Browsing by Subject "water treatment"
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Item Open Access Implementing Electrochemical Impedance Spectroscopy for the In Situ Analysis Of Conducting-Membrane Fouling(2020) DuToit, Marielle McCallenAbstract
As natural resource scarcity and industrial productivity continue to rise, membrane filtration technologies provide a compelling solution for the production of clean water. Membranes are versatile, energy-efficient, and highly effective, yet they suffer from the indomitable problem of fouling. Abundant research has been conducted on this topic, but new methods for understanding and assessing fouling are still emerging, and are nevertheless needed. The present work endeavors to study membrane fouling from yet another perspective using a powerful electrochemical technique known as electrochemical impedance spectroscopy (EIS). EIS is a non-destructive electrical perturbative method, thus it can be performed during filtration. While some research groups have applied EIS for membrane characterization, none have yet incorporated conductive polymeric membranes into the electrochemical setup.
The primary objectives of this study were to (1) synthesize a robust and sufficiently conductive polymeric membrane for use as a working electrode; (2) develop a non-invasive non-Faradaic EIS method to characterize membrane fouling in real time; (3) separate contributions to total fouling from processes happening on the membrane surface and within the interior pore network. Membrane fouling was studied using three model foulants, bovine serum albumin (BSA), humic acid, and colloidal silica in a supporting electrolyte of phosphate buffered saline (PBS) and potassium nitrate (KNO3), respectively. To better understand the spatial position and magnitude of fouling, EIS spectra were interpreted by fitting equivalent circuits informed by the physical structures of the membrane surface and interior.
Data from the EIS fouling tests showed good agreement between changes in impedance, conductance, and capacitance and reduction in permeate flow, which is the conventional parameter used to monitor fouling severity. The conductive coating also allowed for fouling to be differentiated between the surface and interior layers of the membrane. Moreover, experiments with feed solutions containing separate foulants in different solution chemistries verified that EIS is sensitive enough to differentiate between various membrane fouling effects as well as irreversible fouling phenomena. These results suggest that conductive membranes can be used alongside EIS to spatially and temporally characterize membrane fouling as it happens in real time without the need to remove or damage the membrane for analysis.
Item Open Access Improving Membrane Distillation Performance by Reducing the Effluent Concentration of Volatile and Semi-Volatile Contaminants(2017) Winglee, JudyDirect contact membrane distillation (DCMD) technology has the potential to disrupt the water treatment industry by greatly reducing the cost of seawater desalination and industrial wastewater treatment. However, in order for DCMD technology to be developed for these applications, better characterizations of DCMD treatment capabilities are needed. Prior research has shown that DCMD technology has high salt rejection, but few studies have addressed the potential for volatile and semi-volatile contaminant accumulation in the DCMD effluent. Accounting for additional treatment processes to reduce the concentration of these volatile contaminants is vital for determining the cost-effectiveness of DCMD systems.
To improve characterizations of DCMD treatment capabilities, the work in this dissertation describes a novel method for predicting the quality of DCMD effluent and develops feed water guidelines for DCMD applications. The DCMD effluent contaminant concentration was modeled using a mass balance approach to account for the Fickian diffusion of contaminants into the permeate collection stream and the contaminant losses due to evaporation and sorption during DCMD operation. This represents a novel approach to modeling the quality of effluent produced by the DCMD system. Validation of the contaminant concentration model showed that the model had good agreement with the results from bench-scale DCMD testing (within 12% average normalized root-mean-squared-error).
The validated contaminant concentration model was used to assess the performance of commercial-scale DCMD systems and identify contaminants that accumulated the most in the DCMD effluent. The results showed that compounds with very low Henry’s constants (Henry’s constants less than 28PaL/mol) were rejected by the DCMD system, while the concentrations of more volatile compounds were magnified in the DCMD effluent. These findings illustrate that contaminant accumulation in DCMD effluent is a significant issue that must be considered when designing DCMD systems.
To address the high contaminant magnification, the operating conditions of the DCMD system were optimized to reduce the contaminant concentration. Operating the DCMD system using conditions that minimized the contaminant accumulation, instead of conditions that maximized the water flux, decreased the accumulation of some contaminants by over 3x. The contaminant accumulation at these conditions was used to identify the maximum feed water contaminant concentrations for two prominent DCMD applications, seawater desalination and oil and gas produced water treatment. These feed water quality guidelines are an important tool for determining what applications DCMD is suitable for.
The contaminant concentrations in representative seawaters and produced waters were compared to the feed water guidelines for a stand-alone DCMD system to determine if these waters were adequately treated by DCMD for either potable water usage or discharge to publicly owned treatment works. The results of the comparison showed that the contaminant concentrations in the seawaters were within the feed water guidelines, indicating that DCMD seawater desalination is a good treatment method for producing potable water. However, the contaminant concentrations in the produced waters were greater than the limits described in the produced water treatment feed water guidelines. This finding indicated that additional treatment should be used in conjunction with DCMD processing of produced waters, which may increase treatment costs. The contaminant concentration model for predicting the contaminant concentration in the DCMD effluent and the feed water quality guidelines provide a significant advance in characterizing the performance capabilities of DCMD systems, and using these tools is vital for determining applications for DCMD technology.