- Email: [email protected]
A novel application of automatic vacuum membrane bioreactor in wastewater reclamation
DESALINATION Desalination 247 (2009) 583–593
www.elsevier.com/locate/desal
A novel application of automatic vacuum membrane bioreactor in wastewater reclamation Mei-zhuo Gaoa, Zhao-bo Chena*, Nan-qi Rena, Zhen-Peng Zhangb a
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China b Beijing Enterprises Water Group, Beijing, 100195, China Fax: +86-451-8628-2008; email: [email protected], [email protected] Received 11 July 2007; accepted 16 April 2009
Abstract An automatic vacuum membrane bioreactor (AVMBR) equipped with an on-line air–water–chemicals backwashing system was developed for a purpose of the reclamation of domestic wastewater (300–800 mg-COD/L) and traditional Chinese medicine (TCM) wastewater (259–12775 mg-COD/L). The experimental setups were operated in pilot scale at hydraulic retention times (HRTs) of 3, 1 and 0.5 h for domestic wastewater, and at HRTs of 1, 3, 5 and 8 h for TCM wastewater. Experimental results indicated that membrane flux was able to resume as the fouled membrane modules being operated for 150 days were repeatedly subject to an air–water– chemicals backwashing procedure. Compared to a new membrane module, the contaminated membrane fluxes were resumed to 94.7% after the first cleaning, to 82.7% after the second cleaning and to 70.0% after the third cleaning at a vacuum value of 0.02 MPa. To meet reuse standard (effluent COD, less than 30 mg/L), the minimum HRT required by the AVMBR system was 1 h for domestic wastewater reclamation, and 1 h for less than 1000 mg-COD/L, 3.0 h for 1000–3000 mg-COD/L, and 5.0 h for 3000–6000 mg-COD/L TCM wastewater reclamation. However, when the TCM wastewater COD was larger than 6000 mg/L, an 8-h HRT operation of the AVMBR system failed to produce an effluent having a COD concentration less than 30 mg/L. It is concluded that the pilot-scale AVMBR system developed is feasible for the reclamation of domestic wastewater and TCM wastewater with a COD concentration less than 6000 mg/L. Keywords: Air–water–chemicals backwashing; Automatic vacuum membrane bioreactor (AVMBR); Domestic wastewater; Traditional chinese medicine (TCM) wastewater; Wastewater reclamation
1. Introduction Membrane bioreactors (MBRs) or combined with other pre-/post-biological treatment systems *Corresponding author.
have been developed for the treatment of municipal wastewater and industrial wastewaters [1–5]. Membrane filtration guarantees the production of a high-quality effluent since microbial sludge and other suspended solids are able to be retained
0011-9164/09/$– See front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.04.003
584
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
by the membrane. However, solid–liquid separation mode is a major concern in MBR operation. Solid and liquid phases are separated through either pump suction or gravity outflow in the conventional submerged MBRs [6–8]. An intermittent permeate is generally produced while employing pump suction or gravity outflow as the solid–liquid separation mode. Obviously, such solid-liquid separation modes impeded the application of MBRs to wastewater treatment in practice as the continuous permeate was required. Membrane fouling was unavoidable over a long operational period of the MBRs, which led to the decrease of the membrane flux and thus enhanced the operating costs. Periodic backwashing on membrane modules is a common approach to alleviate membrane fouling. Different backwashing strategies have been developed to clean the membrane modules with water and/or air [9–12]. Water backwashing alone has been proved to be an inherently weak cleaning technique since it has to rely predominantly upon hydrodynamic shear forces for cleaning [13]. Air backwashing is an alternative choice derived from the operation of filters, which combines regular water backwashing. Backwashing cleaning efficiency is able to be enhanced by combining air scour as a result of the improved hydrodynamic and medium collision. However, the collision is generally confined to at or near the filter membrane surfaces, resulting in a limited cleaning effectiveness of fouled membrane. Automatic vacuum membrane bioreactor (AVMBR) is a novel-type reactor used for wastewater reclamation. It features the continuous effluent discharge derived from conventional submerged MBR through an automatic vacuum suction system. To authors’ knowledge, AVMBR system does not appear to have been reported in literature. In this study, an AVMBR equipped with a novel on-line airwater-chemicals backwashing system was
developed for a purpose of the reclamation of domestic wastewater and traditional Chinese medicine (TCM) wastewater. The experimental setups were operated in the pilot scale at different hydraulic retention times (HRTs). In addition, cleaning effectiveness of air–water– chemicals backwashing on the membrane modules were evaluated by examining the resume degree of membrane flux.
2. Materials and methods 2.1. Wastewater Domestic wastewater and traditional Chinese medicine (TCM) wastewater were used as the sole carbon sources, respectively. The domestic wastewater had a COD concentration of 300– 800 mg/L, total nitrogen of 25–80 mg/L, total phosphorus of 15–35 mg/L, suspended solids (SS) of 70–120 mg/L and also contained 5–28 mg ammonia nitrogen per liter wastewater. The TCM wastewater had been treated with a two-phase anaerobic digester before it entered the present experimental setups. The major characteristics of the digester effluent were as follows: COD, 1259.1–12776.5 mg/L; total nitrogen, 7–25 mg/L; total phosphorus, 5–9 mg/L; and suspended solids (SS), 1000–1600 mg/L. 2.2. Seed sludge Two kinds of seed sludge were used as the inocula for the treatment of domestic wastewater and TCM wastewater, respectively. Inoculum of domestic wastewater was collected from the secondary settling tank, while the seed sludge of TCM wastewater was taken from the aeration tank and sludge collector in Harbin traditional Chinese medicine company. The mixed liquor suspended solids (MLSS) concentrations were estimated to be 1980 mg/L for the domestic wastewater inoculum, and 2140 mg/L for the TCM wastewater inoculum.
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
2.3. Experimental apparatus and operation
585
was created as gas stored in the vacuum reservoir was pumped out, which would push wastewater to flow into vacuum reservoir from the bioreactor through the membrane module. As the liquid surface in the vacuum reservoir reached 80% of the maximum height, the effluent stored in the reservoir was drained out by Pump 2. The vacuum pump and Pump 2 would alternatively work so that the AVMBR effluent was continuously discharged. The AVMBR system was equipped with an air–water–chemicals backwashing system. Numbers 1–14 in Fig. 1 represent electron-magnetic valve to control water level and air flow. The cleaning scheme of the membrane modules was proposed as follows: The membrane modules were first cleaned with air for 10 min at a flow rate of 500 L/min. Next, the modules were subject to a simultaneous air wash (flow rate, 500 L/h) and water wash (flow rate, 20 L/min) for 10 min; then, the modules were further cleaned with chemicals (flow rate, 5 L/min) and air (flow rate, 500 L/h)
The schematic diagram of the AVMBR system is shown in Fig. 1. The AVMBR had a working volume of approximately 3.2 m3 and was installed with four submerged hollow-fiber PVDF microfiltration (MF) membrane modules (Tianjin Motian Membrane Engineering and Technology Co. Ltd, China). The MF membrane modules are characterized with a pore size of 0.22 mm and an effective surface area of 12.5 m2. The wastewater was fed continuously by a peristaltic pump 1 (Fig. 1). A level controller and an automatic vacuum effluent system (Fig. 1) were used to control the rector working volume at a constant value. The automatic vacuum effluent system consisted of a vacuum reservoir, a vacuum pump, a gas–water segregator, a level sensor, a peristaltic pump (Pump 2), an electron-magnetic valve and a power control device (Fig. 1). When the vacuum pump (Pump 2) worked, a negative-pressure condition
Timer Piezometer
Pressure control valve
Level controller
Flow meter 3 Flow meter 1
7
1
8
6
Valve
4
Flow meter 2
Peristaltic pump 1
CODmix
Clapboard
sampling
Membrane module 1
Valve
3
Membrane module 2
Aerator Bioreactor Valve
2 Gas flow meter
Vacuum pot
Water 10
Filter Air-water separator Filtrate (effluent)
Medicinebackwashing
Valve
12 Vacuum meter Vacuum pump Air 13 Level
Water backwashing
Feed tank Valve Level sensor
Air compressor
Backwashing pump
Pressure control valve
5 Influent
11
9
14 Power control device Peristaltic pump 2 Overfall
Sludge withdrawing pump Water tank Medicine tank Electron-magnetic valve
Fig. 1. Scheme of the pilot-scale AVMBR.
Valve
Pressure control valve
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
for 30 min simultaneously. Finally the membrane modules were backwashed with water for 20 min at a flow rate of 20 L/min. Chemicals used in the present work was a mixed solution containing 95% ethanol and 5% sodium hypochlorite. Membrane permeate was used as washing and backwashing water. The water temperature is 20–258C when cleaning is performed. Cleaning periods of membrane module were 150 days. Two serials of experimental runs were conducted to examine the COD and nitrogen removal performance of the AVMBR systems targeting on respective domestic wastewater and digested TCM wastewater. An AVMBR system was operated for around 300 days at HRTs of 3, 1 and 0.5 h in series while treating domestic wastewater. Degradation of the digested TCM wastewater was examined with the AVMBR systems operated at HRTs of 1, 3, 5 and 8 h, respectively. Each HRT was run about 150 days. The change of vacuum pressure and cleaning effect of air–water–chemicals backwashing were evaluated as the submerged membrane modules were contaminated with the treatment of the digested TCM wastewater at respective HRTs of 3, 5 and 8 h.
2.4. Analytical methods Measurements of MLSS, MLVSS, COD, ammonium nitrogen, total nitrogen (TN), and nitrate and nitrite nitrogen were performed according to Standard methods [14]. Dissolved oxygen (DO) was measured by a hand-held oxygen meter (COM 381, Shanghai Light Industry Research Institute, China) equipped with a DO probe (COS 381, Shenzhen Futai Instrument Co. Ltd, China). Vacuum values were recorded by an electro-pressure meter. Membrane permeate flux was measured by a rotor flowmeter.
3. Results and discussion 3.1. Effect air-water-chemical backwashing on membrane flux Figure 2 depicts the changes of membrane flux and corresponding vacuum value with time. Traditional Chinese medicine wastewater was used as the feedstock in this part of work. Membrane flux was controlled at levels of 8, 12 and 20 L/m2 h, respectively. A new membrane module was operated at a permeate flux
30
22 membrane flux
-2
25
vacuum value
16 20
14 12
15
10 8
10
6 4
5
2 0
0
50
100
150
200
250
300
350
400
0 450
Time (day)
Fig. 2. Changes of vacuum value and membrane flux with time in the pilot scale AVMBR.
3
18
-1
Membrane flux (L•m •h )
20
Vacuum value (10 Pa)
586
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
of 8 L/m2 h for 150 days of operation, and it was cleaned through air–water–chemicals backwashing. The cleaned membrane module continued to be tested at another two higher permeate fluxes, that is 12 and 20 L/m2 h. Each level of membrane flux remained constant for around 150 days by adjusting vacuum pressure. Without exception, the vacuum value increased rapidly, as a result of membrane fouling, over 150 days of operation at different membrane fluxes. It was observed that, as the permeate fluxed were enhanced to 12 and 20 L/m2 h, a lower value of vacuum pressure was needed at the initial stage of each flux to maintain the enhanced membrane fluxes, indicating that the membrane fouling had been relieved by the backwashing to a certain extent (Fig. 2). Overall, membrane modules were only washed twice during the operation of 450 days, but they were able to be well performed at a low pressure (only up to 0.26 MPa) for such long-term operation. The maintenance of the permeate flux was the criterion used to evaluate the effect of
backwashing. The contaminated membrane modules were cleaned through the air–water– chemicals backwashing, and were further evaluated quantitatively through examining the permeate flux with H2O as the liquid medium. Cleaning periods of membrane module were 150 days. Figure 3 shows the variation of permeate flux of the new and cleaned membrane modules under different operating pressures. It appeared that operating pressure has an important effect on the recovery of permeate flux. Overall, the resume degree of membrane flux was improved with the operating pressure. At the highest vacuum pressure tested (0.02 MPa), compared to the water flow rate of new membrane module, the cleaned membrane modules exhibited a 94.7% resume degree after the first cleaning, an 82.7% resume degree after the second cleaning and a 70% resume degree after the third cleaning, respectively. In contrast, the resume degrees of membrane flux corresponded to 88.4%, 44.9% and 13.0% at the lowest vacuum
35 30
30
28.4 New membrane 24.8
afer the first cleaning 22.2 21
after the second cleaning 20.4
afer the third cleaning
-2
-1
Membrane flux (L•m •h )
25
20 16 13.8
15
11.8
12.2
10 6.2
5
0
1.8
20
10
587
2
Vacuum value (103 Pa)
Fig. 3. Changes of membrane flux at variation of vacuum value after three air–water–chemicals backwashing.
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
pressure of 0.002 MPa. These findings indicate that the air–water–chemicals backwashing scheme is a feasible approach to resuming the membrane performance. 3.2. Domestic wastewater reclamation In China, the standard of water reuse in car washing and land watering (PR China CJ25.1– 89) has been legislated. Total organic substance content (in terms of COD) and ammonia nitrogen are the main concerns of this standard (PR China
a)
800
40
95
35
90
700 COD (mg/L)
100
85
600
80
500
75 400
70
300
65
Influent COD
200
Effluent COD
100
COD removal rate (%)
40
60
30
55
80
20
50
15
40 30
10
20 10
0
100
0 0
60
50
TN (mg/L)
50
40
40 30 30 20
20
10
10 0
0
20
40
60
Time (day)
80
100
0
Nitrite and nitrate concentration (mg/L)
Influent TN Effluent TN TN removal rate (%)
TN removal rate (%)
60
c)
70 60
20
Time (day) 70
80
25
5
50 20
90 Influent ammonia nitrogen Effluent ammonia nitrogen Ammonia nitrogen removal rate (%)
60
0 0
100 b)
COD removal rate (%)
900
Ammonia nitrogen removal rate (%)
CJ25.1–89) which requires the reused water having a COD concentration less than 30 mg/L and ammonia nitrogen less than 10 mg/L. It is generally accepted that HRT is a key operating parameter affecting the performance and efficiency of a biological process. The HRT for the AVMBR system was optimized in order to produce permeate meeting the standards of wastewater reuse. The performance of the AVMBR system was evaluated at HRTs of 3, 1 and 0.5 h, respectively. The removal of COD, ammonia nitrogen and TN at an HRT of 3 h is illustrated in Fig. 4. At an
Ammonia nitrogen (mg/L)
588
40
60 80 Time (day)
100
14 d) 12 10 8 6 Effluent nitrite
4
Effluent nitrate
2 0 0
20
40
60
80
100
Time (day)
Fig. 4. COD and nitrogen removal at HRT of 3 h in the pilot scale AVMBR: (a) COD removal; (b) Ammonia nitrogen removal; (c) TN removal; and (d) Effluent nitrite and nitrate.
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
HRT of 3 h, the effluent COD concentration decreased gradually from 50 mg/L initially to 28 mg/L at day 18, associated with an increase in COD removal rate from 89% to 96%. This indicated that the inocula had been acclimated with domestic wastewater and were adapted to new environments. The effluent COD concentration stabilized at around 10 mg/L since day 22, even though the influent COD concentration flocculated between 300 and 800 mg/L. The removal of ammonia nitrogen was significantly improved after 26 days of operation as the ammonia nitrogen removal rate increased from 14% to 100% during this period of operation. Thereafter, ammonia nitrogen was found in a very low range between 0 and 2.0 mg/L, indicating that a good nitrification was achieved with the AVMBR system. Total nitrogen removal rate was not high as it varied between 11% and 60% (Fig. 4c). Most ammonia was converted into nitrate with a concentration of 0.1– 13.1 mg/L. In contrast, only a smaller amount of nitrite, generally less than 2.5 mg/L, was detected (Fig. 4d). Low TN removal rates were probably owing to the presence of a high-level DO concentration in the AVMBR system. The DO concentration was nearly consistent around 5 mg/L throughout the study (data not shown). Nitrogen removal processes require both aerobic and anoxic/anaerobic stages. Simultaneous nitrification and denitrification could not occur in a high DO environment, resulting in a poor TN removal performance of the AVMBR system. The removal of COD, ammonia nitrogen, TN at an HRT of 1 h is illustrated in Fig. 5. As the HRT was shortened from 3 to 1 h, the reactor performance in terms of COD and ammonia nitrogen removal was comparable. The effluent COD concentration stabilized 10 mg/L with a COD removal rate of 94.1–98.5% since day 18. Almost all ammonia nitrogen was removed since day 26. Comparing to 3 h HRT, TN removal performance was enhanced as evidenced by a TN removal rate of 60–85.9% at
589
an HRT of 1 h. It was observed that shortening HRT resulted in an increase in MLSS, but a decrease in DO concentration. As a consequence, anoxic niche might be created where denitrification occurred. In addition, nitrate (0.3–3.9 mg/L) was significantly predominated over nitrite (less than 0.3 mg/L) in the effluent. The COD removal rates at an HRT of 0.5 h were still high, varying between 86.7% and 95.9% (Fig. 6). The MLSS and DO concentrations were around 16,700 and 1.2 mg/L at an HRT of 0.5 h, respectively. Comparing to an HRT of 1.0 h, shortening HRT to 0.5 did not significantly affect the removal of ammonia nitrogen and TN. However, the effluent quality substantially deteriorated with the HRT reducing to 0.5 h as the COD concentration was always found larger than 30 mg/L. This indicated that some soluble substance was unable to be completely degraded at an HRT of 0.5 h in the AVMBR system and simultaneously passed through the membrane. As for domestic wastewater reclamation, an HRT of 1 h was required by the AVMBR system to guarantee the permeate quality. This HRT was slightly lower than the lowest HRT (1.5 h) reported in literature [15] with a lab-scale MBR system, and also lower than 2.7 h HRT reported by Gu et al. with a pilot-scale system while treating synthetic domestic wastewater [16].
3.3. Digested TCM wastewater reclamation Figure 7 depicts the COD removal of digested TCM wastewater at HRTs of 8.0, 5.0, 3.0 and 1.0 h, respectively. As indicated in Fig. 7, digested effluent of TCM wastewater can be divided into four ranges in terms of COD strength, that is, less than 1000, 1000–3000, 3000–6000 and above 6000 mg/L. It was found that the effluent COD was always less than 30 mg/L at all tested HRTs as the influent concentration of the digested TCM effluent was
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593 100
40
800
95
35
700
90 85
600
80
500
75 400
70
300
65
Influent COD
200
Effluent COD
60
100
COD removal rate (%)
55
0 20
40
60
80
90 Influent ammonia nitrogen Effluent ammonia nitrogen Ammonia nitrogen removal rate (%)
30
60 50
15
40 30
10
20 10
0
0 0
100
20
60
TN (mg/L)
60 Influent TN
50
Effluent TN TN removal rate (%)
40 30
20 20 10
10
0
0 20
40
60
80
100
Time (day)
TN removal rate (%)
70 50
Nitrite and nitrate concentration (mg/L)
80
0
60
80
100
10
90
30
40
Time (day)
c)
40
70
20
Time (day) 70
80
25
5
50 0
100
b) Ammonia nitrogen (mg/L)
a)
COD removal rate (%)
COD (mg/L)
900
Ammonia nitrogen removal rate (%)
590
d) 9 8 7 Effluent nitrite
6
Effluent nitrate
5 4 3 2 1 0 0
20
40
60
80
100
Time (day)
Fig. 5. COD and nitrogen removal at HRT of 1 h in the pilot scale AVMBR: (a) COD removal; (b) Ammonia nitrogen removal; (c) TN removal; and (d) Effluent nitrite and nitrate.
below 1000 mg/L. This is quite similar to the COD removal of domestic wastewater by the AVMBR system, albeit the wastewater composition might be apparently different between domestic wastewater and TCM wastewater. This indicates that wastewater strength might be one of important considerations while operating the AVMBR system. To meet reuse standard (effluent COD, less than 30 mg/L), the
minimum HRT required was highly related to the wastewater strength. Overall, the minimal HRT required was 1 h for less than 1000 mgCOD/L TCM wastewater, 3.0 h for 1000– 3000 mg-COD/L TCM wastewater and 5.0 h for 3000–6000 mg-COD/L TCM wastewater. It is easy to understand that lengthening HRT improved the contact between microbial cells and organic pollutants, and facilitated biochemical
100
40
800
95
35
700
90 85
600
80
500
75 400
70
300
65
Influent COD
200
Effluent COD
100
COD removal rate (%)
0
0
20
40
60
100
b) Ammonia nitrogen (mg/L)
a)
COD removal rate (%)
COD (mg/L)
900
90 80
30
70
25
60
20
50
15
40 30
10
Influent ammonia nitrogen Effluent ammonia nitrogen Ammonia nitrogen removal rate (%)
60
80
5
55 100
0
50
60
70 TN (mg/L)
50
60 Influent TN
50
Effluent TN TN removal rate (%)
40 30
20
20 10
10
0
0 20
40
60
80
100
TN removal rate (%)
80
Nitrite and nitrate concentration (mg/L)
90
c)
0
10
20
40
60
80
100
Time (day)
70
30
20
0 0
Time (day)
40
591 Ammonia nitrogen removal rate (%)
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
10
d)
9 8 7
Effluent nitrite
6
Effluent nitrate
5 4 3 2 1 0 0
20
Time (day)
40
60
80
100
Time (day)
Fig. 6. COD and nitrogen removal at HRT of 0.5 h in the pilot scale AVMBR: (a) COD removal; (b) Ammonia nitrogen removal; (c) TN removal; and (d) Effluent nitrite and nitrate.
reactions. As the influent COD was larger than 6000 mg/L, extending HRT, however, was not effective as the effluent COD was larger than 30 mg/L even at the longest HRT tested (8 h). It is assumed that too many soluble substances, either original components of the raw wastewater or the released materials while biodegrading, passed through the MF and resulted in an inferior COD removal at an HRT of 8 h. To guarantee the effluent quality, a much longer HRT and/or a preor post-process are required for the AVMBR sys-
tem treating TCM wastewater having a COD concentration larger than 6000 mg/L. 4. Conclusions An automatic vacuum membrane bioreactor (AVMBR) equipped with an on-line air–water– chemicals backwashing system was demonstrated in pilot scale for the reclamation of domestic wastewater (300–800 mg-COD/L) and traditional Chinese medicine (TCM)
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593 450 420 390 360 330 300 270 240 210 180 150 120 90 60 30 0
13000 Effluent COD at HRT of 1.0 h
12000
Effluent COD at HRT of 3.0 h
11000
Effluent COD at HRT of 5.0 h
10000
Effluent COD at HRT of 8.0 h
9000
Influent COD
8000 7000 6000 5000 4000
Influent COD (mg/L)
Effluent COD (mg/L)
592
3000 2000 1000 0
20
40
60
80
100
120
140
160
0
Time (day)
Fig. 7. COD removal of the digested TCM effluent at different HRTs in the pilot scale AVMBRs.
wastewater (259–12,775 mg-COD/L). Based on the results obtained from this study, the following conclusions could be drawn: 1. Membrane flux was able to resume as the fouled membrane modules being operated for 150 days were repeatedly subject to an air–water–chemicals backwashing procedure. Compared to a new membrane module, the contaminated membrane fluxes were resumed to 94.7% after the first cleaning, to 82.7% after the second cleaning and to 70.0% after third cleaning at a vacuum value of 0.02 MPa. 2. For a purpose of water reuse (PR China CJ25.1–89; effluent COD, less than 30 mg/L), the minimum HRT required by the AVMBR system was 1 h for the domestic wastewater reclamation. 3. The minimum HRT required was highly related to the strength of TCM wastewater while considering the AVMBR effluent as reuse water. Overall, the minimal HRT required was 1 h for less than 1000 mg-COD/L TCM wastewater, 3.0 h for 1000–3000 mg-COD/L TCM wastewater
and 5.0 h for 3000–6000 mg-COD/L TCM wastewater. 4. To guarantee the effluent quality, a much longer HRT than 8 h and/or a pre- or postprocess are recommended for the AVMBR system treating TCM wastewater having a COD concentration larger than 6000 mg/L. Acknowledgments The authors are grateful to Research Center of Environmental Biotechnology in Harbin Institute of Technology for their technical and logistical assistance during this work which was supported by State Key Lab of Urban Water Resource and Environment (HIT-QAK200808) and China National ‘‘863’’ Hi-Tech R & D Program (grant no. 2007AA06Z348). References [1] K.H. Krauth, K.F. Staab, Pressurized bioreactor with membrane filtration for wastewater treatment. Water Research, 27(1993), 405–411. [2] M.D. Knoblock, P.M. Sutton, P.N. Mishra, K. Gupta, A. Janson, Membrane biological reactor
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
[3]
[4]
[5]
[6]
[7]
[8]
[9]
system for treatment of oily wastewaters. Water Environment Research, 66(1994), 133–139. G.T. Seo, T.S. Lee, B.H. Moon, K.S. Choi, H.D. Lee, Membrane separation activated sludge for residual organic removal in oil wastewater. Water Science and Technology, 36(1997), 275–282. D. Mallon, F. Steen, K. Brindle, S.J. Judd, Performance on a real industrial effluent using a Zenochem MBR. In: Brown, E.D. (Ed.), MBR2Proceedings of the Second Meeting on Membrane Bioreactors for Wastewater Treatment, 2 June 1999. School of Water Sciences, Cranfield University, Cranfield, UK (1999), pp. 7–9. N. Ren, Z. Chen, X. Wang, D. Hu, A. Wang, Optimized operational parameters of a pilot scale membrane bioreactor for high-strength organic wastewater treatment. International Biodeterioration & Biodegradation, 56(2005), 216–223. U. Tatsuki, Domestic wastewater treatment by a submerged membrane bioreactor with gravitational filtration. Water Research, 33(12)(1999), 505–515. S.J. Kie, Y. Ik-Keun, L.Y. Moo, Design and operation considerations for wastewater treatment using a flat submerged membrane bioreactor. Process Biochemistry, 38(2002), 279–285. M. Li, L. Xiufen, D. Guocheng, C. Jian, S. Zhisong, Influence of the filtration modes on colloid adsorption on the membrane in submerged membrane bioreactor. Colloids and Surfaces A: Physicochem. Eng. Aspects, 264(2005), 120–125. D. Hall, C.S.B. Fitzpatrick, Spectral analysis of pressure variations during combined air and
[10]
[11]
[12]
[13]
[14]
[15]
[16]
593
water backwash of rapid gravity filters. Water Research, 33(17)(1999), 3666–3672. M. Kennedy, S. Siriphannon, S. van Hoof, J. Schippers, Improving the performance of deadend ultrafiltration systems: comparing air and water flushing. Water Science and Technology: Water Supply, 1 (5/6)(2001), 97–106. C. Guigui, M. Mougenot, C. Cabassud, Air sparging backwash in ultrafiltration hollow fibres for drinking water production. Water Science and Technology: Water Supply, 3(5/6)(2003), 415– 422. H. Nagaoka, A. Tnaka, Y. Toriizuka, Measurement of effective shear stress working on flatsheet membrane by air-scrabing. Water Science and Technology: Water Supply 3(5/6)(2005), 423–428. R. Addicks, Examining the backwashing of rapid granular media filters. Filtration and Separation, 28(1)(1991), 38–41. APHA, Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC (1995). H. Stefan, T. Walter, Treatment of urban wastewater in a membrane bioreactor at high organic loading rates. Journal of Biotechnology, 92(2001), 95–101. G.W. Gu, Y.L. He, Study and Application of the Membrane Bioreactor in Wastewater Treatment. China Chemistry Industry Press, Beijing (2002).
www.elsevier.com/locate/desal
A novel application of automatic vacuum membrane bioreactor in wastewater reclamation Mei-zhuo Gaoa, Zhao-bo Chena*, Nan-qi Rena, Zhen-Peng Zhangb a
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China b Beijing Enterprises Water Group, Beijing, 100195, China Fax: +86-451-8628-2008; email: [email protected], [email protected] Received 11 July 2007; accepted 16 April 2009
Abstract An automatic vacuum membrane bioreactor (AVMBR) equipped with an on-line air–water–chemicals backwashing system was developed for a purpose of the reclamation of domestic wastewater (300–800 mg-COD/L) and traditional Chinese medicine (TCM) wastewater (259–12775 mg-COD/L). The experimental setups were operated in pilot scale at hydraulic retention times (HRTs) of 3, 1 and 0.5 h for domestic wastewater, and at HRTs of 1, 3, 5 and 8 h for TCM wastewater. Experimental results indicated that membrane flux was able to resume as the fouled membrane modules being operated for 150 days were repeatedly subject to an air–water– chemicals backwashing procedure. Compared to a new membrane module, the contaminated membrane fluxes were resumed to 94.7% after the first cleaning, to 82.7% after the second cleaning and to 70.0% after the third cleaning at a vacuum value of 0.02 MPa. To meet reuse standard (effluent COD, less than 30 mg/L), the minimum HRT required by the AVMBR system was 1 h for domestic wastewater reclamation, and 1 h for less than 1000 mg-COD/L, 3.0 h for 1000–3000 mg-COD/L, and 5.0 h for 3000–6000 mg-COD/L TCM wastewater reclamation. However, when the TCM wastewater COD was larger than 6000 mg/L, an 8-h HRT operation of the AVMBR system failed to produce an effluent having a COD concentration less than 30 mg/L. It is concluded that the pilot-scale AVMBR system developed is feasible for the reclamation of domestic wastewater and TCM wastewater with a COD concentration less than 6000 mg/L. Keywords: Air–water–chemicals backwashing; Automatic vacuum membrane bioreactor (AVMBR); Domestic wastewater; Traditional chinese medicine (TCM) wastewater; Wastewater reclamation
1. Introduction Membrane bioreactors (MBRs) or combined with other pre-/post-biological treatment systems *Corresponding author.
have been developed for the treatment of municipal wastewater and industrial wastewaters [1–5]. Membrane filtration guarantees the production of a high-quality effluent since microbial sludge and other suspended solids are able to be retained
0011-9164/09/$– See front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.04.003
584
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
by the membrane. However, solid–liquid separation mode is a major concern in MBR operation. Solid and liquid phases are separated through either pump suction or gravity outflow in the conventional submerged MBRs [6–8]. An intermittent permeate is generally produced while employing pump suction or gravity outflow as the solid–liquid separation mode. Obviously, such solid-liquid separation modes impeded the application of MBRs to wastewater treatment in practice as the continuous permeate was required. Membrane fouling was unavoidable over a long operational period of the MBRs, which led to the decrease of the membrane flux and thus enhanced the operating costs. Periodic backwashing on membrane modules is a common approach to alleviate membrane fouling. Different backwashing strategies have been developed to clean the membrane modules with water and/or air [9–12]. Water backwashing alone has been proved to be an inherently weak cleaning technique since it has to rely predominantly upon hydrodynamic shear forces for cleaning [13]. Air backwashing is an alternative choice derived from the operation of filters, which combines regular water backwashing. Backwashing cleaning efficiency is able to be enhanced by combining air scour as a result of the improved hydrodynamic and medium collision. However, the collision is generally confined to at or near the filter membrane surfaces, resulting in a limited cleaning effectiveness of fouled membrane. Automatic vacuum membrane bioreactor (AVMBR) is a novel-type reactor used for wastewater reclamation. It features the continuous effluent discharge derived from conventional submerged MBR through an automatic vacuum suction system. To authors’ knowledge, AVMBR system does not appear to have been reported in literature. In this study, an AVMBR equipped with a novel on-line airwater-chemicals backwashing system was
developed for a purpose of the reclamation of domestic wastewater and traditional Chinese medicine (TCM) wastewater. The experimental setups were operated in the pilot scale at different hydraulic retention times (HRTs). In addition, cleaning effectiveness of air–water– chemicals backwashing on the membrane modules were evaluated by examining the resume degree of membrane flux.
2. Materials and methods 2.1. Wastewater Domestic wastewater and traditional Chinese medicine (TCM) wastewater were used as the sole carbon sources, respectively. The domestic wastewater had a COD concentration of 300– 800 mg/L, total nitrogen of 25–80 mg/L, total phosphorus of 15–35 mg/L, suspended solids (SS) of 70–120 mg/L and also contained 5–28 mg ammonia nitrogen per liter wastewater. The TCM wastewater had been treated with a two-phase anaerobic digester before it entered the present experimental setups. The major characteristics of the digester effluent were as follows: COD, 1259.1–12776.5 mg/L; total nitrogen, 7–25 mg/L; total phosphorus, 5–9 mg/L; and suspended solids (SS), 1000–1600 mg/L. 2.2. Seed sludge Two kinds of seed sludge were used as the inocula for the treatment of domestic wastewater and TCM wastewater, respectively. Inoculum of domestic wastewater was collected from the secondary settling tank, while the seed sludge of TCM wastewater was taken from the aeration tank and sludge collector in Harbin traditional Chinese medicine company. The mixed liquor suspended solids (MLSS) concentrations were estimated to be 1980 mg/L for the domestic wastewater inoculum, and 2140 mg/L for the TCM wastewater inoculum.
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
2.3. Experimental apparatus and operation
585
was created as gas stored in the vacuum reservoir was pumped out, which would push wastewater to flow into vacuum reservoir from the bioreactor through the membrane module. As the liquid surface in the vacuum reservoir reached 80% of the maximum height, the effluent stored in the reservoir was drained out by Pump 2. The vacuum pump and Pump 2 would alternatively work so that the AVMBR effluent was continuously discharged. The AVMBR system was equipped with an air–water–chemicals backwashing system. Numbers 1–14 in Fig. 1 represent electron-magnetic valve to control water level and air flow. The cleaning scheme of the membrane modules was proposed as follows: The membrane modules were first cleaned with air for 10 min at a flow rate of 500 L/min. Next, the modules were subject to a simultaneous air wash (flow rate, 500 L/h) and water wash (flow rate, 20 L/min) for 10 min; then, the modules were further cleaned with chemicals (flow rate, 5 L/min) and air (flow rate, 500 L/h)
The schematic diagram of the AVMBR system is shown in Fig. 1. The AVMBR had a working volume of approximately 3.2 m3 and was installed with four submerged hollow-fiber PVDF microfiltration (MF) membrane modules (Tianjin Motian Membrane Engineering and Technology Co. Ltd, China). The MF membrane modules are characterized with a pore size of 0.22 mm and an effective surface area of 12.5 m2. The wastewater was fed continuously by a peristaltic pump 1 (Fig. 1). A level controller and an automatic vacuum effluent system (Fig. 1) were used to control the rector working volume at a constant value. The automatic vacuum effluent system consisted of a vacuum reservoir, a vacuum pump, a gas–water segregator, a level sensor, a peristaltic pump (Pump 2), an electron-magnetic valve and a power control device (Fig. 1). When the vacuum pump (Pump 2) worked, a negative-pressure condition
Timer Piezometer
Pressure control valve
Level controller
Flow meter 3 Flow meter 1
7
1
8
6
Valve
4
Flow meter 2
Peristaltic pump 1
CODmix
Clapboard
sampling
Membrane module 1
Valve
3
Membrane module 2
Aerator Bioreactor Valve
2 Gas flow meter
Vacuum pot
Water 10
Filter Air-water separator Filtrate (effluent)
Medicinebackwashing
Valve
12 Vacuum meter Vacuum pump Air 13 Level
Water backwashing
Feed tank Valve Level sensor
Air compressor
Backwashing pump
Pressure control valve
5 Influent
11
9
14 Power control device Peristaltic pump 2 Overfall
Sludge withdrawing pump Water tank Medicine tank Electron-magnetic valve
Fig. 1. Scheme of the pilot-scale AVMBR.
Valve
Pressure control valve
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
for 30 min simultaneously. Finally the membrane modules were backwashed with water for 20 min at a flow rate of 20 L/min. Chemicals used in the present work was a mixed solution containing 95% ethanol and 5% sodium hypochlorite. Membrane permeate was used as washing and backwashing water. The water temperature is 20–258C when cleaning is performed. Cleaning periods of membrane module were 150 days. Two serials of experimental runs were conducted to examine the COD and nitrogen removal performance of the AVMBR systems targeting on respective domestic wastewater and digested TCM wastewater. An AVMBR system was operated for around 300 days at HRTs of 3, 1 and 0.5 h in series while treating domestic wastewater. Degradation of the digested TCM wastewater was examined with the AVMBR systems operated at HRTs of 1, 3, 5 and 8 h, respectively. Each HRT was run about 150 days. The change of vacuum pressure and cleaning effect of air–water–chemicals backwashing were evaluated as the submerged membrane modules were contaminated with the treatment of the digested TCM wastewater at respective HRTs of 3, 5 and 8 h.
2.4. Analytical methods Measurements of MLSS, MLVSS, COD, ammonium nitrogen, total nitrogen (TN), and nitrate and nitrite nitrogen were performed according to Standard methods [14]. Dissolved oxygen (DO) was measured by a hand-held oxygen meter (COM 381, Shanghai Light Industry Research Institute, China) equipped with a DO probe (COS 381, Shenzhen Futai Instrument Co. Ltd, China). Vacuum values were recorded by an electro-pressure meter. Membrane permeate flux was measured by a rotor flowmeter.
3. Results and discussion 3.1. Effect air-water-chemical backwashing on membrane flux Figure 2 depicts the changes of membrane flux and corresponding vacuum value with time. Traditional Chinese medicine wastewater was used as the feedstock in this part of work. Membrane flux was controlled at levels of 8, 12 and 20 L/m2 h, respectively. A new membrane module was operated at a permeate flux
30
22 membrane flux
-2
25
vacuum value
16 20
14 12
15
10 8
10
6 4
5
2 0
0
50
100
150
200
250
300
350
400
0 450
Time (day)
Fig. 2. Changes of vacuum value and membrane flux with time in the pilot scale AVMBR.
3
18
-1
Membrane flux (L•m •h )
20
Vacuum value (10 Pa)
586
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
of 8 L/m2 h for 150 days of operation, and it was cleaned through air–water–chemicals backwashing. The cleaned membrane module continued to be tested at another two higher permeate fluxes, that is 12 and 20 L/m2 h. Each level of membrane flux remained constant for around 150 days by adjusting vacuum pressure. Without exception, the vacuum value increased rapidly, as a result of membrane fouling, over 150 days of operation at different membrane fluxes. It was observed that, as the permeate fluxed were enhanced to 12 and 20 L/m2 h, a lower value of vacuum pressure was needed at the initial stage of each flux to maintain the enhanced membrane fluxes, indicating that the membrane fouling had been relieved by the backwashing to a certain extent (Fig. 2). Overall, membrane modules were only washed twice during the operation of 450 days, but they were able to be well performed at a low pressure (only up to 0.26 MPa) for such long-term operation. The maintenance of the permeate flux was the criterion used to evaluate the effect of
backwashing. The contaminated membrane modules were cleaned through the air–water– chemicals backwashing, and were further evaluated quantitatively through examining the permeate flux with H2O as the liquid medium. Cleaning periods of membrane module were 150 days. Figure 3 shows the variation of permeate flux of the new and cleaned membrane modules under different operating pressures. It appeared that operating pressure has an important effect on the recovery of permeate flux. Overall, the resume degree of membrane flux was improved with the operating pressure. At the highest vacuum pressure tested (0.02 MPa), compared to the water flow rate of new membrane module, the cleaned membrane modules exhibited a 94.7% resume degree after the first cleaning, an 82.7% resume degree after the second cleaning and a 70% resume degree after the third cleaning, respectively. In contrast, the resume degrees of membrane flux corresponded to 88.4%, 44.9% and 13.0% at the lowest vacuum
35 30
30
28.4 New membrane 24.8
afer the first cleaning 22.2 21
after the second cleaning 20.4
afer the third cleaning
-2
-1
Membrane flux (L•m •h )
25
20 16 13.8
15
11.8
12.2
10 6.2
5
0
1.8
20
10
587
2
Vacuum value (103 Pa)
Fig. 3. Changes of membrane flux at variation of vacuum value after three air–water–chemicals backwashing.
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
pressure of 0.002 MPa. These findings indicate that the air–water–chemicals backwashing scheme is a feasible approach to resuming the membrane performance. 3.2. Domestic wastewater reclamation In China, the standard of water reuse in car washing and land watering (PR China CJ25.1– 89) has been legislated. Total organic substance content (in terms of COD) and ammonia nitrogen are the main concerns of this standard (PR China
a)
800
40
95
35
90
700 COD (mg/L)
100
85
600
80
500
75 400
70
300
65
Influent COD
200
Effluent COD
100
COD removal rate (%)
40
60
30
55
80
20
50
15
40 30
10
20 10
0
100
0 0
60
50
TN (mg/L)
50
40
40 30 30 20
20
10
10 0
0
20
40
60
Time (day)
80
100
0
Nitrite and nitrate concentration (mg/L)
Influent TN Effluent TN TN removal rate (%)
TN removal rate (%)
60
c)
70 60
20
Time (day) 70
80
25
5
50 20
90 Influent ammonia nitrogen Effluent ammonia nitrogen Ammonia nitrogen removal rate (%)
60
0 0
100 b)
COD removal rate (%)
900
Ammonia nitrogen removal rate (%)
CJ25.1–89) which requires the reused water having a COD concentration less than 30 mg/L and ammonia nitrogen less than 10 mg/L. It is generally accepted that HRT is a key operating parameter affecting the performance and efficiency of a biological process. The HRT for the AVMBR system was optimized in order to produce permeate meeting the standards of wastewater reuse. The performance of the AVMBR system was evaluated at HRTs of 3, 1 and 0.5 h, respectively. The removal of COD, ammonia nitrogen and TN at an HRT of 3 h is illustrated in Fig. 4. At an
Ammonia nitrogen (mg/L)
588
40
60 80 Time (day)
100
14 d) 12 10 8 6 Effluent nitrite
4
Effluent nitrate
2 0 0
20
40
60
80
100
Time (day)
Fig. 4. COD and nitrogen removal at HRT of 3 h in the pilot scale AVMBR: (a) COD removal; (b) Ammonia nitrogen removal; (c) TN removal; and (d) Effluent nitrite and nitrate.
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
HRT of 3 h, the effluent COD concentration decreased gradually from 50 mg/L initially to 28 mg/L at day 18, associated with an increase in COD removal rate from 89% to 96%. This indicated that the inocula had been acclimated with domestic wastewater and were adapted to new environments. The effluent COD concentration stabilized at around 10 mg/L since day 22, even though the influent COD concentration flocculated between 300 and 800 mg/L. The removal of ammonia nitrogen was significantly improved after 26 days of operation as the ammonia nitrogen removal rate increased from 14% to 100% during this period of operation. Thereafter, ammonia nitrogen was found in a very low range between 0 and 2.0 mg/L, indicating that a good nitrification was achieved with the AVMBR system. Total nitrogen removal rate was not high as it varied between 11% and 60% (Fig. 4c). Most ammonia was converted into nitrate with a concentration of 0.1– 13.1 mg/L. In contrast, only a smaller amount of nitrite, generally less than 2.5 mg/L, was detected (Fig. 4d). Low TN removal rates were probably owing to the presence of a high-level DO concentration in the AVMBR system. The DO concentration was nearly consistent around 5 mg/L throughout the study (data not shown). Nitrogen removal processes require both aerobic and anoxic/anaerobic stages. Simultaneous nitrification and denitrification could not occur in a high DO environment, resulting in a poor TN removal performance of the AVMBR system. The removal of COD, ammonia nitrogen, TN at an HRT of 1 h is illustrated in Fig. 5. As the HRT was shortened from 3 to 1 h, the reactor performance in terms of COD and ammonia nitrogen removal was comparable. The effluent COD concentration stabilized 10 mg/L with a COD removal rate of 94.1–98.5% since day 18. Almost all ammonia nitrogen was removed since day 26. Comparing to 3 h HRT, TN removal performance was enhanced as evidenced by a TN removal rate of 60–85.9% at
589
an HRT of 1 h. It was observed that shortening HRT resulted in an increase in MLSS, but a decrease in DO concentration. As a consequence, anoxic niche might be created where denitrification occurred. In addition, nitrate (0.3–3.9 mg/L) was significantly predominated over nitrite (less than 0.3 mg/L) in the effluent. The COD removal rates at an HRT of 0.5 h were still high, varying between 86.7% and 95.9% (Fig. 6). The MLSS and DO concentrations were around 16,700 and 1.2 mg/L at an HRT of 0.5 h, respectively. Comparing to an HRT of 1.0 h, shortening HRT to 0.5 did not significantly affect the removal of ammonia nitrogen and TN. However, the effluent quality substantially deteriorated with the HRT reducing to 0.5 h as the COD concentration was always found larger than 30 mg/L. This indicated that some soluble substance was unable to be completely degraded at an HRT of 0.5 h in the AVMBR system and simultaneously passed through the membrane. As for domestic wastewater reclamation, an HRT of 1 h was required by the AVMBR system to guarantee the permeate quality. This HRT was slightly lower than the lowest HRT (1.5 h) reported in literature [15] with a lab-scale MBR system, and also lower than 2.7 h HRT reported by Gu et al. with a pilot-scale system while treating synthetic domestic wastewater [16].
3.3. Digested TCM wastewater reclamation Figure 7 depicts the COD removal of digested TCM wastewater at HRTs of 8.0, 5.0, 3.0 and 1.0 h, respectively. As indicated in Fig. 7, digested effluent of TCM wastewater can be divided into four ranges in terms of COD strength, that is, less than 1000, 1000–3000, 3000–6000 and above 6000 mg/L. It was found that the effluent COD was always less than 30 mg/L at all tested HRTs as the influent concentration of the digested TCM effluent was
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593 100
40
800
95
35
700
90 85
600
80
500
75 400
70
300
65
Influent COD
200
Effluent COD
60
100
COD removal rate (%)
55
0 20
40
60
80
90 Influent ammonia nitrogen Effluent ammonia nitrogen Ammonia nitrogen removal rate (%)
30
60 50
15
40 30
10
20 10
0
0 0
100
20
60
TN (mg/L)
60 Influent TN
50
Effluent TN TN removal rate (%)
40 30
20 20 10
10
0
0 20
40
60
80
100
Time (day)
TN removal rate (%)
70 50
Nitrite and nitrate concentration (mg/L)
80
0
60
80
100
10
90
30
40
Time (day)
c)
40
70
20
Time (day) 70
80
25
5
50 0
100
b) Ammonia nitrogen (mg/L)
a)
COD removal rate (%)
COD (mg/L)
900
Ammonia nitrogen removal rate (%)
590
d) 9 8 7 Effluent nitrite
6
Effluent nitrate
5 4 3 2 1 0 0
20
40
60
80
100
Time (day)
Fig. 5. COD and nitrogen removal at HRT of 1 h in the pilot scale AVMBR: (a) COD removal; (b) Ammonia nitrogen removal; (c) TN removal; and (d) Effluent nitrite and nitrate.
below 1000 mg/L. This is quite similar to the COD removal of domestic wastewater by the AVMBR system, albeit the wastewater composition might be apparently different between domestic wastewater and TCM wastewater. This indicates that wastewater strength might be one of important considerations while operating the AVMBR system. To meet reuse standard (effluent COD, less than 30 mg/L), the
minimum HRT required was highly related to the wastewater strength. Overall, the minimal HRT required was 1 h for less than 1000 mgCOD/L TCM wastewater, 3.0 h for 1000– 3000 mg-COD/L TCM wastewater and 5.0 h for 3000–6000 mg-COD/L TCM wastewater. It is easy to understand that lengthening HRT improved the contact between microbial cells and organic pollutants, and facilitated biochemical
100
40
800
95
35
700
90 85
600
80
500
75 400
70
300
65
Influent COD
200
Effluent COD
100
COD removal rate (%)
0
0
20
40
60
100
b) Ammonia nitrogen (mg/L)
a)
COD removal rate (%)
COD (mg/L)
900
90 80
30
70
25
60
20
50
15
40 30
10
Influent ammonia nitrogen Effluent ammonia nitrogen Ammonia nitrogen removal rate (%)
60
80
5
55 100
0
50
60
70 TN (mg/L)
50
60 Influent TN
50
Effluent TN TN removal rate (%)
40 30
20
20 10
10
0
0 20
40
60
80
100
TN removal rate (%)
80
Nitrite and nitrate concentration (mg/L)
90
c)
0
10
20
40
60
80
100
Time (day)
70
30
20
0 0
Time (day)
40
591 Ammonia nitrogen removal rate (%)
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
10
d)
9 8 7
Effluent nitrite
6
Effluent nitrate
5 4 3 2 1 0 0
20
Time (day)
40
60
80
100
Time (day)
Fig. 6. COD and nitrogen removal at HRT of 0.5 h in the pilot scale AVMBR: (a) COD removal; (b) Ammonia nitrogen removal; (c) TN removal; and (d) Effluent nitrite and nitrate.
reactions. As the influent COD was larger than 6000 mg/L, extending HRT, however, was not effective as the effluent COD was larger than 30 mg/L even at the longest HRT tested (8 h). It is assumed that too many soluble substances, either original components of the raw wastewater or the released materials while biodegrading, passed through the MF and resulted in an inferior COD removal at an HRT of 8 h. To guarantee the effluent quality, a much longer HRT and/or a preor post-process are required for the AVMBR sys-
tem treating TCM wastewater having a COD concentration larger than 6000 mg/L. 4. Conclusions An automatic vacuum membrane bioreactor (AVMBR) equipped with an on-line air–water– chemicals backwashing system was demonstrated in pilot scale for the reclamation of domestic wastewater (300–800 mg-COD/L) and traditional Chinese medicine (TCM)
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593 450 420 390 360 330 300 270 240 210 180 150 120 90 60 30 0
13000 Effluent COD at HRT of 1.0 h
12000
Effluent COD at HRT of 3.0 h
11000
Effluent COD at HRT of 5.0 h
10000
Effluent COD at HRT of 8.0 h
9000
Influent COD
8000 7000 6000 5000 4000
Influent COD (mg/L)
Effluent COD (mg/L)
592
3000 2000 1000 0
20
40
60
80
100
120
140
160
0
Time (day)
Fig. 7. COD removal of the digested TCM effluent at different HRTs in the pilot scale AVMBRs.
wastewater (259–12,775 mg-COD/L). Based on the results obtained from this study, the following conclusions could be drawn: 1. Membrane flux was able to resume as the fouled membrane modules being operated for 150 days were repeatedly subject to an air–water–chemicals backwashing procedure. Compared to a new membrane module, the contaminated membrane fluxes were resumed to 94.7% after the first cleaning, to 82.7% after the second cleaning and to 70.0% after third cleaning at a vacuum value of 0.02 MPa. 2. For a purpose of water reuse (PR China CJ25.1–89; effluent COD, less than 30 mg/L), the minimum HRT required by the AVMBR system was 1 h for the domestic wastewater reclamation. 3. The minimum HRT required was highly related to the strength of TCM wastewater while considering the AVMBR effluent as reuse water. Overall, the minimal HRT required was 1 h for less than 1000 mg-COD/L TCM wastewater, 3.0 h for 1000–3000 mg-COD/L TCM wastewater
and 5.0 h for 3000–6000 mg-COD/L TCM wastewater. 4. To guarantee the effluent quality, a much longer HRT than 8 h and/or a pre- or postprocess are recommended for the AVMBR system treating TCM wastewater having a COD concentration larger than 6000 mg/L. Acknowledgments The authors are grateful to Research Center of Environmental Biotechnology in Harbin Institute of Technology for their technical and logistical assistance during this work which was supported by State Key Lab of Urban Water Resource and Environment (HIT-QAK200808) and China National ‘‘863’’ Hi-Tech R & D Program (grant no. 2007AA06Z348). References [1] K.H. Krauth, K.F. Staab, Pressurized bioreactor with membrane filtration for wastewater treatment. Water Research, 27(1993), 405–411. [2] M.D. Knoblock, P.M. Sutton, P.N. Mishra, K. Gupta, A. Janson, Membrane biological reactor
Mei-zhuo Gao et al. / Desalination 247 (2009) 583–593
[3]
[4]
[5]
[6]
[7]
[8]
[9]
system for treatment of oily wastewaters. Water Environment Research, 66(1994), 133–139. G.T. Seo, T.S. Lee, B.H. Moon, K.S. Choi, H.D. Lee, Membrane separation activated sludge for residual organic removal in oil wastewater. Water Science and Technology, 36(1997), 275–282. D. Mallon, F. Steen, K. Brindle, S.J. Judd, Performance on a real industrial effluent using a Zenochem MBR. In: Brown, E.D. (Ed.), MBR2Proceedings of the Second Meeting on Membrane Bioreactors for Wastewater Treatment, 2 June 1999. School of Water Sciences, Cranfield University, Cranfield, UK (1999), pp. 7–9. N. Ren, Z. Chen, X. Wang, D. Hu, A. Wang, Optimized operational parameters of a pilot scale membrane bioreactor for high-strength organic wastewater treatment. International Biodeterioration & Biodegradation, 56(2005), 216–223. U. Tatsuki, Domestic wastewater treatment by a submerged membrane bioreactor with gravitational filtration. Water Research, 33(12)(1999), 505–515. S.J. Kie, Y. Ik-Keun, L.Y. Moo, Design and operation considerations for wastewater treatment using a flat submerged membrane bioreactor. Process Biochemistry, 38(2002), 279–285. M. Li, L. Xiufen, D. Guocheng, C. Jian, S. Zhisong, Influence of the filtration modes on colloid adsorption on the membrane in submerged membrane bioreactor. Colloids and Surfaces A: Physicochem. Eng. Aspects, 264(2005), 120–125. D. Hall, C.S.B. Fitzpatrick, Spectral analysis of pressure variations during combined air and
[10]
[11]
[12]
[13]
[14]
[15]
[16]
593
water backwash of rapid gravity filters. Water Research, 33(17)(1999), 3666–3672. M. Kennedy, S. Siriphannon, S. van Hoof, J. Schippers, Improving the performance of deadend ultrafiltration systems: comparing air and water flushing. Water Science and Technology: Water Supply, 1 (5/6)(2001), 97–106. C. Guigui, M. Mougenot, C. Cabassud, Air sparging backwash in ultrafiltration hollow fibres for drinking water production. Water Science and Technology: Water Supply, 3(5/6)(2003), 415– 422. H. Nagaoka, A. Tnaka, Y. Toriizuka, Measurement of effective shear stress working on flatsheet membrane by air-scrabing. Water Science and Technology: Water Supply 3(5/6)(2005), 423–428. R. Addicks, Examining the backwashing of rapid granular media filters. Filtration and Separation, 28(1)(1991), 38–41. APHA, Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC (1995). H. Stefan, T. Walter, Treatment of urban wastewater in a membrane bioreactor at high organic loading rates. Journal of Biotechnology, 92(2001), 95–101. G.W. Gu, Y.L. He, Study and Application of the Membrane Bioreactor in Wastewater Treatment. China Chemistry Industry Press, Beijing (2002).