Team:NOVA LxPortugal/Implementation

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Proposed Implementation

A Dual Approach to Microplastic Pollution: Clean the Ocean & Plug the Source

The presence of microplastics in the ocean can be traced to two major origins: fragmentation and decomposition of plastic pollutants in the ocean and release of microplastics directly in the ocean.

Bottles, straws and a huge assortment of plastic items float in the world’s oceans nowadays due to pollution and Humanity’s disregard of Nature. Although we all heard that “plastics take forever to degrade in the environment”, that is not really accurate since plastics do break apart and do degrade due to collisions, waves, chemical reactions, interaction with sunlight (mostly UV rays) and biodegradation. All these processes contribute to raising the concentration of microplastics in the waters, and if we add to that the degradation of fishing materials and the occasional spill of raw plastic materials from cargo ships, we have ourselves a recipe of disaster. But the stacking of contributors does not end here, all our waste waters are constantly pouring into the ocean with tons of microplastics from our innocent and naïve daily activities, from washing synthetic clothes in our energy saving washing machines to just brushing our teeth. We all have said to ourselves, “that’s ok because we have wastewater treatment plants”, but unfortunately, these plants fail to remove a significant amount of plastic from the water, before releasing it into rivers, creeks and oceans.

So, we are targeting both the existing problem (microplastics in the ocean) and one easy to access source to prevent the problem from growing. We present below our proposed implementations for a microplastics collecting, separating and biodegrading plant, to take the plastics out of the ocean, and for an add-on to improve the capacity of wastewater treatment plants to deal and retain microplastics, effectively plugging one of the sources.

Clean the Ocean

In this section, we detail our proposal to implement a method capable of removing microplastics from the oceans, rivers, creeks, lakes and other bodies of water. In our prototype, the removed microplastics - both from the water column and from the underlying sediments - are then safely disposed of. We envision the possibility of even taking advantage of the removed microplastics to produce a useful chemical compound. Thus, contributing to a more circular economy and further improving the energetic efficiency of the prototype.

This method is divided in three major steps:

  1) The removal of microplastics from the environment using NanoBubbles technology;

  2) The separation of microplastics from residual sediments using decantation enhanced with the use of high-density aqueous solutions;

  3) The biodegradation of the microplastics in a bioreactor with consequent production of added value compound(s) by our bioengineered machine.

1) NanoBubble Technology

Figure 1: Our prototype for microplastics extraction from natural sediments using nanobubbles.

The entry of plastics into aquatic environments due to discharges from wastewater treatment plants and their subsequent degradation into microplastics change their physical and chemical characteristics. For example, microplastics can, and will, easily associate with solid constituents and organic matter that make up sediments and other natural solids like clays. This leads to the fast accumulation of microplastics in aquatic sediments and consequently, relatively low abundances of microplastics could be found at the water surface [1].

Currently the majority of processes used to separate microplastics make use of a separation of microplastics through the employment of floatation methods that use high-density aqueous solutions. However, due to the accumulation of microplastics in sediments the floatation salt solution is not sufficient to fully release all the microplastics from the soil matrix [2]. As such, we propose the use of nanobubbles technology as a pre-treatment of sediments that will allow to separate microplastics from the organic/soil matrix, therefore making the separation with high-density aqueous solutions more efficient [3].

The separation mechanism is similar to the air flotation methods that exist because it is based on the adsorption of gas bubbles (while rising) upon the surface of finely suspended particles. This mechanism reduces the specific gravity of the particles and makes the contaminants rise up to the surface (increase the rising velocity of contaminants) while leaving sediments behind [4]. Nanobubbles preferentially form on the surface of hydrophobic particles and thus, generation and attachment of nanobubbles on the surfaces like those of the microplastics can greatly reduce the gravity of these microparticles and increase the separation of these contaminants from sediments [4-6].

Through the use of ships equipped with our NanoBubble prototype, we will separate the microplastics from the various organic and inorganic materials present in riverbeds or seabeds sediments, without major sediment displacement. The microplastics, as well as some contaminant sediments, will float to the surface and will then be collected by a net towed behind the ship. Finally, this mixture will be sent to a separation plant described ahead.

2) Separation Plant

Figure 2: Overall view of our proposed implementation plan.

Many wastewater treatment plants use decanters to remove fine sediments in wastewater. Introducing in the decanter the sediment with microplastics, the microplastics would float and exit in the decanted fluid and the sediment would sink effectively separating them. But many plastics are denser than water and would sink along with the sediment. To overcome this, the water would have dissolved salts such as zinc chloride that could raise the water’s density allowing the microplastics to float and the sediment to sink (since the latter is much denser).

The separated microplastics need then to be filtered from the floatation solution and then washed with fresh water so as to remove the salts used in the separation that could influence bacterial growth in a bioreactor with the plastic-degrading bacteria. The sediments coming from the decanter cannot be directly returned to sea. They have traces of salts, used in the separation of microplastics, that can harm the environment and so they must be washed.

Figure 3: In this decantation tank, through the use of high density solutions, the microplastics will float and exit the tank unlike the sediments that will sink.

Figure 4: After the decantation, the sediments will pass through this washing tank to clean the sediments of salt traces so they can be safely returned to the environment.

Figure 5: Through this filtration chamber, the microplastics in suspension are removed from the separation solution and collected. The solution is returned to the decantation tank to be reused.

Figure 6: Before entering the bioreactor, microplastics will pass through this tank, where the salt traces will be cleaned. The washing solution, still with considerable amounts of salts, can then be concentrated and reused in the decantation process.

Separation by Water Density Modulation with Salts

Density separation is one of the methods that are frequently applied to extract microplastics from sediments [7-10]. Like the name suggests, this method is based on the difference in specific density of the microplastics and the environmental matrices [7-9]. To achieve this, solutions with a fixed density (floating solutions) are prepared by dissolving specific salts in distilled/filtered water. The density of microplastics ranges from 0.8 g/cm3 to 1.58 g/cm3, while the average density of sediments is 2.65 g/cm3, by preparing solutions with densities higher than the microplastics density but lower than the sedimental matrix density, the microplastics will float while the sediments will sink [8-11]. Once the separation is completed the microplastics can be extracted from the supernatant by filtration [8-10].

This method can be summed up in 3 steps, the first being the mixing of the sample with floating solution, the second consisting of a period of inactivity to allow the settling of the sediments and the third comprising the removal of the microplastics rich supernatant [7].

Despite appearing as an easily applicable method, the selection of the floatation solution’s density is a crucial step which requires the selection of the most appropriate salt, and certain salts have issues regarding their associated cost and/or dangers. The amount of microplastics that are extracted is also influenced by other factors like volume of floatation solution, mixing time, mixing method, etc. Meaning, for each case, these factors need to be optimized [7,8].

In addition, during the process of separation it is possible for organic matter to float alongside with the microplastic particles, meaning further optimization of the process may be required and further treatments may need to be considered [8].

Decantation Tank

In our prototype we propose retrieving the pre-treated mixture of sediments and microplastics collected by the nets, using the nanobubbles system, and then suspend this crude material in the high-density salt solution and introduce the resulting suspension in a decanter, so we can separate our microplastics from the cruder materials. Using a solution of Zinc Chloride (ZnCl2), allows for a simple yet effective separation [12]. The low density of the microplastics, allows them to float in the medium, while denser sediments sink. Thus, our high-density solution, with microplastics in suspension can be continuously drained from the top of the decanter and the sediments deposited on the bottom can be removed using a conveyor belt. The use of ZnCl2 also means there will not be any damage to those microplastics, while also being an eco-friendly and cheap idea (no production of gases or toxic matter), the solutions can be reutilized after washing – described further ahead. The only known problems with this solution is that even though its efficiency is quite high, it cannot separate lighter particles from microplastics mixed in our samples, and it causes the plastics to become salted, as such we need further cleaning steps.


To remove the suspended microplastics from the high-density solution a step of filtration is needed. Here we use membrane filtration to perform the separation of the microplastics, as it is an efficient method and allows the recovery of the salt solution after the process. Using the microfiltration method at low pressures (membrane with pores that allow the separation of particles down to 1 - 0.1 µm) [13-14], we are only left with our microplastics still salted due to the decantation solution.

Sediments Washer

After the separation of microplastics and sediment, these sediments have traces of metal salts that could damage the environment and so they must be removed. Given the scale of the process we had to look to other sediment washing processes like the leaching plants [15]. They are used in metallurgy where a solution is used to remove precious metal traces from mining waste rock, resulting in a solution with these metals and increasing the yield of metal extraction [16,17]. Our idea would be to use this technique but using water instead to wash the salt off the sediment and retrieve it so we can return the sediments to the environment.

Plastic Washer

Finally, there is the need to remove the salt contamination still left on the microplastics, not to contaminate the bioreactor. In this step the plastics are sent to a new container where they will be washed with new clean water, and undergo a new filtration step, similar to the one described before, yielding purified microplastics. Hence, we guarantee that the salts are fully cleared from the plastics, and once again the salt can be recovered from this water and reused in the decanter.

Decantation Solution Return

The salt solution obtained so far can be reconcentrated and used again for the decantation process, which makes this process considerably less harmful for the environment as it reutilizes both the water and the salt.

3) Bioreactor

Figure 7: Schematic representation of engineered B. subtilis degrading microplastic in a bioreactor.

After successfully transforming B. subtilis to be able to efficiently degrade PE & PET, there is the need to implement it on a large scale to impact the microplastics scourge. Thus, the implementation of a suitable bioprocess requires the design of bioreactors, so our engineered strain has the necessary environmental conditions for efficient microplastic degradation.

Using PE & PET as substrates, the genetically modified B. subtilis will produce a well-known commercially desirable product, ethanol. Since ethanol negatively impacts the cell growth it will be a challenge to design a bioprocess to counteract this problem.

Since our project is in a pivotal state, there is insufficient data from the laboratory experiments, a fact that impairs further design of the bioprocess. However, we plan to start with spinner flasks and validate essential parameters that allow maximizing growth and productivity, calculating the cell growth rate; specific productivity; product yield; and rate of microplastic degradation [18-19]. Once essential parameters are known, we plan to scale-up the process using a bench top bioreactor with an equipped instrument to measure culture conditions. Our final plan is to scale up into a pilot scale bioreactor so we can revolutionize this worldwide issue [18-20].

Plug the Source

In this chapter we detail our add-on to upgrade the microplastic retention and degradation capabilities of common wastewater treatment plants. We will analyze the flow of microplastics in these infrastructures and show that we can easily and precisely act on a single process to achieve great results when deploying our bioengineered machine.

Microplastic Flux in a Wastewater Treatment Facility

The effluents from wastewater treatment plants (WWTP) have been reported as one of the sources that introduces microplastics to aquatic environments [21-23]. The presence of microplastics in WWTP has been reported in several countries, such as Australia, Finland, Germany, Ireland, Netherlands, UK, US, and Sweden [23-24].

Although the WWTP are able to remove some microplastic fragments of bigger dimensions (above 300 micrometers), the ones of smaller dimensions are still able to leave the system in the effluent stream ending into the aquatic environment [25-26]. Still, reports showed that after all the treatment processes, the WWTP are able remove about 90 % of the microplastics present in the stream by their sequestration in the water biomass [25].

Despite being able to reduce the number of microplastics present in the water, there is a high concentration of microplastics in the waters downstream of WWTPs, thus introducing microplastics in inland waters [23-24,26]. Nonetheless 90 % of the microplastics are retained in the organic sludge but are still later released to the environment. This happens since a high percentage of microplastics settle in the bottom of the facilities’ tanks, being accumulated in the sludges, are then used in agricultural fields as fertilizer around the world, without undergoing effective microplastics removing processes, hence pointlessly contaminating agricultural soils and consequently all water bodies downstream [24-26].

Anaerobic Digester

One particularly interesting part of wastewater treatment plant is the anaerobic digestion, implemented in order to produce energy through the digestion of organic matter, which resulted from previous settling, filtering and thickening processes.

Since wastewater treatment plants are already somewhat capable of retaining microplastics, we propose the inclusion of a microplastic degradation step in the anaerobic digestion as a simple and effective way to remove the microplastics from the sludges before they are used to fertilize fields. This process was already somewhat discussed in the literature [27] as a suitable way of microplastic degradation, as several thousands of microplastic particles per kg were found in the sludge that resulted from anaerobic digestion. The European law authorizes this sludge to be used in the fertilization of fields, which reinforces the need to implement microplastic degradation in the anaerobic digestion [28]. So, we propose to integrate our modified bacteria in the mix culture already present in these anaerobic digesters, hence greatly reducing the output of microplastics in a way that is extremely simple and easy to implement.

As we mentioned, our degradation model was built around Bacillus subtilis. One important aspect is that it grows under an aerobic environment, however, it has been demonstrated that B. subtilis can survive in an anaerobic environment, as long as it uses nitrate as an electron acceptor as an alternative to oxygen, with only an increase of its lag phase [29].


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