New generation of low fouling nanofiltration membranes

The nanofiltration (NF) membranes and their properties are quite diverse, but can generally be described as having rejection characteristics that range from “loose” RO to “tight” ultrafiltration. The uniqueness of these membranes is highlighted by their ability to selectively reject different dissolved salts, and have high rejection of low molecular weight, dissolved components. Nanofiltration membranes are mainly used to partially soften potable water, allowing some minerals to pass into the product water and thus increase the stability of the water and prevent if from being aggressive to distribution piping material. Additionally, NF membranes are finding increasing use for purifying industrial effluents, and minimizing waste discharge. The key to using NF membranes for particular application is the selection of a membrane with appropriate rejection characteristic and the design of a suitable process. NF membranes are generally characterized by a high charge density and pore sizes in the range of nanometers. The surface charge is most often negative and has the greatest effect on the selective passage nature of these membranes. The design of a NF system can often be more complex than a RO system. Because the required transmembrane pressure is so low, a system design that will result in balanced flux throughout the array is difficult to achieve. One common approach is to design NF systems with multiple element types. Such so called “hybrid” designs can help more finely control flow in the NF system. Also, membrane fouling can cause a change of rejection selectivity and/or reduction of membrane permeability that could affect both the water quality and operating cost due to increased energy requirement and higher consumption of chemicals needed for stabilization of permeate. The rejection characteristics of NF membranes depend on chemistry and surface charges of the separating barrier layer, ionic composition and concentration of feedwater and operating parameters of the NF system: recovery rate and permeate flux. Although, theoretical models have been developed to calculate selective ion rejection of NF membranes, they are mostly adequate only for simple water systems in limited range of water compositions and concentrations. In algorithms applied for projection of permeate composition in commercial NF systems, the theoretical models are augmented by empirical correction coefficients developed from field results. Selection of proper NF membrane type and prediction of performance is further complicated by membrane fouling. In NF projects involving treatment of feedwater with high concentration of dissolved organics the membrane selection process usually include pilot unit operation to evaluate stability of salt rejection and water permeability of candidate membrane elements. The paper will discuss present offering of commercial NF membranes, case studies and models applied to calculate their performance. This will also include information on recently developed NF membranes with low fouling characteristic that are successfully applied in the world largest (150,000 m3/d, 40 mgd) NF plant in Boca Raton, FL. The process of extensive field testing and parallel membrane performance optimization that led to selection of these nanofiltration elements for the Boca Raton plant will be described as well.