(19b) Self-Adaptive Ultrafiltration with Optimal Coagulant Dosing

RO desalination has emerged as a leading method for desalting seawater, inland brackish water, and water reuse applications.  However, membrane fouling remains a major challenge for operation of RO plants.  Membrane fouling increases the overall resistance to water permeation through the membranes and thus elevates the pressure required for a given rate of water production in addition to reducing membrane lifespan.  Therefore, robust pre-treatment of RO feed water is required to ensure effective long-term operation of RO plants.  In this regard, the use of ultrafiltration (UF) membranes in combination of chemical treatment of the feed stream using coagulants has been shown to consistently produce significantly higher quality water compared to other pre-treatment options (e.g., cartridge filters, sad filters) and thus prolonging lifespan of the RO membranes downstream.  The use of UF for RO feed pretreatment is particularly attractive since UF membrane permeability, which declines due to fouling, can be recovered with effective backwashing (reversing the flow direction).  However, the backwash process often does not result in complete removal of the foulant layer, and a portion of the foulant remains on the membrane surface as an irreversible foulant layer. UF filtration and backwash effectiveness can be increased with suitable coagulant dosing strategy.  However, coagulant dosing can increase the cost of chemical consumption in the overall UF-RO process. In order to address the above challenges a novel strategy was developed for optimizing UF feed coagulant dosing. The approach relies on a backwash effectiveness metric, determined online in real-time by the system onboard control system, which was correlated with backwash effectiveness. A self-adaptive control algorithm was then developed and demonstrated in a field study for optimizing UF coagulant dose for improved filtration performance as feed pretreatment for RO seawater desalination. In this approach the UF control system adjusts the coagulant dose to achieve the optimal backwash effectiveness while reducing chemical consumption. This algorithm was implemented in an 18,000 gallons/day seawater desalination plant and its robustness was demonstrated in a long-term operation even when confronted with variable feed water fouling propensity.