Phosphorus recovery as struvite

One of the new challenges of our society is to move the focus from nutrient removal to nutrient recovery. In this sense, P is a paradigmatic element since it has many useful applications but, according to its current extraction rate, P sources are going to be limited in this century. Anthropogenic wastes contain 3 MtP/yr, which stand for 20% of the global P needs. In this context, there is a clear need for the design of new WWTP configurations that allow the implementation of this new P-recovery paradigm. Because nutrients discharge requirements are becoming more stringent and fertilizer prices are increasing, recovery and reuse of nutrients from wastewater is becoming a need. Waste streams offer an attractive opportunity to recover phosphorus and 15-20% of global demand for phosphate rock in theory could be satisfied by recovering phosphorus flows only from domestic waste.

Beyond the conventional processes for P-removal as ferric chloride or PAC precipitation and biological EBPR, struvite precipitation is a promising technique for the extraction and recovery of nutrients from wastewater. The process is based on co-precipitation of ammonia, magnesium and phosphate reacting to form a precipitate called MAP or struvite (MgNH4PO4·6H2O). Struvite precipitation is cost effective compared to chemical and biological nutrient removal due to savings from the reduction of chemicals used for precipitation and sludge disposal and the decrease of operational problems due to uncontrolled struvite precipitation in some areas of the WWTP. This process can be accepted as a sustainable method of treatment, and allows the removal of nutrients from wastewater and recovery as a slow-release fertilizer.

The potential for struvite formation depends on the concentrations of phosphate, ammonium and magnesium in the wastewater. Typically, wastewaters tend to be rich in ammonium and phosphate but deficient in magnesium, so magnesium supplements are required. If an increase of pH is required, magnesium oxide or magnesium hydroxide can be used. Struvite formation in aqueous solutions occurs after the development of supersaturation, the driving force of all crystallization processes. Therefore, the integration of struvite formation as a specific treatment of wastewater with high concentrations of ammonium and phosphate is becoming a common practice. For example, struvite crystallization is the most recommended treatment for reject water after anaerobic digestion of sludge resulting from WWTP with EBPR.

Our research group is working in the redesing of WWTP configurations to integrate struvite recovery and in the evaluation of the posibilities for P-recovery in different industrial wastewaters. Both modelling studies and experimental evaluation are performed in this research line.

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