Variations of effective volume and removal rate under different water levels of constructed wetland

• Both e and λm decrease with increase of water depth from 20 cm to 60 cm.
• A combination of e and N better reflects the effect of water depth on removal efficiency.
• The reaction rate has a larger effect on the removal efficiency than residence time.
• Increasing water depth does little to improve removal efficiency.

Water depth is a key parameter in the design and operation of constructed wetlands. In this study, the effect of water depth on the effective volume and the removal rate was explored through dye tracer experiments in water depths of 20 cm, 40 cm, and 60 cm at a constructed wetland located at the Center Station of Irrigation Experiment, Nanchang City, China. The One-dimensional Transport with Inflow and Storage (OTIS) model and a so-called CSTRs + PFD (continuously stirred tank reactors + plug flow with dispersion) model were calibrated for analysis of the hydraulic parameters. The results are summarized as follows. (1) When the water depth increased from 20 cm to 60 cm, the hydraulic efficiency λm decreased, as did the effective volume ratio e determined by both moment analysis and the OTIS model. (2) A combination of effective volume ratio and the number of continuously stirred tank reactors, N could better reflect the effect of water depth on the removal efficiency. (3) Theoretical analysis showed that the removal efficiency decreased with increasing water depth, which was caused by a decrease of reaction rate. Therefore, the reaction rate has a larger effect on the removal efficiency than residence time. (4) Dye tracer tests showed that the mean residence time and effective volume first rose and then fell slightly with increasing water depth. The changes in the mean residence time and reaction rate suggest that increasing the water depth would do little to improve the removal efficiency.