This paper focused on the removal of free fatty acids from the ester phase. Transesterification was stopped by neutralization of the catalyst (KOH) by phosphoric acid addition to the reaction mixture. The advantages of this method are a very fast separation, an ester phase that does not contain potassium ions and free glycerol, and a relatively high purity of the glycerol phase. The disadvantage is the formation of free fatty acids, which were removed by the addition of calcium hydroxide, a reaction that formed solid calcium soaps that were removed by centrifugation. The conditions of removal (independent variables), such as the acid number of the input ester phase, molar ratio of calcium to FFAs and addition of water, were studied. The relations between independent and dependent variables (acid number, amount of calcium ions and water in the ester phase) are described. The model allowed for the calculation of conditions, so that the biodiesel fulfilled EN14214: a molar ratio of calcium to FFAs of 3.33:1, the addition of 1.45 mu l water to 1 g of the ester phase and a reaction time of 3 h. This preparation of methyl ester is nearly without waste and does not use water for purification. (C) 2016 Elsevier Ltd. All rights reserved.