Membrane Mineral Scaling and its Mitigation in Reverse Osmosis Desalination of Brackish Water

The treatment and desalination of inland water via reverse osmosis (RO) technology is gaining momentum for upgrading brackish groundwater and developing supplemental fresh water for various regions. In brackish RO plants, high water recovery is critical in order to minimize the volume of residual RO concentrate (brine), given the economic and environmental challenges of concentrate management. However, high recovery may be limited by mineral salt scaling resulting from supersaturation of sparingly soluble minerals (e.g., CaSO4, BaSO4, CaCO3, SiO2). Mineral scaling results in membrane surface blockage, reduction of permeate flux and shortening of membrane lifetime.In order to control or prevent mineral scaling, effective mitigation methods must be developed and tested (i.e., the relationship between RO operating conditions and mineral scaling). To accomplish the above, early detection of mineral scaling is essential. To meet the above challenges, a novel high-pressure RO membrane monitoring system was developed to allow direct membrane surface imaging. The membrane monitoring system (MMS) was interfaced with a slipstream from an RO plant, whereby captured membrane surface images were analyzed online. The present monitoring approach demonstrated, for the first time, early detection of silica scale formation and growth kinetics. Detailed silica scaling studies revealed that scaling consisted of discrete silica particles embedded in a silica “gel” layer. The membrane monitoring approach also served to evaluate the impact of a membrane biofilm on concentration polarization (CP) and mineral scaling kinetics. It was shown that biofilms enhanced CP within the biofilm and significantly exacerbated mineral scaling. The membrane monitoring approach was subsequently deployed in a field study of desalination of brackish agricultural drainage (AD) water. In these studies, the membrane monitor was integrated with a mobile RO pilot plant, developed at UCLA, for real-time field optimization of RO operating conditions for averting mineral scaling. The above approach demonstrated that effective feed filtration for removal of suspended particles is critical for mitigating mineral scaling and reducing nucleation triggered by surface deposited particles. Moreover, antiscalant selection and dosage optimization was feasible under field conditions which also enabled determination of the maximum feasible water recovery level