Open Access Dissertation
Date of Award
Civil and Environmental Engineering
ANALYSIS OF MICROMIXERS TO MINIMIZE SCALING EFFECTS ON REVERSE OSMOSIS MEMBRANES
Advisor: Dr. Shawn McElmurry
Major: Civil Engineering
Degree: Doctor of Philosophy
Structural elements, typically mesh spacers, are required between membrane leaves in spiral
wound elements to ensure flow through reverse osmosis (RO) modules. The standard diamond-shaped
mesh spacer results in non-ideal hydrodynamics that can lead to fouling, which ultimately reduces the
flux of water through the membrane and the operational life of the unit. Additionally, traditional mesh
feed spacers do not allow for reverse flow cleaning due to obstructed flow paths and, once fouled, the
entrapment of scale. To address this shortcoming, a novel method for separating RO membrane leaves
in spiral wound elements is developed and evaluated.
Three-dimensional (3-D) printing was utilized to manufacture micromixers directly on
membrane swatches. To enhance performance, a two-dimensional computational fluid dynamic model
was used to select the optimal geometry and pattern of 3-D printed micromixers. The optimal geometry
selected created unhindered flow between 0.2 m/s and 0.3m/s, using an inlet flow velocity of 0.104 m/s,
across 40% of the membrane surface.
Laboratory experiments were conducted to evaluate the performance of micromixers and
compared to unmodified membranes with a standard 20 mil (0.508 mm) mesh feed spacer. Pure water
flux and salt rejection were found to be similar to standard membranes, indicating the 3-D printing
process did not damage intrinsic membrane properties. Calcium sulfate scaling experiments were
conducted. Scale initially began to form within 2 hours of treatment resulting in a flux decline of
approximately 10% for both modified and unmodified membranes. Over 14 hours, an average flux
decrease of 24% was observed for modified membranes compared to an average flux decrease of 78%
for the unmodified membranes. This demonstrated the improved resistance to fouling created by the
open channel design with optimal flow conditions.
Based on the open-channel flow paths created using 3-D printed micromixers, improved scale
removal by reverse flow cleaning procedures was evaluated. The modified membranes showed 5-10%
more removal for calcium sulfate compared to the unmodified membrane utilizing a 20 mil (0.508 mm)
mesh feed spacer. Following cleaning, all three unmodified membranes and feed spacers had significant
remaining scale, while all three modified membranes showed minimal scale.
Micromixers printed directly to the membrane surface offers the ability to optimize feed
channel hydrodynamics, reduce scale formation, minimize flux decline, and allow for reverse flow
cleaning of fouled membranes, representing a significant advancement in membrane technology.
Walker, Jeremy, "Analysis Of Micromixers To Minimize Scaling Effects On Reverse Osmosis Membranes" (2018). Wayne State University Dissertations. 2132.