TECHNOLOGY AREA(S): Materials
OBJECTIVE: Explore the use of diffusiophoresis to remove suspended particles and bacteria from water.
DESCRIPTION: Military operations frequently take place where safe drinking water is not available. Water has been 30 to 40% of the daily logistical burden in Iraq and Afghanistan, risking soldier lives with every water supply convoy. Soldier dehydration of 3 to 4% can reduce solider performance up to 48%. The universal unit level average water requirement is 25 L (25kg) of water per solider per day, with the requirement raising to 60L (60 kg) per solider per day for a fully developed theater. Currently, the Army uses a number of water purification technologies to meet this logistical burden, but these technologies introduce additional supplies into the logistics train (e.g. filters, membranes, and/or flocculation agents) for proper function and either require high energy costs associated with pumping or low efficiency by relying on sedimentation. An alternative methodology, based on diffusiophoresis, may be capable of providing continuous removal of suspended particulates and solutes from an aqueous stream. Diffusiophoresis is the induction of motion of suspended particles as a result of the presence of a concentration gradient. Membraneless water filtration has been recently demonstrated through the use of dissolved carbon dioxide (CO2) into a colloidal suspension. The dissociation of CO2 forms ions with substantial differences in diffusivities, leading to diffusion potentials significantly larger than ordinary salt gradients. The currently demonstrated process was conducted in micro-channels, which produced 2 µL/h at an estimated energy consumption (for a single channel and assuming 50% clean water recovery) on the order of 0.1 mW·h/L. In comparison to other filtration processes, this represents a potential decrease in filtration energy required by three orders of magnitude. Even when considering power for a total system based on diffusiophoresis, this represents a potentially revolutionary savings in power requirements. Such a process could also be used in conjunction with traditional membrane filtration to mitigate fouling. This filtration methodology is potentially scalable to outputs capable of meeting the above operational goals by creating arrays of micro-channels that share CO2 sources. Optimization of channel dimensions and concentration gradients, performance in the presence of salts, and removal of proteins or bacteria are all potential areas for improvements.
PHASE I: Demonstrate and optimize continuous diffusiophoresis without the use of membranes or filters to maximize particle removal from aqueous streams to achieve <1.0 nephelometric turbidity units (NTU). In addition, demonstrate capability to provide bacterial filtration for common water contaminants and demonstrate a path to remove 95% of bacteria from real-world freshwater sources (e.g. lake/river). Address basic scaling requirements of diffusiophoresis filtration for production of 25L of clean water per day from real-world freshwater sources, including size, weight, and total system power (to include CO2 generation from the atmosphere).
PHASE II: Continue optimization efforts to reduce turbidity to <0.5 nephelometric turbidity units (NTU). Demonstrate performance that meets or exceeds current state of the art (99.9% or better) reduction of bacterial contaminants from real world freshwater sources (e.g. lake/river). Sample source selection(s) should be coordinated with the Army prior to demonstration. Examine feasibility of purification from salinated sources. Examine feasibility of diffusiophoresis to provide filtration of other toxins, to include insecticides and heavy metal contaminants. Provide methodologies for deriving device designs from operational requirements on filtered output, power input and device size/weight.
PHASE III: Development of practical devices for both civilian (e.g. FEMA) and DoD use that represent a substantial reduction in either size, power, or both at output levels comparable to current technologies.
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KEYWORDS: Water Purification, Diffusiophoresis, Turbidity