Converting Municipal Wastewater to Biogas

Nutrient laden wastewater has fueled toxic algal blooms which have subsequently clogged freshwater systems worldwide, including Florida's estuaries, the Mississippi River Basin, lakes in China, and everywhere in-between. When fertilizer-laden runoff or nutrient-rich wastewater effluent is discharged into rivers it provides a source of food that allows these organisms to flourish and multiply. Toxic algal blooms are not only an ecological threat, they also pose a risk to human health. The photosynthetic cyanobacteria, known as blue-green algae, that make up these blooms produce toxins harmful to wildlife, pets and humans. Scientists in the bioenergy field have been exploring ways to exploit the nutrient-loving cyanobacteria as a useful resource. It could be used as the biomass needed to produce biofuels and energy. Now, a researcher from the Idaho National Laboratory has found a unique method of growing cyanobacteria to produce bioenergy that will also improve the quality of water discharged from wastewater treatment facilities at the same time. His findings were recently published in the scientific journal BioEnergy Research.

Cyanobacteria are Power-packed Microorganisms

"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 lead author, Carlos Quiroz-Arita, a Postdoctoral Research Associate at Idaho National Laboratory 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 one problem -- in order to grow the quantities of cyanobacteria needed to produce biofuels, it would require a substantial volume of water in addition to copious amount of nutrients. This got Quiroz-Arita and his team thinking: why not exploit those pesky algal 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."

Working closely with the Drake Water Reclamation Facility (DWRF) in Fort Collins, Colorado, the research team modeled the best approach to growing cyanobacteria in wastewater that would also improve the quality of water discharged from the wastewater treatment plant and reduce atmospheric emissions. Wastewater typically undergoes various treatment processes before the remaining effluent is of a quality that can be discharged safely. Quiroz-Arita chose the stage in the treatment process where the solids are separated from the liquid wastewater using a centrifuge. The solids are then dried and sent off to a landfill for disposal, while the nutrient-rich liquid portion of the waste, known as centrate, is returned back to the wastewater treatment facility.

"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."

Biomass Production is a Step-by-Step Process

According to Quiroz-Arita, this stage of the wastewater treatment process is ideal for plant operators to control the nutrient levels needed for cultivating cyanobacteria. Once the solids have been separated from the centrate by the centrifuge, the centrate can be pumped to a photobioreactor, which uses the nutrient-laden centrate together with sunlight to facilitate the growth of cyanobacteria, while reducing the levels of nitrates and phosphates (the primary nutrients needed for their growth) from the centrate. By removing these nutrients, they improve the water quality in line with those required by state and federal authorities for wastewater effluent discharges into the environment. The nutrients together with the sunlight in the photobioreactor fuels the growth of the cyanobacteria allowing them to flourish and multiply. The cyanobacteria biomass are then separated out of the water using another centrifuge. The biomass is then moved to a biodigester, where anaerobic bacteria break it down, producing methane during the decomposition process. The methane is captured and used as a biogas that can be used to generate heat or power. Carbon dioxide, which is produced as a byproduct of the process, is fed back into the photobioreactor where it provides the CO2 needed by the algae for photosynthesis. Capturing and reusing the CO2 and methane produced in these processes also helps reduce atmospheric emissions associated with wastewater treatment.

 

Life-Cycle Assessment Shows Win-Win Scenario

If one tallies up all the benefits this process has to offer and generate a life-cycle assessment, it shows improved water quality of the wastewater effluent, and a reduction in energy used as well as a reduction in CO2 emissions compared to conventional wastewater treatment processes. The process also results in a precipitate, known as struvite, being separated from the centrate, which is an excellent fertilizer that can be sold to generate an additional source of revenue for the wastewater treatment plant. This innovative method of treating wastewater has captured the interest of the wastewater treatment industry, as well as the US Department of Energy, whose Bioenergy Technologies Office has earmarked municipal wastewater treatment plants as a promising source of water and nutrients for algae-based biofuel production.

 

Journal Reference

Quiroz-Arita, C., Sheehan, J.J., Baral, N.R. et al. A Cyanobacterial Sidestream Nutrient Removal Process and Its Life Cycle Implications. Bioenerg. Res. (2019) 12: 217. doi/10.1007/s12155-019-9963-2

Featured Image by US Department of Energy via EurekAlert!