Raising fish and plants in cleaner water: aquaponics and advanced oxidation processes for small-scale farmers

Luis Andrés Leal, PhD student in Engineering with Emphasis in Chemical Engineering. Credit: Édgar Bejarano, Communications Office, Faculty of Engineering.

There are currently systems that contribute to strengthening food sovereignty in fishing areas for small communities. This is the case of aquaponics, a method that allows the cultivation of fish and plant species autonomously in one place. However, the quality of the water used to operate these systems has deteriorated due to the existence of toxic particles from external contamination. An investigation takes aquaponics as a basis for the creation of a method of water treatment by means of advanced oxidation processes, in order to guarantee the sustainability of these systems, improve the quality of the water and the quality of their crops.

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Aquaponics and its research benefits 

For many, fishing has been an activity that has provided them with food throughout their lives. Over time, the widespread use in communities around the world made it necessary to go further and further into open waters to obtain fish that were previously found on the shores of rivers, seas and oceans. This circumstance gave rise to what is now known as aquaculture, including aquaponics, which later emerged as a form of treatment linked to Recirculating Aquaculture Systems (RAS), and is a method which combines techniques for planting aquatic species (fish, mollusks, crustaceans, shrimp and plants) in a given area, thus contributing to sustainable and autonomous food production practices, especially for small producers.   

However, the high levels of concentration of toxic particles in water, coming from human activities, have given rise to what are known as "emerging contaminants", such as those coming from drugs widely used in the population, which due to their chemical characteristics make it very difficult to eliminate them by means of conventional water cleaning treatments, which makes them a vector of toxicity that affects the quality of what is produced in systems such as aquaponics.  

Seeking to offer an innovative and low-cost solution, chemical engineer Luis Andrés Leal, as part of his research for his PhD in Chemical Engineering, seeks to take advantage of the benefits of aquaponics and put it at the service of water treatment. The research, which is directed by professor and researcher Fiderman Machuca Martínez, from the School of Chemical Engineering, and co-directed by professor and researcher José Antonio Lara, also from the School of Chemical Engineering, and researcher Juan García Garay, is presented as an alternative for small communities to have access to a stable and safe source of food for their consumption.  

Emulating the integration put into practice with aquaponics, this doctoral proposal seeks to evaluate the usefulness of an advanced oxidation process for water treatment, based on the degradation of some emerging pollutants which will be mentioned below. 

Systems integration: aquaponics and advanced oxidation processes 

After conducting a literature search to know the state of the art for his research, the researcher Luis Andrés Leal understood that, although aquaponics (or fish farming and hydroponics, components of this type of activities, which in turn are included in a practice called aquaculture) is an activity both amateur and on a larger scale, there was no complete model such as the one proposed.  

According to the researcher, there are three methods to carry out aquaponics processes: 1) by the method known as Nutrient Film Technique (NTF), which consists of using a pipe with holes along its surface, through which the roots of the plants begin to obtain nutrients from the water tank with the fish; 2) by the method known as Deep Water Culture (DWC), where a surface with the same type of holes is used on top of the tank containing the fish, so that the plant roots absorb the nutrients derived from inside the tank; and 3) by the substrate culture bed method, which consists of the addition of a surface (composed of volcanic lava stones, since their small size facilitates water flow) on top of the tank, and a subsequent process of periodically increasing and decreasing the water level inside the tank. The latter was used for the research. 

"What is the benefit? You feed the fish, the fish in their metabolic process release certain substances (called ammonia). Bacteria that are contained in the roots arranged on those supports, which allows them to grow and transform ammonia into nitrite. Other nitrifying bacteria transform nitrite into nitrate. And nitrate is the nitrogenous spice that plants absorb for their growth. It is an integrative process," explains researcher Leal. 

Researcher Luis Andrés Leal explaining the mechanism of the aquaponic system. He currently grows lettuce and red tilapia in his prototype. Credit: Édgar Bejarano, Communications Office, Faculty of Engineering.

To make this process possible, it was necessary to acquire a main tank for the disposal of the fish, with a capacity of 700 liters. This tank is connected, in turn, by means of communicating vessels, to a smaller tank (200 liters), which in turn has a biofilter, so that the fecal matter of the fish reaches the tank, passes through the filter and returns to the tank where the fish with a lower concentration of waste are being cultivated. Finally, before entering the tank there is a last filter, which performs an additional filtering process of the water.  

The water recirculation system operates for 10 hours, six minutes each, from 8:00 a.m. to 5:00 p.m. In the event of a power outage, an emergency aeration system is also available to operate independently for an additional 8 hours. In the event of a power outage, there is also an emergency aeration system that allows it to operate independently for an additional 8 hours.  

"We are talking about water recirculation. In this recirculation, what we do is that the water passes through the hydroponic bed where the bacteria are and transforms the ammonia into nitrate to feed the plants. And then, as the system works with a siphon, it fills up to a certain volume and when it reaches that volume it starts to drain. Then the water column begins to fall, and when there is emptiness it is covered and stops falling, so the system is automatically controlled", explains the researcher Luis Andrés Leal, and adds that the measurement of the feed provided to the fish has also been standardized, by means of an automatic feeder, which has allowed him to calculate the percentage of transformation of the nitrogen added with the feed into ammonia and nitrate, after it is expelled by the fish. 

Once the aquaponics process was consolidated, the research focused on determining to what extent this mechanism could be useful to test a third level water treatment through advanced oxidation processes. Some of the compounds existing in the water, within the so-called emerging contaminants, were found in tests within the same research: "In the system we have, as in many other systems in Colombia, water is taken from the rivers, so the rivers are being contaminated with these species. And in our case there was no exception. We found drugs, which are Gabapentin and Ibuprofen", says the researcher.  

The existence of traces of these drugs in the water produces a vicious circle, as they accumulate in fish and plants through a process of bioaccumulation. "They (the fish) can become enriched with that drug and, at some point, that drug is going to become a problem. We want to treat these emerging contaminants without altering the nitrogen cycle and, mainly, without affecting plants and fish. 

Fish and plants in a cleaner environment: promising results 

Currently, the doctoral research of the doctoral candidate Luis Andrés Leal is in the phase of preliminary integration tests for the treatment of traces of drugs in the water used by fish. During this period some tests have been carried out, starting from the use of electrolytic cells to which electric current has been supplied. The electrodes in these cells react to the stimulus, forming a compound called -OH radical (hydroxyl radical), a compound whose potency attacks all surrounding elements, including emerging contaminants, through a process of decomposition of their structures.  

"The research is aimed at determining how we can break down these molecules, and that the remaining particles are not harmful to fish or plants. The objective is to mineralize and transform all those structures into carbon dioxide and water", explains the researcher, and adds that, as part of the process, there was a preliminary test, where it was found that there was a degradation of more than 50% in the traces of Ibuprofen, although he clarifies that the times for the quantification of these drugs have taken a longer time because in the country there are still no systems to do these processes, which has forced him to send the samples to laboratories in Germany. 

This research proposes a novel approach, incorporating the system of advanced oxidation processes to an aquaponic process in an economical method, in a way that is accessible to communities of small producers. To this end, conversations have been held with indigenous communities located in the Sierra Nevada de Santa Marta. "The objective is that: to bring them a model with which they can set up their aquaponic systems and improve their food sovereignty. If you can provide a way for someone to cultivate their aquatic species and plants for their own benefit, so much the better," says researcher Leal. 

If interested in being in touch with the PhD's student or any further information about the investigation, please write the Faculty of Engineering Communications Office: comunicaingenieria@correounivalle.edu.co.