Development of single well push–pull tests for measuring local rate constants in horizontal subsurface flow constructed wetlands

Abstract

The single well push–pull test (SWPPT) involves injecting a solution into a well and later extracting it to assess reaction rates by analyzing changes in concentration over time. This study first optimized the conditions for conducting SWPPTs in a horizontal subsurface flow constructed wetland (HSSFCW) and then determined in situ heterotrophic aerobic and anoxic reaction rates in the influent zone of the HSSFCW. The optimal tracer concentration and injection solution volume for the SWPPT were determined to be 0.3 g NaCl L^-1 and 10 L, respectively. Rate constants were derived using two different models: well-mixed and plug-flow. The average well-mixed and plug-flow first-order oxygen rate constants were 0.95 ± 0.20 and 0.66 ± 0.14 min⁻¹, respectively. After adding an easily degradable carbon source to the aerated injection solution, the heterotrophic yield coefficient was calculated to be 0.50 gCOD gCOD^-1. First-order denitrification rate constants varied across the HSSFCW influent zone, ranging between 0.005–0.035 min⁻¹ for the well-mixed model and 0.004–0.027 min⁻¹ for the plug-flow model. The heterotrophic denitrification yield coefficient Yd_H values averaged at 0.55 ± 0.33. Different oxygen utilization routes were identified, with the bulk consumed in multiple biological processes, a small portion lost to trapped gas bubbles and an insignificant amount to chemical reactions. The results of this study demonstrate the potential of using SWPPTs to measure heterotrophic aerobic and denitrifying degradation rate constants and yield in situ in HSSFCWs.