Abstract:
The present invention provides a treating unit of activated sludge for wastewater treatment and a treating apparatus having the same. The treating unit is constructed by a cage-shaped supporting structure which defines an interior space for containing the microbial cell therein. The present invention provides a measure for simultaneous removal of organics and nitrogen compounds from the wastewater under a condition of controlled aeration, and makes the configuration of treating apparatus as well as the treating process more simplified. It is also an alternative to replace the traditional A2O process. According to the present invention, the design of sludge return, which is essential for the conventional activated sludge treatment, is not needed. Moreover, the present invention is compatible with the conventional activated sludge treatment process and is advantageous in its short start-up period during which a stable operation is achievable.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a biological treatment apparatus for wastewater, and more particularly to a biological treatment apparatus for wastewater adopting the activated sludge process. 
     2. Description of the Related Art 
     Biological treatment is a secondary procedure of wastewater treatment, which is typically employed for the wastewater being primarily treated. The wastewater from the primary sedimentation tank is biologically treated and settled, and then discharged. The organics contained in the wastewater is degraded by means of biological metabolism, so as to remove the organics (the so-called biochemical oxygen demand, BOD, and chemical oxygen demand, COD) as well as nitrogen compounds and phosphorous compounds therefrom. 
     Typically, the biological treatment is classified into such as aerobic treatment and anaerobic treatment. The aerobic treatment is carried out to remove the organic carbon from the wastewater and to make the organic nitrogen and ammonia nitrogen bio-oxidize into nitrates. The removal of nitrates from wastewater is carried out by means of anaerobic treatment, which is so-called as denitrification. The microbes employed in biological treatment are consisting of bacteria, fungus, algae and protozoa, where the community thereof shall be well-acclimated for various kinds of wastewater to be treated and substances to be removed. It is so difficult to well-control the treatment efficiency, since the treatment efficiency is highly depending upon the community of microbes and treating condition. In this case, the operation of such process is much complicated and needs technicality. 
     In an activated sludge process, which is one kind of aerobic treatment, wastewater being primarily treated is directed into an aeration tank to contact and mix with the activated sludge therein, and is subsequently directed therefrom into a settlement tank for sludge separation. A portion of the separated sludge is returned into the aeration tank (i.e. the returned sludge) while the remaining portion thereof is discharged to be further processed (i.e. subjected to the so-called sludge treatment and disposal). In addition to the difficulty in acclimation of microbial community, it needs to precisely control a variety of factors and conditions, such as the retention time, the return ratio of sludge, the aeration condition . . . etc. for the activated sludge process, so as to optimize the operation condition and achieve an improved efficiency. 
     Typically, the activated sludge composed of suspended aerobic microbial community is easily peeling off, which may cause the concentration of suspended solids of water and thus the loading of the activated sludge process to significantly increase. Moreover, the peeling-off of microbial aggregation also makes the control of operation condition of such process much difficult. 
     Due to the selectivity nature of biological degradation, it is necessary for the biological treatment to adopt multiple stages of sludge return and wastewater return for controlling the anoxic mode, anaerobic mode and aerobic mode in different reactors or tanks, so as to carry out the nitrification of ammonia nitrogen and removal of organic carbons (e.g. BOD and COD) under the aerobic condition, to carry out the denitrification and removal of nitrate nitrogen under the anaerobic condition. Such process is also termed as an A2O process, which is complicated, land-consuming, and cost-inefficient. 
     On the other hand, the membrane separation process is commonly used in treating the industrial wastewater and municipal wastewater. Some membrane modules, such as microfiltration (MF) modules, are capable of being combined with the biological treatment for the bio-sludge separation in a membrane bioreactor (MBR), so as to obtain the effluent of good quality. Nevertheless, the suspended solids caused by the peeling-off of microbial aggregation as well as the adsorption of sludge floe onto the membrane may result in the membrane fouling, and hence the membrane needs to be frequently replaced or cleaned. Accordingly, such process is not cost-efficient and needs to be improved. 
     SUMMARY OF THE INVENTION 
     It is a first aspect of the present invention to provide a fixable treating unit of activated sludge, which is applicable in a wastewater treatment reactor and capable of eliminating the peeling of microbial aggregations. 
     It is a second aspect of the present invention to provide a biological treatment apparatus capable of removing the organics and nitrogen compounds from wastewater under a controlled operation condition. 
     It is a third aspect of the present invention to provide a biological treatment apparatus in which the design of sludge return is not needed. 
     It is a fourth aspect of the present invention to provide a biological treatment apparatus, whose operation is stabilized in a short period. 
     It is a fifth aspect of the present invention to provide a wastewater treatment system capable of removing the organics, nitrogen compounds and suspended solids from wastewater under a controlled aeration mode. 
     It is a sixth aspect of the present invention to provide a wastewater treatment system, in which the efficiency of membrane treatment is enhanced and the expense thereof is reduced. 
     In accordance with the mentioned aspects, a treating unit for simultaneously removing organics and nitrogen compounds from wastewater is provided. The provided treating unit includes a microbial cell containing conditioned activated sludge and a supporting device constructed by at least two rings located on different planes and having an interior space defined thereby, where the microbial cell is contained in the interior space. 
     Preferably, the microbial cell has a solid content of activated sludge ranged from 10% to 20%. 
     Preferably, each of the rings is made of a thermoplastic material such as polyethylene (PE). 
     Preferably, the supporting device is cage-shaped or sphere-shaped. 
     Preferably, the supporting device is constructed by three rings located on different planes. 
     Preferably, the rings are adhered and fixed to each other by means of thermal melting or other methods. 
     Preferably, the inner diameter of the ring is ranged from 5 mm to 30 mm. 
     Preferably, the inner diameter of the ring is 25 mm. 
     Preferably, the inner diameter of the ring is 16 mm. 
     Preferably, the inner diameter of the ring is 10 mm. 
     In accordance with the mentioned aspects, a biological treatment apparatus for simultaneously removing organics and nitrogen compounds from wastewater is provided. The provided biological treatment apparatus includes a biological reactor containing plural treating units filled therein, and is characterized by that each of the treating units is constructed by a supporting cage and a microbial cell contained in the supporting cage. 
     Preferably, the supporting cage is constructed by at least two rings located on different planes. 
     Preferably, the supporting cage is a sphere-shaped cage constructed by three rings located on different planes. 
     Preferably, the biological reactor is one of a columnar reactor, a rectangular reactor and a fluidized bed. 
     In accordance with the mentioned aspects, a wastewater treatment apparatus including a reactor having plural biological treating units contained therein is provided. The provided wastewater treatment apparatus is characterized by that each of the biological treating units includes a supporting device defining an interior space and a microbial cell contained in the interior space, by which a simultaneous removal of organics and nitrogen compounds from wastewater is carried out under a controlled aeration mode such as a continuous mode or an on/off aeration mode. 
     Preferably, the reactor has a volume filling ratio of the biological treating units of 30% substantially. 
     In accordance with the mentioned aspects, a wastewater treatment is provided. The provided wastewater treatment system includes a biological reactor having a plurality of treating units therein, each of the treating units being formed by a microbial cell and supporting cage containing the microbial cell therein; and a membrane reactor having at least one membrane therein, where the inlet of the membrane reactor is fluidly connected to the outlet of the biological reactor. 
     Preferably, the biological reactor is one of a columnar reactor and a rectangular reactor. 
     Preferably, the rectangular reactor includes at least one treating case containing the treating units. 
     In accordance with the mentioned aspects, a treating apparatus for simultaneously removing organics, nitrogen compounds and suspended solids from wastewater is provided. The provided treating apparatus includes a reactor containing the wastewater to be treated therein; a plurality of treating units distributed in the wastewater within the reactor, each of the treating units being formed by a microbial cell and a supporting cage containing the microbial cell therein; and at least one membrane module configured in the reactor and dipped in the wastewater, wherein the organics and nitrogen compounds are removed from said wastewater by said treating units under a controlled aeration mode, and wherein the suspended solids are filtered from the wastewater by the at least one membrane module. 
     Preferably, the treating units are fluidizedly distributed with the wastewater in the reactor. 
     Preferably, the reactor includes at least one treating case, and wherein the treating units are contained in the treating case. 
     Preferably, the treating case and the membrane module in the reactor are arranged in series. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed descriptions and accompanying drawings, in which: 
         FIG. 1  is a schematic view showing the treating unit for wastewater according to a preferred embodiment of the present invention; 
         FIG. 2  is a flowchart illustrating the procedures for fabricating the treating unit for wastewater according to the preferred embodiment of the present invention; 
         FIG. 3A  and  FIG. 3B  are schematic views showing the wastewater treatment apparatus according to a preferred embodiment of the present invention; 
         FIG. 4A  and  FIG. 4B  are schematic views showing the wastewater treatment apparatus according to another preferred embodiment of the present invention; 
         FIG. 5  is a schematic view showing the wastewater treatment apparatus according to a further preferred embodiment of the present invention; 
         FIG. 6  shows the treating efficiency for synthesized influent with the columnar reactor according to the present invention; 
         FIG. 7  shows the treating efficiency for actual wastewater with the columnar reactor according to the present invention; 
         FIG. 8  is a schematic view showing the wastewater treatment system with the membrane module according to one preferred embodiment of the present invention; 
         FIG. 9  is a schematic view showing the wastewater treatment system with the membrane module according to another preferred embodiment of the present invention; 
         FIG. 10  is a schematic view showing the wastewater treatment system with the membrane module according to one another preferred embodiment of the present invention; 
         FIG. 11  shows the treating efficiency for synthesized influent with the wastewater treatment system with the membrane module according to the present invention; 
         FIG. 12  shows the permeate flux for synthesized influent with the wastewater treatment system with the membrane module according to the present invention; and 
         FIG. 13  shows the treating efficiency for actual wastewater with the wastewater treatment system with the membrane module according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the following disclosures combined with the accompanying drawings, the treating unit and apparatus having the same according to the present invention are illustrated and understood. It should be noted that the elements shown in the drawings are merely provided for illustration, but not limitation to the present invention, and the elements known by the skilled person in this art are omitted from the drawings for clarity. 
     Please refer to  FIG. 1 , which schematically shows the treating unit for wastewater according to a preferred embodiment of the present invention. The treating unit  10  is constructed by a microbial cell  12  and a supporting device  14 . According to the present invention, the microbial cell  12  is composed of conditioned activated sludge, while the supporting device  14  is a supporting cage having an interior space defined therein. The microbial cell  12  is contained in the interior space, and is retained therein with the aid of supporting cage of the supporting device  14 . In a specific embodiment, the supporting device  14  is a sphere-shaped cage constructed by three rings  142 ,  144  and  146  that are located on different planes and intersecting with one another. The three rings  142 ,  144  and  146  are made of thermoplastic material such as polyethylene (PE), and are adhered and fixed with one another by means of thermal melting. According to the present invention, the inner diameter of the respective rings is ranged from 5 mm to 30 mm, and is preferably 25 mm, 16 mm or 10 mm. 
     In the preferred embodiment of the present invention, the microbial cell is prepared from the acclimated and conditioned activated sludge, obtained from the bottom of secondary settling tank of an actual wastewater treatment plant, by using the procedures as shown in  FIG. 2 . The prepared microbial cells are contained and supported by respective supporting devices, whereby the treating units for wastewater according to the present invention are fabricated. 
     In step  21 , the solid content of activated sludge obtained from the bottom of settling tank is adjusted, by means of decanter centrifuge, to a value ranged from 10% to 20%. According to a preferred embodiment of the present invention, cellulose triacetate (CTA), a water-permeable polymer, is used as a gel material to confine the migration of microorganism as a microbial cell. In step  22 , an organic solution containing 100 gram of CTA dissolved in 1 L of methylene chloride is prepared, and is stirred by stir bars for 4˜5 hours. In such case, the concentration of the prepared CTA/methylene chloride organic solution is 10% (v/w), and the amount thereof is sufficient for confining 1000 gram of activated sludge. It is appreciated for the skilled person in this art that the respective amounts of methylene chloride and CTA are variable, depending upon amount of activated sludge to be confined. 
     Subsequently, a mixture of 100 mL of CTA/methylene chloride organic solution and 100 gram of activated sludge having a solid content of 10%˜20% is prepared in step  23 . In step  24 , the mixture is stirred to become gelled, and is contained in an interior space of respective supporting devices. Stirring of the mixture is carried out until the respective supporting device is fully filled with the mixture to form the microbial cell of treating unit of the present invention. In step  25 , the prepared microbial cells are immersed with toluene for 5˜10 seconds and then air-dried in atmosphere. In step  26 , the respective microbial cells are rinsed with water, to remove the toluene residual therefrom, so that the treating units according to the present invention are fabricated. 
     In accordance with a preferred embodiment of the present invention, the supporting device is a supporting cage constructed by three polyethylene (PE) rings that are intersecting with one another. Accordingly, the size of treating unit of the present invention is determined by the inner diameter of PE ring. Preferably, the inner diameter of PE ring of the present invention is ranged from 5 mm to 30 mm. As the inner diameter of PE ring reduces, the size of the supporting cage constructed by the PE rings reduces correspondingly, and thus the microbial cell contained therein is relatively dense. That is, treating units of different sizes can be prepared by selecting PE rings of different inner diameters, which actually depends upon the condition of wastewater to be treated. 
     In a specific embodiment of the present invention, the supporting cage of treating unit is constructed by three PE rings in such a way that the supporting cage containing the microbial cell therein will be sphere-shaped. Nevertheless, it should be appreciated that the amounts and material of rings to construct the supporting cage are variable. It is possible to use rings made of other plastic materials, ceramic materials, or even metals to construct the supporting device. Furthermore, the use of biodegradable materials such as green plastic to construct the supporting device is also possible, which is advantageous for subsequent waste treatment of the treating units. 
     With reference to  FIGS. 3A and 3B , the wastewater treatment apparatus in accordance with a preferred embodiment of the present invention is schematically illustrated. As shown in  FIG. 3A , the wastewater treatment apparatus  30  includes at least a columnar reactor  32  in which plural treating units  34  (shown in  FIG. 3B ) as illustrated above are filled to remove the organics, i.e. carbon compounds, and nitrogen compounds from wastewater. For example, but not limitation, the wastewater treatment apparatus  30  is an aeration basin having at least one fixing frame  36  disposed therein, so that the columnar reactor  32  of the present invention is fixed therewith. The wastewater is fed into the aeration basin through the inlet  382 , and drains therefrom through the outlet  384  after proceeding with the removal of organics and nitrogen compounds within the basin. 
     With reference to  FIGS. 4A and 4B , the wastewater treatment apparatus in accordance with a further preferred embodiment of the present invention is schematically illustrated. As shown in  FIG. 4A , the wastewater treatment apparatus  40  includes a perforated cassette-shaped reactor  42  in which a plurality of treating units  44  (shown in  FIG. 4B ) as mentioned are filled to remove the organics and nitrogen compounds from the wastewater. Similarly, the wastewater treatment apparatus  40  is, for example but not limitation, an aeration basin, in which a plurality of perforated cassette reactors  42 , spaced apart from each other with a predetermined distance, are arranged. Such design enables the improvement in replacing and maintaining of the treating units. 
     According to the present application, the treating units are arranged in the reactor or basin with an adjustable filling ratio, which is adjusted depending upon actual desires. For example, the filling amount of 0.3 cm 3  for a basin volume of 1 cm 3  indicates that the volume filling ratio is substantially 30%. Moreover, the treating units are filled within the basin in a densely-stacking manner as shown in  FIGS. 3A and 4A . Alternatively, the treating units may be filled within the basin in a non-densely stacking manner where the so that the basin may function as a fluidized bed reactor. In this case, the collision among treating units and thus the damage thereof can be avoided, as shown in  FIG. 5 . 
       FIG. 6  shows the treating efficiency for synthesized influent with the columnar reactor according to the present invention, where the amount of total organic carbon (TOC), ammonia nitrogen (NH 3 —N) and chemical oxygen demand (COD) of influent and effluent are measured, respectively. In this case, the inner diameter of the supporting device of treating units contained in the columnar reactor is 25 mm, and the volume filling ratio thereof is substantially 30%, i.e. a filling amount of 0.3 cm 3  for a basin volume of 1 cm 3 . The hydraulic retention time (HRT) is adjustable between 6 and 24 hours, depending upon the condition of influent to be treated. An air flow of 1 L/min per liter of reactor volume is provided to the columnar reactor. The measurement results show that the start-up period of the columnar reactor according to the present invention, during which a stable operation is achievable, is extremely short, and the removal efficiency of TOC and COD is above 95%. The columnar reactor also provides significantly improved removal of NH 3 —N, though the efficiency thereof may be varied with the aeration condition of reactor. For example, a removal efficiency of 55% is achieved by the present invention while the amount of nitrate nitrogen (NO 3 —N) contained in the effluent is less than 0.1 mg/L. 
       FIG. 7  shows the treating efficiency for actual wastewater with the columnar reactor according to the present invention where the amount of total organic carbon (TOC), ammonia nitrogen (NH 3 —N) and chemical oxygen demand (COD) of influent and effluent are respectively measured. In this case, the inner diameter of supporting device of treating units contained in the columnar reactor is 16 mm, and the volume filling ratio thereof is substantially 30%, i.e. a filling amount of 0.3 cm 3  for a basin volume of 1 cm 3 . The hydraulic retention time (HRT) is adjustable between 6 and 24 hours, depending upon the condition of influent to be treated. An air flow of 1 L/min per liter of reactor volume is provided to the columnar reactor. The measurement results show that a stable operation of columnar reactor according to the present invention is achieved. The removal efficiency of COD almost achieves 100%, and the removal efficiency of TOC approaches 90%. When the columnar reactor stably operates, e.g. after an operation period of 17 days, the removal efficiency of NH 3 —N up to 70% is achieved while the amount of NO 3 —N contained in the effluent is less than 0.1 mg/L. 
     Please refer to  FIG. 8 , which is a schematic view showing the wastewater treatment system with the membrane module according to one preferred embodiment of the present invention. The wastewater treatment system  800  is constructed by a biological reactor  810  and a membrane reactor  820  fluidly connected thereto. The biological reactor  810  contains plural treating units  815  therein, while the membrane reactor  820  includes at least one membrane module  825  arranged therein. According to the present invention, each of the treating units  815  is sphere-shaped and formed by a supporting cage and a microbial cell contained therein, as shown in  FIG. 1 . 
     As shown in  FIG. 8 , the wastewater to be treated is pumped from the influent reservoir  840  into the biological reactor  810  with the aid of pump  830 . According to the preferred embodiment, the biological reactor  830  is a columnar reactor in which plural treating units  815  are contained. The wastewater to be treated is directed into the reactor through the inlet  810 A at the bottom, and is discharged through the outlet  810 B at the top of reactor after the removal of organics and nitrogen compounds is carried out. 
     According to the present invention, the inlet  810 B of the biological reactor  810  is fluidly connected with the inlet  820 A of the membrane reactor  820 . That is, the wastewater being treated to remove the nitrogen compounds and organics therefrom is directed into the membrane reactor  820  from the biological reactor  810  through the inlet  820 A, so that the suspended solids contained therein is filtered with the membrane modules  825  arranged in membrane reactor  820 . The filtered permeate is pumped from the membrane reactor  820  to the effluent reservoir  880 , so as to obtain the clear discharge. 
     According to the present invention, the filling ratio of treating units  815  with respect to the biological reactor  810  is adjustable, depending upon the actual demand for treatment. Moreover, with the aid of compressor or blower  850  and air regulator (not shown), the air flow supplied to the biological reactor  810  is adjustable, so as to control the dissolved oxygen (DO) condition for the reactor. The increase of air flow also helps to completely mixing the treating units  815  with the wastewater inside the biological reactor  810 . 
     Please refer to  FIG. 9 , which is a schematic view showing the wastewater treatment system with the membrane module according to another preferred embodiment of the present invention. The wastewater treatment system  900  is constructed by a biological reactor  910  and a membrane reactor  920  fluidly connected thereto. In this embodiment, the biological reactor  930  is a rectangular reactor in which at least one treating cassette  912  is arranged. That is, the plurality of treating units  915  are filled within the treating cassette  912 , which is arranged in the rectangular reactor for the removal of organics and nitrogen compounds from wastewater. The membrane reactor  920  includes at least one membrane module  925  arranged therein. According to the present invention, each of the treating units  915  is sphere-shaped and formed by a supporting cage and a microbial cell contained therein, as shown in  FIG. 1 . 
     As shown in  FIG. 9 , the wastewater to be treated is pumped from the influent reservoir  940  into the biological reactor  910  with the aid of pump  930 . The wastewater to be treated is directed into the reactor through the inlet  910 A at the bottom, and is discharged through the outlet  910 B at the top of reactor after the removal of organics and nitrogen compounds is carried out. 
     According to the present invention, the inlet  910 B of the biological reactor  910  is fluidly connected with the inlet  920 A of the membrane reactor  920 . That is, the wastewater being treated to remove the nitrogen compounds and organics therefrom is directed into the membrane reactor  920  from the biological reactor  810  through the inlet  920 A, so that the suspended solids contained therein is filtered with the membrane modules  925  arranged in membrane reactor  920 . The filtered permeate is pumped from the membrane reactor  920  to the effluent reservoir  980 , so as to obtain the clear discharge. 
     Similarly, the filling ratio of treating units  915  with respect to the biological reactor  910  is adjustable, depending upon the actual demand for treatment. Moreover, with the aid of compressor or blower  950  and air regulator (not shown), the air flow supplied to the biological reactor  910  is adjustable, so as to control the DO condition for the reactor. The increase of air flow also helps to completely mixing the treating units  915  with the wastewater inside the biological reactor  910 . 
     Please refer to  FIG. 10 , which is a schematic view showing the wastewater treatment system with the membrane module according to one another preferred embodiment of the present invention. In this embodiment, the wastewater treatment system  1000  is constructed by a hybrid reactor  1010  in which at least one treating perforated cassette containing plural treating units  1015  is arranged for the removal of organics and nitrogen compounds from wastewater. In addition to the treating perforated cassette containing the treating units  1015 , the reactor  1010  is also provided with at least one membrane module  1025  which is dipped in the wastewater for the filtration of suspended solids. According to the present invention, each of the treating units  1015  is sphere-shaped and formed by a supporting cage and a microbial cell contained therein, as shown in  FIG. 1 . 
     As shown in  FIG. 10 , the wastewater to be treated is pumped from the influent reservoir  1040  into the hybrid reactor  1010  with the aid of pump  1030 . The wastewater to be treated is directed into the hybrid reactor  1010  through the inlet  1010 A thereof. The removal of organics and nitrogen compounds from the wastewater is carried out in the hybrid reactor  1010  by the treating units  1015 , and moreover, the suspended solids contained in the wastewater are filtered by means of the membrane module  1025  arranged therein. The permeate flow is pumped from the hybrid reactor  1010  to effluent reservoir  1080 , so as to obtain the clear discharge. 
     Similarly, with the aid of compressor or blower  1050  and air regulator (not shown), the air flow supplied to the reactor  1010  is adjustable, so as to control the DO condition for the reactor. The increase of air flow also helps to cause the wastewater to form a completely mixed flow or plug flow inside the hybrid reactor  1010 . 
     There are a variety of membrane modules capable of being employed in the wastewater treatment system according to the present invention. For example, but not limitation,  FIGS. 11 and 12  show the treating efficiency for synthesized influent and for the actual wastewater with the wastewater treatment system employing a membrane module of MF hollow fiber according to the present invention, respectively. The pore size of such membrane is 100 kDa, i.e. 0.01 μm. 
       FIG. 6  demonstrates the respective removal efficiency of COD, NH 3 —N and NO 3 —N for the synthesized influent. In this case, the treating units having a size of 25 mm are adopted in the wastewater treatment system and the filling ratio thereof is substantially 30%. The hydraulic retention time is adjusted to 12 hours, and an air flow of 1 L/min for per liter of reactor volume is supplied. The air flow is supplied for one hour with a two-hour suspending (indicated by 1:2). The measurement results show that the start-up period of the wastewater treatment system according to the present invention, during which a stable operation is achievable, is extremely short, and the removal efficiency of COD and NO 3 —N is above 90%, and furthermore, a removal efficiency of ammonia nitrogen of 100% is achievable by the present invention. 
       FIG. 7  demonstrates the respective removal efficiency of COD, NH 3 —N and NO 3 —N for the actual wastewater. In this case, the treating units having a size of 25 mm are adopted for treating the food industrial wastewater and the filling ratio thereof is substantially 30%. The hydraulic retention time is adjusted to 12 hours, and an air flow of 1 L/min for per liter of reactor volume is supplied. The air flow is supplied for one hour with a two-hour suspending (indicated by 1:2). The measurement results show that the start-up period of the wastewater treatment system according to the present invention, during which a stable operation is achievable, is extremely short, and the removal efficiency of COD and NO 3 —N is above 85%, and furthermore, a high removal efficiency of ammonia nitrogen above 95% is achieved by the present invention. 
     In addition to the improved treating efficiency as mentioned, the wastewater treatment system according to the present invention also provides excellent filtration efficiency with respect to the suspended solids. In a preferred embodiment of the present invention, with the treatment by the provided system, an effluent containing no suspended solids is obtained from an influent whose concentration of suspended solids is up to 10˜15 mg/L, and a permeate flux up to 14˜32 L/m 2 /hr is achieved as well, as the measurement results shown in  FIG. 8  demonstrates. Furthermore, since the microbial cell is contained within the supporting cage, the peeling-off of the aggregation can be avoided. By the present invention, the sludge age of activated sludge of treating units is increased to several tens to hundreds days, which significantly improves the treating efficiency for the pollutant in the water and reduces the amount of excess sludge. Therefore, the problem of waste sludge treatment and disposal is addressed. 
     Moreover, the wastewater treatment system according to the present invention is easy to operate, which is cost-efficient and toxicity-tolerant. The present wastewater treatment system is capable of removing the organics and nitrogen compounds from the wastewater under a controlled aeration mode, and enhancing the efficiency of membrane treatment. The expense for membrane module is thus reduced. 
     While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the present invention which is defined by the appended claims.