Nitrification

Objectives

After carbon, nitrogen is the main element to cycle. A crew member consumes 9–19 g of protein-nitrogen per day and excretes roughly the same amount, mainly as urea in urine (7–16 g N per day). Nitrification-based processes serve several purposes, ranging from stabilising waste streams to producing useful compounds such as nitrate, nitrogen gas and water.

The main nitrogen source within the waste stream is urine, derived largely from protein metabolism. Closing the nitrogen cycle means solving several challenges:

Stabilising urine, so that it does not become a risk to the astronaut.
Transforming urine into a substrate suitable for higher-plant and micro-algae cultivation.
Removing or transforming any chemical elements that could be toxic to other MELiSSA processes.

All of this has to be achieved within the ALiSSE criteria: low mass, low energy, low risk and low crew time. Within MELiSSA, this nitrogen transformation task is carried out mainly through nitrification.

Challenges & Approach to the Research

The first set of R&D challenges lies in achieving the best possible process performance: closing the nitrogen and carbon mass balance while minimising the process’s mass, volume and requirements, such as energy and chemicals.

Mechanistic understanding matters just as much for a reliable, predictable process. Two approaches run alongside the process technology itself: detailed characterisation of the microbial conversions at the physiological and molecular level, and mathematical modelling for process control, automation and integration with the other units in the treatment or production system.

For space adaptation, the focus is on understanding radiation effects and developing a gravity-independent system. The latter requires a suitable gas/liquid transfer step to supply oxygen to the microbes, and a specific gas/liquid separation step to remove gaseous compounds from the process and the resulting liquid.

Flight Experiments

The space environment adds a few more constraints, mainly reduced gravity and radiation. Gravity mainly affects multi-phase issues, gas/liquid/solid mixing and separation, while radiation can affect the bacteria at a molecular level (DNA, RNA).

Two MELiSSA flight experiments, BISTRO and NITRIMEL, have already been carried out successfully, with two more currently in preparation. The first two demonstrated the reactivation potential of a microbial consortium after exposure to elevated radiation and fluctuating gravity, promising findings for nitrification-based processes more generally.

The challenge now being addressed in preparing the upcoming URINIS flights is to obtain in-flight activity on the International Space Station (ISS), in both batch incubations and a first continuous reactor test.

Overview of the BISTRO, NITRIMEL and URINIS A/B nitrification flight experiments: research topic, reactor type, ISS orbit, radiation dose and duration.

Ground Demonstration

Any space hardware has to be robust and efficient, and this matters even more for a life support system whose job is to keep an astronaut alive. Before any flight hardware is developed, it is essential to demonstrate the robustness of MELiSSA processes on the ground.

This demonstration began with an engineering phase to study the nitrification requirements and design the nitrifying bioreactor itself. For aeration, both sparging with air and oxygen, and gravity-independent membrane aeration, have been shown to work; effluent is typically withdrawn through a membrane, producing a bacterium-free stream.

Beyond validating key performance figures, the effect of key parameters (oxygen, mixing, nitrite concentration) has been studied, and a predictive control law has been implemented: the mathematical prediction of nitrite concentration matched experimental measurements closely across piloted dissolved-oxygen steps, for instance from 20% to 80%. Long-term stability was demonstrated at the MELiSSA Pilot Plant in Barcelona for more than a year.

Terrestrial Applications

Research on nitrification has produced several terrestrial applications. In the early 1990s, ESA and the R&D centre of Générale des Eaux (today Veolia Water Technologies) worked together to study and compare physical support materials for nitrification in fixed-bed reactors. That work led to the BIOSTYR® support material, now widely deployed across Veolia wastewater treatment plants.

MELiSSA has also started a collaboration with Eawag and Vuna on nitrification-based systems for recovering nutrients from source-separated urine.

HYDROHM’s URIDIS urine-treatment system, demonstrated in Ghent in collaboration with Ghent University.

Education & Communication

Nitrification has received frequent coverage in science communication, including television interviews, documentaries, newspapers and magazines.

The researchers behind it are also regularly invited to speak to general audiences, from primary and secondary school students through to higher education and working professionals. A number of technologists and scientists have trained in this area through MSc and PhD studies, and a substantial body of work has been published in peer-reviewed journals covering the technology, microbiology, modelling and integration involved.

MELiSSA-related work has also been recognised externally: nitrification research placed first and third for Best Master’s Thesis in the Belgian Senate’s 2016 competition.