The aim of the project is to define a treatment line for the recovery of phosphorus from sewage sludge ash.

The main source of phosphorus is phosphate rock mining, and three quarters of the world’s currently active mines are located in Morocco and North Africa, China, the US and Russia. In 2014, the European Union included phosphorus in its list of 20 critical raw materials, i.e. those with a high economic importance and a high supply risk.

Two possible treatment lines were envisaged depending on the type of acid used, divided into three process steps:

• Step 1 – Ash leaching: acidification of the effluent (with HCl or H2SO4) aimed at solubilising the phosphorus present in the ash, with subsequent separation of the solid content to obtain a supernatant as rich as possible in dissolved phosphorus and low in other types of solids.
• Step 2 – Separation of phosphorus: basification of the effluent by addition of lime, leading to sedimentation of phosphorus as calcium phosphate Ca3(PO4)2.
• Step 3 – Final Finishing: to be evaluated according to the destination of the effluent.

In addition, a pilot-scale plant was subsequently hypothesised to evaluate the process.