Abstract: Embodiments of a gravity-flow filter cartridge are described for removing water-borne parasites, and other contaminants such as chlorine, lead, and organics from water. The filtration cartridge includes a liquid porous plastic filter, which has pore diameters large enough to let water pass at acceptable flow rates under only the force of gravity, while the matrix creates a tortuous path which traps parasites and other microorganisms that are typically in the approximately 3-9 micron size range. The filter may include interstitial carbon and other additives to improve performance. The filter may be shaped to provide a large surface and an optimized wall thickness for providing beneficial filter capacity and water flow rate.