Wastewater and its application for tomorrow's fertilizers

Daniela María Rivera Velasco, Master's student in Engineering with an emphasis in Sanitary and Environmental Engineering. Credit: Édgar Bejarano, Communications Office, Faculty of Engineering.

Although wastewater goes through different treatment systems for the removal of organic matter, processes for the removal of nutrients such as nitrogen and phosphorus are not considered. These nutrients, which are present in the water, are not usually used. A research project aims to find new uses for them, this time as possible fertilizers of organic origin, through a chemical process. Its results are expected to contribute to food security and fertilizer autonomy in the region.

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Closing cycles: the purpose of research

Aware that the various wastewater treatment systems do not have an approach in their processes oriented to the total removal of these nutrients and their pollution associated with eutrophication processes in water bodies, the environmental engineer Daniela María Rivera Velasco, in the framework of the Master's Degree in Engineering with emphasis in Sanitary and Environmental Engineering, carried out jointly by Universidad del Valle and Universidad Autónoma de Occidente, proposed a project aimed at assessing the feasibility of the production of an organo-mineral fertilizer by means of a chemical process applied to wastewater. The research, directed by Universidad del Valle professor and researcher Inés Restrepo Tarquino, associated to CINARA, and Universidad Autónoma de Occidente professor and researcher Nancy Vásquez Sarria, is carried out at the Universidad Autónoma de Occidente's wastewater treatment plant, where a treatment system called "activated sludge with extended aeration" is used. 

This process seeks to valorize the water that enters such systems, recovering from it nutrients such as nitrogen and phosphorus, which are essential for plant and crop growth. At the same time, it proposes to respond to two complex scenarios that are currently being experienced, related to the environment and food security for the coming years.

"One is associated with fertilizers as such, with something known as chemical fertilizers, which are those normally used by industry, and which have an impact on the soil," explains researcher Daniela María Rivera Velasco. According to her, this type of fertilizer, when used excessively on soils, produces a leaching effect -the dissolution of waste agents- towards water sources, producing a polluting effect on nature. The second is related to mining for chemical fertilizers. Many are rich in phosphorus, which is usually obtained from phosphate rock. "And, according to research, this material could be depleted in 50 to 100 years," adds the researcher.

To carry out this research, Daniela María Rivera Velasco conducted a physicochemical characterization of the wastewater generated at the Universidad Autónoma de Occidente, where she measured parameters such as COD (Chemical Oxygen Demand), BOD5  (Biochemical Oxygen Demand), Solids (ST, SV, SST, SSV), Alkalinity, Hardness, Total Nitrogen, Ammonia Nitrogen, Nitrites, Nitrates and total phosphorus (PO4-3).

Once the characterization was done, preliminary jar test trials were started, a common method used in processes related to coagulation and flocculation. Since this is a chemical precipitation process, magnesium chloride was used as a reagent and sodium hydroxide to adjust the pH. During this phase, the optimum dosage and pH of the process were identified according to the characteristics of the water used. Subsequently, the recovery process was carried out in the crystallization reactor with a capacity of 12 liters under the previously defined experimental conditions. 

Master's student in Engineering with an emphasis in Sanitary and Environmental Engineering performing laboratory tests. Credit: personal archive.

Preliminary results

The research, currently in the preliminary phase, has shown promising results. "We found that the production of organo-mineral fertilizer is viable", acknowledges researcher Daniela María Rivera Velasco, and explains that, at a chemical level, in water there are alloys of nitrogen and phosphorus with the other elements present, which eventually give way to the formation of a compound technically called "salts" (calcium phosphates and ammonium and magnesium phosphates), mainly struvite. This group has a special importance, since it is one of the active components of some of the most environmentally friendly fertilizers.  

"Why is this struvite important within the concept of fertilizer? Because, unlike other fertilizers of chemical origin, these have an absorption process such that when the plants assimilate it, the consumption of this nutrient is much slower (slow release fertilizer). As it is slower, there will be no excess in the soil, and therefore less contamination of water sources, reducing in turn the impact associated with this type of chemical fertilizers", explains the researcher.  

With evidence indicating the formation of these "salts" in the treated water, the research is heading towards its second stage, in which the aim is to add to these salts what in chemical crystallization processes is known as "seed". "We have chosen to have a "seed" of biological origin, which is called "mixed liquor", which is obtained within the wastewater treatment process. It is added so that the crystals in formation have a better support for their growth," says the researcher, and explains that the "mixed liquor" refers to a mixture of wastewater and activated sludge that is generated in the biological treatment process, in which the active microorganisms (biomass) degrade the organic matter present in the wastewater. This process seeks to increase the size of the crystals so that the fertilizer has greater consistency. This makes the nutrients available for plant absorption. 

Formation of crystalline structures seen through the optical microscopy process. Credit: personal archive.

What's next for research

In addition to techniques such as optical microscopy, used to classify the morphology and size of the salts formed in the organo-mineral fertilizer, future stages of the research plan optical techniques such as X-ray diffraction (XRD), ScanningElectron Microscopy (SEM), and Energy Dispersive X-ray Electroscopy (EDS), for the identification and observation of the structures. 

"What we are looking for is to obtain information about this "seed", especially about its proportion, because we have already seen that a growth of these salts is really being achieved", says researcher Daniela María Rivera Velasco, and says that, once this stage is concluded, they hope to scale up the research to the pilot plant. "The idea is that, at the beginning of next year, the pilot plant for nutrient recovery with the crystallization reactor, which will have a capacity of 250 liters, will be located at the experimental station located at the WWTP of the Universidad Autónoma de Occidente".

At the same time, it is expected to have by this time the ideal proportion of seed for the production of organo-mineral fertilizer, and that the process can be scaled up.

Its usefulness in the region  

Political and social events such as the war between Ukraine and Russia have made reference to an aspect that until then had gone unnoticed, and that is related to food demand and the uncertainty that the decisions of one government or another can cause in the lives of many people in different parts of the world. Such was the case of fertilizers, imports of which were threatened with the advent of the conflict between these countries. This, coupled with population growth, has given fertilizers an importance that was not previously considered, which has also made it necessary to explore new alternatives. 

For researcher Daniela María Rivera Velasco, this is an opportunity to think about the usefulness of her research. "The insufficient or inadequate management of wastewater has triggered pollution problems associated with the increase of nutrients in water bodies and soils, generating a negative impact on the environment. Why not derive it to a process from which we can obtain something, this time the creation of a fertilizer, that can supply future needs?", she proposes, and mentions that the research is also in relation to the UN Sustainable Development Goals, especially number 2, "Zero Hunger", associated to food insecurity with population growth, lack of food and deficiency of products such as fertilizers.

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