Modeling framework for control of bacterial nitrification in aquaponics-inspired hydroponic system

Abstract

Aquaponics is an agriculture technique that combines recirculating aquaculture systems (fish farming) with hydroponics (growing plants without soil) in an integrated system. The elegant principle behind aquaponics is that fish waste is used to fertilize plants while plants are used to filter toxins from the fish water. Both fish and plant crops may be harvested for food. This concept relies on nitrifying bacteria to convert Nitrogen from toxic to beneficial forms. The apparent trade-offs between energy and water used in aquaponic systems versus other horticulture and aquaculture techniques is not well understood and has motivated the development in this thesis of a system-level model of aquaponic systems with emphasis on defining input streams used, such as: energy, water, and chemical additives; and output streams produced, such as: fish and plant mass. Input streams were identified as the control effort needed to maintain acceptable conditions. An analysis framework was devised using the costs (or values) of input and output streams identified in the model. A long-term goal is to use this framework to increase system efficiency. A lab was designed to provide preliminary experimentation. Observation of lab start-up has informed experimental procedures including the chemical simulation of aquatic life. The affects of water temperature set-point changes over steady state operation were investigated experimentally by growing Mesclun cultures from seed to harvest at 80 [degrees] F and 70 [degrees] F. Results show that the 70 [degrees] F set-point resulted in a more efficient system than the 80 [degrees] F system due to decreased energy and water use even though plant biomass was also decreased. Nitrification rates were increased at 70 [degrees] F versus 80 [degrees] F as measured by the increased use of chemical additives and is attributed to an increase in measured dissolved Oxygen at 70 [degrees] F. These results provide useful information for future aquaponic system efficiency studies.