During the past few centuries, phosphorus demand for fertilizers has never stopped to grow due to the exponential rise of human population and its food needs. However, phosphorus is a limited resource. There are only two sources of phosphorus: guano (bird droppings) and phosphate rock (apatite). In fact, many studies predict its exhaustion within the next 100 to 250 years and the price increases every year. Within this framework and aligned with European policies to boost the so-called “circular economy”, it is more necessary than ever to recover phosphorus from renewable sources such as manure. Manure has been applied directly to soil as fertilizer for decades. Nevertheless, conducting this practice excessively and indiscriminately has a negative environmental impact. This has introduced the need to carry out a pre-treatment, which results in phosphorus loss.
The doctoral thesis “Assessment of struvite and K-struvite recovery from digested manure” by Elena Tarragó Abella seeks to recover phosphorus from manure as struvite (MgNH4PO4·6H2O) or K-struvite (MgKPO4·6H2O). Two phosphate minerals that also contain magnesium, nitrogen and potassium, and that are produced by spontaneous crystallization in wastewater treatment plants. The ultimate goal is to obtain them through a controlled process and manufacture a slow-release fertilizer rich in phosphorous and with a lower impact on water quality. Research work was conducted at LEQUIA laboratories with two types of substrates: manure digested after applying solid/liquid separation and manure after applying a treatment to remove nitrogen by Anammox. Experiments were focused on three challenges: 1) deepening the knowledge on struvite and k-struvite crystallisation, 2) evaluating the quality of the product recovered and 3) optimizing the process regarding efficiencies of nutrients recovery and particle sizes.
Results obtained have been more than satisfactory. The process was optimized in terms of nutrient recovery efficiencies and particle size recovered, proving the viability of using the up-flow velocity as a control parameter for the growth of struvite particles, and to adjust particle size to the customers’ requirements for fertilizer application. Moreover, the viability for struvite recovery from digested manure with high solid content is proven, recovering a stable product, and demonstrating that solid particles not only did not inhibit struvite formation, but they acted as nuclei enhancing heterogeneous nucleation, and favored the aggregation and/or agglomeration of struvite crystals. Finally, a methodology to recover the most limited macronutrient in soils (potassium) as K-struvite from swine manure was provided.
To sum up, this doctoral thesis directed by Drs Maria Dolors Balaguer, Sebastià Puig and Maël Ruscalleda proves the feasibility of recovering nutrients from manure, a complex matrix with high concentration of solids. This is a significant progress toward the systematic recovery of essential macronutirents for plants’ growth (potassium, phosphorous and nitrogen) from renewable sources.