PENGARUH PENGGUNAAN ION EXCHANGER DENGAN KARBON AKTIF TERHADAP PENURUNAN KADAR Ca DAN Mg PADA AIR DANAU TLOGOSARI SEMARANG ( Effect of the use Ion Exchangers with Activated Carbon Levels to decrase as Ca and Mg contain Lakes Water Tlogosari Semarang ) - BANJARNAHOR, EDI BAHTRA SETIAWAN. (2015)