MEMBRANES FOR WATER TREATMENT
The modern era of membrane technologies for water purification was launched in the late 1950s with the development of asymmetric cellulose acetate (CA) membranes for reverse osmosis (RO).¹The commercial implementation of water treatment systems has grown steadily with further development of new membrane materials, configurations, and applications. Membranes are used for a wide range of commercial applications due to their small modular size, low energy usages, and low operating costs. While membranes are manufactured from a variety of materials, polymeric membrane materials dominate the commercial products because of low cost and ease of processibility. The materials that followed from CA and its derivatives (cellulose diacetate and cellulose triacetate) are polyamide (PA), aromatic polyamides, polyetheramides, polyetheramines, poly-etherurea, polysulfone, polyethersulfone, polyvinyli-dene fluoride (PVDF), and polypropylene. Thin-film composite (TFC) membranes may be made from a variety of polymers consisting of several different materials for the substrate, the thin film, and other functional layers.

Membrane technologies offer great promise to meet the increasingly stringent regulatory requirements for potable water production. While other technologies can achieve similar treatment objectives, membranes offer notable advantages particularly with respect to the EPA’s Enhanced Surface Water Treatment Rule and Disinfection/Disinfection By-products Rule. For example, microfiltration (MF) and ultrafiltration (UF) membranes can be configured to provide high levels of pathogen removal without dependence on chemical pretreatment and provide a smaller pore size absolute barrier, in contrast to media filtration which relies on chemical pretreatment for adequate pathogen removal. Providing additional pathogen removal credits at lower disinfectant dosages reduces disinfection by-product formation. Moreover, nanofiltration (NF) and RO membranes have made alternative water reclamation (i.e., brackish water and seawater) and wastewater reuse possible solutions to address the growing global scarcity of traditional water sources.

The presence of microorganisms in feed water can further exacerbate fouling due to the accumulation of microorganisms on the membrane surface and on the feed spacer between the envelopes, or biofoul-ing. Microorganisms transported to the membrane element can attach to the feed side of the membrane and the spacer. Attachment depends on Van der Waals forces, hydrophobic interactions, and electrostatic interactions between the microorganisms and the surface. Biofouling control has been attempted via biocide additions; however, while a biocide may kill the biofilm organisms, it usually will not remove the bio-fouling layer² and may cause bacteria that survive disinfection to potentially become more resistant.³ It is critical to both detect potential fouling bacteria or pathogens and reduce fouling of water treatment membranes to reduce operating cost, extend membrane life, and allow application in challenging environments. This could extend the commercial application of membrane-based water treatment systems.

IMPROVING MEMBRANE FOULING CONTROL
Membrane replacement due to fouling is the single largest operating cost when membranes are used in water separation applications^4,5 and, thus, the greatest hindrance to the widespread use of membranes. Fouling [the irreversible (adhesive) macromolecular adsorption] refers to specific intermolecular interactions between macrosolutes present in the feed water and the membrane that occur even in the absence of filtration. These materials on the membrane surface, which cannot be removed by cross-flow operation, backflushing, or back-pulsing, result in permanent flux decline and lead to fouling. Many researchers agree that organic matter is a major contributor to abiotic membrane fouling in water separation applications.^6-14

As described below, several methods have been used to modify the membrane surface chemistry which has led to various claims of “low-fouling” membranes. The surface properties that have been targeted for modification are hydrophilicity, roughness, and charge.

Ion Beam Irradiation
Ion beam irradiation was used to modify the surface of a sulfonated polysulfone water treatment membrane. A beam of 25 keV H+ions with three irradiation fluences (1 1013ions/cm², 5x10¹³ions/cm², and 1 10¹⁴ions/cm2) was used for membrane irradiation. Sulphonic and C-H bonds were broken and new C-S bonds were formed after irradiation; further, membrane roughness decreased after irradiation. A significant increase in flux after ion beam irradiation was also observed, while the amount of cake accumulation on the membrane was decreased after ion beam irradiation. Hydrophobicity, pore size distribution, and selectivity of the membrane were not affected by ion beam irradiation.Results are described in Chennamsetty et al.^15,16 and King et al.¹⁷