Direct Contact Membrane Distillation

Abstract

Direct Contact Membrane Distillation (DCMD) is a primary membrane treatment process of the Direct Osmotic Concentration System (DOC), designed to recycle spacecraft wastewater. The DOC combines three separate processes to generate potable water from source separated hygiene water, urine, and humidity condensate. The DOC can achieve a high water recovery rate, has relatively low power requirements and can accommodate wastewater generated by six crewmembers. This system is designed for long-term human space missions, such as Mars exploration, because it minimizes the need to resupply fresh water for crewmember use. The DCMD reclaims water from a feed solution consisting of urine, urine flush, and humidity condensate by creating a 15ºC temperature difference between the concentrated urine loop (at 40ºC) and the product water loop (at 25ºC). This induces a vapor pressure differential across the hydrophobic membrane, allowing water vapor to cross the microporous membrane, while rejecting urea and ammonia nitrogen, among other foulants. The water produced during pretreatment by both the Forward/Reverse Osmosis subsystems (FO/RO) and the DCMD is combined and undergoes secondary treatment by the Aqueous Phase Catalytic Oxidizer (APCO). A Total Organic Carbon (TOC) Analyzer determined the quality of the water produced by the DOC system. The TOC of water produced by the APCO was less than 3mg/L, which fulfills NASA drinking water standards. Testing resulted in an average of 96% water recovery with 98% TOC rejection. The average water production rate was 1.2 L/hr.