Team:Calgary/VFA Fermentation
VFA Fermentation
Overview
In the first step of the process, astronauts’ feces are collected into a storage tank using a vacuum toilet that uses a minimal amount of water, which can be recovered at the end of the process. From the storage tank, feces are transferred into another tank and left to ferment for 3 days at 22°C with bacteria naturally found in human feces to increase the concentration of volatile fatty acids (VFA) that are later consumed by engineered bacteria to produce PHB. This stage of the process was designed while considering NASA’s Life Support Baseline Values and Assumption and Equivalent System Mass (ESM) analysis was performed to evaluate the feasibility of this stage.
Vacuum Toilet and Vacuum Pumps
Although a vacuum toilet requires about 0.5 milliliters of water per flash, which is a valuable resource on Mars, the used water can later be recovered as described here. The toilet was not included in the ESM analysis because an assumption was made that astronauts on Mars will be using a toilet regardless of our proposed process.
Two vacuum pumps are required for this stage: one pump connected to the toilet to collect feces into a storage tank and another pump to transfer feces from the storage tank into the VFA fermenter. ESM analysis was performed using Jets Vacuumarator 10NT DC vacuum pump as a baseline. This vacuum pump weights 18 kg and uses a 0.9 kW motor. Based on provided external dimensions of 163 by 361 by 294 mm, the volume of the pump was calculated to be 0.017 m3, assuming rectangular shape.
Storage Tank
According to NASA requirements, a fecal collection system must be capable of collecting an average of 150 milliliters of fecal matter per defecation for two defecation per crewmember per day. The maximum design values should also be capable of containing 1.5 liters of diarrhea discharge (Anderson, 2015). Since VFA fermentation, which is the next step after the storage tank, is run for 3 days, the storage tank should be capable of containing at least 3 days’ worth of fecal matter. Assuming a crew of 6 astronauts and taking into account NASA’s requirements for a fecal collection system, the minimum required volume for the storage tank is 5.4 liters. We propose a 10 liter storage tank to ensure enough volume to contain potential diarrhea discharge and extra days’ worth of fecal matter in case of unforeseen circumstances such as loss of power. The mass of an empty storage tank was assumed to be 15 kg based on specifications of commercially available cylinders capable of storing 10 liters of water. There are no power requirements for the storage tank.
Fermentation of Feces to Produce VFA
Equivalent System Mass Analysis
