Implementing a Membrane Bioreactor – Membrane Distillation Pilot-Scale System for Decentralized Potable Water Reuse

Abstract

Membrane distillation (MD) is a promising water treatment technology capable of producing high-quality potable water from concentrated or contaminated streams. However, its widespread adoption has been hindered, in part, by challenges such as fouling and scaling, which can undermine the rejection mechanism during long-term, real-world operations. In MD, a hydrophobic, microporous membrane prevents water from flowing through the pores in liquid form. When the water is heated, some of the water evaporates and transports through the membrane pores in vapor form, leaving behind any non-volatile contaminants and condensing as high-quality distillate. Over time, the membrane can become susceptible to liquid pore flow as foulants and scalants deposit on the membrane surface, reducing the hydrophobicity and increasing pore size. Currently, there is a lack of research on methods to sustain high water flux and water quality when MD is used in the field with contaminated waters. As a result, MD has typically been studied in isolation, with limited integration into broader treatment systems that reflect real-world treatment processes. This study identifies the most problematic foulants and scalants that affect MD performance for water reuse, evaluates various cleaning strategies in a pilot-scale system over extended operation, and links a pilot-scale MD system with a pilot-scale membrane bioreactor (MBR) system for potable water reuse. Organic compounds can interfere with MD by adhering to the membrane through hydrophobic-hydrophobic interactions, but determining the hydrophobicity of organic matter in wastewater is challenging. High-performance liquid chromatography (HPLC) was employed to analyze the organic compounds in water reuse reverse osmosis (RO) concentrate, revealing that they were primarily hydrophilic. When the concentrate was used to foul bench-scale membranes, calcium salts were identified as the primary cause of water flux decline. Cleaning with acid proved effective at recovering most of the lost water flux. A pilot-scale MD system was operated for 90 days to treat reclaimed water to potable standards. Routine acid cleaning was performed during the testing period to mitigate calcium carbonate scaling and maintain high water flux, although some decline in water flux could not be fully recovered by this method. The use of a chelating agent improved water flux recovery and even reversed the trend of increased pore wetting by removing calcium sulfate surface scaling. In contrast, chlorination proved ineffective as a cleaning strategy and led to the formation of disinfection byproducts in the distillate. The MD system was integrated with a pilot-scale MBR housed in a CONEX shipping container, creating a compact, mobile system designed to treat wastewater to potable standards. The MBR subsystem consistently produced water of higher quality than a nearby conventional wastewater reclamation facility, achieving over 6-log removal of bacteriophages when paired with a small UV reactor. The MD system itself demonstrated over 4-log removal of the tested bacteriophages, resulting in a total of 10-log removal across the entire treatment process. While the system could be transported and operated successfully at another facility, higher levels of solids in the wastewater at the second site occasionally led to system shutdowns, impacting water quality.