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Nitrogen recovery from wastewater: Optimization of the membrane contactor system and crystallization of ammonium salt.
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School of Chemical Engineering |
Master's thesis
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en
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76
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Abstract
This thesis focuses on the recovery of ammoniacal nitrogen from synthetic wastewater using acid-based stripping solutions, followed by the crystallization of ammonium sulfate. The literature review covers various nitrogen recovery methods, with special focus on hydrophobic membrane contactors due to their high selectivity and low energy use. Basic principles of crystallization were introduced, and the solubility behavior of ammonium sulfate was discussed.
Sulfuric acid (1–5 M) and oxalic acid (0.5 M) were tested in a hydrophobic membrane contactor system treating a 15 g/L ammonium chloride solution. After 8 hours, NH4+–N concentrations between 4634–6374 mg/L were achieved. However, no clear trend was observed with increasing acid strength. Oxalic acid formed visible precipitates confirmed as ammonium-containing solids by ammonia analysis, XRD, and Raman spectroscopy.
Speciation analysis using Visual MINTEQ modelling showed that ammonia release from the feed is highest at pH 12 and 60 °C, while sulfuric acid is most effective below pH 3. For oxalic acid, the optimal range for ammonia capture was pH 3–6. In the second part, evaporative and cooling crystallization of ammonium sulfate were studied. Evaporative crystallization yielded up to 21.49 kg at 1.97 kWh/kg from 100 kg of 35 wt% ammonium sulfate solution, while cooling crystallization produced 9.61 kg with an estimated energy use of 3.96 kWh/kg. Though cooling produced larger crystals (up to 3000 µm), evaporative crystallization was more favorable due to its higher yield and lower energy consumption.
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Supervisor
Louhi-Kultanen, MarjattaThesis advisor
Kaljunen, Juho UzkurtHaimi, Henri