Turbidity-based scaling detection system for batch reverse osmosis combined with gypsum crystal recovery

Current research into scaling detection and mitigation focuses on analyzing membrane surfaces to determine the onset of scaling. In this research a novel scaling detection system was tested that analyzes the concentrate bulk solution to determine precursors to scaling in a batch-operated reverse osmosis system. Synthetic solutions of gypsum and calcium bicarbonate were successfully concentrated up to a saturation ratio of 6 and 2, respectively, without scaling the membrane. For a simulated agricultural runoff water from the San Luis Demo plant with gypsum as the dominant scalant, a scaling-free water recovery of 68 % was achieved without antiscalants while conventional RO achieved only 50 % water recovery with use of antiscalants. The high gypsum supersaturation ratio and absence of antiscalants facilitated gypsum crystallization and settling in a reactor in 25 and 15 min, respectively. Up to 46 % gypsum crystals were recovered from the brine for potential reuse in agriculture. Recycling the supernatant after crystal settling to batch RO twice allowed an increase in water recovery from 68 to 93 %. The results showcase the effectiveness of instream real-time turbidity analysis to detect precursors of gypsum and calcium bicarbonate scaling with single salt solutions and brackish water.