AWWA ACE59937

The primary objective of this investigation is to optimize a microfiltration process to pretreat agricultural drainage water prior to membrane desalination. A back-pulse optimization approach has been developed to greatly extend fouling free microfiltration of model drainage waters, and is expected to reduce chemical cleaning frequency, operating costs, and membrane replacement. The back-pulse optimization scheme is based on the hypothesis that foulants are brought into close contact with MF membrane surfaces under force of permeation drag, but repulsive electrostatic interactions may prevent irreversible adhesion for monolayer foulant coverages. Optimization of a microfiltration process to treat agricultural drainage water consists of four steps. Step one is to determine the critical coverage point on the membrane surface using the direct microscopic observation system's in situ visualization, thus determining the forward filtration time. Step two is to estimate the intensity of back-pulse required to dislodge deposited foulant particles at the membrane surface through analysis of hydrodynamic drag and lift forces, as well as surface forces arising from van der Waals and electrostatic interactions. Step three is to determine the back-pulse duration using direct microscopic observation to ensure complete removal of particles. Step four is to apply the optimized forward filtration time and back-pulse duration/frequency to a bench-scale hollow fiber microfiltration system. The optimized scheme will be applied to model drainage waters prepared in our lab, as well as real drainage water samples from the Alamo River. Samples of Alamo River water are analyzed to determine the nature of potential foulants. A size fractionation procedure was employed with solids, organics, size distribution, and zeta potential determined for each size fraction (8.0, 1.0, 0.4, and 0.1 µm). For synthetic water experiments, each size fraction was simulated using model inorganic and organic foulants such as colloidal silica, clay, latex, and microorganisms. The MF back-pulsing system, used to evaluate the optimized operating scheme utilizes bench-scale, hollow fiber membrane filters with 100 dual asymmetrically skinned, polysulfone fibers per filter. The MF back-pulsing system is capable of any combination of inside-out, outside-in, dead-end, cross-flow, constant pressure, and constant flux modes of operation. The direct microscopic observation system consists of a flow cell constructed from polycarbonate and glass, mounted on a microscope stage to allow direct visual observation of microbial and particle deposition optical microscopy as described elsewhere. The direct microscopic observation utilizes a flat sheet polysulfone membrane that is chemically and physically analogous to the polysulfone membrane used in the bench-scale hollow fiber system. Includes 6 references, figures.