(142f) High Recovery Desalination with a Flexible RO-NF Configuration Operating in the Mode of Partial Concentrate Recycle with Energy Recovery

Despite the maturity of reverse osmosis (RO) technology, management of the RO concentrate (i.e., “brine”) and membrane fouling/mineral scaling remain as major impediments for inland deployment of RO desalination. High recovery RO desalting operation is critical, particularly in inland water desalination, for minimization of the volume of generated RO concentrate. However, high recovery RO requires a large number of RO membrane elements, which is a challenge particularly for the small, distributed desalination systems. Moreover, the concentration of sparingly soluble mineral salts, and suspended and dissolved foulant species (along the RO feed channel) with rising recovery which can exacerbate membrane mineral scaling/fouling. The consequence is reduced permeate productivity and increased operation and maintenance costs. In order to address the above challenges, a novel flexible system configuration that integrates nanofiltration (NF) and RO elements (Flex-RONF) was developed with the ability to operate in the mode of partial concentrate recycle, along with an installed pressure intensifier to reduce the required feed pump outlet pressure. Modeling of system operation and optimal membrane arrangement was assessed via process modeling, and the resulting configuration was evaluated for desalting of high salinity agricultural drainage (AD) water (~3,000-30,000 mg/L total dissolved solids) of high mineral scaling propensity (primarily gypsum and calcite). The study demonstrated that optimal configuration of NF and RO elements in a given system, along with a pressure intensifier, can effectively reduce the required applied pressure suitable for achieving an acceptable level of permeate quality, while also significantly reducing the system footprint relative to conventional RO system design. In order to reduce mineral scaling, automated periodic freshwater (permeate) flush was implemented at an optimal frequency. Evaluation of the FLEX-RONF system and the implemented operational protocol suggest the applicability of the approach for distributed water desalination given the reduced system footprint and high recovery operation.