Team:Leiden/Bioreactor

Bioreactor

The Bioreactor

The bioreactor is the final product of our project. We aim to develop a closed system that can – for the most part – sustain itself, through a combination of our modified E. coli and an autotroph bacterium (such as cyanobacteria) that can convert carbon dioxide to simple sugars. An example of a cyanobacterium that can excrete its formed sugars is the PCC 7942 strain modified by Niederholtmeyer et al. (2010).  ¹

Regolite (Martian soil) will enter the bioreactor after which it seals shut. Water will then flow through the regolite, exploiting the excellent dissolving property of perchlorate to cleanse the soil. The challenge in cleansing the water from perchlorate, which we achieve through our modified  E. coli . The contaminated water will pass through a series of tubes, where a carbon and a nitrogen source are added for incubation. When the mixture is complete, the perchlorate containing medium will travel to the  E. colonizer  compartment, where incubation will take place until perchlorate is reduced to sub toxic levels. ^1,2 To achieve minimal toxic levels, the incubated medium (containing mainly water,  E. colonizer  waste products and remaining perchlorate) will pass through a liquid filter, while the gasses pass through a high-efficiency particulate arrestance (HEPA) filter, to reduce remaining perchlorate to trace amounts.

The filtered gasses produced during incubation (mainly carbon dioxide and oxygen) will be transported to the compartment where the autotroph bacterium resides, to saturate oxygen levels and turn carbon dioxide to simple sugars. These sugars can then be used to complement the medium of  E. colonizer . This way, we recycle gasses and liquids as much as possible, avoiding the extremely high costs of transportation to Mars. The concentrated oxygen is transported to a tank for use in the habitation module.

Figure 1: An exemplary bioreactor. Original image from: http://www.b2bdirectory.pk/company/1529/algae-bioreactor.jpg

Both tanks (that of E. colonizer and the cyanobacteria) will be covered by a thermal jacket, which acquires heat from the desalination unit’s waste heat. This unit ensures that chloride and other ions do not accumulate over consecutive runs. The cleansed, desalinated water will dissolve the perchlorate in a new batch of regolite optimally. The perchlorate-free regolite can be used safely for growth of crops.

The outer hull will provide resistance against the harshest radiation, while allowing some light to enter the compartment of the autotroph bacterium.

For this bioreactor one bacterium is not enough despite the fact that one bacterium is perfectly capable of breaking down perchlorate. To be able to compete with industrial process of breaking down perchlorate, we would need to have very large amounts of bacteria, working simultaneously. Furthermore, we can do this continuously. An ordinary filter has to be replaced after it is saturated with perchlorate. During this replacement perchlorate reduction stops. In addition, the bacteria have to be contained very strictly. They are not wildtype bacteria, hence they are classified as a G.M.O.; a genetically modified organism. All of these conditions point our product to a bioreactor.

A bioreactor is very similar to any chemical reactor, the difference being that the processes taking place in it are biochemical. A bioreactor allows a lot of control on the reduction of perchlorate. One can monitor and control condition inside it. These include: pressure, temperature, water flow rate, etc. An example of a large bioreactor is shown in figure 1. To be able to break down perchlorate, the bacteria have to be in the same vessel. To make them blend, this central vessel contains a stirrer. The bacteria have to be nourished, so there are flows containing sugars, carbon and nitrogen sources, etc. These are also directed towards the central vessel. The bioreactor has to contain sensors as well. We will equip a chloride and oxygen sensor, to be able to calculate how much perchlorate was removed from the broth.

We have already made designs for our bioreactor on Mars. The total system is displayed in figure 2.

Figure 2: The design for our bioreactor

Each part drawn has its own individual task. All of them will be clarified individually. In principle, only components A-C are needed for the perchlorate reduction. To try and make the system more self-sustainable, D and E can be added. D contains cyanobacteria that are able to secrete simple sugars, which the E. coli can use. For this, we use sunlight. E is used to recycle carbon sources and direct them towards B.

A: Rotary vacuum-drum filter.
The perchlorate reduction starts with this component.

Figure 3: A model drum filter. From: https://en.wikipedia.org/wiki/Rotary_vacuum-drum_filter

A cross-section of this component is shown in figure 3. The drum filter rotates around a central axis. This axis is formed by the central duct, which is a pipe leading to the main bioreactor. The surface of the drum turns through a tank, to which Martian soil and water are added. The inside of the drum has lower pressure than the outside. This way, water is sucked into the central duct. The surface of the drum is porous, so it acts like a filter. Large compounds, like soil, stick to the surface. During one rotation of the drum, the soil stuck to the drum is washed, dried and finally scraped off the drum. After the soil is removed from the drum, it is already purified. The perchlorate is dissolved, and carried into the bioreactor via the central duct. As a result, the Martian soil can directly be used for farming.

B: Main bioreactor.
This is the most important part of the system. The perchlorate is carried in here, to be reduced. Figure 4 shows a sketch of this bioreactor. In here, the water is continuously stirred to maximize the rate of perchlorate reduction. Medium, and base/acid are added to control the biochemical reactions taking place. After the amount of perchlorate is minimized, the waste water is filtered. The filter is used to remove cell debris from the broth. After the filter, the water is lead to the desalination tank.

C: Desalination tank. When the water enters this tank, it contains all kinds of minerals. These minerals entered the water when the Martian soil was suspended in the tank beneath the drum filter. In addition, chloride atoms are formed when perchlorate is broken down. To prevent accumulation in the water, they have to be removed. This can be done by distillation, precipitation and crystallization. Upon adding a sodium salt, kitchen salt can be formed. This can then be removed with a filter. After the desalination stage, the water is decontaminated, and can be used in the drum filter again.

D: Photobioreactor.
This is part of the optional setup. This bioreactor contains cyanobacteria that secrete simple sugars. We utilize this extra reactor to be able to feed the E. coli in the main bioreactor. The oxygen produced during perchlorate reduction is sent here. Additionally, it is made transparent. In this fashion, cyanobacteria can use the sunlight to produce the sugars. These sugars are sent into the cell culture tank. An example of a photobioreactor that uses algae is shown in figure 5.

Figure 4: A 3D rendering of the bioreactor

Figure 5: Cylindrical photobioreactors containing algae. Image from: http://pre01.deviantart.net/28ff/th/pre/f/2012/123/a/7/photobioreactors_by_silentcenter-d4ydxjj.jpg

E: Cell culture tank.
Here, the simple sugars are mixed with other components to form the cell culture for the E. coli. These are gradually sent into the main bioreactor.

References

1. The Environmental Protection Agency (EPA) suggests a Reference Dose (RfD) of 0.0007 mg/kg/day for perchlorate, corresponding to a drinking water equivalent level (DWEL) of 24.5 ppb. For more detail, we refer to the report by the Agency for Toxic Substances & Disease Registry.