Real-time monitoring of chemical water quality in closed-loop hydroponics

Through the MELiSSA program, the European Space Agency aims at a closed regenerative life support system for long-term space missions, inspired by a terrestrial ecosystem. Towards detailed understanding and prediction of plant growth and metabolism, a Plant Characterisation Unit for crop experiments was designed and manufactured. The hydroponic sub-system requires sensors for real-time monitoring and subsequent control. Based on foreseen use of the hydroponic cultivation facility, requirements for monitoring of chemical water quality were established, and various analytical techniques were assessed. Commercially available in-line and at-line technologies were selected for laboratory testing, covering all mandatory parameters (electrical conductivity, pH, temperature, dissolved O₂ and dissolved CO₂), and multiple macro- and micronutrients. Laboratory sensor evaluation emphasized accuracy and stability and included long-term monitoring of closed-loop hydroponics. A sensor set was identified within a limited budget, allowing precise monitoring and automatic control of the nutrient solution pH and conductivity, in addition to monitoring of dissolved oxygen and carbon dioxide. Additionally, sensors for macronutrients and some micronutrients were identified within given limitations. Although requiring dilution of the nutrient solution, an optical NO₃^- sensor demonstrated promising accuracy, stability and selectivity. Results based on ion specific electrodes demonstrate the potential of reasonably priced, commercially available sensor systems for real-time monitoring of nutrients, especially when accompanied by automatic recalibration. However, results also illustrate challenges for long-term use, for example as input to automatic control systems or for harvesting accurate, scientific data. Important challenges include drift, selectivity and stability, with biofouling adding complications upon continuous long-term use. Improvement of sensor performance was achieved through refined calibration protocols, sensor cleaning and recalibration. A list of sensors implemented in the Plant Characterisation Unit is presented, in addition to results and experiences from sensor testing, representing a possible starting-point or comparison for instrumentation of similar cultivation systems.