Removal of calcium from water by zeolites with gravity-driven membrane filtration for water treatment without electricity†

Hard water creates issues due to scaling that impacts industrial and domestic applications. Zeolite adsorption is an effective method for calcium removal. However, treated water requires additional filtration to remove suspended solids, which uses energy under traditional methods. Considering applications at sites where electricity is unavailable, this study removed calcium from water using zeolite adsorption and gravity-driven membrane (GDM) filtration. Four zeolite types were assessed: powdered (P1 and P2), bead-shaped (B) and granular (G). The properties of the zeolites significantly affected calcium removal and GDM filtration performance. Zeolites with low Si/Al ratios (high Al content) exhibited high Ca removal because negatively charged lattice sites were created by replacing Si with Al. P1 and B with Si/Al ratios of 1.40 and 1.49, respectively, exhibited high Ca-removal efficiencies of 92.6% and 99.8%. In contrast, P2 and G with high Si/Al ratios from 4.71–5.35 showed lower removal efficiency of 29.8–43.7%. The average sizes of P1, P2, B, and G were 9.68, 5.73, 2134, and 3639 μm, respectively. At similar Si/Al ratios, smaller particles exhibited faster adsorption rates. However, as the zeolite size decreased, the GDM flux decreased. During GDM filtration after zeolite adsorption, the permeate flux of water treated with large B and G zeolites was higher than that of water treated with small P1 and P2 zeolites. Finally, the zeolites were evaluated based on three criteria: Ca removal, GDM water flux, and price, which provides useful guidance for selecting appropriate zeolites.