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Behaviour of soluble microbial products in a membrane bioreactor
Process Biochemistry 36 (2000) 401 – 406 www.elsevier.com/locate/procbio
Behaviour of soluble microbial products in a membrane bioreactor Xia Huang *, Rui Liu, Yi Qian Department of En6ironmental Science and Engineering, En6ironment Simulation and Pollution Control, State Key Joint Laboratory, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Received 2 February 2000; received in revised form 24 May 2000; accepted 3 June 2000
Abstract Synthetic wastewater was treated with a submerged membrane bioreactor to investigate the organic removal performance, as well as the behaviour of soluble microbial products during long-term operation. Satisfactory chemical oxygen demand (COD), total organic carbon (TOC) and biological oxygen demand (BOD) removal efficiencies were achieved, averaging over 90, 94 and 95%, respectively. The accumulation of TOC in the supernatant of the bioreactor and its degradation after 5 months of operation was observed. Among this accumulation, macromolecules with a molecular weight (MW) \ 100 000 accounted for 34%. A decrease in the supernatant TOC corresponded to a shift from large to small molecules. The organic substances with a MW\100 000 decreased from 34 to 16%, whereas those with a MWB 30 000 increased from 33 up to 52%. The accumulation proved to be inhibitory towards the metabolic activity of the activated sludge, as well as contributing to the poor membrane permeability of the mixed liquor. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Membrane bioreactor; Microbial product; Accumulation; Molecular weight; Metabolic activity; Membrane permeability
1. Introduction The membrane bioreactor, a technological combination of biological treatment with a membrane separation device, has many advantages due to the efficient interception performance of the membrane. Benefits of the membrane bioreactor include excellent effluent quality free of suspended solids and bacteria, a highly retentive activated sludge concentration and consequent compactness of treatment equipment, less surplus sludge production and easy management [1 – 3]. The study on its applications in treating various types of waste streams, such as domestic wastewater [4,5], industrial wastewater [6] and human excrement [7] has therefore attracted great attention. By substituting the final clarifier in the conventional activated sludge process, a membrane device acts as a screen for the interception of suspended solids and microorganisms. In addition, various soluble organic substances, which might be either undecomposed organic substances contained in raw wastewater or micro* Corresponding author. E-mail address: [email protected] (X. Huang).
bial metabolic products produced during biological reactions, could also be selectively retained within the bioreactor and this can upgrade the effluent quality. The behaviour of these accumulated soluble organic substances and especially their influence on microbial activity in the membrane bioreactor has been a controversial issue and has been reported under different operation conditions [5,8,9]. Zhang and Yamamoto mentioned that these accumulated microbial products might be one of the limiting factors to bacterial activity and viability [9]. However, most of these studies were performed during short-term operation and there are few reports of long-term operations. The purpose of this study is, therefore, to elucidate the behaviour of the accumulated soluble organic compounds in a submerged membrane bioreactor during a long-term run over 250 days to treat synthetic wastewater containing glucose and starch. The components of the soluble microbial products in the biological process are complex, and may include humic substances, fulvic acid, polysaccharide, protein, nucleic acid, etc. [10,11] almost independent on the influent wastewater [12]. Since it is not easy to identify each component, the total organic carbon (TOC), a comprehensive index was
0032-9592/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 2 0 6 - 5
X. Huang et al. / Process Biochemistry 36 (2000) 401–406
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Fig. 1. Diagram of the experimental system.
used here to represent the total concentration of soluble microbial products. The supernatant TOC concentration was continuously monitored and the molecular weight distribution of the soluble organic substances in the mixed liquor and their influences on the metabolic activity of activated sludge and the permeability of the mixed liquor were investigated. The results are useful for the maintenance of the membrane bioreactor process.
2. Material and methods
2.1. The experimental system and conditions The experimental system consists of an activated sludge bioreactor and a bundle of hollow fibre membranes (Fig. 1). The bioreactor was 30× 22 ×23 cm, having an effective volume of 15 l. The membrane was made of polypropylene with a pore size of 0.1 mm and a filtration area of 0.4 m2 and was submerged in the bioreactor. Raw wastewater supplied from the storage tank was fed into the bioreactor, where organic pollutants were biologically decomposed. An air diffuser located beneath the membrane module provided aeration and mixed the wastewater. The membrane-filtered effluent was continuously removed with a suction pump (Diaphragm pump, model X030-XB-AAAA365, PULTable 1 Components and quality of synthetic wastewater used in the study (mg l−1 except pH) Component
Concentration
Items
Concentration
Industrial glucose Starch Protein KH2PO4 MgSO4 · 7H2O MnSO4 · 7H2O FeSO4 CaCl2 (NH2)2CO NaHCO3
278 278 28 52.8 66 6 0.3 6 167 111
COD TOC BOD5 NH3-N PH
300–600 110–125 150–300 36–72 6.5
SAFEEDER) connected to the membrane module. An influent pump of the same make and model as the suction pump was controlled by a water level sensor to maintain a constant water level in the bioreactor over the experimental period. The water temperature in the membrane bioreactor was maintained at 25°C with a temperature controller. A synthetic wastewater comprising of glucose, starch and trace nutrients were used. Its components and quality are shown in Table 1. Due to the slight sedimentation of starch and degradation of organic materials in the wastewater storage tank, chemical oxygen demand (COD), TOC, biological oxygen demand (BOD) and NH3-N concentrations in the influent showed some fluctuations but were in the range of the typical municipal sewage of China. Since large molecular weight polysaccharides are the main components contained in municipal sewage, a mixture of starch, glucose and protein reflects the actual situation. The experiment was conducted at a hydraulic retention time of 8.3 h, a sludge retention time of 20 day and a dissolved oxygen concentration in bioreactor of 5 mg l − 1. In order to investigate the behaviour of accumulated soluble organic substances in the membrane bioreactor during long-term operation, the test was run over 250 days. Periodically the membrane module was taken from the bioreactor and cleaned with tap water to remove sludge deposited on the membrane surface.
2.2. Analytical items and methods COD, BOD, TOC and pH of the influent, supernatant liquor, membrane effluent and the suspended solids (SS) concentration in the mixed liquor were measured, respectively, by adopting the Chinese SEPA standard methods [13]. The supernatant samples were obtained by centrifuging the mixed liquor (LG10-2.4A, Beijing Medical Centrifuge Corporation) at 4000 rpm for 15 min, followed by filtration through a membrane of 0.45 mm. For the analysis of the molecular weight (MW) distribution of organic substances in the supernatant liquor, four types of circular ultrafiltration membranes with molecular weight cut-off ranges of 3000, 30 000, 60 000 and 100 000 were used. The filtrate permeating through each membrane with a different MW cut-off range was collected and the TOC concentration measured. The percentage of organic substances with a different MW range in the total amount, in terms of TOC, was calculated using the mass balance concept. The concentration of polysaccharides in the supernatant liquor was measured using an anthrone method [14]. The metabolic activity of activated sludge in the bioreactor was characterised by measuring the dehydrogenase activity [15].
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keep constant the characters of the mixed liquor. The flux could be calculated from the increasing rate of this accumulated volume. In order to eliminate the influence of temperature on the membrane flux, all fluxes J (T) measured at temperature T were corrected to values J(25) at temperature 25°C by the following equation [16]. J(25)= J(T)× 1.02525 − T
(1)
3. Results and discussion Fig. 2. Changes of COD concentrations in the influent and effluent.
3.1. O6erall organic remo6al performance
Fig. 3. Variation of sludge concentration in the bioreactor over the operation time.
The COD removal performance of the submerged membrane bioreactor was investigated and the results are shown in Fig. 2. A satisfactory organic removal performance was achieved. Over the whole experimental period, the membrane-filtered effluent COD varied in the range 20–80 mg l − 1, and the average value was around 50 mg l − 1. The COD (80–95%) component in the wastewater could be removed, with an average of over 90%. For TOC and BOD items, the average removal efficiencies were 94 and 95%, respectively. The stable effluent quality and satisfactory organic removal performance mentioned above were ensured by the efficient interception performance of the membrane device incorporated within the biological reactor.
3.2. Sludge concentration in the bioreactor
Fig. 4. Variation of the supernatant TOC concentration over the operation time.
The membrane permeability of the mixed liquor in the bioreactor was investigated using a mini-membrane module with a filtration area of 0.03 m2 and the same pore size of that used in the membrane bioreactor process tested. Mixed liquor samples of about 2 l volume was taken from the bioreactor at different operation times, and a filtration test of 30 min was conducted using the mini-module. The transmembrane pressure exerted on the membrane was maintained between 25 and 40 kPa, while the change in the accumulated volume of the filtrate with time was monitored with a graduated cylinder. The filtrate was returned to the mixed liquor before the graduation reached 80 ml, less than 5% of the total filtration sample, in order to
Sludge concentration changes in the bioreactor during the experiment were monitored (Fig. 3). After seeding the sludge and feeding wastewater, the growth of biomass occurred and a steady state was observed after 50 days. A sludge concentration of 3.0–4.0 g l − 1 was achieved over the experiment period and this guaranteed the efficient removal of pollutants.
3.3. TOC 6ariation of the supernatant liquor TOC concentration in the supernatant liquor might be attributed to either undecomposed organic substances contained in the raw wastewater or the microbial metabolic products produced in biological reactions. Its variation over the experimental period is shown in Fig. 4. Since a synthetic wastewater comprising of glucose and starch was adopted, most of organic substances contained in the wastewater were supposed to be easily decomposed and hence the supernatant TOC concentration might be an indicator of metabolic matter produced in biological reactions. Compared with stable membrane effluent COD in Fig. 2, the supernatant TOC appeared to increase after the 50th
X. Huang et al. / Process Biochemistry 36 (2000) 401–406
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3.4. Organic MW profile in the supernatant liquor
Fig. 5. TOC percentage profiles of organic substances with different MW in the supernatant at the 132nd and 245th days. Table 2 Polysaccharide concentration in the supernatant at different operation times Itemsa
Operation time (day) 100th–140th
Polysaccharide concentration (TOC mg l−1) Supernatant TOC(mg l−1) Polysaccharide/supernatant TOC (%) a
3.3–9.4 (5.71) 22.5–89.47 (45.79) 4.91–38.24 (19.04)
201st–250th 0.19–8.39 (3.69) 8.2–32.9 (20.5) 1.6–31.9 (17.65)
Data in parentheses are mean values.
day in operation. Relatively high TOC values, which varied in the range 60 – 90 mg l − 1 were observed from the 80th to 150th day. These were approximately three to four times the initial experimental value. As the operation time was prolonged, however, the supernatant TOC declined. Higher values appearing in the supernatant TOC over the operation period from the 80th to 150th day suggested that soluble organic substances, most of which were probably attributed metabolic products, could be cut-off by membrane filtration and accumulated in the bioreactor. Concerning this phenomenon, similar results have been reported in other research under different operation conditions [5,8]. However, from prior research the behaviour of the accumulated TOC in the supernatant liquor over a longer operation period was unclear. In the present study, the test was run for up to a total of 250 days. As a result, the supernatant TOC after 5 months of operation appeared to decline. It was shown that the accumulated soluble metabolic products in the membrane bioreactor could be biologically degraded, although quite a long acclimation period was required. Rittmann [17] also reported that the soluble microbial products were biodegradable, but at a low rate.
As described above, the supernatant TOC in the bioreactor appeared to accumulate over the period from the 80th to 150th day, but afterward declined. In order to investigate the transformation of organic substances corresponding to the change of the supernatant TOC, the organic MW profiles in the supernatant liquor, in terms of the TOC percentage, at the TOC accumulating period and declining period were measured, respectively. The results are shown in Fig. 5. At the 132nd day of operation, when the supernatant TOC was in the higher range, near 70% of the organic substances retained within the bioreactor was higher than 3000 of MW, among which large organic substances with a MW\ 100 000 accounted for 34%. However, at the 245th day of operation, when the supernatant TOC was in the stable range, large organic substances with a MW\ 100 000 decreased from 34 to 16% and substances with a MW between 3000 and 30 000 decreased from 33 to 23%. Correspondingly, small molecules with a MWB 3000 increased from 33 to 52%. This might explain why the decrease of the supernatant TOC after long-term operation, as described in Fig. 4, was a process of large substances being decomposed to smaller ones. The accumulation of soluble organic substances in the supernatant liquor of the membrane bioreactor was due to the membrane interception performance. However, since the pore size of the membrane used in the study was as large as 0.1 mm, it could not cut-off small components of molecular weights as low as 3000. It was assumed that the cut-off performance of the membrane with respect to low molecular weight materials might be related to the gel layer developed on the membrane surface, which was mainly composed of large soluble metabolic products acting as a dynamic membrane. Chiemchaisri et al. also reported that the gel layer developed on the membrane surface could improve the rejection of COD components and viruses [18].
3.5. Polysaccharide concentration in the supernatant liquor It has been reported that polysaccharides are one of the major components of the metabolic products in membrane bioreactor processes [9]. In the process used in this study, polysaccharide concentrations were monitored when the supernatant TOC concentrations were in high and low ranges, respectively. The results are summarised in Table 2. Both the polysaccharide concentration and the ratio held in the supernatant TOC at the experimental period of the 100th–140th day were higher than those from the 201st to 250th day. It was assumed that the accumulation of the supernatant TOC over the 100th–140th day was related to the increased
X. Huang et al. / Process Biochemistry 36 (2000) 401–406
Fig. 6. Influence of supernatant TOC on the metabolic activity of activated sludge.
405
cleaned three times with tap water. Dehydrogenase activities were measured by 2,3,5-triphenyltetrazolium chloride (TTC) reduction. The dehydrogenase activity of the sludge declined with an increase of supernatant TOC (Fig. 6). This suggested that accumulated metabolic products in the membrane bioreactor were inhibitory to the metabolic activity of the activated sludge. The higher the supernatant TOC concentration, the stronger the influence. Inhibition of the accumulated metabolites was also reported in other research [9]. Based on these data, the change of the metabolic activity due to the influence of supernatant TOC can be estimated. In the present study, since the observed maximum supernatant TOC was about 90 mg l − 1, the metabolic activity of the sludge would decline by 15% over the period from the 80th to the 130th day of operation.
3.7. Influence of supernatant TOC on membrane permeability
Fig. 7. Membrane permeability of the mixed liquor with different supernatant TOC.
Fig. 8. Relationship between membrane flux J20 and the supernatant TOC.
polysaccharide concentration, although other components might also be involved. Further identification of the components in the supernatant liquor is necessary.
Three samples of the mixed liquor were taken from the membrane bioreactor on the 37th, 147th and 258th days of operation to conduct filtration tests with a mini-membrane module. The changes of membrane flux as a function of filtration time are shown in Fig. 7. Initially membrane fluxes for all samples fell sharply, and then levelled off. Difference between the stable membrane fluxes of the three samples was observed. Since the suspended solid concentrations in these three samples were similar and in a range 3.0–4.0 g l − 1, the differences in their permeabilities might be related to the supernatant TOC concentration. To investigate a possible correlation between the supernatant TOC and the permeability of the mixed liquor, the membrane flux at a filtration time of 20 min, represented as J20, was plotted against the supernatant TOC. As the supernatant TOC increased, the lower the value of J20 observed (Fig. 8). These results suggested that accumulated soluble organic substances in the membrane bioreactor had negative influence on the membrane permeability of the mixed liquor.
4. Conclusions
3.6. Influence of supernatant TOC on the metabolic acti6ity of acti6ated sludge Dehydrogenase activity can be taken as an indicator of metabolic activity. To investigate the influence of accumulated metabolic products on this, supernatant samples were concentrated by vacuum evaporation before being added to activated sludge. The activated sludge used here was taken from the bioreactor and
1. A satisfactory organic removal performance in a submerged membrane bioreactor could be achieved to treat a synthetic wastewater. COD, TOC and BOD removal efficiencies were 90, 94 and 95%, respectively. 2. Accumulation of the supernatant TOC in the membrane bioreactor and its subsequent degradation were observed.
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3. Among various substances accumulated in the bioreactor, large molecular weight substances with a MW\ 100 000 accounted for 34%. Along with the decrease of the supernatant TOC, these large molecules decreased to 14%, whereas small ones with a MWB3000 increased from 33 to 52%. 4. Accumulated soluble organic substances in the bioreactor were inhibitory to the metabolic activity of the activated sludge and had a negative influence to the membrane permeability of the mixed liquor.
[8]
[9]
[10] [11] [12]
References [1] Brindle K, Stephenson T. Application of membrane biological reactors for the treatment of wastewaters. Biotechnol Bioeng 1996;49(6):601– 10. [2] Kimura S. Japan’s aqua renaissance 90’ project. Water Sci Technol 1991;25(10):95–105. [3] Ueda T, Hata K, Kikuoka Y. Treatment of domestic sewage from rural settlements by a membrane bioreactor. Water Sci Technol 1996;34(9):189–96. [4] Trouve E, Urbain V, Manem J. Treatment of municipal wastewater by a membrane bioreactor: results of a semi-industrial pilot-scale study. Water Sci Technol 1994;0(4):151–7. [5] Yamamoto K, Hiasa M, Mahmood T, Matuso T. Direct solid – liquid separation using hollow fiber membrane in an activated sludge aeration tank. Water Sci Technol 1989;21:43–54. [6] Knoblock MD, Sutton PM, Mishra PN, Gupta K, Janson A. Membrane biological reactor system for treatment of oily wastewaters. Water Environ Res 1994;66(2):133–9. [7] Magara Y, Itoh M. The effect of operational factors on solid/liquid separation by ultra-membrane filtration in a biological deni-
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trification system for collected human excrete treatment plants. Water Sci Technol 1991;23:1583 – 90. Chaize S, Huyard A. Membrane bioreactor on domestic wastewater treatment sludge production and modeling approach. Water Sci Technol 1991;23:1591 – 600. Zhang B, Yamamoto K. Seasonal change of microbial population and activities in a building wastewater reuse system using a membrane separation activated sludge process. Water Sci Technol 1996;34(5):295– 302. Namkung E, Rittmann BE. Soluble microbial products (SMP) formation kinetics by biofilms. Water Res 1986;20(6):795–806. Rebhun M, Manka J. Classification of organics in secondary effluents. Environ Sci Technol 1971;5(7):606 – 9. Tambo N, Kamei T, Takahashi M. Behaviour and characteristics of metabolic products in an aerobic biological process (I) — evaluation of mass balance using gel chromatograph. Gesuidoukyoukaishi (in Japanese) 1981;18(210):48– 57. Chinese SEPA. Water and Wastewater Monitoring Methods, third ed., Chinese Environmental Science Publishing House, 1997. Cai W-Ch, Yuan H-J. Chemical Analysis Methods for Organic Matter (in Chinese). Science Publishing House, 1982. Yu Y-X, et al. Microorganisms Monitoring Manual for Environmental Engineering (in Chinese). Chinese Environmental Science Publishing House, 1989. Sato T, Ishii Y. Effects of activated sludge properties on water flux of ultrafiltration membrane used for human excrement treatment. Water Sci Technol 1990;23:1601 – 8. Rittmann BE, Bae W, Namkung E, Lu CJ. A critical evaluation of microbial product formation in biological processes. Water Sci Technol 1987;19:517 – 28. Chiemchaisri C, Wong YK, Urase T, Yamamoto K. Organic stabilization and nitrogen removal in membrane separation bioreactor for domestic wastewater treatment. Water Sci Technol 1992;25(10):231– 40.
Behaviour of soluble microbial products in a membrane bioreactor Xia Huang *, Rui Liu, Yi Qian Department of En6ironmental Science and Engineering, En6ironment Simulation and Pollution Control, State Key Joint Laboratory, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Received 2 February 2000; received in revised form 24 May 2000; accepted 3 June 2000
Abstract Synthetic wastewater was treated with a submerged membrane bioreactor to investigate the organic removal performance, as well as the behaviour of soluble microbial products during long-term operation. Satisfactory chemical oxygen demand (COD), total organic carbon (TOC) and biological oxygen demand (BOD) removal efficiencies were achieved, averaging over 90, 94 and 95%, respectively. The accumulation of TOC in the supernatant of the bioreactor and its degradation after 5 months of operation was observed. Among this accumulation, macromolecules with a molecular weight (MW) \ 100 000 accounted for 34%. A decrease in the supernatant TOC corresponded to a shift from large to small molecules. The organic substances with a MW\100 000 decreased from 34 to 16%, whereas those with a MWB 30 000 increased from 33 up to 52%. The accumulation proved to be inhibitory towards the metabolic activity of the activated sludge, as well as contributing to the poor membrane permeability of the mixed liquor. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Membrane bioreactor; Microbial product; Accumulation; Molecular weight; Metabolic activity; Membrane permeability
1. Introduction The membrane bioreactor, a technological combination of biological treatment with a membrane separation device, has many advantages due to the efficient interception performance of the membrane. Benefits of the membrane bioreactor include excellent effluent quality free of suspended solids and bacteria, a highly retentive activated sludge concentration and consequent compactness of treatment equipment, less surplus sludge production and easy management [1 – 3]. The study on its applications in treating various types of waste streams, such as domestic wastewater [4,5], industrial wastewater [6] and human excrement [7] has therefore attracted great attention. By substituting the final clarifier in the conventional activated sludge process, a membrane device acts as a screen for the interception of suspended solids and microorganisms. In addition, various soluble organic substances, which might be either undecomposed organic substances contained in raw wastewater or micro* Corresponding author. E-mail address: [email protected] (X. Huang).
bial metabolic products produced during biological reactions, could also be selectively retained within the bioreactor and this can upgrade the effluent quality. The behaviour of these accumulated soluble organic substances and especially their influence on microbial activity in the membrane bioreactor has been a controversial issue and has been reported under different operation conditions [5,8,9]. Zhang and Yamamoto mentioned that these accumulated microbial products might be one of the limiting factors to bacterial activity and viability [9]. However, most of these studies were performed during short-term operation and there are few reports of long-term operations. The purpose of this study is, therefore, to elucidate the behaviour of the accumulated soluble organic compounds in a submerged membrane bioreactor during a long-term run over 250 days to treat synthetic wastewater containing glucose and starch. The components of the soluble microbial products in the biological process are complex, and may include humic substances, fulvic acid, polysaccharide, protein, nucleic acid, etc. [10,11] almost independent on the influent wastewater [12]. Since it is not easy to identify each component, the total organic carbon (TOC), a comprehensive index was
0032-9592/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 2 0 6 - 5
X. Huang et al. / Process Biochemistry 36 (2000) 401–406
402
Fig. 1. Diagram of the experimental system.
used here to represent the total concentration of soluble microbial products. The supernatant TOC concentration was continuously monitored and the molecular weight distribution of the soluble organic substances in the mixed liquor and their influences on the metabolic activity of activated sludge and the permeability of the mixed liquor were investigated. The results are useful for the maintenance of the membrane bioreactor process.
2. Material and methods
2.1. The experimental system and conditions The experimental system consists of an activated sludge bioreactor and a bundle of hollow fibre membranes (Fig. 1). The bioreactor was 30× 22 ×23 cm, having an effective volume of 15 l. The membrane was made of polypropylene with a pore size of 0.1 mm and a filtration area of 0.4 m2 and was submerged in the bioreactor. Raw wastewater supplied from the storage tank was fed into the bioreactor, where organic pollutants were biologically decomposed. An air diffuser located beneath the membrane module provided aeration and mixed the wastewater. The membrane-filtered effluent was continuously removed with a suction pump (Diaphragm pump, model X030-XB-AAAA365, PULTable 1 Components and quality of synthetic wastewater used in the study (mg l−1 except pH) Component
Concentration
Items
Concentration
Industrial glucose Starch Protein KH2PO4 MgSO4 · 7H2O MnSO4 · 7H2O FeSO4 CaCl2 (NH2)2CO NaHCO3
278 278 28 52.8 66 6 0.3 6 167 111
COD TOC BOD5 NH3-N PH
300–600 110–125 150–300 36–72 6.5
SAFEEDER) connected to the membrane module. An influent pump of the same make and model as the suction pump was controlled by a water level sensor to maintain a constant water level in the bioreactor over the experimental period. The water temperature in the membrane bioreactor was maintained at 25°C with a temperature controller. A synthetic wastewater comprising of glucose, starch and trace nutrients were used. Its components and quality are shown in Table 1. Due to the slight sedimentation of starch and degradation of organic materials in the wastewater storage tank, chemical oxygen demand (COD), TOC, biological oxygen demand (BOD) and NH3-N concentrations in the influent showed some fluctuations but were in the range of the typical municipal sewage of China. Since large molecular weight polysaccharides are the main components contained in municipal sewage, a mixture of starch, glucose and protein reflects the actual situation. The experiment was conducted at a hydraulic retention time of 8.3 h, a sludge retention time of 20 day and a dissolved oxygen concentration in bioreactor of 5 mg l − 1. In order to investigate the behaviour of accumulated soluble organic substances in the membrane bioreactor during long-term operation, the test was run over 250 days. Periodically the membrane module was taken from the bioreactor and cleaned with tap water to remove sludge deposited on the membrane surface.
2.2. Analytical items and methods COD, BOD, TOC and pH of the influent, supernatant liquor, membrane effluent and the suspended solids (SS) concentration in the mixed liquor were measured, respectively, by adopting the Chinese SEPA standard methods [13]. The supernatant samples were obtained by centrifuging the mixed liquor (LG10-2.4A, Beijing Medical Centrifuge Corporation) at 4000 rpm for 15 min, followed by filtration through a membrane of 0.45 mm. For the analysis of the molecular weight (MW) distribution of organic substances in the supernatant liquor, four types of circular ultrafiltration membranes with molecular weight cut-off ranges of 3000, 30 000, 60 000 and 100 000 were used. The filtrate permeating through each membrane with a different MW cut-off range was collected and the TOC concentration measured. The percentage of organic substances with a different MW range in the total amount, in terms of TOC, was calculated using the mass balance concept. The concentration of polysaccharides in the supernatant liquor was measured using an anthrone method [14]. The metabolic activity of activated sludge in the bioreactor was characterised by measuring the dehydrogenase activity [15].
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keep constant the characters of the mixed liquor. The flux could be calculated from the increasing rate of this accumulated volume. In order to eliminate the influence of temperature on the membrane flux, all fluxes J (T) measured at temperature T were corrected to values J(25) at temperature 25°C by the following equation [16]. J(25)= J(T)× 1.02525 − T
(1)
3. Results and discussion Fig. 2. Changes of COD concentrations in the influent and effluent.
3.1. O6erall organic remo6al performance
Fig. 3. Variation of sludge concentration in the bioreactor over the operation time.
The COD removal performance of the submerged membrane bioreactor was investigated and the results are shown in Fig. 2. A satisfactory organic removal performance was achieved. Over the whole experimental period, the membrane-filtered effluent COD varied in the range 20–80 mg l − 1, and the average value was around 50 mg l − 1. The COD (80–95%) component in the wastewater could be removed, with an average of over 90%. For TOC and BOD items, the average removal efficiencies were 94 and 95%, respectively. The stable effluent quality and satisfactory organic removal performance mentioned above were ensured by the efficient interception performance of the membrane device incorporated within the biological reactor.
3.2. Sludge concentration in the bioreactor
Fig. 4. Variation of the supernatant TOC concentration over the operation time.
The membrane permeability of the mixed liquor in the bioreactor was investigated using a mini-membrane module with a filtration area of 0.03 m2 and the same pore size of that used in the membrane bioreactor process tested. Mixed liquor samples of about 2 l volume was taken from the bioreactor at different operation times, and a filtration test of 30 min was conducted using the mini-module. The transmembrane pressure exerted on the membrane was maintained between 25 and 40 kPa, while the change in the accumulated volume of the filtrate with time was monitored with a graduated cylinder. The filtrate was returned to the mixed liquor before the graduation reached 80 ml, less than 5% of the total filtration sample, in order to
Sludge concentration changes in the bioreactor during the experiment were monitored (Fig. 3). After seeding the sludge and feeding wastewater, the growth of biomass occurred and a steady state was observed after 50 days. A sludge concentration of 3.0–4.0 g l − 1 was achieved over the experiment period and this guaranteed the efficient removal of pollutants.
3.3. TOC 6ariation of the supernatant liquor TOC concentration in the supernatant liquor might be attributed to either undecomposed organic substances contained in the raw wastewater or the microbial metabolic products produced in biological reactions. Its variation over the experimental period is shown in Fig. 4. Since a synthetic wastewater comprising of glucose and starch was adopted, most of organic substances contained in the wastewater were supposed to be easily decomposed and hence the supernatant TOC concentration might be an indicator of metabolic matter produced in biological reactions. Compared with stable membrane effluent COD in Fig. 2, the supernatant TOC appeared to increase after the 50th
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3.4. Organic MW profile in the supernatant liquor
Fig. 5. TOC percentage profiles of organic substances with different MW in the supernatant at the 132nd and 245th days. Table 2 Polysaccharide concentration in the supernatant at different operation times Itemsa
Operation time (day) 100th–140th
Polysaccharide concentration (TOC mg l−1) Supernatant TOC(mg l−1) Polysaccharide/supernatant TOC (%) a
3.3–9.4 (5.71) 22.5–89.47 (45.79) 4.91–38.24 (19.04)
201st–250th 0.19–8.39 (3.69) 8.2–32.9 (20.5) 1.6–31.9 (17.65)
Data in parentheses are mean values.
day in operation. Relatively high TOC values, which varied in the range 60 – 90 mg l − 1 were observed from the 80th to 150th day. These were approximately three to four times the initial experimental value. As the operation time was prolonged, however, the supernatant TOC declined. Higher values appearing in the supernatant TOC over the operation period from the 80th to 150th day suggested that soluble organic substances, most of which were probably attributed metabolic products, could be cut-off by membrane filtration and accumulated in the bioreactor. Concerning this phenomenon, similar results have been reported in other research under different operation conditions [5,8]. However, from prior research the behaviour of the accumulated TOC in the supernatant liquor over a longer operation period was unclear. In the present study, the test was run for up to a total of 250 days. As a result, the supernatant TOC after 5 months of operation appeared to decline. It was shown that the accumulated soluble metabolic products in the membrane bioreactor could be biologically degraded, although quite a long acclimation period was required. Rittmann [17] also reported that the soluble microbial products were biodegradable, but at a low rate.
As described above, the supernatant TOC in the bioreactor appeared to accumulate over the period from the 80th to 150th day, but afterward declined. In order to investigate the transformation of organic substances corresponding to the change of the supernatant TOC, the organic MW profiles in the supernatant liquor, in terms of the TOC percentage, at the TOC accumulating period and declining period were measured, respectively. The results are shown in Fig. 5. At the 132nd day of operation, when the supernatant TOC was in the higher range, near 70% of the organic substances retained within the bioreactor was higher than 3000 of MW, among which large organic substances with a MW\ 100 000 accounted for 34%. However, at the 245th day of operation, when the supernatant TOC was in the stable range, large organic substances with a MW\ 100 000 decreased from 34 to 16% and substances with a MW between 3000 and 30 000 decreased from 33 to 23%. Correspondingly, small molecules with a MWB 3000 increased from 33 to 52%. This might explain why the decrease of the supernatant TOC after long-term operation, as described in Fig. 4, was a process of large substances being decomposed to smaller ones. The accumulation of soluble organic substances in the supernatant liquor of the membrane bioreactor was due to the membrane interception performance. However, since the pore size of the membrane used in the study was as large as 0.1 mm, it could not cut-off small components of molecular weights as low as 3000. It was assumed that the cut-off performance of the membrane with respect to low molecular weight materials might be related to the gel layer developed on the membrane surface, which was mainly composed of large soluble metabolic products acting as a dynamic membrane. Chiemchaisri et al. also reported that the gel layer developed on the membrane surface could improve the rejection of COD components and viruses [18].
3.5. Polysaccharide concentration in the supernatant liquor It has been reported that polysaccharides are one of the major components of the metabolic products in membrane bioreactor processes [9]. In the process used in this study, polysaccharide concentrations were monitored when the supernatant TOC concentrations were in high and low ranges, respectively. The results are summarised in Table 2. Both the polysaccharide concentration and the ratio held in the supernatant TOC at the experimental period of the 100th–140th day were higher than those from the 201st to 250th day. It was assumed that the accumulation of the supernatant TOC over the 100th–140th day was related to the increased
X. Huang et al. / Process Biochemistry 36 (2000) 401–406
Fig. 6. Influence of supernatant TOC on the metabolic activity of activated sludge.
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cleaned three times with tap water. Dehydrogenase activities were measured by 2,3,5-triphenyltetrazolium chloride (TTC) reduction. The dehydrogenase activity of the sludge declined with an increase of supernatant TOC (Fig. 6). This suggested that accumulated metabolic products in the membrane bioreactor were inhibitory to the metabolic activity of the activated sludge. The higher the supernatant TOC concentration, the stronger the influence. Inhibition of the accumulated metabolites was also reported in other research [9]. Based on these data, the change of the metabolic activity due to the influence of supernatant TOC can be estimated. In the present study, since the observed maximum supernatant TOC was about 90 mg l − 1, the metabolic activity of the sludge would decline by 15% over the period from the 80th to the 130th day of operation.
3.7. Influence of supernatant TOC on membrane permeability
Fig. 7. Membrane permeability of the mixed liquor with different supernatant TOC.
Fig. 8. Relationship between membrane flux J20 and the supernatant TOC.
polysaccharide concentration, although other components might also be involved. Further identification of the components in the supernatant liquor is necessary.
Three samples of the mixed liquor were taken from the membrane bioreactor on the 37th, 147th and 258th days of operation to conduct filtration tests with a mini-membrane module. The changes of membrane flux as a function of filtration time are shown in Fig. 7. Initially membrane fluxes for all samples fell sharply, and then levelled off. Difference between the stable membrane fluxes of the three samples was observed. Since the suspended solid concentrations in these three samples were similar and in a range 3.0–4.0 g l − 1, the differences in their permeabilities might be related to the supernatant TOC concentration. To investigate a possible correlation between the supernatant TOC and the permeability of the mixed liquor, the membrane flux at a filtration time of 20 min, represented as J20, was plotted against the supernatant TOC. As the supernatant TOC increased, the lower the value of J20 observed (Fig. 8). These results suggested that accumulated soluble organic substances in the membrane bioreactor had negative influence on the membrane permeability of the mixed liquor.
4. Conclusions
3.6. Influence of supernatant TOC on the metabolic acti6ity of acti6ated sludge Dehydrogenase activity can be taken as an indicator of metabolic activity. To investigate the influence of accumulated metabolic products on this, supernatant samples were concentrated by vacuum evaporation before being added to activated sludge. The activated sludge used here was taken from the bioreactor and
1. A satisfactory organic removal performance in a submerged membrane bioreactor could be achieved to treat a synthetic wastewater. COD, TOC and BOD removal efficiencies were 90, 94 and 95%, respectively. 2. Accumulation of the supernatant TOC in the membrane bioreactor and its subsequent degradation were observed.
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3. Among various substances accumulated in the bioreactor, large molecular weight substances with a MW\ 100 000 accounted for 34%. Along with the decrease of the supernatant TOC, these large molecules decreased to 14%, whereas small ones with a MWB3000 increased from 33 to 52%. 4. Accumulated soluble organic substances in the bioreactor were inhibitory to the metabolic activity of the activated sludge and had a negative influence to the membrane permeability of the mixed liquor.
[8]
[9]
[10] [11] [12]
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