WO 2012/019338 PCT/CN2010/075823 AEROBIC AND ANAEROBIC SYSTEM FOR TREATING WASTEWATER FIELD OF THE INVENTION The present invention relates to systems and methods for treating wastewater, and more particularly, to an aerobic and anaerobic systems and methods for treating wastewater by using algae and anaerobic microorganisms to remove chemical oxygen demand (COD), nitrogen and phosphorus in a convenient, cost effective, and environmental friendly manner. BACKGROUND OF THE INVENTION A large number of systems for treatment of contaminants in water, such as effluent from wastewater treatment facilities, are variously known in the prior art. Many of the various prior art type wastewater treatment facilities may utilize aerodiffusor systems (i.e., aerobic systems), septic systems (i.e., anaerobic systems) or other designs that are meant to reduce various contaminant counts within the effluent. A number of these prior art systems are considered to be advance treatment systems that provide an initial treatment of effluent directly from the source. However, these designs require additional or background treatment facilities due to the poor removal of nitrogen and phosphorus. The removal of nitrogen and phosphorus depends on both of aerobic microorganisms and anaerobic microorganisms. In general, anaerobic systems are not suitable for optimizing the growth of aerobic microorganisms, while aerobic systems are not suitable for optimizing the growth of anaerobic microorganisms. In addition, these designs do not utilize efficiently sunlight in the daytime and darkness at night. Algae are microorganisms like plants, which use sunlight and carbon dioxide to grow and in turn release oxygen. They are the fastest growing microorganisms on our planet and converted almost 70% carbon dioxide into oxygen. Algae can assimilate nitrogen and phosphorus. But optimized conditions for algae growth have been much more stringent than usual aerobic microorganisms, so that establishing optimized conditions in wastewater for algae growth has not been cost effective up to the present. Therefore, Algae are not suitable for removal of carbon contaminant or chemical oxygen demand (COD) in wastewater. Although there exist a number of anaerobic microorganisms growing in wastewater, which can be used to remove nitrogen by reversion of nitration and assimilate carbon contaminant, they WO 2012/019338 PCT/CN2010/075823 generally release phosphorus. Therefore, anaerobic microorganisms are not suitable for removal of phosphorus in wastewater. Accordingly, the conventional means for treating wastewater fail to remove chemical oxygen demand (COD), nitrogen and phosphorus in a convenient, cost effective, and environmental friendly manner. Therefore, there exists a need for removal of chemical oxygen demand (COD), nitrogen and phosphorus in wastewater in a convenient, cost effective, and environmental friendly manner. DISCLOSURE OF THE INVENTION In view of the disadvantages inherent in conventional systems and methods mentioned above for removal of chemical oxygen demand (COD), nitrogen and phosphorus in wastewater, the general purpose of the present invention is to provide aerobic and anaerobic systems and methods for treating wastewater, by using algae and anaerobic microorganisms to remove chemical oxygen demand (COD), nitrogen and phosphorus in a convenient, cost effective, and environmental friendly manner. The present invention includes advantages of conventional systems and methods mentioned above, and overcomes the drawbacks inherent therein. In first aspect, the present invention provides an aerobic and anaerobic system for treating wastewater, comprising: a photo-bioreactor capable of exposing to sunlight and/or artificial daylight, which is connected to pipelines for transferring wastewater and air into the photo-bioreactor; an anaerobic tank without light for anaerobic fermentation by anaerobic microorganisms, which is connected to a pipeline for transferring CO 2 into the anaerobic tank; and a culture fermenter for growing photosynthetic algae at a required temperature and light, which is connected to pipelines for transferring culture medium and CO 2 into the culture fermenter. Oxygen is harmful to anaerobic microorganisms in the anaerobic tank. Preferably in the anaerobic tank, there is no light source so as to prevent producing oxygen from algae by photosynthesis. And if necessary, oxygen or air in the anaerobic tank may be dispelled by continuously feeding CO 2 into the anaerobic tank for anaerobic fermentation. So in a preferable embodiment of first aspect of the invention, there is at least a nozzle at the joint of the anaerobic tank and the pipeline for continuously feeding CO 2 into the anaerobic tank. The pipeline for transferring
CO
2 preferably connects a nearby source of CO 2 gas to the anaerobic tank via a heat exchanger to 2 WO 2012/019338 PCT/CN2010/075823 lower the temperature of the incoming gas. Algae in the culture fermenter convert CO 2 to oxygen for growth by photosynthesis. So the culture fermenter is equipped with at least a light source such as sunlight and artificial daylight to continue algae growth, and if necessary, CO 2 and/or nutrient are transferred into the culture fermenter. So in a preferable embodiment of first aspect of the invention, the culture fermenter comprises at least an artificial daylight used for adjusting light intensity suitable for algae growth. In another preferable embodiment of first aspect of the invention, there is at least a nozzle at the joint of the culture fermenter and the pipeline for feeding CO 2 and/or nutrient into the culture fermenter. The pipeline for transferring CO 2 preferably connects a nearby source of CO 2 gas to the culture fermenter via a heat exchanger to lower the temperature of the incoming gas. In addition, preferably the pipeline for transferring culture medium further comprises a peristaltic pump for accurately dispensing the nutrient amount into the culture fermenter for preventing saturation of the growing media with a particular nutrient by uncontrolled dispensing of nutrients. In a further embodiment, in the system of first aspect of the invention, the entry of the photo-bioreactor is connected to the exit of the photo-bioreactor via a pipeline comprising at least one peristaltic pump and at least one valve so as to maintain continuous flow of the wastewater in the photo-bioreactor; the exit of the photo-bioreactor is connected to the entry of the anaerobic tank via a pipeline comprising at least one valve so as to evacuate the wastewater from the photo-bioreactor into the anaerobic tank; the exit of the anaerobic tank is connected to the entry of the photo-bioreactor via a pipeline comprising at least one pump and at least one valve so as to evacuate the wastewater from the anaerobic tank into the photo-bioreactor; and the exit of the culture fermenter is connected to the entry of the photo-bioreactor via a pipeline comprising at least one pump and at least one valve for transferring algae into the photo-bioreactor. In a preferable embodiment of first aspect of the invention, the pipeline connected with the photo-bioreactor for transferring air transports air to flush the photo-bioreactor at night so as to evacuate the wastewater from the photo-bioreactor at night; and/or the pump on the pipeline between the photo-bioreactor and the anaerobic tank transports air to flush the anaerobic tank in the daytime so as to evacuate the wastewater from the anaerobic tank in the daytime. 3 WO 2012/019338 PCT/CN2010/075823 In another preferable embodiment of first aspect of the invention, the exit of the photo-bioreactor is located at the bottom of the photo-bioreactor so as to thoroughly evacuate the wastewater from the photo-bioreactor; and/or the exit ofthe anaerobic tank is located at the bottom of the anaerobic tank so as to thoroughly evacuate the wastewater from the anaerobic tank. When the valve between the entry and the exit of the photo-bioreactor is open and other valves are closed, the peristaltic pump between the entry and the exit of the photo-bioreactor can maintain continuous flow of the wastewater in the photo-bioreactor. In a preferable embodiment of first aspect of the invention, the photo-bioreactor is capable of providing enough exposure of sunlight to the algae and maintaining continuous flow of the wastewater. For example, the photo-bioreactor is set up on the roof of a house for enough exposure of sunlight, while the anaerobic tank and the culture fermenter is in the house. Preferably the photo-bioreactor is separated by a plurality of vertical columns as mentioned in PCT application W02008010737. The wastewater enters the photo-bioreactor from the top to the bottom of a vertical column, and then travels through an adjacent column to the top. The photo-bioreactor may be manufactured using any transparent material including glass and plastic of any kind as long as it has sufficient tensile strength. The material must be UV resistant or treated to prolong its use in open environment. The thickness may vary from 1.5 inch to 4.5 inch depending on the transparency and stability of the material. Although the wastewater in the photo-bioreactor can be pressed out by using air to flush the photo-bioreactor, the pipeline between the photo-bioreactor and the anaerobic tank preferably comprises at least one pump, which can evacuate the wastewater by using air to flush the photo-bioreactor. At night the valve between the exit of the photo-bioreactor and the entry of the anaerobic tank is open and other valves are closed, the pump in the pipeline between the exit of the photo-bioreactor and the entry of the anaerobic tank runs to evacuate the wastewater by using air to flush the photo-bioreactor, and then the wastewater enters the anaerobic tank for anaerobic fermentation. In the daytime, the valve between the exit of the anaerobic tank and the entry of the photo-bioreactor is open and other valves are closed, the pump in the pipeline between the exit of the anaerobic tank and the entry of the photo-bioreactor runs to evacuate the wastewater by using air to flush the anaerobic tank, and then the wastewater enters the photo-bioreactor for photosynthesis to 4 WO 2012/019338 PCT/CN2010/075823 remove nitrogen and phosphorus contaminants by algae. If necessary (e.g., when the wastewater initially enters the photo-bioreactor, there are not algae in the wastewater), the valve between the exit of the culture fermenter and the entry of the photo-bioreactor is open and other valves are closed, the pump in the pipeline between the exit of the culture fermenter and the entry of the photo-bioreactor runs to transfer the culture of algae into the photo-bioreactor and mix with the wastewater. In a further embodiment, in the system of first aspect of the invention, the anaerobic tank comprises a cooling device for controlling the temperature of the inside of the anaerobic tank; and/or the culture fermenter preferably comprises a cooling device for controlling the temperature of the inside of the culture fermenter. Preferably the anaerobic tank is capable of culturing the anaerobic microorganism at a required temperature. The cooling device (e.g., refrigerant compressor) capable of cooling the inside of the anaerobic tank at a required level, especially in summer, neutralizes heat from anaerobic fermentation. The cooling device can be used for controlling (i.e., decreasing) the rate of fermentation, keeping the inside of the anaerobic tank clean, and preventing clogging. Preferably the culture fermenter is capable of growing the algae at a required temperature. The cooling device capable of cooling the inside of the culture fermenter at a required level, especially in summer, neutralizes heat from algae growth. The cooling device can be used for controlling (i.e., decreasing) the rate of algae growth, keeping the inside of the culture fermenter clean, and preventing clogging. In a further embodiment, in the system of first aspect of the invention, there is at least a controller to monitor and control a plurality of variables used in the process of treating wastewater, including amount and temperature of water, amount and temperature of air and/or C0 2 , nutrient levels, time of day, and amount of sunlight intensity. For example, the controller is selected from any one of a Programmable Logic Controller (PLC), a computer, and any systems that are capable interacting with a plurality of different sensors and take appropriate measures. A plurality of sensors are respectively connected with the controller for measuring various variables used in the process of treating wastewater. For example, there is at least one sensor in the culture fermenter for continuously monitor the nutrient levels and CO 2 levels. The controller is capable of controlling all of the pumps and valves to take automatically the steps of the process of treating wastewater as programmed by the 5 WO 2012/019338 PCT/CN2010/075823 user. In a further embodiment, in the system of first aspect of the invention, the photo-bioreactor is equipped with at least a light source to continue algae growth in the absence of adequate sunlight during the daytime. In a further embodiment, in the system of first aspect of the invention, the photo-bioreactor is equipped with a separator for separating the treated water from algae and anaerobic microorganisms. The separator comprises at least one valve, which can not be open until getting the desired outcome of treating wastewater. When the valve in the separator is open and other valves are closed, the treated wastewater flows out of the photo-bioreactor. Generally in the separator, there is at least one filter for separating the treated water from algae and anaerobic microorganisms. In a preferable embodiment of first aspect of the invention, the separator is connected to a dryer via a conveyor belt. The dryer is used for drying the water out from microorganisms and algae, which can be recirculated back into the system. For example, algae gathered by the dryer can be added to the culture fermenter. The dryer is preferably coupled with humidity sensor and ventilation fans for automatically ventilating inside air at a predefined level of humidity. In second aspect, the present invention provides a method of treating wastewater, which comprises the steps of: putting culture medium and algae in a culture fermenter to grow algae at a convenient condition of C0 2 , temperature and light; putting wastewater into a photo-bioreactor capable of exposing to sunlight; pumping out the culture of algae from the culture fermenter to the photo-bioreactor; putting air into the photo-bioreactor if necessary; circulating the wastewater in the photo-bioreactor in the daytime; evacuating the wastewater from the photo-bioreactor into an anaerobic tank without light for anaerobic fermentation by anaerobic microorganisms at night; putting CO 2 into the anaerobic tank if necessary; evacuating the wastewater from the anaerobic tank into the photo-bioreactor to circulate the wastewater in the photo-bioreactor in the daytime; and separating the treated water from algae and anaerobic microorganisms. Any one of steps in the method of second aspect of the invention can be repeated until the 6 WO 2012/019338 PCT/CN2010/075823 wastewater is treated to get the desired outcome. For example, at any time that the amount of algae in the photo-bioreactor is lower than the amount required, the culture of algae can be pumped out once more from the culture fermenter to the photo-bioreactor. Preferably the method of second aspect of the invention is the method by using the system of first aspect of the invention, so the present invention also provides a use of the system of first aspect of the invention in carrying out the method of second aspect of the invention. For a better understanding of the invention, it will now be described in greater detail by reference to specific examples and drawings. It should be noted that the examples and drawings only exemplify the invention, and should not be construed as limiting the scope of the invention. According to the description of the application, various modifications and alterations of the invention are obvious to a skilled person in the art. The publications cited in the application are used to illustrate the invention, the contents of which are incorporated herein by reference, as if they have been written down herein. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram illustrating an aerobic and anaerobic system for treating wastewater according to an example of the present invention. EXAMPLES Example 1: Aerobic and Anaerobic System for Treating Wastewater Fig. 1 shows an aerobic and anaerobic system for treating wastewater, comprising: a photo-bioreactor 10 capable of exposing to sunlight and artificial daylight 11, which is connected to pipelines for transferring wastewater and air into the photo-bioreactor 10; an anaerobic tank 20 without light for anaerobic fermentation by anaerobic microorganisms, which is connected to a pipeline for transferring CO 2 into the anaerobic tank; and a culture fermenter 30 for growing photosynthetic algae at a required temperature and light, which is connected to pipelines for transferring culture medium and CO 2 into the culture fermenter 30. In the system, the photo-bioreactor 10 is equipped with at least an artificial daylight 11 to continue algae growth in the absence of adequate sunlight during the daytime. 7 WO 2012/019338 PCT/CN2010/075823 In the anaerobic tank 20, there is no light source so as to prevent producing oxygen from algae by photosynthesis. And if necessary, oxygen or air in the anaerobic tank may be dispelled by continuously feeding CO 2 into the anaerobic tank 20 for anaerobic fermentation. So there is at least a nozzle at the joint of the anaerobic tank 20 and the pipeline for continuously feeding CO 2 into the anaerobic tank 20. The pipeline for transferring CO 2 preferably connects a nearby source of CO 2 gas to the anaerobic tank 20 via a heat exchanger to lower the temperature of the incoming gas. The culture fermenter 30 is equipped with at least an artificial daylight 31 to continue algae growth, and CO 2 and/or nutrient may be transferred into the culture fermenter 30. The artificial daylight 31 in the culture fermenter 30 is used for adjusting light intensity suitable for algae growth. There is at least a nozzle at the joint of the culture fermenter 30 and the pipelines for feeding CO 2 and nutrient (e.g., culture medium) into the culture fermenter 30. The pipeline for transferring CO 2 connects a nearby source of CO 2 gas to the culture fermenter 30 via a heat exchanger to lower the temperature of the incoming gas. In addition, the pipeline for transferring culture medium further comprises a peristaltic pump 32 for accurately dispensing the nutrient amount into the culture fermenter for preventing saturation of the growing media with a particular nutrient by uncontrolled dispensing of nutrients. In the system, the entry 12 of the photo-bioreactor 10 is connected to the exit 13 of the photo-bioreactor 10 via a pipeline comprising at least one peristaltic pump 14 and at least one valve 15 so as to maintain continuous flow of the wastewater in the photo-bioreactor 10; the exit 13 of the photo-bioreactor 10 is connected to the entry 21 of the anaerobic tank 20 via a pipeline comprising at least one valve 22 so as to evacuate the wastewater from the photo-bioreactor 10 into the anaerobic tank 20; the exit 23 of the anaerobic tank 20 is connected to the entry 12 of the photo-bioreactor 10 via a pipeline comprising at least one pump 24 and at least one valve 25 so as to evacuate the wastewater from the anaerobic tank 20 into the photo-bioreactor 10; and the exit 33 of the culture fermenter 30 is connected to the entry 12 of the photo-bioreactor 10 via a pipeline comprising at least one pump 34 and at least one valve 35 for transferring algae into the photo-bioreactor 10. In the system, the pipeline connected with the photo-bioreactor 10 for transferring air transports air to flush the photo-bioreactor 10 at night so as to evacuate the wastewater from the photo-bioreactor 10 at night; the pump 24 on the pipeline between the photo-bioreactor 10 and the 8 WO 2012/019338 PCT/CN2010/075823 anaerobic tank 20 transports air to flush the anaerobic tank 20 in the daytime so as to evacuate the wastewater from the anaerobic tank 20 in the daytime; the exit 13 of the photo-bioreactor 10 is located at the bottom of the photo-bioreactor 10 so as to thoroughly evacuate the wastewater from the photo-bioreactor 10; and the exit 23 of the anaerobic tank 20 is located at the bottom of the anaerobic tank 20 so as to thoroughly evacuate the wastewater from the anaerobic tank 20. When the valve 14 between the entry 12 and the exit 13 of the photo-bioreactor 10 is open and other valves are closed, the peristaltic pump 15 between the entry 12 and the exit 13 of the photo-bioreactor 10 can maintain continuous flow of the wastewater in the photo-bioreactor 10. In addition, the photo-bioreactor 10 is capable of providing enough exposure of sunlight to the algae and maintaining continuous flow of the wastewater. The photo-bioreactor 10 is set up on the roof of a house for enough exposure of sunlight, while the anaerobic tank 20 and the culture fermenter 30 is in the house. As shown in Fig. 1, the photo-bioreactor 10 is separated by a plurality of vertical columns. The wastewater enters the photo-bioreactor 10 from the top to the bottom of a vertical column, and then travels through an adjacent column to the top. The photo-bioreactor 10 is made of transparent glass material. The material is UV resistant. The thickness may vary from 1.5 inch to 2.5 inch. Although the wastewater in the photo-bioreactor 10 can be pressed out by using air to flush the photo-bioreactor 10, the pipeline between the photo-bioreactor 10 and the anaerobic tank 20 preferably comprises at least one pump 26, which can evacuate the wastewater by using air to flush the photo-bioreactor 10. In the system, the anaerobic tank 20 comprises a cooling device for controlling the temperature of the inside of the anaerobic tank 20. The cooling device (e.g., refrigerant compressor) capable of cooling the inside of the anaerobic tank at a required level, especially in summer, neutralizes heat from anaerobic fermentation. Also in the system, the culture fermenter 30 comprises a cooling device for controlling the temperature of the inside of the culture fermenter 30. The cooling device capable of cooling the inside of the culture fermenter at a required level, especially in summer, neutralizes heat from algae growth. In the system, there is at least a controller to monitor and control a plurality of variables used in the process of treating wastewater, including amount and temperature of water, amount and 9 WO 2012/019338 PCT/CN2010/075823 temperature of air and C0 2 , nutrient levels, time of day, and amount of sunlight intensity. The controller is selected from any one of a Programmable Logic Controller (PLC), a computer, and any systems that are capable interacting with a plurality of different sensors and take appropriate measures. A plurality of sensors are respectively connected with the controller for measuring various variables used in the process of treating wastewater. There is at least one sensor in the culture fermenter 30 for continuously monitor the nutrient levels and CO 2 levels. The controller is capable of controlling all of the pumps and valves to take automatically the steps of the process of treating wastewater as programmed by the user. In the system, the photo-bioreactor 10 is equipped with a separator 40 for separating the treated water from algae and anaerobic microorganisms. The separator 40 comprises at least one valve 41, which can not be open until getting the desired outcome of treating wastewater. When the valve 41 in the separator 40 is open and other valves are closed, the treated wastewater flows out of the photo-bioreactor 10. Generally in the separator 40, there is at least one filter for separating the treated water from algae and anaerobic microorganisms. The separator 40 is connected to a dryer 42 via a conveyor belt. The dryer 42 is used for drying the water out from microorganisms and algae, which can be recirculated back into the system. Algae gathered by the dryer 42 can be added to the culture fermenter 30. In addition, the dryer 42 is coupled with humidity sensor and ventilation fans for automatically ventilating inside air at a predefined level of humidity. Example 2: Method of Treating Wastewater by Using the System Described in Example 1 The system described in Example 1 can be used in a method of treating wastewater, which comprises the steps of: putting culture medium and algae in a culture fermenter 30 to grow algae at a convenient condition of C0 2 , temperature and light; putting wastewater into a photo-bioreactor 10 capable of exposing to sunlight; pumping out the culture of algae from the culture fermenter 30 to the photo-bioreactor 10; putting air into the photo-bioreactor 10 if necessary; circulating the wastewater in the photo-bioreactor 10 in the daytime; evacuating the wastewater from the photo-bioreactor 10 into an anaerobic tank 20 without light for anaerobic fermentation by anaerobic microorganisms at night; 10 WO 2012/019338 PCT/CN2010/075823 putting CO 2 into the anaerobic tank 20 if necessary; evacuating the wastewater from the anaerobic tank 20 into the photo-bioreactor 10 to circulate the wastewater in the photo-bioreactor 10 in the daytime; and separating the treated water from algae and anaerobic microorganisms. More specially, at night the valve 22 between the exit 13 of the photo-bioreactor 10 and the entry 21 of the anaerobic tank 20 is open and other valves are closed, the pump 26 in the pipeline between the exit 13 of the photo-bioreactor 10 and the entry 21 of the anaerobic tank 20 runs to evacuate the wastewater by using air to flush the photo-bioreactor 10, and then the wastewater enters the anaerobic tank 20 for anaerobic fermentation. In the daytime, the valve 25 between the exit 23 of the anaerobic tank 20 and the entry 12 of the photo-bioreactor 10 is open and other valves are closed, the pump 24 in the pipeline between the exit 23 of the anaerobic tank 20 and the entry 12 of the photo-bioreactor 10 runs to evacuate the wastewater by using air to flush the anaerobic tank 20, and then the wastewater enters the photo-bioreactor 10 for photosynthesis to remove nitrogen and phosphorus contaminants by algae. When the wastewater initially enters the photo-bioreactor 10, the valve 35 between the exit 33 of the culture fermenter 30 and the entry 12 of the photo-bioreactor 10 is open and other valves are closed, the pump 34 in the pipeline between the exit 33 of the culture fermenter 30 and the entry 12 of the photo-bioreactor 10 runs to transfer the culture of algae into the photo-bioreactor 10 and mix with the wastewater. In addition, at any time that the amount of algae in the photo-bioreactor 10 is lower than the amount required, the culture of algae can be pumped out once more from the culture fermenter 30 to the photo-bioreactor 10. 11