Pilot study on hardness removal from groundwater using a combined crystallization pellet-fluidized bed, ultrafiltration, and reverse osmosis process

The application of ultrafiltration (UF) reverse osmosis (RO) (a dual-membrane process) faces the key challenges of fouling of the RO membrane and scaling on the concentrate pipeline. However, this challenge could potentially be addressed by combining the dual-membrane process with the chemical crystallization circulating pellet-fluidized bed (CrystPFB) process and using it as a pretreatment step to soften the feed water into the dual-membrane process. To demonstrate and validate the proposed approach, a pilot study of the combined crystallization pellet-fluidized bed, ultrafiltration, and reverse osmosis process (CrystPFB-UF-RO) was performed at a water treatment plant in Shaanxi Province, China, using the dual-membrane process to treat high-hardness groundwater. At a sodium hydrate dosage of 100–350 mg/L, the CrystPFB reduced the calcium ion concentration and total hardness of the raw water by 55–98 % and 40–65 %, respectively. The calcium ion crystallization efficiency reached up to 80 %. With hydrochloric acid dosed in the softened water, the pH of the softened water was maintained between 6.5 and 8.5, and the turbidity remained < 5 NTU (nephelometric turbidity unit). The operating time of the UF-RO process fed with softened water with a calcium removal ratio of 70 % before reaching the thresholds for chemical cleaning, as defined by the normalized water yield, normalized membrane pressure decrease, and normalized salt passage ratio was extended by 3.4, 3.4, and 5.0 times, respectively, compared to when the UF-RO process was fed with raw water. Moreover, the calculated scaling indices of the concentrate from the UF-RO process indicated that the concentrate pipeline would not be scaled. RO membranes that reached the thresholds for cleaning were characterized and analyzed. Scaling increased along the water flow direction of the membranes, and the scaling on the membrane surface was dominated by CaCO₃. We proposed a modeling method for the deposition of CaCO₃ on the surface of RO membranes in a combined system, offering a practical mathematical model for applying combined process systems in engineering practice. Our results provide theoretical criteria and data support for the application of CrystPFB-dual-membrane combined process in real-world engineering.