Abstract
1,4-Dioxane is a Group 2B carcinogen that poses immediate risk to human health and is a byproduct of many common industrial processes1. For over two decades, 1,4-dioxane was improperly disposed of into a tributary that feeds into the Huron River, the primary water supply for the city of Ann Arbor, Michigan3. The current remediation method is expensive and inefficient. Additionally, it introduces bromate, another carcinogen, to the water in the process10,11. Our team engineered and optimized a freshwater bacteria (Pseudomonas putida S16) with a tetrahydrofuran monooxygenase gene complex (THFMO) known to naturally degrade 1,4-dioxane14. Our bacteria is suited to operate within a bioreactor, which we aim to implement in tandem with existing infrastructure. Our ultimate goal is to improve the sustainability, efficiency, and efficacy of the current water treatment process.
Project Description
The Problem
1,4-Dioxane is a Group 2B Carcinogen and is a common byproduct of industrial processes1. As a cyclic ether, 1,4-dioxane does not break down easily and is prevalent in water sources and soil. Since the compound has been identified as possibly carcinogenic to humans, 1,4-dioxane poses an immediate risk to human health2. From 1966-1986, a medical device manufacturer improperly disposed of wastewater containing 1,4-dioxane into a tributary that feeds into the Huron River, the primary water supply for Ann Arbor, Michigan3.
Ann Arbor is home to over 120,000 residents and is the location of our home institution, the University of Michigan, which enrolls nearly 50,000 students4,5. Currently there is a large groundwater plume containing 1,4-dioxane covering approximately 6 square miles, about 15.5 square kilometers, of the Ann Arbor area6. Despite over 30 years of cleanup efforts, the plume is actively spreading, threatening our community. 1,4-Dioxane does not stick to soil and migrates into groundwater with the potential to reach surface water7. There remain concentrations exceeding 85 parts per billion (ppb) and even reaching up to 2200 ppb in the plume8. These are concentrations over 300 times Michigan’s legal limit of 7.2 ppb in residential drinking water, which was set by the state in 20179.
Current Methods
Currently, the status quo solution involves extraction wells that are used to bring up contaminated water to the surface. Then, ozone/hydrogen peroxide (O3/H2O2) advanced oxidation and UV irradiation are used to treat the contaminated water before release into the Huron River10. Due to this process being only 40-70% efficient and costing upwards of 1.2 million dollars per year, only one-eighth of the total 1,4-dioxane in the water supply had been removed as of 201910. Furthermore, bromate, another likely human carcinogen, is released as a byproduct of the oxidation process11. As the 1,4-dioxane plume continues to spread through the groundwater of Ann Arbor, Scio Township, and surrounding areas, there exists an imminent threat to residents12.
Our Solution
There is a need for a more efficient degradation process for 1,4-dioxane. Our solution capitalizes on the ability of genetically engineered aqueous bacteria to metabolize 1,4-dioxane with efficiency topping 55%13. Our proposed system of treatment involves synthetically modifying freshwater bacteria (Pseudomonas putida S16) to include a tetrahydrofuran monooxygenase gene (THFMO), first isolated from a bacteria found in industrial sludge and known to naturally degrade 1,4-dioxane14. We achieved this modification through the use of a plasmid vector containing the THFMO gene, an inducible arabinose promoter, and a gentamicin antibiotic resistance gene. Our team employed genetic engineering principles to degrade 1,4-dioxane in order to design a bioreactor supporting a groundwater treatment system for use in the affected communities.
The bioreactor will function via a two-part “scrubbing” process, using both an initial biodegradative co-metabolic step and a secondary chemical treatment to safely achieve high degradation efficiency. In contrast to the current system to remove 1,4-dioxane, this system can be scaled up and implemented alongside existing infrastructure at wastewater treatment facilities. Specifically, our bioreactor will be placed after the secondary steps, which involves removing any existing biomass from the water. Once our bioreactor does its work, the tertiary steps, which serve to remove nutrients, bacteria, and other such smaller contaminants, will remove anything that leaves our bioreactor. This removes a significant amount of cost that comes with creating an independent treatment system15. Furthermore, our transformed bacteria metabolize 1,4-dioxane into non-harmful products. Our system does not create and release toxic chemicals such as bromate, which is a downside of the existing remediation process. Therefore, by implementing our bioreactor alongside existing infrastructure and not introducing other contaminants into processed water, our system is ultimately more environmentally and fiscally conscious than existing methods.
Our bioreactor system is also designed to be low maintenance. With regular nutrient supplementation, the biofilm will continue forming on a rotating disk component without the need to frequently replace components. This sustainable aspect of our bioreactor is in contrast to the existing system where one must continuously replace reagents, such as those for O3/H2O2 advanced oxidation. After initial construction into the existing water treatment facility, the cost of nutrients to support bacterial growth is the primary cost differentiator between ours and existing processes. We estimate the electric cost for running the mechanical components of the bioreactor to be comparable to the cost for running the current 1,4-dioxane cleanup method15. Given that our experimental results are promising for higher degradation efficiency than existing methods, coupled with the lower marginal operating costs, we are hopeful that our system is a marked improvement on traditional processes used to clean up the plume. Ultimately, our bioreactor is designed to be less expensive, lower maintenance, more efficient, and more sustainable than the current 1,4-dioxane treatment methods, which ultimately brings us closer to our goal of providing a better method for providing clean drinking water to every resident of the Ann Arbor community and beyond.
Generalizability
The development of a bioreactor utilizing Pseudomonas putida S16 offers a promising solution for the degradation of 1,4-dioxane in contaminated water sources. As water treatment continues to rise in price, it is becoming increasingly important to develop alternative low-cost methods that are accessible to communities affected by water contamination. Reliable water treatment should be accessible to all, particularly as the 1,4-dioxane plume spreads. Furthermore, we designed our bioreactor to be easily modified to target other difficult-to-remove water contaminants, such as nitrites and nitrates. The affordability of our method could specifically appeal to underfunded communities suffering from the effects of water contamination. Overall, the utilization of genetic engineering to enhance the efficiency of water treatment could ultimately improve accessibility to clean water.
Our bioreactor system is designed around the central theme of scalability, thereby enabling its implementation on a larger level to combat the expanding plume in the Ann Arbor area. In the center of our design is a set of rotating disks that have biofilms of our engineered bacteria that can degrade 1,4-dioxane. Our system passes water through several times to improve efficiency. Multiple filtration and sanitizing steps also ensure no bacteria are unintentionally released with the remediated water. Since the disks are designed to hold almost any biofilm-compatible bacteria, it is possible to generalize this design to target other contaminants that current water treatment processes inefficiently eliminate or cannot eliminate at all. Bacteria targeting such contaminants can be engineered and directly used in our bioreactor design. Even within the Huron River watershed, we envision these adapted bioremediation solutions being implemented on a large-enough scale to facilitate extensive decontamination.
Screen capture of our CAD implementation of a rotating bioreactor
