Currently, researchers are exploring alternative phosphorus sources for agricultural production that are more sustainable than rock phosphate. In this context, the recovery of phosphorus from organic wastes as struvite can constitute an important tool for promoting circular economy practices and reducing the risk of phosphorus contamination through eutrophication. Struvite recovery has been widely developed using different organic wastes with high concentrations of N and P, such as industrial, municipal and animal wastes, mainly in the form of effluents. However, little information is available concerning phosphorus recovery in the form of struvite from sewage sludge samples, these processes being mainly based on chemical procedures. Therefore, the main aim of this work was to study phosphorus recovery from three sewage sludge samples from different wastewater treatment plants (SS1, SS2 and SS3), in comparison with the solid fraction of pig manure (M), through an optimized bioacidification process, as well as to evaluate the potential for struvite precipitation from the recovered P-rich supernatants. Protein recovery through alkaline treatment of the remaining precipitates was also studied. The results obtained showed the feasibility of the optimized bioacidification process for P recovery, especially in the samples M and SS3, which showed the highest P recovery yields (65.7% and 69%, respectively) and the best results regarding struvite formation. In addition, the protein recovery efficiency of the remaining solid residues ranged from 59.3% to 67.4%, without showing a clear influence of the type of organic waste used.

The European Union (EU) advocates for a sustainable agricultural model with reduced synthetic fertiliser use. This study compares different high-P organo-mineral pellet fertilisers (OMFs) and their effects on crop yield. A trial was conducted under controlled conditions in ryegrass (Lollium perenne L.) pots with different organo-mineral fertilisation strategies at sowing with adjusted doses of P (120 kg P ha⁻¹) and N (200 kg N ha⁻¹). Pellets were developed from compost enriched with bone meal (OMF-BON), struvite (OMF-STR), and monoammonium phosphate (OMF-MAP). Conventional fertilisers (Complex15 and MAP) and alternative unpelletised/pelletised sources (STR and BON) were also tested. The experimental design included an unfertilised control (C), and treatments were carried out in triplicate (N = 24). Over 40 days, three cuttings (10, 25, and 40 days) were collected to determine fresh/dry biomass, nutrient content, and N, P, and K extraction efficiency. Soil labile parameters were influenced by the application of fertilisers especially OMF-MAP, OMF-STR, and MAP. MAP and STR yielded the highest nutrient extraction and biomass production, followed by their pelletised forms (OMF-MAP and OMF-STR). These results highlight the potential of pelletised organo-mineral fertilisers as sustainable alternatives to conventional sources.

In this study several phosphorus fertilizers were evaluated under controlled production conditions using Lactuca sativa var. baby leaf and a clay-loam soil of pH 6.5 as a plant–soil model system. Various inorganic (phosphate rock, monoammonium phosphate, struvite), organic (bone meal and bone meal pelletized with compost) and organomineral fertilizers (phosphate rock, monoammonium phosphate, struvite pelletized with compost) were compared. The soil properties, crop yield, morphological aspects and metabolomics of the plants were analyzed. After 45 days of the growing cycle, the organomineral fertilizers (OMFs) composed of compost and monoammonium phosphate (OMF₂(MAP+C)) or struvite (OMF₃(STR+C)) exhibited the best yield results: 101.37 g and 83.21 g, respectively. These treatments also exhibited the best phosphorus use efficiency (PUE) results: 7.40% and 8.33%, respectively. The yield of plants treated with MAP was 56.01 g, and its PUE was 5.33%. The yield of plants treated with STR was 62.10 g and the PUE was 4.67%. Accordingly, the development of OMFs with compost had a positive effect regarding MAP and STR fertilization. Lettuce fertilized with organic bone meal fertilizers had the lowest yield and nutrient use efficiency. The non-targeted metabolic study of green tissue revealed an overactivation of the TriCarboxylic Acids-TCA cycle and amino acid biosynthesis in plants fertilized with bone meal and phosphate rock treatments, likely as a plant stress response. The overall conclusion of this work is that the development of OMFs with compost is a good strategy to increase soil P availability and, accordingly, plant P uptake and %PUE.

Olive mill wastewater is a polluting residue generated from the olive oil industry and is one which constitutes an environmental concern in Mediterranean countries. Composting has been reported as a viable valorization alternative, as it reduces the volume and the phytotoxic characteristics of OMW. In this study, several composts derived from OMW were evaluated under controlled conditions over two growing season pot experiments using Lactuca sativa as a test crop. The analysis focused on soil quality changes, crop yield, and plant development. Additionally, potential phytotoxicity was also evaluated through a direct acute toxicity plant growth test. Application of OMW composts improved soil fertility indicators, including oxidizable carbon, Kjeldahl total nitrogen, Olsen phosphorous, and plant availability. Crop yields were comparable to those obtained with other organic amendments such as vermicompost and fresh cattle manure in both growing seasons and plant development (in terms of chlorophyll content and canopy cover) was not negatively affected. Nutrient uptake (NPK) was consistent during both growing seasons, with similar nitrogen use efficiency to that achieved in other organic treatments. Regarding the potential toxic effect, the OMW composts tested enhanced seed germination when mixed with coconut fiber at weight ratios below 29.2%. No half-maximal effective concentration (EC50) values were detected, even at 100% compost concentration, while half-maximal inhibitory concentration (LC50) values ranged between 65–75%. These results indicate that OMW composts can serve as an effective short-term source of plant-available nitrogen and a medium-term source of phosphorus, without risk of finding inhibitory or phytotoxic effects on crops.