Integrated water treatment: softening and ultrafiltration
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Integrated water treatment with softening and ultrafiltration is proposed as a promising option for hard waters, as a means to balance risks from microorganisms and disinfection/disinfectant by-products in drinking water systems. The biggest impediment for applying membrane processes is to control fouling. Therefore, the objectives of this research were to understand the nature of the fouling mechanisms for ultrafiltration when used for hard waters and to use that understanding to determine options for the use of softening as a pretreatment before ultrafiltration. To understand fouling mechanisms in the integrated system, three conditions in softening were selected: standard softening, enhanced softening, and Mg softening conditions based on results from two natural waters (i.e., Lake Austin water and Missouri River water). Each condition corresponded to three different levels of softening performance in terms of removal of inorganics and organic matter. Experiments were performed using both the natural waters and synthetic waters with similar (but separable) inorganic, organic, and particulate characteristics. Based on their behavior in softening, alginic acid and dextran with nominal molecular weight of 60 kD were chosen as reasonable surrogates for natural organic matter (NOM). Four possible fouling mechanisms were investigated: inorganic fouling by precipitates, organic fouling, particle fouling, and combined fouling by particle and organic matter. The organic fouling and the combined fouling by particle and organic matter were the major fouling mechanisms. The integrated treatment with softening and ultrafiltration proves to be a promising option for hard waters since softening pretreatment effectively reduced the foulants prior to ultrafiltration. The degree of softening to improve water flux should be determined with the raw water to be applied because it depends on the raw water characteristics. Fouling was investigated with flux decline and extents of recovery by three different cleaning methods. Surface analyses of fouled membranes were performed with scanning electron microscopy and X-ray photoelectron spectroscopy.