Phosphorus recovery from digestates by crystallization to produce fertilizers

Abstract

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