Inferential control of the phosphate concentration in hydroponic systems via measurement of the nutrient solution's pH-buffering capacity

Urban hydroponics is expanding on a global scale. This provides environmentalists with an ideal opportunity to couple sustainability principles to the development of the sector. A primary concern in hydroponics is the generation of large amounts of nutrient laden wastewater. Unlike conventional agriculture, the water effluent typically exits the urban farm via a dedicated outlet, where water quality can be monitored and regulated. It is therefore imperative to develop processes in which the quality of the exit water is part of the design objective. Minimisation of nitrogen and phosphorous in the effluent is key in this regard. This study presents an affordable technique to control phosphate levels in a hydroponic unit. The technique relies solely on continuous pH measurement from which the pH-buffering capacity of the nutrient solution is calculated. The pH-buffering capacity is then used as an inferential measurement to control the phosphate concentration in solution. This concentration can then be controlled at significantly lower levels and thereby reduce phosphate spillage proportionally. Experiments were performed in which the phosphate concentration was controlled at 1 ±0.08 mM (representing standard protocol), 0.2 ±0.03 mM and 0.1 ±0.03 mM. No significant variation in plant growth rate or leaf mass fraction was observed in the runs with lower phosphate concentrations, suggesting that optimal growth is possible at phosphate levels ten times lower than conventional practice. Since the phosphate concentration in the hydroponic outlet is directly proportional to the load of phosphate spillage, the results are encouraging from a nutrient pollution perspective.