Solid-Liquid separation

Overview

In the second stage of the process, solid partices from human feces are separated to obtain a sterile, VFA-rich liquid stream that can be passed to the next stage of the process. Sterility is essential, since genetically engineered E. coli in the next stage of the process, where PHB is produced, 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 removed remaining small particles.

Design options considered

Considering 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 insufficient water recovery. Under 55% of water was recover for all, even diluted trials, though dilution did improve the water recovery efficiency. Staged filtration 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 initial water in the sample was recovered highlighting 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 water, production of sterile and VFA-rich output stream and production of solid by product - char, which can in turn be a used as a building material, radiation shielding, and as a food subtract (A. Serio, E. Cosgrove & A. Wojtowicz, 2016). However, the product stream leaving the torrefaction processing unit contains only water and VFA resulting in low pH and low amount of nutrients. We have conducted the "PHB synthesis using pure VFAs as feedstock" experiment to evaluate weather or not E coli can survive in the following conditions (condition 5). The experiment showed that E coli it is unable to produce PHB in the VFA rich liquid, meaning that torrefaction can't be used as a primary technique for solid liquid separation.

Screw-press dewatering system required a large number of consumables as well had the largest mass when combined together with the ESM parameters for multi-filtation ((Jones, Fisher, Delzeit, Flynn & Kliss, 2016). This lead to a very high ESM value which meant that despite low power consumption the technology is not feasible for implementation on space.

Finally we conducted experiments to test the liquid recovery efficiency using a centrifuge and found 75% liquid recovery using the "Centrifugation for solid liquid separation" experiment . Literature search on the application of centrifugal separators showed that it would be a good match for the required task. We have further contacted Russel Finex Company to get their advice on the application of their centrifugal separators to our process and received a confirmation of applicability. We have also received required ESM parameters from the Russel Finex representative.

Estimated ESM parameters for these process designs are summarized in Table 1.

After evaluating 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 by-products after solid-liquid separation and the sludge from wastewater treatment on Mars to recover additional water and produce char.