Implementation Strategy
Figure 1: Graphical representation of the implementation strategy for pHish and CHIPS.
Device design specifications
Our team quickly realized that due to the high volume of ultrapure water that is used in the semiconductor industry (1), we would require a system that can process large volumes of wastewater rapidly. We determined that a large scale bioreactor would be the best strategy that balances the need to maintain an engineered organism with the necessity for continuous cycling of wastewater through the system. Additionally, the use of a large scale bioreactor would allow routine sampling of the contents to monitor conditions within the bioreactor, including nutrient and waste levels, cell density and growth rate, and pH and other chemical contaminant measurements. We also decided to integrate depth filtration systems to ensure that our engineered cells would not escape in neutralized water effluent.
Device placement in wastewater stream
We discussed at length how we would safely integrate our devices into a manufacturing setting without causing major disruptions. Mr. Jim Ajello informed us that the addition of new, in-line devices mid-process can be a real barrier to companies choosing to adopt new technologies. We came to the conclusion that the best solution would be to design and integrate a bioreactor at the end of the production process with internalized filtration systems that would serve as a last resort to prevent accidental release. We also discussed our options with Dr. Donald Hughes. He agreed that a large-scale bioreactor would be the best approach for our system. He suggested to us that we should integrate our device into the manufacturing process such that it was immediately downstream of the fab and upstream of water treatment facilities. This would reduce dilution, mixing, and the spread of chemical contaminants, making remediation easier. We integrated this feedback into our implementation plan.
Expansion of remediation capabilities
From the beginning of the project, we recognized that building additional functionality into our device would be beneficial to improve the sustainability of semiconductor fabrication. A potential additional function that we identified would be copper recapture. We learned from interviews with staff from NY Creates that copper, as well as many other heavy metals are used in the manufacturing process, and is remediated from wastewater using ion exchange technology. Many iGEM teams have added heavy metal sequestration parts to the iGEM parts repository. We considered whether this system would improve heavy metal removal from wastewater by concentrating these molecules in cells from our bioreactor. We abandoned this idea after our Human Practices team calculated the value of copper that could be reclaimed from the wastewater based on the data they collected on waste composition. They determined that the copper concentration was low enough that the value added would not be worth the cost of the addition of this functionality to our device. We did, however, identify a more desirable target for expanding our device’s capabilities.
The higher value target we would want our system to address is the problem with PFAS entering the water supply during the fabrication process. We learned from Dr. Donald Hughes that this is the primary pollution concern for community members. PFAS can be removed from water using activated carbon filtration, reverse osmosis, and ion exchange resins (2). However, once removed from water, there is currently no environmentally friendly way to destroy PFAS at scale (3). We met with team UNC-Chapel Hill to discuss the PFAS-related issues and to learn about their team's strategy for removing PFAS from the human body. They are designing a recombinant albumin that could sequester PFAS, and then be excreted. Such a technology could be adapted in our system to scrub and sequester PFAS from wastewater, but still leaves us without a sustainable way to destroy these forever chemicals. Research is ongoing, and it appears that there are species in nature with some capacity to destroy PFAS (4). When specific genes or biochemical pathways are identified, such genes could be integrated into our synthetic organism to provide this ability.
Containment considerations
An additional concern we had with our device is possible environmental contamination if it escaped containment from the bioreactor. Full development of our device would include a mechanism for ensuring the organism would not survive in the wild. Several mechanisms could be employed to prevent survival of engineered cells should they escape the bioreactor. Potential options include the following:
1. The glucose-mediated death sensor kill-switch gene ( BBa_K3634012 from the iGEM repository) could be added to our system. Our device would be grown in a glucose-rich environment, which reduces the expression of the toxic ccdB protein, which forms a complex with the enzyme DNA gyrase and causes DNA cleavage and transcriptional inhibition. In the absence of glucose, ccdB would be expressed, killing the bacteria.
2. Using an auxotrophic E. coli chassis (5). By choosing a strain of E. coli that lacks genes involved in essential amino acid synthesis, we could ensure the cells will only grow in the bioreactor with media supplemented with that amino acids.
Option 2 may be preferable, as we learned from interviews with Dr. Neil Dalvie and Mr. Kenneth Scherrieble that acid production due to glucose metabolism can kill a bioreactor (or make pH adjustment even more challenging), so an organism grown in an environment conducive to producing base may be preferable.
Dr. Dalvie informed us that since our device would function in a bioreactor, a closed system, cells can be physically removed by water via settling. This is common for sewage and wastewater systems, which mitigates the need for a genetic failsafe. However, our team is of the opinion that its inclusion would provide an important safeguard against escape into the environment. Dr. Dalvie agreed that inclusion of a genetic control of survival is a sensible preventative measure.
Application to other industries
The semiconductor industry has not cornered the market on industrial waste. Many industries produce waste that includes excessive acid or base (5). Our device could therefore be implemented in a variety of industries. Mr. Kenneth Scherrieble, who is President of a waste management service, indicated that our system would be of use to any industry that must monitor and adjust pH, as problems with acidification of these systems is a huge challenge that could be corrected by our organism.
References
1. Wang, Qi, et al. “Water strategies and practices for sustainable development in the semiconductor industry.” Water Cycle, vol. 4, 2023, pp. 12–16, https://doi.org/10.1016/j.watcyc.2022.12.001.
2. Speth, Thomas. “PFAS Treatment in Drinking Water and Wastewater – State of the Science.” U.S. Environmental Protection Agency, 16 Sept. 2020, www.epa.gov/research-states/pfas-treatment-drinking-water-and-wastewater-state-science.
3. Meegoda, Jay N., et al. “A review of Pfas Destruction Technologies.” International Journal of Environmental Research and Public Health, vol. 19, no. 24, 7 Dec. 2022, p. 16397, https://doi.org/10.3390/ijerph192416397.
4. Karimi Douna, B., and H. Yousefi. “Removal of PFAS by Biological Methods”. Asian Pacific Journal of Environment and Cancer, Vol. 6, no. 1, Mar. 2023, pp. 53-68, doi:10.31557/apjec.2023.6.1.53-68.
5. Iwasaki, Toshio, et al. “Escherichia Coli Amino Acid Iwasaki, Toshio, et al. “Escherichia Coli Amino Acid Auxotrophic Expression Host Strains for Investigating Protein Structure–Function Relationships.” OUP Academic, Oxford University Press, 8 Dec. 2020, academic.oup.com/jb/article/169/4/387/6026966?login=true.
6. Typical Wastes Generated by Industry Sectors, U.S. Environmental Protection Agency, 17 Jan. 2024, www.epa.gov/hwgenerators/typical-wastes-generated-industry-sectors.