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UBC Theses and Dissertations

Development of control strategies for the operation of a struvite crystallization process Fattah, Kazi Parvez


In this study, a dynamic control model for struvite crystallization process was developed that incorporated both chemistry and control software, which could be used to increase the efficiency and ease process operation. This process model was the basis of an automatic controller that had the capability to manipulate flows and chemical additions, and thereby control the system at a desired set point. The control model was then used as a prediction tool to determine conditions that influence the supersaturation ratio of the process. A pilot scale crystallizer was operated at a local treatment plant to test the model. The struvite produced from the operation of the process was then examined to determine the influence of various operating parameters on its quality. Supersaturation ratio (SSR) and upflow velocity in the crystallizer were found to influence the size and shape of the pellets developed. Mid-sized pellets (2.0-2.5 mm), had the highest crushing strengths; SSR did not appear to influence the crushing strength of pellets formed. High concentration of magnesium in the crystallizer was related to the formation of pellets having greater crushing strengths. To determine a single solubility constant for struvite, a study was conducted under varying experimental conditions. Results showed that, for a particular temperature, and in the working pH range of 7.0-9.0, the solubility constant was independent of the pH and water matrix. These experimental values, along with values found in literature, were used to derive a universal constant and a linear equation relating solubility product (pKsp) with temperature. In this study, the effectiveness of two carbon dioxide strippers, in reducing caustic usage, were evaluated. Results showed that carbon dioxide stripping was efficient in reducing caustic costs, by as much as 46%. The potential saving in caustic cost due to CO₂ stripping was calculated to be as high as 38 cents per thousand liters treated. The determination of the concentration of Mg in a struvite crystallization process is important because of its influence on SSR, the associated operational cost and its struvite forming potential when unused chemical is passed back to the treatment plant. In this study, methods were tested, with acceptable degree of accuracy, which can provide information, on-site, on the concentration and rate of application of the element.

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