Filter Out Heavy Metals
Our system needed to allow diffusion of heavy metal ions across a filter boundary without letting the genetically engineered cells out into the environment. To satisfy this requirement, we bought a hollow fiber reactor with a molecular weight cut-off (MWCO) of 20 kd @ 50%, effectively isolating the E.coli cells (0.7-1.4 micrometers) from the outlet,[1] while allowing diffusion of smaller molecules such as ions across the membrane of the fibers.
Isolated System
The entire filter system is housed in a watertight box to prevent contaminants from entering. To filter out debris that could enter via the collection bucket, a carbon filter is placed in the system before the fiber reactor. This ensures that only water with microscopic contaminants enters the fiber reactor. Thus, the system is as isolated from the outside environment as possible.
Effective Flow Rates
A carbon water filter was added before the hollow fiber reactor in order to filter out large particles and debris that would clog the hollow fiber reactor and to purify the water of certain compounds which would harm the cells. Entering water needed to have an initial flow rate that would allow it to pass through both filters while still allowing time for heavy metal diffusion across the fiber reactor membrane and uptake by the cells. The pump was placed before the carbon fiber in order to move it effectively through the carbon fiber and still have a slow flow rate through the fiber reactor.
Saturation Detection
A subsystem of reporter constructs was set up inside the box in order to detect when the metallothionein in the fiber reactor is saturated with heavy metals and the cells need replacing. The constructs consisted of a chromoprotein, amilCP, downstream from a specific heavy metal inducible promoter. When the metallothioneins became concentrated, the cells turn visibly blue, indicating that the cells need to be replaced.
Maintenance
Our system is powered by a high capacity car battery that is recharged by a compact solar panel. Since our flow requirements are not high, the system can last for up to 2 weeks without any sunlight at all. With several hours of sunlight a day, the system can retain charge for months at a time. Combined with the tough, weatherproof design, this allows our system to purify water fully autonomously for very long periods.
References
Nelson DE, Young KD. Penicillin binding protein 5 affects cell diameter, contour, and morphology of Escherichia coli. J Bacteriol. 2000 Mar182(6):1714-21 p.1719.
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