams of (1) the BDDM reactor and (2) the configuration of the BDDM support module.
Average VSS/SS ratio of the BDDM and the biodiatomite mixed liquor at the end of filtration process are shown in Fig. 3. These data were derived from multiple well reproduced repetitions of different filtration flux. It can be seen from Fig. 3 that the average VSS/SS ratio of the BDDM is less than that of the biodiatomite mixed liquor in the reactor, which indicate that some microorganisms in the BDDM were sloughed off during the experiment process. So the VSS/SS ratio variation of the BDDM with operation pressure was studied under the filtration flux 50 L/m2 h. Experiment results are presented in Table 3.
546
D.-W. Cao et al. / Desalination 250 (2010) 544–547
Fig. 2. ESEM pictures of the BDDM: (1 and 2) the surface near the mixed liquor; (3) the back surface; (4) the side of the cross section near the support mesh, respectively.
As shown in Table 3, the VSS/SS ratio of the BDDM is basically stable after the precoating stage. Microorganisms in the BDDM were sloughed off during the precoating stage and there was no change in the filtration process. The VSS/SS ratio of the BDDM was less than that of the conventional activated sludge for the main component of diatomite in the BDDM. During the precoating stage the biodiatomite particles deposited on the surface of the support mesh. Particles and microorganisms adhered together by the adhesive force between them [12]. In the precoating stage, the hydraulic scour of the high filtration flux caused some microorganisms of the biodiatomite to slough off. In the filtration process, the filtration flux became small and microorganisms on the surface of the biodiatomite could not be removed. At the same time, microorganisms in the mixed liquor could not permeate into the BDDM for its surface interception. Therefore, the VSS/SS ratio of the BDDM was invariable after the precoating stage. 3.3. Dynamic semicompressibility of the BDDM 3.3.1. Specific resistance of the biodiatomite Specific resistance is a parameter of sludge filterability [11]. The average specific resistance of the biodiatomite is 1.15× 1011 m kg− 1 in this experiment. It was reported that the activated sludge specific
resistance was 1.65–2.83 × 1014 m kg− 1 [13]. So the average specific resistance of the biodiatomite is lower than that of activated sludge 3 orders of magnitude, which indicates the biodiatomite have good filterability and dewatering. It also suggests that the BDDM may have good filterability and low filtration resistance. 3.3.2. Filtration resistance of the BDDM The BDDM was removed completely by air backwash. Therefore, we conclude that the total filtration resistance of the BDDM (Rt) is composed of cake layer resistance (Rc) and clean stainless steel mesh resistance (Rm), which can be expressed as follows: ð2Þ
Rt = Rm + Rc
A series of resistance of the BDDM with flux 55 L/m2 h are calculated by Eqs. (1) and (2) and summarized in Table 4. The results from Table 4 clearly show that the cake layer resistance was the major part of the total filtration resistance, and the ratio of the Table 3 VSS/SS ratio variation of the BDDM with the filtration flux 50 L/m2 h. Mixed After Operation pressure (kPa) liquor precoating 0.25 0.5 6 15 VSS/SS(%)
42.04
39.90
20.5
30
57.5
39.86 39.88 39.75 39.70 39.74 39.92 39.74
Table 4 A series of resistance of the BDDM with flux 55 L/m2 h.
Fig. 3. Average SS/VSS ratio of the BDDM and the biodiatomite mixed liquor.
Filtration time (h)
0
0.2
0.8
1.7
2.3
3.0
3.3
3.9
5.0
5.7
6.1
Rm (×1011m− 1) Rc (× 1011m− 1) Rt (×1011m− 1) Rc/Rt (%)
0.1 0.8 0.9 11.1
0.1 0.9 1.0 10
0.1 1 1.1 9.1
0.1 1.5 1.6 6.3
0.1 2.7 2.8 3.6
0.1 4.5 4.6 2.2
0.1 5.7 5.8 1.7
0.1 8.6 8.7 1.1
0.1 15.8 15.9 0.6
0.1 20.5 20.6 0.5
0.1 23.5 23.6 0.4
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Air backwash was adopted for the BDDM at the end of the filtration stage, and the BDDM could be removed completely. The dynamic semicompressibility of the BDDM could avoid the severe adhesion between the cake layer and the support mesh, which was beneficial for the backwash. 4. Conclusions
Fig. 4. Schematic description of the dynamic semicompressibility of the BDDM.
The BDDM on the surface of the support mesh connected through bridging action with thickness of 1mm–2mm. Its surface was fine, homogeneous, and of high porosity. The hydraulic scour of the precoating stage caused asymmetric pore structure of the cake layer. When the precoating stage was finished, the average VSS/SS ratio of the BDDM was less than that of the biodiatomite mixed liquor. The average specific resistance of the biodiatomite was low, which was 1.15 × 1011 m kg− 1. Cake layer resistance was the major part of the total filtration resistance of the BDDM. The existence of diatomite and microorganisms caused the BDDM to have the dynamic semicompressibility. All the characteristics of the BDDM make it have advantages of high SS retention efficiency, high filtration flux and easy backwash in municipal wastewater treatment. Acknowledgements
Fig. 5. Operation pressure as a function of filtration time with flux 55 L/m2 h.
clean stainless steel mesh resistance to the total filtration resistance of the BDDM decreased when the cake layer resistance increased. The cake layer resistance increased gradually in the fore part and increased rapidly in the aft part. These results also indicated that the total filtration resistance of the BDDM at the end of the filtration process was lower than that of the conventional membrane bioreactor 1–2 orders of magnitude [14], which tested the above prediction made by the specific resistance of biodiatomite. According to the filtration resistance, morphology and components of the BDDM, it can be concluded that the BDDM exhibits dynamic semicompressibility. The schematic description of the dynamic semicompressibility of the BDDM is shown in Fig. 4. Diatomite has certain mechanical robustness with Mohs hardness of 1–1.5 for its main chemical composition amorphous SiO2. Because diatomite is the main substance in the BDDM, so the BDDM has the property of incompressibility. For the easy deformation and compressibility of microorganisms [15–17], the microorganisms on the surface of diatomite made the BDDM also have some compressibility. So the BDDM has semicompressibility. In the beginning of filtration stage, the cake layer resistance was small and the BDDM was not compressed (Fig. 4-A). As the filtration time was extended, the cake layer resistance increased obviously. When the operation pressure surpassed the limit of mechanical robustness of microorganisms, the BDDM was compressed partly (Figs. 5 and 4-B). The semicompressibility of the BDDM had variation and increased with dynamic feature. Therefore, the BDDM has dynamic semicompressibility.
This research was financially supported by the National Key Technologies R & D Program “Drinking Water Safety Ensuring Technologies and Instruments Development for Small Cities and Towns” (2006BAJ08B02). We also thank the Gao Tingyao Environmental Science and Technology Development Foundation of Tongji University, Shanghai, China. References [1] E.A. Tsapiuk, J. Membr. Sci. 116 (1996) 17–29. [2] Y. Kiso, Y.-J. Jung, T. Ichinari, M. Park, T. Kitao, Water Res. 34 (2000) 4143–4150. [3] W. Fuchs, C. Resch, M. Kernstock, M. Mayer, P. Schoeberl, R. Braun, Water Res. 39 (2005) 803–810. [4] F.G. Meng, H.M. Zhang, Y.S. Li, X.W. Zhang, F.L. Yang, J.N. Xiao, Sep. Purif. Technol. 44 (2005) 250–257. [5] K.P. Langé, W.D. Bellamy, D.W. Hendricks, G.S. Logsdon, JAWWA 76 (1986) 76–84. [6] J.E. Ongerth, P.E. Hutton, JAWWA 93 (2001) 54–63. [7] Y. Zhao, D. CAO, L. Liu, W. Jin, Water Environ. Res. 78 (2006) 392–396. [8] W. Jin, Y.-P. Zhao, Z.-X. Xu, T.-Y. Gao, J. Tongji Univ., Nat. Sci. 33 (2005) 1626–1629. [9] H.-q. Chu, D.-w. Cao, W. Jin, B.-z. Dong, J. Membr. Sci. 325 (2008) 271–276. [10] Chinese NEPA, Water and Wastewater Monitoring Methods4th ed., Chinese Environmental Science Publishing House, Beijing, 1997. [11] P. Aarne Vesilind, Treatment and Disposal of Wastewater Sludge2nd ed., Ann Arbor Science Publishers, Inc, 1979. [12] A. Broeckmann, J. Busch, T. Wintgens, W. Marquardt, Desalination 189 (2006) 97–109. [13] Z. Zhang, Theories and Designs of Wastewater Treatment, China Architecture and Building Press, Beijing, 2003. [14] B. Fan, X. Huang, X. Wen, Y. Yu, Environ. Sci. 24 (2003) 91–97. [15] W.M. Lu, K.L. Tung, S.M. Hung, J.S. Shiau, K.J. Hwang, Powder Technol. 116 (2001) 1–12. [16] K. Nakanishi, T. Tadokoro, R. Matsuno, Chem. Eng. Commun. 62 (1987) 187–201. [17] K.-J. Hwang, Y.-H. Yu, W.-M. Lu, J. Membr. Sci. 194 (2001) 229.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Characteristics of the biodiatomite dynamic membrane (cake layer) for municipal wastewater treatment☆ Da-Wen Cao a, Hua-Qiang Chu b, Wei Jin b, Bing-Zhi Dong c,⁎ a b c
National Engineering Research Centre for Urban Pollution Control, Shanghai 200092, China School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai 200092, China
a r t i c l e
i n f o
Available online 7 October 2009 Keywords: Biodiatomite dynamic membrane Morphology Dynamic semicompressibility
a b s t r a c t According to the morphology investigation of the biodiatomite dynamic membrane (BDDM), the deposits on its surface was fine, homogeneous and of high porosity. The ratio of volatile suspended solids (VSS) to suspended solids (SS) of the BDDM was less than that in the biodiatomite mixed liquor. The research results showed that the hydraulic scour of the precoating stage caused microorganisms in the biodiatomite at the side of the cake layer near the support mesh to slough off, and therefore enhanced the porosity and augment the asymmetric pore structure of the cake layer. Diatomite, the core of biodiatomite, is the key factor of the dynamic semicompressibility of the BDDM. Thus, the BDDM has characteristics of good retention efficiency, high filtration flux and easy backwash. © 2009 Elsevier B.V. All rights reserved.
1. Introduction When filtering a solution containing fine particles, a dynamic membrane is formed on the underlying support mesh, which improved intrinsic separation efficiency of the underlying support mesh [1]. Dynamic membrane bioreactors are advantageous in low costs of investment for membrane modules, high filtration flux, low operation pressure, and easy backwash as well as with the advantages of membrane bioreactors for municipal wastewater treatment [2,3]. The characteristics of the dynamic membrane, such as morphology and components, can affect its performances in wastewater treatment process. Meng et al. [4] found that the activated sludge cake layer seemed to be dense and nonporous, and was easily compressed. Its pore distribution was strongly influenced with cake compressibility by the high transmembrane pressure. Diatomite, which consists primarily of amorphous SiO2, is the sedimentary fossils of ancient diatom and has the characteristics of light weight, high porosity and high chemical stability. The diatomite filtration process has been used for water supply in a long history. The diatomite dynamic membrane could not be compressed easily, and could achieve high flux, good effluent quality and easy backwash [5,6]. Zhao et al. [7] and Jin et al. [8] used the diatomite as the carriers for miscroorganisms to form biodiatomite. The biodiatomite combined with anoxic and/or aerobic processes was called biodiatomite reactor whose effluent quality was very good.
Combined with the dynamic membrane technology and the biodiatomite reactor, a biodiatomite dynamic membrane (BDDM) reactor for municipal wastewater treatment was investigated [9]. The results showed that the BDDM had the advantages of high filtration flux (50 L/m2 h–130 L/m2 h) and good retention efficiency of suspended solid with effluent turbidity less than 1 NTU. The good performance of the BDDM was determined by its own characteristics. Therefore, this work was focused on the investigation of the BDDM characteristics including cake layer morphology, components structure and filtration resistance variation, which could shed light on mechanisms of the BDDM for wastewater treatment. 2. Materials and methods 2.1. BDDM reactor As shown in Fig. 1-1, the BDDM reactor consists of three parts: an anoxic tank, an aerobic tank and a dynamic membrane filter (DMF). Each part has the same effective volume of 10 L. The BDDM support module (Fig. 1-2) seems like that of flat sheet membranes, which was made of 74 μm stainless steel mesh. It was fixed in submerged mode with a double-sided effective filtration area of 0.084 m2. Raw municipal wastewater (Table 1) for this experiment was derived from a local municipal wastewater treatment plant in Shanghai. 2.2. Typical experimental procedure
☆ Presented at the Conference on Membranes in Drinking and Industrial Water Production, 20–24 October 2008, Toulouse, France. ⁎ Corresponding author. Tel.: +86 21 65982691; fax: +86 21 65982313. E-mail addresses: [email protected] (H.-Q. Chu), [email protected] (B.-Z. Dong). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.020
2.2.1. Bioreactor operation The BDDM process consists of three basic stages: precoating, filtration, and backwash. In the precoating stage, the biodiatomite mixed liquor was recirculated to form the dynamic membrane. In the
D.-W. Cao et al. / Desalination 250 (2010) 544–547 Table 1 Quality of raw municipal wastewater.
Nomenclature R ΔP μ J Rt Rc Rm
545
filtration resistance (m− 1) transmembrane pressure (Pa) filtrate viscosity (Pa s) filtration flux of BDDM (m/s) total filtration resistance of BDDM (m− 1) cake layer resistance (m− 1) clean stainless steel mesh resistance (m− 1)
Items
Concentration
COD (mg/L) NH4–N (mg/L) SS (mg/L) Water temperature (°C) pH
260–600 7.6–11.2 30–360 23–33 6.5–7.4
Table 2 Operation parameters of the BDDM reactor.
filtration stage, a constant flux was applied and the operation pressure of the BDDM showed a continuous increase. A high precision vacuum pressure gauge was connected to the effluent pipe to measure the operation pressure which was used to calculate the transmembrane pressure and the filtration resistance of the BDDM. The filtration process was stopped once the operation pressure reached 40 kPa, and then backwash stage was started with air backwash mode. The operation parameters of the BDDM reactor are shown in Table 2.
Operation parameters
Value
MLSS (mg/L) Design flux (L/m2 h) Sludge retention time (d) Hydraulic retention time (h) Water temperature (°C) Aerobic tank DO (mg/L) Anoxic tank DO (mg/L)
11000 50–130 70 2.8–7.1 26–32 3–4 <0.15
2.2.2. Morphology analysis of the BDDM The cake layer on the support mesh was sampled after the BDDM module taken out of the reactor at the end of filtration stage. The cake layer was cut from the support mesh, and the disturbance to it was avoided to the most extent to keep the original morphology and structure of the cake layer in the sampling process. The cake surface and cross section structure was observed with the help of an environmental scanning electron microscope (ESEM) (XL-30ESEM, Philips, Holland).
where R=filtration resistance (m− 1), ΔP=transmembrane pressure (Pa), μ=filtrate viscosity (Pa s), and J=filtration flux of the BDDM (m/s). The specific resistance of the biodiatomite was measured with the vacuum filtration method [11].
2.2.3. Components analysis of the biodiatomite mixed liquor and the BDDM Suspended solid (SS) and volatile suspended solid (VSS) in the biodiatomite reactor and in the BDDM were analyzed according to the standard methods from Chinese NEPA Standard Methods [10]. The VSS/SS ratio variation of the BDDM was studied with the filtration flux 50 L/m2 h and the samples were taken at different operation pressure value.
The ESEM pictures of the surface near the mixed liquor, the back surface (binding surface of support mesh) and the cross section of the BDDM are shown in Fig. 2. As can be seen from Figs. 2-1 and 2-2, the deposits on the surface of the cake layer was fine, homogeneous and of high porosity. These properties are beneficial for improving filtration flux. It can also be observed that the cake layer surface was a little roughness, but this did not affect its SS retention capability. The SS BDDM effluent was zero at the most time during the experiment. The stainless steel mesh lines embedded deeply into the BDDM and left impressions on its back surface (Fig. 2-4). The support mesh intercepts the biodiatomite particles at the beginning of the dynamic membrane formation. Meanwhile, the biodiatomite particles depositing on the surface of the support mesh connect through bridging action. Though the size of the diatomite particles is smaller than the aperture of stainless steel mesh, microorganisms and extracellular polymer substances which adhere to the surface of diatomite (Fig. 2-5) make the biodiatomite particles bind tightly. Once the BDDM is formed, the support mesh does little contribution to the dynamic membrane retention capacity, and it only plays the role of supporting the dynamic membrane and improving the dynamic membrane strength.
2.2.4. Filtration resistance analysis of the BDDM Filtration resistance of the clean stainless steel mesh module submerged into clean water and of the BDDM were calculated by the Darcy's law as follows: R=
ΔP μJ
ð1Þ
3. Results and discussions 3.1. Morphology of the BDDM
3.2. Components variation of the BDDM and the biodiatomite mixed liquor
Fig. 1. Diagrams of (1) the BDDM reactor and (2) the configuration of the BDDM support module.
Average VSS/SS ratio of the BDDM and the biodiatomite mixed liquor at the end of filtration process are shown in Fig. 3. These data were derived from multiple well reproduced repetitions of different filtration flux. It can be seen from Fig. 3 that the average VSS/SS ratio of the BDDM is less than that of the biodiatomite mixed liquor in the reactor, which indicate that some microorganisms in the BDDM were sloughed off during the experiment process. So the VSS/SS ratio variation of the BDDM with operation pressure was studied under the filtration flux 50 L/m2 h. Experiment results are presented in Table 3.
546
D.-W. Cao et al. / Desalination 250 (2010) 544–547
Fig. 2. ESEM pictures of the BDDM: (1 and 2) the surface near the mixed liquor; (3) the back surface; (4) the side of the cross section near the support mesh, respectively.
As shown in Table 3, the VSS/SS ratio of the BDDM is basically stable after the precoating stage. Microorganisms in the BDDM were sloughed off during the precoating stage and there was no change in the filtration process. The VSS/SS ratio of the BDDM was less than that of the conventional activated sludge for the main component of diatomite in the BDDM. During the precoating stage the biodiatomite particles deposited on the surface of the support mesh. Particles and microorganisms adhered together by the adhesive force between them [12]. In the precoating stage, the hydraulic scour of the high filtration flux caused some microorganisms of the biodiatomite to slough off. In the filtration process, the filtration flux became small and microorganisms on the surface of the biodiatomite could not be removed. At the same time, microorganisms in the mixed liquor could not permeate into the BDDM for its surface interception. Therefore, the VSS/SS ratio of the BDDM was invariable after the precoating stage. 3.3. Dynamic semicompressibility of the BDDM 3.3.1. Specific resistance of the biodiatomite Specific resistance is a parameter of sludge filterability [11]. The average specific resistance of the biodiatomite is 1.15× 1011 m kg− 1 in this experiment. It was reported that the activated sludge specific
resistance was 1.65–2.83 × 1014 m kg− 1 [13]. So the average specific resistance of the biodiatomite is lower than that of activated sludge 3 orders of magnitude, which indicates the biodiatomite have good filterability and dewatering. It also suggests that the BDDM may have good filterability and low filtration resistance. 3.3.2. Filtration resistance of the BDDM The BDDM was removed completely by air backwash. Therefore, we conclude that the total filtration resistance of the BDDM (Rt) is composed of cake layer resistance (Rc) and clean stainless steel mesh resistance (Rm), which can be expressed as follows: ð2Þ
Rt = Rm + Rc
A series of resistance of the BDDM with flux 55 L/m2 h are calculated by Eqs. (1) and (2) and summarized in Table 4. The results from Table 4 clearly show that the cake layer resistance was the major part of the total filtration resistance, and the ratio of the Table 3 VSS/SS ratio variation of the BDDM with the filtration flux 50 L/m2 h. Mixed After Operation pressure (kPa) liquor precoating 0.25 0.5 6 15 VSS/SS(%)
42.04
39.90
20.5
30
57.5
39.86 39.88 39.75 39.70 39.74 39.92 39.74
Table 4 A series of resistance of the BDDM with flux 55 L/m2 h.
Fig. 3. Average SS/VSS ratio of the BDDM and the biodiatomite mixed liquor.
Filtration time (h)
0
0.2
0.8
1.7
2.3
3.0
3.3
3.9
5.0
5.7
6.1
Rm (×1011m− 1) Rc (× 1011m− 1) Rt (×1011m− 1) Rc/Rt (%)
0.1 0.8 0.9 11.1
0.1 0.9 1.0 10
0.1 1 1.1 9.1
0.1 1.5 1.6 6.3
0.1 2.7 2.8 3.6
0.1 4.5 4.6 2.2
0.1 5.7 5.8 1.7
0.1 8.6 8.7 1.1
0.1 15.8 15.9 0.6
0.1 20.5 20.6 0.5
0.1 23.5 23.6 0.4
D.-W. Cao et al. / Desalination 250 (2010) 544–547
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Air backwash was adopted for the BDDM at the end of the filtration stage, and the BDDM could be removed completely. The dynamic semicompressibility of the BDDM could avoid the severe adhesion between the cake layer and the support mesh, which was beneficial for the backwash. 4. Conclusions
Fig. 4. Schematic description of the dynamic semicompressibility of the BDDM.
The BDDM on the surface of the support mesh connected through bridging action with thickness of 1mm–2mm. Its surface was fine, homogeneous, and of high porosity. The hydraulic scour of the precoating stage caused asymmetric pore structure of the cake layer. When the precoating stage was finished, the average VSS/SS ratio of the BDDM was less than that of the biodiatomite mixed liquor. The average specific resistance of the biodiatomite was low, which was 1.15 × 1011 m kg− 1. Cake layer resistance was the major part of the total filtration resistance of the BDDM. The existence of diatomite and microorganisms caused the BDDM to have the dynamic semicompressibility. All the characteristics of the BDDM make it have advantages of high SS retention efficiency, high filtration flux and easy backwash in municipal wastewater treatment. Acknowledgements
Fig. 5. Operation pressure as a function of filtration time with flux 55 L/m2 h.
clean stainless steel mesh resistance to the total filtration resistance of the BDDM decreased when the cake layer resistance increased. The cake layer resistance increased gradually in the fore part and increased rapidly in the aft part. These results also indicated that the total filtration resistance of the BDDM at the end of the filtration process was lower than that of the conventional membrane bioreactor 1–2 orders of magnitude [14], which tested the above prediction made by the specific resistance of biodiatomite. According to the filtration resistance, morphology and components of the BDDM, it can be concluded that the BDDM exhibits dynamic semicompressibility. The schematic description of the dynamic semicompressibility of the BDDM is shown in Fig. 4. Diatomite has certain mechanical robustness with Mohs hardness of 1–1.5 for its main chemical composition amorphous SiO2. Because diatomite is the main substance in the BDDM, so the BDDM has the property of incompressibility. For the easy deformation and compressibility of microorganisms [15–17], the microorganisms on the surface of diatomite made the BDDM also have some compressibility. So the BDDM has semicompressibility. In the beginning of filtration stage, the cake layer resistance was small and the BDDM was not compressed (Fig. 4-A). As the filtration time was extended, the cake layer resistance increased obviously. When the operation pressure surpassed the limit of mechanical robustness of microorganisms, the BDDM was compressed partly (Figs. 5 and 4-B). The semicompressibility of the BDDM had variation and increased with dynamic feature. Therefore, the BDDM has dynamic semicompressibility.
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