Solid-Liquid separation

Overview

In the second stage of the process, solid particles are separated from human feces to obtain a sterile, VFA-rich liquid stream that can be passed to the next stage of the process. Sterility is essential, since our PHB-producing E. coli in the next stage of the process might be outcompeted if other types of bacteria are present. The separation of solids is achieved using centrifugation, which removes large solid particles, followed by filtration, which removes remaining small particles and bacteria.

Design options considered

Considering the limited availability of resources on Mars including power, the initial experiments focused on mechanical and gravity-driven separation process:

• Gravity-driven filtration
• Settling
• Pressure filtration

Laboratory experiments (see "Gravity Driven Filtration" and "Gravity Driven Sedimentation" on the Experiments page) provided insufficient sterility and water recovery. Under 55% of the initial water was recovered for all trials, though dilution did improve the water recovery efficiency. The Staged Filtration for Solid-Liquid Separation experiment was a modification to the original experiments and provided sufficient sterility (the sample was passed through a 0.2 micron filter paper), yet insufficient water recovery - only 10% of the initial water in the sample was recovered. These results highlighted the need for more energy-intensive methods.

More advanced solid-liquid separation techniques were then considered:

• Torrefaction (mild pyrolysis)
• Centrifugation followed by filtration
• Screw-press dewatering system followed by multi-filtration

Initially, torrefaction appeared to be the best solution due to it's ability to recover natural and pyrolytic (chemically bound) water, the production of sterile and VFA-rich output stream and the production of solid by product known as char. Char can be a used as a building material, radiation shield, and as a food supplement (A. Serio et al., 2016). However, the product stream leaving the torrefaction processing unit contains only water and VFAs, resulting in a low pH and a low amount of nutrients. We have conducted an experiment (PHB Synthesis using Pure VFAs as Feedstock) to evaluate weather or not E. coli can survive in the previously mentioned conditions. The experiment showed that E. coli it is unable to produce PHB in the VFA-rich liquid, meaning that torrefaction cannot be used as a primary technique for solid liquid separation.

The screw-press dewatering system required a large number of consumables, and had the largest mass when combined with the ESM parameters for multi-filtation (Jones et al., 2016). This led to a very high ESM value which meant that despite low power consumption, the technology is not feasible for implementation on Mars.

Finally, we conducted experiments to test the liquid recovery efficiency using a centrifuge and found 75% liquid of initial water was recovered using the Centrifugation for Solid-Liquid Separation experiment. A literature search on the application of centrifugal separators showed that it would be a good match for the required task on Mars. We have further contacted the Russel Finex Company to obtain their advice on the application of their centrifugal separators to our process, and received their confirmation of applicability. We have also received required ESM parameters from the Russel Finex representative.

After evaluating the advantages and disadvantages of the proposed designs and considering the ESM analysis, centrifugation followed by filtration was chosen as the preferred method for the solid-liquid separation step. In addition, the team proposes to adapt torrefaction to treat the solid byproducts after solid-liquid separation and the sludge from wastewater management on Mars to recover additional water and produce reusable char.