Submitted to: Environmental Engineering Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2013
Publication Date: 3/25/2013
Publication URL: http://handle.nal.usda.gov/10113/56627
Citation: Blersch, D., Kangas, P., Mulbry III, W.W. 2013. Autonomous benthic algal cultivator under feedback control of ecosystem metabolism. Environmental Engineering Science. 60:53-60. Interpretive Summary: Algal turf scrubbing (ATS) is an engineered wastewater treatment system in which lawn-like, filamentous algae are grown in shallow sloping raceways. Algae grow using the nutrients in the wastewater and are removed from the system by weekly harvesting. Beyond their practical application for wastewater treatment, ATS systems are also useful for answering much more basic questions in biology and ecology. Some species of algae grow very quickly within algal turf scrubbers. However, which species dominate and how fast they grow is partially dependent on the water flow rate through the system. The general goal of this research is to develop automated systems in which a characteristic (such as color, growth rate, or size) of the organisms being grown is used to control the system in which the organisms are grown (for example, by affecting the feeding rate, temperature, or amount of light). The specific objective of these experiments was to evaluate how well algal growth rates could be used to automatically optimize water flow rates in an ATS. Results from computer modeling experiments showed that algal growth rates could be used in concert with a feedback control system to optimize water flow rates. However, in practice, the variability of algal growth rates between weekly harvests frequently confused the feedback control system. Consequently, some experimental trials resulted in optimal flow rates but other trials were not successful. Nonetheless, these results will be useful for scientists trying to develop automated control systems and for companies seeking to grow crops of algae using the least amount of energy.
Technical Abstract: An autonomous and internally-controlled techno-ecological hybrid was developed that controls primary production of algae in a laboratory-scale cultivator. The technoecosystem is based on an algal turf scrubber (ATS) system that combines engineered feedback control programming with internal feedback patterns within the ecosystem. An ATS is an engineered, highly-turbulent aquatic system used to cultivate benthic filamentous algae while removing pollutants from an overlying water stream. The feedback control system is designed to monitor the carbon metabolism of the algal turf and manipulate the turbulence regime experienced by the algae to automatically optimize for maximum biomass production. The feedback control algorithm converged quickly on the state of maximum primary production when the variance of the input data was low. However, the convergence rate was slow at moderate levels of input variance. Results show that the system can converge on the maximum algal productivity at the lowest level of turbulence, the most efficient state from an engineering perspective, but in practice the system was confounded by measurement noise. Investigation into the species composition of the dominant algae showed shifting relative abundance for those units under automated control, suggesting that certain species are more suited for utilizing the technological feedback pathways for manipulating the energy signature of their environment. The results have implications for further design of techno-ecological hybrids for the testing of principles of organization for self organizing systems.