Welcome to iGEM 2021
This year our team developed an enhanced "plastic-eating" biofilm system to treat the PET micro-/nanoplastics in water. In order to make our project more reasonable, we have done a series of HP work. From the HP work, we have gained a lot of constructive suggestions to help us clarify our thinking more deeply. Taking into account the safety of engineering bacteria, we finally set the working scene as a sewage treatment plant. We modified the traditional sewage treatment system to have an additional function of treating microplastic pollutants at a relatively low transformation cost.
Through background surveys and field trips, we knew that the sewage treatment method mainly depends on the receiving water body. At present, the main treatment method of modern sewage treatment plant generally involves biochemical treatment, such as activated sludge method and MBR technology (Figure 1). From the field trip to the Lin'an Qingshan Sewage Treatment Co., Ltd., we have a deeper understanding of the design of the actual sewage treatment facility (Figure 2). In brief, the whole process can be divided into three stages of processing.
- Primary treatment (namely physical treatment): The whole process is that the raw sewage is raised by the sewage lifting pump through the coarse filter, and then goes through the filter or sieving device, and then enters the sedimentation tank. After sand water separation of wastewater into the settling basin for the first time, more than for primary treatment.
- Secondary treatment: The pond water into biological processing equipment, at the beginning of the activated sludge and biological membrane method, (including the reactor of activated sludge process with aeration tank, oxidation ditch, etc., biological membrane method, including biological filter, biological rotating disc, etc.). The effluent from the biological treatment equipment enters the secondary sedimentation tank, and the effluent from the secondary sedimentation tank is discharged after disinfection or enters the tertiary treatment.
- Tertiary treatment: Part of the sludge of the secondary sedimentation tank returns to the primary sedimentation tank or biological and physicochemical treatment equipment, part entering the sludge thickening tank, and then entering the sludge digestion tank. After dehydration and drying equipment, the sludge is finally used.
Figure 1 Typical sewage treatment process flow chart in a traditional sewage treatment plant .
Figure 2 Field trip of Lin'an Qingshan Sewage Treatment Co., Ltd. (A) Aerial view of the plant, (B-I) Photos of swirl grit tank, primary sedimentation tank, MBR treatment, sludge conditioning tank, denitrification reaction tank, effluent, sampling room, and central control system.
2. ASTWS Sewage Treatment System Design
It is reported that most large particles will be removed from the water, however, particles including microplastics may still remain in water or sludge. Thus, we need to solve three issues for micro-/nanoplastics pollutants.
- How to capture the micro-/nanoplastic in water and degrade by our dual-enzyme system?
- How to treat the plastic residues in sludge?
- How to ensure biosafety and prevent the microorganism escaping
2.1 Treatment of micro-/nanoplastics in water
As we know, conventional sewage plant generally involves biological treatment process, such as nitrification-denitrification treatment to remove nitrogen in sewage using nitrifying and denitrifying bacteria. In order to meet the needs of polyculture of different microorganisms, the equipment usually contains anaerobic tanks, aerobic tanks, and facultative aerobic tanks. Therefore, we designed a new processing flow based on this biological treatment process to treat the micro-/nano-plastics in water.
Firstly, we designed a device that contained our engineered bacteria (Figure 3A-B). This E. coli will produce strengthen biofilms by overexpressing OmpR234 gene. And at the same time, express our dual enzyme degradation system (PETase and MHETase) to treat the PET microplastics around. There may be some biological carriers inside the container that help the biofilm attach (see the inserted picture in Figure 3B). Commercial biological vectors can be easily found from local suppliers. The device can be placed in an aerobic tank and/or a secondary sedimentation tank. It can biodegrade PET microplastics while removing microparticles. As shown in Figure 3B, the cylinder at the upper left is the water inlet, and the outlet at the lower right. The core of the device is the intermediate reaction device with the engineered bacteria. When the contaminated water enters the reaction part, we will block the discharge site of the water outlet with slices, and degrade the PET microplastics in sewage. The rotating device was also designed to help increase our reaction rate and achieve the degradation of the maximum possible possibility.
Figure 3 (A) Schematic diagram of bioreactor (version 1), and 3D modeling diagram of bioreactor (B) Version 1 and (C) Version 2 (Lifting method).
Subsequently, we discussed the actual problems may encounter, such as how to detect the content of microplastics in the water, how to judge the end of the reaction, how to solve the problem of sinking, and the problem of replacing consumables. On these issues, we got help from Lai Qilong, a member of HUST2-China. Most importantly, regarding the problems such as prolonged reaction time caused by sinking and difficulty in replacing consumables, he suggested that our reaction device be designed as the lifting device. In accordance with the guidelines discussed in the collaboration, we improved the device to lifting model (Figure 3C).
As the biocarrier is a very important device in our bioreactor, how to ensure that the bacteria grow, adhere and not easily fall off is a problem that we urgently need to solve. Fortunately, we are very honored to have invited Engineer Bian who is from Wenhan Environmental Co., Ltd. From Engineer Bian, we got useful advice to select a combination packing (Figure 4B-C). Its advantage is that it has a biological selectivity - it can adhere to most bacteria and be shock-resistant. Moreover, the bacteria that can remain on the combined packing are highly active since the weak one will be directly eliminated, similar to a natural selection process. In addition, according to the expert’s advice, we can control the speed between 30 and 35 rotations per second, so as to ensure that the bacteria do not fall off and the normal reaction.
Figure 4 (A) 3D modeling of bio carrier, (B-C) Photos of combination packing .
Outlet valve design
Considering the implementation ability of the outlet valve mentioned above, Engineer Bian suggested we using an electric butterfly valve (Figure 5), which can not only adjust the flow size but also prevent sewage leakage (See Video 1). Secondly, expert Han Wen recommends that an electric vent valve should be added to prevent the sinking of bacteria. Relays can be used to accomplish automatic discharge to prevent sinking.
Figure 5 Photo of electric butterfly valve .
Video 1 Demonstration of Electronic Butterfly Valve
Final 3D modeling and Hardware
Combining all the above data, we drew the final integrated device design (Figure 6A). We added aerobic tank system and aeration system to keep the concentration of dissolved oxygen and the supply of nutrients. Figure 6B is the model of our specific degradation device. And Figure 6C is the overall model made using 3D printing technology. Figure 6D demonstrates the 3D printing model of combination packing. The disc combined with the white linear structure is conducive to the attachment of bacteria and the full mixing with sewage, so as to more effectively treat the microplastic pollution in the wastewater.
Figure 6 (A) Final integrated device design including anoxic tank, aerobic tank, and our degradation device; (B) model of our specific degradation device; (C) the overall model and (D) combination packing model made using 3D printing technology.
Video 2 Demonstration of final overall model.
2.2 Treatment of sludge
In our initial vision, the sludge should be collected and then used our enhanced degradable biofilm to treat the plastic residues in it. However, from the interviews with the Environment Bureau and the sewage treatment plant, we learned that sludge treatment is high cost, usually by means of dehydration, incineration, etc. The harmful exhaust gas caused by this process is also under strict control by our country. Therefore, considering the transformation cost and the problem of our system adaptation, the sludge treatment needs to find other more suitable methods.
In addition, we also carefully consider biosafety issues. The E. coli we use is a modified bacteria with a resistance gene. These bacteria cannot be released into the environment to cause secondary pollution. So, we put the application scenario in a closed sewage treatment plant which already has disinfection steps in itself. In other words, there is a disinfection process before the water comes out to make sure that the effluent is harmless -- the modified bacteria will be killed. At the same time, from the interview result with Expert Bai, a sewage water quality inspection expert, E. coli is one of the important criteria of water quality in sewage treatment plants. All treatment plants will install equipment to test for E. coli before the water is utilizing and discharging to make sure the water is harmless. After several steps of disinfection and monitoring, the treated water is released for recycling to supply nearby water bodies or agriculture.
3. The future development
If the device will be used in a sewage treatment plant, the material we use is polyethylene pipe (PE), a new environmentally friendly building material. The pipe has good push-pull resistance, shear resistance, and corrosion resistance. This is currently vigorously advocated and popularized the use of the pipe. We will also design the detection devices needed to monitor dissolved oxygen concentration and PH value to ensure the normal growth of E. coli.
In summary, compared with the traditional sewage treatment process, the ASTWS-China process can not only effectively remove microplastics from wastewater, but also use biotechnology to completely degrade PET microplastics to TPA and EG to achieve non-toxic and harmless discharge.
 Nang feng bei gang. City sewage treatment process flow chart. Zhihu, 2019. <https://www.zhihu.com/people/chu-men-hou-guai>.