A new type of biofilm reactor, which is adapted to Brazilian conditions and reduces costs through the use of polyurethane foam, can reduce the amount of nitrogen compounds in wastewater by up to 70% article in environmental technology. The researchers who conducted the study developed a mathematical model to analyze and predict the nitrogen removal mechanism. The biofilm consisted of bacteria that converted nitrogen compounds into environmentally benign nitrogen gas.
The study was led by Bruno Garcia Silva during his PhD in hydraulic engineering and sanitation at the University of São Paulo (USP) in Brazil, with Eugenio Foresti as the supervisor. Foresti is a professor at the São Carlos School of Engineering (EESC-USP). The study was supported by FAPESP.
The article was one of the outcomes of the thematic project “Biorefinery concept in biological wastewater treatment plants: Pollution control coupled with material and energy recovery”, for which Marcelo Zaiat, also a professor at EESC-USP, was principal investigator. Researchers from the Federal University of São Carlos (UFSCar) and the Mauá Institute of Technology (IMT) collaborated.
“Nitrogen removal is still achieved by only a few wastewater treatment plants in Brazil, while it is regularly performed in Europe and the United States,” Garcia told Agência FAPESP. “The idea is to adapt [ the necessary infrastructure ] to our reality. The usual process here is based on anaerobic reactors, which produce effluents with low levels of organic matter, making nitrogen removal difficult.”
The removal of nitrogen compounds (including nitrite, nitrate and ammonia) from domestic and industrial wastewater is essential as they contaminate surface waters (lakes, reservoirs and streams) as well as aquifers and other groundwater and encourage the growth of bacteria, algae and plants in a manner called eutrophication known process out of control.
In addition, consuming nitrate-contaminated water can lead to diseases such as infant methemoglobinemia (blue baby syndrome), which causes headaches, dizziness, fatigue, lethargy, breathlessness, and in severe cases, neurological changes such as seizures and coma.
“When algal blooms spread, like you see in reservoirs like Billings [ one of the main water sources for São Paulo ]For example, a lack of oxygen in the water leads to the death of fish and the loss of water supplies and recreational areas. It’s very difficult to remove algae from reservoirs,” said Foresti, who leads the group.
distinguishing features
One of the main distinguishing features of this new reactor model is the biofilm formed by a biological process in which bacteria form a film on the polyurethane foam. Another reason is the configuration of the equipment to allow what is known as counter-diffusion, where oxygen is introduced on the opposite side of the contaminants.
“Oxygen is transported into the foam because it only stays where it is needed for the reaction,” explains Garcia. “We don’t want oxygen to constantly come into contact with organic material. If this were the case, the bacteria would use up all the oxygen to break it down and there would be nothing left to use up the nitrite and nitrate. So we introduce the oxygen on the other side of the biofilm. The goal is that the organic matter that reaches the biofilm on the opposite side is oxidized not only by oxygen but also by nitrite and nitrate.”
If no oxygen enters the reactor, the ammonia remains unchanged. However, if ammonia enters the reactor space with the addition of oxygen, it is converted into nitrite and nitrate. “The only way out is through the biofilm, and the compounds overcome this barrier by diffusing in the opposite direction to the organic matter. Their counter-current collision with organic matter creates optimal conditions for nitrite and nitrate removal as there is no longer any oxygen and there is enough organic matter for denitrification,” Garcia said.
Foresti explained that in Brazil, because of the prevailing climate, which is warmer than the northern hemisphere, anaerobic reactors (which break down organic matter with bacteria that don’t need oxygen to survive) are increasingly being used by municipal wastewater treatment companies. Bacteria break down organic matter faster in warm weather. In Europe and the US, where average temperatures are lower, the process is different. The organic matter present in the liquid phase after sludge removal is oxidized aerobically (by oxygen).
In Brazil, however, nitrogen compounds are not completely removed and released directly into nature for cost reasons. The new type of reactor developed by the researchers is expected to add a second, simpler and cheaper stage to wastewater treatment for development with future technologies and partnerships.
Grant for research stays in the USA
Researchers working in the lab of Robert Nerenberg, a professor at the University of Notre Dame in the US, collaborated with Garcia, who was a visiting scientist there in 2019-20 FAPESP’s support.
“The difference between my project and theirs is that instead of polyurethane foam, they use a semi-permeable membrane that resembles a drinking straw filled with air. When this capillary comes in contact with water, it lets oxygen through but not water, so the biofilm adheres to the surface and grows on it. In other words, oxygen is supplied to the bacteria through the walls of this thin tube. The oxygen escapes and the water yields ammonia and organic matter. It’s the same system as counter-diffusion, except the material we use is simpler and cheaper,” Garcia said.
“The bacteria grow on the surface and form a biofilm, but it is not a filter in the true sense because it offers no mechanical resistance to the passage of particles. What the reactor actually does is help the bacteria grow and consume soluble organic matter and nitrogen compounds.”
Next Steps
According to Foresti, the reactor’s new configuration inspires further research by the group. In a collaborative program between the São Paulo State Basic Sanitation Corporation (SABESP) and FAPESP, the researchers plan to test the new model with real sewage that has been passed through an aerobic reactor at the SAAE municipal sewage treatment plant in São Carlos. Researchers from UFSCar and IMT are also part of the program and will develop additional systems to be tested.
“Bruno’s research is the first to use counter-diffusion in this way here in Brazil,” Foresti said. “It’s a proof of concept for synthetic wastewater. The efficiency found in this reactor configuration was far superior to that observed in previous research, but we still need to evaluate several factors.”
The new configuration was tested in the laboratory. Efficiency will be measured in further projects as it is unpredictable how the equipment will perform when processing large volumes of wastewater and the system needs to be tested with actual domestic and industrial effluents. So far, it has only been tested on samples of synthetic waste made by the researchers themselves.
“We may need to improve the design and geometry,” Garcia said. “How can the design be optimized to get the largest optimal surface area per reactor volume to reduce costs? The study provides a basis, a foundation on which we can continue to think about the process and the mathematical tool.”
Relation: Silva BG, Perez-Calleja P, Foresti E, Nerenberg R. Unique biofilm structure and mass transfer mechanisms in the foam-aerated biofilm reactor (FABR). environmental technology. 2022:1-15. doi: 10.1080/09593330.2022.2058422