Spatial Variation of Longitudinal Dispersion in LECA-Based Vegetated Beds

The evaluation of the dispersion in vegetated beds may allow indentifying mechanisms that affect the transport and reaction of solutes, namely organic and nitrogen compounds. A set of non-reactive tracer experiments (slag injection) was performed in a vegetated bed (a mesocosm with a LECA-based substratum and colonized with Phragmites australis) used for the removal of organic and nitrogen pollutant loads. Loads of approximately 300 mg COD/L and 30 mg NH4-N/L and a hydraulic loading rate of 3.5 cm/d were used. The results showed a delay in all the residence time distribution (RTD) curves and a variation in the dimensionless residence time (mu((m,theta))) of the E(theta) curves, which means that the mass centre of the impulse was late relatively to the expected one. A strong dispersion and tracer retention (due to the presence of stagnated areas and internal recirculation) was observed, especially in the first 33 cm of the bed, which seems to have been related to the presence of complex clusters of roots, solid material, biofilm and LECA particles. An analytical solution of the Multiple-Tanks-in-Series (MTS) model well represents the RTD curves obtained in the tracer experiments. The detected dispersion and dead volume ratios (7% to 12%) did not affect the performance of the bed, which presented mean removal efficiencies of 85% and 60.4% for COD and NH4-N, respectively.