Water Purification Needs The BorderTransportationSystem will be carrying approximately 2,000,000 people per year and each person will use about 6 gallons of water for toilets, drinking, washing up, etc.  There will also be a considerable amount of waste or Graywater and it too needs to be considered and reused where possible. Graywater Systems - Phoenix Arizona - AquaCell water recycling Human Waste Solids Putting Human Waste to Use Reuse of excreta - Wikipedia Glossary of Terms for Waste Products - Pennsylvania chapter 271 Chlorine Storage Chlorine is used for final purification of water, it kills bacteria that may have passed through the filters.  It will be required at each purifier location.  State of Hawaii. Pamphlet 155 Water and Wastewater Operators Chlorine Handbook  {*.PDF - external file} Fresh Water for All EPA and Mexico Index of ...