Byproduct Recovery from Reclaimed Water Reverse Osmosis Concentrate Using Lime and Soda-Ash Treatment

Lime and soda-ash softening of reclaimed water reverse osmosis concentrates as a pretreatment step for concentration by seawater reverse osmosis was the focus of this study. The objectives were removal of the potential fouling minerals of calcium, magnesium, and silica by selective precipitation, while producing byproducts with potential resale value. Three different bench-scale lime-soda processes were evaluated. The traditional method produced low-quality magnesium hydroxide [Mg(OH)₂] and calcium carbonate (CaCO₃) byproducts. A modified process with pre-acidification to eliminate carbonate removed 98 to 99% of calcium and magnesium and produced CaCO₃ that was >94% pure. To prevent the contamination of byproducts with calcium sulfate (CaSO₄) in high-sulfate concentrates, a CaSO₄ crystallization step was added successfully to the modified process to precipitate CaSO₄ before Mg(OH)₂ precipitation and produce gypsum that was 92% pure. The modified lime-soda process also removed 94 to 97% silica, 72 to 77% barium, and 95 to 96% strontium, which are known as reverse osmosis membrane foulants.