Turning municipal wastewater into bioenergy

Cyanobacteria blooms have choked waterways throughout the world, from estuaries in Florida to the Mississippi River Basin to lakes in China.

And, the toxins that cyanobacteria produce are harmful to humans, pets and wildlife alike. These photosynthetic organisms, also called blue-green algae, thrive on human sources of nitrogen and phosphorus, including effluent from waste treatment plants and fertilizers that get washed into watersheds from farms.

But bioenergy researchers have long recognized an upside to this nutrient-loving, waterborne menace: They could provide an excellent supply of biomass for biofuels and power.

Now, a researcher at Idaho National Laboratory has developed a novel way to grow cyanobacteria for bioenergy, while at the same time cleaning up water from wastewater treatment plants.

The findings appear in the journal BioEnergy Research.

A power-packed microorganism

“The scientific community got interested in producing biofuels from algae because the amount of oil from algae is 10 times that of palm oil and 131 times that of soybeans,” said Carlos Quiroz-Arita, who started his research as a graduate student at Colorado State University. “Well, cyanobacteria has four times the energy productivity as algae under laboratory-scale conditions.”

But there’s a problem: Growing that much cyanobacteria would take a lot of water and a lot of nutrients.

So, Quiroz-Arita and his colleagues started thinking about cyanobacteria blooms. “It doesn’t make sense to use more water and more fertilizers to make biofuels,” he said. “If we grow cyanobacteria at a wastewater treatment facility, we can not only use cyanobacteria and algae for growing biofuels, but also for reducing algae and cyanobacteria blooms downstream.”

The inner workings of a wastewater treatment plant

The researchers worked with the Drake Water Reclamation Facility (DWRF) in Fort Collins, Colorado, to model the best approach to make cyanobacteria from wastewater. Since DWRF officials were most interested in improving water quality and reducing CO2 emissions, Quiroz-Arita designed his approach to meet those goals.

Wastewater in a modern wastewater treatment plant like DWRF typically goes through several different processes before the treated effluent can be safely discharged.

Quiroz-Arita settled on the point in the process where a centrifuge is used to separate the solid waste from the liquid waste. The solid waste is dried and sent to a landfill, and the nutrient-rich liquid waste, called the centrate, is recycled back into the wastewater treatment plant before it is discharged.

“Wastewater treatment plants cannot release the centrate into the environment,” Quiroz-Arita said. “It would kill everything. What they do is just keep recycling the centrate back into the process with pumps. It’s an energy-intensive process to clean the nitrogen and phosphorous, and in many cases, it is not enough to meet the water quality criteria.”

A step-by step process for producing biomass

It’s at this step in the wastewater treatment process that plant operators could best control nutrient concentrations for growing cyanobacteria.

Once the centrifuge separates the solids from the centrate, the centrate is pumped into a device called a photobioreactor—a device where the cyanobacteria is cultivated using nutrients and sunlight, clearing the nitrogen and phosphorous from the centrate to levels consistent with state and federal water quality standards.

The cyanobacteria multiplies, and then another centrifuge separates the cyanobacteria biomass from the water.

That biomass then moves to a biodigester—a device that uses microbes to turn biomass into biogas, which is then burned for heat and power. The resulting CO2 is pumped back into the photobioreactor to aid with photosynthesis and reduce the carbon footprint.

Trade-offs and the life-cycle assessment

Since cyanobacteria grow best with just the right amount of nutrients, the researchers started by testing different centrate/effluent concentrations. “We found the best centrate total nitrogen concentration to obtain the highest growth rate and nutrient uptake rate for this cyanobacteria strain,” Quiroz-Arita said.

Finding the right recipe for each wastewater treatment plant depends on its individual wastewater and centrate characteristics. Each plant would likely require its own biological and engineering analyses, he said.

When the researchers started to look at nitrogen concentration and cyanobacteria growth, there were some trade-offs. At lower nitrogen concentrations, with a slower growth rate, the water reached the water quality standards faster. The trade-off is that a slower growth rate at lower nitrogen concentrations requires more acreage for the photobioreactor which, in turn, consumes more electricity.
When all the benefits of this process are tallied in a life-cycle assessment, the result is cleaner effluent, lower CO2 emissions and reduced energy consumption relative to conventional wastewater treatment processes. (The process also produces a fertilizer, struvite, which precipitates from the centrate before it enters the photobioreactor. The facility can sell the struvite as a coproduct.)

The process has garnered interest from industry, and the U.S. Department of Energy’s Bioenergy Technologies Office has highlighted municipal wastewater facilities as a promising water and nutrient source for algae-based biofuel production.

Next, Quiroz-Arita is looking for funding to continue researching ways to improve the cyanobacteria growth rate and nutrients uptake rate, to optimize the nutrient removal process and metrics of sustainability, and to collaborate with municipal wastewater treatment facilities to scale up the process under different conditions.