Dynamic and equilibrium precipitation of struvite from the concentrated cellulosic ethanol stillage

The phosphate rock mineral is the main source of P-fertilizer production. It is estimated to become depleted in next century. Thus, the recovery of phosphorus from waste streams has attracted great interest. The cellulosic ethanol production is seen as more and more important in future. During the production of cellulosic ethanol, the phosphorus element is released from lignocellulosic biomasses and ends up dissolved as phosphate ions in the stillage stream. In this study, the struvite (MgNH₄PO₄ · 6 H₂O) recovery from the concentrated cellulosic ethanol stillage (ES) was conducted under room conditions with an initial pH at 7-9. The effect of Mg²⁺, PO₄^3-, NH₄⁺ and Ca²⁺ during struvite precipitation tests was investigated. The optimized pH value for struvite recovery is estimated at 8.5, by which 85% of PO₄^3- and 46% of Mg²⁺ are removed from the liquid stream. The mass fraction of struvite in recovered crystal sample reaches 82 wt.%. The economic evaluation of struvite recovery from ES was also investigated. This work proves that the struvite is potentially to be recovered with high purity from the concentrated cellulosic ethanol stillage.