Date of Award

12-2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Science

Advisor

Ladner, David A

Committee Member

Karanfil , Tanju

Committee Member

Freedman , David L

Abstract

Membrane fouling is a major operational issue in reverse osmosis (RO) desalination plants. In particular, plants treating brackish groundwater can encounter troublesome inorganic scales, including carbonates, sulfates, and silicates. A novel cleaning method is proposed to remove inorganic scales from fouled RO membranes usinag dissolved CO 2 . As CO2 molecules encounter membrane foulants, the surfaces serve as nucleation sites for small bubbles to form and shear off foulants. Dissolved CO2 solutions were prepared by bubbling CO2 gas into water held in a pressure vessel. Gas dissolution was confirmed by enhanced exit velocities for water containing CO2 , due to the increase in volume from exsolution, when compared to water containing less soluble N2 . A dissolved CO2 solution was effective in removing scale from RO membranes through bubble nucleation. Membranes scaled with CaCO3 were cleaned for 10 minutes with a once-through dissolved CO2 solution of approximately pH 4.5, achieving an average 80% flux recovery. Controls were performed with other cleaning regimes to isolate effects from pH and air scouring present in CO2 cleaning. An HCl solution at pH 3 provided an average flux recovery of 79% after circulating through the system for 30 minutes, while an HCl solution at pH 4 only gave an average 20% flux recovery. Trials using N2 gas in place of CO2 only produced a 6% flux recovery on average. Lowering the pH of the N2 solution to pH 4 with HCl boosted cleaning slightly to an average 8% flux recovery. Thus, the low pH of the CO2 solution at pH 4.5 and bulk phase air scouring are minor mechanisms in scale removal. In addition, membranes scaled with calcium silicates were not cleaned using dissolved CO2 - only NaOH at pH 12 plus sodium dodecyl sulfate provided significant cleaning. Future work should be done with additional scale types to narrow in on the mechanism for cleaning by dissolved CO2 .

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