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More Efficient Wastewater Treatment with Less Energy

AUTHOR: ND Staff

PUBLISHED: June 18, 2014

Robert Nerenberg
Robert Nerenberg, Associate Professor, Civil and Environmental Engineering and Geological Sciences

Wastewater treatment is essential to protecting human health and the environment, but because of the burgeoning global population, and the increasing need to reuse wastewater, treatment plants are being required to meet even more stringent standards. Most plants will need upgrades to comply with these new regulations. Not only do the changes mean significant capital investments, higher chemical costs, and the production of more green-house gases, but they also signal increased energy requirements. A Notre Dame team has developed a novel water treatment technology that helps existing plants meet the new standards using less space, less energy, and less capital, while producing fewer emissions. Equally important, this technology may be adapted to convert a full-sized wastewater treatment plant from an energy sink into an energy source.

HMBP Biofilm
Shown here in a laboratory system, an HMBP biofilm grows on a hollow-fibermembrane. The needle-shaped object on the right is a microsensor used to measure microgradients of contaminants within the biofilm, a key tool for understanding this process.

Wastewater treatment plants discharge enormous volumes of water, exceeding 100 million gallons of water per day in some larger cities. Although the treatment process varies depending upon the sophistication and age of a plant, it requires a tremendous amount of energy, mainly to aerate the chambers and pump the water.

Conventional wastewater treatment systems use approximately three percent of all of the electrical energy produced in the United States. With the new regulations for nitrogen and phosphorus removal, the process will become even more of an energy drain. It will require larger facilities and more chemicals, making the process more expensive. Greater amounts of greenhouse gases will also be produced as a result of meeting the new standards.

Associate Professor Robert Nerenberg and his team of researchers may have a solution. They have developed a process that reduces the energy requirements for a treatment plant by up to 50 percent while minimizing emissions of nitrous oxide (N2O), a potent greenhouse gas. The Hybrid Membrane-Biofilm Process (HMBP) they have designed features air-filled hollow-fiber membranes, which are incorporated into a plant’s activated sludge tank. Once in the tank, a nitrifying biofilm develops on the membranes, producing nitrite and nitrate. By suppressing bulk aeration (instead of supplying it), the liquid becomes anoxic, and the nitrite/nitrate can be reduced with influent bio-chemical oxygen demand (BOD).

Its hybrid nature is what distinguishes the HMBP from other membrane-aerated processes. Heterotrophic bacteria are kept in suspension by maintaining low bulk liquid BOD concentrations, while nitrifying bacteria form a biofilm on the fiber, getting their oxygen by passive diffusion through the air-filled fibers. Thus, the HMBP can save up to 50 percent of the electrical energy required to run the plant, while preventing N2O emissions and reducing the need for additional chemical additives. Another major advantage is the ability to retrofit existing infrastructure, rather than expand it or replace it with new systems.

After a successful bench-scale study, the team built a pilot reactor, which was tested at the 26th Ward Water Pollution Control Plant in Brooklyn in conjunction with New York’s Applied Research Facility. The pilot study confirmed the ability of the HMBP to achieve total nitrogen removal from an actual wastewater treatment plant in scalable concentrations. Ongoing research, funded by the National Science Foundation, will provide a more basic understanding of the unique microbial processes used in the HMBP.

According to Nerenberg, with a few tweaks the HMBP can also function like a microbial fuel cell, so that in addition to removing the nitrogen and other impurities from the water, it could convert the chemical energy contained in biodegradable compounds into electrical energy. This could allow wastewater treatment plants to send electrical power back to the grid.

SUGGESTED READING

Shea, Caitlyn and Nerenberg, Robert, “Performance and Microbial Ecology of Air-Cathode Microbial Fuel Cells with Layered Electrode Assemblies,” Applied Microbiology and Biotechnology, 2010, 86, 5, 1399.

Downing, Leon S.; Bibby, Kyle J.; Esposito, Kathleen; Fascianella, Tom; Tsuchihashi, Ryujiro; and Nerenberg, Robert, “Nitrogen Removal from Wastewater Using a Hybrid Membrane-biofilm Process: Pilot-scale Studies,” Water Environment Research, 2010, 82, 3, 195-201.

Downing, Leon S. and Nerenberg, Robert, “Effect of Bulk Liquid BOD Concentration on Activity and Microbial Community Structure of a Nitrifying, Membrane-aerated Biofilm,” Applied Microbiology ­­­­and Biotechnology, 2008, 81,153-162.

Downing, Leon S. and Nerenberg, Robert, “Total Nitrogen Removal in a Hybrid, Membrane-aerated Activated Sludge Process,” Water Research, 2008, 42, 3697-3708.

Downing, Leon S. and Nerenberg, Robert, “Effect of Oxygen Gradients on the Activity and Microbial Community Structure of a Nitrifying, Membrane-aerated Biofilm,” Biotechnology and Bioengineering, 2008, 101, 1193-1204.