Treatment at different depths and vertical mixing within a 1-m deep horizontal subsurface-flow wetland

The majority of BOD5 removal occurred within the first-third of the bed where vegetation cover was poor. Performance of the bed declined over time from Trial 1 to Trial 2, possibly due to a cumulative build-up of organic matter within the substrate as a result of limited oxygen transfer throughout the 1.0 m depth of substrate via root leakage or diffusion across the air-water interface. Root penetration was limited to the upper 0.4 m of the substrate, with the majority of below-ground biomass forming a dense mat in the upper 0.2 m. A comparison of two-parameter (K-C*) first-order volumetric rate constant (Kv20) with those obtained from 0.4 to 0.6 m deep HSSF-CW in the same region indicate that a doubling of the wetted depth resulted in no improvement in BOD5 removal and a decline in TN removal on an areal basis. Further investigations are warranted, comparing the performance of replicated beds spanning a range of depths (e.g. 0.25, 0.5 and 1.0 m) in order to quantitatively determine the optimal depth of HSSF-CWs treating domestic wastewater.