Effect of controlled aeration on COD and nitrogen removal in aerated constructed wetlands used for effluent polishing

Aerated wetland (AW) technology is typically used in high oxygen demand situations. This study however investigated its applicability for further polishing of effluents of both a municipal and an industrial wastewater treatment plant. Different aeration and pollutant removal strategies were tested, all under high hydraulic loading rate conditions (0.69 m3.m−2.d−1, 12h hydraulic retention time). The experiments were done on two 350 m² horizontal subsurface flow constructed wetlands (HSSF CW), filled with expanded clay aggregates (Argex™) and planted with common reed (Phragmites australis). Each CW was divided into three equal zones, each equipped with forced bed aeration (FBA™). For this study, continuous (100% on) and time-based (50%-time on/off) aeration were compared versus set dissolved oxygen levels controlled by oxygen sensors (3–4, 2–3 and 1–2 mgO2. L−1 in each zone, and 2–3 mgO2. L−1 in zone 1 and no aeration in zones 2 and 3, designated as 2–3/0/0). Results showed near 100% nitrification for all aeration regimes. Chemical oxygen demand (COD) removal was between 18% and 33% during high aeration (100%, 50%, 3–4 and 2–3 mgO2. L−1) but was reduced (1–15%) during the most limiting aeration modes (1–2 and 2–3/0/0 mgO2. L−1). Denitrification was limited for municipal effluent, however, reasonable NO3-N removal (31–72%) was noted for industrial effluent. The optimal balance between removal efficiency and energy consumption was found to be for the 2–3 mgO2. L−1 aeration setting, consuming 6.9–11.2 Wh.m−3. The outcomes of this study can be helpful for implementing aerated CWs as a tertiary treatment.