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Microfiltration-membrane-coupled bioreactor for urban wastewater reclamation
DESALINATION ELSEVIER
Desalination 141 (2001) 63-73 www.elsevier.com/locate/desal
Microfiltration-membrane-coupled bioreactor for urban wastewater reclamation C.-H. Xing”‘, X.-H. Wena, Y. Qiana, E. Tardieub “State Key Joint Laboratory of Environmental Simulation and Pollution Control, Department of Environmental Science and Engineering, Tsinghua University Beijing, 100084, P. R China bDirection Dspartementale de I’Agriculture et de la Forgt, Centre AdminishatifCondP, 18013 Bourges Cedex, France
Received 30 March 200 1; accepted 23 April 200 1
Abstract A microfiltration-membrane-coupled bioreactor (MMB) on pilot scale was operated for 135 days to investigate its technical feasibility in urban wastewater reclamation. Different operation parameters such as sludge retention time from 5 to 30 days, hydraulic retention time from 3.75 to 7.5 h, and membrane flux from 50 to 100 L.m-*.h-‘, were tested. The highest sludge concentration attained was 15.1 g.L-’ (as suspended solids, SS) and 8.9 g.L-’ (as volatile suspended solids, VSS); the VSS to SS ratio of MMB sludge was 0.55 on average. The ratio of sludge VSS to sludge COD was found to be 1.45. The mass loading rates of MMB were close to those of conventional activated sludge process (CASP) but the volumetric loading rates, two to four times those of CASP. Up to 95% of COD, 97.7% of ammonia nitrogen and 100% of suspended solids were removed on average. Further analysis indicated that the bioreactor was responsible for 87% of the total COD removal and only 8% were attributed to the membrane separation. The reclaimed water could be reused either directly or indirectly for municipal or industrial purposes. Keywords: Activated sludge; Bioreactor; Ceramic membrane; Reclamation; Microfiltration; Urban wastewater
1. Introduction The microfiltration membrane bioreactor (MMB) is actually an integration of advanced membrane separation and a conventional bio*Corresponding author. Currently at Environmental Technology Institute, Innovation Centre (NTU), BLK 2, Unit 237,18 Nanyang Drive, Singapore 637723. Tel: +65 7943 126; Fax: +65 7921291; Email: [email protected]
reactor [ 11. Since its emergence, MMB has been successfully applied to the biology industry [2,3], the chemical industry [4,5], biotechnological leaching of heavy metals [6] and wastewater treatment [7]. With more serious environmental pollution, MMB’s application in wastewater treatment is receiving more and more attention and in this case, generally featured as the replacement of secondary clarifier by a
001 l-9164/01/$- See front matter Q 2001 Elsevier Science B.V. All rights reserved PII: SO01 l-9164(01)00389-7
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C.-H. Xing et al. /Desalination 141 (2001) 63-73
microfiltration (MF) membrane [S]. Due to the absence of a secondary clarifier, the overall MMB plant size can be remarkably reduced in comparison with that of the conventional activated sludge process (CASP) [9]. MMB provides not only complete retention of all microorganisms and an increase of sludge concentration but also a complete disinfection of treated water [l]. As a consequence, MMB offers the absolute separation of hydraulic retention time (HRT) and sludge retention time (SRT), which facilitates a more flexible control of operation parameters [ 1,lo]. High sludge concentration maintained in the bioreactor of the MMB makes it possible to treat high strength wastewater efficiently [ 111.The entire retention of activated sludge extends the sludge-substrate contact time and therefore is conducive to removing lowbiodegradable pollutants contained in wastewater
[121. A semi-industrial MMB trial on domestic wastewater has demonstrated that extremely high nitrification can be achieved, and removal of COD, ammonia nitrogen and SS reached 96%, 97% and 99.9%, respectively. The filtration system maintained a good flux of 60 to 80 L. m-*.h-’ at 20°C for over 15 days without chemical cleaning [ 131. It is reported that for rural settlement treatment, over 90% of organic matter, SS, and coliform were removed during 336 days of study [ 141.To further examine their effectiveness for municipal wastewater, two pilot MMB systems have been studied in California for 5 months and in Paris for 1 year [ 151.Preliminary results show that a high degree of treatment in terms of SS (100%) and organic matter (greater than 96% for COD) can be achieved and better than 6 log removal of total coliform and 4 log removal of naturally occurring bacteriophages were observed. However, as most of the aforementioned membranes within MMB are made from polymeric materials sensitive to caustic cleaning reagents, much difficulty has been encountered in membrane cleaning [9]. A
possible solution is to substitute these polymeric membranes by ceramic MF modules that are easy to be chemically cleaned in situ. Little is yet known about the performance of ceramic MMB when applied to urban wastewater reclamation. The objective of this paper was to examine the long-term performance of a ceramic MMB system at pilot scale for urban wastewater reclamation. The impact of operation parameters on effluent quality was carefully investigated. The roles of the membrane and bioreactor were analyzed in terms of COD removal efficiency. Finally, the reuse potential of reclaimed water was extensively discussed in this paper.
2. Materials and methods 2.1. Process description The process diagram is described in Fig. 1.
The bioreactor filled with activated sludge had a working volume of 30 L. A level controller and pump were operated together to keep the working volume stable. The bioreactor, centrifugal pump 2 and MF module constituted a loop where the MMB sludge was circulated at a high speed. To enhance the circulation, centrifugal pump 3 was installed. The flowrate of MMB effluent was precisely controlled by a PLC unit, pneumatic valve and an electromagnetic flowmeter. By
E-m A Stirrermn Awata r”) He.1 exchanger w
Vdw
I
Flow-mater
k
Convolve
Fig. 1. Schematics of the MMB pilot.
0 *
Level controller Pmasurepln
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C.-H. Xing et al. /Desalination 141 (2001) 63-73
altering the flowrate of excess sludge discharge, the magnitude of SRT was changed. In cleaning mode, however, the cleaning tank would replace the bioreactor to form a cleaning loop with the microtiltration membrane module. The MF membrane was a ceramic tubular KerasepTM X6. The membrane skin layer and support material were made of zirconia (ZrO,) and alumina (y-A&O,), respectively. Each membrane had seven channels and the diameter of each channel was 4.5 mm. The membrane was 40 cm in length while the surface area of each single membrane was 0.04 m*. Membrane pore size was about 0.45 pm. Initial permeability of the new membrane was about 22 L.m-*.h-‘. (kPa)-’ on the basis of a tap water test at 25°C. 2.2. Urban wastewater The urban wastewater was piped from a local sewage station. As shown in Table 1, it could be classified as typical low-strength urban wastewater. 2.3. Operation parameters The experiment was carried out in three phases: Run 1, Run 2, and Run 3. The operation parameters are listed in Table 2. SRT was
sequentially extended from 5 and 15 to 30 days while HRT varied from 3.75,4.4,5 to 7.5 h. The cross-flow velocity within membrane channels was set at 2-3 m.s-‘. To compare the effluent quality under different fluxes, the membrane flux was increased from 50, 75, 85 to 100 L.m-*.h-’ by raising the flowrate of effluent. 2.4. Analytical methods The MMB system was monitored with daily measurements of redox (by Monet 8935) and temperature. Turbidity was measured by model 965- 10 turbidity meter (Orbeco analytical System Inc. USA). Samples for supernatant COD of activated sludge was taken after 15 minutes of
Table 1 Characteristics of urban wastewater Items
Typical
Range
COD, mg.L-’ SS, mg.L-’ NH,-N, mg.L-’ Coliform, number L-’ Turbidity, NTU pH value Temp., ‘C
100-300 20-200 17-23 105-lo6 50-60 7.5-8.5 15-25
30-1424 o--483 13-25 105-10’ 50-80 7.5-8.5 15-25
Table 2 Operation parameters Items
Start-end, d
HRT, h
SRT, d
Flux, L.m-*.h“
Run 1
l-5 6-29
5-15 7.5
30 5
50-150 50
Run 2
30-62 63-75
5.0 3.75
15 15
75 100
Run 3
76-97 98-l 16 117-135
5 4.4 3.75
30 30 30
75 85 100
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C.-H. Xing et al. /Desalination
centrifugation at 4500 rpm and subsequent standard filter filtration. All the items in Table 3 were measured as reference [ 161.
141 (2001) 63-73 100000
60
s p loo0 g 0
3. Results and discussions
60
100
40
10
20
1
0
50
100
40
60
5 d E
30
60
* x"
20
40
10
20
0
0
3.1. Removal of COD, NH,-N and SS
Fig. 2 represents the evolution of COD, NH,-N and SS vs. time. It could be seen that at the start-up stage (days 1 to 5), the effluent COD of the MMB system attained 4347 mg.L-’ when the influent COD changed between 1lo160 mg.L-‘. This was because the newly inoculated sludge could not immediately adapt to the urban wastewater so the activity of activated sludge remained relatively low. Another high effluent COD was observed at 25-30 mg.L-’ on days 81 to 83 due to the breakdown of the influent pump. However, taking the 135 days of performance as a whole, 92% of effluent COD data were equal to or less than 12 mg.L-’ when influent COD fluctuated between 30 and 1424mg.L-‘. On average, the COD of MMB effluent was as low as 7.5 mg.L-’ while total COD removal was as high as 95%. Obviously, the conclusion could be drawn that MMB had great potential in removing organic pollutants (as COD), and its stability in COD removal was rather satisfactory during the long-term pilot operation of MMB for urban wastewater treatment. Comparing results of days 30 to 62 (Run 2) with days 76 to 97 (Run 3), it was seen that effluent COD remained good when SRT was sequentially extended from 5, 15, to 30 days at a constant HRT of 5 h. In other words, the prolongation of SRT had no discernible effect on COD removal of the MMB system. Additionally, at a constant SRT of 30 days, from days 98 to 116 to days 117 to 135, the membrane flux went up from 85 to 100 L.m-*.h-’ whilst the HRT was shortened from 4.4 to 3.75 h. However, there was still no observable change of MMB effluent
100
10000
z
500
100
400
60
LG B
3oo
60
$
200
40
100
20
0
0 0
15
+-Runl+Run2
30
45
60 75 Time,d
90
105
120
135
---rtc--_Run3----+
Fig. 2. COD, NH,-N and SS vs. time. l influent (left Y), 0 effluent (left Y), A removal (right Y).
COD. These results indicated that due to the “physical separation” of the MF membrane, the performance of MMB was sound and stable in spite of the variation of SRT, HRT and membrane flux during the 135 days of experiment. With regard to ammonia nitrogen (NH,-N), both the influent and effluent were sampled weekly. As shown in Fig. 2, the effluent NH,-N remained generally as low as 0.2-1.1 mg.L-’ when influent NH,-N varied from 13 to 25 mg. L-‘. On the whole, 97% of effluent NH,-N data were lower than 0.6 mg.L-‘. The removal efficiency of NH,-N was found to be 97.7% on average, which indicated that most ammonia nitrogen had been either nitrified or stabilized within the MMB system. It could be explained from the following two aspects: (1) the nitrifying bacteria with a long generation time were completely confined within the bioreactor instead
C.-H. Xing et al. /Desalination
of being washed out, and therefore the concentration of nitrifying bacteria was kept relatively high. In contrast, however, the nitrifLing bacteria in CASP were often washed out, especially when SRT was set too short [ 171. (2) As the sludge production was low in MBR processes, the autotrophic nitrifying bacteria met less competition from those hetertrophic bacteria that were strong competitors for consuming ammonia nitrogen [8,18]. As a result, high nitrification was achieved in MMB even if SRT was as short as 5 days during Run 1 of this study. Similar to the above analysis on COD, it could be easily concluded from Fig. 2 that NH,-N removal during Runs 1, 2 Run 3 was high and stable as well when SRT, I-IRT and membrane flux varied between 5-30 days, 3.75-7.5 h and 50-100 L.mm2.h-‘,respectively. It may imply that the membrane separation physically improved the growth of nitrifying bacteria, and the removal efficiency of NH,-N was therefore no longer relevant to the operation parameters such as SRT, HRT and flux. Though the influent SS had changed between 2-483 mg.L-‘, no SS was detected in the MMB effluent in the 135-day experiment. In other words, the removal efficiency of SS attained was 100%. Similar results could be found in Manem and Sanderson [l], Xing et al. [9] and Chiemchaisri et al. [lo]. 3.2. Sludge concentration Activated sludge, concentrated before addition into the MMB bioreactor, was taken from secondary clarifier in a local wastewater treatment plant. The initial sludge SS was approximately 1.5 g.L-‘. As listed in Table 2, the start-up stage lasted for only 5 days. At this stage, HRT was prolonged from 5 to 15 h at a constant SRT of 30 days, while the membrane flux was set between 50 and 150 L.m-‘.h-‘. The evolution of sludge SS and VSS over time are depicted in Fig. 3. As SRT was
141 (2001) 63-73
67
30
0.75
24
0.90 0.45
iI9
%
3 ‘2 UJ
0.30
6
0.15
0
K
0.00 0
15
30
45
60
76
90
105
120
135
Time, d j--Runl+
Run2 +.
Run34
Fig. 3. Sludge SS, VSS and VSSISS vs. time.
sequentially extended from 5, 15 to 30 days; at each SRT, HRT was shortened therewith, and the sludge concentration (SS or VSS) was steadily increased from Runs 1 to 3. On day 105, there was a sudden increase of influent COD. The highest sludge concentration of MMB was achieved at 15.1 g.L-’ (as SS) and 8.9 g.L-’ (as VSS) that was about 5-8 times that in CASP. This suggested that by treating the same wastewater, the volume of the MMB bioreactor could be significantly reduced in comparison with that of CASP. A large amount of investment could be saved herewith. The sludge VSS to SS ratio was on average 0.55 despite that the MMB sludge concentration had observable changes with sequential extension of SRT during Runs 1,2 and 3. For treatment of urban wastewater by CASP, the sludge VSWSS ratio was known as 0.5-0.8 in most cases. Evidently, there was no discernible difference between MMB and CASP from the viewpoint of sludge VSS/SS ratio. .This implied that sludge activity in MMB was nearly the same as that in CASP. On the other hand, the sludge VSS/SS ratio in MMB was very stable (see Fig. 5) that indicated a dynamic balance between the active biomass and inorganic residues. No inert fractions were accumulated in the bioreactor of the MMB system. This can partially explain why the performance of the MMB was stable in longterm operation.
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C.-H. Xing et al. /Desalination 141 (2001) 63-73
Comparative research on Figs. 2 and 3 revealed that the variation of sludge concentration in the MMB bioreactor had no negative effect on the removal of COD, NH,-N, and SS. In fact, sludge concentration was just an inevitable outcome of the biomass passive adaptation to influent strength and operation parameters. At constant SRT, the higher strength the influent was, the higher the sludge concentration would be in the MMB bioreactor. Similarly, at constant influent strength, the longer the SRT was, the higher the sludge concentration would be, and vice versa. A high sludge concentration can often lead to poor effluent quality in CASP. However, the effluent quality was proven to be irrelevant to the sludge concentration in MMB when applied to urban wastewater reclamation. During the 135-day operation of the MMB, the evolution of sludge VSS vs. sludge COD is seen in Fig. 4. The correlation coefficient (R2)of 0.9655 indicated a linear relationship between sludge COD and sludge VSS. The CODNSS ratio of sludge was found to be 1.45, which was close to the theoretical value of 1.42 when the bacteria were formulated as C,H,NO, upon complete oxidation. Namely, the difference between the experimental and theoretical value was only 2.1%. This may suggest an alternative method for indirectly and promptly determining sludge concentration. In other words, once we measured the sludge COD, the sludge VSS could
be reasonably calculated by conversion between sludge COD and sludge VSS. The precision was sound enough to meet the requirements of scientific research because the linear relationship between sludge VSS and sludge COD was definitely favorable. On the other hand, the measurement of sludge VSS normally required one working day when using the traditional burn/weigh method. As the standard COD analysis took only 2-3 h, however, the sludge VSS could be available within 3 h by the aboverecommended conversion from sludge COD. A similar idea had been practiced in Japan for years where the sludge VSS was indirectly determined by the difference between total organic carbon and soluble organic carbon [ 191. 3.3. Sludge loading rates With the fluctuation of raw wastewater quality, loading rates of MMB changed accordingly during Runs 1, 2 and 3. As presented in Fig. 5, the highest mass loading rate was obtained at
0
0 0
2
4 Sludge
6 vss,
g.L”
Fig. 4. Sludge COD vs. sludge VSS.
a
-
1;
60
75
QO
105
Time, d
10 /-RunI+
Run2 s_
Fig. 5. sludge loading rates vs. time.
RunS-4
120
135
C.-H. Xing et al. /Desalination 141 (2001) 63-73
6.85 kgCOD.kgVSS’.d-’ on day 68, which was 17-68 times that of CASP (0.1-0.4 kgCOD. kgVSS’.d-‘). The highest volumetric loading rate was also achieved on the same day at 9.11 kg COD/m3.d, approximately 22-45 times that of CASP (0.4-0.8 ckgCOD/m3.d). Note that influent COD was measured at 1424mg.L-’ on day 68; however, the effluent COD was only 8 mg.L-‘. It indicated that MMB had strong ability to resist the shock loading and the performance remained sound and stable. In addition, the lowest mass loading rate was observed on day 114 at 0.036 kg COD.kgVSS’.d- (corresponding influent COD 44.9 mg.L-‘, effluent COD 4.1 mg.L-‘) and the lowest volumetric loading rate, on day 10 at 0.1 kg COD/m3 (corresponding influent COD 31 mg.L-‘, effluent COD 3.8 mg.L-‘). Taking Runs 1,2 and 3 as a whole, the average loading rates were found to be 0.83 kgCOD.kgVSS’.d-’ and 1.6 kg COD/m3, which were about 2-8 times and 24 times of those of CASP, respectively. For CASP, too low or high a mass loading rate would inevitably result in poor effluent quality. However, due to the membrane separation, the variation of mass loading rate in MMB had no effect on effluent quality. The mass loading rate, one of the most important parameters in CASPdesign, was no longer crucial to the fullscale design of MMB system when applied to urban wastewater treatment. From the viewpoint of reducing plant size, the application of MMB could not only eliminate the secondary clarifier but also reduce the volume of bioreactor by two to four times. Therefore, MMB was a spacesaving process [ 11. At a constant HRT of 5 h, results of day 3062 and 76-97 were chosen to examine the impact of SRT on mass loading rates. It could be seen that the average mass loading rates of MMB were decreased from 1.04 to 0.5 kgCOD.kgVSS’.d-’ when SRT was extended from 15 to 30 days (see Fig. 5). This was due to the fact that a long SRT led to high sludge VSS, and as a result, the mass loading rates decreased. The lower the mass
69
loading rates, the better the treated water quality of MMB. Moreover, the longer the SRT, the less the excess sludge production. Thus, considerable sludge disposal costs would be saved. In other words, MMB provided the possibility of reducing excess sludge production in urban wastewater reclamation. However, high sludge concentration may result in an increase of energy consumption. A comprehensive investigation was actually necessary to determine whether the operation of MMB at longer SRT was technically and economically feasible. 3.4. Analysis on removal eficiency distribution It is well known that MMB comprises a bioreactor and membrane modules, yet little information is available about their respective roles in pollutant removal when applied to urban wastewater reclamation. As illustrated in Fig. 6, COD removal efficiency was only 60-80% at the startup stage. However, COD removal of 95% was on average achieved in the 135-day experiment where 87% was performed by the bioreactor and only 8% was done by membrane separation. It could be concluded that the removal of organic pollutants (as COD) was mainly attributed to the bioreactor unit. The role of membrane module was to confine activated sludge within the bioreactor and provide a complete separation of
100
0 0
15
30
45
60
75
90
Time-d ~Runl--rtc_Run2
-Run3~-~
Fig. 6. COD removal distribution vs. time.
105
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135
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C.-H. Xing et al. /Desalination
activated sludge and treated water. It proved that the bioreactor was still the most important functional unit in the MMB application for urban wastewater reclamation. It was noticed that on days 9,10,114-l 16 and 128, COD removal by the bioreactor was on average 67.5% while the COD removal by the membrane module went up to l&7%, which represents one-fifth of the total COD removal efficiency by the MMB. This was due to the fact that the influent COD had a sudden decrease to 46.2 mg.L-’ on average when the COD of MMB effluent and sludge supernatant remained relatively stable. The amount of COD removal by the bioreactor was remarkably reduced while the COD removal by direct membrane separation remained unchanged. Consequently, COD removal by MMB was found to be only 86.2% in total. On the other hand, when influent COD was much higher, for instance, on days 52,65,68,70, 93, 104-105, and 122-125, total COD removal by the MMB reached 96% on average. Only 3.1% was attributed to membrane separation while 92.9% was removed by the bioreactor. It revealed that the membrane module played a minor role in COD removal if the influent concentration was high and vice versa (see Fig. 6). 3.5. Reuse potential of MMB effluent The quality of MMB effluent is summarized in Table 3. It can be seen that all the listed items met the current drinking water standards of China. Except the nitrate concentration (see gray row in Table 3), other monitored items met the guidelines for drinking water standards issued by the World Health Organization (WHO) as well. From the viewpoint of drinking water standards, MMB effluent might be good enough for potable uses. However, a concern whether drinking water standards were adequate to ensure the safety of all waters “regardless of source” is still under dispute. Some argue that drinking water standards applied only to - and were designed only
141 (2001) 63-73
for - waters derived from relatively pristine sources. Although this had a long-standing basis in normal sanitation practice, it was becoming more difficult to determine what the best available source water was. On the other hand, despite technological advances and high quality of treated effluents, public reactions to direct potable reuse and even indirect potable water reuse have not been encouraging at present [20]. In fact, different countries had developed different approaches and guidelines to protect public health and the environment. The major difference was the affordability and health risk level [25]. Regarding direct/indirect potable reuse of reclaimed wastewater, the applicable solution was to allow different nations to adopt their own regulations and guidelines. A uniform standard may not be realistic at this stage because different drinking water standards were being used in different countries. The California Water Recycling Regulations (CWRR) was the best known conservative/high-cost/low-risk example [26]. However, due to the difference in state jurisdictions, the CWRR may not be applicable for European and Asian countries. Comparatively, reuse of MMB effluent for municipal purposes such as toilet flushing, plant watering, car washing and so on, were more prone to public acceptance [9]. For densely populated areas, the MMB effluent may be a good substitution of drinking water that was used for non-potable purposes, and therefore, large amounts of drinking water could be saved. Similar reuse practices in large commercial buildings and apartment complexes have become more popular in Japan [20]. However, for those arid and semi-arid regions, MMB effluent could be a sound alternative source for groundwater recharge and finally augment drinking water supply potential [27]. In addition, the MMB effluent can be applied to irrigation of crops, flowers and landscaped areas. After simple pretreatment, the MMB effluent may also be applied to cooling tower, stack gas scrubbing and
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C.-H. Xing et al. /Desalination 141 (2001) 63-73 Table 3 MMB effluent quality and drinking water standards Items
MMB effluent
Color, TCU
c2.5
<15
15
Turbidity, NTU
<2
5
5
Taste and odor
Acceptable
Acceptable
Acceptable
Temperature, “C
Acceptable
Acceptable
pH value
8.2
Acceptable 6.5-8.5
Chloride as Cl-, mg.L-’
250
200
Fluoride as F-, mg.L-’
50 0.3
1.0
1.5
Nitrate as N, mg.L-’ Nitrate as N, mg.L-’
19.4 0.01
20
10.3
Hardness as CaCO,, mg.L-’ Phenols, mg.L-’
331 co.002
450
0.91 -
0.002
0.001
Cyanide as CN-, mg.L-’ Sulfate as SO:-, mg.L-’
co.002 25.5
0.05
0.07
250
200
0.05 1.0
0.01 1.0
co.004 co.05 co.01
0.05
0.05
0.1
0.5
0.05
0.01
co.05 _c
0.3
0.3
I 3 counts L-’
0 counts 100 mL-’
Arsenic, mg.L-’ Mercury, ug.L-’ Chromium as Cr6’, mg.L-’ Manganese, mg.L-’ Lead, mg.L-’ Iron, mg.L-’ Total coliforms
GB-5749-85’
WHO guidelinesb
7.0-8.5
TCU, true color units; NTU, nephelometric turbidity units. “Values represent the current drinking water standards in P.R. China. bValues represent the guidelines for drinking water quality of the World Health Organization [22,23]. The microfiltration membrane could effectively retain the bacteria (size from 0.5-5 urn) and viruses (size from 0.01-0.3 urn), and as a result, there was no coliform or MS-2 viruses detected [1,24]. From the biological viewpoint, the effluent of the MMB system was safe for potable purposes.
processing etc. [26]. Further purified by both reverse osmosis and ion exchange, MMB effluent can be used for wafer manufacture and, moreover, in consideration of the high price of ultrapure water, normally 5-l 0 US$/m3, the reuse potential of MMB effluent in this case might be promising. In a word, with the economic development and global industrialization, the conflict between water demand and water supply will continue to increase each year. As a result, water reuse as a proven alternative, either for metal
direct or indirect purposes, has been gaining worldwide acceptance in order to make the water supply more sustainable [2 11. Additionally, Table 3 indicates that to meet the strict WHO drinking water guidelines, a denitritication unit was required to remove the excessive nitrogen from MMB effluent. On the other hand, Muller et al. reported that denitrification might be realized in the bioreactor of the MMB even if there were no specifically designed anoxic step [ 171. As a matter of fact, further
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study is needed to uncover the profile of nitritication and denitrification that occurred naturally within the MMB system.
4. Conclusions The reclamation of urban wastewater by MMB was technically feasible. The reclaimed water could be reused for either municipal or industrial purposes, However, public reactions to direct potable reuse and even indirect potable water reuse have not been encouraging at present. The variation of operation parameters, sludge concentration and loading rates had no discernible effect on the stable performance of the MMB system. During the 135day experiment, COD removal efficiency was on average as high as 95%, in which 87% was attributed to the bioreactor and only 8% from membrane separation. Average removal of ammonia nitrogen and suspended solids attained 97.7% and loo%, respectively. The highest concentration of activated sludge was obtained at 15.1 g.L-’ (as SS) and 8.9 g.L-’ (as VSS) while the sludge VSS/SS ratio slightly vacillated around 0.55, corresponding to that of CASP (0.5-0.8). The relation between sludge VSS and sludge COD was soundly linear. The VSWOD ratio of sludge was about 1.45, which was quite close to the theoretical value 1.42. The mass loading rates of MMB fell within the CASP range, but the volumetric loading rates were two to four times those of CASP. This implied that treating the same wastewater, the impact of MMB could be remarkably reduced compared to that of CASP. Acknowledgments
The authors wish to acknowledge the CIRSEE-Suez-Lyonnaise des Eaux, France and the State Key Laboratory of Environmental Simulation & Pollution Control at Tsinghua University, China.
141 (2001) 63-73
References PI J. Manem and R. Sanderson, in: Water Treatment Membrane Process, J. Mallevialle, P.E. OdendaaI and M.R. Wiesner, eds., McGraw-Hill, New York, 1996, pp. 17.1-17.27. PI H. Moueddeb, J. Sanchez, C. Bardot and M. Fick, J. Membr. Sci., 114 (1996) 59. 131 M.G. Roig, J.F. Bello, S. Rodriguez, J.F. Kennedy and D.W. Taylor, Polymer Int., 39 (1996) 17. [41 U.H. Chun and P.L. Rogers, Desalination, 70 (1998) 353. PI S. Kise and M. Hayashida, J. Biotech., 14 (1990) 221. 161 E.S. Van Leeuwen, Wat. Sci. Tech., 24 (1991) 289. 171 K. Brindle and T. Stephenson, Biotech. Bioeng., 49 (1996) 601. PI S. Chaize and A. Huyard, Wat. Sci. Tech., 23 (1991) 1591. [91 C.-H. Xing, Y. Qian, X.-H. Wen and Y.-B. Meng, Proc. WEFTEC Asia’98, Singapore, 1 (1998) 119. 1101 C. Chiemchaisri, K. Yamamoto and S. Vigneswaran, Wat. Sci. Tech., 27 (1994) 171. Pll J.A. Scott, D.J. Neilson, W. Liu and P.N. Boon, Wat. Sci. Tech., 38 (1998) 413. C.-H. Xing, Ceramic membrane bioreactor for urban P21 wastewatertreatment and membrane fouling mechanism, Ph.D. Thesis, Tsinghua University (in Chinese), Beijing, 1998. F.W. Trouve, V. Urbain and J. Manem, Wat. Sci. [I31 Tech., 30 (1994) 151. P41 T. Ueda, K. Hataand Y. Kikuoka, Wat. Sci. Tech., 34 (1996) 189. WI P. Cote, H. Buisson, C. Pound and G. Arakaki, Desalination, 113 (1997) 189. 1161 Standard Methods for the Examination of Water and Wastewater, American Public Health Association/ American Water Works Association/Water Environment Federation, Washington, DC, USA, 1992. E.B. Muller, A.H. Stouthamer, H. W. van Verseveld P71 and D.M. Eikeiboom, Wat. Res., 29 (1995) 1179. [18] K. Hanaki, C. Wantawin and S. Ohgaki, Wat. Res., 24 (1996) 289. [19] K. Fujie, H.-K. Hu, X. Huang, Y. Tanaka, K. Urano and H. Ohtake, Wat. Sci. Tech., 33 (1996) 173. [20] T. Asano, Recycling oftreatedwastewater for indirect potable and urban reuse - treatment options and challenges. Presented at the Euro-CASE Workshop,
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[21]
[22] [23] [24]
Wastewater as a Resource, Insitut de France, Paris, 2000. T. Hedberg, Attitudes to traditional and alternative sustainable sanitary systems, Wat. Sci. Tech., 39 (1999) 9. T.H.Y. Tebbutt, Principles of Water Quality Control, 5th ed., Butterworth-Heinemann, Oxford, UK, 1998. W.J. Masschelein, Unit Processes in Drinking Water Treatment, Marcel Dekker, New York, 1992. N. Cicek, H. Winnen, M.T. Suidan, B.E. Wrenn, V. Urbain and J. Manem, Wat. Res., 32 (1998) 1553.
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[25] C. Devis, R. Hultquist, B. Jimenez-Cisneros, W. Kennedy, B. Sheikh and B. van der Mewe, Proc. 1st World Water Congress of the International Water Assoc., Paris, Vol. 8,2000,9-16. [26] J. Crook, D.A. Okun and A.B. Pincince, Water reuse, an assessment report prepared by Camp Dresser & McKee Inc., Alexandria, USA, 1994. [27] Joint Task Force of Water Environment Federation and American Water Works Association, Using reclaimed water to augment potable water resources, Washington, DC, USA, 1998.
Desalination 141 (2001) 63-73 www.elsevier.com/locate/desal
Microfiltration-membrane-coupled bioreactor for urban wastewater reclamation C.-H. Xing”‘, X.-H. Wena, Y. Qiana, E. Tardieub “State Key Joint Laboratory of Environmental Simulation and Pollution Control, Department of Environmental Science and Engineering, Tsinghua University Beijing, 100084, P. R China bDirection Dspartementale de I’Agriculture et de la Forgt, Centre AdminishatifCondP, 18013 Bourges Cedex, France
Received 30 March 200 1; accepted 23 April 200 1
Abstract A microfiltration-membrane-coupled bioreactor (MMB) on pilot scale was operated for 135 days to investigate its technical feasibility in urban wastewater reclamation. Different operation parameters such as sludge retention time from 5 to 30 days, hydraulic retention time from 3.75 to 7.5 h, and membrane flux from 50 to 100 L.m-*.h-‘, were tested. The highest sludge concentration attained was 15.1 g.L-’ (as suspended solids, SS) and 8.9 g.L-’ (as volatile suspended solids, VSS); the VSS to SS ratio of MMB sludge was 0.55 on average. The ratio of sludge VSS to sludge COD was found to be 1.45. The mass loading rates of MMB were close to those of conventional activated sludge process (CASP) but the volumetric loading rates, two to four times those of CASP. Up to 95% of COD, 97.7% of ammonia nitrogen and 100% of suspended solids were removed on average. Further analysis indicated that the bioreactor was responsible for 87% of the total COD removal and only 8% were attributed to the membrane separation. The reclaimed water could be reused either directly or indirectly for municipal or industrial purposes. Keywords: Activated sludge; Bioreactor; Ceramic membrane; Reclamation; Microfiltration; Urban wastewater
1. Introduction The microfiltration membrane bioreactor (MMB) is actually an integration of advanced membrane separation and a conventional bio*Corresponding author. Currently at Environmental Technology Institute, Innovation Centre (NTU), BLK 2, Unit 237,18 Nanyang Drive, Singapore 637723. Tel: +65 7943 126; Fax: +65 7921291; Email: [email protected]
reactor [ 11. Since its emergence, MMB has been successfully applied to the biology industry [2,3], the chemical industry [4,5], biotechnological leaching of heavy metals [6] and wastewater treatment [7]. With more serious environmental pollution, MMB’s application in wastewater treatment is receiving more and more attention and in this case, generally featured as the replacement of secondary clarifier by a
001 l-9164/01/$- See front matter Q 2001 Elsevier Science B.V. All rights reserved PII: SO01 l-9164(01)00389-7
64
C.-H. Xing et al. /Desalination 141 (2001) 63-73
microfiltration (MF) membrane [S]. Due to the absence of a secondary clarifier, the overall MMB plant size can be remarkably reduced in comparison with that of the conventional activated sludge process (CASP) [9]. MMB provides not only complete retention of all microorganisms and an increase of sludge concentration but also a complete disinfection of treated water [l]. As a consequence, MMB offers the absolute separation of hydraulic retention time (HRT) and sludge retention time (SRT), which facilitates a more flexible control of operation parameters [ 1,lo]. High sludge concentration maintained in the bioreactor of the MMB makes it possible to treat high strength wastewater efficiently [ 111.The entire retention of activated sludge extends the sludge-substrate contact time and therefore is conducive to removing lowbiodegradable pollutants contained in wastewater
[121. A semi-industrial MMB trial on domestic wastewater has demonstrated that extremely high nitrification can be achieved, and removal of COD, ammonia nitrogen and SS reached 96%, 97% and 99.9%, respectively. The filtration system maintained a good flux of 60 to 80 L. m-*.h-’ at 20°C for over 15 days without chemical cleaning [ 131. It is reported that for rural settlement treatment, over 90% of organic matter, SS, and coliform were removed during 336 days of study [ 141.To further examine their effectiveness for municipal wastewater, two pilot MMB systems have been studied in California for 5 months and in Paris for 1 year [ 151.Preliminary results show that a high degree of treatment in terms of SS (100%) and organic matter (greater than 96% for COD) can be achieved and better than 6 log removal of total coliform and 4 log removal of naturally occurring bacteriophages were observed. However, as most of the aforementioned membranes within MMB are made from polymeric materials sensitive to caustic cleaning reagents, much difficulty has been encountered in membrane cleaning [9]. A
possible solution is to substitute these polymeric membranes by ceramic MF modules that are easy to be chemically cleaned in situ. Little is yet known about the performance of ceramic MMB when applied to urban wastewater reclamation. The objective of this paper was to examine the long-term performance of a ceramic MMB system at pilot scale for urban wastewater reclamation. The impact of operation parameters on effluent quality was carefully investigated. The roles of the membrane and bioreactor were analyzed in terms of COD removal efficiency. Finally, the reuse potential of reclaimed water was extensively discussed in this paper.
2. Materials and methods 2.1. Process description The process diagram is described in Fig. 1.
The bioreactor filled with activated sludge had a working volume of 30 L. A level controller and pump were operated together to keep the working volume stable. The bioreactor, centrifugal pump 2 and MF module constituted a loop where the MMB sludge was circulated at a high speed. To enhance the circulation, centrifugal pump 3 was installed. The flowrate of MMB effluent was precisely controlled by a PLC unit, pneumatic valve and an electromagnetic flowmeter. By
E-m A Stirrermn Awata r”) He.1 exchanger w
Vdw
I
Flow-mater
k
Convolve
Fig. 1. Schematics of the MMB pilot.
0 *
Level controller Pmasurepln
65
C.-H. Xing et al. /Desalination 141 (2001) 63-73
altering the flowrate of excess sludge discharge, the magnitude of SRT was changed. In cleaning mode, however, the cleaning tank would replace the bioreactor to form a cleaning loop with the microtiltration membrane module. The MF membrane was a ceramic tubular KerasepTM X6. The membrane skin layer and support material were made of zirconia (ZrO,) and alumina (y-A&O,), respectively. Each membrane had seven channels and the diameter of each channel was 4.5 mm. The membrane was 40 cm in length while the surface area of each single membrane was 0.04 m*. Membrane pore size was about 0.45 pm. Initial permeability of the new membrane was about 22 L.m-*.h-‘. (kPa)-’ on the basis of a tap water test at 25°C. 2.2. Urban wastewater The urban wastewater was piped from a local sewage station. As shown in Table 1, it could be classified as typical low-strength urban wastewater. 2.3. Operation parameters The experiment was carried out in three phases: Run 1, Run 2, and Run 3. The operation parameters are listed in Table 2. SRT was
sequentially extended from 5 and 15 to 30 days while HRT varied from 3.75,4.4,5 to 7.5 h. The cross-flow velocity within membrane channels was set at 2-3 m.s-‘. To compare the effluent quality under different fluxes, the membrane flux was increased from 50, 75, 85 to 100 L.m-*.h-’ by raising the flowrate of effluent. 2.4. Analytical methods The MMB system was monitored with daily measurements of redox (by Monet 8935) and temperature. Turbidity was measured by model 965- 10 turbidity meter (Orbeco analytical System Inc. USA). Samples for supernatant COD of activated sludge was taken after 15 minutes of
Table 1 Characteristics of urban wastewater Items
Typical
Range
COD, mg.L-’ SS, mg.L-’ NH,-N, mg.L-’ Coliform, number L-’ Turbidity, NTU pH value Temp., ‘C
100-300 20-200 17-23 105-lo6 50-60 7.5-8.5 15-25
30-1424 o--483 13-25 105-10’ 50-80 7.5-8.5 15-25
Table 2 Operation parameters Items
Start-end, d
HRT, h
SRT, d
Flux, L.m-*.h“
Run 1
l-5 6-29
5-15 7.5
30 5
50-150 50
Run 2
30-62 63-75
5.0 3.75
15 15
75 100
Run 3
76-97 98-l 16 117-135
5 4.4 3.75
30 30 30
75 85 100
66
C.-H. Xing et al. /Desalination
centrifugation at 4500 rpm and subsequent standard filter filtration. All the items in Table 3 were measured as reference [ 161.
141 (2001) 63-73 100000
60
s p loo0 g 0
3. Results and discussions
60
100
40
10
20
1
0
50
100
40
60
5 d E
30
60
* x"
20
40
10
20
0
0
3.1. Removal of COD, NH,-N and SS
Fig. 2 represents the evolution of COD, NH,-N and SS vs. time. It could be seen that at the start-up stage (days 1 to 5), the effluent COD of the MMB system attained 4347 mg.L-’ when the influent COD changed between 1lo160 mg.L-‘. This was because the newly inoculated sludge could not immediately adapt to the urban wastewater so the activity of activated sludge remained relatively low. Another high effluent COD was observed at 25-30 mg.L-’ on days 81 to 83 due to the breakdown of the influent pump. However, taking the 135 days of performance as a whole, 92% of effluent COD data were equal to or less than 12 mg.L-’ when influent COD fluctuated between 30 and 1424mg.L-‘. On average, the COD of MMB effluent was as low as 7.5 mg.L-’ while total COD removal was as high as 95%. Obviously, the conclusion could be drawn that MMB had great potential in removing organic pollutants (as COD), and its stability in COD removal was rather satisfactory during the long-term pilot operation of MMB for urban wastewater treatment. Comparing results of days 30 to 62 (Run 2) with days 76 to 97 (Run 3), it was seen that effluent COD remained good when SRT was sequentially extended from 5, 15, to 30 days at a constant HRT of 5 h. In other words, the prolongation of SRT had no discernible effect on COD removal of the MMB system. Additionally, at a constant SRT of 30 days, from days 98 to 116 to days 117 to 135, the membrane flux went up from 85 to 100 L.m-*.h-’ whilst the HRT was shortened from 4.4 to 3.75 h. However, there was still no observable change of MMB effluent
100
10000
z
500
100
400
60
LG B
3oo
60
$
200
40
100
20
0
0 0
15
+-Runl+Run2
30
45
60 75 Time,d
90
105
120
135
---rtc--_Run3----+
Fig. 2. COD, NH,-N and SS vs. time. l influent (left Y), 0 effluent (left Y), A removal (right Y).
COD. These results indicated that due to the “physical separation” of the MF membrane, the performance of MMB was sound and stable in spite of the variation of SRT, HRT and membrane flux during the 135 days of experiment. With regard to ammonia nitrogen (NH,-N), both the influent and effluent were sampled weekly. As shown in Fig. 2, the effluent NH,-N remained generally as low as 0.2-1.1 mg.L-’ when influent NH,-N varied from 13 to 25 mg. L-‘. On the whole, 97% of effluent NH,-N data were lower than 0.6 mg.L-‘. The removal efficiency of NH,-N was found to be 97.7% on average, which indicated that most ammonia nitrogen had been either nitrified or stabilized within the MMB system. It could be explained from the following two aspects: (1) the nitrifying bacteria with a long generation time were completely confined within the bioreactor instead
C.-H. Xing et al. /Desalination
of being washed out, and therefore the concentration of nitrifying bacteria was kept relatively high. In contrast, however, the nitrifLing bacteria in CASP were often washed out, especially when SRT was set too short [ 171. (2) As the sludge production was low in MBR processes, the autotrophic nitrifying bacteria met less competition from those hetertrophic bacteria that were strong competitors for consuming ammonia nitrogen [8,18]. As a result, high nitrification was achieved in MMB even if SRT was as short as 5 days during Run 1 of this study. Similar to the above analysis on COD, it could be easily concluded from Fig. 2 that NH,-N removal during Runs 1, 2 Run 3 was high and stable as well when SRT, I-IRT and membrane flux varied between 5-30 days, 3.75-7.5 h and 50-100 L.mm2.h-‘,respectively. It may imply that the membrane separation physically improved the growth of nitrifying bacteria, and the removal efficiency of NH,-N was therefore no longer relevant to the operation parameters such as SRT, HRT and flux. Though the influent SS had changed between 2-483 mg.L-‘, no SS was detected in the MMB effluent in the 135-day experiment. In other words, the removal efficiency of SS attained was 100%. Similar results could be found in Manem and Sanderson [l], Xing et al. [9] and Chiemchaisri et al. [lo]. 3.2. Sludge concentration Activated sludge, concentrated before addition into the MMB bioreactor, was taken from secondary clarifier in a local wastewater treatment plant. The initial sludge SS was approximately 1.5 g.L-‘. As listed in Table 2, the start-up stage lasted for only 5 days. At this stage, HRT was prolonged from 5 to 15 h at a constant SRT of 30 days, while the membrane flux was set between 50 and 150 L.m-‘.h-‘. The evolution of sludge SS and VSS over time are depicted in Fig. 3. As SRT was
141 (2001) 63-73
67
30
0.75
24
0.90 0.45
iI9
%
3 ‘2 UJ
0.30
6
0.15
0
K
0.00 0
15
30
45
60
76
90
105
120
135
Time, d j--Runl+
Run2 +.
Run34
Fig. 3. Sludge SS, VSS and VSSISS vs. time.
sequentially extended from 5, 15 to 30 days; at each SRT, HRT was shortened therewith, and the sludge concentration (SS or VSS) was steadily increased from Runs 1 to 3. On day 105, there was a sudden increase of influent COD. The highest sludge concentration of MMB was achieved at 15.1 g.L-’ (as SS) and 8.9 g.L-’ (as VSS) that was about 5-8 times that in CASP. This suggested that by treating the same wastewater, the volume of the MMB bioreactor could be significantly reduced in comparison with that of CASP. A large amount of investment could be saved herewith. The sludge VSS to SS ratio was on average 0.55 despite that the MMB sludge concentration had observable changes with sequential extension of SRT during Runs 1,2 and 3. For treatment of urban wastewater by CASP, the sludge VSWSS ratio was known as 0.5-0.8 in most cases. Evidently, there was no discernible difference between MMB and CASP from the viewpoint of sludge VSS/SS ratio. .This implied that sludge activity in MMB was nearly the same as that in CASP. On the other hand, the sludge VSS/SS ratio in MMB was very stable (see Fig. 5) that indicated a dynamic balance between the active biomass and inorganic residues. No inert fractions were accumulated in the bioreactor of the MMB system. This can partially explain why the performance of the MMB was stable in longterm operation.
68
C.-H. Xing et al. /Desalination 141 (2001) 63-73
Comparative research on Figs. 2 and 3 revealed that the variation of sludge concentration in the MMB bioreactor had no negative effect on the removal of COD, NH,-N, and SS. In fact, sludge concentration was just an inevitable outcome of the biomass passive adaptation to influent strength and operation parameters. At constant SRT, the higher strength the influent was, the higher the sludge concentration would be in the MMB bioreactor. Similarly, at constant influent strength, the longer the SRT was, the higher the sludge concentration would be, and vice versa. A high sludge concentration can often lead to poor effluent quality in CASP. However, the effluent quality was proven to be irrelevant to the sludge concentration in MMB when applied to urban wastewater reclamation. During the 135-day operation of the MMB, the evolution of sludge VSS vs. sludge COD is seen in Fig. 4. The correlation coefficient (R2)of 0.9655 indicated a linear relationship between sludge COD and sludge VSS. The CODNSS ratio of sludge was found to be 1.45, which was close to the theoretical value of 1.42 when the bacteria were formulated as C,H,NO, upon complete oxidation. Namely, the difference between the experimental and theoretical value was only 2.1%. This may suggest an alternative method for indirectly and promptly determining sludge concentration. In other words, once we measured the sludge COD, the sludge VSS could
be reasonably calculated by conversion between sludge COD and sludge VSS. The precision was sound enough to meet the requirements of scientific research because the linear relationship between sludge VSS and sludge COD was definitely favorable. On the other hand, the measurement of sludge VSS normally required one working day when using the traditional burn/weigh method. As the standard COD analysis took only 2-3 h, however, the sludge VSS could be available within 3 h by the aboverecommended conversion from sludge COD. A similar idea had been practiced in Japan for years where the sludge VSS was indirectly determined by the difference between total organic carbon and soluble organic carbon [ 191. 3.3. Sludge loading rates With the fluctuation of raw wastewater quality, loading rates of MMB changed accordingly during Runs 1, 2 and 3. As presented in Fig. 5, the highest mass loading rate was obtained at
0
0 0
2
4 Sludge
6 vss,
g.L”
Fig. 4. Sludge COD vs. sludge VSS.
a
-
1;
60
75
QO
105
Time, d
10 /-RunI+
Run2 s_
Fig. 5. sludge loading rates vs. time.
RunS-4
120
135
C.-H. Xing et al. /Desalination 141 (2001) 63-73
6.85 kgCOD.kgVSS’.d-’ on day 68, which was 17-68 times that of CASP (0.1-0.4 kgCOD. kgVSS’.d-‘). The highest volumetric loading rate was also achieved on the same day at 9.11 kg COD/m3.d, approximately 22-45 times that of CASP (0.4-0.8 ckgCOD/m3.d). Note that influent COD was measured at 1424mg.L-’ on day 68; however, the effluent COD was only 8 mg.L-‘. It indicated that MMB had strong ability to resist the shock loading and the performance remained sound and stable. In addition, the lowest mass loading rate was observed on day 114 at 0.036 kg COD.kgVSS’.d- (corresponding influent COD 44.9 mg.L-‘, effluent COD 4.1 mg.L-‘) and the lowest volumetric loading rate, on day 10 at 0.1 kg COD/m3 (corresponding influent COD 31 mg.L-‘, effluent COD 3.8 mg.L-‘). Taking Runs 1,2 and 3 as a whole, the average loading rates were found to be 0.83 kgCOD.kgVSS’.d-’ and 1.6 kg COD/m3, which were about 2-8 times and 24 times of those of CASP, respectively. For CASP, too low or high a mass loading rate would inevitably result in poor effluent quality. However, due to the membrane separation, the variation of mass loading rate in MMB had no effect on effluent quality. The mass loading rate, one of the most important parameters in CASPdesign, was no longer crucial to the fullscale design of MMB system when applied to urban wastewater treatment. From the viewpoint of reducing plant size, the application of MMB could not only eliminate the secondary clarifier but also reduce the volume of bioreactor by two to four times. Therefore, MMB was a spacesaving process [ 11. At a constant HRT of 5 h, results of day 3062 and 76-97 were chosen to examine the impact of SRT on mass loading rates. It could be seen that the average mass loading rates of MMB were decreased from 1.04 to 0.5 kgCOD.kgVSS’.d-’ when SRT was extended from 15 to 30 days (see Fig. 5). This was due to the fact that a long SRT led to high sludge VSS, and as a result, the mass loading rates decreased. The lower the mass
69
loading rates, the better the treated water quality of MMB. Moreover, the longer the SRT, the less the excess sludge production. Thus, considerable sludge disposal costs would be saved. In other words, MMB provided the possibility of reducing excess sludge production in urban wastewater reclamation. However, high sludge concentration may result in an increase of energy consumption. A comprehensive investigation was actually necessary to determine whether the operation of MMB at longer SRT was technically and economically feasible. 3.4. Analysis on removal eficiency distribution It is well known that MMB comprises a bioreactor and membrane modules, yet little information is available about their respective roles in pollutant removal when applied to urban wastewater reclamation. As illustrated in Fig. 6, COD removal efficiency was only 60-80% at the startup stage. However, COD removal of 95% was on average achieved in the 135-day experiment where 87% was performed by the bioreactor and only 8% was done by membrane separation. It could be concluded that the removal of organic pollutants (as COD) was mainly attributed to the bioreactor unit. The role of membrane module was to confine activated sludge within the bioreactor and provide a complete separation of
100
0 0
15
30
45
60
75
90
Time-d ~Runl--rtc_Run2
-Run3~-~
Fig. 6. COD removal distribution vs. time.
105
120
135
70
C.-H. Xing et al. /Desalination
activated sludge and treated water. It proved that the bioreactor was still the most important functional unit in the MMB application for urban wastewater reclamation. It was noticed that on days 9,10,114-l 16 and 128, COD removal by the bioreactor was on average 67.5% while the COD removal by the membrane module went up to l&7%, which represents one-fifth of the total COD removal efficiency by the MMB. This was due to the fact that the influent COD had a sudden decrease to 46.2 mg.L-’ on average when the COD of MMB effluent and sludge supernatant remained relatively stable. The amount of COD removal by the bioreactor was remarkably reduced while the COD removal by direct membrane separation remained unchanged. Consequently, COD removal by MMB was found to be only 86.2% in total. On the other hand, when influent COD was much higher, for instance, on days 52,65,68,70, 93, 104-105, and 122-125, total COD removal by the MMB reached 96% on average. Only 3.1% was attributed to membrane separation while 92.9% was removed by the bioreactor. It revealed that the membrane module played a minor role in COD removal if the influent concentration was high and vice versa (see Fig. 6). 3.5. Reuse potential of MMB effluent The quality of MMB effluent is summarized in Table 3. It can be seen that all the listed items met the current drinking water standards of China. Except the nitrate concentration (see gray row in Table 3), other monitored items met the guidelines for drinking water standards issued by the World Health Organization (WHO) as well. From the viewpoint of drinking water standards, MMB effluent might be good enough for potable uses. However, a concern whether drinking water standards were adequate to ensure the safety of all waters “regardless of source” is still under dispute. Some argue that drinking water standards applied only to - and were designed only
141 (2001) 63-73
for - waters derived from relatively pristine sources. Although this had a long-standing basis in normal sanitation practice, it was becoming more difficult to determine what the best available source water was. On the other hand, despite technological advances and high quality of treated effluents, public reactions to direct potable reuse and even indirect potable water reuse have not been encouraging at present [20]. In fact, different countries had developed different approaches and guidelines to protect public health and the environment. The major difference was the affordability and health risk level [25]. Regarding direct/indirect potable reuse of reclaimed wastewater, the applicable solution was to allow different nations to adopt their own regulations and guidelines. A uniform standard may not be realistic at this stage because different drinking water standards were being used in different countries. The California Water Recycling Regulations (CWRR) was the best known conservative/high-cost/low-risk example [26]. However, due to the difference in state jurisdictions, the CWRR may not be applicable for European and Asian countries. Comparatively, reuse of MMB effluent for municipal purposes such as toilet flushing, plant watering, car washing and so on, were more prone to public acceptance [9]. For densely populated areas, the MMB effluent may be a good substitution of drinking water that was used for non-potable purposes, and therefore, large amounts of drinking water could be saved. Similar reuse practices in large commercial buildings and apartment complexes have become more popular in Japan [20]. However, for those arid and semi-arid regions, MMB effluent could be a sound alternative source for groundwater recharge and finally augment drinking water supply potential [27]. In addition, the MMB effluent can be applied to irrigation of crops, flowers and landscaped areas. After simple pretreatment, the MMB effluent may also be applied to cooling tower, stack gas scrubbing and
71
C.-H. Xing et al. /Desalination 141 (2001) 63-73 Table 3 MMB effluent quality and drinking water standards Items
MMB effluent
Color, TCU
c2.5
<15
15
Turbidity, NTU
<2
5
5
Taste and odor
Acceptable
Acceptable
Acceptable
Temperature, “C
Acceptable
Acceptable
pH value
8.2
Acceptable 6.5-8.5
Chloride as Cl-, mg.L-’
250
200
Fluoride as F-, mg.L-’
50 0.3
1.0
1.5
Nitrate as N, mg.L-’ Nitrate as N, mg.L-’
19.4 0.01
20
10.3
Hardness as CaCO,, mg.L-’ Phenols, mg.L-’
331 co.002
450
0.91 -
0.002
0.001
Cyanide as CN-, mg.L-’ Sulfate as SO:-, mg.L-’
co.002 25.5
0.05
0.07
250
200
0.05 1.0
0.01 1.0
co.004 co.05 co.01
0.05
0.05
0.1
0.5
0.05
0.01
co.05 _c
0.3
0.3
I 3 counts L-’
0 counts 100 mL-’
Arsenic, mg.L-’ Mercury, ug.L-’ Chromium as Cr6’, mg.L-’ Manganese, mg.L-’ Lead, mg.L-’ Iron, mg.L-’ Total coliforms
GB-5749-85’
WHO guidelinesb
7.0-8.5
TCU, true color units; NTU, nephelometric turbidity units. “Values represent the current drinking water standards in P.R. China. bValues represent the guidelines for drinking water quality of the World Health Organization [22,23]. The microfiltration membrane could effectively retain the bacteria (size from 0.5-5 urn) and viruses (size from 0.01-0.3 urn), and as a result, there was no coliform or MS-2 viruses detected [1,24]. From the biological viewpoint, the effluent of the MMB system was safe for potable purposes.
processing etc. [26]. Further purified by both reverse osmosis and ion exchange, MMB effluent can be used for wafer manufacture and, moreover, in consideration of the high price of ultrapure water, normally 5-l 0 US$/m3, the reuse potential of MMB effluent in this case might be promising. In a word, with the economic development and global industrialization, the conflict between water demand and water supply will continue to increase each year. As a result, water reuse as a proven alternative, either for metal
direct or indirect purposes, has been gaining worldwide acceptance in order to make the water supply more sustainable [2 11. Additionally, Table 3 indicates that to meet the strict WHO drinking water guidelines, a denitritication unit was required to remove the excessive nitrogen from MMB effluent. On the other hand, Muller et al. reported that denitrification might be realized in the bioreactor of the MMB even if there were no specifically designed anoxic step [ 171. As a matter of fact, further
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study is needed to uncover the profile of nitritication and denitrification that occurred naturally within the MMB system.
4. Conclusions The reclamation of urban wastewater by MMB was technically feasible. The reclaimed water could be reused for either municipal or industrial purposes, However, public reactions to direct potable reuse and even indirect potable water reuse have not been encouraging at present. The variation of operation parameters, sludge concentration and loading rates had no discernible effect on the stable performance of the MMB system. During the 135day experiment, COD removal efficiency was on average as high as 95%, in which 87% was attributed to the bioreactor and only 8% from membrane separation. Average removal of ammonia nitrogen and suspended solids attained 97.7% and loo%, respectively. The highest concentration of activated sludge was obtained at 15.1 g.L-’ (as SS) and 8.9 g.L-’ (as VSS) while the sludge VSS/SS ratio slightly vacillated around 0.55, corresponding to that of CASP (0.5-0.8). The relation between sludge VSS and sludge COD was soundly linear. The VSWOD ratio of sludge was about 1.45, which was quite close to the theoretical value 1.42. The mass loading rates of MMB fell within the CASP range, but the volumetric loading rates were two to four times those of CASP. This implied that treating the same wastewater, the impact of MMB could be remarkably reduced compared to that of CASP. Acknowledgments
The authors wish to acknowledge the CIRSEE-Suez-Lyonnaise des Eaux, France and the State Key Laboratory of Environmental Simulation & Pollution Control at Tsinghua University, China.
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