Effect of different bypass rates and unit area ratio in hybrid constructed wetlands.

Abstract

This study presents the performance of a hybrid constructed wetland (Bp(VF + HF)2:1) system which consists of an unsaturated vertical flow (VF) unit followed by a saturated down-flow unit simulating horizontal flow (HF) with HF/VF area ratio of 0.5 and influent bypass to the HF unit. Treating synthetic wastewater simulating municipal wastewater, optimum total nitrogen (TN) removal (57%) was reached at 39% bypass and surface loading rate (SLR) of 33g BOD5/m2day and 9.7g TN/m2day (overall system). On the other hand, treating actual municipal wastewater, the system reached 63% TN removal at 30% bypass and SLR of 18g BOD5/m2day and 4.7g TN/m2day. Surface removal rates reached 5.5 and 3.0g TN/m2day for synthetic and municipal wastewater. Surface nitrification rate in the VF unit was in the range of 5.0-7.4 and 3.6-3.8gN/m2day for synthetic and municipal wastewater, respectively, indicating a large effect of wastewater characteristics on the nitrification process. Infiltration rate in the VF unit remained high and far from clogging risk. Overall greenhouse gas emissions were 0.11 (N2O) and 0.41 (CH4) g/m2day which corresponded to emissions factors (relative to total organic carbon and TN influent) of 0.7% (N2O) and 3.6% (CH4). Compared with a similar system with a different HF/VF area ratio of 2.0, organic matter and nitrogen removal efficiency was similar, but surface removal rates were about 3 times higher.