Team:Linkoping Sweden/Implementation
Industrial setup
Since the beginning of our project, the manner in which our project could be implemented on an industrial scale has been a central focus for us. We have aimed to do the groundwork for a future product that would be appreciated by the ultimate users, in this case farmers, as well as fill all the requirements they seek. This brought us to the Swedish island of Gotland, where we met with farmers and discussed their incitements regarding our project and what they would like our industrial setup to look like.
The consensus of the farmers' opinions was that a small-scaled, cost-effective setup that could be implemented locally on each farm was preferred. This led us to create the design seen in Figure 1.
The idea behind our industrial setup for desalination is to utilize gravity, as well as Pascal's law of communicating vessels. This way, we can achieve an eco-friendly approach by reducing energy consumption and thereby desalination costs. The tanks would be placed underground, below sea level. A channel would lead the seawater into tank No.1 and fill it until it is at the same level as the sea. Inspired by vein anatomy, we inserted multiple valves and a gate to prevent the water from running from the tank back into the ocean with gene-modified phototrophic organisms, something that may disrupt the ecosystem. The cone-shaped tanks would maximize the capture of sunlight for the phototrophic organisms to proliferate and desalinate. This would also reduce the ground blasting costs by minimizing the need for unnecessary volume. Meanwhile, the tank would also capture rainwater which would dilute the salt concentration in the tanks.
In tank No.1, the modified phototrophic organisms would be added and cultivated with nutrition from the seawater. There would be a pipe connecting tank No.1 with tank No.2. This pipe would be placed close to the water surface so that gravity can move the saturated waste material (dead cells) towards the bottom. This could create a gradient where the desalinated water would be closer to the surface. In addition, the shape would also capture rainwater to lower the salinity of the desalinated water furthermore, something that may contribute to the lysis of the genetically modified phototrophic organisms. Multiple filters would be placed in the pipe between tank No.1 and tank No.2 to isolate the organisms from the desalinated water.
The waste biomaterials of phototrophic organisms would precipitate to the bottom level of the tank, as it does in the ocean. Therefore, an additional pipe, that leads these waste biomaterials to a waste tank, would be placed at the bottom level of the tank. This pipe would also have valves and a gate towards the gatherer tank inserted that the farmers could access when needed, to remove the waste biomaterial. Here the biomass would dry out, preparing it for biofuel production.
Size and time required for desalination
Based on the input from the Gotland farmers from Sweden and the Shanghai Jiao Tong University (SJTU) 2015 iGEM team that inspired our project, we have established a hypothetical scale for the total area and time needed for our industrial setup [1]. The most water craving period in a year for the Gotland farmers is from May to August. Within this period, approximately 30 000 m3 water is consumed on one farm, meaning that each farm needs 333 m3 water per day.
The data from the SJTU BioX Shanghai team suggests that the genetically modified Synechocystis sp. PCC 6803 with halorhodopsin, has a desalination rate of approximately 0.25% every 12 hours. However, due to the lack of information regarding the density of the Synechocystis sp. PCC 6803 BioX Shanghai team used, we cannot predict the exact rate of desalination in relation to phototrophic organisms' density. Therefore, we estimate that the density in tank No.1 would have 25% of the density that the SJTU BioX Shanghai team had in their lab. Based on that, we can estimate the rate of our desalination process to approximately 0.063% every 12 hours. The salinity of Gotland seawater is approximately 1%, indicating that we would need about one week to desalinate a given amount of seawater to water with a salt level that is acceptable for agricultural use. This calculation is based on a scenario with stable sunlight viability throughout the entire desalination period and assumes constant phototrophic organism density.
As the ice starts to melt in early spring, some diatoms and dinoflagellates bloom. Although the bloom is not highly dense, the time it arrives at full capacity is much earlier in the year than when the farmers need the water [2]. Due to the tough growth conditions during the spring, a lower density of modified organisms would be generated and thereby the desalination process would be slower. However, desalination of a tank that could account for part of the early water usage would still be possible during this period. This, in combination with the early rainwater, would create a favorable water situation before the season starts. The early bloom could also provide a good initial organism density as the water warms and sunlight increases, which would turn the desalination speed up to its maximum quicker. Due to the outlet pipes being placed at the top of the tank, water deprived of nutrients would also be replaced by nutrient-rich water from the sea. The manipulation of the water movement would ensure that the blooming diatoms, dinoflagellates, cyanobacteria, and other microalgae receive a sufficient amount of nutrients to support higher cell densities. As the top water layer usually experiences a too high intensity of sunlight, that layer could be transported out of the first tank without removing too many phototrophic organisms, putting less strain on the filters [3].
The volume and scale of the tanks designed for the farmers are shown in Table 1. Based on how much water the farmers need, they could select the tank size that suits them. To provide the farmers with their full, daily water needs between May and August, a tank size with a radius of 33 m would be needed. The tank would be 15 m deep, to be able to sustain all phototrophic organisms [3]. In order to improve the efficiency of our setup further, an aspect that could perhaps be further researched is the possibility to modify the organisms further to gain a faster growth rate.
Table 1.The table shows the relationship between the tank size and desalination capacity per day at 100 % and 50 % desalination efficiency. The sizes of the desalination tanks have been established with consideration to the ground area each farmer has. The depth of the tanks would be 15 m. The top water layer depth used to calculate data in the table is 2 m.
| Tank top radius (m) | Tank top water layer volume (m3) | Desalinated water at 100 % desalination efficiency (m3/day) | Desalinated water at 50 % desalination efficiency (m3/day) |
|---|---|---|---|
| 33 | 2281 | 326 | 163 |
| 35 | 2566 | 367 | 183 |
| 40 | 3351 | 479 | 239 |
| 50 | 5236 | 748 | 374 |
| 70 | 10263 | 1466 | 733 |
| 100 | 20944 | 2992 | 1496 |
Cell separation by filters
We plan to express a carbohydrate-binding domain, also known as CBD, on the surface of our genetically modified phototrophic organisms. The CBD will grant the phototrophic organisms the ability to bind to cellulose filters with high affinity, and thereby separate them from the desalinated water.
Currently, a considerable volume of research promotes cellulose-based materials due to their durability. Cellulose-based materials can also be recycled, making them much more sustainable compared to existing alternatives [4–7]. As the most abundant biomaterial on earth, cellulose exhibits excellent advantages due to its wide availability as a natural resource [6]. Its unique physical and chemical properties allow cellulose to be recycled. All these properties make cellulose a cost-effective raw material used in many industrial setups. In addition, natural cellulose has relatively low solubility, making it durable in water; and the cellulose filter waste can also be used as biofuel material [4, 9, 10].
The CBD molecule we are using originates from a previous Linköping 2019 iGEM team. Their results confirm that the CBD has a high affinity for cotton-based cellulose [11]. Cotton is a cost-effective cellulose source, but due to the high water consumption used in cotton production, another alternative may be more suitable for our purpose. After some research, we found a possible replacement for the cellulose source, lignocellulose. Lignocellulose can be found in agricultural waste and crop residuals such as corn stover and wheat straw; even wood chips, dead branches, fallen leaves, and grasses can be used for lignocellulose extraction [12]. However, the affinity of the CBD to lignocellulose still needs investigation.
Costs for the desalinated water
An approximate calculation of the expected costs of our desalination method, applied according to what has been described here, can be seen in Table 2. Our theoretical industrial set-up was designed to meet the farmer’s requirement of providing 333 m3 water per day during the May-August period. Based on the estimated cost calculations for our desalination system, the cost for 1 m3 water would be 0.31199 €, if dividing the cost for ground blasting over a span of 120 years. However, if not considering the ground blasting cost, the cost for 1 m3 would be as little as 0.00199 €. Filter costs are not included in this calculation due to insufficient experimental data regarding the amounts required. This is something that needs to be further researched. However, it is plausible to assume that the costs of cellulose-containing filters, for example cotton, would not be that high. Compared to what the farmers pay today, 0.4-0.6 €/m3 water, the estimated costs for our desalination system is considerably lower. These prices qualify for what the farmer’s we met presented as a requirement for our desalination system to be economically beneficial to them.
Table 2.The calculations for the purchase costs of the pumps are based on the system using two pumps simultaneously that each have a lifespan of 15 years, and are operated during four months of the year. The calculations for the solar panel costs are based on them being operated during four months of the year, and having to provide 2 kW to each pump. The ground blasting costs are calculated for two tanks with the top radius 33 m and a depth of 15 m. Therefore, the amount of desalinated water obtained per day has been assumed to be 326 m3. All costs are calculated based on the desalination system being used for 120 years and does not take economic inflation into consideration.
| Expense | Purchase cost (€/unit) | Cost for 1 m3 desalinated water (€) |
|---|---|---|
| Ground blasting | 1 500 000 | 0.31 |
| Pump | 150 | 0.00049 |
| Solar Panels | 7 240 | 0.0015 |
| Total | 1 507 390 | 0.31199 |
| Total without ground blasting cost | 7 390 | 0.00199 |
Conclusion
Having mechanical engineers evaluate our proposed design and run tests to ensure its efficiency in practice would be preferable. Furthermore, any input from companies producing chemical and biological equipment would also be valuable. Due to the lack of experimental data, most aspects of our industrial setup are based on assumptions. Our desalination hypothesis does not consider all factors, for instance, rain volume and sunshine hours. However, we have established that, based on our estimations, our industrial set-up would provide a cost-efficient desalination method that succeeds the requests made by the farmers we met and interacted with. Furthermore, it is applicable on a small scale which was also one of the main concerns for them. The desalination set-up would also work on other coasts around the world although the water has a higher salt concentration. Because of the higher salt concentration, the desalination process may take longer. However, the construction is expected to be less expensive depending on the geology where the desalination structure would be located. Conclusively, CyaSalt is a method that would rely solely on sunlight as its energy source and that would leave no environmental footprints.
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