Factors Controlling Oxygen Delivery in ALS Hydroponic Systems

Future large-scale ALS systems may use hydroponic nutrient delivery systems (NDS) for growing staple and salad crops. Crop performance is strongly dependent on dissolved O₂ concentration (DOC), pH, and nutrient content of the hydroponic solution. DOC is influenced by solution temperature and flow rate, growth rate of the crop, and the bacterial community present in the solution. Solution temperature determines the solubility of O₂ in water, and may increase as solution volume is reduced to minimize overall system mass. Flow may be altered when the height of the solution bathing the root zone is reduced because of dense root mats. These factors may produce anaerobic pockets where N₂ losses by denitrification occur, but without yield losses, as long as sufficient O₂ to meet root respiration is supplied. Bacterial communities may compete for DOC with the plants when the NDS is used for recycling gray water streams. Aeration experiments were conducted on a hydroponic system to calculate mass transfer coefficients for O₂ as a function of flow rate. These coefficients decreased as flow rate increased, and flow rate was more important than solution temperature in controlling DOC. This analysis was used to suggest minimum flow rates and maximum solution temperatures for meeting gray water recycling O₂ demand for a crop growing hydroponically at low light. (Supported by NASA’s Fundamental Space Biology NRA program through NCC-0027).