- Email: [email protected]
Filtration characteristics of activated sludge in hybrid membrane bioreactor with porous suspended carriers (HMBR)
Desalination 249 (2009) 507–514
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Filtration characteristics of activated sludge in hybrid membrane bioreactor with porous suspended carriers (HMBR) Qi-Yong Yang ⁎, Tao Yang, Hui-Juan Wang, Kang-Qiang Liu School of Chemistry and Chemical Engineering, Jiujiang University, Jiujiang 332005, China
a r t i c l e
i n f o
Article history: Accepted 19 August 2008 Available online 4 October 2009 Keywords: Hybrid membrane bioreactor Suspended carriers Filtration resistance Particle size Activated sludge Membrane fouling
a b s t r a c t The suspended carriers were efficient in controlling membrane fouling in hybrid membrane bioreactor with porous suspended carriers (HMBR). The purpose of this study consisted in investigating the effect of suspended carriers on the sludge suspension, especially the filterability of sludge suspension. The filterability of sludge suspension in HMBR and general membrane bioreactor (MBR) were investigated and compared in parallel conditions by dead-end filtration for better evaluating the influence of suspended carriers on the sludge suspension. Several aspects of sludge suspension such as filtration resistance, specific cake resistance and particle size were discussed. During long-term operation the filtration resistances rose gradually in the early stage (about 100 days) and then increased rapidly, but there was a slight difference between MBR and HMBR with the prolongation of operation time. The granulometric analysis revealed that the mean particle size of sludge suspension of HMBR decreased more sharply than that of MBR, because the fluidized carriers in HMBR would impose shear stresses on sludge flocs and induce the destruction of the network of sludge zoogloea. Dead-end filtration experiments indicated that the resistance-increasing rates of three portions of sludge suspension were in the order of supernatant N dissolved organics N microbial flocs. In order to further understand the filterability of sludge suspension, the specific cake resistance (α or α.C) of sludge suspension and supernatant in HMBR and MBR were determined. During long-term operation the α and α.C increased with operation time. These results revealed that the suspended carriers in HMBR had appreciably negative effect on the biological characteristics and filterability of the sludge suspension, but they were efficient in controlling membrane fouling during continuous operation of HMBR. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The developments of cost-effective membrane manufacturing technology and increasingly stringent regulations for the discharge of effluents have given an impetus to the application of membrane bioreactors (MBR) for wastewater and reuse [1]. However, membrane fouling, which results in smaller permeate flux and higher operational costs, has been the main obstacle to the widespread application of MBR. More recent investigations focus on more effective and economical techniques to prevent or mitigate membrane fouling, especially the reduction of cake formation on membrane surfaces. The particle accumulation on membrane surface could be effectively mitigated when MBRs operated below critical flux [2,3] and in intermittent mode [4,5]. Air-scouring, which formed back-transport detaching particles from membrane surface into bulk solution, also was the major method of preventing the formation of cake layer [6–8]. Recently powdered activated carbon (PAC) was used to reduce membrane surface fouling ⁎ Corresponding author. School of Chemistry and Chemical Engineering, Jiujiang University, Jiujiang 332005, PR China. Tel./fax: +86 792 8314448. E-mail address: [email protected] (Q.-Y. Yang). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.08.013
[9,10]. In addition, backpulsing and backwashing were introduced to remove cake layer after its formation [11]. However, air-scouring had no effect on the cake removal beyond the critical value of the air flow rate [6,7]. Furthermore, the shear stress would cause biological flocs breakage and consequently increase pore blocking and compressibility of cake [4,12,13]. It also should be noted that the main effect of PAC was to change the composition and permeability of the cake layer [9,14], not to mitigate the particles accumulation on membrane surface. So the porous suspended carriers were added into MBR to enhance the scouring effect of removing the cake layer over membrane [15]. Our previous study [15] investigated filtration resistance online during continuous operation of hybrid membrane bioreactor with porous suspended carriers (HMBR) and revealed that suspended carriers were efficient in controlling membrane fouling. However, it was the lack of comprehensive investigation of fouling in HMBR. In order to go further into the fouling of HMBR, the filterability of the activated sludge suspension was determined in this study. There were some differences between the sludge suspension of HMBR and that of MBR because of the presence of suspended carriers in HMBR. Therefore, the HMBR and MBR were operated in parallel by feeding
508
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
Fig. 1. Schematic diagram of experimental system (a) and configuration of the HMBR or MBR (b) (1—Wastewater tank; 2—Influent pump; 3—Hydrolysis-acidification bioreactor; 4— Water level sensor; 5—Electric agitator; 6—Fixed carrier; 7—Aeration bioreactor; 8—Aeration pipe; 9—Suspended carrier; 10—Membrane module; 11—Gas flow meter; 12—Air compressor; 13—Pressure gauge; 14—Relay; 15—Peristaltic pump; 16—Baffle wall).
the same wastewater and the filterability of sludge suspension in both systems were investigated and compared in same conditions.
particles were determined by LS 13 320 laser diffraction particle size analyzer (Beckman Corporation, USA). 2.3. Fractionation of activated sludge suspension
2. Material and methods 2.1. Membrane bioreactor systems The experimental system consisted of three bioreactors: hydrolysis-acidification bioreactor and two aeration bioreactors as shown in Fig. 1. The hydrolysis-acidification bioreactor was equipped with fixed carriers and an electric agitator, which had a working volume of 25 L (250 × 250 × 400 mm). A flat-shaped hollow fiber membrane module was immersed in each aeration bioreactor with a working volume of 10 L (220 × 140 × 325 mm). Hollow fiber membranes were made of polyethylene with a pore size of 0.1 – 0.2 µm (Hangzhou Zheda Hyflux Hualu Membrane Technology Co. Ltd., China) and the total membrane surface area of module was 0.4 m2. The aeration bioreactor with porous suspended carriers was HMBR, and the other was called MBR. The porous suspended carrier was made of polyurethane and couldn't induce any erosion on membrane due to its flexibility. Its volume accounted for a proportion of 10–20% of total working volume. The characteristics of the carrier were shown in Table 1. The membrane permeate was intermittently suctioned by a peristaltic pump under a constant flux, and the transmembrane pressure (TMP) was monitored by a pressure gauge. A suction mode of 10-minutes-on and 3-minutes-off was adopted. The HMBR and MBR were operated in parallel by feeding the same wastewater for better evaluating the influence of suspended carriers on the sludge suspension. The operation parameters of HMBR and MBR were shown in Table 2. 2.2. Analyses The analysis of chemical oxygen demand (COD) and the measure of mixed liquor suspended solids (MLSS and MLVSS) were according to standard methods (Editorial Board of Environment Protection Bureau of China, 1997). The size distribution of activated sludge Table 1 Characteristics of porous suspended carriers. Parameter
Density/kg m− 3
Porosity/%
Average pore size/mm
Configuration size/mm
Value
30
90
1.0–1.5
10 × 10 × 10
The mixed liquor of activated sludge was fractionated into several portions (microbial flocs, supernatant and dissolved organics) as follows [16]: the mixed liquor was centrifuged (1430 g, 15 min) to separate the microbial flocs from the supernatant. Some portion of the supernatant was then filtered with a membrane filter (0.45 µm pore size), and permeate was the soluble organic. The microbial flocs was put into the Ringer's solution and centrifuged again, and then these steps were repeated 2–3 times to get the purified flocs. Finally, the microbial flocs were suspended in a saline solution so that their concentration was equal to that in the original suspension. 2.4. Determination of filterability In order to investigate the filterability (including filtration resistance and specific cake resistance) of the activated sludge suspension and its portions, a special membrane filtration cell with a filtration area of 0.00332 m2 was introduced (made by Shanghai Institute of Applied Physics, Chinese Academy of Sciences). As shown in Fig. 2, the module in dead-end filtration mode was suitable for highly viscous biological suspensions, especially. The membrane sheet used in the experiment was made of cellulose acetate with a pore size of 0.22 µm. Permeate flux was determined by weighing the permeate mass with an electronic balance and the operating pressure ranged from 0.1 to 1 MPa. The permeate mass were weighed and noted every 15 s. 2.4.1. Filtration resistance of the activated sludge suspension and its portions According to the resistance-in-series model, the permeate for MBR can be expressed by ΔP μRt
ð1Þ
R t = Rm + Rf
ð2Þ
J=
where J is permeate flux, ΔP is transmembrane pressure (TMP), µ is the viscosity of permeate, and Rt is the total resistance, Rm is the intrinsic membrane resistance, and Rf is the filtration resistance of the activated sludge suspension or its portions.
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
509
Table 2 Operation parameters of HMBR and MBR. Operation time/d
Wastewater
Influent COD /mg L− 1
Air flow rate /m3 h− 1
DO /mg L− 1
Constant flux /L m− 2 h− 1
SRT /d
HRT /h
1 – 87 96 – 162 172 – 201 202 – 235 236 – 254
Synthetic a Synthetic b Industrial c Industrial c Industrial c
250 – 860 980 – 1470 760 – 1830 1100 – 1700 880 – 1760
0.15 0.15 0.15 0.15 0.15
3.0–5.0 3.0–5.0 3.0–5.0 3.0–5.0 3.0–5.0
3.0 3.0 3.0 4.5 6.0
No waste 50 50 50 50
10.8 10.8 10.8 7.2 5.4
DO: Dissolved oxygen; SRT: Sludge retention time; HRT: Hydraulic retention time. a Synthetic wastewater of terephthalic acid as sole substrate. b Synthetic wastewater of terephthalic acid and ethylene glycol. c Industrial wastewater discharged from polyester fabric alkali-peeling process (Xianglong Dyeing & Printing Co. Ltd. Suzhou City, China).
Flux (J) and TMP data are used to calculate resistances by Eq. (1). Rt is calculated from the final flux and TMP values at the end of operation; Rm is determined by filtration of pure water with new membrane; and Rf can be obtained by Eq. (2). 2.4.2. Specific cake resistance of the activated sludge suspension and supernatant The specific cake resistance is usually defined as the resistance per unit mass of cake layer deposited per unit membrane surface area. In this study specific cake resistances were employed to indicate the filterability of activated sludge suspension. In the experiment specific cake resistances were measured with the membrane filtration module under unstirred condition. According to the filtration equation, the relationship between filtrate volume and time can be expressed by Eq. (3). Specific cake resistance and compressibility of the cake layer could be calculated by Eqs. (3) and (4), respectively. t μ⋅Rm μ⋅α⋅C⋅V = + V S⋅ΔP 2S2 ⋅ΔP α = α0 ⋅ΔP
n
in two systems was similar. The ratio of MLVSS/MLSS was equal to 0.72–0.85. The MLSS increased rapidly to about 15,000 mg.L− 1 on the 87th day because no excess sludge was extracted from both systems. In a conventional MBR system, new sludge was continuously generated with the consumption of feed organic materials while some sludge mass was decayed by endogenous respiration. Due to the slower sludge decay rate, the sludge generation and reduction couldn't reach an equilibrium state within the reasonable MLSS range. From the 88th to the 96th day the sludge was disintegrated without feed and some sludge was removed at the same time, so on the 97th day MLSS decreased to 7100 and 6750 mg L− 1 for MBR and HMBR, respectively. Then the excess sludge was removed to maintain proper MLSS level and SRT was equal to 50 days. With the prolongation of operation time the final sub-steady-state MLSS reached 12,000 mg L− 1.
ð3Þ ð4Þ
where V is the volume of filtrate at time t, S is the filtration area, Rm is the membrane resistance, C is the suspended solids concentration, ΔP is transmembrane pressure (TMP) drop, µ is the viscosity of permeate, α is the specific cake resistance, α0 is the specific cake constant, and n is the compressibility of the cake layer. 3. Results and discussion 3.1. Variation of mixed liquor suspended solids (MLSS) and volatile suspended solids (MLVSS) Figs. 3 and 4 showed the variation of MLSS and MLVSS in two systems. It was obvious that the increasing rate of MLSS and MLVSS
Fig. 3. Variation of MLSS in MBR and HMBR.
Fig. 2. Schematic diagram of experimental setup for filterability.
Fig. 4. Variation of MLVSS in MBR and HMBR.
510
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
3.2. Variation of TMP for MBR and HMBR during long-term continuous operation The membrane permeate was intermittently suctioned by a peristaltic pump. The transmembrane pressure (TMP) was monitored under a constant flux condition, which indicated the extent of membrane fouling. Fig. 5 illustrated the increase of TMP for MBR and HMBR. The operation was stopped when the suction pressure reached 30 kpa because it was difficult to maintain the flux at constant level at high TMP (for example, it was needed to adjust the peristaltic pump on the 144th day when TMP reached 30.92 KPa to maintain a constant flux). Then the membrane module was taken out from the bioreactor for cleaning. When membrane flux was 3.0 L m− 2 h− 2, TMP increased slowly for both systems. However, TMP increased rapidly at high flux such as 4.5 or 6.0 L m− 2 h− 2. For MBR, all the equipment and operating parameters were exactly the same as HMBR except the adding of suspended carriers. The increase rate of TMP for HMBR was far lower than that for MBR, which indicated that the suspended carriers were the main contributors to improve the filtration performance of submerged HMBR. 3.3. Characteristics of filtration flux and resistance of sludge suspension Although MBR and HMBR operated in parallel under the same feed and operation conditions, there were slightly different in the concentration of suspended solids (SS) for the two systems. In order to better evaluate the filterability of sludge suspensions in MBR and HMBR, the SS drawn from two systems were made to equal concentration by saline solution for resistance measurement. The variation of filtration flux and resistance of sludge suspensions (on the 247th day) in MBR and HMBR were shown in Fig. 6. The permeate flux decreased sharply in the early stage and then almost leveled off after about 1200 s of filtration time. While the resistance increased rapidly with the operation time and also leveled off after about 1200 s. The resistance was calculated by Eqs. (1) and (2) every 15 s. These results indicated that the sharp flux decline and resistance increase in the early stage be mostly relevant to the formation of cake layer. Furthermore, it was worth noting that the profiles of filtration flux and resistance variation with time exhibited similar trends between MBR and HMBR. The conclusion may be obtained that the filterability of suspension in MBR was similar to that in HMBR. 3.4. Variation of filtration resistance of sludge suspension The filtration resistances of sludge suspension in the two systems were similar in early stage just as mentioned above. However, Fane et al. [17] observed that fouling increased when bacteria lacked
nutrients and their growth slowed down. This indicated that the bacterial physiological condition have an effect on fouling. Therefore, it was necessary to investigate and compare the filtration performance of sludge suspensions in the two systems because of the presence of porous suspended carriers in HMBR, which may lead to some changes in aspects such as particle size distribution, viscosity, constituents and bacterial physiology. The variations of filtration resistance of sludge suspension in the two systems during long-term operation were shown in Fig. 7. The values of filtration resistances in Fig. 7 were average resistances during the dead-end filtration time (about 1400 s) after the initial data (about 300 s) were eliminated. As shown in Fig. 7, the filtration resistances rose gradually during early stage (about 100 days), and then increased rapidly from the 103rd day to the 247th day. In early stage the filtration resistances of sludge suspension in the two systems were very similar, but from the 103rd day to the 247th day the resistances of sludge suspension in MBR were appreciably different from that in HMBR. On the one hand, it may be the difference in the microbes and morphological composition between HMBR and MBR due to the porous suspended carriers in HMBR. The carriers supplied anoxic or anaerobic condition for anaerobe such as denitrifying bacteria. Lim et al.[18] observed a slight difference in microbial community structure between the anoxic and the aerobic condition. On the other hand, the shear stresses induced by suspended carriers imposed on the suspension could destruct the activated sludge flocs, which was the result of physicochemical interactions between microorganisms, inorganic particles, extracellular polymeric substances and multivalent cations. Therefore, the shear stresses modified the biological characteristics and filterability of sludge suspension in HMBR.
3.5. Variation of mean particle size of sludge suspension As mentioned above, one of the effects of suspended carriers on the biological characteristics of sludge suspension in HMBR was modifying the particle size of sludge suspension. In addition to air bubbling, the suspended carriers in HMBR flowed upward together with the fluid along the membrane surfaces and induced shear stresses that generated the back-transport of sludge suspension from the membrane surfaces. According to Choo et al. [19], the smaller the particle diameter was, the greater the net particle velocity toward the membrane became and the more particle deposition on membrane surfaces would occur. Therefore, it was necessary to take into consideration the size distribution and mean particle diameter of sludge suspension in the two systems during long-term operation. The granulometric analysis, given in Fig. 8, revealed the size distribution of the sludge suspension in MBR and HMBR. The results indicated that the sludge flocs presented nearly a unimodal
Fig. 5. Comparison of TMP between MBR and HMBR (A: washing membrane surface with tap water; B: cleaning membrane model with sodium hypochlorite of 0.4% (w/w) for 16 h after washing).
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
511
Fig. 6. Variation of filtration flux and resistance of sludge suspensions in MBR and HMBR (measured at 0.1 MPa).
distribution and the size distribution of sludge suspension shifted to a higher value with operation time for both systems. Fig. 9 presented the change of mean particle size of the sludge suspension in the two systems with operation time. It was obvious that the mean particle size of sludge suspension decreased with operation time for both systems. For MBR the mean particle size decreased from 252.9 (18th day) to 176.4 µm (156th day), while for HMBR it decreased from 249.9 (18th day) to 67 µm (156th day). It was obvious that the mean particle size of HMBR decreased more sharply than MBR. It may correspond to the suspended carriers in HMBR. Just as MBR, the recirculation of sludge suspension in HMBR would destruct the sludge flocs and reduce the mean particle size. Furthermore, the fluidized carriers in HMBR would impose shear stresses on sludge flocs. The shear stresses would induce the destruction of the network of sludge zoogloea and thus generate more fine colloidal particles.
Fig. 7. Comparison of resistances of sludge suspension between MBR and HMBR (measured at 0.1 MPa, significance level α was 0.05 and measure times were 3).
However, it should be noted that for MBR and HMBR the mean particle size on the 243rd day increased to 214.5 and 162.2 µm respectively. It was due to the leakage of sludge suspension on the 228th day for both systems and addition of fresh activated sludge. In contrast to the literatures, the mean particle size of sludge suspension in this experiment was slightly greater. It was attributed to the low ratio of air to permeate flow.
3.6. Filtration resistances of sludge suspension portions in MBR and HMBR As mentioned above, the filtration of sludge suspension increased sharply from the 103rd day to the 247th day for both MBR and HMBR. To further investigate the factors leading to the drastic increase in resistance of suspension, the sludge suspensions were divided into three portions: microbial flocs, supernatant and dissolved organics. Batch filtration experiments with a stirred cell were employed to measure the filtration resistance of each portion for MBR and HMBR. The results and the confidence intervals were shown in Table 3. The results on the 103rd day were not significantly different between MBR and HMBR, but on the 247th day the results for MBR were appreciably different from that for HMBR. These observations coincided well with the above results of the filtration resistances of sludge suspension. During the long-term run the shear stress of air bubble and suspended carriers on activated sludge would break biological flocs and release biological colloids to bulk suspension. Then a dense layer was built up on the membrane surface and resulted in lower filterability in dead-end filtration. Therefore, the accumulation of biological colloids in the suspension, especially in the supernatant, resulted in the bad filterability of activated sludge. Considering the membrane filtration only, the activated sludge should be discharged at intervals to keep better filterability. For the MBR system, the filtration resistance of suspension rose from 4.10 × 1012 m− 1(103rd day) to 1.06 × 1013 m− 1(247th day), with an increase of 158%. While the suspension resistance increased from
512
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
Fig. 8. Variation of size distribution of sludge suspensions in MBR and HMBR with operation time.
4.18 × 1012 m− 1 (103rd day) to1.15 × 1013 m− 1 (247th day) for HMBR, with an increase of 175%. The resistances increasing rate of supernatant in MBR and HMBR were 156% and 174%, respectively. The resistances of soluble organic in MBR and HMBR increased with 92% and 110%, respectively. As for microbial flocs, there were only increasing rates of 50% and 71% for MBR and HMBR, respectively. Therefore, the resistance-increasing rates of three portions were in the order of supernatant N dissolved organics N microbial flocs.
These results indicated that the sharp increase in filtration resistance of sludge suspension could be contributed mainly to the supernatant fraction, which played a preponderant role in filtration resistance of sludge suspension. 3.7. Specific cake resistances of the suspension and its supernatant The filtration resistance of suspension and its portions were determined in a fixed pressure of 0.1 MPa as above. In this study the filtration resistance of suspension and its portions were mainly the cake resistance. It was well known to us that the cake resistance was related to the specific cake resistance, the membrane area and the
Table 3 Filtration resistance of various fractions of sludge suspension in MBR and HMBR. Operation time /d
Reactor
103rd day
MBR HMBR MBR
247th day
HMBR
Fig. 9. Variation of mean particle diameter of sludge suspensions in MBR and HMBR with operation time.
Filtration resistancea/1012 m− 1 Suspension
Microbial flocs
Supernatant
Soluble organics
4.10 ± 0.13 4.18 ± 0.11 10.6 ± 0.21 (158%)b 11.5 ± 0.34 (175%)
2.84 ± 0.12 2.65 ± 0.10 4.23 ± 0.11 (50%) 4.52 ± 0.17 (71%)
3.18 ± 0.37 3.53 ± 0.19 8.14 ± 0.52 (156%) 9.67 ± 0.26 (174%)
0.75 ± 0.08 0.89 ± 0.06 1.44 ± 0.23 (92%) 1.87 ± 0.18 (110%)
a All measurements were mean values of 3 time measurements in the condition of 0.05 of significance level (α ). All measurements were obtained in the filtration pressure of 0.1 MPa except that soluble organics was measured in 0.02 MPa. b Increased percentage to the resistances of the 103rd day.
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
mass of cake deposited on the membrane surface. That is, specific cake resistance (α) was the cake resistance normalized to the mass of materials deposited per unit of membrane surface area and it was a unique property of particles consistent with their size and conformation [20]. Therefore the specific cake resistance was determined to be a better comparison between the filterability of suspension and supernatant in MBR and HMBR. In this study α was employed to indicate the filterability of the sludge suspension, while the filterability of supernatant was characterized by α.C due to the solids concentration (C) in supernatant is not precisely measurable (only 3– 6% of the suspended solid content in the supernatant). The specific cake resistance (α or α.C) of cake layers of suspension and supernatant was measured by changing the operating pressure in the range from 10 to 200 KPa. The results were shown in Fig. 10. Whether MBR or HMBR, the α of suspension on the 247th day were about 2 times higher than that on the 103rd day and α.C of supernatant on the 247th day was about one order of magnitude higher than that on the 103rd day through the entire pressures test. The results indicated that the specific cake resistances of suspension and supernatant increased with the operation time of biological treatment, and it also revealed that the increasing rate of specific cake resistances of supernatant was higher than that of suspension. It may be the release of polymers from cells to the supernatant and the accumulation of cells fragment in suspension due to the lysis of microorganism during long-term operation. Furthermore, it could be easily seen from Fig. 10 that the α and α.C between MBR and HMBR in both days were very similar. As the results for MBR and HMBR were not significantly different from a statistical
513
point of view, it was difficult to determine whether the suspended carriers in HMBR had any effect on the sludge suspension and its supernatant from α and α.C only. However, the experiments of filtration resistance of sludge suspension and its portions indicated that the suspended carriers had appreciably negative effect on the filterability of the sludge suspension and its supernatant. 4. Conclusion In this study the filterability of sludge suspension in HMBR and MBR were investigated and compared in the same conditions by dead-end filtration for better evaluating the influence of suspended carriers on the sludge suspension. The results revealed that the suspended carriers in HMBR had appreciably negative effect on the biological characteristics and filterability of the sludge suspension. The following conclusions could be obtained: 1. The profiles of permeate flux and resistance of sludge in dead-end filtrations leveled off after about 1200 s of filtration time for both systems. 2. The increase rate of TMP for HMBR was far lower than that for MBR during long-term operation. 3. Dead-end filtration experiments showed that the filtration resistances of sludge suspension rose gradually in the early stage and then increased rapidly. In early stage the variation of resistance for both systems was similar, but from the 103rd day to the 247th day there was a slight difference between MBR and HMBR. 4. The mean particle size of sludge suspension in HMBR decreased more sharply than that in MBR during long-term operation. 5. The sharp increase in filtration resistance of sludge suspension could be contributed mainly to the supernatant fraction. The resistance-increasing rates of three portions of sludge suspension were in the order of supernatantN dissolved organics N microbial flocs. Dead-end filtration experiments indicated that the resistanceincreasing rates of three portions of sludge suspension in HMBR were appreciably greater than that in MBR with the prolonging of operation time. 6. The specific cake resistance of sludge suspension and supernatant increased with the operation time in both systems and the variations were similar. However, the increasing rate of specific cake resistances of supernatant was higher than that of suspension.
Acknowledgement The authors would like to acknowledge the financial support from the Education Department of Jiangxi Province, China (No. [2007]336). References
Fig. 10. Specific cake resistances and compressibility of the suspension and the supernatant in MBR and HMBR.
[1] W.B. Yang, Cicek Nazim, J. Ilg, State-of-the-art of membrane bioreactors: worldwide research and commercial application in North America, J. Membr. Sci. 270 (2006) 201–211. [2] B.D. Cho, A.G. Fane, Fouling transients in nominally sub-critical flux operation of a membrane bioreactor, J. Membr. Sci. 209 (2002) 391–403. [3] S. Ognier, C. Wisniewski, A. Grasmick, Membrane bioreactor fouling in sub-critical filtration condition: a local critical flux concept, J. Membr. Sci. 229 (2004) 171–177. [4] S.P. Hong, T.H. Bae, T.M. Tak, A. Randall, Fouling control in activated sludge submerged hollow fiber membrane bioreactors, Desalination 143 (2002) 219–228. [5] P. Gui, X. Huang, Y. Chen, et al., Effect of operational parameters on sludge accumulation on membrane surfaces in a submerged membrane bioreactor, Desalination 151 (2002) 185–194. [6] T. Ueda, K. Hata, Y. Kikuoka, O. Seino, Effects of aeration on suction pressure in a submerged membrane bioreactor, Wat. Res. 31 (3) (1997) 489–494. [7] E.H. Bouhabila, R.B. Aim, H. Buisson, Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment), Desalination 118 (1998) 315–322. [8] I.S. Chang, S.J. Judd, Air sparging of a submerged MBR for municipal wastewater treatment, Process Biochem. 37 (2002) 915–920.
514
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
[9] G.T. Seo, C.D. Moon, S.W. Chang, S.H. Lee, Long term operation of high concentration powdered activated carbon membrane bioreactor for advanced water treatment, Water Sci. Technol. 50 (2004) 81–87. [10] Y.Z. Li, Y.L. He, Y.H. Liu, S.C. Yang, G.J. Zhang, Comparison of the filtration characteristics between biological powdered activated carbon sludge and activated sludge in submerged membrane bioreactors, Desalination 174 (2005) 305–314. [11] H. Ma, L.F. Hakim, C.N. Bowman, R.H. Davis, Factors affecting membrane fouling reduction by surface modification and backpulsing, J. Membr. Sci. 189 (2001) 255–270. [12] C. Wisniewski, A. Grasmick, Floc size distribution in a membrane bioreactor and consequences for membrane fouling, Colloids surf., A Physicochem. Eng. Asp. 138 (1998) 403–411. [13] W. Lee, S. Kang, H. Shin, Sludge characteristics and their contribution to microfiltration in submerged membrane bioreactors, J. Mem. Sci. 216 (2003) 217–227. [14] J.S. Kim, C.H. Lee, H.D. Chun, Comparison of ultrafiltration characteristics between activated sludge and BAC sludge, Wat. Res. 32 (11) (1998) 3443–3451.
[15] Q.Y. Yang, J.H. Chen, F. Zhang, Membrane fouling control in submerged membrane bioreactor with porous, flexible suspended carriers, Desalination 189 (2006) 292–302. [16] J.S. Kim, C.H. Lee, I.S. Chang, Effect of pump shear on the performance of a crossflow membrane bioreactor, Wat. Res. 35 (9) (2001) 2137–2144. [17] A.G. Fane, C.J.D. Fell, P.H. Hodgson, G. Leslie, K.C. Marshall, Microfiltration of biomass and biofluids effect of membrane morphology and operating conditions, Filtr. Sep. 28 (1991) 332–340. [18] B.-R. Lim, K.-H. Ahn, P. Songprasert, S.-H. Lee, M.-J. Kim, Microbial community structure in an intermittently aerated submerged membrane bioreactor treating domestic wastewater, Desalination 161 (2004) 145–153. [19] K.H. Choo, C.H. Lee, Hydrodynamic behavior of anaerobic biosolids during crossflow filtration in the membrane anaerobic bioreactor, Wat. Res. 32 (12) (1998) 3387–3397. [20] J.A. Howell, V. Sanchez, R.W. Field, Membranes in bioprocessing — theory and applications, Blackie Academic & Professional, Glasgow, UK, 1993.
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Filtration characteristics of activated sludge in hybrid membrane bioreactor with porous suspended carriers (HMBR) Qi-Yong Yang ⁎, Tao Yang, Hui-Juan Wang, Kang-Qiang Liu School of Chemistry and Chemical Engineering, Jiujiang University, Jiujiang 332005, China
a r t i c l e
i n f o
Article history: Accepted 19 August 2008 Available online 4 October 2009 Keywords: Hybrid membrane bioreactor Suspended carriers Filtration resistance Particle size Activated sludge Membrane fouling
a b s t r a c t The suspended carriers were efficient in controlling membrane fouling in hybrid membrane bioreactor with porous suspended carriers (HMBR). The purpose of this study consisted in investigating the effect of suspended carriers on the sludge suspension, especially the filterability of sludge suspension. The filterability of sludge suspension in HMBR and general membrane bioreactor (MBR) were investigated and compared in parallel conditions by dead-end filtration for better evaluating the influence of suspended carriers on the sludge suspension. Several aspects of sludge suspension such as filtration resistance, specific cake resistance and particle size were discussed. During long-term operation the filtration resistances rose gradually in the early stage (about 100 days) and then increased rapidly, but there was a slight difference between MBR and HMBR with the prolongation of operation time. The granulometric analysis revealed that the mean particle size of sludge suspension of HMBR decreased more sharply than that of MBR, because the fluidized carriers in HMBR would impose shear stresses on sludge flocs and induce the destruction of the network of sludge zoogloea. Dead-end filtration experiments indicated that the resistance-increasing rates of three portions of sludge suspension were in the order of supernatant N dissolved organics N microbial flocs. In order to further understand the filterability of sludge suspension, the specific cake resistance (α or α.C) of sludge suspension and supernatant in HMBR and MBR were determined. During long-term operation the α and α.C increased with operation time. These results revealed that the suspended carriers in HMBR had appreciably negative effect on the biological characteristics and filterability of the sludge suspension, but they were efficient in controlling membrane fouling during continuous operation of HMBR. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The developments of cost-effective membrane manufacturing technology and increasingly stringent regulations for the discharge of effluents have given an impetus to the application of membrane bioreactors (MBR) for wastewater and reuse [1]. However, membrane fouling, which results in smaller permeate flux and higher operational costs, has been the main obstacle to the widespread application of MBR. More recent investigations focus on more effective and economical techniques to prevent or mitigate membrane fouling, especially the reduction of cake formation on membrane surfaces. The particle accumulation on membrane surface could be effectively mitigated when MBRs operated below critical flux [2,3] and in intermittent mode [4,5]. Air-scouring, which formed back-transport detaching particles from membrane surface into bulk solution, also was the major method of preventing the formation of cake layer [6–8]. Recently powdered activated carbon (PAC) was used to reduce membrane surface fouling ⁎ Corresponding author. School of Chemistry and Chemical Engineering, Jiujiang University, Jiujiang 332005, PR China. Tel./fax: +86 792 8314448. E-mail address: [email protected] (Q.-Y. Yang). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.08.013
[9,10]. In addition, backpulsing and backwashing were introduced to remove cake layer after its formation [11]. However, air-scouring had no effect on the cake removal beyond the critical value of the air flow rate [6,7]. Furthermore, the shear stress would cause biological flocs breakage and consequently increase pore blocking and compressibility of cake [4,12,13]. It also should be noted that the main effect of PAC was to change the composition and permeability of the cake layer [9,14], not to mitigate the particles accumulation on membrane surface. So the porous suspended carriers were added into MBR to enhance the scouring effect of removing the cake layer over membrane [15]. Our previous study [15] investigated filtration resistance online during continuous operation of hybrid membrane bioreactor with porous suspended carriers (HMBR) and revealed that suspended carriers were efficient in controlling membrane fouling. However, it was the lack of comprehensive investigation of fouling in HMBR. In order to go further into the fouling of HMBR, the filterability of the activated sludge suspension was determined in this study. There were some differences between the sludge suspension of HMBR and that of MBR because of the presence of suspended carriers in HMBR. Therefore, the HMBR and MBR were operated in parallel by feeding
508
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
Fig. 1. Schematic diagram of experimental system (a) and configuration of the HMBR or MBR (b) (1—Wastewater tank; 2—Influent pump; 3—Hydrolysis-acidification bioreactor; 4— Water level sensor; 5—Electric agitator; 6—Fixed carrier; 7—Aeration bioreactor; 8—Aeration pipe; 9—Suspended carrier; 10—Membrane module; 11—Gas flow meter; 12—Air compressor; 13—Pressure gauge; 14—Relay; 15—Peristaltic pump; 16—Baffle wall).
the same wastewater and the filterability of sludge suspension in both systems were investigated and compared in same conditions.
particles were determined by LS 13 320 laser diffraction particle size analyzer (Beckman Corporation, USA). 2.3. Fractionation of activated sludge suspension
2. Material and methods 2.1. Membrane bioreactor systems The experimental system consisted of three bioreactors: hydrolysis-acidification bioreactor and two aeration bioreactors as shown in Fig. 1. The hydrolysis-acidification bioreactor was equipped with fixed carriers and an electric agitator, which had a working volume of 25 L (250 × 250 × 400 mm). A flat-shaped hollow fiber membrane module was immersed in each aeration bioreactor with a working volume of 10 L (220 × 140 × 325 mm). Hollow fiber membranes were made of polyethylene with a pore size of 0.1 – 0.2 µm (Hangzhou Zheda Hyflux Hualu Membrane Technology Co. Ltd., China) and the total membrane surface area of module was 0.4 m2. The aeration bioreactor with porous suspended carriers was HMBR, and the other was called MBR. The porous suspended carrier was made of polyurethane and couldn't induce any erosion on membrane due to its flexibility. Its volume accounted for a proportion of 10–20% of total working volume. The characteristics of the carrier were shown in Table 1. The membrane permeate was intermittently suctioned by a peristaltic pump under a constant flux, and the transmembrane pressure (TMP) was monitored by a pressure gauge. A suction mode of 10-minutes-on and 3-minutes-off was adopted. The HMBR and MBR were operated in parallel by feeding the same wastewater for better evaluating the influence of suspended carriers on the sludge suspension. The operation parameters of HMBR and MBR were shown in Table 2. 2.2. Analyses The analysis of chemical oxygen demand (COD) and the measure of mixed liquor suspended solids (MLSS and MLVSS) were according to standard methods (Editorial Board of Environment Protection Bureau of China, 1997). The size distribution of activated sludge Table 1 Characteristics of porous suspended carriers. Parameter
Density/kg m− 3
Porosity/%
Average pore size/mm
Configuration size/mm
Value
30
90
1.0–1.5
10 × 10 × 10
The mixed liquor of activated sludge was fractionated into several portions (microbial flocs, supernatant and dissolved organics) as follows [16]: the mixed liquor was centrifuged (1430 g, 15 min) to separate the microbial flocs from the supernatant. Some portion of the supernatant was then filtered with a membrane filter (0.45 µm pore size), and permeate was the soluble organic. The microbial flocs was put into the Ringer's solution and centrifuged again, and then these steps were repeated 2–3 times to get the purified flocs. Finally, the microbial flocs were suspended in a saline solution so that their concentration was equal to that in the original suspension. 2.4. Determination of filterability In order to investigate the filterability (including filtration resistance and specific cake resistance) of the activated sludge suspension and its portions, a special membrane filtration cell with a filtration area of 0.00332 m2 was introduced (made by Shanghai Institute of Applied Physics, Chinese Academy of Sciences). As shown in Fig. 2, the module in dead-end filtration mode was suitable for highly viscous biological suspensions, especially. The membrane sheet used in the experiment was made of cellulose acetate with a pore size of 0.22 µm. Permeate flux was determined by weighing the permeate mass with an electronic balance and the operating pressure ranged from 0.1 to 1 MPa. The permeate mass were weighed and noted every 15 s. 2.4.1. Filtration resistance of the activated sludge suspension and its portions According to the resistance-in-series model, the permeate for MBR can be expressed by ΔP μRt
ð1Þ
R t = Rm + Rf
ð2Þ
J=
where J is permeate flux, ΔP is transmembrane pressure (TMP), µ is the viscosity of permeate, and Rt is the total resistance, Rm is the intrinsic membrane resistance, and Rf is the filtration resistance of the activated sludge suspension or its portions.
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
509
Table 2 Operation parameters of HMBR and MBR. Operation time/d
Wastewater
Influent COD /mg L− 1
Air flow rate /m3 h− 1
DO /mg L− 1
Constant flux /L m− 2 h− 1
SRT /d
HRT /h
1 – 87 96 – 162 172 – 201 202 – 235 236 – 254
Synthetic a Synthetic b Industrial c Industrial c Industrial c
250 – 860 980 – 1470 760 – 1830 1100 – 1700 880 – 1760
0.15 0.15 0.15 0.15 0.15
3.0–5.0 3.0–5.0 3.0–5.0 3.0–5.0 3.0–5.0
3.0 3.0 3.0 4.5 6.0
No waste 50 50 50 50
10.8 10.8 10.8 7.2 5.4
DO: Dissolved oxygen; SRT: Sludge retention time; HRT: Hydraulic retention time. a Synthetic wastewater of terephthalic acid as sole substrate. b Synthetic wastewater of terephthalic acid and ethylene glycol. c Industrial wastewater discharged from polyester fabric alkali-peeling process (Xianglong Dyeing & Printing Co. Ltd. Suzhou City, China).
Flux (J) and TMP data are used to calculate resistances by Eq. (1). Rt is calculated from the final flux and TMP values at the end of operation; Rm is determined by filtration of pure water with new membrane; and Rf can be obtained by Eq. (2). 2.4.2. Specific cake resistance of the activated sludge suspension and supernatant The specific cake resistance is usually defined as the resistance per unit mass of cake layer deposited per unit membrane surface area. In this study specific cake resistances were employed to indicate the filterability of activated sludge suspension. In the experiment specific cake resistances were measured with the membrane filtration module under unstirred condition. According to the filtration equation, the relationship between filtrate volume and time can be expressed by Eq. (3). Specific cake resistance and compressibility of the cake layer could be calculated by Eqs. (3) and (4), respectively. t μ⋅Rm μ⋅α⋅C⋅V = + V S⋅ΔP 2S2 ⋅ΔP α = α0 ⋅ΔP
n
in two systems was similar. The ratio of MLVSS/MLSS was equal to 0.72–0.85. The MLSS increased rapidly to about 15,000 mg.L− 1 on the 87th day because no excess sludge was extracted from both systems. In a conventional MBR system, new sludge was continuously generated with the consumption of feed organic materials while some sludge mass was decayed by endogenous respiration. Due to the slower sludge decay rate, the sludge generation and reduction couldn't reach an equilibrium state within the reasonable MLSS range. From the 88th to the 96th day the sludge was disintegrated without feed and some sludge was removed at the same time, so on the 97th day MLSS decreased to 7100 and 6750 mg L− 1 for MBR and HMBR, respectively. Then the excess sludge was removed to maintain proper MLSS level and SRT was equal to 50 days. With the prolongation of operation time the final sub-steady-state MLSS reached 12,000 mg L− 1.
ð3Þ ð4Þ
where V is the volume of filtrate at time t, S is the filtration area, Rm is the membrane resistance, C is the suspended solids concentration, ΔP is transmembrane pressure (TMP) drop, µ is the viscosity of permeate, α is the specific cake resistance, α0 is the specific cake constant, and n is the compressibility of the cake layer. 3. Results and discussion 3.1. Variation of mixed liquor suspended solids (MLSS) and volatile suspended solids (MLVSS) Figs. 3 and 4 showed the variation of MLSS and MLVSS in two systems. It was obvious that the increasing rate of MLSS and MLVSS
Fig. 3. Variation of MLSS in MBR and HMBR.
Fig. 2. Schematic diagram of experimental setup for filterability.
Fig. 4. Variation of MLVSS in MBR and HMBR.
510
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
3.2. Variation of TMP for MBR and HMBR during long-term continuous operation The membrane permeate was intermittently suctioned by a peristaltic pump. The transmembrane pressure (TMP) was monitored under a constant flux condition, which indicated the extent of membrane fouling. Fig. 5 illustrated the increase of TMP for MBR and HMBR. The operation was stopped when the suction pressure reached 30 kpa because it was difficult to maintain the flux at constant level at high TMP (for example, it was needed to adjust the peristaltic pump on the 144th day when TMP reached 30.92 KPa to maintain a constant flux). Then the membrane module was taken out from the bioreactor for cleaning. When membrane flux was 3.0 L m− 2 h− 2, TMP increased slowly for both systems. However, TMP increased rapidly at high flux such as 4.5 or 6.0 L m− 2 h− 2. For MBR, all the equipment and operating parameters were exactly the same as HMBR except the adding of suspended carriers. The increase rate of TMP for HMBR was far lower than that for MBR, which indicated that the suspended carriers were the main contributors to improve the filtration performance of submerged HMBR. 3.3. Characteristics of filtration flux and resistance of sludge suspension Although MBR and HMBR operated in parallel under the same feed and operation conditions, there were slightly different in the concentration of suspended solids (SS) for the two systems. In order to better evaluate the filterability of sludge suspensions in MBR and HMBR, the SS drawn from two systems were made to equal concentration by saline solution for resistance measurement. The variation of filtration flux and resistance of sludge suspensions (on the 247th day) in MBR and HMBR were shown in Fig. 6. The permeate flux decreased sharply in the early stage and then almost leveled off after about 1200 s of filtration time. While the resistance increased rapidly with the operation time and also leveled off after about 1200 s. The resistance was calculated by Eqs. (1) and (2) every 15 s. These results indicated that the sharp flux decline and resistance increase in the early stage be mostly relevant to the formation of cake layer. Furthermore, it was worth noting that the profiles of filtration flux and resistance variation with time exhibited similar trends between MBR and HMBR. The conclusion may be obtained that the filterability of suspension in MBR was similar to that in HMBR. 3.4. Variation of filtration resistance of sludge suspension The filtration resistances of sludge suspension in the two systems were similar in early stage just as mentioned above. However, Fane et al. [17] observed that fouling increased when bacteria lacked
nutrients and their growth slowed down. This indicated that the bacterial physiological condition have an effect on fouling. Therefore, it was necessary to investigate and compare the filtration performance of sludge suspensions in the two systems because of the presence of porous suspended carriers in HMBR, which may lead to some changes in aspects such as particle size distribution, viscosity, constituents and bacterial physiology. The variations of filtration resistance of sludge suspension in the two systems during long-term operation were shown in Fig. 7. The values of filtration resistances in Fig. 7 were average resistances during the dead-end filtration time (about 1400 s) after the initial data (about 300 s) were eliminated. As shown in Fig. 7, the filtration resistances rose gradually during early stage (about 100 days), and then increased rapidly from the 103rd day to the 247th day. In early stage the filtration resistances of sludge suspension in the two systems were very similar, but from the 103rd day to the 247th day the resistances of sludge suspension in MBR were appreciably different from that in HMBR. On the one hand, it may be the difference in the microbes and morphological composition between HMBR and MBR due to the porous suspended carriers in HMBR. The carriers supplied anoxic or anaerobic condition for anaerobe such as denitrifying bacteria. Lim et al.[18] observed a slight difference in microbial community structure between the anoxic and the aerobic condition. On the other hand, the shear stresses induced by suspended carriers imposed on the suspension could destruct the activated sludge flocs, which was the result of physicochemical interactions between microorganisms, inorganic particles, extracellular polymeric substances and multivalent cations. Therefore, the shear stresses modified the biological characteristics and filterability of sludge suspension in HMBR.
3.5. Variation of mean particle size of sludge suspension As mentioned above, one of the effects of suspended carriers on the biological characteristics of sludge suspension in HMBR was modifying the particle size of sludge suspension. In addition to air bubbling, the suspended carriers in HMBR flowed upward together with the fluid along the membrane surfaces and induced shear stresses that generated the back-transport of sludge suspension from the membrane surfaces. According to Choo et al. [19], the smaller the particle diameter was, the greater the net particle velocity toward the membrane became and the more particle deposition on membrane surfaces would occur. Therefore, it was necessary to take into consideration the size distribution and mean particle diameter of sludge suspension in the two systems during long-term operation. The granulometric analysis, given in Fig. 8, revealed the size distribution of the sludge suspension in MBR and HMBR. The results indicated that the sludge flocs presented nearly a unimodal
Fig. 5. Comparison of TMP between MBR and HMBR (A: washing membrane surface with tap water; B: cleaning membrane model with sodium hypochlorite of 0.4% (w/w) for 16 h after washing).
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
511
Fig. 6. Variation of filtration flux and resistance of sludge suspensions in MBR and HMBR (measured at 0.1 MPa).
distribution and the size distribution of sludge suspension shifted to a higher value with operation time for both systems. Fig. 9 presented the change of mean particle size of the sludge suspension in the two systems with operation time. It was obvious that the mean particle size of sludge suspension decreased with operation time for both systems. For MBR the mean particle size decreased from 252.9 (18th day) to 176.4 µm (156th day), while for HMBR it decreased from 249.9 (18th day) to 67 µm (156th day). It was obvious that the mean particle size of HMBR decreased more sharply than MBR. It may correspond to the suspended carriers in HMBR. Just as MBR, the recirculation of sludge suspension in HMBR would destruct the sludge flocs and reduce the mean particle size. Furthermore, the fluidized carriers in HMBR would impose shear stresses on sludge flocs. The shear stresses would induce the destruction of the network of sludge zoogloea and thus generate more fine colloidal particles.
Fig. 7. Comparison of resistances of sludge suspension between MBR and HMBR (measured at 0.1 MPa, significance level α was 0.05 and measure times were 3).
However, it should be noted that for MBR and HMBR the mean particle size on the 243rd day increased to 214.5 and 162.2 µm respectively. It was due to the leakage of sludge suspension on the 228th day for both systems and addition of fresh activated sludge. In contrast to the literatures, the mean particle size of sludge suspension in this experiment was slightly greater. It was attributed to the low ratio of air to permeate flow.
3.6. Filtration resistances of sludge suspension portions in MBR and HMBR As mentioned above, the filtration of sludge suspension increased sharply from the 103rd day to the 247th day for both MBR and HMBR. To further investigate the factors leading to the drastic increase in resistance of suspension, the sludge suspensions were divided into three portions: microbial flocs, supernatant and dissolved organics. Batch filtration experiments with a stirred cell were employed to measure the filtration resistance of each portion for MBR and HMBR. The results and the confidence intervals were shown in Table 3. The results on the 103rd day were not significantly different between MBR and HMBR, but on the 247th day the results for MBR were appreciably different from that for HMBR. These observations coincided well with the above results of the filtration resistances of sludge suspension. During the long-term run the shear stress of air bubble and suspended carriers on activated sludge would break biological flocs and release biological colloids to bulk suspension. Then a dense layer was built up on the membrane surface and resulted in lower filterability in dead-end filtration. Therefore, the accumulation of biological colloids in the suspension, especially in the supernatant, resulted in the bad filterability of activated sludge. Considering the membrane filtration only, the activated sludge should be discharged at intervals to keep better filterability. For the MBR system, the filtration resistance of suspension rose from 4.10 × 1012 m− 1(103rd day) to 1.06 × 1013 m− 1(247th day), with an increase of 158%. While the suspension resistance increased from
512
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
Fig. 8. Variation of size distribution of sludge suspensions in MBR and HMBR with operation time.
4.18 × 1012 m− 1 (103rd day) to1.15 × 1013 m− 1 (247th day) for HMBR, with an increase of 175%. The resistances increasing rate of supernatant in MBR and HMBR were 156% and 174%, respectively. The resistances of soluble organic in MBR and HMBR increased with 92% and 110%, respectively. As for microbial flocs, there were only increasing rates of 50% and 71% for MBR and HMBR, respectively. Therefore, the resistance-increasing rates of three portions were in the order of supernatant N dissolved organics N microbial flocs.
These results indicated that the sharp increase in filtration resistance of sludge suspension could be contributed mainly to the supernatant fraction, which played a preponderant role in filtration resistance of sludge suspension. 3.7. Specific cake resistances of the suspension and its supernatant The filtration resistance of suspension and its portions were determined in a fixed pressure of 0.1 MPa as above. In this study the filtration resistance of suspension and its portions were mainly the cake resistance. It was well known to us that the cake resistance was related to the specific cake resistance, the membrane area and the
Table 3 Filtration resistance of various fractions of sludge suspension in MBR and HMBR. Operation time /d
Reactor
103rd day
MBR HMBR MBR
247th day
HMBR
Fig. 9. Variation of mean particle diameter of sludge suspensions in MBR and HMBR with operation time.
Filtration resistancea/1012 m− 1 Suspension
Microbial flocs
Supernatant
Soluble organics
4.10 ± 0.13 4.18 ± 0.11 10.6 ± 0.21 (158%)b 11.5 ± 0.34 (175%)
2.84 ± 0.12 2.65 ± 0.10 4.23 ± 0.11 (50%) 4.52 ± 0.17 (71%)
3.18 ± 0.37 3.53 ± 0.19 8.14 ± 0.52 (156%) 9.67 ± 0.26 (174%)
0.75 ± 0.08 0.89 ± 0.06 1.44 ± 0.23 (92%) 1.87 ± 0.18 (110%)
a All measurements were mean values of 3 time measurements in the condition of 0.05 of significance level (α ). All measurements were obtained in the filtration pressure of 0.1 MPa except that soluble organics was measured in 0.02 MPa. b Increased percentage to the resistances of the 103rd day.
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
mass of cake deposited on the membrane surface. That is, specific cake resistance (α) was the cake resistance normalized to the mass of materials deposited per unit of membrane surface area and it was a unique property of particles consistent with their size and conformation [20]. Therefore the specific cake resistance was determined to be a better comparison between the filterability of suspension and supernatant in MBR and HMBR. In this study α was employed to indicate the filterability of the sludge suspension, while the filterability of supernatant was characterized by α.C due to the solids concentration (C) in supernatant is not precisely measurable (only 3– 6% of the suspended solid content in the supernatant). The specific cake resistance (α or α.C) of cake layers of suspension and supernatant was measured by changing the operating pressure in the range from 10 to 200 KPa. The results were shown in Fig. 10. Whether MBR or HMBR, the α of suspension on the 247th day were about 2 times higher than that on the 103rd day and α.C of supernatant on the 247th day was about one order of magnitude higher than that on the 103rd day through the entire pressures test. The results indicated that the specific cake resistances of suspension and supernatant increased with the operation time of biological treatment, and it also revealed that the increasing rate of specific cake resistances of supernatant was higher than that of suspension. It may be the release of polymers from cells to the supernatant and the accumulation of cells fragment in suspension due to the lysis of microorganism during long-term operation. Furthermore, it could be easily seen from Fig. 10 that the α and α.C between MBR and HMBR in both days were very similar. As the results for MBR and HMBR were not significantly different from a statistical
513
point of view, it was difficult to determine whether the suspended carriers in HMBR had any effect on the sludge suspension and its supernatant from α and α.C only. However, the experiments of filtration resistance of sludge suspension and its portions indicated that the suspended carriers had appreciably negative effect on the filterability of the sludge suspension and its supernatant. 4. Conclusion In this study the filterability of sludge suspension in HMBR and MBR were investigated and compared in the same conditions by dead-end filtration for better evaluating the influence of suspended carriers on the sludge suspension. The results revealed that the suspended carriers in HMBR had appreciably negative effect on the biological characteristics and filterability of the sludge suspension. The following conclusions could be obtained: 1. The profiles of permeate flux and resistance of sludge in dead-end filtrations leveled off after about 1200 s of filtration time for both systems. 2. The increase rate of TMP for HMBR was far lower than that for MBR during long-term operation. 3. Dead-end filtration experiments showed that the filtration resistances of sludge suspension rose gradually in the early stage and then increased rapidly. In early stage the variation of resistance for both systems was similar, but from the 103rd day to the 247th day there was a slight difference between MBR and HMBR. 4. The mean particle size of sludge suspension in HMBR decreased more sharply than that in MBR during long-term operation. 5. The sharp increase in filtration resistance of sludge suspension could be contributed mainly to the supernatant fraction. The resistance-increasing rates of three portions of sludge suspension were in the order of supernatantN dissolved organics N microbial flocs. Dead-end filtration experiments indicated that the resistanceincreasing rates of three portions of sludge suspension in HMBR were appreciably greater than that in MBR with the prolonging of operation time. 6. The specific cake resistance of sludge suspension and supernatant increased with the operation time in both systems and the variations were similar. However, the increasing rate of specific cake resistances of supernatant was higher than that of suspension.
Acknowledgement The authors would like to acknowledge the financial support from the Education Department of Jiangxi Province, China (No. [2007]336). References
Fig. 10. Specific cake resistances and compressibility of the suspension and the supernatant in MBR and HMBR.
[1] W.B. Yang, Cicek Nazim, J. Ilg, State-of-the-art of membrane bioreactors: worldwide research and commercial application in North America, J. Membr. Sci. 270 (2006) 201–211. [2] B.D. Cho, A.G. Fane, Fouling transients in nominally sub-critical flux operation of a membrane bioreactor, J. Membr. Sci. 209 (2002) 391–403. [3] S. Ognier, C. Wisniewski, A. Grasmick, Membrane bioreactor fouling in sub-critical filtration condition: a local critical flux concept, J. Membr. Sci. 229 (2004) 171–177. [4] S.P. Hong, T.H. Bae, T.M. Tak, A. Randall, Fouling control in activated sludge submerged hollow fiber membrane bioreactors, Desalination 143 (2002) 219–228. [5] P. Gui, X. Huang, Y. Chen, et al., Effect of operational parameters on sludge accumulation on membrane surfaces in a submerged membrane bioreactor, Desalination 151 (2002) 185–194. [6] T. Ueda, K. Hata, Y. Kikuoka, O. Seino, Effects of aeration on suction pressure in a submerged membrane bioreactor, Wat. Res. 31 (3) (1997) 489–494. [7] E.H. Bouhabila, R.B. Aim, H. Buisson, Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment), Desalination 118 (1998) 315–322. [8] I.S. Chang, S.J. Judd, Air sparging of a submerged MBR for municipal wastewater treatment, Process Biochem. 37 (2002) 915–920.
514
Q.-Y. Yang et al. / Desalination 249 (2009) 507–514
[9] G.T. Seo, C.D. Moon, S.W. Chang, S.H. Lee, Long term operation of high concentration powdered activated carbon membrane bioreactor for advanced water treatment, Water Sci. Technol. 50 (2004) 81–87. [10] Y.Z. Li, Y.L. He, Y.H. Liu, S.C. Yang, G.J. Zhang, Comparison of the filtration characteristics between biological powdered activated carbon sludge and activated sludge in submerged membrane bioreactors, Desalination 174 (2005) 305–314. [11] H. Ma, L.F. Hakim, C.N. Bowman, R.H. Davis, Factors affecting membrane fouling reduction by surface modification and backpulsing, J. Membr. Sci. 189 (2001) 255–270. [12] C. Wisniewski, A. Grasmick, Floc size distribution in a membrane bioreactor and consequences for membrane fouling, Colloids surf., A Physicochem. Eng. Asp. 138 (1998) 403–411. [13] W. Lee, S. Kang, H. Shin, Sludge characteristics and their contribution to microfiltration in submerged membrane bioreactors, J. Mem. Sci. 216 (2003) 217–227. [14] J.S. Kim, C.H. Lee, H.D. Chun, Comparison of ultrafiltration characteristics between activated sludge and BAC sludge, Wat. Res. 32 (11) (1998) 3443–3451.
[15] Q.Y. Yang, J.H. Chen, F. Zhang, Membrane fouling control in submerged membrane bioreactor with porous, flexible suspended carriers, Desalination 189 (2006) 292–302. [16] J.S. Kim, C.H. Lee, I.S. Chang, Effect of pump shear on the performance of a crossflow membrane bioreactor, Wat. Res. 35 (9) (2001) 2137–2144. [17] A.G. Fane, C.J.D. Fell, P.H. Hodgson, G. Leslie, K.C. Marshall, Microfiltration of biomass and biofluids effect of membrane morphology and operating conditions, Filtr. Sep. 28 (1991) 332–340. [18] B.-R. Lim, K.-H. Ahn, P. Songprasert, S.-H. Lee, M.-J. Kim, Microbial community structure in an intermittently aerated submerged membrane bioreactor treating domestic wastewater, Desalination 161 (2004) 145–153. [19] K.H. Choo, C.H. Lee, Hydrodynamic behavior of anaerobic biosolids during crossflow filtration in the membrane anaerobic bioreactor, Wat. Res. 32 (12) (1998) 3387–3397. [20] J.A. Howell, V. Sanchez, R.W. Field, Membranes in bioprocessing — theory and applications, Blackie Academic & Professional, Glasgow, UK, 1993.