A hybrid anaerobic membrane bioreactor coupled with online ultrasonic equipment for digestion of waste activated sludge

Bioresource Technology 102 (2011) 5617–5625

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A hybrid anaerobic membrane bioreactor coupled with online ultrasonic equipment for digestion of waste activated sludge Meilan Xu a, Xianghua Wen a,⇑, Zhiyong Yu a, Yushan Li b, Xia Huang a a b

Environmental Simulation and Pollution Control State Key Joint Laboratory, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China Department of Municipal and Environmental Engineering, School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

a r t i c l e

i n f o

Article history: Received 7 November 2010 Received in revised form 7 February 2011 Accepted 8 February 2011 Available online 13 February 2011 Keywords: Anaerobic membrane bioreactor Waste activated sludge Membrane fouling Ultrasound

a b s t r a c t Anaerobic membrane bioreactor and online ultrasonic equipment used to enhance membrane filtration were coupled to form a hybrid system (US-AnMBR) designed for long-term digestion of waste activated sludge. The US-AnMBR was operated under volatile solids loading rates of 1.1–3.7 gVS/Ld. After comprehensive studies on digestion performance and membrane fouling control in the US-AnMBR, the final loading rate was determined to be 2.7 gVS/Ld with 51.3% volatile solids destruction. In the US-AnMBR, the improved digestion was due to enhanced sludge disintegration, as indicated by soluble matter comparison in the supernatant and particle size distribution in the digested sludge. Maximum specific methanogenic activity revealed that ultrasound application had no negative effect on anaerobic microorganisms. Furthermore, implementing ultrasound effectively controlled membrane fouling and successfully facilitated membrane bioreactor operation. This lab-scale study demonstrates the potential feasibility and effectiveness of setting up a US-AnMBR system for sludge digestion. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction As wastewater treatment in China continues to expand, waste activate sludge generation is increasing. Excess sludge poses a significant threat towards ecology systems, therefore requiring proper treatment and disposal protocols, an expensive process estimated to account for 20–50% of the total investment in wastewater treatment in China, while 50–70% in Europe (Gyßer, 2007). Therefore, there is an increasing interest in the reduction of excess sludge while retaining the effluent quality criteria (Rai et al., 2004). Anaerobic digestion is the most applied technique for sludge stabilization, a process that reduces sludge volatile solids and produces biogas (Tiehm et al., 2001). In a conventional anaerobic sludge digester, the solids retention time (SRT) is identical to the hydraulic retention time (HRT) which results in a large reactor volume, since a long SRT (20–30d) is required for effective volatile solids destruction (Pillay et al., 1994). Anaerobic membrane bioreactor (AnMBR) employs membrane as an efficient separation method to separate solid from sludge suspension. When decoupling SRT from HRT through complete retention of the solid, a shorter HRT could be achieved, therefore reducing the reactor volume of an AnMBR to that smaller than a conventional anaerobic digester. Effective physical screening of microorganism and solids by the membrane allows their retention in the system (Sharrer et al., 2010), enhancing

⇑ Corresponding author. Tel.: +86 10 62772837; fax: +86 10 62771472. E-mail address: [email protected] (X. Wen). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.02.038

degradation of pollutants. Furthermore, conventional sludge processing includes a thickening step following anaerobic digestion, which generally requires coagulants to be added for efficient thickening. The membrane filtration process is capable of thickening municipal sewage sludge (Benítez et al., 1995). In that sense, thickening can be incorporated into the AnMBR digestion process, eliminating the need for coagulant addition and reducing operating cost (Pierkiel and Lanting, 2005). Therefore, AnMBR is expected to be advantageous for excess sludge digestion. However, membrane fouling, a major drawback of AnMBR, degrades membrane filtration performance, hindering the application of AnMBR. Fouling stems from the adsorption of organic matter, precipitation of inorganic matter, and adhesion of microbial cells to the membrane surface (Choo and Lee, 1996). The researches on the membrane fouling in AnMBR are much fewer than that in aerobic MBR (membrane bioreactor). With increasing application of AnMBR in various wastewater treatments, more attention is needed on anaerobic membrane fouling control. Some strategies for membrane fouling control includes appropriate operation conditions, high shear across membrane, modification of mixed liquor characteristics and membrane materials, and cleaning methods. Presently, external configuration is the most common for AnMBR. In general, a high cross-flow velocity, creating high shear stress, is used to reduce fouling rate in external membrane units of AnMBR. Strohwald and Ross (1992) found that the cross-flow velocity should be maintained at a value larger than 1.5 m/s. However, Padmasiri et al. (2007) indicated that an increase in cross-flow velocity resulted in poor anaerobic digestion performance. In order to ensure

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operability of sludge digestion with AnMBR, other methods must be explored to control membrane fouling effectively. In some studies, ultrasound has been proved effective for enhancement of membrane filtration for different solutions such as peptone, whey, paper industrial wastewater and aqueous milk solution (Chai et al., 1999; Muthukumaran et al., 2004; Kyllönen et al., 2006; Latt and Kobayashi, 2006). Lamminen et al. (2004) determined that the mechanism behind ultrasonic cleaning lies in cavitation effect and acoustic streaming. In a study, Sui et al. (2008) firstly used ultrasound to control membrane fouling in an AnMBR treating synthesized glucose wastewater and confirmed the feasibility of using ultrasound. Is ultrasound also applicable in membrane fouling control in an AnMBR during sludge digestion? Work should be done in order to determine the applicability of such an approach. In conjunction, the influence of ultrasound on sludge properties must be taken into account. It is well known that ultrasound disintegration technology has been widely used for cell lysis pretreatment in order to improve anaerobic sludge digestion. This is because hydrolysis is the rate-limiting step of sludge digestion process. Zhang et al. (2007) found that sonication at 25 kHz and 0.5 W/ml for 30 min disintegrated the sludge flocs by 30.1%. Nickel and Neis (2007) also applied the ultrasound to disintegration of waste activated sludge to improve subsequent biodegradation. The results demonstrated that the anaerobic degradation process was considerably accelerated by ultrasonic sludge pre-treatment with a frequency of 31 kHz, a power intensity of 10 W/cm2, and a sonication time of 90 s. In those studies, the imposed ultrasound power intensity is generally high. Other studies have tested the effects of ultrasound on the enhancement of sludge dewaterability. In a study about the ultrasonic post-treatment after anaerobic digestion, Na et al. (2007) used capillary suction time (CST) to evaluate the dewatering behaviors. It was clearly observed that CST of digested sludge decreased with increasing volumetric supplied energy, except in the range of 0–800 kJ/L of energy. Furthermore, the ultrasonic improvement of biological activity has been investigated in some studies. Schläfer et al. (2000) observed an increase in biological activity of the process when irradiated by discontinuous ultrasound with a frequency of 25 kHz and a power input of 0.3 W/L. Another study also indicated that the sludge OUR (oxygen uptake rate) increased by 28% and the biomass growth rate increased by 12.5% under the optimal sonication conditions with a frequency of 25 kHz, a power density of 0.2 W/mL and a sonication time of 30 s (Zhang et al., 2008). The impact of ultrasound on the sludge is principally determined by the ultrasound parameters used and the other operational conditions. In this study, when induced into the membrane module of the AnMBR, the ultrasound with relatively lower intensity was intentionally used for membrane fouling control, the effect of which on the anaerobic sludge and the overall digestion performance was unknown and must be evaluated. Based upon the above discussion, this study implements an anaerobic membrane bioreactor coupled with online ultrasonic equipment (US-AnMBR) for digestion of waste activated sludge. During the long-term operational experiment, the performance of waste activated sludge digestion, the effect of ultrasound on the sludge, and the performance of membrane filtration with the ultrasound unit were investigated in order to evaluate the feasibility and obtain the viable operational parameters of a hybrid US-AnMBR applied to waste activated sludge digestion.

membrane module. The membrane used was hollow fiber polyethylene membrane with 0.4 lm nominal pore size and a total surface area of 0.012 m2 (Mitsubishi Rayon, Japan). The filtration mode was outside-in. The membrane module was submerged in a water bath (30 L  20 W  15 H cm3) attached at the bottom with six piezoelectric ultrasonic transducers which was linked to an ultrasound generator that could irradiate with a frequency of 28 kHz and an adjustable power output of 0–300 W (JDX-03, Jinxing Co., Ltd., China). The mixed liquor from the bioreactor was pumped with a centrifugal pump into the membrane and the retentate was recirculated back to the bioreactor. The temperature was kept at 35 ± 2 °C with hot water recirculation. An AnMBR without online ultrasonic equipment operating in parallel under the same condition was also used as a control. Both filtration systems employed a low cross-flow velocity of 1.0 m/s to alleviate the possible negative effect caused by high shear. For the US-AnMBR, ultrasound parameters were adjusted by the power intensity and the time mode. The power intensity is defined as the power output supplied to per unit area of the transducer surface (unit: W/cm2). The time mode related to the continuous irradiation time in an operation period (e.g., 3 min/ 60 min meant irradiating 3 min in 60 min). After a period of filtrating, fouled membranes of both reactors were replaced with new ones for further analysis. At the beginning, two systems were inoculated with anaerobic sludge taken from another lab-scale AnMBR and started with a low volatile solids (VS) loading rate of 1.1 gVS/Ld. In order to screen for the optimal loading rate, the HRT was reduced gradually from 8 to 3 days, enabling two systems operating under an incremental volumetric loading rate of 1.5, 2.0, 2.8 and 3.7 gVS/Ld. As the HRT reduced, the designed permeate flux was accordingly adjusted in the range of 1.3–3.5 L/m2 h. The SRT was controlled by sampling digested sludge regularly from the system. Generally, 200 mL (8.3% of the total volume) samples were taken once every 3 days. The mass of samples was considered into the calculation of SRT and VS reduction rate.

2. Methods

where, l (Pa s) is the viscosity of the permeate assumed as the viscosity of pure water under the given temperature; TMP (Pa) is the trans-membrane pressure measured with mercury manometer and J (m3/m2 s) is the permeate flux through the membrane. To monitor digestion process, the total volatile fatty acid (VFA) concentration, bicarbonate alkalinity, pH, dissolved organic carbon

2.1. US-AnMBR and its operation The US-AnMBR system (Fig. 1) consisted of a 2.4 L anaerobic bioreactor with a mixer (complete mixing) and an external

2.2. Feedstock The feedstock for this study was the waste activated sludge collected from the secondary sediment tank of a wastewater treatment plant in Beijing. To prevent the clogging of rough particles, the waste activated sludge was pre-filtered by a 1 mm screen and then stored at 4 °C. Before adding to the reactor, the feedstock was thickened with the supernatant discarded to approximate desired concentration except for Run 6, where the unthickened waste activated sludge was directly fed. The properties of pre-treated feedstock are presented in Table 1. The feedstock was fed semicontinuously and equably through a peristaltic pump into the reactor, the amount of which was determined by the designed HRT. 2.3. Analytical methods To characterize membrane fouling, the total membrane filtration resistance (Rtotal) were evaluated daily. Based on Darcy’s law, Rtotal (m1) was calculated as follows:

Rtotal ¼

TMP lJ

ð1Þ

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1 6 2

11

12

11

12

10 3

11

12

7 9 4

8

5

1. Feed stock tank 2. Feed pump 3. Anaerobic digester (CSTR with mixer) 4. Hot water bath 5 . Hot water recirculation pump 6. Gas collector 7. Mixed liquor recirculation pump 8. Ultrasonic cleaningequipment 9. Hollow fiber membrane 10. Suction pump 11. Valve 12. Manometer Fig. 1. The schematic diagram of the US-AnMBR.

Table 1 Feedstock characteristics. Run

TS (mean, g/L)

VS (mean, g/L)

VS/TS ratio (mean, %)

DOC (mean, mg/L)

NHþ 4 AN (mean, mg/L)

1 2 3 4 5 6

13.7 11.9 11.2 13.0 18.8 6.2

9.0 8.7 8.2 8.3 10.8 4.0

65.6 73.1 73.2 63.8 57.4 64.5

50.9 44.0 35.1 64.5 84.7 30.5

45.5 39.0 31.1 23.2 42.9 37.7

(DOC) and NHþ 4 AN in the reactor and in the effluent were measured about once every 3 days. The mixed liquor sample from the reactor was centrifuged at 20,000g for 30 min at 4 °C, and then the supernatant was filtered through a 0.45 lm membrane. The resulted filtrate was used for analysis. Total VFA concentration and bicarbonate alkalinity were measured by a simple titration method (Anderson and Yang, 1992). Total VFA concentration was converted as acetic acid (HAc) concentration (unit: mgHAc/l). To evaluate the performance of digestion, the parameter ‘‘a’’ was calculated by using Eq. (2):

a¼

VA BA

ð2Þ

where, VA (mmol/L) is the VFA concentration and BA (mmol/L) is the bicarbonate alkalinity. This parameter has been used to monitor the reactor stability (Poggi-Varaldo and Oleszkiewicz, 1992). pH was determined with pH meter (FE20, METTLER TOLEDO, Switzerland). DOC was analyzed with TOC (Total organic carbon) equipment (TOC-V CPH, SHIMADZU, Japan). NHþ 4 AN was determined with UV–VIS spectrophotometer (DR 5000, HACH, USA). The biogas production was measured daily with liquid displacement method. The particle size distribution of digested sludge was analyzed using a laser particle size analyzer (Mastersizer 2000, Malvern, UK). Total solids (TS), volatile solids (VS), mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) in the reactor were determined according to the standard methods (APHA, 1998). The efficiency of sludge digestion was evaluated with average VS reduction rate that was calculated as follows:

VSr ¼

ðVSf  VSd  VSe  DVSÞ  100% VSf

ð3Þ

where, VSr is the VS reduction rate, VSf (g) is the cumulative VS amount of feedstock, VSd (g) is the cumulative VS amount of discharged digested sludge (including samples), VSe (g) is the cumulative VS amount of effluent and DVS (g) is the increased VS amount in the reactor, which is calculated by subtracting the VS amount in the reactor at the original state from that at the terminal state. The maximum specific methanogenic activity (MSMA) of digested sludge with glucose as the sole carbon source was analyzed. For the MSMA test, 50 mL sludge collected from the reactors was added to a 250 mL serum bottle and incubated in 100 mL nutrient solution with its composition as glucose (18.75 g/L), CO(NH2)2 (1.08 g/L), KH2PO4 (0.44 g/L), NaCl (0.01 g/L), Na2MoO42H2O (0.01 g/L), FeCl2 (0.003 g/L), MnSO42H2O (0.01 g/L), MgSO47H2O (0.1 g/L) and CaCl22H2O (0.01 g/L). The MLSS and MLVSS of sludge were measured before adding to the bottles. The pH and final volume of the mixed liquor in the serum bottle was adjusted to 7.0 by using NaHCO3 and to 200 mL with deionized water. The serum bottles were sealed and incubated at 110 rpm and 35 °C. Methane production was measured by liquid displacement method with 2 M NaOH as the solution. The MSMA was calculated as below:

lmax;CH4 ¼

KT 0 XVT 1

ð4Þ

where, lmax;CH4 (mL/gh) is the maximum specific methanogenic activity, K (mL/h) is the maximum methane production rate evaluated as the slope of the curve (accumulative methane production versus time) in the linear range, T 0 (K) is the absolute temperature at standard state (273 K), T 1 (K) is the ambient absolute temperature, X (g/L) is the MLVSS concentration of sludge added and V (L) is the volume of sludge added. Moreover, anaerobic digested sludge was observed by scanning electron microscope (SEM, QUANTA 200, FEI, Holland). For the SEM analysis, the sample was processed by a series of procedures including immobilization with 2.5% glutaraldehyde and 1% osmium tetroxide, and subsequent dehydration using ethanol (30–100%) followed by drying with critical point drier (CPD 030, BAL-TEC, Switzerland). Then, gold sputtering on the sample was carried out with sputter coater (SCD 005, BAL-TEC, Switzerland). Finally, the prepared sample was scanned at selected magnifications.

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Table 2 Digestion efficiency of the US-AnMBR and the AnMBR. Run Day

VS loading Average VS removal rate (g VS/Ld) (%)

1 2 3 4 5 6

1.1 1.5 2.0 2.8 3.7 2.7

1–48 49–112 113–195 196–225 251–304 305–390

Average gas production rate (mL biogas/g VS added)

US-AnMBR AnMBR US-AnMBR

AnMBR

46.6 45.7 46.2 47.0 49.0 51.3

144 176 174 176 207 195

45.3 42.8 43.1 45.3 48.1 50.7

162 156 179 210 227 229

3. Results and discussion 3.1. Performance of waste activated sludge digestion in the US-AnMBR To maximize the capability of the hybrid US-AnMBR system, six runs were conducted during the long-term operation of 390 days. The VS loading rate was gradually increased up to the highest and then adjusted to be the optimal value (Table 2). The details of each run are presented in the following sequence: During the beginning of operation (day 1–48, Run 1), although the reactor started with a low volumetric loading of 1.1 gVS/Ld, VFA concentration in the two reactors increased gradually until the 28th day when VFA was up to 1940 mgHAc/L in the US-AnMBR while 1480 mgHAc/L in the AnMBR (Fig. 2a). Generally speaking, for an anaerobic process, feeding should be reduced or suspended when the total VFA concentration was above 1000 mgHAc/L (Anderson and Yang, 1992). Moreover, the calculated parameter a in both systems also increased gradually with the highest value of 2.7 and 2.8 for the AnMBR and the US-AnMBR, respectively. Poggi-Varaldo and Oleszkiewicz (1992) indicated that an a value less than 1 was required for stable digestion of municipal solids waste and wasted activated sludge. As for the pH, it was relatively stable at 7.4 ± 0.2. Based on the VFA concentration and the a, it was indicated that the two systems were in an unstable state at the beginning. Fortunately, as the digestion proceeded, VFA concentration and parameter a decreased gradually and a low a of below 0.7 was obtained on and after the 31st day in the AnMBR while a of below 0.6 was obtained on and after the 37th day in the USAnMBR, indicating that both reactors had already operated stably. Similar to the VFA concentration, the DOC also increased at the beginning of the reactor operation (Fig. 2b). The highest DOC value of approximately 1350 and 1400 mg/L were obtained in the US-AnMBR and the AnMBR, respectively, on the 16th day. The DOC in the reactor was much more than that in the supernatant of the feed sludge (below 100 mg/L). It indicated that the organic matter was released from the sludge into the liquid phase but was unable to be degraded timely, therefore accumulating in the reactor. After the 16th day, the DOC in the reactor decreased and finally maintained at a relatively low and stable level of about 350 mg/L. In addition, as rejected by the membrane and the cake layer covering over the membrane, the DOC in the effluent was always lower than that in the reactor, differing from the result obtained in a study about a hybrid MBR process with the same membrane for treatment of coke wastewater (Zhao et al., 2010). For the NHþ 4 AN concentration, it also increased gradually at the beginning and maintained at a relatively stable state after the 7th day in both systems (Fig. 2c). The biogas production increased tardily in both systems as the digestion progressed (Fig. 2d). Because of the long SRT of 36 days, the MLSS was accumulated in the reactors. The MLSS concentration increased up to about 27 g/ L finally in the US-AnMBR under this loading rate (Fig. 2e). The efficiency of digestion is shown in Table 2. It was calculated on

the basis of data during the term of steady digestion process, when the a was lower than 1. The average VS reduction efficiency in the US-AnMBR reached to 46.6% under this loading rate. In general, the degree of degradation of organic matter in anaerobic sludge digestion varies between 25% and 60% (Nickel and Neis, 2007). According to vector attraction control requirements, a minimum of 38% reduction should be achieved in volatile solids during biosolids treatment (US EPA, 1999). To obtain an optimal operation condition, the VS loading rate was elevated in the next run. During day 49–112 (Run 2), the volumetric loading rate was increased up to 1.5 gVS/Ld. In the US-AnMBR, the VFA concentration still remained below 1000 mg/L and the a was much lower than 1, suggesting successful digestion. It indicated that waste activated sludge was well digested under a loading rate of 1.5 VS/Ld. The soluble organic matter in the reactor increased slightly up to about 500 mg/L as the loading rate increased. The NHþ 4 AN concentration varied from 180–370 mg/L. On the 59th day, due to an accidental leak in the outer plexiglass pipe sleeve of the membrane module, a loss of digested sludge from the US-AnMBR caused a decrease in MLSS concentration. The MLSS concentration increased up to approximately 40 g/L finally under this loading rate (Fig. 2e). During Run 3 (day 113–195), the volumetric loading rate was further increased to 2.0 gVS/Ld. Despite an increase (below 1000 mg/L) during day 152–156, the VFA concentration and the a remained stable and low in the US-AnMBR, indicating that waste activated sludge was well digested. In the US-AnMBR, the MLSS concentration increased up to about 50 g/L finally for this run (Fig. 2e). The DOC in the reactor increased quickly up to about 1200 mg/L and then fluctuated in the range of 890–1120 mg/L, indicating that more organic matter was released and accumulated in the reactor (Fig. 2b). This similar phenomenon was also observed in the control AnMBR. During Run 4 (day 196–225), some digested sludge was discarded from the two systems to reduce the MLSS concentration to about 30 g/L. Then the two systems were operated at a loading rate of 2.8 gVS/Ld. The VFA concentration and the a stayed in the normal range suggested successful digestion. In the US-AnMBR, the MLSS concentration quickly increased up to about 62 g/L finally under this loading rate. The soluble organic matter in the reactor also increased and then maintained relatively stable in the range of 1050–1160 mg/L. The NHþ 4 AN concentration increased and varied from 410–550 mg/L. Under this loading rate, the gas production ability was improved (Table 2). During Runs 1–4 (day 1–225), the VS reduction efficiency was not distinctly affected by the volatile solids volumetric loading rate (Table 2). It must be noted that the SRT stayed at 36 days for all four runs. Within the experimental conditions tested, the digestion efficiency was principally determined by the SRT, rather than by the VS loading rate. The VS loading rate could be further heightened based upon the digestion results of the above four runs. However, along with the elevation of the VS loading rate, the accumulation of MLSS in the reactor was aggravated, bringing the tougher challenge in membrane fouling control. In considering a proper performance of the sludge digestion and the membrane filtration, the MLSS accumulation problem must be also considered when maximizing the treatment capability of the biological system. Due to problems with the centrifugal pumps for sludge recirculation between the membrane module and the anaerobic reactor on the 226th day, the two systems were cultivated at anaerobic mesophilic condition without membrane filtration during day 226–250. When the recirculation pump was replaced with a screw pump on the 251st day, the systems were restarted with a high loading rate of 3.7 gVS/Ld (day 251–304, Run 5). Before the restart of the systems, partial digested sludge was discarded to maintain the same MLSS concentration in two reactors. Similar to Run 1,

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Fig. 2. Digestion performance data of the two systems: (a) VFA, (b) DOC, (c) NHþ 4 AN, (d) biogas productions and (e) MLSS (‘‘S’’ and ‘‘E’’ in the legend of figure (b and c) represent the supernatant and the effluent).

the VFA concentration in the US-AnMBR increased gradually at the beginning until the 274th day, when the value reached to approximately 1300 mgHAc/L, and then decreased to below 1000 mg/L. The parameter a was below 1 after the 274th day, indicating that the US-AnMBR could maintain stable operation under this high loading rate. As for the soluble organic matter in the reactor, it rapidly peaked to approximately 2160 mg/L on the 290th day. Meanwhile, the NHþ 4 AN concentration also increased to a peak value of about 1010 mg/L in the US-AnMBR, which was much more than that in other runs (Fig. 2c). The higher amount of biogas was also

produced (Fig. 2d). An average VS reduction efficiency of 49.0% indicated that the sludge was digested well under this high loading rate. However, the MLSS concentration in the reactor increased sharply to a final value of about 101 g/L (Fig. 2e). After increasing the VS loading rate up to 2.0 gVS/Ld, due to the high MLSS concentration, severe fouling occurred in the AnMBR (discussed in the Section 3.3). The membrane must be taken out for water cleaning, resulting in a shutdown of system operation. Therefore, considering the performance of membrane filtration, the conventional AnMBR was incapable of operating under such

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a high loading rate. In the US-AnMBR, membrane filtration was feasible under the high VS loading rate of 3.7 gVS/Ld. However, according to the filtration resistance development tendency (discussed in the Section 3.3), it was forecasted that the membrane fouling control required a higher energy to maintain filtration process for the long term. In order to ensure comprehensive feasibility of the hybrid system, high energy consumption and potential membrane damage must be avoided. Therefore, the US-AnMBR should be operated under a loading rater lower than 3.7 gVS/Ld. To ensure long term operation of membrane filtration, the systems were adjusted to operate under a loading rate of 2.7 gVS/Ld during Run 6. VFA, pH, DOC, NHþ 4 AN and MLSS concentration gradually became stable and maintained at relatively low levels. The VS reduction achieved a high efficiency of 51.3% in the US-AnMBR. Compared with the previous 225 days, the performance of digestion was improved slightly during day 251–390 since that the actual SRT was prolonged to about 40 days. It indicated the significance of SRT for sludge digestion. Based upon the operation data discussed above, the waste activated sludge digestion could operate feasibly in the US-AnMBR at a high volumetric loading rate of 2.7 gVS/Ld. If the feedstock TS was calibrated to 40 g/L (typical value for present digesters), the HRT was calculated as 9 days under this loading rate, which was much shorter than that used for conventional anaerobic digester (20–30 days). Furthermore, without considering ultrasound energy used for membrane fouling control, greater sludge treatment ability (higher volumetric loading rate such as 3.7 gVS/Ld) could be expected for the US-AnMBR. The comparison of digestion efficiency under various operation conditions for both systems (Table 2) indicated that the ultrasound used in this study had no negative effect and even slightly improved the sludge digestion. 3.2. Effect of the ultrasound on the digested broth The digestion results suggested better digestion performance in the US-AnMBR, the reason of which was worthy investigating. The present studies on the improvement of sludge disintegration and biological activity have opened some research directions. In this study, soluble matter, particulate matter and anaerobic microbial activity for both systems were respectively compared to examine the effect of ultrasound, thereby explaining why the sludge digestion was improved by the ultrasound. The variation profile of the parameters of soluble matter in digested broth, including VFA, DOC and NHþ 4 AN concentration, were discussed below. In most cases, the higher VFA, DOC and NHþ 4 AN concentration were observed in the US-AnMBR in contrast to those in the AnMBR (Fig. 2a–c). To evaluate exactly the effect of

ultrasound on the digestion process, the data from the two systems were statistically analyzed by using the software SPSS 16.0 (SPSS Incorporation, USA). The nonparametric tests were conducted to examine whether the characteristics of the digested broth differed significantly between the US-AnMBR and AnMBR. Generally, the nonparametric test result is expressed as the ‘‘Exact Sig. (2-tailed)’’. A value below 0.05 suggests a distinct difference existing between the two samples’ data. The ‘‘Exact Sig. (2-tailed)’’ values for VFA, DOC and NHþ 4 AN concentration were all 0.000, indicating that a distinct difference occurred between the two systems when referring to VFA, DOC and NHþ 4 AN concentration. DOC concentration was higher in the US-AnMBR since the ultrasound used in this study accelerated the dissolving of organic matter, which mainly included protein and polysaccharide, and then resulted in more production of VFA. The higher NHþ 4 AN concentration was caused by the hydrolyzation of more protein. Except the parameters for liquid phase discussed above, the particle size distribution of sludge was also analyzed. As shown in Fig. 3, the mean particle size of digested sludge in the US-AnMBR was smaller, indicating that the ultrasound facilitated the disintegration of sludge, causing the release of more organic matter. It also explained the data for VFA, NHþ 4 AN and DOC in the former paragraph. However, the greater disintegration of sludge in the US-AnMBR might aggravate the disruption of the interaction between the different microbial species and consequently lower the anaerobic microbial activity. The ratio of MSMA during the digestion process (lmax;CH4 ðtÞ ) to the original value (lmax;CH4 ð0Þ ) was calculated in this study. After the startup, the ratio in both systems decreased and always maintained lower than 1.0. The lowest value reached to 0.37 in the US-AnMBR, while 0.33 was achieved in the AnMBR. On the 251st day when the two systems were restarted, the ratio value was reset to be 1.0. It also decreased and then fluctuated at 0.54 ± 0.08 and 0.53 ± 0.08 in the US-AnMBR and the AnMBR, respectively. During the digestion process, in most cases, there was no distinct difference in the ratio of MSMA between both systems. As indicated by the above results, although it decreased during the digestion process, the specific methanogenic activity of the digested sludge was not negatively influenced by the ultrasound in contrast to that without the ultrasound. In the SEM images, sludge samples from both reactors showed small quantities of microorganisms in comparison with the feed waste activated sludge, due to bacteria decomposition during the sludge digestion process. It was discovered that cocci and bacilli were dominating anaerobic microorganisms in both reactors. As an interface directly exposed to the ultrasound irradiation, the biomass adhering to the surface of membrane was subject to

Fig. 3. Particle size distribution of digested sludge in reactors.

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ultrasound irradiation. However, cocci and bacilli were also observed clearly on the membrane surface. In order to further understand the ultrasonic effect on the microorganisms in details, the ongoing works concerning microbial community dynamics are pursued with molecular biological methods. Together with the results of methanogenic activity discussed above, it can be concluded that the intentional use of ultrasound for membrane fouling control in this study did not distinctly affect the microorganisms in the anaerobic digestion process. Principally contributed to the greater sludge decomposition, the digestion performance in the US-AnMBR was slightly improved. However, as viewed from membrane filtration, some properties of the bulk sludge in the US-AnMBR, such as higher DOC and smaller flocs, playing key roles the cake formation and membrane fouling (Lin et al., 2010, 2011), might conversely facilitate membrane fouling and increase the hardness of ultrasonic control. It would be investigated in our other study. 3.3. Overall performance of membrane fouling control by the ultrasound During the startup of the filtration system, ultrasound with power intensity of 0.18 W/cm2 and time mode of 3 min/60 min was imposed on membrane M1 according to our previous study (Xu et al., 2010). Then, the ultrasound parameters were adjusted to adapt to the performance of membrane filtration. Several groups of ultrasound parameters applied during 390 days’ operation are presented in Table 3. On the 226th day, the replacement of centrifugal pumps caused a cease of membrane filtration during day 226–250. The overall performance of the membrane filtration in the two systems was evaluated using total membrane filtration resistance and is compared as shown in Fig. 4. Higher membrane filtration resistance implied severer fouling. The extremely low resistances for the two systems on certain days were due to the replacement of the membrane modules. In addition, the sharp drop in filtration resistance for the AnMBR was due to periodic cleaning of the membrane module without the ultrasound. During day 1–156, the highest resistance reached to 29.0  1012 m1 in the AnMBR, while the membrane fouling was well controlled with a resistance of below 11.5  1012 m1 in the US-AnMBR. After the 156th day (with VS loading rate above 2.0 gVS/Ld), the membrane in the AnMBR was more severely fouled than the former 156 days and must be taken out to be cleaned with water. The frequency of cleaning the membrane module without the use of ultrasound gradually increased until day 216–225 when the cleaning frequency was once every day. In contrast, the

filtration resistance of the membrane with the ultrasound could be controlled below 19.4  1012 m1 without any additional cleaning procedure. In the filtration experiment during day 251–304, for the purpose of finding the limit of the membrane performance, the offline cleaning procedure was not conducted for the AnMBR. The highest filtration resistance reached to 105.1  1012 m1 for the membrane without the ultrasound while the membrane resistance in the US-AnMBR was controlled below 23.5  1012 m1. For the US-AnMBR, with the increase of the MLSS concentration, the ultrasound energy input was gradually elevated to effectively control the membrane fouling during day 253–304 (Table 3). Based upon the filtration results, a higher energy was required to maintain long-term filtration under such high concentration. It is well known that higher MLSS concentration aggravates membrane fouling owing to increasing opportunity of cake layer formation. It causes a greater amount of fouling to be controlled. Furthermore, a higher particle concentration produces a greater attenuation of the sound waves as they pass through the crossflow suspension in virtue of increased acoustic impedance. The degree of attenuation is considered to be an important parameter controlling the efficiency of the ultrasonic field (Wakeman and Tarleton, 1991). To alleviate the difficulty of membrane fouling control, the lower VS loading rate was adopted in the subsequent experiment. During day 305–390 (Run 6), as unthickened wasted activated sludge was fed, a shorter HRT of 1.5 days was adopted and thereby the flux permeate was adjusted to 7.0 L/m2h. The MLSS concentration decreased with the lower VS loading rate (Fig. 2e). For the AnMBR, owing to the still high MLSS concentration accumulated from the last run, the membrane fouling rate was so rapid that the resistance ascended to 114.4  1012 m1 on the 307th day, 2 days after the use of the new membrane. During day 305–332, the membrane of AnMBR must be frequently cleaned once every 5 days. Then the tendency of membrane fouling in the AnMBR alleviated as the MLSS concentration decreased and the membrane was cleaned only on the day 374th. Benefited by the previous behavior of membrane cleaning, the membrane resistance for the membrane in the AnMBR showed slight difference in contrast to that in the US-AnMBR after the 338th day. During the same 85 days’ filtration (day 305–390), the membrane fouling was well controlled without any additional cleaning in the US-AnMBR. Finally, the membrane filtration resistance stabilized at a level of below 12.0  1012 m1. The overall results indicated that severe membrane fouling occurred in the conventional AnMBR when applied to sludge digestion. The membrane with the ultrasound showed lower filtration resistance. It demonstrated the feasibility of the ultrasonic

Table 3 Ultrasound parameters for the US-AnMBR. Membrane

M1 M2 M3 M4 M5 M6 M6 M6 M7 M8 M8 M8 M9

Day

1–26 27–47 48–77 78–112 113–156 157–207 207–216 216–225 251–252 253–263 264–271 272–304 305–390

Ultrasound parameters Power intensity (W/cm2)

Time mode (min/min)

Energy (kJ/h)

0.18 0.18 0.18 0.18 0.18 0.24 0.4 0.5 0.5 0.18 0.3 0.5 0.4

3/60 5/60 5/60 5/60 4/60 3/60 2/60 2/60 1/10 1/10 1/10 1/10 1/10

16.2 27.0 27.0 27.0 21.6 21.6 24.0 30.0 90.0 32.4 54.0 90.0 72.0

Flux (L/m2h)

MLSS (g/L)

1.3 1.3 1.7 1.7 2.5 2.5–3.5 3.5 3.5 3.5 3.5 3.5 3.5 7.0

16.7 ± 3.0 25.8 ± 3.1 24.4 ± 3.0 24.8 ± 3.8 29.3 ± 5.6 40.5 ± 4.5 53.2 ± 3.4 60.9 ± 2.1 23.8 ± 1.4 36.2 ± 6.2 52.9 ± 4.3 81.6 ± 12.8 71.6 ± 16.3

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Fig. 4. Total membrane filtration resistances for the two systems.

approach for membrane fouling control in AnMBR when applied to sludge digestion. With the increase of the MLSS concentration, the ultrasound energy input must be elevated accordingly to control the membrane fouling. 4. Conclusions (1) At a high VS loading rate of 2.7 gVS/Ld, the US-AnMBR achieved an average of 51.3% VS reduction. This lab-scale study shows that US-AnMBR is expected to be feasible and superior for sludge digestion. (2) The ultrasound intentionally used for membrane fouling control did not affect the anaerobic microorganisms negatively. The ultrasound enhanced sludge disintegration in the US-AnMBR, resulting in the slightly improved digestion performance. (3) The membrane fouling in the US-AnMBR was controlled effectively by the ultrasound. It ensured the successful operation of the system. As the MLSS concentration increased, higher ultrasound energy must be adopted to control membrane fouling.

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