Sustainable resource recovery in closed loop system

Abstract

Demands on water supplies and carbon dioxide emissions are increasing due to population growth. Water scarcity has led to an increasing need for robust waste-water treatment technologies that can be integrated into water reuse systems. Long term space habitation is currently limited by the frequency at which resources can be delivered. Loop closure through regenerative processes is a method to greatly reduce resupply needs. Resources currently reused on the International Space Station (ISS) are limited to water and air revitalization. The technologies used are based on physical and chemical processes some of which are expendable and non-regenerable. Non-regenerable processes require significant supply of the active material to replenish those lost in regeneration. The expansion of human presence in space necessitates a reliable, sustainable, carbon dioxide adsorption system and water source. Water is a critical life support element, representing at minimum, 65% of the daily mass input for crew members. A reliable water source is essential for space habitation. Water recovery allows mission duration to be uncoupled from a dependence on initial water resources or resupply scenarios. However, current water recovery systems require a significant use of consumable resources such as hazardous chemicals needed for urine stabilization. These systems also yield toxic end products that create additional challenges regarding storage or disposal.
In contrast, biological treatment uses living cells to recycle energy harnessed from the oxidation of compounds in waste water for cell growth and maintenance, which can result in a sustainable process. Membrane aerated biological reactors (MABRs) provide an efficient and sustainable alternative process for treating a space-based waste stream. Research on the use of biological waste water treatment for space habitation has culminated in the development of the rectangular Counter-diffusion Membrane Aerated Nitrifying Denitrifying Reactor (rCoMANDR) and a graphene based carbon dioxide removal technology at Texas Tech University (TTU). This work demonstrates the ability to treat space-based waste streams using a microgravity compatible biological reactor. We have demonstrated the continuous operation of the reactor over almost 3 years. The CoMANDR system was able to operate in both an oxic mode and anoxic mode. Both modes were able to achieve carbon and nitrogen oxidation although in the anoxic mode performance was reduced and reaction rates were smaller. The anoxic mode did allow for increased reduction of oxidized nitrogen through denitrification. CoMANDR was also able to operate without the need for a feed tank by accepting waste-waters as they are produced with no reduction in efficiency. The system required little if any maintenance although bio-solids did accumulate over the operational period. The system does require O2 as a consumable but at a relatively low rate compared to human consumption. The system also produces some N2 gas, which is a useful product, and CO2, a contaminant that must be removed. In order to efficiently remove CO2 without consumption of resources and in a manner that would facilitate its use as a resource by producing a more concentrated waste gas, graphene based adsorbents were evaluated. Joule heating / electric swing adsorption (ESA) was evaluated as a mode of regeneration. Pristine graphene films and reduced graphene oxide (rGO) aerogels were both tested for their ability to remove CO2. The amount of CO2 captured by graphene films and rGO aerogels was at the upper range of carbon adsorbents and was found to be stable over multiple samples and multiple cycles of adsorption and electrical current stimulated desorption. The use of similar graphene based adsorbents in space-based systems could provide a lower mass adsorbent and more efficient regeneration method.

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Keywords

Wastewater Recycle, Air Pollution, Carbon Dioxide (CO2) Removal, Nano Particles, Space Habitation, Graphene, Biological Wastewater Treatment

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