Performance evaluation of a pilot-scale fluidized pellet bed reactor for the treatment of RO/NF concentrates

Disposal of reverse osmosis and nanofiltration membrane concentrate is regulated under several federal and state laws and regulations. The methods used to dispose reverse osmosis/nanofiltration membrane concentrate determine which regulatory requirement must be met. The main problem associated with the membrane concentrate disposal is its high total dissolved solids (TDS) concentrations. Calcium, sulfate and bicarbonate present in excess amounts are the main source of high total dissolved solids in membrane concentrate. The TDS can be lowered by crystallizing calcium carbonate into the surface of sand in a fluidized medium.; The objective of the study was to evaluate the performance of pilot-scale fluidized pellet bed reactor (FPBR) operated under various conditions. The pilot-scale tests each were conducted for 36 hours. The pH of the feed water solution was raised using sodium hydroxide or calcium hydroxide to crystallize CaCO3 onto the sand. The performance of the pilot scale FPBR was evaluated at pH 9 and 9.5 and at antiscalant concentrations of 0 mg/l, 12 mg/l and 24 mg/l. An effort was also made to check the difference in performance by using calcium hydroxide as the pH raising solution instead of sodium hydroxide.; The results of pilot scale tests showed that the removal of TDS can be achieved by crystallizing calcium carbonate onto the surface of the seeding material as a pellet in a fluidized medium. A tapered rubber inlet increased the performance of the FPBR by removing a greater percentage of calcium hardness, total hardness, alkalinity and TDS by more efficiently mixing the injected chemicals and fluidizing the media. Well graded and clean sand must be chosen as a seeding material for the crystallization of calcium carbonate. Sodium hydroxide as pH raising solution effectively fostered pellet growth in the fluidized pellet bed reactor whereas hydrated lime produced large amount of suspension solids by precipitating calcium carbonate into the solution.; The adverse effect of antiscalant on removal of TDS is greater at pH 9.0 (NaOH as pH raising chemical) compared to 9.5 (NaOH and Ca(OH)2 as pH raising chemical). The statistical analysis of results showed that addition of antiscalant concentration in the feed water hindered the performance of the FPBR. The removal percent of calcium hardness, total hardness, and alkalinity dropped significantly after adding antiscalant in the feed water at pH 9 and 9.5. The adverse effects of antiscalant can be minimized by using a granular activated carbon (GAC) filter. Although the concentration of carbon in GAC effluent was less than the tap water concentration, the antiscalant still hinders the performance of FPBR for removing calcium hardness, total hardness, alkalinity and TDS.