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.

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 using algal photosynthesis yields more biomass than the sludge output of current systems," Lammers said.

"The economic key to the POWER system is converting algal biomass into liquid fuel and electric power."

 

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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 enclosed 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 intense summer heat from 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.