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Author
Vidal i Tomás, Maria
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Abstract
Phosphorus is an essential but scarce component, used worldwide to obtain fertilisers. Nowadays, the current excessive extraction of mineral phosphorus in combination with the fact that these natural resources are dwindling, has prompted the need to develop ways to recover and reuse surplus phosphorus from residual waste streams.
In particular, every year the Spanish olive oil industry generates millions of tons of residues, in which the P concentration is considerably high. In the same way, these residues are highly rich in potassium which is an essential nutrient for the olive oil tree growth. Therefore, after considering different options, and on an ecological basis, the removal and recovery of this residual P via MPP precipitation has been considered to be the optimal way to avoid wasting phosphorus and potassium while reusing both in the olive oil sector.
In view of the above, this undergraduate thesis has comprised the previous batch study in order to finally design a continuous crystalizing industrial process. Therefore, in this context, we have studied the optimal precipitation conditions of MPP, using the aqueous phase of wastewater and wastes from olive oil mills as phosphorous and potassium sources. To do so, 3 main experiments have been carried out.
In the first experiment, the effect of 3 operational parameters was studied. These were: the growth temperature (4ºC and 25ºC), pH (10 and 11), and the stirring time (15 and 30 minutes). It turned out that most of the potassium struvite precipitated during the stirring time, and that the longer the phase, the higher the MPP % precipitated. In the same way, it was observed that when the stirring and growing temperature was the same, the final MPP precipitate was more abundant.
In the second experiment, the effect of the pH (10 and 11) and the reaction temperature (25ºC and 30ºC) was studied. The trial results showed that 25ºC was the optimal temperature and that Ca2+ interference increased when the pH was 11.
Finally, in the third experiment, mineral phosphorus was added in order to simulate the total P present in the initial residue (not only in the aqueous phase). It transpired out that neither a Mg2+: K+: PO43- ratio of 1:10:1 nor 3:10:1 favoured the MPP precipitation.
Furthermore, once the operational parameters were established, a first proposal for a continuous crystallizing MPP process was done. Likewise, an approximate economic and environmental study was done. Furthermore, even though this new valoration process of residual aqueous streams from the olive oil industry by obtaining MPP would increase the final product value, it turns out to be neither an eco-friendly nor a viable process - at least, with the current P concentration of the starting residues, and the expected costs.
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