Of the biofuel feedstocks investigated to reduce the nation's dependence on foreign oil, algae are the most promising due to their high lipid content, rapid growth rate, and ability to sequester CO2. Algae do not compete with existing food commodities, can grow on marginal lands not suitable for agriculture, and do not require large volumes of fresh water. Moreover, algae can remove nitrogen and phosphorus from nutrient-rich wastewaters, thus reducing eutrophication of receiving water bodies that cause gross disruptions of ecosystems, such as the Gulf of Mexico hypoxia. Algal production has been estimated to yield 3,200 to 14,600 gallons of oil/acre/year, but mass culture efforts have yielded 10-20 times less oil than expected. Outdoor bioreactors that are subject to natural variations and function as ecosystems have not mimicked pure laboratory reactors, and sustainable production of algae will require an abundant and nutrient-rich water source, which is unlikely to support the mass cultivation of pure cultures. This project incorporates ecological and reactor engineering concepts to develop a sustainable algal growth system in wastewater-fed, outdoor bioreactors. Four 2600-gallon continuous-flow reactors with a ten day hydraulic residence time were operated at the Lawrence, KS Wastewater Treatment Plant for six months in 2009. Top-down ecological control practices were applied to two of the reactors in order to increase and stabilize algal production.
The growth of algae growing in mixed-species, outdoor community cultures will be regulated by their supplies of macronutrients (nitrogen and phosphorus), carbon dioxide, and incident light. These algal species will vary significantly in size and shape; some of them will be small and edible by algae-eating zooplankton, while some of them will be large and inedible. The amount of lipid that can be produced from these algae will be constrained by zooplankton grazing in the bioreactor. During the past 25 years, a very large and globally-accepted body of literature has been developed which indicates that in natural lakes, algal biomass losses to zooplankton grazing can be minimized by including zooplanktivorous fish in the food web (top-down ecological control; see Fig. 2 from Smith et al. 2010). Likewise, algal biomass in outdoor bioreactors can be strongly affected by food web structure. Fishless bioreactors can be expected to develop large populations of large-bodied herbivorous zooplankton, thus exhibiting significantly lower algal biomass than would be predicted from their nutrient supply, relative to bioreactors that contain zooplanktivorous fish and smaller, less-efficient grazers.
One of our primary research goals was to determine whether the ecological concept of top-down control applies to microalgae grown in outdoor bioreactors. If this concept indeed applies, then careful control of food-web structure will allow for the design and operation of future large-scale outdoor bioreactors or ponds for algal biomass production – without the danger of algal population crashes due to predation.
Highlights of Results • Algae may be successfully cultivated in open pond reactors using wastewater effluent as an N and P source. • Top-down ecological control practices can decrease losses of algae to zooplankton predation. • The % lipid content of algae cultivated in the field is consistent with laboratory studies.
References Smith VH, Sturm BSM, Billings S, deNoyelles FJ. 2010. “Ecological aspects of algal biodiesel production”. Trends in Ecology & Evolution, 25(5), 301-309.