J. Sihombing, A. . Barus, J. Y. Pasaribu, and I. J. 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”, REPROKIMIA, vol. 4, no. 1, pp. 38–43, Jun. 2025.