Feasibility Study of Indigenously Developed Fly Ash Membrane in Municipal Wastewater Treatment

Available online at www.sciencedirect.com

ScienceDirect Aquatic Procedia 4 (2015) 1492 – 1499

INTERNATIONAL CONFERENCE ON WATER RESOURCES, COASTAL AND OCEAN ENGINEERING (ICWRCOE 2015)

Feasibility Study of Indigenously Developed Fly Ash Membrane in Municipal Wastewater Treatment ManeeshNamburatha, GanapatiJoshia, MuraliCholemaria, ChandrahasSheta, SreekrishnanT Rband SrinivasVeeravallia,* a

Department of Applied Mechanics, Indian Institute of Technology Delhi, New Delhi, India, 110016, b Department of Biochemical engineering and Biotechnology, Indian Institute of Technology Delhi, New Delhi, India, 110016

Abstract Afly ash membrane developed by The Energy and Resources Institute (TERI), New Delhi was studied for its applicability in municipal wastewater treatment. In particular, the effect of aeration on preventing membrane fouling was studied. The velocity field generated by aeration was studied to understand how the rising bubbles would efficiently scour the membrane filter and prevent fouling. Particle image velocimetry was used to monitor the air bubble movement along the membrane surface. The optimal reactor configuration (membrane module orientation) for which the aeration would impart maximum shear over the membrane was determined using potable water. This reactor configuration was later used for the biological treatment of synthetic wastewater. The second aspect of the study involved designing a support system to improve the strength of the membrane. Membrane modules without any internal support were able to withstand trans-membrane pressures (TMP) up to 270 mmHg. Two types of frames/seperators were used to increase membrane strength. In one type of the frame, support was unidirectional and in another, bidirectional. Bidirectionallysupported membranes were able to withstanda TMP of 760mmHg for a period of 7 days. At a constant filtration rate, a membrane bioreactor with more than one membrane in parallel operation was able to delay the fouling process than in a single membrane system due to lesser pressure across the membranes. As expected, membrane fouling took longer time in the systems operated at higher air flowrate due to better scouring action of the air bubbles. © Published by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license ©2015 2015The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015. Peer-review under responsibility of organizing committee of ICWRCOE 2015 Keywords:Fly ash membrane; municipal wastewater treatment; fouling reduction; aeration

* Corresponding author. Tel.: +91-11-26591182; Fax: +91-11-26591182. E-mail address:[email protected]

2214-241X © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015 doi:10.1016/j.aqpro.2015.02.193

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1. Introduction The membrane bioreactor (MBR) used in this study was a continuous flow stirred-tank reactor (CSTR)with a membrane unit to retain biomass. The MBR retains the biomass inside the system and helpsmaintain high biomass concentration. It also produces good quality effluent and occupies a smaller area compared to conventional activated sludge process(Devendraet al., 2008,Santos et al., 2011,Reeta Rani et al., 2012). Widespread use of membrane technology in the area of wastewater treatment has been hindered by its disadvantages like fouling, cost of membrane and operational complexicity(AnjaDrews, 2010,Kraemer et al., 2012). Fouling is the main problem with all MBRs and there is no way to eliminate fouling completely. To limit fouling, different methods like adsorbents andalteration of biomass characteristicshave beenattempted (Pierreet al., 2006). Increasing the surface shear on the membranes helps in reducing fouling and techniques like turbulence promoters, pulsatile flow and vortex generation have also been found to be effective. Aeration is the commonly used method for the removal of foulants from membrane surfaces in a MBRfor aerobic treatment processes(Pierreet al.,2006, ZF zuiet al., 2003). Use of computational fluid dynamics (CFD)and experimental techniques such as particle image velocimetry(PIV) has also been introduced to study the effect of hydrodynamics to optimise MBR design (Braakaet al., 2011). Membrane cost is another important parameter to consider while using a MBR.Fly ash generated during combustion of coal is a waste byproduct.Membranes developed using fly ash helps in managing the waste generated and also bring down the cost of the membrane. Fly ash membranes are ceramic membranes, which have many advantages such as high thermal and chemical stability, pressure resistance, long lifetime, and catalytic properties from their intrinsic nature(Jedidiet al., 2009).In the present work, a microfiltration membrane made of coal fly ash was used to study its efficacy in treating municipal wastewater. The effect of aeration, which was used to supply oxygen to the system, on reducing fouling in the membrane was studied.PIV wasused to study the movement of air bubbles along the surface of membrane to estimate its effectiveness in preventing membrane fouling. Modifications in the membrane scaffold was studied to enhance membrane strength. 2. Method and Materials Fly ash membranes (15cm x 15cm and 10cm x 10cm) manufactured by The Energy and Resources Institute (TERI),New Delhiwas used in the study. The work was conducted in two phases. The first phase used pure water as the medium and involved an investigation of the velocity field generated by the rising bubbles used for aeration. The objective here was to determine the optimal configuration whereby the bubble flow would efficiently scour the membrane filter and prevent fouling. Experiments were conducted in a glass tank of size 38.6cm x18.1cm x19.8cm, filled with 13 L water. The experimental setup is shown in Figure 1. The bubble source was placed at the bottom of the tank and it was connected to an aerator and the total airflow was monitored with the help of a rotameter placed online. A high speed camera (Mega Speed MS50K) was used to capture the details of the bubble flow. Uniform lighting was obtained using two halogen lamps (100W, 12VDC). Bubble velocities were calculated using the Particle Image Velocimetry (PIV) technique with an in-house code. The details of the parameters used for PIV are given in Appendix A. Once the optimal operating parameters were obtained from the efficacy of the system was tested in the presence of biomass, in a lab-scale bioreactor operating on synthetic wastewater. Experiments were conductedin the same glass tank as above. Airflow was monitored using a rotameter (Japsin, range 0 to 10 lpm) attached to the air line. The dissolved oxygen was monitored using a DO probe (Sonde DN19) immersed in the reactor. The filtered water was extracted from the module by means of a peristaltic pump (AttoSJ 1220) and vacuum gauges (H.Guru) were used to monitor the trans-membrane pressure. 3. Results and Discussion 3.1 The velocity field The filter module was suspended in the water tank as shown in Figure 1. A glass plate was placed at a small

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distance from the filter and parallel to it inorder toforce the passage of air bubbles close tothe filter surfacethereby enhancing the shear close to the filter module. Experiments were conducted for 3 different configurations with a view to obtain a uniform flow of bubbles over the filter surface. x x x

Configuration A: The membrane filter was placed directly on the bubble source. Configuration B: The filter was placed directly on the bubble source and two additional glass end plates were attached to the filter. Configuration C: Configuration B was further modified to create a gap of 10mm between the filter bottom and the bubble source. (This configuration is shown in Figure 1.) Filter

Light source

Front guide plate Side guide plates Glass tank

ROTAMETER

Bubble

COMPUTER

source CAMERA

AIR PUMPS

Light source 1

Fig. 1 Schematic view of the experimental setup.

Figure 2 shows the velocity fields observed for the three different configurations. In Figure 2(a) we see that a large re-circulating flow is set up near the top-right corner of the filter. The velocity magnitudes and hence the shear is very small in the re-circulating region. Figure 2(b) shows that the presence of the end plates effectively remove the re-circulating region; however, the flow is still not uniform in the gap between the filter and the guide plate. Figure 2(c) shows an essentially uniform flow field. Thus configuration C achieves the required objective of a uniform scouring flow over the filter surface. Hence, further quantitative analysis was carried out only with configuration C. The velocity field obtained for configuration C was analysed to estimate the shear available at the filter surface for scouring biomass away from the filter surface. If we assume that the velocity gradient in the water at the filter surface scales with the bubble velocity gradient then it is quite straightforward to estimate the shear as the average value of Vb(x)/(d/2). Here, d is the gap between the front guide plate and filter surface and Vb(x) is the averagebubble velocity. Various gaps between the filter surface and the front guide plate (15 mm, 10 mm, 8 mm, 5 mm and 3 mm) were tried to get an optimal gap. The results are shown in Figure 3 for an airflow of 1.96 lpm. We find that the shear has a maximum at the lowest gap of 3 mm. In the actual filtration experiments we expect a film of biomass to be present at the filter surface, thus, it would not be useful to decrease the gap between the filter and the guide plate below 3 mm. Figure 3 shows a comparison of the shear obtained from two different runs with the same configurations and parameters. We see that the results are repeatable to a very high degree

(a): Configuration A

(b): Configuration B

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(c): Configuration C

Shear Rate (s-1)

Fig. 2 Velocity vector plots obtained from PIV for the three different guide plate arrangements. The length of the vector is proportional to the local velocity magnitude (in the plane of the measurements).

d (mm) Fig. 3. Estimate of the shear in the vicinity of the filter surface.

3.2Experiments with synthetic wastewater In the second phase, the efficacy of the membrane was tested in the presence of biomass placed in synthetic wastewater. The membrane modules immersed in the reactor were placed above the aeration source (as shown in Figure 1). Membrane modules of two different dimensions viz. 15 cm x 15 cm and 10 cm x 10 cm were tested. Airflow was monitored using a rotameter attached to the air line, as before. The dissolved oxygen was monitored using a DO probe immersed in the reactor contents. Aeration is required for maintaining the required DO level in the reactor for microbial growth; it also helps in controlling biomass (sludge) deposition on the membrane surface. The composition of the wastewater (0.1% glucose medium) is given in the Table 1. Wastewater was fed into the reactor using a centrifugal pump (Sinex). The flow rate was adjusted to 10 L/d (corresponding to a hydraulic retension time i.e. HRT of 1 day) or 5 L/d (corresponding to HRT of 2 days). Membrane modules were immersed in the reactor and the outlet (permeate tube) was connected to a peristaltic pump (Atto) for withdrawing the treated wastewater. Vacuum gauges were connected in the permeate line to monitor the build-up of trans-membrane pressure (TMP). The parameters that were monitored during the study were: Flow rate (feed and permeate); TMP, airflow rate, dissolved oxygen levels and chemical oxygen demand (COD) of the feed and permeate. The reactor was inoculated using sludge from the activated sludge unit of the Okhla municipal wastewater treatment plant, Delhi. The system was started in a batch mode to acclimatize and grow the inoculum. As synthetic wastewater was used, the acclimatization was rapid and the reactor was ready for continuous mode of operation within 3 days. As the biomass concentration increased in the reactor, it accumulated on the membrane surface. Membrane cleaning by back washing was done after every 24 hours. Excess biomass were removed from the bioreactor to keep the concentrations of 6-7 gram/Litre (dry weight), in the experiments done at constant biomass concentration. 3.3 Membranestrength The strength of the membranes is very important in the operation of MBR. The filter modules consisted of two filter elements separated by a frame. The outlet tube was fitted to the frame. No internal supports were used in the

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first set of experiments. As the biomass concentration increased in the course of operation, there was a corresponding increase in the TMP (Figure 4). Within 24 h of operation, complete COD removal was obtained. The operation was continued till the membrane ruptured. No sludge was removed from the system during the course of this experiment. As seen in Figure 4, the membranes perform slightly better at the lower flow rate (5L/d). However, all the membranes ruptured below 300 mm Hg (TMP). Table 1 Composition of synthetic wastewater Constituents Concentration (g/L) Glucose

1

Yeast extract

0.34

KH2PO4

0.136

K2HPO4

0.234

MgCl2.6 H2O

0.084

FeCl3

0.05

Trans Membrane Pressure (mmHg)

In order to enhance the ability of the filter module to withstand larger TMPs, it was decided to provided internal supports on the frame placed between the two filter elements. Two different configurations were tested, one with bi-directional struts and the other with uni-directional struts (Figure 5). To afford direct comparison under identical conditions, both types of modules were placed in the tank with synthetic wastewater and tested. The flow rate for wastewater was maintained at 10L/d. The results are shown in Figure 6. While both modules were visibly superior to the one without internal support, it appears that the module with bi-directional support (module 1) performs much better than that with uni-directional support (module 2). We note that the experiment had to be interrupted before module 1 ruptured since the TMP had reached the upper limit of our vacuum gauges. Finally, since the purpose of these initial experiments was to push the modules to rupture, guide plates were not used and thus the scouring action, due to bubble flow in the vicinity of the filter surface, was not present.

300

250 200

150 100 50

0 0

20 10litre/Day

40

60

Time (hours) 5litre/day (1)

80

100 5litre/day (2)

Figure 4: Variation of trans-membrane pressure with time for the filter modules without internal support. The experiments were stopped when the filter ruptured. A

D B

C

(a).Bi-directional support

(A=B= C=14 mm) (b). Uni-directional support

Figure 5: Details of the separators used to provide bi-directional and uni-directional support to the filter elements.

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Figure 6.Comparison of performance of filter modules with internal supports. Membrane 1 has bi-directional supports (Figure 5a) while membrane 2 has uni-directional supports (Figure 5b).

3.4 Effect of airflow on fouling The next set of experiments were designed to study the efficacy of bubble flow in reducing membrane fouling. Configuration C which, as discussed in section 3.1, provides uniform bubble flow, was used here. Experiments were done at a flow rate of 10 L/d with two membrane modules in parallel. Air flow rates of 7 L/h and 9 L/h were used. 500 ml of settled sludge was removed daily to maintain approximately 6-7g/L (dry weight) of the sludge in the tank. The results are shown in Figure 7. As seen in Figure 7 the reponse is almost the same for the first 20 hours. The sudden rise in TMP at this time indicates that fouling is not significant before this time. It appears that scouring becomes effective only after the sludge layer reaches a certain minimum thickness. This is why the initial behaviour is the same for both flowrates. After this the behaviour is very different. The TMP slowly builds up at the lower flow rate, while it levels off very quickly for the higher (9 litre/hour) flow rate. It is clear that at the higher flow rate the filter module can be operated for very long periods without cleaning or back-flushing. We note that both flow rates were more than adequate considering the oxygen demand of the bio-reactor. Therefore, in actual operation, the cost of periodic maintenance should be compared with the extra operational cost incurred in providing higher air flow rates. A second series of runs were conducted to determine the optimal gap between the filter module and the front guide plate. In these runs the air flow rate was kept constant at 7L/hour and the wastewater flow was maintained at 10L/day. Since we were interested in enhancing the fouling, sludge was not removed and it was allowed to build up in the tank. Two filter modules were placed in the tank. Three different gaps (3mm, 4mm and 5mm) were tested. The results are summarised in Figure 8. From the development of TMP,it is clear that the performance is best for a gap of 4mm. We note that the experiments with pure water indicated an optimal gap of 3mm or less. The present result indicates that scouring is effective only after the sludge layer build up to a thickness of more than 1 mm.

Trans Membrane Pressure (mmHg)

350

300 250 200 150 100

50 0 0

50

100

150

200

Time (Hours) 9 Litre/ Hour

7 litre/ Hour

Figure 7.Effect of airflow on membrane fouling.

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Trans Membrane Pressure (mmHg)

800 700 600 500 400 300 200 100 0

0

50

100

150

200

250

Time (Hours) 4mm

5mm

3mm

Figure 8. Effect of gap between the filter module and the front guide plate for configuration C . The air flow ratehas been maintained at 7L/hour and the wastewater flow at 10L/day.

4. Conclusions The flow field studies indicate that with a proper arrangement of guide plates around the filter module, and by providing asmall gap between the bubble source and the filter module, we can achieve a uniform flow of bubbles in the vicinity of the filter surface to provide good scouring action. Estimates of the shear at the filter surface due to the bubble flow showed that the optimal gap between the filter surface and the front guide plate appeared to be below 3mm. This is consistent with the later studies in the presence of biomass which indicated that a gap of approximately 4mm is optimal. Flyash membranes are britlleand unsupported membranes cannot withstand even moderate TMP (300 mm Hg). The performance was greatly improved with the use of internal support especially bi-directional internal support. Experiments with synthetic wastewater indicated that with a proper arrangement to guide the bubble flow and with a sufficiently high air flow rate membrane fouling can be kept under check and the system can be operated without interruption for membrane maintenance (say by back flushing). However, the flow rate required is significantly higher than that required to provide the oxygen demanded by the bio-reactor. Thus, in actual practice, the gain in operation time must be offset against the extra cost incurred in providing greater air flow rates. Acknowledgements This project was funded by DST under grant No.: DST/TDT/WTI/2k9/138. We wish to thank Mr. R. P. Bhogal and the other staff members of the Gas Dynamics Laboratory, IIT Delhi for their help in conducting the experiments. Appendix A Table 2.Details of the experimental conditions & parameters used for PIV Sl. Experimental Conditions & Parameters used for Configuration A No. PIV

Configuration B

Configuration C

A

Experimental Conditions

1

Tank size

38.6cm x 18.1cm x 19.8cm

38.6cm x 18.1cm x 19.8cm

38.6cm x 18.1cm x 19.8cm

2

Water in the tank (liters)

13

13

13

3

Air flow rate (lpm)

1.96

1.96

1.96

4

Distance between the front guide plate and tank front-wall

10 cm

10 cm

10 cm

5

Gap between filter bottom side and bubble source

0

0

10 mm

6

Distance between front- glass wall and camera front-surface

90 cm

90 cm

90 cm

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7

Ambient Pressure (cm of Hg.)

73.5

73.5

73.5

8

Ambient Temperature (oC)

30

30

30

9

Side guide plates

No

Yes

Yes

10

Front guide plates

Yes

Yes

Yes

B

Parameters used for PIV

11

Frame rate (FPS)

350

350

503

12

Exposure time (μs)

2000

2000

1988

13

Gain

925

925

925

14

Offset

6

6

6

15

Aperture

4

4

4

16

Image size (pixels)

960 x 960

960 x 960

1280 x 1020

17

Gama

1

1

1

18

Interrogation window size (pixels)

32x32

32x32

32x32

19

Color setting during downloading images from the camera

Grayscale

Grayscale

Grayscale

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