By Jay A. Rodman
What if a new system could be used to treat wastewater without the high input of electricity that regular sewage treatment plants require? What if this water treatment system, fed by nutrients in the wastewater, were able to use photosynthesis to produce renewable surplus energy to help meet the world’s growing energy demand?
Just such a win-win approach is currently being designed and tested by researchers at New Mexico State University. If successful, their system will provide a more sustainable method for treating wastewater, a new viable approach to producing electric power and liquid biofuels, and a revenue stream to offset infrastructure improvements.
The approach is being called the POWER – photosynthetically oxygenated waste-to-energy recovery – system, according to Peter Lammers, research professor and technical director of NMSU’s Algal Bioenergy Program.
Readers of previous issues of this publication are probably already aware that intensive research on algae and algal biofuels has been ongoing at NMSU for several years, much of it funded by large federal grants.
Lammers and his colleagues have found that certain types of algae are highly effective in removing carbon, nitrogen and phosphorus compounds from municipal and agricultural wastewater.
“Unlike traditional wastewater plants that use bacteria for this purpose, processing wastewater with algae yields about 100 times more biomass than the sludge output of current systems,” Lammers said. “The economic key to the POWER system is the photosynthetic energy of the newly expanded biomass when converted back into fuel and electric power.”
The Desert Southwest offers an ideal environment, in many ways, for the production of algae. Abundant sunlight, mild temperatures, and wide open spaces for algae cultivation facilities have attracted new energy companies to the state, including Sapphire Energy, El Dorado Biofuels and Joule Energy.
But one essential component is in short supply in this arid environment: water.
It is this reality that has led researchers to explore using alternatives to fresh water that are less in demand, such as brackish water and municipal wastewater.
The conceptual breakthrough was to go beyond thinking about wastewater as an ingredient in algae production to thinking about algae as an ingredient in a newly designed sewage treatment system.
How will the new system work?
Algae will be cultivated in sewage water outdoors in large enclosed plastic bag containers – “closed photobioreactors” – that prevent evaporative water loss. They heat up much like a greenhouse but are much cheaper. Such PBRs also retain carbon dioxide, a nutrient for the algae, thus fostering high-density algae production, and they keep wastewater odors and potentially harmful microbes contained.
The main inputs, in addition to sunlight, are the wastewater, the algae and the CO₂.
The PBRs, given the appropriate strains of algae, will produce large amounts of algal biomass in a short period of time, while removing nutrients to yield clean discharge water.
The output of these PBRs is a broth that is then separated into biosolids that move into the fuel production components.
The success of the above system as a sustainable approach relies on efficiency at every stage. One challenge of the enclosed PBRs in desert environments is to prevent the mixture from overheating in the summer and killing the algae.
The standard PBR solution to this problem is to regulate the temperature of the water using a cooling system, but of course that requires lots of electricity.
Lammers and his colleagues are exploring an alternative approach, specifically testing strains of algae that have evolved in high-temperature geothermal environments. Such algae thrive in the hotter PBR temperatures. As an added benefit, these strains provide an acidified environment that, at the high daytime temperatures, promises to neutralize the pathogenic microbial elements in the wastewater. The algae are also mixotrophic, which means they can thrive during periods of low sunlight by processing carbon in the wastewater.
Lammers and his colleagues hope that this PBR-based system could be adapted worldwide by employing a variety of different algae types with optimum temperature profiles compatible with different climates. It should also be scalable for communities of various sizes and, since it is a net energy producer, should be adaptable for communities in developing countries with inadequate or non-existent wastewater treatment facilities and severe resultant public health challenges.
“Modern sewage treatment processes were invented in an era of cheap energy,” Lammers said. “‘Business as usual’ will not supply seven billion humans with clean air, clean water and energy.”
Lammers also stressed that this basic approach could be adapted for waste treatment on large-scale dairy and livestock operations.
By the time this article is published, several designs of the PBR will be installed near existing algae testbeds at NMSU’s Fabian Garcia Science Center west of the Las Cruces campus. These new PBRs will be deployed for initial testing in a hoop house to maximize system heat during the winter months. Full outdoor testing will commence in the late spring. The performance of the various PBR designs and algae types will be evaluated to determine which work best in our local environment.
The multidisciplinary team of researchers on the POWER system includes Shuguang Deng in chemical engineering, Nirmala Khandan in civil engineering, Adrian Unc in plant and environmental sciences and Wayne Van Voorhies in molecular biology.
For more information about algal biofuels research at NMSU go to http://research.nmsu.edu/erl/algalbiofuels/testbed/.
Economics: making algae energy economically viable
Pools of green algae have loads of potential, from creating sustainable fuel to providing renewable sources of nutrients for agriculture and even treating wastewater. The trick is doing it in a way that makes economic sense.
“Making the price competitive for the cultivating, harvesting and processing of algae versus traditional fuels is still something we are working on,” said C. Meghan Downes, an associate professor of economics and international business in the College of Business at NMSU, specializing in sustainability. “Technically, the promise is still there. We just need to overcome some key cost barriers.”
She said researchers are still working to produce enough algae per acre to process the amount of oil needed to be profitable. A continued push from the military to develop these resources is helping to advance the effort, however. Using algae in additional ways, such as in agriculture and in sewage treatment, can also help make the finances more attractive to investors.
“Right now we are working with Boeing on sustainable aviation fuels,” Downes said. “Students in our Doctor of Economic Development program are gaining real-world experience developing models for measuring sustainability.”
She is also assisting researchers using algae and fish to clean wastewater in shrimp farming operations. That process not only produces shrimp for human consumption and cleaner wastewater, but also algae oil for fuel.
“There are still barriers,” Downes said. “But there is nothing so undoable to keep this from becoming a reality.”