Engineering success

The initial idea

Our initial idea was to conceptualize a Photobioreactor (PBR) to show how our modified Physcomitrella patens could be applied for treat water. Originally the application was meant for wastewater treatment facilities and over time shifted to a more agricultural focused application.

Research results

With the research we did for the PBR we looked into the reactors used for industrial, as well as laboratory work. While researching we found that most commercial PBRs are mostly used with cyanobacteria or algae. Though most reactors can also be used for mosses or require only a little adjusting to be used with those.

What we found is that there are two broad classifications for Photobioreactor. These are the open reactor systems and the closed ones. These, as the name implies, differ in one major way. The open systems have no barrier between the culturing medium and the environment, whereas the closed systems do not allow the culture to be in contact with its surroundings. A closed reactor also allows for the possibility to monitor and adjust the growth related factors of the organism more closely and accurately.

Examples for the open systems are reactor types like an open pond, which is basically a pool filled with medium. These pools can be just a basin like structure with no extras or be arranged as a raceway pond. These differ in that they are a circular track through which the medium is continually pushed by a paddle mechanism. The constant motion in these helps with both reducing sedimentation and stirring the medium so that the light distribution between the cells is more even. The downside to these open systems is that with no barrier between the inside and environment it is nigh on impossible to control the release of a genetically modified organism (GMO).

First expert meeting

After our initial research was done we decided it would be beneficial to contact someone with more experience in the field. This was so we could discuss our thoughts about the best approach for the reactor and if we overlooked any important things it should be capable of.

The expert we contacted for this was Dr. Holger Klose the head of the institute for alternative biomass at the Forschungszentrum Juelich.

In the meeting he gave us some general feedback for our idea. For example he mentioned that most of the water treatment in a wastewater treatment facility is done seasonally though it would also be possible to do it in the winter months. Another point he touched upon was that even though the natural light in a wastewater treatment facility should be sufficiently available he suggested to try if we could grow our moss under artificial light - when growing it on our PBR - at least for our initial trial runs and optimizations.

He then said he would refer us to one of his colleagues, Ladislav Nedbal who builds the PBRs for the institute.

Dr. Klose also gave us some contacts for startups, which build photobioreactors, so we could get feedback from an expert in the industry.

Talk with Phytolinc

After the meeting with Dr. Klose, we decided to contact Dennis Prausse from Phytolinc. He explained the general mechanisms behind the membrane reactors and how they regulate parts of the reactor.

Mr. Prausse said that we could feasibly build a prototype of a reactor like this though he did not know what materials would be necessary for this. But he assured us that a membrane reactor would be upscalable rather easily.

On some points he was unsure though if a membrane reactor would be a good fit for our project. For example the shearing force that would be applied to the moss could be a hindrance in ensuring good growth. The membrane could also lead to a reduction in the amount of phosphate that the moss would be able to accumulate from wastewater.

First Design ideas

After the talks with Dr. Holger Klose and Dennis Prausse, we started on the first more definite concept. For this we started by choosing a few of the reactor types we thought might be a good fit for the project and that we could feasibly be able to build.

We came up with a list of three potential candidates.

Though there were concerns that the growth of the moss might be slow and the yield of phosphate low we decided that we could try to realize a membrane reactor.This was mostly due to its good scalability and how easy it would be to harvest the moss and apply new moss.

The second reactor type we decided would be a good fit was a tube reactor. This was mostly for a great distribution of light over all the cells and that it would result in a potentially high yield of phosphate. Downsides to this reactor would be the amount of possible sedimentation and a difficulty in the regulation of the system due to the required materials. The biggest drawback in our opinion though was that it would be very difficult to harvest and replace the moss that would be currently in the system.

Lastly we choose the open pond as a possible reactor. This type of reactor would have the problem of not having a barrier to the environment which would mean it would be susceptible for outside contamination. Which also would force us to work with a kill switch in the moss if we were to apply it to the reactor, so that we could ensure that no GMO would be released. On the upside an open pond would be easy and cost efficient to build, would be really easy to upscale and could have a high yield.

We decided that we would build a prototype of a PBR to show that our concept is sound and on the basis of this list we decided on a few factors that would be necessary for the reactor.

These points were that it has circulation of both water and gases, especially O2 and CO2. It would also need a light source to help with the growth of the moss. It would also be important to have a system that ensures that the GMO is not released, so it would either need to be closed or we would have to work with a genetic kill switch. For us the most important for our proof of concept was a way to easily harvest the moss and get new moss into the system.

Meeting with Ladislav Nedbal

With the list of the core features for our reactor we contacted Ladislav Nedbal, the colleague of Dr. Klose. Even before the meeting proper he sent us some things for further consideration. Specifically a video of a DIY turf scrubber he built, a reactor type we had until then not been aware of because they are a rather new development.

In the meeting we then went into more detail on the turf scrubber and how it compares to the open pond which is the most widely used reactor type. Here the turf scrubber has quite a few advantages over the open pond, for example it has better gas exchange because the body of water has more motion and is smaller in general. The turf scrubber also has better mass conversion, even though it requires less water to be pumped. And lastly the harvesting is a lot easier with the turf scrubber because the organism is placed on a mesh.

We also got a few tips on how to build the turf scrubber and what we could anticipate such as aggressive algae growth.

Lastly he helped us with ideas on how we could detect the accumulated phosphate. He mentioned that it should be possible with spectroscopy methods, for example raman spectroscopy or NMR spectroscopy could be viable options.

The turf scrubber

After the meeting we had with Ladislav Nedbal we decided a turf scrubber would be the best approach for us due to it being easy to build and satisfying our criteria for a reactor, most importantly having an easy way to harvest the moss.

We then started designing our own turf scrubber by starting with the example given to us by Ladislav Nedbal and thinking about what needed to be changed in order for it to work effectively with moss.

One of these changes was that we wanted to elevate the mesh on which the moss sits , so that the moss’ roots can grow into the water. But with this setup it would not be at risk of being torn off the mesh even if the flow were stronger. Another change we considered was to have the mesh hang in a small body of water, which is continually cycled by pumping new water in. The water would be exiting this basin like structure because we planned to have it propped up on one end so it stands in a slight angle.

Otherwise we considered that the example given to us should be suitable for our project.

The materials we had to get for our turf scrubber were easily acquired in a hardware store. We needed wood, a waterproof plastic tarp, a fine mesh and waterproof tape and a garden hose. We were fortunate that we had access to the other things like a basin in which to collect the water, a pump for a garden pond and we even had an old frame of a table on which we placed the system.

For the mesh we decided to use a mesh that is normally used to cover up the holes in the ground through which light is provided for the basement rooms. We chose this type of mesh for its small mesh size, which is about 2mm x 2mm. The plastic tarp we used is one for covering the bottom of a garden pond. The wood was used for the frame of the turf scrubber, so we used it for its base, the side railings and to prop it up into an angle.

To distribute the water of the length of the reactor we decided it would suffice if we closed up the end at the top and drilled some holes in the side of the hose through which the water could flow.

Building the turf scrubber

We were not able to build the prototype of our turf scrubber to the exact specifications we detailed in our planning and modeling. This was partly due to a lack of time and a lack of expertise and materials.

We started by building the basis. For this we fixated the two railings on the sides of our wooden plank. We covered this with the waterproof tarp and fixated it with nails. We then stretched the mesh above this created indent and fastened it in place. We decided that we did not need a way to easily remove the mesh yet because we first wanted to see if the whole setup would work in general.

Underneath this basis we put some small pieces of wood so that it has the desired angle. We then fixed the hose to the higher end with the waterproof tape, after we drilled multiple holes in its side and put a cap on the end of the hose so that the water would be spread over the length of the turf scrubber.

We placed this construction on top of the old table frame and placed the basin under the lower end of the reactor. The pump was then placed into the basin and the hose's open end attached to its outlet.

Fig. 2: The finished prototype
Fig. 1: Sketch of a basic turf scrubber

Improvements and problem solving

There are a few possible improvements that can be done to our prototype. These consist of lowering the pressure with which the water is pumped through the system. This could be achieved with a less powerful pump e.g. a pump for aquariums. Another improvement is a way to easily replace the mesh on the turf scrubber so that instead of having to scrape the moss of the used mesh, the mesh can be taken out with the moss and a new one can be installed. /p>

If we were to continue with this version of the turf scrubber where there is just a constant flow of water underneath the moss and no standing body of water it, we would also need a way of preventing the formation of small pools of water. This problem could be caused by the tarp just not being applied tight enough and could be fixed easily.

Another problem that might arise is that of potential pathogens that could hinder or kill our moss. This could be circumvented through by sterilizing the water beforehand, for example by treating it with UV light. This would also help with reducing other organisms from growing inside the system.

There might also be other factors we could tweak to make the reactor even more viable, but it would probably be a better idea to consider these after having addressed the more important and obvious flaws.