In biodiesel production through transesterification, glycerin is produced as a valuable co-product. Some biodiesel plants have incorporated a glycerin purification process since the beginning of operations. The objective of this study is to evaluate the benefits of the incorporation of a glycerin purification process to a biodiesel plant where has already been constructed without the initial consideration of including an extra process for glycerin. A cost-benefit analysis is applied to evaluate the potential benefit from the incorporation of this process, in cases of a high plant cost and a low plant cost, with and without a glycerin facility installation. Our results show that the installation of a glycerin purification process could benefit a biodiesel plant through glycerin recovery and a decrease in wastewater treatment cost. The current downward trend in the market price of glycerin could be offset by encouraging the development of new applications for glycerin in order to expand the existing market and to stabilize the market price. However, biodiesel plants using waste cooking oil in Japan still need government subsidy to sustain their business activity. (C) 2010 Elsevier Ltd. All rights reserved.