Liquid Radwaste Processing wiri~ Spiral Wound Reverse Osmosis

Abstract

Two different reverse osmosis systems were investigated in this work. The first was a 50-element plant-scale system that is used to treat 2200 cubic metres of AECL (low to intermediate level) liquid radwastes annually. It uses thin film composite (TFC) membranes and operates at an applied pressure of 2760 kPa, with a fixed crossflow of about 40 L/min. The other system uses the same thin film composite membranes for waste processing, but is a 2-element pilot-scale system. It is operated at pressures ranging between 1500 kPa and 7000 kPa, at a fixed crossflow of 55 L/min. The average lifetime of the thin film composite membranes in the plant-scale processing application at AECL is about 3000 hours. After this service life has expired the rejection efficiency (for bulk conductivity) declines rapidly from 99.5% to about 95% as the membranes become impaired from chemical cleaning procedures that are required after each hundred cubic metres of waste are treated. The permeation flux for the plant-scale system decreases from about 2.2 L/min/element to below 0.5 L/min/element at the end of the membrane's useful service life. The plant-scale membrane elements, fouled by an assortment of chemicals including calcium phosphate, ferric oxide, and various organics, were successfully regenerated by exposing them to a three-step chemical cleaning procedure (in the pilot-scale system), using detergent, HC1, and an alkaline- based cleaning with EDTA respectively. The 3-step procedure was successful in elevating the flux from 0.5 L/min for the spent membrane, to 1.2 L/min after the three step cleaning procedure. The 1.2 L/min post-cleaning flux could be maintained at a crossflow velocity of 55 ~/min/vessel. The decontamination factor (DF), which is the feed to permeate concentration ratio, was determined for cesium and strontium. For the plant-scale system (at the operating pressure of 2760 kPa), the DF of cesium decreased from about 100 when the membranes were new, to about 30 after they were replaced. After cleaning the fouled membranes with the pilot-scale system, the DF for cesium increased from about 30 to 50, as the applied pressure to the system was increased from 1500 kPa to 5500 kPa. By comparison, the strontium DF increased for the fouled membranes at the operating pressure of 2760 kPa, from about 1000 (when they were new), to about 4000 for the spent membranes. The strontium DF was unaffected by the applied pressure, however. The increase of strontium DF is believed to be due to the exchange of strontium with deposited calcium on the fouled membrane.