Sihombing, Juna, Adil Barus, Jahartap Yustin Pasaribu, and Immanuel Julius Sirait. “The Effectiveness of Activated Carbon Made from Shallot Skin in Reducing Calcium Levels and Total Hardness in Hard Water: Efektivitas Karbon Aktif Berbahan Kulit Bawang Merah Terhadap Penurunan Kadar Calsium Dan Total Hardness Pada Air Sadah”. Jurnal Rekayasa, Teknologi Proses dan Sains Kimia (REPROKIMIA) 4, no. 1 (June 18, 2025): 38–43. Accessed October 24, 2026. http://akses.ptki.ac.id/jurnal/index.php/reprokimia/article/view/230.