Green Carbon for Hydro-Logic Merlin Garden Pro Filter
The Carbon Filter is the carbon block pre-filter that will remove sediment, chlorine, organic chemicals, etc.
The Sediment Filter is a pre-filter that will remove silt, rust, sand, etc.
The Merlin-Garden Pro comes with two of these RO membranes elements. As long as you keep the pre-filteres clean or your inlet water is not beyond the recommended parameters, these should last 2-4 years. When it is time to replace them, you need to get a pair.