Abstract
Chemical precipitation is a consolidated technique applied in wastewater treatment to remove and recover phosphorous and ammonium that remain in the effluent after the anaerobic digestion treatment. The precipitate is magnesium ammonium phosphate hexahydrate (MgNH4PO4·6H2O), also known as struvite, and it is sold as a slow-release fertiliser. However, the value of struvite is quite low and has a limited market. Furthermore, it precipitates with heavy metals and other impurities that need to be removed to make the fertiliser commercially viable. This study looked at the thermal decomposition of struvite to recover added value products and recycle the magnesium for further precipitation. A kinetic study was carried out to understand the mechanism of decomposition and the formation of the different solid phases, which is fundamental for the design and optimisation of the technology. The thermogravimetric study confirmed that thermal decomposition is possible, but ammonia could not be completely released below 250 °C. The thermal analysis also led to the determination of the energy required for the decomposition, found to be 1.87 kJ g−1, which also includes the evaporation of water and ammonia. The kinetic study through the isoconversional method showed the presence of two major reactions, and the model-fitting approach identified the diffusion model as the best fit for the first reaction. The activation energy of the first reaction found with this method was 0.24 kJ g−1, comparable with the data obtained from the isoconversional method. The two-stage decomposition reactions were proposed, and the final calcination product was confirmed as magnesium pyrophosphate, which could be used in agriculture or dissolved in diluted mineral acids solution to separate the phosphate from the magnesium.
Highlights
The level of pollution of all water sources continues to rise in line with the increase in global population
Struvite decomposition in the solid state was investigated to understand whether ammonia could be released and recovered as a more valuable product, and the magnesium source could be recycled in the system, saving part of the costs of the expensive chemical used
Thermogravimetric analysis showed the influence of the heating rate in the decomposition of struvite: the calcination onset temperature decreased at lower heating rates, confirming results from previous studies
Summary
The level of pollution of all water sources continues to rise in line with the increase in global population. High concentration of nutrients such as phosphorous and nitrogen compounds (e.g. nitrates) causes eutrophication, which affect the aquatic life. This pollution requires treatment; a higher level of contaminants translates into higher use of energy and chemicals for their removal. Ammonia is one of the nitrogen-based compounds that can be found in wastewater and sludge It must be removed because it is toxic and harmful to the environment, but it is a valuable resource used in industry to produce nitrogen fertilisers; as such, its production has been rising constantly in the past decades, exceeding 140 million tonnes in 2016 [4]. This process is typically powered by coal or methane; it is very energy intensive and accounts for more than 50% of the total world hydrogen consumption, which corresponds to 3.6 M tonnes [6, 7]
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