A new low-cost anaerobic reactor model, which works with a bacterial biofilm adhered to a polyurethane foam, can reduce the concentration of nitrogen compounds in sanitary sewage by up to 70%, according to a study published in the journal Environmental Technology.
The researchers perfected a mathematical model that makes it possible to understand and predict the mechanism of nitrogen removal in the biofilm formed by bacteria that transform nitrogen compounds into nitrogen gas, which is harmless to the environment, contributing to future research.
The work was carried out by Bruno Garcia Silva during his doctorate in hydraulic engineering and sanitation at the University of São Paulo (USP), under the guidance of Professor Eugenio Foresti, from the School of Engineering of São Carlos (EESC-USP), and with the support of FAPESP scholarship.
The article is one of the results of the thematic project “Application of the biorefinery concept to biological wastewater treatment plants: the control of environmental pollution combined with the recovery of matter and energy”, coordinated by Professor Marcelo Zaiat. It also involves teams from the Federal University of São Carlos (Ufscar) and the Instituto Mauá de Tecnologia.
“Nitrogen removal is still achieved in a few sewage treatment plants in Brazil, while in Europe and the United States it already happens more easily. The idea is to bring [a infraestrutura necessária] to our reality. Here, the anaerobic reactor is usually used, which generates an effluent with a low organic load, and this makes the nitrogen removal process difficult”, explains Garcia to Agência FAPESP.
The removal of so-called nitrogen compounds (including nitrite, nitrate and ammonia) from both domestic and industrial sewage is essential, as they can contaminate surface (lakes, dams, streams and streams) and underground (such as large aquifers) bodies of water. , favoring the uncontrolled growth of bacteria, algae and plants — a process known as eutrophication.
In addition, consumption of water contaminated by nitrates can lead to the development of diseases such as methemoglobinemia, known as blue baby syndrome. More common in children, it can cause headache, dizziness, fatigue, lethargy or even shock, severe respiratory depression and neurological changes such as seizures and coma in severe cases.
“When algae blooms, one of the consequences that we have seen in dams like Billings, for example, is the death of fish due to lack of oxygen in the water. excess algae, which are very difficult to remove from the liquid environment”, highlights Foresti, coordinator of the group.
Differentials
As the researchers explain, the main differentiator of the new reactor model is the biofilm, which forms after a biological process in which bacteria create a kind of film over a foam, in this case, polyurethane. In addition, the equipment’s configuration allows what the researchers call “counterdiffusion”, that is, the oxygen is inserted on the opposite side of the contaminants.
“Oxygen will be transported into the foam because it only stays where it’s needed for the reaction to take place. We didn’t want this gas in contact with the organic matter all the time, as the bacteria would consume all the oxygen to degrade it and there would be nothing left to consume the nitrite and nitrate. That’s why we insert the oxygen on the other side of the biofilm. The idea is that the organic matter that reaches the biofilm from the opposite side can be oxidized not only by oxygen, but also by nitrite and nitrate “, says Garcia.
When there is no oxygen entering the reactor, the ammonia remains unchanged. But when it gets to the part where oxygen comes in, it starts to turn into nitrite and nitrate.
“As the only way out is through the biofilm, the compounds cross this barrier by diffusion in the opposite direction to that of organic matter. The encounter of organic matter in the counterflow creates optimal conditions for the removal of this nitrite and nitrate, because there is no more oxygen and there is organic matter necessary for denitrification”, adds the researcher.
Foresti explains that, in Brazil, anaerobic reactors (in which organic matter is degraded by bacteria that do not need oxygen to live) are being increasingly used by municipalities because of our climate, which is warmer than that of the Northern Hemisphere. The high temperatures allow a greater activity of the bacteria to decompose the organic matter.
In Europe and the United States it is the opposite, because with low temperatures the process is different. The organic matter present in the liquid phase, after sludge removal, is oxidized by an aerobic process (which involves oxygen).
However, because of costs, nitrogen compounds end up not being completely removed here in Brazil and end up being released directly into nature. This new reactor model is intended to develop a second stage for the treatment of sewage plants, easier and cheaper, aiming at future technologies and partnerships.
partnerships
Garcia had the collaboration of colleagues from the laboratory of Professor Robert Nerenberg, at the University of Notre Dame, in the United States, where he served as a visiting researcher between 2019 and 2020.
“The difference between my project and theirs is that, instead of using polyurethane foam, there they use a semipermeable membrane — similar to a straw filled with air inside. In contact with water, this straw allows the passage of oxygen, but not from the water, and the biofilm grows attached to this surface. In other words, it is through the walls of this straw that oxygen is supplied to the bacteria. So, the oxygen comes from the inside to the outside and the water is supplying the ammonia and organic matter. It’s the same system of counter-diffusion. The difference is that here we use a simpler and cheaper material”, ponders Garcia.
“In the biofilm, the bacteria grow attached to the surface. But it would not be a filter itself, because it does not offer mechanical resistance to the passage of a particle. What this reactor does, in fact, is to serve as a support material for the bacteria to grow and consume soluble organic matter and nitrogenous compounds”, explains the scientist.
Next steps
Foresti adds that the new reactor configuration is already inspiring more recent studies by the group. In a cooperation program between the Basic Sanitation Company of the State of São Paulo (Sabesp) and FAPESP, the researchers intend to test the new model with real sewage from the city of São Carlos, which has already passed through an anaerobic reactor at the treatment operated by the São Carlos Autonomous Water and Sewage Service (Saae).
Researchers from UFSCar and Instituto Mauá are also part of this cooperation program and will develop other systems to be tested.
“Bruno’s research is the first to use the counterdiffusion process in this way here in Brazil. He proved the concept for synthetic wastewater. The efficiency found with this reactor configuration was much higher than that observed in previous research, but we still need to evaluate several factors”, highlights Foresti.
The new configuration has been tested for the time being in the lab. New projects still need to validate efficiency, as it is not possible to predict how the equipment will behave with large amounts of effluents. In addition, it is necessary to test the system using real, domestic and industrial sewage, since, until then, the samples were of synthetic sewage, prepared by the team itself.
“Perhaps it is necessary to improve the design and geometry. How can I optimize this design to have a greater surface area per volume of reactor to make it cheaper? , which is the mathematical model”, concludes Garcia.
06/28/2022 – Ambiente