Pretreatment of Reverse Osmosis Concentrate from Reclaimed Water for Conventional and High-Efficiency Reverse Osmosis and Evaluation of Electrochemical Production of Reagents

The primary goal of this research was to investigate several water treatment unit operations for converting RO concentrate produced from treated municipal wastewater into potable water. The secondary goal was to evaluate the use of an electrochemical cell for producing the reagents needed to operate a fluidized bed crystallization reactor (FBCR), regenerate ion exchange media, and produce a ferric iron coagulating agent. The effectiveness of the pretreatment processes to prevent membrane fouling were evaluated for conventional and high-efficiency reverse osmosis (HERO). Fluidized bed crystallization removed 93 to >97% of hardness ions, 42% of silica, and 6.5% of total organic carbon. Membrane fouling during HERO was lower than that for conventional RO for pretreatment using fluidized bed crystallization and ion exchange. However, conventional RO with ferric iron coagulation following fluidized bed crystallization and ion exchange showed the least membrane fouling and increased recovery in the second stage RO by 470%. The use of an electrochemical cell for generating the reagents needed for the pretreatment processes was evaluated. Energy costs for operating the electrochemical cell for making acid, base, and ferric iron coagulant were 4.1 kWh per m³ of RO concentrate. The use of electrochemically generated reagents combined with fluidized bed crystallization produces no waste solutions from the pretreatment processes.