Solid-Liquid separation

Overview

The second stage of the process is required to remove all the solids from the feces and produce a sterile VFA rich liquid stream. Sterility is essential, since the genetically engineered e coli can’t compete with other bacteria types. Considering very limited availability of resources (including power) on Mars, the initial experiments focused on mechanical and gravity-driven separation process:

• simple filtration
• settlement
• pressure filtration

It was found that low power requirement process either do not provide sufficient sterility or could not recover more than 55% of initial water present. The design that performed the best was the sequential pressure filtration – giving sterile liquid, but only 10% of initial water recovery.

More advanced solid liquid separation technologies were then considered:

• torrefaction (mild pyrolysis)
• centrifugal separation followed by filtration
• and the screw-press dewatering system followed by a multi-filtration

Originally torrefaction appeared to be the best solution, since it allowed the recovery of natural and pyrolytic water, produced the liquid stream which was sterile and contained the required VFAs and produced char as by product. Char can be used in building, radiation shielding, as a food subtract and hence was a desired by-product. However, the ESM analysis showed that the torrefaction ESM parameter is larger than that of centrifugal separator.

ESM Analysis of different Solid-liquid separation technologies

It was chosen to proceed with centrifugal separator followed by a filter as a technology for the solid-liquid separation.

The team however also proposes to adapt torrefaction technology for the treatment of the by-product stream - sludge and the general sludge formed durign the wastewater treatment on Mars.