- Email: [email protected]
Membrane bioreactor performances: effluent quality ofcontinuous and sequencing systems for water reuse
Desalination 204 (2007) 39–45
Membrane bioreactor performances: effluent quality of continuous and sequencing systems for water reuse Jorge Lobos*, Christelle Wisniewski, Marc Heran, Alain Grasmick Laboratoire Génie des Procédés d’Elaboration des Bioproduits, UMR-CIRAD 016, Université Montpellier II- CC005, 34095, Montpellier Cedex 05, France Tel. +33 (4) 67 14 38 45; Fax +33 (4) 67 14 47 87; email: [email protected]
Received 20 February 2006; accepted 15 March 2006
Abstract The aim of this work is to study and compare the performances of two immersed membrane bioreactors especially respect to effluent quality for water reuse; one is operating in a sequencing way (MSBR), the other one in a continuous way (MCBR). Organic matter removal and membrane permeability were studied and analysed. The COD removal efficiency was higher for the continuous operation and reached a value — after a period of biomass adaptation — higher than 97% with a COD in the effluent lower than 50 mg/L. The COD removal efficiency was close to 94% with a COD in the effluent lower than 125 mg/L for the sequential operation. The filtration performance was better for the continuous operation, with a nearly constant evolution of transmembrane pressure. In this case the filtration conditions allowed a long time operation period (3600 h) without the requirement of membrane regeneration. The results show better performance for the continuous membrane reactor respect to the organic matter removal and filtration performances. In both systems the quality of water treated was conform to the legislative France constrains (COD < 125 mg/L), but in the context of water reutilization the potential of the continuous system is higher than the sequential one. Keywords: Membrane bioreactor; Water reuse; Sludge reduction *Corresponding author.
Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.534
40
J. Lobos et al. / Desalination 204 (2007) 39–45
1. Introduction Nowadays, membrane bioreactors present several advantages over the conventional process, like small footprint and high sanitary effluent quality [1]. The performances are well known in respect to quality of pollutant removal from wastewater including possibility of treated water reuse. Moreover, high biomass concentrations can be reached, without any negative effect on the quality of the treated wastewater because of the membranes presence. In this context, the aim of this work is to study and compare the performances of two immersed membrane bioreactors especially respect to the effluent quality for water reuse; one is operating in a sequencing way (MSBR), the other one in a continuous way (MCBR). Organic matter removal and membrane permeability were studied and analysed. The behaviour of the system was studied without any sludge extraction (except for sampling), during long operational running periods (150 d). The influent used was representative of a complex influent with respect to soluble and suspended fractions (acetate + extract of meat). Substrate removal, biomass production, respirometric activity and membrane fouling were quantified. 2. Material and methods 2.1. Experimental set-up The experiments were performed on two immersed membrane bioreactors with a working volume of 50 L (Fig. 1). The suspension was stirred by a horizontal rotating mixer at 400 rpm. The system was continuously aerated at 80 L/min flow rate. The pH was maintained at 7.5±0.5. The bioreactor was kept at a regulated temperature of 20± 0.5°C. 2.2. Membranes Capillary membranes made of polysulphone (2 mm in external diameter, 0.1 µm in pore size, initial membrane resistance 1012 m2/m3) were used
(Fig. 1). The filtration surface area was 0.3 m2 with 3 membrane modules (each module of 0.1 m2). Microfiltration was operated in an outside to inside mode at a low permeate flux (2.3–7.2 L/m2 h), which was maintained constant by peristaltic pumps. Air was supplied by distributors placed at the bottom of the reactor, under each membrane module. The aeration provided oxygen for the biological process needs and helped to decrease external membrane fouling owing to the turbulence around the membrane modules. 2.3. Substrate and culture The substrate was representative of a complex effluent (acetate + extract of meat). For the extract of meat a commercial product (Viandox®) was used. Nitrate of ammonium ((NH4)NO3) and diammonium hydrogen phosphate ((NH4)2HPO4) were added to maintain an acceptable COD/N/P ratio and so, to avoid any nutrient limitation. The pH was adjusted using NaHCO3 at 0.5 g/L. The average feed concentrations for the two operations are given in Table 1. The systems were operated under the operational conditions in Table 2. For the MSBR the cycle time was 12 h/cycle, with 0.3 h of feeding time, 6.2 h of reaction time, 5 h of filtration time and 0.5 of relaxation time. An aerobic mixed culture of heterotrophic organisms from a municipal wastewater treatment plant was used to inoculate the bioreactor. No sludge was taken out of the pilot unit during the whole experimental period except for sampling. 2.4. Analysis The performance of the membrane bioreactors was studied by following the degradation of the substrates and the evolution of the biomass concentration. Several parameters were determined in the influent, the effluent and the mixed liquor suspension. The supernatant was separated from the mixed liquor by filtrating through 1.2 µm
J. Lobos et al. / Desalination 204 (2007) 39–45
41
Fig. 1. Pilot bioreactor and membranes used for the experiments.
Table 1 Mean concentrations of the feed solutions
CODTotal (mgCOD/L) Influent MCBR Influent MSBR
1830 2090
CODParticular (mgCOD/L)
CODSoluble (mgCOD/L) Acetate
Viandox®
Viandox®
1500 1500
239 553
91 37
Table 2 Operating conditions of the membrane bioreactor
Operating condition Working volume, L Hydraulic flow rate, L/d Volumetric organic load, g/L·d Hydraulic retention time, d Temperature, °C pH Dissolved oxygen, mg/L
50 24 1.0 2 20±0.5 7.0±0.5 >2
microfibre glass filters. The analytical methods used are given in Table 3. 3. Results and discussion 3.1. Overall performance Fig. 2 shows the evolution of the effluent total
SS (mg/L)
MLVSS (mg/L)
93 38
56 23
COD (CODTE) and the COD removal efficiency for the two systems. For the MCBR and MSBR, two periods can be distinguished with respect to the COD removal efficiency. In the case of MCBR, the first one goes until day 85 and during this period the removal efficiency was under 95% afterwards, the removal efficiency was higher than 97%. In the case of MSBR, in the last 20 days of operation, the removal efficiency was higher than 94%. In both periods, no acetate was detected in the effluent and so we can suppose its total degradation. In both systems, an important difference between the mixed liquor suspension soluble COD (CODSR) and CODTE was observed in the first period. The presence of some soluble and colloidal compounds in the system could be explained by the non-complete elimination of all the organic compounds of the Viandox® fraction of substrate
42
J. Lobos et al. / Desalination 204 (2007) 39–45
Table 3 Measured parameters and analytical methods
Parameters
Methods
CODS, CODT Soluble proteins
Digestion method and colorimetric determination Spectrophotometric method using bovine serum albumin as standard, at a wavelength of 280 nm According to APHA (1992) Enzymatic kit Electrode oxymeter WTW oxi 340 Hannah Instrument pH meter
MLSS, MLVSS Acetate Dissolved oxygen pH
3000
100 95 90 85
2000
80 1500
75 70
1000
65
CODT removal efficiency
CODT effluent (mg/L)
2500
60
500
55 0 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
50 150
Time (d)
Fig. 2. CODTE: (S) MCBR and (U) MSBR; CODT removal efficiency: (z) MCBR and ({) MSBR.
or the release of soluble microbial compounds. A slight decrease between CODSR and CODTE was observed during the firsts period while a biomass acclimation for substrate degradation seemed to take place. In the second period, no difference between the CODSR and CODTE was observed and a nearly complete degradation of several substrate compounds occurred. With respect to the effluent quality, a COD concentration in the MCBR effluent lower than 50 mg/L was observed in the second period. While for the MSBR, a COD concentration lower than
125 mg/L was observed. It seems, after a cycle microbial behaviour study, that the low COD performance of the MSBR system is linked with a more important release of soluble microbial compounds and not with lower substrate degradation. These results showed that the presence of particular and non-easily degradable compounds in the effluent did not limit the MBR COD removal performance. The high solids retention time was achieved in the system because no sludge extraction made possible the growth of all types the micro-organism, including the micro-organism
J. Lobos et al. / Desalination 204 (2007) 39–45
with a very low specific growth rate [2]. Consequently, in this condition a big mixed population exists and a bigger capacity of complex substrate degradation seems possible. On the other hand, a high solids retention time is beneficial for the hydrolysis of suspended solids from the feed streams. We can notice that the treated effluent was free of suspended solids. This shows the effective action of the membrane separation.
43
aerobic treatment step, i.e. activated sludge, concern the sludge conditioning and disposal [4,5]. According to the very low conversion yield obtained in the MCBR, the substrate would essentially be consumed in order to ensure the maintenance cell requirements. However, according to the cycles of increase and stabilisation observed in the MLVSS, a natural cell lysis phenomenon could occur in the system. This lysis could induce the presence of particular compounds in the reactor and certainly the release of soluble products that were then consumed by the present and viable cells as a neo-substrate (i.e. cryptic growth) [6]. The higher biomass production in the sequencing reactor is presumably for the optimum conditions for biomass replication during the feed period. Though the daily organic loading was the same, the principal metabolic process seemed to be different. The overall performances of the two systems are given in Table 4.
3.2. Sludge production During the overall operational period, 0.044 g MVS/g CODT (0.058 g CODP/g CODT) and 0.091 g MVS/g CODT (0.092 gCODP/gCODT) observed sludge yields were obtained for the continuous and sequencing reactor respectively (Fig. 3). These values are approximately 5–10 times lower than those measured in a conventional activated sludge process [3]. The results point out clearly the lower sludge productions of the systems, which is a critical factor in the sustainable wastewater system operation. Generally speaking, 60% of the total plant operating costs of a conventional wastewater treatment plant (WWTP) based on an
3.3. Filtration performances Fig. 4 shows the evolution of the transmembrane pressure (TMP) for two modules in the
16.00 14.00
y = 0.091x + 3.1474 R2 = 0.9738
MLVSS (g/L)
12.00 10.00
MSBR
8.00 6.00 4.00
y = 0.0442x + 3.3011 2 R = 0.9278
MCBR
2.00 0.00 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Organic matter removed (gCODT/L)
Fig. 3. MLVSS vs. organic matter removed for biomass yield determination. Operation in MCBR (S) and MSBR (¡).
44
J. Lobos et al. / Desalination 204 (2007) 39–45 normal operation
0.50
Washing of one module
normal operation
0.45 0.40
TMP (bar)
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Time (d)
Fig. 4. Transmembrane pressure evolution (TMP) of two membrane modules in the MCBR operation.
Table 4 Overall performance of membrane bioreactors Operating condition/ Overall performances
MCBR MSBR
Volumetric organic load, g/L·d Effluent concentration, gCOD/L Organic matter removal rate, gCOD/L·d Biomass production rate, gMLVSS/L·d Organic matter removal efficiency, % Biomass yield conversion, gCOD/gCOD OUR stabilised value, gO2/L·d Oxygen need, gO2/gCOD
0.92 < 0.05 0.87
1.04 0.125 0.94
0.04 >97 0.058 0.50 0.57
0.086 >94 0.091 0.48 0.51
MCBR operation. In spite of the continuous increase in MLVSS, the membrane behaviour was nearly constant (TMP constant) and the hydrodynamic conditions avoided major fouling even at a high MLVSS concentration. 4. Conclusion Biological and filtration performances were investigated in membrane bioreactors using a
complex substrate. The results showed a high COD degradation performance for both systems. A biomass adaptation period was necessary for the consumption of the non easily biodegradable compounds. A biological degradation seemed to take place on the organic suspended solids present in the acetate–Viandox® substrate as well. The results show better performance for the continuous membrane reactor with respect to the organic matter removal. In both cases the treated water quality conformed to the legislative France constrains (COD < 125 mg/L), but in the context of water reuse the effluent of continuous operation has a lot of potential. The biomass conversion yields were very low compared to those of the conventional process. Despite a continuous increase in the MLVSS concentration, a stabilisation in terms of removal efficiency and respiratory activity was achieved. In any case, a small sludge extraction from the system seems to be necessary for a stable long-term operation of the membrane bioreactors. The filtration performance was better for the continuous operation, where a nearly constant
J. Lobos et al. / Desalination 204 (2007) 39–45
transmembrane pressure was observed. In this case the filtration conditions allowed a long time operation period (3600 h) without the requirement of the membrane regeneration. Consequently, the performances of the MCBR were better than the MSBR in respect to COD degradation performance, quality of the treated water, sludge production and filtration stability. Then, the membrane bioreactors can be considered as an interesting option in the field of wastewater treatment, especially if water reuse is a critical factor to involve. References [1] T. Stephenson, S. Judd, B. Jefferson and K. Brindle, Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London, 2000.
45
[2] S. Rosenberger, U. Krüger, R. Witzig, W. Manz, U. Szewzyk and M. Kraume, Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water. Wat. Res., 36 (2002) 413– 420. [3] P. Pitter and J. Chudoba, Biodegradability of Organic Substances in the Aquatic Environment. CRC Press, Boca Raton, USA, 1990. [4] R.D. Davis and J.E. Hall, Production, treatment and disposal of wastewater sludge in Europe from a UK perspective. Eur Water Pollution Control, 7(2) (1997) 9–17. [5] N.J. Horan, Biological Wastewater Treatment System. Wiley, Chichester, 1990. [6] J. Lobos, C. Wisnieswki, M. Héran and A. Grasmick, Effects of starvation conditions on biomass behaviour for minimization of sludge production in membrane bioreactors, Wat. Sci. Tech., 51(6–7) (2005) 35–44.
Membrane bioreactor performances: effluent quality of continuous and sequencing systems for water reuse Jorge Lobos*, Christelle Wisniewski, Marc Heran, Alain Grasmick Laboratoire Génie des Procédés d’Elaboration des Bioproduits, UMR-CIRAD 016, Université Montpellier II- CC005, 34095, Montpellier Cedex 05, France Tel. +33 (4) 67 14 38 45; Fax +33 (4) 67 14 47 87; email: [email protected]
Received 20 February 2006; accepted 15 March 2006
Abstract The aim of this work is to study and compare the performances of two immersed membrane bioreactors especially respect to effluent quality for water reuse; one is operating in a sequencing way (MSBR), the other one in a continuous way (MCBR). Organic matter removal and membrane permeability were studied and analysed. The COD removal efficiency was higher for the continuous operation and reached a value — after a period of biomass adaptation — higher than 97% with a COD in the effluent lower than 50 mg/L. The COD removal efficiency was close to 94% with a COD in the effluent lower than 125 mg/L for the sequential operation. The filtration performance was better for the continuous operation, with a nearly constant evolution of transmembrane pressure. In this case the filtration conditions allowed a long time operation period (3600 h) without the requirement of membrane regeneration. The results show better performance for the continuous membrane reactor respect to the organic matter removal and filtration performances. In both systems the quality of water treated was conform to the legislative France constrains (COD < 125 mg/L), but in the context of water reutilization the potential of the continuous system is higher than the sequential one. Keywords: Membrane bioreactor; Water reuse; Sludge reduction *Corresponding author.
Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2006.03.534
40
J. Lobos et al. / Desalination 204 (2007) 39–45
1. Introduction Nowadays, membrane bioreactors present several advantages over the conventional process, like small footprint and high sanitary effluent quality [1]. The performances are well known in respect to quality of pollutant removal from wastewater including possibility of treated water reuse. Moreover, high biomass concentrations can be reached, without any negative effect on the quality of the treated wastewater because of the membranes presence. In this context, the aim of this work is to study and compare the performances of two immersed membrane bioreactors especially respect to the effluent quality for water reuse; one is operating in a sequencing way (MSBR), the other one in a continuous way (MCBR). Organic matter removal and membrane permeability were studied and analysed. The behaviour of the system was studied without any sludge extraction (except for sampling), during long operational running periods (150 d). The influent used was representative of a complex influent with respect to soluble and suspended fractions (acetate + extract of meat). Substrate removal, biomass production, respirometric activity and membrane fouling were quantified. 2. Material and methods 2.1. Experimental set-up The experiments were performed on two immersed membrane bioreactors with a working volume of 50 L (Fig. 1). The suspension was stirred by a horizontal rotating mixer at 400 rpm. The system was continuously aerated at 80 L/min flow rate. The pH was maintained at 7.5±0.5. The bioreactor was kept at a regulated temperature of 20± 0.5°C. 2.2. Membranes Capillary membranes made of polysulphone (2 mm in external diameter, 0.1 µm in pore size, initial membrane resistance 1012 m2/m3) were used
(Fig. 1). The filtration surface area was 0.3 m2 with 3 membrane modules (each module of 0.1 m2). Microfiltration was operated in an outside to inside mode at a low permeate flux (2.3–7.2 L/m2 h), which was maintained constant by peristaltic pumps. Air was supplied by distributors placed at the bottom of the reactor, under each membrane module. The aeration provided oxygen for the biological process needs and helped to decrease external membrane fouling owing to the turbulence around the membrane modules. 2.3. Substrate and culture The substrate was representative of a complex effluent (acetate + extract of meat). For the extract of meat a commercial product (Viandox®) was used. Nitrate of ammonium ((NH4)NO3) and diammonium hydrogen phosphate ((NH4)2HPO4) were added to maintain an acceptable COD/N/P ratio and so, to avoid any nutrient limitation. The pH was adjusted using NaHCO3 at 0.5 g/L. The average feed concentrations for the two operations are given in Table 1. The systems were operated under the operational conditions in Table 2. For the MSBR the cycle time was 12 h/cycle, with 0.3 h of feeding time, 6.2 h of reaction time, 5 h of filtration time and 0.5 of relaxation time. An aerobic mixed culture of heterotrophic organisms from a municipal wastewater treatment plant was used to inoculate the bioreactor. No sludge was taken out of the pilot unit during the whole experimental period except for sampling. 2.4. Analysis The performance of the membrane bioreactors was studied by following the degradation of the substrates and the evolution of the biomass concentration. Several parameters were determined in the influent, the effluent and the mixed liquor suspension. The supernatant was separated from the mixed liquor by filtrating through 1.2 µm
J. Lobos et al. / Desalination 204 (2007) 39–45
41
Fig. 1. Pilot bioreactor and membranes used for the experiments.
Table 1 Mean concentrations of the feed solutions
CODTotal (mgCOD/L) Influent MCBR Influent MSBR
1830 2090
CODParticular (mgCOD/L)
CODSoluble (mgCOD/L) Acetate
Viandox®
Viandox®
1500 1500
239 553
91 37
Table 2 Operating conditions of the membrane bioreactor
Operating condition Working volume, L Hydraulic flow rate, L/d Volumetric organic load, g/L·d Hydraulic retention time, d Temperature, °C pH Dissolved oxygen, mg/L
50 24 1.0 2 20±0.5 7.0±0.5 >2
microfibre glass filters. The analytical methods used are given in Table 3. 3. Results and discussion 3.1. Overall performance Fig. 2 shows the evolution of the effluent total
SS (mg/L)
MLVSS (mg/L)
93 38
56 23
COD (CODTE) and the COD removal efficiency for the two systems. For the MCBR and MSBR, two periods can be distinguished with respect to the COD removal efficiency. In the case of MCBR, the first one goes until day 85 and during this period the removal efficiency was under 95% afterwards, the removal efficiency was higher than 97%. In the case of MSBR, in the last 20 days of operation, the removal efficiency was higher than 94%. In both periods, no acetate was detected in the effluent and so we can suppose its total degradation. In both systems, an important difference between the mixed liquor suspension soluble COD (CODSR) and CODTE was observed in the first period. The presence of some soluble and colloidal compounds in the system could be explained by the non-complete elimination of all the organic compounds of the Viandox® fraction of substrate
42
J. Lobos et al. / Desalination 204 (2007) 39–45
Table 3 Measured parameters and analytical methods
Parameters
Methods
CODS, CODT Soluble proteins
Digestion method and colorimetric determination Spectrophotometric method using bovine serum albumin as standard, at a wavelength of 280 nm According to APHA (1992) Enzymatic kit Electrode oxymeter WTW oxi 340 Hannah Instrument pH meter
MLSS, MLVSS Acetate Dissolved oxygen pH
3000
100 95 90 85
2000
80 1500
75 70
1000
65
CODT removal efficiency
CODT effluent (mg/L)
2500
60
500
55 0 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
50 150
Time (d)
Fig. 2. CODTE: (S) MCBR and (U) MSBR; CODT removal efficiency: (z) MCBR and ({) MSBR.
or the release of soluble microbial compounds. A slight decrease between CODSR and CODTE was observed during the firsts period while a biomass acclimation for substrate degradation seemed to take place. In the second period, no difference between the CODSR and CODTE was observed and a nearly complete degradation of several substrate compounds occurred. With respect to the effluent quality, a COD concentration in the MCBR effluent lower than 50 mg/L was observed in the second period. While for the MSBR, a COD concentration lower than
125 mg/L was observed. It seems, after a cycle microbial behaviour study, that the low COD performance of the MSBR system is linked with a more important release of soluble microbial compounds and not with lower substrate degradation. These results showed that the presence of particular and non-easily degradable compounds in the effluent did not limit the MBR COD removal performance. The high solids retention time was achieved in the system because no sludge extraction made possible the growth of all types the micro-organism, including the micro-organism
J. Lobos et al. / Desalination 204 (2007) 39–45
with a very low specific growth rate [2]. Consequently, in this condition a big mixed population exists and a bigger capacity of complex substrate degradation seems possible. On the other hand, a high solids retention time is beneficial for the hydrolysis of suspended solids from the feed streams. We can notice that the treated effluent was free of suspended solids. This shows the effective action of the membrane separation.
43
aerobic treatment step, i.e. activated sludge, concern the sludge conditioning and disposal [4,5]. According to the very low conversion yield obtained in the MCBR, the substrate would essentially be consumed in order to ensure the maintenance cell requirements. However, according to the cycles of increase and stabilisation observed in the MLVSS, a natural cell lysis phenomenon could occur in the system. This lysis could induce the presence of particular compounds in the reactor and certainly the release of soluble products that were then consumed by the present and viable cells as a neo-substrate (i.e. cryptic growth) [6]. The higher biomass production in the sequencing reactor is presumably for the optimum conditions for biomass replication during the feed period. Though the daily organic loading was the same, the principal metabolic process seemed to be different. The overall performances of the two systems are given in Table 4.
3.2. Sludge production During the overall operational period, 0.044 g MVS/g CODT (0.058 g CODP/g CODT) and 0.091 g MVS/g CODT (0.092 gCODP/gCODT) observed sludge yields were obtained for the continuous and sequencing reactor respectively (Fig. 3). These values are approximately 5–10 times lower than those measured in a conventional activated sludge process [3]. The results point out clearly the lower sludge productions of the systems, which is a critical factor in the sustainable wastewater system operation. Generally speaking, 60% of the total plant operating costs of a conventional wastewater treatment plant (WWTP) based on an
3.3. Filtration performances Fig. 4 shows the evolution of the transmembrane pressure (TMP) for two modules in the
16.00 14.00
y = 0.091x + 3.1474 R2 = 0.9738
MLVSS (g/L)
12.00 10.00
MSBR
8.00 6.00 4.00
y = 0.0442x + 3.3011 2 R = 0.9278
MCBR
2.00 0.00 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Organic matter removed (gCODT/L)
Fig. 3. MLVSS vs. organic matter removed for biomass yield determination. Operation in MCBR (S) and MSBR (¡).
44
J. Lobos et al. / Desalination 204 (2007) 39–45 normal operation
0.50
Washing of one module
normal operation
0.45 0.40
TMP (bar)
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Time (d)
Fig. 4. Transmembrane pressure evolution (TMP) of two membrane modules in the MCBR operation.
Table 4 Overall performance of membrane bioreactors Operating condition/ Overall performances
MCBR MSBR
Volumetric organic load, g/L·d Effluent concentration, gCOD/L Organic matter removal rate, gCOD/L·d Biomass production rate, gMLVSS/L·d Organic matter removal efficiency, % Biomass yield conversion, gCOD/gCOD OUR stabilised value, gO2/L·d Oxygen need, gO2/gCOD
0.92 < 0.05 0.87
1.04 0.125 0.94
0.04 >97 0.058 0.50 0.57
0.086 >94 0.091 0.48 0.51
MCBR operation. In spite of the continuous increase in MLVSS, the membrane behaviour was nearly constant (TMP constant) and the hydrodynamic conditions avoided major fouling even at a high MLVSS concentration. 4. Conclusion Biological and filtration performances were investigated in membrane bioreactors using a
complex substrate. The results showed a high COD degradation performance for both systems. A biomass adaptation period was necessary for the consumption of the non easily biodegradable compounds. A biological degradation seemed to take place on the organic suspended solids present in the acetate–Viandox® substrate as well. The results show better performance for the continuous membrane reactor with respect to the organic matter removal. In both cases the treated water quality conformed to the legislative France constrains (COD < 125 mg/L), but in the context of water reuse the effluent of continuous operation has a lot of potential. The biomass conversion yields were very low compared to those of the conventional process. Despite a continuous increase in the MLVSS concentration, a stabilisation in terms of removal efficiency and respiratory activity was achieved. In any case, a small sludge extraction from the system seems to be necessary for a stable long-term operation of the membrane bioreactors. The filtration performance was better for the continuous operation, where a nearly constant
J. Lobos et al. / Desalination 204 (2007) 39–45
transmembrane pressure was observed. In this case the filtration conditions allowed a long time operation period (3600 h) without the requirement of the membrane regeneration. Consequently, the performances of the MCBR were better than the MSBR in respect to COD degradation performance, quality of the treated water, sludge production and filtration stability. Then, the membrane bioreactors can be considered as an interesting option in the field of wastewater treatment, especially if water reuse is a critical factor to involve. References [1] T. Stephenson, S. Judd, B. Jefferson and K. Brindle, Membrane Bioreactors for Wastewater Treatment. IWA Publishing, London, 2000.
45
[2] S. Rosenberger, U. Krüger, R. Witzig, W. Manz, U. Szewzyk and M. Kraume, Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water. Wat. Res., 36 (2002) 413– 420. [3] P. Pitter and J. Chudoba, Biodegradability of Organic Substances in the Aquatic Environment. CRC Press, Boca Raton, USA, 1990. [4] R.D. Davis and J.E. Hall, Production, treatment and disposal of wastewater sludge in Europe from a UK perspective. Eur Water Pollution Control, 7(2) (1997) 9–17. [5] N.J. Horan, Biological Wastewater Treatment System. Wiley, Chichester, 1990. [6] J. Lobos, C. Wisnieswki, M. Héran and A. Grasmick, Effects of starvation conditions on biomass behaviour for minimization of sludge production in membrane bioreactors, Wat. Sci. Tech., 51(6–7) (2005) 35–44.