Team:ASTWS-China/Implementation

Summary

A typical sewage treatment plants use a series of tanks to treat the wastewater, the process usually contains 6 steps and generally involves three stages, called primary, secondary and tertiary treatment, besides pretreatments. (See Fig.1 and Fig.2-A) After grate and grit screening, wastewater will usually be treated by micro-organism (in anaerobic and aerobic tanks) between twice sedimentation. Most of large particles will be cleaned out from the water, however, the microparticles including microplastics may remain. Therefore, we add three more steps and design the new treatment process. (See Fig.2-B)

Figure 2 A conceptual diagram showing the sources of microplastic pollution and their environmental, ecological and health related impacts[4].

Fig.2. Typical sewage treatment process flow chart in a traditional sewage treatment plant (A) and ASTWS-China (B)

First, we designed a device which may contains our engineered bacteria. These E. coli will manufacture biofilm and PETase in the container. Inside of the container may have some biocarriers which can help biofilm attach to. The commercial biocarriers could easily find from local supplier. (See Fig.3) The device could be placed in aerobic tank and/or secondary sedimentary tank. (See Fig.4.) It may help remove the microparticles and biodegrade PET microplastics at same time.

Fig.3 Example of commercial biocarriers (From Hangzhou Nihao Environmental Tech Co. Ltd.)

Fig.4 Aerobic tank (left) and secondary sedimentary tank at Hangzhou city-west (Jiangcun) sewage treatment plant (West Lack District, Hangzhou, Zhejiang Province, China)

Second, we designed a filter tank after secondary sedimentary tank, which can help physically remove remaining microparticle from water by ultrafiltration membrane. General speaking, the vast majority (about 80 ~ 99.9%) of microparticles have been removed secondary sedimentary[1], however, the last 1 ~ 35% of microparticles are relatively hard to treat (Fig.5). Ultrafiltration technology has been broad used for drinking water purification. In recent years, it begun to been applied to civil water treatment plants and received courageous results.[2](See Fig.6) Therefore we try to apply this technology to sewage treatment as well.

Fig. 5 Estimated flow, shapes and types of MPs in different stages of WWTPs[1].

Fig.6 Ultrafiltration system at Dongying Waterwork Company (Dongying City, Shandong Province, China, Picture retrieved from https://www.meipian.cn/1kjcuyr9 at 2020/10/17)

Third, we plan use activated sludge method to treat the microplastics (especially PET) from sludge. Sewage treatment plants collect the sludge from primary sedimentary tank, anaerobic tank, aerobic tank as well as secondary sedimentary tank and it contains a large part of microplastic particles from wastewater. By activated sludge process, we add engineered E.coli into the sludge aeration reactor together with other functional micro-organisms. (See Fig.7.) Then our biofilm-PETase system will biodegrade the microplastics from the sludge during the process.

Fig.7 A schematic diagram of conventional activated sludge treatment process [3]

Comparing to traditional sewage treatment process, we believe our ASTWS-China process would help not only clean the microparticles out but also biodegrade PET microplastics from wastewater in an efficient way.

Moreover, we consider the biosafety issue as well. Because most of sewage treatment plant use micro-organism during the process, and they have already a disinfection process which is also suitable for our engineered E. coli. Common methods of disinfection include ozone, chlorine, ultraviolet light (See Fig.8), or sodium hypochlorite. After multiple steps of disinfection, the treated water is ready to be released back into the water cycle by means of the nearest body of water or agriculture.

Fig. 8 Ultraviolet light disinfection process at Hangzhou city-west (Jiangcun) sewage treatment plant (West Lack District, Hangzhou, Zhejiang Province, China)

References:

[1] Liyuan Hou, Deepak Kumar, Chang Geun Yoo, et al. Conversion and removal strategies for microplastics in wastewater treatment plants and landfills. Chemical Engineering Journal, 2021, 406: 126715.

[2] Yue Peng, Ding Yun, Yang Qing et al. Research progress and application of ultrafiltration technology in urban drinking water treatment. Water Purification Technology, 2017, 36(4): 36-42 

[3]Jiang, Xi, "Evaluation of an anaerobic granule biosensor for upset early warning detection" (2008). Retrospective Theses and Dissertations. 15472.https://lib.dr.iastate.edu/rtd/15472