Keywords

Membrane, Productivity, Fouling, Surface Characteristics, Modeling

Abstract

The reduction of membrane productivity (i.e. membrane fouling) during operation occurs in virtually all membrane applications. Membrane fouling originates from the method by which membranes operate: contaminants are rejected by the membrane and retained on the feed side of the membrane while treated water passes through the membrane. The accumulation of these contaminants on the feed side of the membrane results in increased operating pressures, increased backwashing frequencies, increased chemical cleaning frequencies, and increased membrane replacement frequencies. The most significant practical implication of membrane fouling is increased operating and maintenance costs. As such, membrane fouling must be properly managed to ensure successful and efficient operation of membrane systems. This document presents four independent studies regarding the fouling of size exclusion and diffusion controlled membranes. A brief description of each study is presented below. The first study systematically investigated the fouling characteristics of various thin film composite polyamide reverse osmosis (RO) and nanofiltration (NF) membranes using a high organic surficial groundwater obtained from the City of Plantation, Florida. Prior to bench-scale fouling experiments, surface properties of the selected RO and NF membranes were carefully analysed in order to correlate the rate and extent of fouling to membrane surface characteristics, such as roughness, charge and hydrophobicity. More specifically, the surface roughness was characterized by atomic force microscopy, while the surface charge and hydrophobicity of the membranes were evaluated through zeta potential and contact angle measurements, respectively. The results indicated that membrane fouling became more severe with increasing surface roughness, as measured by the surface area difference, which accounts for both magnitude and frequency of surface peaks. Surface roughness was correlated to flux decline; however, surface charge was not. The limited range of hydrophobicity of the flat sheet studies prohibited conclusions regarding the correlation of flux decline and hydrophobicity. Mass loading and resistance models were developed in the second study to describe changes in solvent mass transfer (membrane productivity) over time of operation. Changes in the observed solvent mass transfer coefficient of four low pressure reverse osmosis membranes were correlated to feed water quality in a 2,000 hour pilot study. Independent variables utilized for model development included: temperature, initial solvent mass transfer coefficient, water loading, ultraviolet absorbance, turbidity, and monochloramine concentration. Models were generated by data collected throughout this study and were subsequently used to predict the solvent mass transfer coefficient. The sensitivity of each model with respect to monochloramine concentration was also analyzed. In the third study, mass loading and resistance models were generated to predict changes in solvent mass transfer (membrane productivity) with operating time for three reverse osmosis and nanofiltration membranes. Variations in the observed solvent mass transfer coefficient of these membranes treating filtered secondary effluent were correlated to the initial solvent mass transfer coefficient, temperature, and water loading in a 2,000 hour pilot study. Independent variables evaluated during model development included: temperature, initial solvent mass transfer coefficient, water loading, total dissolved solids, orthophosphorous, silica, total organic carbon, and turbidity. All models were generated by data collected throughout this study. Autopsies performed on membrane elements indicated membranes that received microfiltered water accumulated significantly more dissolved organic carbon and polysaccharides on their surface than membranes that received ultrafiltered water. Series of filtration experiments were systematically performed to investigate physical and chemical factors affecting the efficiency of backwashing during microfiltration of colloidal suspensions in the fourth study. Throughout this study, all experiments were conducted in dead-end filtration mode utilizing an outside-in, hollow-fiber module with a nominal pore size of 0.1 µm. Silica particles (mean diameter ~ 0.14 µm) were used as model colloids. Using a flux decline model based on the Happel's cell for the hydraulic resistance of the particle layer, the cake structure was determined from experimental fouling data and then correlated to backwash efficiency. Modeling of experimental data revealed no noticeable changes in cake layer structure when feed particle concentration and operating pressure increased. Specifically, the packing density of the cake layer (l-cake porosity) in the cake layer ranged from 0.66 to 0.67, which corresponds well to random packing density. However, the particle packing density increased drastically with ionic strength. The results of backwashing experiments demonstrated that the efficiency of backwashing decreased significantly with increasing solution ionic strength, while backwash efficiency did not vary when particle concentration and operating pressure increased. This finding suggests that backwash efficiency is closely related to the structure of the cake layer formed during particle filtration. More densely packed cake layers were formed under high ionic strength, and consequently less flux was recovered per given backwash volume during backwashing.

Notes

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Graduation Date

2007

Semester

Fall

Advisor

Taylor, James S.

Degree

Doctor of Philosophy (Ph.D.)

College

College of Engineering and Computer Science

Department

Civil and Environmental Engineering

Degree Program

Environmental Engineering

Format

application/pdf

Identifier

CFE0001854

URL

http://purl.fcla.edu/fcla/etd/CFE0001854

Language

English

Release Date

December 2007

Length of Campus-only Access

None

Access Status

Doctoral Dissertation (Open Access)

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