Acoustically activated trapping for colloidal filtration: a multiscale experimental investigation using laser-based optical diagnostics

Colloidal filtration is widely used in water and wastewater treatment to remove solid, microscopic particles (known as colloids) including inorganic particles, bacteria, and other aggregates present in drinking water sources and wastewater. This process uses pressure to drive a colloid-bearing fluid (e.g., contaminated water) through a microfiltration/ultrafiltration membrane that consists of a series of microscopic channels that allows the fluid to pass through but blocks the passage of colloids.

Over time, the channels become clogged with colloids until the pressure required to drive the fluid through the membrane becomes too great and the membrane must be replaced. Membrane replacement costs represent 50% of the total operational cost of a filtration system. The expense required to operate membrane-type filters is a significant contribution to water access inequality.

To address these challenges, the Principal Investigators (PIs) of this project propose to explore the use of acoustic waves (sound) as a to break up clogs and capture colloids before they reach the membrane interface with the goal of extending the operational lifetime of the membrane and reduce the overall operating costs. The successful completion of this project will benefit society through the generation of fundamental knowledge on membrane fouling and a potential reduction in operating costs which could increase access to clean water in rural areas or communities with inadequate infrastructure.

Additional benefits to society will be achieved through student education and training including the mentoring of one graduate student at Iowa State University (ISU).

Membranes are used in a wide variety of applications to separate solid particles (colloids) from water and waste streams. Fouling during colloidal filtration processes results from the build-up of colloidal particles at a membrane interface, forming what is known as a cake layer. The pressure required to drive fluid through a cake layer increases with time, thus requiring a significant increase in pumping power input.

The overarching goal of this proposal is to develop experimentally validated 2D and 3D models of cake layer formation and break-up at membrane interfaces due to acoustic field interactions. To advance this goal, the Principal Investigators (PIs) propose to test the hypothesis that the application of an acoustic field generates a body force at the membrane interface that modifies the particle distribution within the cake layer.

The specific research objectives are to (1) measure cake layer growth without an acoustic field to understand membrane surface forces; (2) investigate and model acoustic field interactions with the membrane; and (3) optically interrogate acoustic mixing in 3D membranes. The successful completion of this research has the potential for transformative impact through the generation of fundamental knowledge of interfacial phenomena related to external, acoustic field-actuated formation of cake layers in membranes.

To implement the education and training goals of the project, the PIs plan to engage with the ISU chapter of the Society of Hispanic Professional Engineers (SHPE) and the Academic Program for Excellence (APEX) program to develop and deliver learning experiences to high school and undergraduate students underrepresented in STEM. In addition, the PIs plan to leverage existing programs at ISU, such as the Women in Science and Engineering program to introduce, recruit, and mentor women in science and engineering.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.