Municipal and Industrial Wastewater Treatment via Reverse Osmosis, Electrocoagulation, and Dissolved Air Flotation

Abstract

This dissertation reports mechanistic investigations of selected technologies employed for physicochemical treatment and reuse of industrial and municipal wastewater. The first part investigates simultaneous biological, colloidal, organic, and inorganic fouling of reverse osmosis (RO) membranes harvested from the world’s largest potable water reuse facility (Orange County Water District, California). This research was based on the hypothesis that combined fouling worsens RO performance and shortens membranes’ lifespan at full-scale. Detailed surface characterization of several end-of-life RO elements from each of the three stages of the full-scale plant revealed copious amounts of biocolloidal, organic, siliceous, and non-siliceous foulants on all membranes but to different extents. Bioorganic foulants dominated the first and second stages whereas inorganic (mainly silicon-related) foulants were more abundant in the third stage resisting chemical cleaning. Presence of organic silica (siloxanes) on membranes was reported for the first time. Difficulties in chemically removing bioorganic foulants were attributed to their adhesion to membrane surfaces via bridging through multivalent calcium, magnesium, and aluminum ions. This work provided the first detailed look into differences in the accumulation of various foulant classes on each element, combined fouling impacts on lead and lag elements of all three stages, as well as fouling impacts on physicochemical characteristics of membrane surfaces and its (ir)reversibility during chemical cleaning. Outcomes revealed that siliceous foulants were the main contributors to membranes’ end-of-life. Hence, silicon control is recommended to extend membrane lifetime during municipal wastewater reclamation and potable reuse.

The second part of the research evaluated three processes for treatment of saline wastewaters: namely electrocoagulation, dissolved air flotation, and sedimentation. Temperature and natural organic matter effects on pitting corrosion and coagulant speciation during aluminum electrocoagulation were evaluated. Results suggested the need for pH adjustment at low temperatures to limit aluminum solubility under enhanced coagulation conditions. Finally, sedimentation and dissolved air flotation were compared to purify turbid and hypersaline produced water generated during unconventional oil and gas exploration and production. A combination of pre-hydrolyzed (aluminum chlorohydrate) and polyacrylamide coagulants were used to develop ultrahigh rate gravity liquid-solid separation processes to generate “clean brine” with total residence times of only 5-6 minutes.