Recovery of nutrients from anaerobic digester supernatant using magnesium-rich waste material

Guo, J 2010, Recovery of nutrients from anaerobic digester supernatant using magnesium-rich waste material, Masters by Research, Civil, Environmental and Chemical Engineering, RMIT University.


Document type: Thesis
Collection: Theses

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Title Recovery of nutrients from anaerobic digester supernatant using magnesium-rich waste material
Author(s) Guo, J
Year 2010
Abstract Formation of magnesium ammonium phosphate (struvite, MgNH4PO4.6H2O), which has commercial value as fertiliser, is a sustainable technology for ammonia and phosphate removal and recovery from anaerobic digester supernatant. Considering that magnesium concentration relative to ammonia and phosphate concentrations in the supernatant is low, magnesium dosage is usually required to force struvite formation. This research was conducted to investigate the optimum reaction conditions and the feasibility of Mg-rich waste material as a magnesium source for ammonia and phosphate removal and recovery as struvite from anaerobic digester supernatant.

Struvite formation was carried out by adding Na2HPO4 and MgCl2 or magnesium-rich waste material at different conditions: (i) pH; (ii) reaction time; (iii) molar ratios (Mg2+:NH4+:PO43-). Confirmation of struvite formation and measurement of struvite precipitation’s particles size were analysed by XRD and ESEM, respectively. Economic analysis was conducted to estimate the cost of using Mg-rich waste material in the process. A chemical equilibrium software Visual MINTEQ was used for predicting struvite formation using high range of the pH and molar ratio.

Increasing the mixing rate and the reaction time had little effect on ammonia and phosphate removal. Crystal growth had only a minor relationship to the reaction time. pH change with time indicated that induction time was extremely short. The optimum pH was between 9 and 9.5. The reaction time of 10 minutes was adequate for struvite formation due to a high reaction rate. The Mg2+:NH4+:PO43- ratio of 1.2:1.0:1.0 was found to be the optimum molar ratio in order to use Mg2+ and PO43- dosage efficiently. The chemical equilibrium software Visual MINTEQ successfully predicted struvite formation. The prediction indicated that struvite formation was at a wide pH range from 6.5 to 11.5. MgHPO4.3H2O, Mg3(PO4)2 and Brucite (Mg(OH)2) formed with struvite, but struvite was still the main product. The removal of ammonia and phosphate was achieved over 98.00% using Mg-rich waste material as a magnesium source at pH 9.5 and the Mg2+:NH4+:PO43- ratio of 1.2:1.0:1.0. Precipitates analysis using XRD showed that nearly pure struvite formed. Economic evaluation indicated that Mg-rich waste material as the magnesium source can reduce the total cost compared with MgCl2.
Degree Masters by Research
Institution RMIT University
School, Department or Centre Civil, Environmental and Chemical Engineering
Keyword(s) struvite
nutrient recovery
magnesium-rich waste material
crystal growth
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