Batch-operation as a method to enhance oxygen supply in a constructed wetland

Abstract

The removal of organic matter and nitrogen in constructed wetlands (CW) is highly dependent on the availability of dissolved oxygen in the treatment system. Therefore it is of great importance to develop innovative methods to enhance oxygen supply in the CW matrix. This paper focuses on and describes the methods used to enhance oxygen supply in CWs. Also, a pilot scale experimental batch-operated lightweight aggregate (LWA) filter system (FS) consisting of eight identical LWA filter cells (with a depth of 1.15 m and an area of 1 m each) was established during the summer of 2005 for the treatment of municipal wastewater. The main objective of this study was to evaluate the performance of the experimental batch-operated FS on the basis of performance indicators. During the experiments, the effect of different operational regimes on the purification efficiency of the filter system was also investigated. The highest purification efficiencies of 96 and 51 for BOD7 and Ntot, respectively, were achieved when the recirculation rate of 200 was applied at the hydraulic retention time of 2 days. The highest organic matter removal rate of the experimental FS as KBHT0.19 m d- is approximately two times higher than the removal rate of typical horizontal-flow CWs. However, the highest aeration capacity of 21.1 gO2 m- d- is somewhat lower than the average aeration capacity of vertical-flow CWs, and substantially lower than the aeration capacity of a VF filter with re-circulation or tidal-flow systems. As the aeration capacity of the system partly depends on the oxygen demand of inflow wastewater, it is necessary to carry out studies of shorter hydraulic loading rates in order to optimize the performance of experimental batch-operated FSs.