Recycling phosphorus as struvite from pig slurry requires an acidification step to dissolve the inorganic solids containing most of the phosphorus. This study focused on the biological acidification of several pig slurries using sucrose as a model organic co-substrate. Lactic acid fermentation occurred systematically, dissolving 60'90% of TP (total phosphorus) and T-Mg (total magnesium) at pH 6 or lower. Optimal pH range for maximum P dissolution aimed at struvite recovery was 5.5'6. A simple model was developed correlating pH, sucrose and buffer capacity to optimize P dissolution and future recovery using real organic waste. © 2017 Elsevier Ltd