Treating municipal wastewater through a vegetation filter with a short-rotation poplar species

The performance of a vegetation filter using a short-rotation coppice of poplars was evaluated over a 3-year period in terms of pollutant removal capacity. The vegetation filter was designed for scattered and small populations with no storage facilities and a wastewater application constrained by the own production of effluent. Wastewater effluent was pre-treated in an Imhoff tank and applied to the vegetation filter. The chemical compositions of drainage water and groundwater were regularly monitored. Surface soil samples at the beginning and the end of the study were also collected. The monitored chemical species in drainage water and groundwater were DOC, COD, NT, NO3-N, NH4-N, PT, PO4-P, and other major ions. Electrical conductivity, organic matter content (%), NO3-N, available P, cation exchange capacity and major cations were analysed for soil. The vegetation filter presented efficient removal of wastewater-originated pollutants. DOC and COD removal reached values of 85%. A correlated increase in soil organic matter content was detected (from 1.0% to almost 2.8%). A similar removal capacity was observed for PT which is interpreted as due to plant uptake mechanisms and PO43− precipitation in the presence of soil Ca2+. Around 73% of NT was removed. However, due to the high applied NT load, the average NT concentration in drainage water was about 41.9 mg/L, higher than the admissible concentration limit. When considering NT mass, about 10% of the cumulative applied NT leached through the vadose zone. Groundwater quality was not affected by the vegetation filter operation.