Novel process derives algae-based biofuel from wastewaterDavid Wagman | May 09, 2019
A researcher at Idaho National Laboratory has developed a way to grow blue-green algae - known as cyanobacteria - for bioenergy. The process could also help clean up water from wastewater treatment plants.
Blue-green algae blooms can clog waterways around the world, from estuaries in Florida to the Mississippi River Basin to lakes in China.
Their toxins can be harmful to humans and wildlife. These photosynthetic organisms thrive on human sources of nitrogen and phosphorus, including effluent from waste treatment plants and fertilizers that wash into watersheds from farms.
But this nutrient-loving algae also can serve as a feedstock for biofuels and power. Researchers say that the amount of oil from algae is 10 times that of palm oil and 131 times that of soybeans. Cyanobacteria have four times the energy productivity as algae under laboratory-scale conditions.
However, growing cyanobacteria at an industrial scale requires a lot of water and nutrients. The Idaho researchers decided to grow cyanobacteria at a wastewater treatment facility, producing algae to grow biofuels and also reducing blooms downstream.
The researchers worked with the Drake Water Reclamation Facility (DWRF) in Fort Collins, Colorado. They settled on the point in the treatment process where a centrifuge is used to separate solid waste from liquid waste. The solid waste is dried and sent to a landfill. The nutrient-rich liquid waste, called the "centrate," is recycled back into the wastewater treatment plant before it is discharged.
Once the centrifuge separates the solids from the centrate, the centrate is pumped into a photobioreactor, a device where the cyanobacteria are cultivated using nutrients and sunlight. This process clears the nitrogen and phosphorous from the centrate to levels consistent with state and federal water-quality standards.
The cyanobacteria multiply and another centrifuge separates the cyanobacteria biomass from the water. That biomass then moves to a biodigester, which uses microbes to turn biomass into biogas. The biogas 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.
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 was that a slower growth rate at lower nitrogen concentrations required more acreage for the photobioreactor which, in turn, consumed more electricity.
The process has generated interest from industry, and the U.S. Department of Energy’s Bioenergy Technologies Office has highlighted municipal wastewater facilities as promising water and nutrient sources for algae-based biofuel production.