… life cycle analyses (LCAs) that consider all energy inputs to algae production indicate that fertilizer inputs may contribute to poor energy gain in algae cultivation including biodiesel production from microalgae produced in outdoor ponds. Most of these studies have assumed that the algal ponds will be supplied with chemical fertilizers in order to meet the N and P requirements for algal growth. It was found that algal biodiesel could actually generate greenhouse gas emissions and require significantly more energy inputs compared to conventional crops. At the same time it has also been proposed that most of the environmental burden associated with algae could be removed if wastewater was used as a nutrient source. … Use of treated wastewater as a fertilizer source for algal growth eliminates the energy used in producing chemical fertilizers while simultaneously reducing eutrophication through removal of nutrients (mainly N and P) from wastewater.

Cultivating algae in open reactors that may be colonized by other algae and by herbivorous invertebrates presents challenges in terms of stability of production. It has been suggested that diverse algal assemblages may contribute to more productive and / or more stable open reactors because of a generally positive relationship among plant diversity, stability, and productivity but tests of this are lacking in algal production systems.

… Because an abundance of planktivorous fish is often associated with suppression of herbivorous zooplankton (that naturally colonize outdoor tanks) and high standing crops of algae, it has been suggested that including planktivorous fish increases algal production.

—Bhattacharjee and Siemann

In their study, the Rice researchers used open tank systems which integrated wastewater treatment and algal biomass production to test whether 1) diverse assemblages of algae are more resistant to invasion and have greater, more stable productivity (mass, lipids, fatty acid methyl ester [FAME]) than low diversity algal assemblages; 2) inclusion of planktivorous fish increases stability and productivity of these open tank systems; and 3) greater algal diversity and / or the presence of planktivorous fish increase the removal of nutrients (N & P) from wastewater.

The Rice study was made possible by the participation of the Houston Department of Public Works and Engineering, which helped Rice’s research team set up a test involving 12 open-topped 600-gallon tanks at one of the city’s satellite wastewater treatment plants in July 2013. The tanks were fed with filtered wastewater from the plant’s clarifiers, which remove suspended solids from sewage. Various formulations of algae were tested in each tank.

Some were monocultures of oil-rich algal strains and others contained mixed cultures, including some with local algal strains from Houston bayous. Some tanks contained fish that preyed upon algae-eating zooplankton.

While the Rice team recorded prolific algal growth in all 12 tanks, the monocultures produced more algal and fatty acid methyl ester (FAME) mass than the diverse tanks. More than 80% of lipids were converted to FAME indicating potentially high production for conversion to biodiesel (up to 0.9 T ha^-1 y^-1). Prolific algal growth lowered temperature and levels of total dissolved solids in the tanks and increased pH and dissolved oxygen compared to supply water. The monocultures were not invaded by other algal species; the presence of fish did not affect any variables.

The results of this pilot experiment are not equivalent to establishing long-term, large-scale, commercial efforts. However, they add to the set of demonstrations that algal biomass can be grown in open tank bioreactors using wastewater as a nutrient source for biodiesel production. Perhaps the most interesting aspect of this study was the successful cultivation of stable and highly productive monocultures in open tanks which challenges the idea that wastewater-fed open pond systems will quickly become contaminated and therefore are not suitable for commercial production systems. Rates of nutrient removal, especially nitrogen, from wastewater were also much higher then what has been previously reported. The marked differences between this study and others motivate additional studies to examine the factors which underlie these large differences in productivity and nutrient removal including variation in wastewater chemistry, temperature, and naturally recruiting predators of zoo- plankton.

—Bhattacharjee and Siemann

Bhattacharjee said more research is needed to determine whether wastewater-based algaculture will be cost-effective and under what circumstances. For instance, the algae in the Rice study was four times more effective at removing phosphorous than were the algae in a Kansas study. She said that could be because the Houston test was performed in summer and fall, and the tanks were about 30 degrees warmer on average than the tanks in Kansas.

The research was supported by Rice University.

Resources

• Meenakshi Bhattacharjee and Evan Siemann (2015) “Low algal diversity systems are a promising method for biodiesel production in wastewater fed open reactors” Algae, vol. 30, no. 1, pp.67-79 doi: 10.4490/algae.2015.30.1.067