Article pubs.acs.org/est
Influence of Different Mesh Filter Module Configurations on Effluent Quality and Long-Term Filtration Performance Christian Loderer,* Anna Wörle, and Werner Fuchs University of Natural Resources and Life Sciences, Vienna, Department for Agrobiotechnology, IFA-Tulln, Institute for Environmental Biotechnology, Konrad Lorenz Str. 20, 3430 Tulln, Austria ABSTRACT: Recently, a new type of wastewater treatment system became the focus of scientific research: the mesh filter activated sludge system. It is a modification of the membrane bioreactor (MBR), in which a membrane filtration process serves for sludge separation. The main difference is that a mesh filter is used instead of the membrane. The effluent is not of the same excellent quality as with membrane bioreactors due to the much lager pore sizes of the mesh. Nevertheless, it still resembles the quality of currently used standard treatment system, the activated sludge process. The new process shows high future potential as an alternative where a small footprint of these plants is required (3 times lower footprint than conventional activated sludge systems because of neglecting the secondary clarifier and reducing the biological stage). However, so far only limited information on this innovative process is available. In this study, the effect of different pore sizes and different mesh module configurations on the effluent quality was investigated varying the parameters crossflow velocity (CFV) and flux rate. Furthermore the long-term filtration performance was studied in a pilot reactor system and results were compared to the full-scale conventional activated sludge process established at the same site. The results demonstrate that the configuration of the filter module has little impact on effluent quality and is only of importance with regard to engineering aspects. Most important for a successful operation are the hydrodynamic conditions within the filter module. The statement “the higher the pore size the higher the effluent turbidity” was verified. Excellent effluent quality with suspended solids between 5 and 15 mg L−1 and high biological elimination rates (chemical oxygen demand (COD) 90−95%, biological oxygen demand (BOD5) 94−98%, total nitrogen (TN) 70−80%, and ammonium nitrogen (NH4-N) 95−99%) were achieved and also compared to those of conventional activated sludge systems. Regarding the air requirement for module aeration, which is the main cost factor in MBR technology, an astonishing optimization could be achieved. During the long-term filtration experiments only 4 N m3/m3 was necessary to keep a stable filtration process for more than 3 weeks (MBR 20−50 N m3/m3).
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INTRODUCTION The activated sludge process is the most commonly used biological wastewater treatment system. However, due to the limitations of the solid−liquid separation by gravitational settling the sludge concentration is only in a range of about 5 g L−1. One common problem is the bulking sludge which leads to poor sludge settling and bad effluent quality.1 On account of this drawback the membrane bioreactor (MBR) has become one of the most promising newer technologies. Not only is higher effluent quality achieved by replacing the settling tank with a membrane filtration unit, but also the reduction of the footprint makes the MBR an attractive process.2,3 Unfortunately, there are some shortcomings, which limit the acceptance of MBR in wastewater treatment. These include increased energy demand (50% of operational costs), high prices for membranes, and membrane fouling (weekly chemical maintenance cleaning).3 Cake formation is unavoidable in cross-flow filtration. However, this phenomenon can be exploited innovatively by forming a purpose-built dynamic membrane. Using such an approach, membranes can be replaced by a coarse-pore filter with a lager pore size than in micro- or ultrafiltration.2 The potential benefit of the dynamic membrane is that it can be formed using inexpensive materials. Once the filter is fouled, © 2012 American Chemical Society
the deposited layer can be removed and a new cake layer can be established.4 Such filters were applied originally for sludge thickening. Recently its application was expanded to wastewater treatment in conjunction with the activated sludge process. It has the following advantages: (i) low cost filter material compared to membranes,5 (ii) high filtrate flux rates and low filtration resistance,6 and (iii) low energy requirement compared to MBR technology.7 Preliminary investigations in our laboratory showed that the effluent quality varies with different operation conditions. The critical issue is the formation of a cake layer on the filter surface that acts as the dynamic membrane. Key parameters for achieving high quality effluent are shear stress at the filter surface, the suction rate, and the suspended solids content. Mesh filtration is gaining certain interest as a low cost version of MBR applications and the number of related publications is increasing. That innovative process is seen as an alternative treatment process for CAS systems that are limited in footprint. Received: Revised: Accepted: Published: 3844
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However, little is known about the ideal operation conditions and the most suitable module configuration. Therefore, in this study, two types of module configuration were tested (inside− out and outside−in modules). In a subsequent experiment in a 500-L bioreactor treating actual municipal wastewater, the suitability of the innovative system for long-term operation was tested and compared to the local full-scale activated sludge plant.
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MATERIALS AND METHODS Filter Material. All filter materials tested were made of woven polyester filaments in plain weave (wires running parallel to the length of textile fabric pass alternately over and under the wires running traversal through the fabric). They differed in the size of the mesh openings which were in the range of 25 to 140 μm. Screening Tests. To identify the most suitable filter material a simplified test procedure was developed where mesh materials made of polyester with different openings were compared. Experiments were conducted using a cylindrical beaker with a plug valve installed at the bottom. The beaker was filled with 500 mL of sludge. Then the opening was covered with the filter fabric (0.005 m2) fixed by a plastic ring. After that the vessel was flipped, so the closed valve was at the top. As soon as the valve was opened, atmospheric pressure was applied and the filtration started. Filtrate was collected in a flask situated on a balance. The total amount of filtrate was 200 g, corresponding to a volume of 200 mL. After each 50 mL the flask was exchanged. The filtration time was noted and the turbidity of each sample was determined with a turbidimeter (IR 3000, MERK Company). Because suspended solids (SS) are the more commonly used parameter in wastewater treatment, the relationship between turbidity and SS content was calculated. For this purpose more than 80 samples were collected during the continuous operation of the 500-L plant described later. Both parameters were determined, and based on the results a linear correlation was calculated. One turbidity unit (NTU) corresponded to 1.38 mg/L SS (R2 = 0.92). For the screening tests, recirculation sludge from the wastewater treatment plant (WWTP) in Tulln was taken. During the development of the test method it was found that the sludge had to be diluted to obtain reasonable filtration times and more reproducible results (a too thick filter layer was built up to quick, which falsified the gained results). Therefore the SS content was fixed at 1.5 g L−1 through dilution with a 0.9% (w/w) NaCl solution. Tests with Different Module Configurations. Whereas the initial screening test provided a first estimate of the effect of pore size on the filtration performance, it is obvious that the chosen test procedure does not resemble the actual conditions in a later filter configuration. Therefore in a second approach two types of filter modules were tested under submerged conditions where a cross-flow was generated through coarse bubble aeration. Figure 1 shows the two module types used. The modules were constructed by MEMOS, Germany, and employed mesh filters in a tubular configuration. All modules were made using the same type of filter material as for the screening tests. The module for inside−outside filtration (Figure 1a) resembled the typical design for tubular cross-flow modules. It consisted of parallel filter tubes encased in a tubular shell for filtrate collection (distance between rigid filter tubes was ∼2 mm). The module for outside−inside
Figure 1. Schematic drawing of the test modules. Operation modes: (a) inside−outside and (b) outside−inside.
filtration (Figure 1b) required a different design. It consisted of filter tubes potted with their ends into two permeate collectors, in the form of five concentric rings interconnected through radial channels. The openings in the permeate collectors (distance between two rings ∼1 cm) allowed the passing of the air sludge mixture under cross-flow operation. Spacers, within the filter tubes (distance between filter tubes ∼3 mm), were used to prevent the collapse of the tubes during filtrate withdrawal. The whole construction was encased by an outer shell. In both systems filtrate flow was controlled by a peristaltic suction pump (Watson Marlow, 505U). An overview of the technical data of the two modules used is presented in Table 1. Table 1. Technical Data of the Two Filtration Modules unit number of filter tubes per module inner diameter of filter tubes effective length of filter tubes total module length filter area filter surface packing densitya a
inside−out module
outside−in module
-
55
186
mm
8.0
8.0
mm
450
190
mm m2 m2filter surface/m3
535 0.64 3.2
210 0.99 3.3b
reactor
For these experimental cases. bm2filter surface/m3nitirification zone.
For module aeration a disk membrane diffuser (AFD 270, Alpin Abwassertechnik, Austria) with the same diameter as the module was placed below the filter module. In case of the inside−out module, which was smaller in diameter, a self-made diffuser of the correct dimension was installed. Module aeration rates were varied in the range of 1−15 L min−1. Tests were carried out in a 200-L tank with recirculation sludge from the CAS Tulln at a SS concentration of 8−9 g L−1 and at ambient temperature. The modules were operated at constant filtrate flow conditions of 70 and 150 L m−2 h−1, respectively, and the module aeration rates were varied in the range of 1−15 L min−1. That leads to an air requirement during the experiments of 0.5−20 N m3air/m3produced permeate compared to the typical 15−50 N m3/m3 needed for MBR. Drawn filtrate was recycled to the tank to maintain constant sludge 3845
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total nitrogen, ammonium nitrogen, and phosphate were determined with commercially available photometrical test kits (Dr. Lange, test kits and digital photometer DR 2800). The absorbance at 254 nm was measured with a standard photometer (Perkin-Elmer, LambdaBio). The BOD5 concentration was measured by the respirometric method (WTW, OxiTop System). The concentration of the total solids was determined with an automatic moisture analyzer (Sartorius, MA35). Suspended solids were analyzed after filtration through a glass fiber filter and subsequent drying according to the standard protocol (DIN EN 872, H33). The principle parameters were determined twice a week.
concentration. Each test run lasted about 30 min, and turbidity and pressure loss were determined. After each trial the modules were mechanically cleaned by intensive flushing of the mesh surface with a water jet. Long-Term Filtration Test. Mesh Filtration Bioreactor. In the final experiment the most suitable module type was tested under practical conditions in a wastewater treatment test facility. Figure 2 shows the plant with a nitrification/
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RESULTS AND DISCUSSION Screening Test. The results of the first screening tests are shown in Figure 3. For 11 meshes with different opening sizes
Figure 2. Test plant for the long-term experiment.
denitrification process implemented in a total working volume of 500 L. The denitrification volume is 1/3 of the total volume. The filtration module including the aeration system was submerged in the nitrification zone. Independently from module aeration, a membrane diffuser was installed for sludge aeration (EMR 6.4 mol, Envicon, Germany). Filtrate flow was measured with an electromagnetic flow meter (EMF) (Endress&Hauser, Germany). The flow rate was automatically adjusted to a preset value through regulation of the filtrate suction pump. To allow a constant filter flow independently of the wastewater feeding rate, part of the filtrate was recycled into the reactor. For this purpose a level switch (L) was positioned in the aeration tank. It controlled an automatic valve installed in the filtrate suction line. The suction head was continuously monitored by a pressure inducer (−1 to 0 bar, Wika, Germany). In the long term experiment filtration was executed in a semicontinuous mode. Automatically every 30 min a backflush step was performed. For this procedure, the filtration was stopped and the module was backwashed with filtrate (3 flushes for 5 s each, applied in reverse direction at approximately the triple suction rate). Subsequently normal filtration was restarted. All data were fed into an automatic controller (FX2N Mitsubishi, Japan) which was connected to a PC for parameter setting and data storage. For feed, wastewater from the local WWTP in Tulln was collected after presettling. Throughout the experiment the SS concentration in the reactor was maintained between 7 and 9 g L−1. The volumetric loading rate during the test run was in a range of 4.5−7.2 kg COD m−3 d−1) and the module aeration was set to 10 L min−1. Analytical Methods. The typical parameters for the characterization of wastewater streams were determined in order to assess the effectiveness of the treatment process. COD,
Figure 3. Results of the initial screening tests using 11 different pore sizes (25−140 μm).
the effluent turbidity was measured within filtrate samples of four times 50 mL, which corresponds to four times 10 L/m2 filter surface. In general and as expected, increasing the pore size increased the turbidity. For the first 50 mL of filtrate, effluent turbidity reached values in the range of 8−12 NTU for mesh openings up to 47 μm. In contrast, pore sizes bigger than 55 μm led to a notable decrease of effluent quality apparent from a turbidity increase up to 600 NTU. However, the difference in quality was more pronounced for the initial 50 mL of filtrate of each test whereas for the subsequent samples this effect was much smaller. In case of the last sample turbidity was always around 1 NTU. The obvious reason is that a certain amount of filtrate is required to build up the filter cake that subsequently acts as the actual filter layer. The findings are in line with other data reported. Kiso et al.5 tested three textile materials in the range of 100−500 μm. Mesh sizes of 500 and 200 μm provided unsatisfactory results and high turbidity. In contrast a mesh size of 100 μm performed much better and after 10 min of filtration NTU values lower than 10 were obtained. The final turbidity after 50 min was around 2 NTU. Pore sizes in the lower range, 15−100 μm, were compared by Wu et al.8 In terms of turbidity best results were obtained with the smallest pore size. But probably due to the retention of very fine particles, they observed formation of a gel layer at the 15 μm mesh, which led to significant flux decline. Also in our experiments filtration times almost doubled comparing the material with the smallest and the biggest mesh opening. Generally, for our experiments, it was concluded that a 3846
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mesh size up to ∼50 μm is most suitable but too small mesh sizes are probably disadvantageous with regard to long-term filtration stability. Experiments with Submerged Modules. Based on this preliminary experience, for the following experiments, two types of filter materials with mesh openings of 30 and 47 μm were chosen. Although it would have been interesting to investigate filters with smaller mesh openings, these materials were not available in sufficient quantity for the production of test modules. As described, two different tubular module configurations, inside−out and outside−in, were designed. The main focus was on the influence of flux rate and cross-flow velocity (CFV) on the filtration behavior. The applied air rates were 1 to 15 L/min. With the given module geometry the superficial gas velocities can be calculated. The superficial velocity is the artificial gas velocity calculated as if the given gas phase were the only one flowing in the given cross sectional area of the module,9 i.e., gas flow divided by the open cross section area inside the tubes (for the inside−out module) or between the tubes and the outer shell (outside−in module). According to that, the superficial gas velocities were 0.006 to 0.09 m s−1, for the inside−outside module. Due to the larger diameter, for the outside−inside module the corresponding values were ten times lower, 0.0006−0.009 ms−1. In addition, the actual velocity of the sludge/air mixture was measured directly at the top of the module using a vane anemometer (MiniAir2, Schiltknecht). The relation between aeration rates and actual sludge/air velocity is shown in Figure 4.
Figure 5. Results of the average effluent turbidity after 30 min filtration tests using the inside−outside filtration module (left: flux rate of 70 L m−2 h−1; right: flux rate of 150 L m−2 h−1).
reported by Ozaki and Yamamoto.10 They investigated sludge layer formation on a flat sheet membrane in an MBR and observed that shear stress induced by aeration intensity played the most significant role on sludge accumulation on the membrane surface. As mentioned, the mesh size played a less important role. Nevertheless, its influence on effluent turbidity was more pronounced when higher CFVs were applied. At a specific flux rate of 70 L m−2 h−1 and a CFV of 0.10 m s−1 a 30% decline in effluent quality was observed when the pore size increased from 30 to 47 μm. With decreasing CFV the difference in turbidity became smaller and the effect even reversed at very low CFV. The reason for the latter observation remains unclear. It is also remarkable that doubling of the flux to 150 L m−2 −1 h had only little effect on the effluent turbidity. However, it was noticed that the higher flux rate led to smaller deviation of effluent turbidity between the two mesh sizes. This can be explained by the fact that higher flux rates led to a higher convective flow of particles to the membrane surface. Hence, more particles accumulate and a thicker and more stable cake layer is formed. It is interesting to note that under all conditions stable filter performance with a nearly constant filter pressure was achieved. This is in contrast to results obtained by Iversen et al.11 who tested 22 different textile materials for their suitability to act as a membrane substitute in MBR technology. But in their investigations, besides module aeration an active forward flow of 0.2 m s−1 by means of an external pump was applied. Generally high turbidities, instable flux rates, and a high increase of the filter pressure are reported. Evidently, the configuration tested here where sludge recirculation is induced only by the raising air bubbles (air lift) is more suitable for the development of an effective dynamic membrane. That can be attributed to the fact that actual sludge velocities occurring in our experiments are significantly lower than in the experiments of Iversen and co-workers.11 Performance of the Outside−In Modules. The implementation of air sparged tubular modules, as tested above, has been proposed for side stream MBR applications.12,13 Nevertheless, the usage of such modules in submerged systems seems problematic due to the voluminous outer casing and the resulting high space demand within the aeration tank. Therefore the second module type with the outside−in configuration was developed. Modules of similar typebut with membranes as a filter materialare already in commercial use for MBR applications. The 30 μm mesh, being the best performing filter material from the previous tests, was chosen for this module.
Figure 4. Mean velocities of the liquid/air mixture measured on the top of the modules. Tests were conducted in clean water (bullets) and in activated sludge (squares).
Performance of the Inside−Out Modules. The findings obtained are summarized in Figure 5. It was observed that under the investigated conditions the aeration rate (that determined the cross-flow velocity) was of much higher influence than the mesh size. The lowering of the CFV from 0.10 to 0.05 m s−1 led to an approximately 50% decrease in effluent turbidity. Further reduction to 0.02 m s−1 even resulted in a 4-fold drop of the effluent turbidity. The reason for a higher turbidity at elevated CFV lies in the less favorable conditions for the build-up of the cake layer. Its growth is hindered by an increased shear stress which enhances the backtransport of particles into the bulk fluid. Apparently, the dynamic membrane acts in a similar way as a deep bed filter where smaller particles are entrapped in the filter bed. Consequently, the thicker the cake layer the better the particle removal capacity. These results are consistent with the findings 3847
dx.doi.org/10.1021/es204636s | Environ. Sci. Technol. 2012, 46, 3844−3850
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Figure 6. Results of the outside−in module filtration tests: left: impact of CFV on average turbidity; right: course of turbidity throughout the 30 min experiment.
Figure 7. Typical example for the filtration performance over a period of 3 weeks (CFV 0.1 m s−1; flux 150 L m−2 h−1), black line: flux; gray line: filter pressure.
As lower aeration rates improve the effluent quality, initial trials were made with an air flow of 1−5 L min−1. However, it turned out that these low aeration rates are not sufficient for long-term operation of the module. Partial blocking of the interspace between the filter tubes due to the agglomeration of sludge was observed. Therefore it was necessary to increase the aeration rate to 10 L min−1. The plant was operated for a total period of 6 months, including 3 months where the operation of the plant was stepwise improved and 3 months under almost constant operation conditions. Typically, it was possible to continuously operate the module for around 3 weeks without any extra measures. Moreover, as discussed later the achieved effluent quality was better then expected from the short-term experiments. A typical example of the long-term filtration performance is presented in Figure 7. The flux was continuously operated at the set point of 150 L m−2 h−1 and an almost constant filter pressure of about 30 mbar was observed. The periodic drop-down of the flow rate as seen in Figure 7 is a result of the back-flush procedure. It should also be noted that much of the observed filter pressure can be attributed to friction losses in the permeate collector and the connecting pipes. According to previous investigations the head loss at the filter surface is less than 10 mbar.6 Despite the implementation of a backflush procedure, after a certain period clogging of the filter surface occurred, announced by a rapid increase of the TMP and a corresponding flux decline. Similar observations were already made in earlier
Generally, for the outside−in module similar observations, as described before, were made (Figure 6). By lowering the CFV a better effluent turbidity was gained with best values at a CFV < 0.06 m s−1 (Figure 6a). Accordingly, regarding effluent quality no major difference between the behavior of the two module configurations was observed. In these trials, not only was the averaged effluent turbidity measured but it was continuously monitored throughout the 30 min filtration period. Therefore the course of the NTU development could be followed more in detail (Figure 6b). Long-Term Experiments in a Bioreactor. To gain more information on long-term filtration performance under practical conditions, the test run in a treatment plant was carried out. In the same experiment, the removal performance of the whole process regarding standard wastewater parameters was monitored. The intention was to compare the mesh process with commonly used wastewater treatment systems. For these investigations the outside−in tubular module was used. This module type was selected for two reasons. On the one hand and as already mentioned, this module configuration is more practicable for use as a submerged filtration system. On the other hand, this configuration allowed the implementation of a periodic backflushing procedure. The mechanical stability of the mesh tubes against pressure applied outside is very low and therefore flow reversion in the inside−out module was not possible. 3848
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experiments with flat sheet modules by Fuchs et al.6 and are also described by Moghaddam et al.2 where also flat sheet modules were used. In contrast, Kiso et al.5 reported permanent filtration without blocking effects for several months at flux rates in a range of 20−40 L m−2 h−1. Nevertheless they also mentioned filter blocking as a critical issue. Whenever blocking occurred the module was removed and chemically cleaned in an external vessel. Two cleaning steps, 0.5% citric acid for 1 h, and hypochlorite 1000 ppm active chlorine for 2 h, were applied. Subsequently the module was flushed with water, repositioned in the aeration tank, and the filtration was restarted. Even though several cleaning actions were applied to the same module no significant change of filtration behavior was observed. Generally, the typical filtration behavior can be divided into three characteristic phases, as depicted in Figure 8: a start-up phase, a stable filtration phase,
mechanically destroyed and squeezed through the mesh filter. Moreover, the high local flux rates accelerate the blocking of the remaining mesh openings and ultimately, a tight coverage of the mesh surface occurs. Regarding the removal capacity of the complete process, it can be pointed out that the effluent quality was very good with exception of a few samples from the clogging period. The parameters COD, TN, and NH 4 −N as well the SS concentration in the effluent (2 h mixed samples) were similar to or even below the values achieved in the conventional full scale activated sludge systems (Table 2). Table 2. Results of the Relevant Wastewater Parameters (Inlet and Effluent) during the Long-Term Experiment Compared to the CAS parameter
unit
inlet
effluent MESH
effluent CAS
COD BOD5 NH4−N NO3−N TN SS
mg/L mg/L mg/L mg/L mg/L mg/L
300−600 150−350 15−55 30−70 300−500
15−30
