Low-Salt-Rejection Reverse Osmosis for Energy-Efficient Brine Management

Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Elimelech, Menachem

Abstract

A steady fresh water supply is essential to national security, industrial operations, and economic growth. However, rising global water stress poses severe challenges for securing the water supply to keep pace with accelerated industrialization, while supporting an increased population. Meanwhile, industrial wastewater discharged into the environment without proper treatment can lead to severe pollution of aquatic ecosystems and contaminate freshwater resources. Brine and wastewater management, therefore, needs to achieve both water recycling and proper disposal of pollutants. Minimal and zero liquid discharge (MLD/ZLD) are brine management strategies that have attracted worldwide attention. While conventional reverse osmosis (RO) has been proposed as a promising technology in desalination and MLD/ZLD processes, its application is limited by the maximum hydraulic pressures that current RO membranes and modules can withstand. Therefore, developing innovative membrane technology with high energy-efficiency and technology readiness is necessary to displace inefficient and expensive thermal technologies in brine management. The overall goal of this dissertation is to develop membrane-based technology to facilitate energy-efficient brine management. Specifically, we proposed and advanced the development of low-salt-rejection reverse osmosis (LSRRO), a novel staged RO process that employs low-salt-rejection membranes to process high salinity feed streams beyond the limit of operating pressures. In this dissertation, we modeled the expected energy consumption of LSRRO and demonstrated that it requires two to three times less energy than thermal desalination. Nevertheless, a number of material and process design challenges must be addressed in the development of LSRRO. These include proving the concept of LSRRO, determining optimal system design under different conditions, understanding transport mechanisms in low-salt-rejection membranes, arriving at controllable modification of membranes to different salt rejection, and examining the influence of mineral scaling. This work aims to provide a roadmap for the academic and industrial development of LSRRO. A critical need for the development of LSRRO is to understand the water and salt transport mechanism of the low-salt-rejection membranes. Furthermore, the application of LSRRO requires the ability to accurately predict the system performance with a transport model customized for the process. Therefore, this dissertation reports on the development of a transport model designed specifically for LSRRO. By combining solution-friction transport theory with the Spiegler-Kedem-Katchalsky model, we developed a transport model that can both accurately describe LSRRO process and explain the fundamental nature of water and salt transport in low-salt-rejection membranes by fundamental physical parameters. Membrane scaling is a practical concern that must be examined when treating high salinity brine, as scaling can dramatically reduce the water production rate and therefore energy efficiency. Although extensive pretreatment can remove scale-forming ionic species, elucidating the scaling behavior of low-salt-rejection membranes is essential. Our experimental results highlight the observation that managing the effective concentrations of scale-forming ions is the key to limit mineral scaling. This dissertation has identified the opportunity for LSRRO to enable energy-efficient management of saline brines while also addressing the most critical research needs for its development. In doing so, a novel process has been developed, fundamental mechanisms have been elucidated, and practical concerns have been addressed. This multi-faceted study of LSRRO has combined fundamental science with industrially relevant process design. As a result, it has guided the study of pressure-driven membrane processes deeper into ZLD application and laid a strong foundation for energy-efficient brine treatment via LSRRO.

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