Up to 70% of environmentally harmful nitrogen removed by new wastewater treatment system

A new type of biofilm reactor adapted to Brazilian conditions and using polyurethane foam to reduce costs can reduce the amount of nitrogen compounds in wastewater by up to 70%, according to a article in Environmental technology. The researchers who conducted the study developed a mathematical model to analyze and predict the mechanism of nitrogen removal. The biofilm included bacteria that converted nitrogen compounds into nitrogen gas, which is harmless to the environment.

The study was led by Bruno Garcia Silva during his doctoral research in hydraulic engineering and sanitation at the University of São Paulo (USP) in Brazil, with Eugenio Foresti as thesis supervisor. Foresti is a professor at the São Carlos School of Engineering (EESC-USP). The study was funded by FAPESP.

The article was one of the results of the thematic project “Biorefinery concept applied to biological wastewater treatment plants: control of environmental pollution coupled with material and energy recovery”, for which Marcelo Zaiat, also a professor at the CESE -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 only carried out by a few treatment plants in Brazil, while it is regularly carried out in Europe and the United States,” Garcia told Agência FAPESP. “The idea is to adapt [the necessary infrastructure] to our reality. The usual method here is based on anaerobic reactors, which produce effluents with low organic content, which makes it difficult to remove nitrogen.

The removal of nitrogen compounds (nitrites, nitrates and ammonia, among others) from domestic wastewater and industrial wastewater is essential because they contaminate surface waters (lakes, reservoirs and streams) as well as aquifers and other groundwater, supporting the growth of bacteria, algae and plants spiral out of control in a process known as eutrophication.

Additionally, drinking nitrate-contaminated water can lead to illnesses such as infantile methemoglobinemia (blue baby syndrome), which causes headaches, dizziness, fatigue, lethargy, shortness of breath and neurological alterations such as seizures and coma in severe cases.

“When algal blooms proliferate, as seen in reservoirs like Billings [one of the main water sources for São Paulo], for example, lack of oxygen in the water leads to the death of fish and the loss of water supplies and recreational areas. It is very difficult to remove algae from reservoirs,” said Foresti, who leads the group.

Differentiators

One of the key differentiators for this new reactor design is the biofilm formed by a biological process in which bacteria create a film on the polyurethane foam. Another is configuring the equipment to allow for what the researchers call counter-diffusion, where oxygen is introduced from the side opposite the contaminants.

“The oxygen is carried in the foam because it ensures that it only stays where it is needed for the reaction to occur,” Garcia explained. “We don’t want oxygen to come into constant contact with organic matter. If it did, the bacteria would use all the oxygen to break it down and there would be nothing left to consume the nitrites and nitrates. Oxygen is therefore inserted on the other side of the biofilm. The objective is that the organic matter that reaches the biofilm on the opposite side is oxidized not only by oxygen but also by nitrites and nitrates.

When oxygen does not enter the reactor, the ammonia remains unchanged. However, when ammonia enters the reactor site with an oxygen supply, it is converted into nitrite and nitrate. “The only way out is through the biofilm, and the compounds cross this barrier by diffusion in the opposite direction from the organic matter. Their countercurrent collision with organic matter creates optimal conditions for nitrite and nitrate removal because there is no more oxygen and there is enough organic matter for denitrification,” Garcia said.

Foresti explained that in Brazil, anaerobic reactors (which break down organic matter using bacteria that don’t need oxygen to survive) are increasingly being used by municipal wastewater treatment companies. due to the predominant climate, which is warmer than that of the northern hemisphere. Bacteria break down organic matter faster in hot weather. In Europe and the United States, 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 eliminated for cost reasons and are directly discharged into nature. The new type of reactor developed by the researchers is designed to add an easier and cheaper second stage to wastewater treatment, for development with future technologies and partnerships.

USA Fellowship

Researchers working in the lab of Robert Nerenberg, a professor at the University of Notre Dame in the United States, collaborated with Garcia, who was there as a visiting researcher in 2019-20 with The support of FAPESP.

“The difference between my project and theirs is that instead of polyurethane foam, they use a semi-permeable membrane, which looks like a straw full of air. When this capillary comes into contact with water, it passes oxygen but not water, so the biofilm adheres to the surface and grows on it. In other words, oxygen is supplied to bacteria through the walls of this thin tube. Oxygen comes out and water provides ammonia and organic matter. It’s the same system as counter-diffusion, except the material we use is simpler and cheaper,” Garcia said.

“Bacteria grow on the surface to form a biofilm, but it is not a filter strictly speaking because it offers no mechanical resistance to the passage of particles. What the reactor actually does is serve as a medium for bacteria to grow and consume soluble organic matter and nitrogen compounds.

Next steps

According to Foresti, the new configuration of the reactor inspires new research in the group. As part of a cooperative program between São Paulo State Basic Sanitation Corporation (SABESP) and FAPESP, researchers plan to test the new model with real wastewater that has passed through an aerobic reactor in the operated sewage treatment plant. by SAAE, the municipal sanitation service of São Carlos. Researchers from UFSCar and IMT are also part of the program and will develop other systems for testing.

“Bruno’s research is the first to use counterdiffusion 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 significantly higher than observed in previous research, but we still need to assess several factors.

The new configuration has been tested in the laboratory. Efficiency will be measured in other projects, as it is not possible to predict how the equipment will perform when treating large volumes of effluent, and the system must be tested with actual domestic and industrial wastewater . So far, it has only been tested on synthetic waste samples prepared by the researchers themselves.

“We may have to improve the design and the geometry,” Garcia said. “How can the design be optimized to achieve the greatest optimal surface area per reactor volume to reduce cost?” The study provides a base, a base on which we can continue to think about the process and the mathematical tool.

Reference: 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

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Irene B. Bowles