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 liters 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.

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 as well as 0.5 liters of water required per flush, the minimum required volume for the storage tank is 7.8 liters. We propose a 15 liters 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 20 kg based on specifications of commercially available cylinders capable of storing 10 - 20 liters of water. There are no power requirements for the storage tank.

Fermentation of Feces to Produce VFA

Fermentation of feces to produce VFA will occur over 3 days with bacteria naturally found in solid human waste. As described above, the volume requirement for containing 3 days’ worth of fecal matter and water is 7.8 liters. We propose an 8 liters fermenter. Although anaerobic conditions are preferred for this step, feces should be well-mixed to prevent settling of solids and bacteria during fermentation. The power required for agitation was estimated at 0.02 kW. Since temperature, pH, and dissolved oxygen will not be controlled at this step, other power requirements were assumed to be negligible. The mass of the empty fermenter was assumed to be 20 kg, the same as the storage tank. Although the fermenter has a smaller volume than the storage tank, it might require thicker walls to withstand potentially higher pressures.

We performed lab experiments to determine the optimal operating temperature for the VFA fermentation step. Due to safety concerns with handling human fecal matter, synthetic feces that mimic the chemical properties of real feces were prepared for the experiments according to the recipe described here. The two temperatures selected for the experiment were 37°C and 22°C corresponding to the optimal bacterial growth temperature and expected room temperature in a Mars habitat, respectively. A detailed description of the experimental design and protocol can be found here. Although experimental results showed higher VFA production at 37°C, more plastic was produced from synthetic feces fermented at 22°C. Due to higher concentrations of VFA, the supernatant from synthetic feces fermented at 37°C was more acidic, which resulted in little to no bacterial growth and, as a result, lower plastic production. As a result, 22°C was chosen as the preferred operating temperature for this step.

Equivalent System Mass Analysis

ESM analysis was performed using NASA’s Advanced Life Support Equivalent System Mass Guidelines accounting for each component’s mass, volume and power requirements (Levri, 2003). ESM results are summarized in Table 1.