Access to decent sanitation remains a problem in developing countries. At the same time, sanitation technology is constantly evolving specifically regarding resource recovery solutions. Some chemical elements found in human excreta derived from non-renewable resources, and the recycling of phosphorous from sewage in particular is a possible solution to the growing issue of resource scarcity. A potential way to recover phosphorous from urine or water-borne sewage is through struvite precipitation. Struvite (MgNH4PO4. 6H2O) is a mineral that can be used as a slowrelease magnesium, ammonium and phosphate based fertilizer and can be produced from urine by adding magnesium to the ammonium and phosphate rich urine. Usually, magnesium is dosed chemically using salts such as MgCl2, MgO, MgSO4 or bittern, together with pH regulating agents but these reactants produce unfavourable chemical by-products and the process tends to be expensive. Previous studies have proven that electrochemical dosing of magnesium is a feasible and reliable method of struvite precipitation. It not only produces high grade struvite that is valuable and marketable, but it also eliminates the need for alkalinity dosing in order to create a suitable pH environment for struvite precipitation. Further to that, electrochemical precipitation does not produce any harmful chemical by-products. Previous work shows that one main challenge that is associated with this method is the formation of a mineral layer on the magnesium anode called nesquehonite (MgCO3 · 3H2O). This leads to increased electrode potentials and hence high energy consumptions and may also lead to system failures of the reactor. Further to that, struvite generally precipitates as small crystals that are difficult to separate from the solution, leading to low mass recoveries of the product. These small crystals are formed as a result of the high supersaturation, which generally occurs for most processes that are employed to make struvite. In view of these problems, this dissertation presents an investigation of the potential improvements to the electrochemical precipitation of struvite from source-separated urine. The main aim is to minimise or eliminate the formation of mineral precipitates on the anode surface. It also looks into ways of increasing the crystal sizes of the struvite being precipitated in the electrochemical system. The methodology for this investigation involved modelling and experimental work. The specific objectives for this study were to: a) Investigate how thermodynamic modelling of struvite precipitation compares to the experimental results from an electrochemical precipitation reactor, b) Employ the aspect of seeding in an electrochemical reactor for struvite production and determine the technical feasibility of the proposed process, c) Establish how to minimise the formation of nesquehonite so that the quality of struvite produced in the electrochemical reactor is not compromised, d) Investigate how the crystal sizes of the struvite particles produced in the seeded electrochemical precipitation batch reactor setup compare to those produced in the continuously stirred reactor setup with a recycle that gives the particles a longer residence time, e) Investigate the economics and energy requirements of the SEP (Seeded electrochemical precipitation reactor). The results of the thermodynamic model suggested that the Mg:P molar ratio (magnesium to phosphorous molar ratio), the pH of the reaction environment and the presence of the constituent ions in adequate quantities play a critical role in the precipitation of struvite with regards to yield and purity. The model predicted that phosphorous conversion to struvite from stored urine can reach up to 98 % at Mg: P molar ratio of 1.2 and at a pH of 9.5. Complete P recoveries were obtainable at higher Mg:P molar ratios of up to 21, in a pH range of 9 to 10. However, nesquehonite also started to form at Mg: P molar ratios greater than 7.8 and between pH values of 6 and 12 due to excess magnesium ions and the presence of carbonate ions in the urine. Theoretically, the implication of this was a compromise on the quality of the struvite produced and, practically, also the likelihood of the formation of a passivation layer of nesquehonite on the sacrificial magnesium anode. As such, the magnesium concentration in the model was kept high enough to recover most of the orthophosphate present in the urine but low enough to avoid the formation of nesquehonite. The optimum Mg:P molar ratio and pH range were tested experimentally using an electrochemical precipitation batch reactor with struvite seeds. A Mg:P molar ratio of 1.2 and pH of 9.5 proved to provide good operating conditions for phosphorous recovery of up to 96 % instead of the 98% that had been predicted by the thermodynamic model. The results of the seeded electrochemical precipitation experiments, at different seeding and stirring conditions, showed that seeding does not affect the rate and extent of phosphate recovery and the rate at which equilibrium is reached. This was due to the fact that the precipitation of struvite occurred extremely rapidly owing to its sparingly soluble nature and the degree of supersaturation of the solution, which also led to the fast precipitation of numerous small struvite crystals. Results of the thermodynamic model and those of the seeded electrochemical precipitation experiment were comparable with regards to the recovery of the orthophosphate from the source separated urine and the conversion of magnesium from the sacrificial anode. However, there were notable discrepancies with regards to the final pH and the conversion of the carbonate ions and thus the likelihood of the formation of nesquehonite. The formation of the passivation layer of nesquehonite on the anode surface over time led to an increase in the magnesium electrode potential which translated to an increase in the cell potential by 130% from 1 V to 2.3 V. It was postulated that this was due to the relatively high level of magnesium supersaturation at the anode surface compared to the rest of the system which favoured the formation of nesquehonite. This was not computed in the thermodynamic model which assumed the system to be well mixed, however the simulations did show that nesquehonite does indeed form at high magnesium concentration. The precipitates that were collected in suspension were analysed for magnesium, orthophosphate and ammonium ions and the results also showed that these ions were present in a 1:1:1 ratio. Further to this, XRD results confirmed that the layer on the anode was indeed nesquehonite. This led to the conclusion that the precipitates that formed in suspension were struvite and most of the nesquehonite that formed only built up on the anode surface and did not affect the quality of the struvite collected. Analysis of the size of the crystals that were produced in the electrochemical precipitation batch reactor over 2 hours showed that there was only a slight increase in the size of the resultant struvite particles after seeding. Two extreme stirring rates were also investigated and it was observed that high stirring rates produced larger particles than the lower stirring rate. This implies that the high stirring rate resulted in a decrease in the local supersaturation at the anode surface. This promotes particle size enlargement through a growth mechanism in the bulk solution rather than the formation of smaller particles through nucleation. Also, higher stirrer speeds could result in higher rate of particle-particle collisions and increased particle shear which would result in the formation of smaller particles. However, this appears not to be the case for this research. When the system was run continuously, i.e. by recycling the struvite particles back into the reactor with fresh urine in order to increase their residence time in the reactor, there was some growth of the crystals after 8 hours. The general shape exhibited by the particles was coffin like in all the images with other spherical and randomly shaped particles. This dissertation has thus shown that the electrochemical precipitation of struvite from source separated urine can be improved in terms of the functionality of the system and the quality of product by i) seeding ii) stirring. The cost of struvite precipitation in the seeded electrochemical precipitation reactor using a magnesium sacrificial anode was evaluated and compared against the cost of struvite precipitation by chemical dosage of magnesium with MgCl2. The electricity and magnesium electrode costs were evaluated for the seeded electrochemical precipitation method while only the chemical costs for the chemical dosage method were considered. Transport costs were neglected as they were assumed to be the same for both. The economic evaluation showed that the electrochemical precipitation method costs about ZAR 3.47 per kilogram of struvite while the chemical dosing method costs about ZAR 3.87 per kilogram of struvite produced. Since these costs are similar, it is recommended that this technology be investigated further. It is also important to note that chemical precipitation is an already proven technique for struvite production while electrochemical precipitation of struvite is relatively new, even though the core technology has existed for some time now. Therefore, there is likely more room for improvement and optimization in an electrochemical precipitation process while this might not be the case for chemical precipitation of struvite. Also, since it is difficult to control the magnesium concentration on the anode surface, it is recommended that in order to suppress the formation of nesquehonite on the anode surface, the urine should be pre-treated in order to remove the carbonate ions which leads to the formation of nesquehonite. Pre-treatment could involve acidifying the solution in order to convert the carbonate slowly into unstable carbonic acid and then to carbon dioxide gas. This would be beneficial especially when the system is run continuously in order to influence crystal growth and essentially avoid system failures which would be caused by the build-up of the nesquehonite layer.

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