Combination of emergent and submerged plants has been proved to be able to enhance pollutant removal efficiency of surface flow-constructed wetland (SFCW) during winter. However, intensive photosynthesis of submerged plants during summer would cause pH increase, which may have adverse effects on emergent plants. In this study, nitrogen transformation of lab-scale SFCW under pH gradient of 7.5, 8.5, 9.5 and 10.5 was systematically investigated. The results showed that total nitrogen (TN) removal efficiency decreased from 76.3 +/- 0.04 to 51.8 +/- 0.04 % when pH increased from 7.5 to 10.5, which was mainly attributed to plant assimilation decay and inhibition of microbe activities (i.e., nitrite-oxidizing bacteria and denitrifiers). Besides, the highest sediment adsorption in SFCW was observed at pH of 8.5. In general, the combination of submerged and emergent plants is feasible for most of the year, but precaution should be taken to mitigate the negative effect of high alkaline conditions when pH rises to above 8.5 in midsummer.

The treatment efficiency of constructed wetlands (CWs) is highly dependent on the stability of the hydraulic flow patterns. To date, general technologies used to study hydraulic flow patterns of CWs mainly include tracer method, model simulation and velocity measurement, which are either expensive, empirical, or having secondary pollution. In this study, a new technology, which was based on the isotopic composition variation in CWs, was applied to detect the hydraulic flow patterns of two different CWs. Results showed that the hydraulic flow patterns of the two studied wetlands could be detected effectively by using hydrogen and oxygen isotopes. Furthermore, the locations of stagnant areas (SAS) and preferential flow areas (PFAs) were also determined. Significant regional difference in isotopic composition existed inside each CW, and two wetland design suggestions are proposed after hydraulic analysis. One is that the influent of CWs is supposed to be distributed uniformly, and another piece of advice is that the vegetation in the direction perpendicular to water flow should be maintained at the same types and density. (C) 2016 Elsevier B.V. All rights reserved.