Struvite precipitation causes important operational problems during the sludge treatment process, especially when EBPR sludge is treated. Predicting struvite formation is critical to be able to design process alternative that best minimises struvite precipitation. With this aim, phosphorus precipitation in an anaerobic digestion pilot plant was studied using experimental data and mass balance analysis. The results obtained showed significant phosphorus precipitation as struvite (58% of the fixed phosphorus) and a low precipitation of calcium phosphates (15%), forming mainly hydroxyapatite (HAP). The rest of the phosphorus fixed in the digester (27%) was attributed to adsorption processes on solid surfaces. The effect of pH and available phosphorus concentration on phosphorus fixation in the digester was also studied, indicating that a decrease of available PO4-P concentration in the digester can minimise phosphorus precipitation, reducing the maintenance problems associated with struvite deposits. (C) 2007 Elsevier B.V. All rights reserved.