Sewage sludge hydrolysates and organic waste as alternative phosphate fertilizers in a circular economy strategy

Phosphorus (P) is one of the main plant nutrients, but it has very low solubility due to strong bonds with Al and Fe in acidic soils and with Ca and Mg in alkaline soils. The higher food demands has led to an increase of the use of phosphate fertilizers, mainly derived from phosphate rocks, which are predicted to be depleted in the next decades. Thus, there is the improving need to close the P cycle in order to reduce the P input from mineral source, recycling P rich organic products such as animal manure and human excreta. The aim of this work was to study the P availability for plants in calcareous soil of different bio-waste and to develop a new fertilizer from sewage sludge, with high P recovery and high P use efficiency. Different bio-waste were tested in a soil–plant system with different results in term of P solubility and plant growth, mainly caused by the metal concentration and the organic matter in the products. Later, two sludge products were used as raw sources for a new liquid fertilizer. Different chemical and enzymatic hydrolyses were tested on both sludge. The best solution in term of P recovery and low heavy metal extraction was then used in a soilless cultivation of lettuce and in a soil-plant system. The soil solved the problems encountered in the soilless cultivation, showing promising results for the hydrolysates as alternative phosphate fertilizers to the chemical ones in a circular economy strategy.