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Access Type

WSU Access

Date of Award

January 2018

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Civil and Environmental Engineering

First Advisor

Shawn McElmurry

Abstract

ABSTRACT

ANALYSIS OF MICROMIXERS TO MINIMIZE SCALING EFFECTS ON REVERSE OSMOSIS MEMBRANES

by

Jeremy Walker

December 2018

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.

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