A Numerical Investigation Concerning the Effect of Step-Feeding on Performance of Constructed Wetlands Operating under Mediterranean Conditions

The effects of wastewater step-feeding (SF) on the performance of horizontal subsurface flow constructed wetlands (HSF CWs) are numerically investigated. The purpose is to check if this alternative feeding technique increases the ability of HSF CWs to remove pollutants. Two methodologies are used: Initially, the tanks-in-series (TIS) methodology, based on the finite volume method (FVM), is analyzed using the volumetric degradation coefficient λ. In this case, the operation of a CW is similar to a series of continuous stirred-tank reactors (CSTRs) operating under steady conditions. Then, the step-feeding (SF) procedure is presented, in which the CW is operated like a plug flow reactor (PFR). For the numerical investigation, the available experimental data for five existing HSF CWs are used. The results show that SF does not improve the performance of HSF CWs in removing biochemical oxygen demand (BOD) operating under Mediterranean conditions. When the HSF CWs operate without the SF procedure, the performance is between 55 and 81% for the TIS method and 60 and 89% for the PFR method, while the ability of the CW tank to remove the BOD decreases and varies from 48 to 79% (TIS) and from 54 to 86% (PFR), respectively.