This work studied the influence of the characteristics of the supernatants on the struvite precipitation process. Eighteen experiments with the supernatants generated in an anaerobic digestion pilot plant were performed in a stirred reactor. In order to obtain the pH control during the crystallization process, a Fuzzy Logic based controller was used. High phosphorus precipitation and recovery efficiencies were obtained. The composition of the supernatants was analyzed in order to study its influence on the solids formed from those solutions. The presence of calcium reduced the percentage of phosphorus precipitated as struvite leading to the formation of amorphous calcium phosphate (ACP), which tended to be lost with the effluent of the reactor. Calcite was also formed when supernatants with high magnesium: phosphorus (Mg/P) and calcium: phosphorus (Ca/P) molar ratios were employed. Some ammonium volatilization by conversion to NH(3) occurred in all the experiments. The use of air to increase the PH to an adequate value showed to be feasible. Aeration cleaned struvite crystals from suspended solids, which makes aeration interesting for struvite separation. However, aeration slightly increased the loss of phosphorus with the effluent of the reactor and promoted ammonium volatilization. (C) 2009 Elsevier Ltd. All rights reserved.