Tracer study of the hydraulic performance of constructed wetlands planted with three different aquatic plant species

•Artemisia selengensis, Juncus effusus and Iris sibirica wetlands are studied.•Juncus effusus wetland exerts the best performance among the tested wetlands.•The degree of plug flow is not the criterion to assess hydraulic performance.•Hydraulic indicators N and λ are not recommended as the appropriate parameters.•e, Imoment, t10 and t90/t10 are suggested as the most representative indicators.

The species, density and distribution of aquatic plant affects the hydraulic performance of surface flow constructed wetlands, and further affects the purification of wastewater. In this study, Tracer tests were conducted in three free surface flow constructed wetlands planted with different species of aquatic plants, i.e. Artemisia selengensis, Juncus effusus and Iris sibirica. These tests were repeated three times in each wetland. The optimal hydraulic indicators measuring the hydraulic performance were measured using a significance test and representative analysis. The results showed that the constructed wetland planted with Juncus effusus, characterized by thinner stalks and a higher planting density, was significantly better than those planted with Artemisia selengensis and Iris sibirica in both the effective volume ratio e and moment index Imoment. There was no significant difference between the wetland planted with Artemisia selengensis and the one planted with Iris sibirica in any of the analyzed hydraulic indicators. The results showed that the wetland planted with Juncus effusus exhibited the maximum values of effective volume rate e, moment index Imoment, short circuiting index t10 and mixing index t90/t10 and had the minimum of number of tanks in the TIS model N. Therefore, the Juncus effusus wetland displayed the best hydraulic performance and the most abundant flow mixture among the tested wetlands. The results also revealed that the degree of plug flow or completely mixed flow in a constructed wetland should not be used as a criterion for evaluating hydraulic performance. N and λ were not recommended as the measurement indicators. The effective volume ratio e, moment index Imoment, short circuiting index t10 and mixing index t90/t10 were selected as the optimal indicators for measuring the flow conditions in free water surface flow constructed wetlands.