<p><strong><em>Abstract</em></strong><strong>: </strong><em>The research investigated the effectiveness of Cogon grass as biomass adsorbent in Calcium (Ca) removal process. Calcium is one of the chemical element that causes hardness in river water. Calcium removal process was based on batch-adsorption principle process. Drying, size reduction and chemical activation were done on cogon grass as pretreatment step. Calcium removal process was conducted in adsorption column where pretreated cogon grass and contaminated river water were placed on together. Operation time was 60 minutes. The result showed different parts of cogon grass and decreasing in size of adsorbent particle gave a significant effect on calcium removal. The biggest percentage of calcium removal was 67.39% with capacity of adsorption was 0.54 mg/gram adsorbent at 250 µm adsorbent particle from root of cogon grass.</em><em></em></p><p><em> </em><strong><em>Keywords</em></strong><strong>:</strong><em> </em><em>cogon grass, biomass adsorbent, calcium concentration, river water</em></p><p> </p>