Abstract:
Phytoremediation technologies are employed worldwide to remove nutrient pollutants
from agricultural and industrial wastewater. Unlike in algae-based nutrient removal, control methodologies for plant-based remediation have not been standardized. Control systems that guarantee
consistently low outlet concentrations of nitrogen and phosphorous often use expensive analytical
instruments and are therefore rarely viable. In this study, pH measurement was used as the sole input
to control the nitrate outlet concentration in a continuously operated Lemna minor (lesser duckweed)
phytoremediation tank. When grown in 20 L batches of modified Hoagland’s solution, it was found
that a constant ratio exists 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 nitrate uptake rates were
determined by standard spetrophotometric method. At critically low nitrate concentrations, this
ratio reduced slightly to 1.08 mol N·(mol H+)
−1
. Assuming a constant nitrogen content, the biomass
growth rate could be predicted based on the acid dosing rate. A proportional-integral controller was
used to maintain pH on 6.5 in a semi-continuously operated 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 from synthetic wastewater while
a constant biomass layer was maintained. This study presents a clear illustration of how advanced
chemical engineering control principles can be applied in phytoremediation processes.
