In fall 2011, one of the largest pit lakes of Eastern Germany was limed with a novel on-site process. Within 16 weeks of operation, its 110 Mm
of water was shifted from the iron buffer to circumneutral pH-values. Due to a thorough consideration of the chemical and hydrodynamic parameters the method obtained 80 % process efficiency. In this paper we will present the details of this novel UNP-process.