Optimization of struvite from sewage sludge ash as phosphorus source

Phosphorus is an essential macronutrient for crop production. Struvite precipitation and reuse from phosphorus-rich sewage sludge are cost-effective measures to improve phosphorus utilization efficiency and decrease its negative environmental impact. In this study, the objectives were to optimize the phosphorus extraction (using sulfuric acid) and recovery (as struvite) processes and determine the most appropriate process conditions using RSM. This was done by evaluating the effect of different parameters such as acid concentration (mol/l) (0.02 to 0.8), H₂SO₄/ISSA ratio (liquid/solid) (20 to 150 ml/g), and time (0.5 to 4 h) for leaching tests and N: P ratio (1 to 2), Mg: P ratio (1 to 2), and pH (8 to 11) for struvite precipitation. The optimization of the factors affecting PO₄^3--P extraction from SSA by acidic leaching showed that applying 0.5 mol/l H₂SO₄ and 57 ml/g L/S ratio at 2 h achieved the highest PO₄^3--P extraction (99.8%). The extraction of phosphorus also leached heavy metals; however, using a cation exchange resin, it was possible to effectively remove heavy metals from P-rich solutions. The optimal phosphorus recovery as struvite (98.5%) was achieved at the lowest pH and N/P ratio, while an increase in Mg/P ratio from 1 to 2 positively affected phosphorus recovery. The obtained struvite had a high content 94.6%, and the heavy metal content in struvite was lower than the value of standard, so that the obtained struvite sample can be used as fertilizer.