Effect of by-pass and effluent recirculation on nitrogen removal in hybrid constructed wetlands for domestic and industrial wastewater treatment

Hybrid constructed wetlands (CWs) including subsurface horizontal flow (HF) and vertical flow (VF) steps look for effective nitrification and denitrification through the combination of anaerobic/anoxic and aerobic conditions. Several CW configurations including several configurations of single pass systems (HF + HF, VF + VF, VF + HF), the Bp(VF + HF) arrangement (with feeding by-pass) and the R(HF + VF) system (with effluent recirculation) were tested treating synthetic domestic wastewater. Two HF/VF area ratios (AR) were tested for the VF + HF and Bp(VF + HF) systems. In addition, a R(VF + VF) system was tested for the treatment of a high strength industrial wastewater. The percentage removal of TSS, COD and BOD5 was usually higher than 95% in all systems. The single pass systems showed TN removal below the threshold of 50% and low removal rates (0.6–1.2 g TN/m2 d), except the VF + VF system which reached 63% and 3.5 g TN/m2 d removal but only at high loading rates. Bp(VF + HF) systems required by-pass ratios of 40–50% and increased TN removal rates to approximately 50–60% in a sustainable manner. Removal rates depended on the AR value, increasing from 1.6 (AR 2.0) to 5.2 g TN/m2 d (AR 0.5), both working with synthetic domestic wastewater. On real domestic wastewater the Bp (VF + HF) (AR 0.5 and 30% by-pass) reached 2.5 g TN/m2 d removal rate. Effluent recirculation significantly improved the TN removal efficiency and rate. The R(HF + VF) system showed stable TN removals of approximately 80% at loading rates ranging from 2 to 8 g TN/m2 d. High TN removal rates (up to 73% TN and 8.4 g TN/m2 d) were also obtained for the R(VF + VF) system treating industrial wastewater. •Single pass systems showed TN removal below 50% at 0.6–1.2 g TN/m2 d.•30–50% by-pass in Bp(VF + HF) system increased TN removal up to 60%.•Removal rates (1.6–5.2 g TN/m2 d) highly depended on HF/VF area ratio in Bp(VF + HF).•Recirculation increased stability and TN removal rates up to 80% and 8 g TN/m2 d.•Recirculating R(VF + VF) system efficiently treated an industrial effluent of 4 g COD/L.