Mild temperature hydrothermal oxidation of anaerobic fermentation filtrate for carbon and nitrogen recovery in a regenerative life support system

Recycling nutrients from organic waste for growing higher plants in the Micro Ecological Life Support System Alternative (MELiSSA) loop during long-term space missions

Space agencies are developing Bioregenerative Life Support Systems (BLSS) in view of upcoming long-term crewed space missions. Most of these BLSS plan to include various crops to produce different types of foods, clean water, and O2 while capturing CO2 from the atmosphere. However, growing these plants will require the appropriate addition of nutrients in forms that are available. As shipping fertilizers from Earth would be too costly, it will be necessary to use waste-derived nutrients. Using the example of the MELiSSA (Micro-Ecological Life Support System Alternative) loop of the European Space Agency, this paper reviews what should be considered so that nutrients recycled from waste streams could be used by plants grown in a hydroponic system. Whereas substantial research has been conducted on nitrogen and phosphorus recovery from human urine, much work remains to be done on recovering nutrients from other liquid and solid organic waste. It is essential to continue to study ways to efficiently remove sodium and chloride from urine and other organic waste to prevent the spread of these elements to the rest of the MELiSSA loop. A full nitrogen balance at habitat level will have to be achieved; on one hand, sufficient N2 will be needed to maintain atmospheric pressure at a proper level and on the other, enough mineral nitrogen will have to be provided to the plants to ensure biomass production. From a plant nutrition point of view, we will need to evaluate whether the flux of nutrients reaching the hydroponic system will enable the production of nutrient solutions able to sustain a wide variety of crops. We will also have to assess the nutrient use efficiency of these crops and how that efficiency might be increased. Techniques and sensors will have to be developed to grow the plants, considering low levels or the total absence of gravity, the limited volume available to plant growth systems, variations in plant needs, the recycling of nutrient solutions, and eventually the ultimate disposal of waste that can no longer be used.

A prototype system for the hydrothermal oxidation of feces

To ensure access to safe sanitation facilities in rural communities, cheap off-grid technologies need to be developed to substitute pit latrines and open defecation. In this study, we present a prototype system based on hydrothermal oxidation, which, under optimal conditions, converts a fecal sludge simulant almost completely to CO 2 and water, leaving behind only a carbon-poor aqueous phase with the minerals. The prototype has been designed to process the feces from two households. This technology does not only enable a fast and complete conversion, but is potentially also very energy efficient, as the feed does not require any pre-treatment or drying. The system was found to effectively remove 97-99% of the total organic carbon within a reaction time of 600 s under an external energy demand of roughly 4 kWh per kilogram of wet feces by using the oxygen in air as an oxidant. A total of ten experiments with varying injection pressure, total solids content of the feed, and residence time in the reactor were performed to find experimental settings with high conversion. Only when the residence time was decreased from 600 to 300 s did the conversion fall significantly below 97%. To reach a target value of 99.9% TOC conversion, the reactor temperature and/or the residence time must be increased further. To achieve a system applicable in regions with no connection to the energy grid, the thermal loss of the reactor insulation needs to be lowered further to achieve an overall thermally self-sustaining operation.

Complete oxidation of organic waste under mild supercritical water oxidation by combining effluent recirculation and membrane filtration

Supercritical water oxidation (SCWO) is a technology that can oxidize various organic (wet) wastes into CO₂. Complete oxidation of specific organics with SCWO goes in tandem with tailored conditions, typically involving elevated operating temperatures, long residence times, high oxidizer-to-waste ratios, or a combination of those, which promote difficulties, e.g., corrosion. These challenges hamper the practical implementation of SCWO, albeit SCWO offers excellent oxidation efficiencies. This work proposes a novel process combining mild supercritical water oxidation (SCWO) with membrane filtration to enhance the oxidation of organics. The modified SCWO works at mild reaction conditions (i.e., 380 °C, 25 MPa and oxidizer equivalence ratios as low as 1.5) to potentially decrease the risks. The membrane filtration discards clean effluent and recycles the retentate (containing incomplete oxidized organics) back to the mild SCWO process for further oxidation, thereafter resulting in near-complete removal of organics. Fresh feed is continuously added, as in the conventional process, along with recycled retentate to guarantee the throughput of the modified SCWO process. A mixture of SCWO-resistant volatile fatty acids (TOC = 4000 mg·L^-1) was studied to validate the proposed process. The proposed process in this study enhances the organic decomposition from 43.2% to 100% at mild conditions with only 10% capacity loss. CO₂ was the dominant gas product with traces of CO and H₂. Carbon output in the gas products increased with recirculation and got close to the carbon input of the freshly added feed ultimately. The results indicated that the proposed process maximized the benefits of both technologies, which allows the development of a technological process for supercritical water oxidation, as well as a new stratagem for waste treatment.

Effluent recirculation enables near-complete oxidation of organics during supercritical water oxidation at mild conditions: A proof of principle

This work presents a continuous set-up for SCWO, which was operated at mild conditions (380 °C, 25 MPa, oxidant equivalence ratio of 2.0 and residence time of 26 s) to oxidize cellulose, lignin, and acetic acid as model compounds. The aim was to oxidize different organics consecutively to near completion in the same mild reaction conditions and set-up. These conditions can overcome some drawbacks associated to SCWO. To combine near complete oxidation with the applied mild process conditions, aqueous effluent from SCWO, containing intermediates from incomplete oxidation, was recycled for consecutive oxidation. Meanwhile, fresh feedstock was continuously fed to retain the process capacity. Upon recycling the aqueous effluent three to four times, depending on the feedstock, the oxidation efficiency increased from 63.9%, 45.3% and 28.3% in a single pass for cellulose, lignin, and acetic acid, respectively, to near 100%. The principle of effluent recirculation should allow a compact set-up to perform almost complete oxidation of different organics at mild conditions. The principles and effects of effluent recirculation are outlined, as well as practical consequences and perspectives of this novel principle to SCWO.

Assessment of carbon recovery from solid organic wastes by supercritical water oxidation for a regenerative life support system

The carbon recovery from organic space waste by supercritical water oxidation (SCWO) was studied to support resource recovery in a regenerative life support system. Resource recovery is of utmost importance in such systems which only have a limited total amount of mass. However, the practical waste treatment strategies for solid space wastes employed today are only storing and disposal without further recovery. This work assesses the performance of SCWO at recovering organic wastes as CO₂ and water, to discuss the superiority of SCWO over most present strategies, and to evaluate the different SCWO reactor systems for space application. Experiments were carried out with a batch and a continuous reactor at different reaction conditions. The liquid and gas products distribution were analyzed to understand the conversion of organics in SCWO. Up to 97% and 93% of the feed carbon were recovered as CO₂ in the continuous and the batch reactor, respectively. Residual carbon was mostly found as soluble organics in the effluent. Compared with the batch reactor, the continuous reactor system demonstrated a ten times higher capacity within the same reactor volume, while the batch reactor system was capable of handling feeds that contained particulate matter though suffering from poor heat integration (hence low-energy efficiency) and inter-batch variability. It was concluded that SCWO could be a promising technology to treat solid wastes for space applications. A continuous reactor would be more suitable for a regenerative life support system.