Bioreactor

To implement the Decyclifier where it is most needed, in the waste plants, we designed a bioreactor (see figure 1) that would be part of the waste remediation process at facilities such as Uppsala Vattenfall’s power plant. In the burning of waste, the incinerated waste is refined in three steps. The first step separates larger metal objects and the later two refines the ashes through electrofiltration and a water-acid filtering systems. Through the process, two types of ash contain considerably higher concentrations of PAHs. We suggest that these two types of ash should be sludged with water and supplied to the Decyclifier bioreactor.

For the Decyclifier reactor we purpose a simple batch-mode model, due to the lengthy process of the PAH degradation (approximately 10 weeks for a 100 m³ batch). With a suggested ratio of 2:1 water to ash-sludge, approximately 33 m³ of ash could be purified per batch (Li et al., 2002). According to Robies-González et al., levels of purification in similar systems reached between 70 and 98% of degraded PAHs (González et al., 2008). With utilization of our biosurfactant system and the optimized activity of our modified Laccase, one might assume that the Decyclifier reactor should be able to reach at least the higher level of purification, possibly also in shorter amount of time. There is however no way to say for certain without testing the system under proper running conditions.

Based on similar reactors the inoculating concentration of the Decyclifier in the reactor should be about 50 g/kg. As such it will be necessary to also include a smaller pilot reactor for growth of the inoculating culture prior to PAH degradation. Preferably this reactor should be run using similar conditions as the main reactor to make sure that the cells are indeed functioning properly.

Judging from our tests in the lab, the Decyclifier is capable of surviving high concentrations of pure napththalene and can thus be assumed to also be able to survive fairly high concentrations of PAHs in the toxic ash. A key issue however is the fact that ash contains many other toxic substances in addition to PAHs. As of yet no tests regarding the cells viability in direct contact with the ash has been performed. Lab tests were planned to assert how well the Decyclifier would survive in cultures with actual ash, however unfortunately we did not receive ash in time for these tests to be performed. This would of course be a key experiment for the future assessment of the Decyclifier system.

Due to the relative toxicity provided through other components of the ash, it is important to remember that the remaining material still needs to be disposed of after the reactor run time has finished. As such the Decyclifier reactor will need to be emptied and the remaining sludge deposited as was previously done, however now no longer containing PAHs.

A major necessity for a well functioning reactor is the maintenance of optimal and homeostatic growing conditions, such as temperature, pH, O₂ and CO₂. This requires a constant rate of stirring throughout the system which can be achieved through a multiple bladed rotor originating from the top of the reactor and baffles attached to the sides. The baffles serve to direct the flow of the liquid phase in an even pattern following the edges of the container. Temperature will be regulated externally using a fluid based coil jacket with a separate heat regulation system coupled with an internal sensor. This would be the best method for specific and swift temperature regulation, as it is far easier to manipulate than a non-coil heat jacket system. Gas levels as well as pressure can be managed using a oxygen supplying gas input line coupled with an oxygen saturation sensor and a pressure regulated output valve. PH can be regulated in a similar fashion using a pH-electrode coupled with a bas/acid supplying system. Finally a stirred tank reactor of this size is bound to create foam and as such a proper anti-foaming agent must be added (Wang et al, 2008). As the NahR/Psal system will indicate the presence of naphthalene, and thus the presence of PAHs in general, our own fluorometer could be implemented to determine the levels of PAHs left in the sludge. This can be achieved by attaching a small cell to the side of the reactor, with a small but constant amount of fluid flowing through it, fitted with our device.

It is hard to asses the actual building and maintenance cost of the system without proper knowledge of local conditions. A decent approximation can be made utilizing purposed cost values for a similarly operating system, such as a biogas chamber reactor. However, even such values are hard to come by and no proper cost approximation could be performed.

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References

Li, P., Gong, Z., Jing, X., Xu, H., Zhang, C., Ma, X., He, Y.. Bioremediation of PAHs contaminated soil using bio-slurry reactor process. Ying Yong Sheng Tai Xue Bao. 2002 Mar;13(3):327-30.

Robles-González, I.V., Héctor, F.F., Poggi-Varaldo, M. A review on slurry bioreactors for bioremediation of soils and sediments. Microbial Cell Factories. 29 February 2008 7:5 doi:10.1186/1475-2859-7-5

Wang, L.K., Pereira, N.C., Hung, Y.-T., Shammas, N.K. Biological Treatment Processes Volume 8. N.p., n.d. Web. 26 Feb. 2014.5