Improving Membrane Distillation Performance by Reducing the Effluent Concentration of Volatile and Semi-Volatile Contaminants

Abstract

Direct 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.