Effect of carbon dosing on denitrification in an aerated horizontal subsurface flow constructed wetland used for effluent polishing

Aerated wetlands (AW) are normally used for high oxygen demand situations and were proven optimal for nitrification. However, denitrification remains a concern in AWs especially due to unfavorable oxygen concen-trations and lack of biodegradable organic matter (OM). In this study, a 350 m2 aerated horizontal subsurface flow constructed wetland (HSSF CW) treating the effluent of a municipal wastewater treatment plant, was tested in a high hydraulic loading rate situation (0.69 m3.m- 2.d-1, 12 h hydraulic retention time), in combination with external carbon dosing to promote denitrification. Forced bed aeration (FBATM) allowed to independently aerate three equally sized zones along the flow direction, and was controlled by a Dissolved Oxygen (DO) sensor in each zone. Zone 1 was destined for nitrification (DO = 2-3 mg.O2.L-1), zone 2 for denitrification (no aeration) with the addition of a carbon source (Brenntaplus VP-1), and zone 3 was again aerated (DO = 2-3 mg.O2.L-1) to degrade any residual OM. Four different carbon dosages (COD/N 0, 2.0, 3.5 and 4.0) were applied at the start of zone 2, based on online measured nitrate data. The results revealed that nitrification remained optimal (+/- 100%) throughout the experiment despite the lower water temperature (10-15 degrees C). Total nitrogen (TN) removal increased from 23 to 59% as the COD/N ratio increased from 0 to 4. However, the COD removal efficiency remained moderate during initial dosing (21 and 18% for COD/N = 0 and 2, respectively) and further decreased with higher carbon dosing (no removal for COD/N = 4.0). The outcomes of this study can be helpful for implementing carbon dosing to enhance nitrate removal in full-scale AWs used for tertiary treatment.

Automated summary

This study successfully enhanced nitrate removal efficiency by adding a carbon source in an aerated horizontal subsurface flow constructed wetland. However, the efficiency of COD removal decreased with higher carbon dosing.