Favorable operating conditions for obtaining high-value struvite product from sludge dewatering filtrate

Struvite (MgNH₄PO₄) is emerging as a potential fertilization product for agriculture. As other phosphate minerals that can be precipitated from wastewater are known to have lower value as fertilizers, the overall fertilization quality of the solids precipitated in struvite recovery systems depends largely on the percentage of struvite in the mixture of precipitated solids. This work focused on determining the most appropriate operating conditions (with a focus on pH and hydraulic retention time [HRT]) for removing >90% of the dissolved phosphate and, at the same time, attaining a precipitate with the highest possible struvite content from typical filtrates of sludge dewatering belt press systems. To this end, a continuous laboratory-scale completely mixed reactor was operated at different pH values and HRTs and the precipitant composition was determined. Theoretical calculations showed that the most cost-effective MgCl₂ dosage to typical Israeli belt press filtrate is ∼10 mM. Using this dosage, ∼8.0 mM of phosphate precipitated under all the operating conditions studied. The shortest retention time (15 min) and lowest pH value (pH 7.4) applied were found to be the most favorable for the attainment of the most homogeneous struvite precipitate (∼85% struvite). At higher pH values and longer HRTs, the overall precipitated mass (in P units) was slightly higher, but unwanted calcium-phosphate and magnesium-phosphate precipitates were observed at higher percentages alongside the struvite crystal. This finding implies that the typical high pH values, previously perceived to be optimal for struvite precipitation, actually result in a less-valuable product.