- Email: [email protected]
Filterability of biological sludge on porous membranes
Desalination 245 (2009) 621–625
Filterability of biological sludge on porous membranes D. Abdessemed*, G. Nezzal Laboratory of the Process Engineering-Environment, University of Sciences and Technology, Houari Boumediene B.P., 32 El Alia 16111, Bab Ezzouar, Algiers, Algeria Tel. +213 771 85 12 43; Fax +213 21 24 79 19; email: [email protected] Received 13 July 2008; revised 02 January 2009; accepted 09 February 2009
Abstract This study aims to find a new die purification for wastewater reuse. It consists of the filterability of biological sludge exits of the wastewater treatment plant at Staoueli (Algeria) on porous membranes. The membrane used is a mineral membrane M2 Carbosep (15 Kg/mol). we are interested for the first time in the influence of the transmembrane pressure on the permeate flux for various concentrations in biomass where we deduced that it is necessary to work with transmembrane pressures lower or equal to 1 bar, because in this field the membrane fouling is minimum and in the second time, we considered the different parameters of evolution (the permeate flux Jv, the turbidity, the biological oxygen demand (BOD5), and the chemical oxygen demand (COD)) for various concentrations in biomass according to time. This process showed good performance in removing the chemical oxygen demand 97.7 % for a concentration in biomass of 50g/L and in removing biological oxygen demand 98.8 %. Keywords: Wastewater; Biological sludge; Ultrafiltration; Fouling; Reuse
1. Introduction The activated sludge process removes dissolved and colloidal, biodegradable organic matter in municipal and industrial wastewater treatment and reclamation processes are the most widespread technology in the field of water treatment, but as the requirements
*Corresponding author.
increase in terms of effluents quality, water reuse, and limitation of the sludge production, the processes with activated sludge must be supplemented or replaced by new technologies. Currently, by replacing a conventional secondary settling tank in the activated sludge process by a membrane separation unit, membrane bioreactors (MBR), the combination of membrane separation and activated sludge process are extensively used. According to Le-Clech et al. [1] an activated sludge is a complex and
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.02.028
622
D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
highly variable suspension containing an array of microorganisms, components present in the feed wastewater, and metabolites such as the extracellular polymeric substances (EPS) produced within the bioreactor. Therefore, Jin et al. [2] have studied that membrane filtration behaviours of the activated sludge are influenced by many complicated constituents. Iritani et al.[3] studied the properties of deadend microfiltration by exploring under constant pressure, using two types of activated sludge, controlled under the condition of different air flow rates. The activated sludge cultured at the air flow rate of 0.15 L/min (the anaerobic condition) exhibited a significant flux decline compared with the air flow rate of 2.33 L/min (the aerobic condition). Matošića et al. [4] have studied that Membrane fouling in membrane bioreactors (MBR), is caused by cake formation on the surface, mainly attributed to suspended solids; and also by surface adsorption connected with pore blocking, attributed to soluble components of activated sludge. Evenbhj et al. [5] have tested membrane ultrafiltration of activated sludge from membrane bioreactors (MBR) and identified that it is usually performed with low fluxes. This is done to prevent membrane fouling and system failure. However, even when operated with low fluxes, filterability of the biomass can fluctuate with time considerably. Among the many causes that are mentioned to explain this phenomenon, changes in influent quality and quantity seem most probable. In the present study, this technique has been applied to treat the domestic wastewater treatment of Staoueli by a bioreactor with membrane. We
are particularly interested in the evolution of the various parameters (the permeate flux Jv, the turbidity, the chemical oxygen demand (COD), and the biological oxygen demand (BOD)) for different concentrations in biomass according to time. 2. Experimental conditions 2.1. Characteristics of raw water Table 1 gives the principal characteristics of the wastewater treatment plant at Staoueli. 2.2. Experimental procedures In the reactor (Fig. 1), we put 10 liters of wastewater, then we inoculate the concentration in biomass from MLSS = 5g/L until 50g/L. Carbosep tubular inorganic membrane M2, with 15 kg/mol cut-off, was used (porous carbon support and a membrane layer of ZrO2). The transmembrane pressure ΔP was 1 bar, the cross flow velocity (U) was 3 m/s, and the temperature 298 K. 3. Results and discussions 3.1. Influence of transmembrane pressure on the permeate flux Fig. 2 represents the permeate flux according to the transmembrane pressure for various concentrations in biomass. In the first part of the curve (ΔP < 0.7 bar), the permeate flux is proportional with the transmembrane pressure — the repulsive strengths are superior to the
SS, mg/L BOD5, mg/L COD, mg/L Turbidity, NTU
(7)
(5)
Table 1 Characteristics of raw water Parameters
(6)
(6)
(5)
(4)
Values 350 300 600 14
Permeate
(3)
(5) Bioreactor
(1) (2)
Fig. 1. Experimental Set up. 1, bioreactor; 2, cooling coil; 3, centrifugal pump; 4, flowmeter; 5, valves; 6, pressure gauge; 7, tubular UF module.
623
80
70
70
60
60
50 Jv (L/hm2)
Jv (L/hm2)
D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
50 40
20 10
30
MLSS = 5g/L MLSS = 10g/L MLSS = 20g/L MLSS = 30g/L MLSS = 50g/L
20
MLSS = 5g/L MLSS = 10g/L MLSS = 20g/L MLSS = 30g/L MLSS = 50g/L
30
40
10 0 0
20
40
60
80
Time (min)
0 0.3
0.6
0.9
1.2
1.5
ΔP (bar)
Fig. 2. Evolution of the permeate flux according to the transmembrane pressure Δp for different concentrations of biomass.
attractive strengths and there is no accumulation of components on the membrane surface. We can assimilate curves obtained in rights which obey DARCY’s law. In the second part of the curve, for 0.7 < ΔP < 1 bar, we observe a light curvature of the rights— it is the appearance of the beginning of a concentration polarization. The solute retained by the membrane tends to accumulate at the surface of the membrane and constitutes an additional barrier to the passage of the solvent. In the third part of the curve for ΔP > 1 bar, we notice that as the transmembrane pressure increases, the permeate flux which aims towards a limit value, Jlim—a formation of an asymptote with the increase in concentration of polarization. We can conclude that it is necessary to work on transmembrane pressures lower or more equal in 1 bar, because in this domain the membrane fouling is minimum. 3.2. Variation of permeate flux and turbidity with time for different concentrations in biomass 3.2.1. Permeate flux
Fig. 3. Evolution of the permeate flux with time.
the permeate is not recycled in the feed tank. The variation in permeate flux during the time is shown in Fig. 3. The permeate flux decreases from 61 L/hm2 to 50 L/hm2 in time, when it reaches a landing for a concentration in biomass of 5g/L. Whereas the limit value of permeate flux is 40 L/hm2 for a concentration in biomass equal to 50g/L. For a weak concentration in biomass, we obtain the best value of permeate flux. We observe a small permeate flux variation with time. It is due on the one hand to the colloid organics and suspended matter retained by the membrane and on the other hand with the fouling partial of the membranes’ pores. 3.2.2. Turbidity The curve of turbidity evolution according to time (Fig. 4) reveals a reduction in turbidity as of the first few minutes of ultrafiltration until obtaining a value of zero at the end of the test, for 1.6
1.2 1 0.8 0.6 0.4 0.2 0
Once the conditions operating the ultrafiltration such as the transmembrane pressure ΔP = 1 bar, we begin the assay of concentration, where
MLSS = 5g/L MLSS = 10g/L MLSS = 20g/L MLSS = 30g/L MLSS = 50g/L
1.4 Turbidity (NTU)
0
0
10
20
30
40
50
60
Time (min)
Fig. 4. Variation of the turbidity in function time.
70
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D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
BOD5 (mg/L)
300 250 BOD5 (Initial) BOD5 (Final)
200 150 100 50 0
MLSS=5g/L MLSS=10g/L MLSS=20g/L MLSS=30g/L MLSS=50g/L
Fig. 5. Variation of the BOD5 for different concentrations of biomass.
a concentration in biomass of 30g/L and 50g/L. This established fact is explained by the strong capacity of the membrane to retain the suspended matter responsible for water turbidity. 3.3. Evolution of the BOD5 according to the process for various concentrations in biomass During the assay of concentration, we were interested in the follow-up of BOD5 of the permeate according to the time of ultrafiltration. The measured values of BOD5 are shown in Fig. 5. The BOD5 decreased from 300 mg/L to 3.6 mg/L with a pollution reduction of 98.8 %. 3.4. Evolution of the COD according to the process for various concentrations in biomass Fig. 6 showed the follow-up of the COD of the permeate for various concentrations in biomass.
This process showed good performance in removing the COD of 97.7 % (where the value passed from 300mg/L to 6.8mg/L for a concentration in biomass of 50g/L). We noticed that whatever is the concentration in biomass, the elimination rate is almost identical. 4. Conclusion The results obtained showed that ultrafiltration is an interesting process. The comparison between the analysis results of traditional purification (BOD5 = 30 mg/L, COD = 90 mg/L) and that using the ultrafiltration of biological sludge makes it possible to notice that the latter is better efficient. The decrease of the COD and the BOD5 in the course of time is due to the rejection of foulant by the membrane CARBOSEP M2 which formed a fouling cake on the membrane surface. In addition, the permeate obtained is of very good quality from the physicochemical and bac-
COD (mg/L)
600 500
COD (Initial) COD (Final)
400 300 200 100 0
MLSS=5g/L MLSS=10g/L MLSS=20g/L MLSS=30g/L MLSS=50g/L
Fig. 6. Variation of the COD for different concentrations of biomass.
D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
teriological point of view, which is not the case with traditional purification. The permeate obtained is limpid, of this fact that humic and mineral substances are not present, whereas in traditional purification, water is coloured because by the humic substances. References [1] P. Le-Clech, V. Chen and A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci., 284 (2006) 17–53. [2] B. Jin, B.-M.Wil´en and P. Lant, A comprehensive insight into floc characteristics and their impact on
625
compressibility and settleability of activated sludge, Chem. Eng. J., 95 (2003) 221–234. [3] E. Iritani, N. Katagiri, T. Sengoku, K.M. Yoo, K. Kawasaki and A. Matsuda, Flux decline behaviors in dead-end microfiltration of activated sludge and its supernatant, J. Membr. Sci., 300 (2007) 36–44. [4] M. Matošića, M.Vukovićb, M. Ćurlina and I. Mijatovića, Fouling of a hollow fibre submerged membrane during long term filtration of activated sludge, Desalination, 219 (2008) 57–65. [5] H. Evenbhj, B. Verrecht, J.H.J.M. Van der Graaf and B. Van der Bruggen, Manipulating filterability of MBR activated sludge by pulsed substrate addition, Desalination, 78 (2005) 193–201.
Filterability of biological sludge on porous membranes D. Abdessemed*, G. Nezzal Laboratory of the Process Engineering-Environment, University of Sciences and Technology, Houari Boumediene B.P., 32 El Alia 16111, Bab Ezzouar, Algiers, Algeria Tel. +213 771 85 12 43; Fax +213 21 24 79 19; email: [email protected] Received 13 July 2008; revised 02 January 2009; accepted 09 February 2009
Abstract This study aims to find a new die purification for wastewater reuse. It consists of the filterability of biological sludge exits of the wastewater treatment plant at Staoueli (Algeria) on porous membranes. The membrane used is a mineral membrane M2 Carbosep (15 Kg/mol). we are interested for the first time in the influence of the transmembrane pressure on the permeate flux for various concentrations in biomass where we deduced that it is necessary to work with transmembrane pressures lower or equal to 1 bar, because in this field the membrane fouling is minimum and in the second time, we considered the different parameters of evolution (the permeate flux Jv, the turbidity, the biological oxygen demand (BOD5), and the chemical oxygen demand (COD)) for various concentrations in biomass according to time. This process showed good performance in removing the chemical oxygen demand 97.7 % for a concentration in biomass of 50g/L and in removing biological oxygen demand 98.8 %. Keywords: Wastewater; Biological sludge; Ultrafiltration; Fouling; Reuse
1. Introduction The activated sludge process removes dissolved and colloidal, biodegradable organic matter in municipal and industrial wastewater treatment and reclamation processes are the most widespread technology in the field of water treatment, but as the requirements
*Corresponding author.
increase in terms of effluents quality, water reuse, and limitation of the sludge production, the processes with activated sludge must be supplemented or replaced by new technologies. Currently, by replacing a conventional secondary settling tank in the activated sludge process by a membrane separation unit, membrane bioreactors (MBR), the combination of membrane separation and activated sludge process are extensively used. According to Le-Clech et al. [1] an activated sludge is a complex and
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.02.028
622
D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
highly variable suspension containing an array of microorganisms, components present in the feed wastewater, and metabolites such as the extracellular polymeric substances (EPS) produced within the bioreactor. Therefore, Jin et al. [2] have studied that membrane filtration behaviours of the activated sludge are influenced by many complicated constituents. Iritani et al.[3] studied the properties of deadend microfiltration by exploring under constant pressure, using two types of activated sludge, controlled under the condition of different air flow rates. The activated sludge cultured at the air flow rate of 0.15 L/min (the anaerobic condition) exhibited a significant flux decline compared with the air flow rate of 2.33 L/min (the aerobic condition). Matošića et al. [4] have studied that Membrane fouling in membrane bioreactors (MBR), is caused by cake formation on the surface, mainly attributed to suspended solids; and also by surface adsorption connected with pore blocking, attributed to soluble components of activated sludge. Evenbhj et al. [5] have tested membrane ultrafiltration of activated sludge from membrane bioreactors (MBR) and identified that it is usually performed with low fluxes. This is done to prevent membrane fouling and system failure. However, even when operated with low fluxes, filterability of the biomass can fluctuate with time considerably. Among the many causes that are mentioned to explain this phenomenon, changes in influent quality and quantity seem most probable. In the present study, this technique has been applied to treat the domestic wastewater treatment of Staoueli by a bioreactor with membrane. We
are particularly interested in the evolution of the various parameters (the permeate flux Jv, the turbidity, the chemical oxygen demand (COD), and the biological oxygen demand (BOD)) for different concentrations in biomass according to time. 2. Experimental conditions 2.1. Characteristics of raw water Table 1 gives the principal characteristics of the wastewater treatment plant at Staoueli. 2.2. Experimental procedures In the reactor (Fig. 1), we put 10 liters of wastewater, then we inoculate the concentration in biomass from MLSS = 5g/L until 50g/L. Carbosep tubular inorganic membrane M2, with 15 kg/mol cut-off, was used (porous carbon support and a membrane layer of ZrO2). The transmembrane pressure ΔP was 1 bar, the cross flow velocity (U) was 3 m/s, and the temperature 298 K. 3. Results and discussions 3.1. Influence of transmembrane pressure on the permeate flux Fig. 2 represents the permeate flux according to the transmembrane pressure for various concentrations in biomass. In the first part of the curve (ΔP < 0.7 bar), the permeate flux is proportional with the transmembrane pressure — the repulsive strengths are superior to the
SS, mg/L BOD5, mg/L COD, mg/L Turbidity, NTU
(7)
(5)
Table 1 Characteristics of raw water Parameters
(6)
(6)
(5)
(4)
Values 350 300 600 14
Permeate
(3)
(5) Bioreactor
(1) (2)
Fig. 1. Experimental Set up. 1, bioreactor; 2, cooling coil; 3, centrifugal pump; 4, flowmeter; 5, valves; 6, pressure gauge; 7, tubular UF module.
623
80
70
70
60
60
50 Jv (L/hm2)
Jv (L/hm2)
D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
50 40
20 10
30
MLSS = 5g/L MLSS = 10g/L MLSS = 20g/L MLSS = 30g/L MLSS = 50g/L
20
MLSS = 5g/L MLSS = 10g/L MLSS = 20g/L MLSS = 30g/L MLSS = 50g/L
30
40
10 0 0
20
40
60
80
Time (min)
0 0.3
0.6
0.9
1.2
1.5
ΔP (bar)
Fig. 2. Evolution of the permeate flux according to the transmembrane pressure Δp for different concentrations of biomass.
attractive strengths and there is no accumulation of components on the membrane surface. We can assimilate curves obtained in rights which obey DARCY’s law. In the second part of the curve, for 0.7 < ΔP < 1 bar, we observe a light curvature of the rights— it is the appearance of the beginning of a concentration polarization. The solute retained by the membrane tends to accumulate at the surface of the membrane and constitutes an additional barrier to the passage of the solvent. In the third part of the curve for ΔP > 1 bar, we notice that as the transmembrane pressure increases, the permeate flux which aims towards a limit value, Jlim—a formation of an asymptote with the increase in concentration of polarization. We can conclude that it is necessary to work on transmembrane pressures lower or more equal in 1 bar, because in this domain the membrane fouling is minimum. 3.2. Variation of permeate flux and turbidity with time for different concentrations in biomass 3.2.1. Permeate flux
Fig. 3. Evolution of the permeate flux with time.
the permeate is not recycled in the feed tank. The variation in permeate flux during the time is shown in Fig. 3. The permeate flux decreases from 61 L/hm2 to 50 L/hm2 in time, when it reaches a landing for a concentration in biomass of 5g/L. Whereas the limit value of permeate flux is 40 L/hm2 for a concentration in biomass equal to 50g/L. For a weak concentration in biomass, we obtain the best value of permeate flux. We observe a small permeate flux variation with time. It is due on the one hand to the colloid organics and suspended matter retained by the membrane and on the other hand with the fouling partial of the membranes’ pores. 3.2.2. Turbidity The curve of turbidity evolution according to time (Fig. 4) reveals a reduction in turbidity as of the first few minutes of ultrafiltration until obtaining a value of zero at the end of the test, for 1.6
1.2 1 0.8 0.6 0.4 0.2 0
Once the conditions operating the ultrafiltration such as the transmembrane pressure ΔP = 1 bar, we begin the assay of concentration, where
MLSS = 5g/L MLSS = 10g/L MLSS = 20g/L MLSS = 30g/L MLSS = 50g/L
1.4 Turbidity (NTU)
0
0
10
20
30
40
50
60
Time (min)
Fig. 4. Variation of the turbidity in function time.
70
624
D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
BOD5 (mg/L)
300 250 BOD5 (Initial) BOD5 (Final)
200 150 100 50 0
MLSS=5g/L MLSS=10g/L MLSS=20g/L MLSS=30g/L MLSS=50g/L
Fig. 5. Variation of the BOD5 for different concentrations of biomass.
a concentration in biomass of 30g/L and 50g/L. This established fact is explained by the strong capacity of the membrane to retain the suspended matter responsible for water turbidity. 3.3. Evolution of the BOD5 according to the process for various concentrations in biomass During the assay of concentration, we were interested in the follow-up of BOD5 of the permeate according to the time of ultrafiltration. The measured values of BOD5 are shown in Fig. 5. The BOD5 decreased from 300 mg/L to 3.6 mg/L with a pollution reduction of 98.8 %. 3.4. Evolution of the COD according to the process for various concentrations in biomass Fig. 6 showed the follow-up of the COD of the permeate for various concentrations in biomass.
This process showed good performance in removing the COD of 97.7 % (where the value passed from 300mg/L to 6.8mg/L for a concentration in biomass of 50g/L). We noticed that whatever is the concentration in biomass, the elimination rate is almost identical. 4. Conclusion The results obtained showed that ultrafiltration is an interesting process. The comparison between the analysis results of traditional purification (BOD5 = 30 mg/L, COD = 90 mg/L) and that using the ultrafiltration of biological sludge makes it possible to notice that the latter is better efficient. The decrease of the COD and the BOD5 in the course of time is due to the rejection of foulant by the membrane CARBOSEP M2 which formed a fouling cake on the membrane surface. In addition, the permeate obtained is of very good quality from the physicochemical and bac-
COD (mg/L)
600 500
COD (Initial) COD (Final)
400 300 200 100 0
MLSS=5g/L MLSS=10g/L MLSS=20g/L MLSS=30g/L MLSS=50g/L
Fig. 6. Variation of the COD for different concentrations of biomass.
D. Abdessemed and G. Nezzal / Desalination 245 (2009) 621–625
teriological point of view, which is not the case with traditional purification. The permeate obtained is limpid, of this fact that humic and mineral substances are not present, whereas in traditional purification, water is coloured because by the humic substances. References [1] P. Le-Clech, V. Chen and A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci., 284 (2006) 17–53. [2] B. Jin, B.-M.Wil´en and P. Lant, A comprehensive insight into floc characteristics and their impact on
625
compressibility and settleability of activated sludge, Chem. Eng. J., 95 (2003) 221–234. [3] E. Iritani, N. Katagiri, T. Sengoku, K.M. Yoo, K. Kawasaki and A. Matsuda, Flux decline behaviors in dead-end microfiltration of activated sludge and its supernatant, J. Membr. Sci., 300 (2007) 36–44. [4] M. Matošića, M.Vukovićb, M. Ćurlina and I. Mijatovića, Fouling of a hollow fibre submerged membrane during long term filtration of activated sludge, Desalination, 219 (2008) 57–65. [5] H. Evenbhj, B. Verrecht, J.H.J.M. Van der Graaf and B. Van der Bruggen, Manipulating filterability of MBR activated sludge by pulsed substrate addition, Desalination, 78 (2005) 193–201.