Batch feed and intermittent recirculation to increase removed loads in a vertical subsurface flow filter

An alternative configuration of Vertical Subsurface Flow Constructed Wetland (VSSF CW) is presented in this paper. The proposed FID-VSSF system (batch Feed + Intermittent recirculation + free Drainage) is the combination of the following features: fill-and-drain, treatment phase with recirculation, idle period and free drainage. The aim of this configuration is to increase the removed loads in order to reduce the land area requirements of VSSFs. The FID-VSSF was tested under three ranges of applied loads (medium-load of 108 L m−2 d−1, high-load of 168 L m−2 d−1 and overload of 224 L m−2 d−1) and compared with a conventional down flow VSSF (C-VSSF, 68 L m−2 d−1). The average removed organic load in the FID-VSSF was 58.5 gCOD m−2 d−1 compared to the 25.9 gCOD m−2 d−1 removed in the C-VSSF. The removed TKN load in the FID-VSSF was 6.7 gTKN m−2 d−1, significantly higher than 3.6 gTKN m−2 d−1 removed in the C-VSSF. Simultaneous nitrification/denitrification and enhanced TN removal was observed in the FID-VSSF as a consequence of saturated and unsaturated conditions due to the intermittent recirculations and saturated water level on the bottom. A limit of the proposed system, which emerged during the overload period, was the decrease of nitrification rate due to the ponding formation. The use of recirculation demonstrates the possibility of reducing the area requirement of VSSF wetlands to 1.4 m2/PE with the FID-VSSF configuration (compared to 3.5 m2/PE of a conventional configuration) or of treating higher loads during seasonal peaks. The FID-VSSF configuration, made up of recirculation pipes, simple automatic valves and a small pumping station, results in low energy consumption yet allows land area reduction and improvements in nitrogen removal within the system.