Effect of step-feeding on the performance of lab-scale columns simulating vertical flow-horizontal flow constructed wetlands

The effect of step-feeding (untreated wastewater by-pass) on the performance of lab-scale columns simulating a hybrid vertical flow (VF)-horizontal flow (HF) constructed wetland (CW) system was studied. Step-feeding strategies have been adopted in several kinds of CW, but this is the first report about the use of step-feeding in VF + HF hybrid systems treating domestic wastewater. Applied loading rates were 7–11 g BOD5/m2 day and 2.1–3.4 g TN/m2 day (overall system). Removal efficiency reached 98% TSS and COD and 99% BOD5 on average, whilst a 50% by-pass improved TN removal from 31 to 50%. Maximum surface nitrification rate (5.5 g N/m2 day) was obtained in VF unit, whilst maximum denitrification rate (1.8 g N/m2 day) was observed in HF unit. Referred to the overall system, maximum surface nitrification and denitrification rates were 2.2 and 1.6 g N/m2 day, respectively. However, potential nitrifying and denitrifying activities (batch assays) were 15.0 and 58.9 g N/m2 day, respectively. Even at 50% by-pass, operational conditions in HF unit (dissolved oxygen, redox, COD/TN ratio) were not suitable enough for denitrification. However, methane emissions were not observed and nitrous oxide emissions were relatively low.