- Email: [email protected]
Membrane fouling control through the change of the depth of a membrane module in a submerged membrane bioreactor for advanced wastewater treatment
Desalination 231 (2008) 35–43
Membrane fouling control through the change of the depth of a membrane module in a submerged membrane bioreactor for advanced wastewater treatment Jun-Young Kima, In-Soung Changb*, Dong-Hwan Shinb, Hun-Hwee Parkb a
Department of Semiconductor and Display Engineering, Hoseo University, Asan, 336-795, South Korea b Department of Environmental Engineering, Hoseo University, South Korea Tel. +82 (41) 540-5744; Fax +82 (41) 540 5748; email: [email protected]
Received 17 May 20007; accepted 20 November 2007
Abstract Membrane fouling is a principal limitation of wide application of membrane bioreactors (MBR) to wastewater treatment. In this study, to control the membrane fouling the position of a membrane module in a submerged MBR was elevated from bottom to top of the reactor. This could divide the reactor into two different zones: upper and lower. Air was not supplied at the lower zone whereas aeration was given to the upper zone where the membrane filtration was carried out. Biosolids concentration was reduced in the upper zone because the mixed liquor could be settled down to the lower zone. Therefore, membrane fouling could be lessened in the upper zone due to the reduced biosolids concentration. In this study, to verify if this newly designed MBR configuration could mitigate membrane fouling, the effect of the vertical position of the membrane module in a lab-scale MBR on membrane fouling was investigated. Furthermore, a pilot plant (50 m3/d) of the membrane coupled biological nutrients removal (BNR) process was designed based on the above configuration and was run for 5 months. In the lab test, the higher the membrane was located in the bioreactor, the less membrane fouling was observed. With the pilot plant operation, MLSS concentration in the upper part was lessened to 16~33% than that in the lower part of the tank where air was not supplied. It indicates that two different zones were successfully formed. The interval of periodical chemical cleanings with NaOCl was extended from 2 to 4 months, indicating that the membrane fouling was mitigated. DO concentration at the upper part was 5.3 mg/L, whereas DO at the lower part was 0.4 mg/L. Therefore, this may result in better denitrrification efficiency in the anoxic tank because the recycled sludge to the anoxic tank has low DO concentration. Nitrate concentration at the lower part of the MBR was 2.8 mg/L, whereas that at the upper part
*Corresponding author. Presented at The 4th IWA Conference on Membranes for Water and Wastewater Treatment May 15–17, 2007, Harrogate, UK 0011-9164/08/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.11.036
36
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
was 5.8 mg/L, suggesting that partial denitrification of nitrate was carried out in the lower part of the MBR. Moreover, overall TN removal was 75%, which was higher than that of the conventional BNR processes, indicating that denitrification rate in the anoxic tank increased due to the low DO level of the returned sludge. Consequently, this newly designed MBR could make it possible to control membrane fouling and to get a better TN removal. Keywords: Fouling; Membrane; Membrane bioreactor (MBR); Wastewater
1. Introduction Application of membrane technologies to biological nutrient removal (BNR) processes has been widely practiced for the purpose of complete solid–liquid separation instead of employing a sedimentation tank. This membrane coupled BNR processes have many advantages over the conventional biological processes, such as high and stable removal of organics and nutrients, excellent effluent water quality, and small footprints [1]. Most membrane coupled BNR processes employ anoxic and/or anaerobic tanks as well as an aeration basin for successful denitrification and phosphorous uptake. The mixed liquors containing high concentration of oxygen in an aeration basin are usually returned to an anoxic tank for denitrification, where a low oxygen level is requisite for maintaining anoxic conditions. Denitrifying bacteria suffer from shock or stress due to the high level oxygen in the returned sludge [2]. Therefore, most practical BNR processes employ a degassing tank prior to the anoxic tank for the purpose of reducing oxygen level in the returned sludge. Installation of the degassing tank requires extra footprints and thereby, the overall investment cost increases. On the other hand, an excess and extensive aeration is given to the membrane surface in the aeration basin of membrane coupled BNR processes. Since the excess air makes turbulence near the membrane surface, membrane fouling can be readily lessened by the excess aeration. But it obviously leads to high oxygen level exceeding the amounts of oxygen required for aerobic metabolism. This results in high concentration of
oxygen in the returned sludge, so that degassing is strongly required to be installed prior to the anoxic tank. Moreover, considering that a significant portion of operation and maintenance costs in wastewater treatment plants is originated from aeration costs, excess and extensive aeration to control membrane fouling should be modified and an alternative strategy for membrane fouling control is currently needed. A membrane cassette is usually immersed at the bottom of the aeration basin. It is comprised of a membrane module and an aeration device. Excess air is directly supplied to the membrane module at the bottom of the reactor. If the membrane cassette is located at the middle of depth, the aeration basin would be divided into two different zones; upper and lower. The upper zone above the cassette becomes aerobic because air is supplied from the cassette which is located in the middle of the basin. On the other hand, air is not supplied beneath the cassette, so that the lower zone becomes anaerobic and the mixed liquor settles down. The lower half of the aeration basin becomes a settling tank and the other half acts as aeration basin. When the mixed liquor in the lower zone is returned to the anoxic tank, the oxygen level in the returned sludge could be lowered. Thus, the degassing tank could be omitted in the processes. Mixed liquor of suspended solids (MLSS) concentration in the lower zone becomes higher due to sludge settling, whereas MLSS concentration in the upper zone becomes lower. MLSS would be lower in the upper zone but higher in the lower zone than the original MLSS concentration measured when the reactor is not divided into two zones. Therefore, membrane
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
fouling could be lessened by the reduced MLSS concentration which is one of the important factors affecting membrane fouling. Reduced sludge concentration might decrease the cake resistance (Rc) which is mainly responsible for the membrane fouling in MBR processes [3]. Therefore, the aims of this study were 1) to verify if this new configuration of the aeration basin having two different MLSS concentrations could mitigate the membrane fouling through the lab-scale test, and 2) to confirm if this newly designed membrane coupled BNR process was properly operated through the pilot plant trial.
2. Materials and methods 2.1. Lab-scale test 2.1.1. Activated sludge suspension with different MLSS To prepare activated sludge suspensions having different MLSS concentrations, 4 separate labscale bioreactors (individual reactor’s working volume = 5 L) were run in parallel with different F/M ratios ranging from 0.2 to 0.8 kg COD/kg MLVSS·d. Using a fill and draw technique, each bioreactor had been fed with synthetic feed solu-
influent
37
tion [4] in which the main carbon and nitrogen sources were glucose and ammonium sulfate, respectively. Compressed air was supplied through a diffuser on the bottom of each reactor to provide dissolved oxygen and turbulence for mixing. After 3 months of operation, each bioreactor reached steady state and showed 4 different MLSS concentrations: 3,500, 6,500, 10,000 and 12,400 mg/L. 2.1.2. Operation of MBR To investigate the influence of the elevation of the membrane module in the MBR, another bioreactor was run. Fig. 1 shows schematic of a 5-L lab-scale submerged MBR. U-shaped hollow fiber membrane modules (membrane material is polyvinyledenedifluodide, pore size = 0.4 μm, E&E Co., Korea) were immersed in the four bioreactors individually in this study. 2 l/min of compressed air was supplied through a diffuser to provide dissolved oxygen and mixing. Transmembrane pressure (TMP) profile as a function of time was monitored as the membrane module was vertically elevated along the depth of the MBR. Elevation of the membrane module was expressed as percentage (%) of the depth of the membrane module to the overall reactor depth.
(a)
(b)
PC Feed
(c)
(d)
permeate air membrane coupledaeration tank
Fig. 1. Schematic of the laboratory scale MBR.
(a) peristaltic pump (b) pressure gauge (c) level sensor (d) solenoid valve
38
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
And then membrane filtration was carried out for several hours to monitor the TMP rise under constant flux condition (30 l/m2.h). The TMP profiles of the individual elevations as a function of time were compared with each other. 2.2. Pilot plant operation The newly designed configuration of the membrane coupled-BNR process comprised of anaerobic, anoxic 1, anoxic 2, aerobic and aerated settling tanks is shown in Fig. 2. The membrane module is located in the middle of the tank. Air is supplied only beneath the membrane module, but not supplied to the bottom of the aerated settling tank. Therefore, the sludge under the membrane module has low DO with high MLSS concentration due to the settled sludge. In contrast, the sludge over the membrane module has low MLSS concentration. The overall air supply to the aeration basin could be reduced in this situation, which could lead to low DO concentration in the returned sludge because the returned line is connected to the bottom of the aerated settling tank where the DO level has already been reduced. The hollow fiber membrane module was immersed in the aerated settling tank. A suction pump worked for 8 min and then stopped for 2 min to give the membrane idle. Under the constant flux condition (36 l/m2.h) the TMP was monitored along with running time. The membrane module
used at the pilot plant is of the same kind as described previously. The pilot plant (Q = 50 m3/d) was run with domestic wastewater in the Chonan City in Korea. Total hydraulic retention time (HRT) in the system was approximately 8 h and solids retention time (SRT) was approximately 20–30 d. Internal recycling was 100–200% of the influent flow rate. External recycling ratio was 50–100%. 0.6 m3/min of aeration was given to the aerated settling tank. 2.3. Analytical methods MLSS and BOD5 concentrations were measured according to the analytical methods described in the Standard Methods [5]. CODmn, TN and TP concentrations were determined using a spectrophometric method with DR 4000 (Hach, USA). To represent the soluble foulants in the bulk phase, total organic carbon (TOC) concentration of the filtrates through GF/C (Whatman, UK) was measured using TOC analyzer (Phoenix 80000, Tekmer Dohrmann, USA). 3. Results and discussion 3.1. Lab-scale test 3.1.1. Effect of MLSS concentration on TMP Prior to verification of the effect of the elevation of the membrane module in the reactor on Blower
Influent
Internal Recycled Sludge
Effluent Effluent
External Recycled Sludge
Anaerobic Tank
Anoxic Tank 1
Anoxic Tank 2
Aerobic Tank
Fig. 2. Schematic of the pilot plant with newly configured membrane coupled BNR.
Aerated Settler
Excess Sludge
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
membrane fouling, the influence of MLSS concentration on membrane fouling was investigated. Membrane filtration of activated sludge suspensions having different MLSS was carried out individually. The TMP rise under the constant flux mode was monitored as a function of time (Fig. 3). The TMP rise became steep as the MLSS concentration increased. The time required to reach 0.5 bar of TMP was shortened as MLSS increased. For example, 200 min was required at MLSS of 3,500 mg/L, but 20 min was needed at MLSS of 12,400 mg/L. The cake layer formed on the membrane surface was comprised of biomass in the bulk solution, and thus, the increase in MLSS in bulk phase was believed to increase the mass of the cake layer (M). According to the following well-known equation [6], the increase in M leads to the increase in the cake layer resistance (Rc).
Rc =
M ⋅ α M 180 (1 − ε ) = Am Am ρ p ⋅ d p2 ⋅ ε3
39
particles diameter (m), M = mass of the cake layer (kg), Am = membrane area (m2). Even though MLSS is not the only factor affecting Rc, microorganisms in the bulk solution with high MLSS concentration could possibly increase the mass of the cake layer (M), which obviously resulted in severe fouling. To compare membrane fouling quantitatively at each MLSS concentration, Rc values were calculated using resistance-in-series model after completing membrane filtration. Table 1 shows that Rc was predominant resistance and linearly proportional to the MLSS concentration, indicating that MLSS directly affected the degree of membrane fouling. Therefore, it was required to confirm if the membrane filterability under reduced MLSS concentration could be enhanced. Reduced MLSS concentration could be obtained by the change of the elevation of the membrane module in the submerged MBR.
(1)
3.1.2. Effect of elevation of membrane module on fouling
where α = specific cake resistance (m/kg), ε = porosity, ρp = density of particles (kg/m3), dp =
Three runs of membrane filtration were performed as the depth of the membrane module was
0.7 0.6
TMP (bar )
0.5 0.4 0.3
MLSS 3,500mg/L MLSS 6,500mg/L
0.2
MLSS 10,000mg/L MLSS 12,400mg/L
0.1 0 0
30
60
90
120 150 Time (min)
180
210
Fig. 3. TMP profile during filtration of activated sludge with different MLSS concentrations.
240
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J.-Y. Kim et al. / Desalination 231 (2008) 35–43
Table 1 A series of resistances calculated by the resistance-in-series model Resistance (1010 m–1)
MLSS (mg/L) 3,500 6,500 10,000 12,400
Cake resistance ratio (%)
Rm
Rc
Rf
RT
Rc/(Rc + Rf)
4 4 3 3
5.2 6.2 9 11
0.8 0.8 1 1
10 11 13 15
87 89 90 92
changed. Run 1 was carried out with the sludge having MLSS of 7,050 mg/L. The membrane module was submerged near the bottom of the reactor. Air was supplied to the bottom of the reactor, so that mixed liquor in the reactor was totally mixed. At run 2, a new membrane module was put to the middle of the reactor, where the membrane module was located at 49% of the whole depth of the reactor. Since air was supplied to the bottom of the membrane module, not the bottom of the reactor, the reactor was divided into two separate zones: upper and lower. Mixed liquor in the lower zone had high MLSS due to settling, whereas MLSS was relatively low in the upper zone. At run 3, another new membrane module was put near the top of the reactor. Thus, MLSS of the upper zone at run 3 was much smaller than that at run 2. Table 2 shows the relative elevation of the membrane module in the reactor and MLSS concentrations of the upper and lower zones.
At run 2, the MLSS concentration in the upper zone, where membrane filtration was carried out, was two times higher than that of run 3 (Table 2). Therefore, the TMP rise at run 1 was faster than that of runs 2 and 3 (Fig. 4). The most rapid TMP rise was observed at run 1, whereas TMP at run 3 increased very slowly and gradually, i.e., the slowest TMP rise was observed at the highest elevation of the membrane module (run 3). TMP rise rates of runs 1, 2 and 3 were 0.21, 0.19, 0.08 bar/min, respectively. The TMP rise profile can provide information on the degree of overall membrane fouling, but it does not give us any information on what is the predominant contributor to the membrane fouling. Therefore, to understand the fouling phenomenon in detail, cake resistance (Rc) and fouling resistance (Rf) were calculated after each run. Table 3 shows that Rc decreased as the membrane module was vertically elevated in the reac-
Table 2 Variation of MLSS and soluble foulants concentrations according to the elevation of the membrane module in the bioreactor Operation
Elevation of membrane module in bioreactor (%)
MLSS (mg/L)
Soluble foulants (mg TOC/L)
Run 1
15
7,050
78
Run 2
49
Upper zone: 6,050 Lower zone:15,400
Upper zone: 76 Lower zone: 85
Run 3
68
Upper zone: 3,050 Lower zone:18,050
Upper zone: 67 Lower zone: 353
J.-Y. Kim et al. / Desalination 231 (2008) 35–43 0.3
41
Run-1 Run-2 Run-3
0.25
TMP (bar )
0.2 0.15 0.1 0.05 0 0
30
60
90
120
150
180
210
240
270
Time (min)
Fig. 4. TMP profile during membrane filtration of activated sludge according to the elevation of the membrane module in the reactor.
Table 3 Calculated resistances according to the elevation of the membrane module Resistance (1010 m–1) Run 1 Run 2 Run 3
Cake resistance ratio (%)
Rm
Rc
Rf
RT
Rc/(Rc + Rf)
1.5 1.3 1.5
5.0 3.8 2.2
0.5 0.9 0.7
7.0 6.0 4.4
91 81 76
tor. Since the MLSS concentration in the upper zone decreased as the membrane module was elevated, Rc in run 3 was the smallest. On the other hand, Rf was not changed as much as Rc. Because the soluble foulant concentrations in the upper zone at each run were similar to each other as shown in Table 2, the individual Rf values at each run were not greatly changed as the membrane module was elevated. Consequently, the vertical position of the membrane module in the submerged MBR affects membrane fouling. The higher it was located in the bioreactor, the less membrane fouling was observed.
3.2. Pilot-plant test 3.2.1. Removal of organic compounds Fig. 5 shows a variation of BOD5 and CODmn of influent and effluent during the pilot plant. Influent BOD5 concentrations varied from 53.6 to 163.7 mg/L (average: 89.1 mg/L) and influent CODmn concentrations varied from 25.9 to 109.2 mg/L, (average: 60 mg/L). Effluent BOD5 concentrations ranged from 0.8 to 5.7 mg/L. The average removal efficiency of BOD5 was 96%. Effluent CODmn concentrations ranged from 3.7 to 10.9 mg/L. The average removal efficiency of CODmn was 87.9%.
42
J.-Y. Kim et al. / Desalination 231 (2008) 35–43 180
120
C OD Mn I nf l ue nt
100
C OD Mn E ff l ue nt
BOD5 Influent
160
BOD5 Effluent
120
Con.(mg/L)
Con.(mg/L)
140 100 80 60 40
(a)
20
80 60 40
(b)
20 0
0 1
30
60 90 Time(day)
120
150
1
30
60 90 Ti me (d a y)
1 20
15 0
Fig. 5. Variation of influent and effluent BOD5 (a) and CODmn (b).
3.2.2. TN and TP removal Total nitrogen (TN) concentrations in the influent ranged from 22.4 to 38.2 mg/L (average: 30.6 mg/L). Ammonia nitrogen (NH+-N) was the main component of the influent TN — approximately 60%. As shown in Fig. 6a, effluent TN concentrations ranged from 3.9 to 12.1 mg/L. It should be noted that the average TN removal was around 75% and furthermore, the removal increased as the operation extended. The TN removal efficiency of the conventional membrane coupled BNR process is around 70%. Therefore, this newly designed BNR configuration was effective for improving TN removal because the denitrification rate in the anoxic tank might be increased due to the low DO level of the returned sludge. Also, nitrate (NO3–) concentration in the lower part of the aerated settling tank was 2.8 mg/L, whereas that in the upper part of the aerated settling tank was 5.8 mg/L, suggesting that partial denitrification of nitrate was carried out in the lower part of the aeration basin. Total phosphorus (TP) concentrations in the influent ranged from 1.96 to 5.44 mg/L (average: 3.36 mg/L). As shown in Fig. 6b, the effluent TP concentrations ranged from 0.62 to 1.49 mg/L. The average removal rate of TP was 66%.
3.2.3. Variation of TMP The rate of TMP build-up is an important factor in evaluating membrane filterability in submerged MBR systems because it is directly related to the extent of membrane fouling. TMP (Fig. 7) increased from 16 to 39 kPa during 5 months. After TMP reached to 39 kPa, chemical cleaning by using 0.2% sodium hypochlorite (NaOCl) was carried out to maintain the steady flux. The extension of periodical cleaning interval from 2 to 4 months was due to low MLSS concentration at the upper part in the aerated settling tank. 4. Conclusions In this study, the influence of the elevation of the membrane module in the lab scale and pilot plant MBR on membrane fouling was investigated. Due to the reduced MLSS concentration in the upper zone of the aeration basin, membrane fouling was mitigated as the membrane module was vertically elevated. The higher the membrane was located in the bioreactor, the less membrane fouling was observed. The interval of periodical chemical cleanings for the pilot plant was extended from 2 to 4 months, indicating that the membrane fouling could be mitigated due to the
J.-Y. Kim et al. / Desalination 231 (2008) 35–43 (a)
40
6
35
43 (b)
TP Influent TP Effluent
5
Con.(mg/L)
Con.(mg/L)
30 25 20
TN Influent
15
TN Effluent
10
4 3 2 1
5 0
0 1
30
60
90
120
150
1
30
60
Time(day)
90
120
150
Time(day)
TMP (kPa)
Fig. 6. Concentration variation of the influent and effluent TN (a) and TP (b). 45 40 35 30 25 20 15 10 5 0
Chemical cleaning
1
30
60
90
120
150
Time (day)
Fig. 7. TMP profile during the pilot plant operation.
low MLSS concentration at the upper part in the aerated settling tank. The overall TN removal efficiency was improved because the returned sludge had a low DO level. Consequently, all of the experimental results show us a possibility of the new MBR design to control membrane fouling economically.
[3]
[4]
References [1] S.R. Chae, T.T. Ahn, S.T. Kang and H.S. Shin, Mitigated membrane fouling in a vertical submerged membrane bioreactor (VSMBR). J. Membr. Sci., 280 (2006) 572–581. [2] Y.Z. Peng, X.l. Wang and B.K. Li, Anoxic biological phosphorus uptake and the effect of excessive
[5]
[6]
aeration on biological phosphorus removal in the A2O process. Desalination, 189 (2006) 155–164. P. Le-Clech, B. Jefferson and S.J. Judd, Impact of aeration, solids concentration and membrane characteristics on the hydraulic performance of a membrane bioreactor. J. Membr. Sci., 218 (2003) 117– 129. I.S. Chang, C.H. Lee and K.H. Ahn, Membrane filtration characteristics in membrane-coupled activated sludge system: The effect of floc structure on membrane fouling. Sep. Sci. Technol., 34(9) (1999) 1743–1758. APHA/AWWA/WEF. Standard Methods for the Examination of Water and Wastewater, 19th ed., Washington, DC, 1995. J.S. Kim, C.H. Lee and I.S. Chang, Effect of pump shear on the performance of a crossflow membrane bioreactor. Water Res., 35(9) (2001) 2137–2144.
Membrane fouling control through the change of the depth of a membrane module in a submerged membrane bioreactor for advanced wastewater treatment Jun-Young Kima, In-Soung Changb*, Dong-Hwan Shinb, Hun-Hwee Parkb a
Department of Semiconductor and Display Engineering, Hoseo University, Asan, 336-795, South Korea b Department of Environmental Engineering, Hoseo University, South Korea Tel. +82 (41) 540-5744; Fax +82 (41) 540 5748; email: [email protected]
Received 17 May 20007; accepted 20 November 2007
Abstract Membrane fouling is a principal limitation of wide application of membrane bioreactors (MBR) to wastewater treatment. In this study, to control the membrane fouling the position of a membrane module in a submerged MBR was elevated from bottom to top of the reactor. This could divide the reactor into two different zones: upper and lower. Air was not supplied at the lower zone whereas aeration was given to the upper zone where the membrane filtration was carried out. Biosolids concentration was reduced in the upper zone because the mixed liquor could be settled down to the lower zone. Therefore, membrane fouling could be lessened in the upper zone due to the reduced biosolids concentration. In this study, to verify if this newly designed MBR configuration could mitigate membrane fouling, the effect of the vertical position of the membrane module in a lab-scale MBR on membrane fouling was investigated. Furthermore, a pilot plant (50 m3/d) of the membrane coupled biological nutrients removal (BNR) process was designed based on the above configuration and was run for 5 months. In the lab test, the higher the membrane was located in the bioreactor, the less membrane fouling was observed. With the pilot plant operation, MLSS concentration in the upper part was lessened to 16~33% than that in the lower part of the tank where air was not supplied. It indicates that two different zones were successfully formed. The interval of periodical chemical cleanings with NaOCl was extended from 2 to 4 months, indicating that the membrane fouling was mitigated. DO concentration at the upper part was 5.3 mg/L, whereas DO at the lower part was 0.4 mg/L. Therefore, this may result in better denitrrification efficiency in the anoxic tank because the recycled sludge to the anoxic tank has low DO concentration. Nitrate concentration at the lower part of the MBR was 2.8 mg/L, whereas that at the upper part
*Corresponding author. Presented at The 4th IWA Conference on Membranes for Water and Wastewater Treatment May 15–17, 2007, Harrogate, UK 0011-9164/08/$– See front matter © 2008 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2007.11.036
36
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
was 5.8 mg/L, suggesting that partial denitrification of nitrate was carried out in the lower part of the MBR. Moreover, overall TN removal was 75%, which was higher than that of the conventional BNR processes, indicating that denitrification rate in the anoxic tank increased due to the low DO level of the returned sludge. Consequently, this newly designed MBR could make it possible to control membrane fouling and to get a better TN removal. Keywords: Fouling; Membrane; Membrane bioreactor (MBR); Wastewater
1. Introduction Application of membrane technologies to biological nutrient removal (BNR) processes has been widely practiced for the purpose of complete solid–liquid separation instead of employing a sedimentation tank. This membrane coupled BNR processes have many advantages over the conventional biological processes, such as high and stable removal of organics and nutrients, excellent effluent water quality, and small footprints [1]. Most membrane coupled BNR processes employ anoxic and/or anaerobic tanks as well as an aeration basin for successful denitrification and phosphorous uptake. The mixed liquors containing high concentration of oxygen in an aeration basin are usually returned to an anoxic tank for denitrification, where a low oxygen level is requisite for maintaining anoxic conditions. Denitrifying bacteria suffer from shock or stress due to the high level oxygen in the returned sludge [2]. Therefore, most practical BNR processes employ a degassing tank prior to the anoxic tank for the purpose of reducing oxygen level in the returned sludge. Installation of the degassing tank requires extra footprints and thereby, the overall investment cost increases. On the other hand, an excess and extensive aeration is given to the membrane surface in the aeration basin of membrane coupled BNR processes. Since the excess air makes turbulence near the membrane surface, membrane fouling can be readily lessened by the excess aeration. But it obviously leads to high oxygen level exceeding the amounts of oxygen required for aerobic metabolism. This results in high concentration of
oxygen in the returned sludge, so that degassing is strongly required to be installed prior to the anoxic tank. Moreover, considering that a significant portion of operation and maintenance costs in wastewater treatment plants is originated from aeration costs, excess and extensive aeration to control membrane fouling should be modified and an alternative strategy for membrane fouling control is currently needed. A membrane cassette is usually immersed at the bottom of the aeration basin. It is comprised of a membrane module and an aeration device. Excess air is directly supplied to the membrane module at the bottom of the reactor. If the membrane cassette is located at the middle of depth, the aeration basin would be divided into two different zones; upper and lower. The upper zone above the cassette becomes aerobic because air is supplied from the cassette which is located in the middle of the basin. On the other hand, air is not supplied beneath the cassette, so that the lower zone becomes anaerobic and the mixed liquor settles down. The lower half of the aeration basin becomes a settling tank and the other half acts as aeration basin. When the mixed liquor in the lower zone is returned to the anoxic tank, the oxygen level in the returned sludge could be lowered. Thus, the degassing tank could be omitted in the processes. Mixed liquor of suspended solids (MLSS) concentration in the lower zone becomes higher due to sludge settling, whereas MLSS concentration in the upper zone becomes lower. MLSS would be lower in the upper zone but higher in the lower zone than the original MLSS concentration measured when the reactor is not divided into two zones. Therefore, membrane
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
fouling could be lessened by the reduced MLSS concentration which is one of the important factors affecting membrane fouling. Reduced sludge concentration might decrease the cake resistance (Rc) which is mainly responsible for the membrane fouling in MBR processes [3]. Therefore, the aims of this study were 1) to verify if this new configuration of the aeration basin having two different MLSS concentrations could mitigate the membrane fouling through the lab-scale test, and 2) to confirm if this newly designed membrane coupled BNR process was properly operated through the pilot plant trial.
2. Materials and methods 2.1. Lab-scale test 2.1.1. Activated sludge suspension with different MLSS To prepare activated sludge suspensions having different MLSS concentrations, 4 separate labscale bioreactors (individual reactor’s working volume = 5 L) were run in parallel with different F/M ratios ranging from 0.2 to 0.8 kg COD/kg MLVSS·d. Using a fill and draw technique, each bioreactor had been fed with synthetic feed solu-
influent
37
tion [4] in which the main carbon and nitrogen sources were glucose and ammonium sulfate, respectively. Compressed air was supplied through a diffuser on the bottom of each reactor to provide dissolved oxygen and turbulence for mixing. After 3 months of operation, each bioreactor reached steady state and showed 4 different MLSS concentrations: 3,500, 6,500, 10,000 and 12,400 mg/L. 2.1.2. Operation of MBR To investigate the influence of the elevation of the membrane module in the MBR, another bioreactor was run. Fig. 1 shows schematic of a 5-L lab-scale submerged MBR. U-shaped hollow fiber membrane modules (membrane material is polyvinyledenedifluodide, pore size = 0.4 μm, E&E Co., Korea) were immersed in the four bioreactors individually in this study. 2 l/min of compressed air was supplied through a diffuser to provide dissolved oxygen and mixing. Transmembrane pressure (TMP) profile as a function of time was monitored as the membrane module was vertically elevated along the depth of the MBR. Elevation of the membrane module was expressed as percentage (%) of the depth of the membrane module to the overall reactor depth.
(a)
(b)
PC Feed
(c)
(d)
permeate air membrane coupledaeration tank
Fig. 1. Schematic of the laboratory scale MBR.
(a) peristaltic pump (b) pressure gauge (c) level sensor (d) solenoid valve
38
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
And then membrane filtration was carried out for several hours to monitor the TMP rise under constant flux condition (30 l/m2.h). The TMP profiles of the individual elevations as a function of time were compared with each other. 2.2. Pilot plant operation The newly designed configuration of the membrane coupled-BNR process comprised of anaerobic, anoxic 1, anoxic 2, aerobic and aerated settling tanks is shown in Fig. 2. The membrane module is located in the middle of the tank. Air is supplied only beneath the membrane module, but not supplied to the bottom of the aerated settling tank. Therefore, the sludge under the membrane module has low DO with high MLSS concentration due to the settled sludge. In contrast, the sludge over the membrane module has low MLSS concentration. The overall air supply to the aeration basin could be reduced in this situation, which could lead to low DO concentration in the returned sludge because the returned line is connected to the bottom of the aerated settling tank where the DO level has already been reduced. The hollow fiber membrane module was immersed in the aerated settling tank. A suction pump worked for 8 min and then stopped for 2 min to give the membrane idle. Under the constant flux condition (36 l/m2.h) the TMP was monitored along with running time. The membrane module
used at the pilot plant is of the same kind as described previously. The pilot plant (Q = 50 m3/d) was run with domestic wastewater in the Chonan City in Korea. Total hydraulic retention time (HRT) in the system was approximately 8 h and solids retention time (SRT) was approximately 20–30 d. Internal recycling was 100–200% of the influent flow rate. External recycling ratio was 50–100%. 0.6 m3/min of aeration was given to the aerated settling tank. 2.3. Analytical methods MLSS and BOD5 concentrations were measured according to the analytical methods described in the Standard Methods [5]. CODmn, TN and TP concentrations were determined using a spectrophometric method with DR 4000 (Hach, USA). To represent the soluble foulants in the bulk phase, total organic carbon (TOC) concentration of the filtrates through GF/C (Whatman, UK) was measured using TOC analyzer (Phoenix 80000, Tekmer Dohrmann, USA). 3. Results and discussion 3.1. Lab-scale test 3.1.1. Effect of MLSS concentration on TMP Prior to verification of the effect of the elevation of the membrane module in the reactor on Blower
Influent
Internal Recycled Sludge
Effluent Effluent
External Recycled Sludge
Anaerobic Tank
Anoxic Tank 1
Anoxic Tank 2
Aerobic Tank
Fig. 2. Schematic of the pilot plant with newly configured membrane coupled BNR.
Aerated Settler
Excess Sludge
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
membrane fouling, the influence of MLSS concentration on membrane fouling was investigated. Membrane filtration of activated sludge suspensions having different MLSS was carried out individually. The TMP rise under the constant flux mode was monitored as a function of time (Fig. 3). The TMP rise became steep as the MLSS concentration increased. The time required to reach 0.5 bar of TMP was shortened as MLSS increased. For example, 200 min was required at MLSS of 3,500 mg/L, but 20 min was needed at MLSS of 12,400 mg/L. The cake layer formed on the membrane surface was comprised of biomass in the bulk solution, and thus, the increase in MLSS in bulk phase was believed to increase the mass of the cake layer (M). According to the following well-known equation [6], the increase in M leads to the increase in the cake layer resistance (Rc).
Rc =
M ⋅ α M 180 (1 − ε ) = Am Am ρ p ⋅ d p2 ⋅ ε3
39
particles diameter (m), M = mass of the cake layer (kg), Am = membrane area (m2). Even though MLSS is not the only factor affecting Rc, microorganisms in the bulk solution with high MLSS concentration could possibly increase the mass of the cake layer (M), which obviously resulted in severe fouling. To compare membrane fouling quantitatively at each MLSS concentration, Rc values were calculated using resistance-in-series model after completing membrane filtration. Table 1 shows that Rc was predominant resistance and linearly proportional to the MLSS concentration, indicating that MLSS directly affected the degree of membrane fouling. Therefore, it was required to confirm if the membrane filterability under reduced MLSS concentration could be enhanced. Reduced MLSS concentration could be obtained by the change of the elevation of the membrane module in the submerged MBR.
(1)
3.1.2. Effect of elevation of membrane module on fouling
where α = specific cake resistance (m/kg), ε = porosity, ρp = density of particles (kg/m3), dp =
Three runs of membrane filtration were performed as the depth of the membrane module was
0.7 0.6
TMP (bar )
0.5 0.4 0.3
MLSS 3,500mg/L MLSS 6,500mg/L
0.2
MLSS 10,000mg/L MLSS 12,400mg/L
0.1 0 0
30
60
90
120 150 Time (min)
180
210
Fig. 3. TMP profile during filtration of activated sludge with different MLSS concentrations.
240
40
J.-Y. Kim et al. / Desalination 231 (2008) 35–43
Table 1 A series of resistances calculated by the resistance-in-series model Resistance (1010 m–1)
MLSS (mg/L) 3,500 6,500 10,000 12,400
Cake resistance ratio (%)
Rm
Rc
Rf
RT
Rc/(Rc + Rf)
4 4 3 3
5.2 6.2 9 11
0.8 0.8 1 1
10 11 13 15
87 89 90 92
changed. Run 1 was carried out with the sludge having MLSS of 7,050 mg/L. The membrane module was submerged near the bottom of the reactor. Air was supplied to the bottom of the reactor, so that mixed liquor in the reactor was totally mixed. At run 2, a new membrane module was put to the middle of the reactor, where the membrane module was located at 49% of the whole depth of the reactor. Since air was supplied to the bottom of the membrane module, not the bottom of the reactor, the reactor was divided into two separate zones: upper and lower. Mixed liquor in the lower zone had high MLSS due to settling, whereas MLSS was relatively low in the upper zone. At run 3, another new membrane module was put near the top of the reactor. Thus, MLSS of the upper zone at run 3 was much smaller than that at run 2. Table 2 shows the relative elevation of the membrane module in the reactor and MLSS concentrations of the upper and lower zones.
At run 2, the MLSS concentration in the upper zone, where membrane filtration was carried out, was two times higher than that of run 3 (Table 2). Therefore, the TMP rise at run 1 was faster than that of runs 2 and 3 (Fig. 4). The most rapid TMP rise was observed at run 1, whereas TMP at run 3 increased very slowly and gradually, i.e., the slowest TMP rise was observed at the highest elevation of the membrane module (run 3). TMP rise rates of runs 1, 2 and 3 were 0.21, 0.19, 0.08 bar/min, respectively. The TMP rise profile can provide information on the degree of overall membrane fouling, but it does not give us any information on what is the predominant contributor to the membrane fouling. Therefore, to understand the fouling phenomenon in detail, cake resistance (Rc) and fouling resistance (Rf) were calculated after each run. Table 3 shows that Rc decreased as the membrane module was vertically elevated in the reac-
Table 2 Variation of MLSS and soluble foulants concentrations according to the elevation of the membrane module in the bioreactor Operation
Elevation of membrane module in bioreactor (%)
MLSS (mg/L)
Soluble foulants (mg TOC/L)
Run 1
15
7,050
78
Run 2
49
Upper zone: 6,050 Lower zone:15,400
Upper zone: 76 Lower zone: 85
Run 3
68
Upper zone: 3,050 Lower zone:18,050
Upper zone: 67 Lower zone: 353
J.-Y. Kim et al. / Desalination 231 (2008) 35–43 0.3
41
Run-1 Run-2 Run-3
0.25
TMP (bar )
0.2 0.15 0.1 0.05 0 0
30
60
90
120
150
180
210
240
270
Time (min)
Fig. 4. TMP profile during membrane filtration of activated sludge according to the elevation of the membrane module in the reactor.
Table 3 Calculated resistances according to the elevation of the membrane module Resistance (1010 m–1) Run 1 Run 2 Run 3
Cake resistance ratio (%)
Rm
Rc
Rf
RT
Rc/(Rc + Rf)
1.5 1.3 1.5
5.0 3.8 2.2
0.5 0.9 0.7
7.0 6.0 4.4
91 81 76
tor. Since the MLSS concentration in the upper zone decreased as the membrane module was elevated, Rc in run 3 was the smallest. On the other hand, Rf was not changed as much as Rc. Because the soluble foulant concentrations in the upper zone at each run were similar to each other as shown in Table 2, the individual Rf values at each run were not greatly changed as the membrane module was elevated. Consequently, the vertical position of the membrane module in the submerged MBR affects membrane fouling. The higher it was located in the bioreactor, the less membrane fouling was observed.
3.2. Pilot-plant test 3.2.1. Removal of organic compounds Fig. 5 shows a variation of BOD5 and CODmn of influent and effluent during the pilot plant. Influent BOD5 concentrations varied from 53.6 to 163.7 mg/L (average: 89.1 mg/L) and influent CODmn concentrations varied from 25.9 to 109.2 mg/L, (average: 60 mg/L). Effluent BOD5 concentrations ranged from 0.8 to 5.7 mg/L. The average removal efficiency of BOD5 was 96%. Effluent CODmn concentrations ranged from 3.7 to 10.9 mg/L. The average removal efficiency of CODmn was 87.9%.
42
J.-Y. Kim et al. / Desalination 231 (2008) 35–43 180
120
C OD Mn I nf l ue nt
100
C OD Mn E ff l ue nt
BOD5 Influent
160
BOD5 Effluent
120
Con.(mg/L)
Con.(mg/L)
140 100 80 60 40
(a)
20
80 60 40
(b)
20 0
0 1
30
60 90 Time(day)
120
150
1
30
60 90 Ti me (d a y)
1 20
15 0
Fig. 5. Variation of influent and effluent BOD5 (a) and CODmn (b).
3.2.2. TN and TP removal Total nitrogen (TN) concentrations in the influent ranged from 22.4 to 38.2 mg/L (average: 30.6 mg/L). Ammonia nitrogen (NH+-N) was the main component of the influent TN — approximately 60%. As shown in Fig. 6a, effluent TN concentrations ranged from 3.9 to 12.1 mg/L. It should be noted that the average TN removal was around 75% and furthermore, the removal increased as the operation extended. The TN removal efficiency of the conventional membrane coupled BNR process is around 70%. Therefore, this newly designed BNR configuration was effective for improving TN removal because the denitrification rate in the anoxic tank might be increased due to the low DO level of the returned sludge. Also, nitrate (NO3–) concentration in the lower part of the aerated settling tank was 2.8 mg/L, whereas that in the upper part of the aerated settling tank was 5.8 mg/L, suggesting that partial denitrification of nitrate was carried out in the lower part of the aeration basin. Total phosphorus (TP) concentrations in the influent ranged from 1.96 to 5.44 mg/L (average: 3.36 mg/L). As shown in Fig. 6b, the effluent TP concentrations ranged from 0.62 to 1.49 mg/L. The average removal rate of TP was 66%.
3.2.3. Variation of TMP The rate of TMP build-up is an important factor in evaluating membrane filterability in submerged MBR systems because it is directly related to the extent of membrane fouling. TMP (Fig. 7) increased from 16 to 39 kPa during 5 months. After TMP reached to 39 kPa, chemical cleaning by using 0.2% sodium hypochlorite (NaOCl) was carried out to maintain the steady flux. The extension of periodical cleaning interval from 2 to 4 months was due to low MLSS concentration at the upper part in the aerated settling tank. 4. Conclusions In this study, the influence of the elevation of the membrane module in the lab scale and pilot plant MBR on membrane fouling was investigated. Due to the reduced MLSS concentration in the upper zone of the aeration basin, membrane fouling was mitigated as the membrane module was vertically elevated. The higher the membrane was located in the bioreactor, the less membrane fouling was observed. The interval of periodical chemical cleanings for the pilot plant was extended from 2 to 4 months, indicating that the membrane fouling could be mitigated due to the
J.-Y. Kim et al. / Desalination 231 (2008) 35–43 (a)
40
6
35
43 (b)
TP Influent TP Effluent
5
Con.(mg/L)
Con.(mg/L)
30 25 20
TN Influent
15
TN Effluent
10
4 3 2 1
5 0
0 1
30
60
90
120
150
1
30
60
Time(day)
90
120
150
Time(day)
TMP (kPa)
Fig. 6. Concentration variation of the influent and effluent TN (a) and TP (b). 45 40 35 30 25 20 15 10 5 0
Chemical cleaning
1
30
60
90
120
150
Time (day)
Fig. 7. TMP profile during the pilot plant operation.
low MLSS concentration at the upper part in the aerated settling tank. The overall TN removal efficiency was improved because the returned sludge had a low DO level. Consequently, all of the experimental results show us a possibility of the new MBR design to control membrane fouling economically.
[3]
[4]
References [1] S.R. Chae, T.T. Ahn, S.T. Kang and H.S. Shin, Mitigated membrane fouling in a vertical submerged membrane bioreactor (VSMBR). J. Membr. Sci., 280 (2006) 572–581. [2] Y.Z. Peng, X.l. Wang and B.K. Li, Anoxic biological phosphorus uptake and the effect of excessive
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aeration on biological phosphorus removal in the A2O process. Desalination, 189 (2006) 155–164. P. Le-Clech, B. Jefferson and S.J. Judd, Impact of aeration, solids concentration and membrane characteristics on the hydraulic performance of a membrane bioreactor. J. Membr. Sci., 218 (2003) 117– 129. I.S. Chang, C.H. Lee and K.H. Ahn, Membrane filtration characteristics in membrane-coupled activated sludge system: The effect of floc structure on membrane fouling. Sep. Sci. Technol., 34(9) (1999) 1743–1758. APHA/AWWA/WEF. Standard Methods for the Examination of Water and Wastewater, 19th ed., Washington, DC, 1995. J.S. Kim, C.H. Lee and I.S. Chang, Effect of pump shear on the performance of a crossflow membrane bioreactor. Water Res., 35(9) (2001) 2137–2144.