A Moon habitat does not become sustainable simply by landing enough drinking water. Every breath, meal, washcloth and spacewalk moves material through a system that must keep people alive while losing as little as practical. The engineering challenge is to recover useful water and oxygen without making maintenance, contamination control or power demand unmanageable. Environmental control and life support systems, or ECLSS, connect those jobs. The International Space Station offers a working example, including a demonstrated 98% total water recovery milestone. But a percentage achieved on an orbiting laboratory is not a certification for a lunar base. Understanding the difference explains why pumps, filters, sensors and spare parts matter as much as a habitat's pressure shell. Historical ISS regenerative ECLSS concept diagram, not a current lunar habitat design or an inventory of installed ISS amenities. Credit: NASA. Hero photo: NASA astronaut Kayla Barron replaces a filter in the ISS Brine Processor Assembly. Credit: NASA. The photograph shows orbital research hardware, not equipment operating on the Moon. What a life support loop actually closes NASA describes ECLSS as more than a recycling plant. Its responsibilities include atmospheric pressure, oxygen levels, ventilation, water supply, waste management, and fire detection and suppression. Three central regenerative elements are the Water Recovery System, Air Revitalization System and Oxygen Generation System. They exchange material, but they do not perform interchangeable jobs. Water recovery turns collected wastewater into water suitable for use. Air revitalization removes carbon dioxide and trace contaminants from circulating cabin air. Oxygen generation supplies breathable oxygen, using water as an input. Keeping one subsystem running does not make the others optional: a cabin can contain enough oxygen and still have an unsafe accumulation of carbon dioxide. The word closed needs a boundary. A water-processing loop may recover most of the water entering it while the wider habitat still consumes food, replacement filters, gases, electrical energy and maintenance supplies. Waste streams can leave the system. Air can escape during operations or leaks. Recovery percentage, total delivered mass and safe operating duration answer different questions. That distinction is useful when evaluating a proposed Moon base. Ask which streams are collected, which are treated, and which are discarded. Then ask how long the quoted performance was sustained and what support was required. A laboratory result, a flight demonstration and a maintainable service for a crew are separate levels of evidence. From wastewater to drinking water The station's water system gathers more than urine. Dehumidifiers recover moisture released into the cabin by breathing and perspiration. Other wastewater joins the treatment stream. NASA's ECLSS reference also identifies water from spacesuit hydration systems among the inputs. Collection is the first practical limit: water that never reaches the recovery equipment cannot benefit from its purification efficiency. The Urine Processor Assembly uses vacuum distillation to separate recoverable water from urine. Distillation leaves concentrated brine that still contains water. The Brine Processor Assembly addresses that remaining loss with a membrane process and warm, dry air. Water evaporating from the brine becomes humidity, which the station's collection equipment can capture again. Collected water then passes through the Water Processor Assembly. Specialized filtration and a catalytic treatment step remove contaminants. Sensors check water quality; water that does not meet the system's acceptance criteria is reprocessed. NASA describes adding iodine to acceptable water to discourage microbial growth during storage. This is controlled treatment, not simply routing a waste line into a drinking-water tank. In its June 2023 milestone report, NASA said the brine processor helped demonstrate 98% total water recovery , compared with 93–94% before that addition. The number refers to the reported station water-recovery configuration. It is not a promise that every future spacecraft will recover 98% of every water stream at all times. Why two NASA percentages can differ NASA's general ECLSS reference page still describes approximately 90% water recovery. Its specific 2023 milestone report documents 98% with the brine processor. For the higher figure, use the demonstration report and identify its configuration rather than treating the general overview as an updated performance record. Why 98% is important, but not independence A simple accounting example shows the benefit. Suppose a recovery system receives 100 kilograms of recoverable water. At 94% recovery, 6 kilograms leave that processing boundary unrecovered. At 98%, 2 kilograms do. For that same input, the unrecovered amount falls by two-thirds. This is illustrative arithmetic, not a forecast of lunar consumption or a measurement of a particular habitat. The remaining two kilograms still need to be replaced if the inventory is to remain constant. Real operations also require an initial fill, stored reserves, water tied up in equipment and protection against unexpected losses. Crew size and activity change the throughput. A percentage without a daily mass balance cannot tell planners how much water a delivery vehicle must bring. High recovery can also shift costs rather than eliminate them. Additional treatment equipment needs power, space, heat rejection and service. Filters and processing components have finite lifetimes. If collecting the last portion of water requires more hardware or crew attention, mission designers must compare that burden with the mass and reliability of carrying more supplies. Mission duration changes the comparison. A short visit may reasonably emphasize stored consumables and simple equipment. Repeated or extended stays strengthen the case for regeneration because the same hardware can reduce many successive deliveries. Neither choice is automatically superior: the decision depends on the whole mission, including the consequences of a failed processor. Water and air are connected, not interchangeable NASA's oxygen-generation description starts with electrolysis. Electricity splits water into oxygen and hydrogen. Oxygen enters the cabin atmosphere. Hydrogen can be vented or sent to a carbon dioxide reduction assembly. That assembly uses a Sabatier reactor to react hydrogen with crew-exhaled carbon dioxide, producing water and methane. NASA describes the methane as being released into space. Returning that water to the system recovers useful material, but the methane waste stream shows why this is not a perfectly closed chemical cycle. Nor does a water-recovery figure establish the percentage of oxygen recovered. Oxygen production, carbon dioxide removal and water purification have different inputs, losses, operating limits and failure modes. Ventilation is equally important. Fans and ducting move cabin air through treatment equipment and occupied spaces. Trace-contaminant removal addresses substances released by materials, electronics and people. Pressure control and fire protection remain essential even when the recycling machinery is working. The habitat is an integrated environmental system, not a collection of independent appliances. For lunar operations, those connections create design questions. How does a water-processing outage affect oxygen production? What reserves keep the crew safe while a component is isolated? Can atmosphere treatment continue when another rack is being repaired? These are engineering questions raised by the coupled architecture, not claims that a particular lunar vehicle has already solved them. The Moon adds an operating environment The ISS provides valuable long-duration operating experience, but its equipment cannot be assumed to work unchanged on the lunar surface. A surface