PHOSPHORUS RECOVERY FROM THE LIQUID FRACTION OF DIGESTATES BY CRYSTALLIZATION OF STRUVITE

Since the second half of the 19th century the meat consumption has globally increased, and it is estimate by the scientists and the global organizations (e.g. FAO) that the trend will continue to increase. In the large retail sector, the growing demand of animal-by products is reflected in the increase of cereals production and the use of fertilizer, but also in the increment of livestock effluents and agricultural waste. Although the common purpose is to limit the nitrogen damages due to the livestock effluents pollution (Nitrates Directive), the attention is now focused on the phosphorus issues. Phosphorus is a non-renewable and scarce natural resource which is going to run out soon. The issues are related to both its concentration in the soil non-labile pool, which is unavailable for plants, and its soluble form which can be a pollutant and may cause the eutrophication of water bodies. This study focuses on one of the alternative methods for phosphorus recovery from the digested sludge. In the specific, phosphorus can be recovered from waste streams by the crystallization of struvite, which is an effective slow-release fertilizer and it offers many advantages versus conventional fertilizers, such as low leach rates and slowly release of nutrients. Many technological approaches are being developed to enhance struvite precipitation but few of them are suitable for livestock effluents, which are characterized by high solids concentration (3-4 % TS) that affect struvite crystallization rate. The struvite crystallizers technology had been developed in different prototypes and put into production successfully abroad. This study aims to elucidate the effect of solids (3-4 % TS) on struvite formation in a complex matrix, such as animal slurries and digestates. Special attention is given to identifying the role of solids in struvite nucleation and growth, and its impact on the quality of the product recovered. The tests were performed inside a lab-scale prototype of airlift reactor (processing 7 L/day for an equivalent of 1.5 ton/year), which shape and functioning are specifically designed to reproduce the best conditions for struvite precipitation. The two input flows are seawater bitter (SWB), which represents the Mg source, and the liquid fraction of digestate, which is the ammonium and magnesium source. In the reactor a continuous air flux is used to create internal recycle flow, allowing the struvite crystals to grow, under a monitored pH of 8.5. Three differentiated zones could be distinguished in the reactor after few days of sperimentation: riser (supersaturation conditions), clarifier (quiet zone with up-flow) and collector (particles settling). Since seawater bitter is a waste brine remaining after salt (NaCl) extraction from seawater, it has no production costs. Thus, economical evaluations estimate that the total cost for a pilot-plant producing struvite with SWB as Mg source, is 0.01-0.22€ kg struvite-1 (about 0.6 € kg P-1) including profit. Struvite technology is nearing commercialization, but chemical input costs are very high when MgCl2 or another Mg source (wood ashes, magnesite, magnesia or by-products of MgO production) are used in the process. Thus, the use of seawater bittern (SWB) instead of MgCl2 salt is a promising strategy for P removal. Seawater bittern is a by-product of sea salt processing mainly composed by magnesium chloride (>95%) and a low-cost magnesium source. Struvite is an effective slow-release fertilizer and it offers many advantages versus conventional fertilizers, such as low leach rates and slowly release of nutrients. It is suitable in grasslands and forests where fertilizers are applied once in several years; it does not damage growing plants when a single high dose is applied; represents an alternative for those crops that require magnesium, such as sugar beets; and, phosphorus uptake is higher in ryegrass when struvite is used as a fertilizer. Moreover, the nucleation of struvite crystals in animal slurries allows the recovery of organic carbon in the final fertiliser collected.