Emerging technologies for urine wastewater recycling in space exploration: A comprehensive review

Efficient water recovery is crucial in long-range space missions, where resupply is limited and a sufficient water supply must be provided for the entire mission duration. Water can be resupplied to the International Space Station (ISS), but for missions outside Low Earth Orbit (LEO), it is almost impossible. Even after 50 years of research, the existing Urine Processor Assembly (UPA) and Water Processor Assembly (WPA) continue to be used on the ISS for water recovery at a suboptimal rate. There is a need to improve the existing system’s efficiency, adapt it to suit the needs of long-range missions, or develop new technology. This review examines the UPA and WPA systems, highlighting their roles, efficiency, and the need for further modification. Membrane-based technologies, including Forward Osmosis (FO) and Membrane Distillation (MD), as well as emerging hybrid technologies such as integrated FO-MD and the Photocatalytic Membrane Reactor (PMR), have been discussed. Special attention has been given to brine treatment technologies, such as the Forward Osmosis Brine Dryer (FOBD), Ionomer Water Processor (IWP), Capillary Brine Residual in Containment (CapiBRIC), and Brine Evaporation Bag (BEB), which various space agencies have identified as potential alternatives. Technological advancements in developing a closed-loop biological life support system, Micro-Ecological Life Support System Alternative (MELiSSA) are also discussed. These technologies can be used as a standalone system or integrated with other treatment methods for water recycling. The review offers a critical evaluation of current water recycling systems and examines innovative technologies that can be integrated to enhance system reliability in space exploration missions.