Browsing by Subject "Water--Purification--Filtration"
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Item Integrated water treatment: softening and ultrafiltration(2002) Kweon, Ji Hyang; Lawler, Desmond F.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.Item Particle aspects of precipitative softening: experimental measurement and mathematical modeling of simultaneous precipitation and flocculation(2006) Nason, Jeffrey Alan; Lawler, Desmond F.Precipitative coagulation processes (i.e., alum or iron sweep-floc coagulation and lime softening) are used nearly ubiquitously in the treatment of drinking water from surface water sources. Although the benefits of such processes are well known, the ability to predict the particle size distributions from such processes is virtually non-existent. The objective of this research was to improve the quantitative understanding of how particle size distributions change due to simultaneous precipitation and flocculation in water treatment through experimental investigation and mathematical modeling. This study focused on one such process, precipitative softening, as an example system. The experimental research used bench-scale calcium carbonate precipitation experiments under a variety of conditions to elucidate how the particle size distribution changes due to simultaneous precipitation and flocculation and to identify the controlling variables. Independent variables included the saturation ratio, pH, ratio of precipitating ions, initial seed type and concentration, and the mixing intensity. Experiments were performed under conditions of constant and declining solution composition, and particle size distributions were measured using a Coulter Counter. Trends in the experimental results were clear; particle size distributions changed dramatically by nucleation, particle growth and flocculation. The saturation ratio, initial seed type and concentration, and the mixing intensity were identified as the most important variables. Where possible, relationships linking the changes in the particle size distribution with the independent variables were delineated. An existing mathematical model for flocculation was modified to include mathematical expressions describing nucleation and particle growth. Flocculation in the revised model was described using three plausible rate expressions. An underlying hypothesis of the research was that the incorporation of the mechanisms of precipitation into the existing flocculation model would allow the prediction of particle size distributions from softening processes. Model predictions were tested against experimental data to determine which representation of flocculation during precipitation was most appropriate. It was found that at high mixing intensities flocculation during precipitation was most accurately modeled using a size-independent flocculation expression. At lower mixing intensities, the shortrange force model was a better predictor of the particle size distribution. The implications of these findings are discussed.Item Physicochemical aspects of particle breakthrough in granular media filtration(2004-05) Kim, Jinkeun, 1968-; Lawler, Desmond F.Granular media filtration is used almost universally to remove particles from drinking water, and is usually the last particle removal process in water treatment plants. Therefore, superb particle removal efficiency is needed during this process to ensure a high quality of drinking water. However, particle breakthrough can occur by either the breakoff of previously captured particles (or flocs) or the direct passage of some influent particles through the filter. It is hypothesized that there are physicochemical differences among the particles, such that better destabilized particles are caught in a filter, while others that are not so well destabilized are allowed through. To investigate these differences, the zeta potential distribution (ZPD) and particle size distribution of effluent samples after filtration were analyzed. Filtration experiments were performed in a laboratory-scale filter using spherical glass beads with diameter of 0.55 mm as collectors. A single type of particle suspension (Min-U-Sil 5, nearly pure SiO2) and three different destabilization methods (pH control, alum and polymer destabilization) were utilized. The operating conditions were similar to those of standard media filtration practice: a filtration velocity of 5 m/h. More favorable particles, i.e., particles with smaller surface charge, were well attached to the collectors especially during the early stage of filtration when surface charge of particles and collectors were both negative. This selective attachment of the lower charged particles caused the ZPD of the effluent to move to a more negative range. On the other hand, the ZPD of effluent did not keep moving from less negative to more negative during the later stages of filtration, and this result was thought to be caused by two reasons: ripening effects and detachment of flocs. At the same time, to assess the possibility of particle detachment during the normal filtration, a hydraulic shock load (20% increase of flow rate) was applied after 4 hours of normal filtration. Less favorable particles, i.e., particles with larger surface charge, were easily detached during the hydraulic shock load. Therefore, proper particle destabilization before filtration is crucial for maximum particle removal as well as minimum particle breakthrough.