6th Stage - The Granular Activated Carbon (GAC) and alkaline filter is fitted externally so you can use the carbon filter (WLP6) or the mineral filter (WLP13) as the 5th stage. Alternatively the 6th stage can comprise of the hydrogen generator to produce the perfect pH and ORP values for a detoxifying water normally only sold as an expensive bottled water. 6th Stage options GAC + Alkaline Hydrogen generator 5th Stage filter for the 5 stage purification system. Option 1 - carbon filter (WLP6) for final finishing of water prior to dispensing - standard setup. Option 2 - mineral filter (WLP13) helps add back minerals to the purified water prior to dispensing. Option 3 - Granular Activated Carbon (GAC) and alkaline filter (WLP12) finishes the purified water and changes the pH value from acidic (the pH of purified water is normally 6.5 depending upon the pH of the supply water) to alkaline with a pH of approximately 8.5 depending upon the pH of the supply water. Option 4 - mixed ion exchange bed deionizing filter which makes the water suitable for autoclave systems and laboratory applications. If using the Hydrogen generator, for the 5th stage use option 1 or option 2 only.