Team:Calgary/SolidLiquidSeparation

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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

After laboratory experiments, these low power requirement processes were unable to remove all solid particles including bacteria or recovered less than 55% of the water present in the sample. Sequential pressure filtration performed the best during lab experiments resulting in a sterile liquid but recovering only 10% of water.

More advanced solid-liquid separation techniques were then considered:

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

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

Initially, torrefaction appeared to be the best solution, since it can recover natural and pyrolytic water, results in a sterile VFA-rich liquid stream and turns solid matter into char, which can be a used as a building material, radiation shielding, and as a food subtract. However, the product stream in the torrefaction process contains only water and VFA resulting in low pH and lack of nutrients and, therefore, cannot sustain bacterial growth during PHB production stage. This was also confirmed by lab experiments. Additionally, torrefaction had higher ESM parameters than centrifugation.

Table 1: ESM analysis for different process designs for the solid/liquid separation stage of the process.

	Screw-press dewatering system	Multifiltration	Torrefaction	Centrifugation
Power (kW)	0.3	1.8	0.9	5
Weight (kg)	179	232	378	5
Volume (m^3)	2	1.8	3.2	0.014
Spares & Consumables (kg/day)	0.0084	0.4	0	0
Spares & Consumables (m^3)	0.01	0.005	0	0
ESM Estimation	1810	1560	1150	443

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.