Phosphorus recovery from digestates by crystallization to produce fertilizers

Abstract

[CAT/VA] El fo¿sfor e¿s un recurs essencial i limitat que s¿extreu de les explotacions mineres. S¿espera que el nivell actual d¿explotacio¿ no es pugui mantenir a llarg termini. El fo¿sfor tambe¿ es troba a les aigu¿es residuals, especialment als fangs procedents de la digestio¿ anaero¿bia, i es necessari extraure¿l d¿elles abans de que es deposite ja que es un dels causants de l¿eutrofitzacio¿. Per tant, en aquest projecte es prete¿n estudiar el proce¿s de recuperacio¿ de fo¿sfor des de la digestio¿ anaero¿bia seguida d¿una etapa d¿osmosi fins a la cristal·litzacio¿ en forma d¿estruvita. En la cristal·litzacio¿ s¿estudien els para¿metres ratio Mg:P, prese¿ncia de Ca i pH. En primer lloc, es va realitzar una exhaustiva investigacio¿ sobre la composicio¿ d'aigu¿es residuals. Una vegada es va triar la composicio¿ de l'aigua a estudiar, amb l¿objectiu de simular una etapa d'osmosi, es va multiplicar la concentracio¿ dels ions que forma¿ven l'aigua per dos i per quatre. Per a aquestes tres situacions (sense osmosi, fase d'osmosi aigua residual concentrada per 2 i fase osmosi aigua residual concentrada per quatre), es van realitzar els mateixos experiments ja que d¿aquesta forma es podrien comparar els resultats. D'una banda, es va estudiar el valor o¿ptim del ratio Mg:P i el pH per a la recuperacio¿ del fo¿sfor en forma d'estruvita mitjanc¿ant simulacions amb els programes Visual Minteq i OLI. Per altra banda, a escala de laboratori, les aigu¿es corresponents a les tres situacions estudiades es van sintetitzar amb el ratio Mg:P o¿ptim determinat en les simulacions i es van preparar diferents mostres de cadascuna que es van analitzar mitjanc¿ant plasma acoblat inductivament. Espectrometria o¿ptima d'emissio¿ i difraccio¿ els raigs X. Amb els resultats obtinguts, es va poder confirmar que el proce¿s de recuperacio¿ de fo¿sfor amb l'etapa d'osmosi millora els resultats obtinguts, que un augment del pH augmenta la quantitat de fo¿sfor recuperat i que quasi el 100% del fo¿sfor es pot recuperar com estruvita.

[EN] Phosphorus is an essential and limited resource that is extracted from mining operations. It is expected that the current level of exploitation will not be maintained on a long term. Phosphorus is also found in wastewater, especially in the sludge from anaerobic digestion, and it is necessary to extract it before its disposal as it is one of the causes of eutrophication. Hence, this project aims to study the process of phosphorus recovery from an anaerobic digestion stage followed by an osmosis stage to a crystallization stage as struvite. At the crystallization stage, the parameters Mg:P ratio, pH and presence of Ca are studied. First, an exhaustive research of wastewater composition was performed. Once the composition of the water to be studied was chosen, in order to simulate a stage of osmosis, the concentration of the ions that made up the water is multiplied by two and four.. For these three situations (without osmosis stage, osmosis stage concentrated by 2 and osmosis stage concentrated by four), the same experiments were carried out in order to compare them. On the one hand, the optimum value of the Mg: P ratio and the pH for the recovery of phosphorus in the form of struvite were studied by simulations with Visual Minteq and OLI software. Then, at laboratory scale, the waters corresponding to the three situations studied were synthesized with the Optimum Ratio determined in the simulations. Different samples were prepared from each experiment and were analyzed by means of Inductively coupled plasma - Optimal emission spectrometry and X-ray diffraction. With the results obtained it was possible to confirm that the phosphorus recovery process with the osmosis stage improves the results obtained, that an increase in the pH increases the amount of phosphorus recovered and that almost 100% of the phosphorus can be recovered as struvite.