Online Control of Lemna minor L. Phytoremediation : Using pH to Minimize the Nitrogen Outlet Concentration

Abstract

Ecological remediation has gained significant attention recently due to the adverse ramifications of anthropogenic waste in water. For several years, natural processes have been incorporated in centralised wastewater treatment. Furthermore, natural wetland systems and constructed wetland systems are instances where aquatic ecosystems are able to facilitate high impact pollutant removal. As such, ecological phytoremediation technologies are employed worldwide to remove nutrient pollutants from agricultural and industrial wastewater. Unfortunately, standard process control methodology in phytoremediation systems has not been fully realised — a common rationale is that plant-based technologies have been limited for use in large open-air environments like the aforementioned wetlands, lagoons, and stabilisation ponds. It is understood that with adequate control system infrastructure, nutrient removal is greatly improved, as is the case with algae-based nutrient removal research. Notwithstanding consistently low outlet concentrations of nitrogen and phosphorous, control systems often involve the use of expensive analytical instruments and are therefore rarely viable. In the current study, Lemna minor (lesser duckweed) was grown in 20 L batches of modified Hoagland’s solution in de-ionised water with pH and level control capabilities. The results present a successful application of phytoremediation process control. Alkalisation of the liquid medium was observed in the pH as a response to the uptake of nitrate. The nitrate uptake was determined by standard spectrophotometric method. Despite the difference in biomass amounts, it was evident that a constant ratio existed between the amount of nitrate removed and the amount of acid dosed (required for pH control), which was equal to 1.25 mol N · (mol H^+)^(−1). The pH response due to the co-absorption of NO^−_3 and H^+ ions made it possible to use the pH measurement as the sole input to control the nitrate outlet concentration. A proportional-integral controller was used to maintain near-neutral pH of 6.5 in a continuously operated phytoremediation tank covered by L. minor. A nitrogen control strategy was developed which exploited this relationship between nitrate uptake and dosing and successfully removed upwards of 80 % of the fed nitrogen. At critically low nitrate concentrations (in the range of 0.05–0.3 mM), the nitrate to proton ratio was reduced to 1.08 mol N · (mol H^+)^(−1). The biomass growth rate was successfully predicted based on the acid-dosing rate. This study demonstrates a clear illustration of how advanced chemical engineering control principles can be applied in phytoremediation processes.