Abstract
Increasing global food demand has led to an intensive use of synthetic fertilisers. In this regard, the use of non-conventional streams such as swine wastewater (SW) and urban sewage sludge liquid fraction (USS) for the production of bio-based fertilisers can increase the sustainability of both the fertiliser industry and agriculture while reducing the reliance on imported nutrients. In this work, USS and SW were assessed for the production of struvite at different PO4 3−:Mg2+ ratios. Significant differences were found in terms of struvite crystals’ shape and size among both feedstocks due to the different saturation indexes, and it was concluded that PO4 3−:Mg2+ ratios of 1:2 for SW and 1:1 for USS were the most suitable for obtaining big crystals suitable to be used for direct fertilisation.
In addition, it was observed that the crystallisation process is highly dependent on the presence of interfering ions (mainly Ca) that can result in the formation of hydroxyapatite instead of struvite. Finally, recovering struvite from SW and USS could potentially reduce the European import of P by up to 6.5%.