Browsing by Subject "membrane distillation"
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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.
Item Open Access Novel Ceramic Membranes for Membrane Distillation: Surface Modification, Performance Comparison with PTFE Membranes, and Treatment of Municipal Wastewater(2011) Hendren, Zachary DoubravaCurrent global water scarcity and the spectre of a future critical shortage are driving the need for novel and energy saving water technology approaches. Desalination of seawater and the reuse of treated wastewater effluent, which have historically been viewed as undesirable water sources, are increasingly being explored as sources for reducing water consumption. Although the dominant technologies for taking these water sources to potable quality, energy consumption still makes them unsustainable for widespread application. Membrane distillation (MD) is an innovative water purification method that has shown promise as a technology that can address several of these issues. MD is a membrane process that produces very high quality product water. However, similarly to other thermal desalting processes, MD utilizes heat as the dominant source of energy rather than pressure, and can potentially be used to produce water at higher recoveries (and therefore less waste) than is feasible with existing approaches. Another important advantage of MD is that the water separation occurs at modest temperatures (<90oC), opening the door for the utilization of currently usable waste heat sources. Despite these advantages, MD is primarily a lab scale technology, and key questions concerning process performance, including flux magnitude, energy efficiency, fouling propensity, membrane performance, and long-term system performance must be addressed to fully vet this technology.
This work is represents an attempt to provide insight into several of these issues. The overarching approach taken throughout this project is the parallel evaluation of ceramic membranes alongside commonly used polymeric (PTFE) membranes. The combined factors of MD being a relatively nascent technology and the fundamental separation mechanism point toward initial real-world applications of MD for the treatment of high concentration water that may necessitate membranes exposure to harsher thermal and chemical environments. The robust and inert nature of ceramics make them ideal candidates for such application, although their hydrophilic surface do allow for direct implementation in MD. The first phase of this work details the evaluation of several candidate surface treatments for modifying ceramic membranes and shows that aluminum oxide ceramic membranes can be successfully modified with perfluorodecyltriethoxysilane to possess the necessary hydrophobicity for MD application. The effectiveness of the surface treatment in modifying the membrane surface chemistry was assessed using a multitude of analytical approaches, which showed that the modified ceramic surface attained high hydrophobicity and thus are suitable for application of the membranes in direct contact membrane distillation (DCMD).
The next phase of research details the development and verification of a model for DCMD performance. The relative membrane performance was compared, with the polymeric membrane consistently outperforming the modified ceramics, which was attributed to a combination of superior thermal and physical membrane characteristics. Beyond attempting to evaluate the performance differences, this model allows the consideration of various operational scenarios, focusing on membrane flux and energy performance as various membrane and operational parameters change to determine conditions that maximize MD performance as well as provide insight critical to develop MD-specific membranes.
Finally, membrane performance was evaluated during the treatment water containing various organic foulants as well for the treatment of municipal wastewater. The results showed that the level of fouling was highly dependent on foulant type, with alginate identified as a component that produces severe fouling under all conditions evaluated, and wastewater fouling being relatively minimal. Membrane cleaning solutions were implemented to show that near-complete flux recovery was attainable, and plain deionized water was shown to be as effective as sodium hypochlorite.