MINIMIZING HYDROPONIC NUTRIENT POLLUTION THROUGH PH-BASED CONTROL ALGORITHMS

Abstract

It is well established that mankind’s current economic practices are unsustainable. Conse-quences such as food shortages and climate change are predicted in the coming decades and efforts to help avert these impending crises remain warranted. This is particularly true for ag-riculture. Not only must food production increase to support the rapidly growing human popu-lation but the environmental impacts caused by agricultural pollution continue to grow in se-verity. Around 50 % of the fertilizer applied to crop fields is washed away into surrounding habitats resulting in eutrophication, biodiversity loss and stratospheric ozone depletion, to name but a few (Kanter et al., 2020). As specified in the Title, the scope of this work surrounds nutrient pollution from hydroponic systems. Soilless agriculture is growing exponentially worldwide and will likely play a key role in the future of sustainable food production. Unlike conventional agriculture, the nutrient solution is physically contained, and thus nutrient discharge can be monitored and controlled. Despite this advantage, hydroponic systems are known to produce large amounts of nutrient laden wastewater. This wastewater results from frequent solution replacements (or high iii throughputs for continuous systems) to maintain high nutrient concentrations and to prevent the build-up of inert and toxic species, which accumulates rapidly due to transpiration. The nutrients of concern are nitrogen and phosphorous. These nutrients are typically limiting in natural ecosystems and thus causes the aforementioned environmental impacts when dis-charged. The aim of this work is therefore to minimize the nitrogen and phosphorous discharge rates from hydroponic systems. This can be accomplished by controlling their concentrations at low levels in solution. This strategy is not new and nutrient concentration control is often employed in hydroponic systems. The electrical conductivity method is the most common but is ill-suited for operation at low concentrations. Ion-selective-electrodes have also been used but these are expensive and generally not economically viable. The novelty of this work lies in the use of pH as the sole measured variable to control the nitrogen and phosphorous concentrations at low levels in hydroponic systems. Nitrogen is sup-plied to hydroponic systems either as nitrate or ammonium, and phosphorous is supplied as phosphate. Separate control methodologies were designed for each of these three nutrients. The control systems were able to reduce nitrogen and phosphorous pollution from the system by around an order of magnitude as compared with traditional hydroponic methods. Advantages and drawbacks are also discussed and compared with existing methods.