Recirculation Effects in a Septic Tank Followed by a Gravel-Based Vertical Flow Constructed Wetland System

High Resolution Image Download MS PowerPoint Slide The recirculation of effluent from vertical flow constructed wetlands (VFCW) to a septic tank (ST) is a strategy for total nitrogen removal through denitrification, given the VFCW’s effective nitrification. Therefore, understanding its effects is essential for optimizing performance and achieving sustainability. This study evaluated the effect of recirculation on a system with an ST and a VFCW with gravel-bed media. Two operational phases were compared for domestic wastewater treatment (flow = 1.45 m 3 d –1 ). In Phase 1, recirculation was not used, while in Phase 2, 50% of the VFCW effluent was recirculated to the ST. Monitoring included flow measurements and physicochemical analyses, enabling mass balance calculations. Next-generation sequencing assessed the recirculation’s impact on bacterial populations. Recirculation reduced hydraulic retention times in both the ST and VFCW but did not affect organic matter and solids removal. Nitrification was effective in the coarse gravel VFCW, though ammonia-oxidizing bacteria were scarce, and nitrite-oxidizing bacteria were absent in the 0–0.50 m layer, suggesting nitrification in deeper layers or via unconventional pathways. Recirculation increased denitrifying bacteria in the ST, enabling high denitrification rates. These results demonstrate recirculation’s potential as a simple, cost-effective method to improve the performance of an ST followed by a gravel-based VFCW, especially for nitrogen removal.