Performance of two small subsurface flow constructed wetlands treating domestic wastewaters in Italy

The performance of a vertical and a horizontal subsurface flow wetland (v-SSF and h-SSF), designed for treating domestic wastewater from a single family, was investigated by monitoring total nitrogen (TN), nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), total phosphorus (TP), chemical oxygen demand (COD) and the dissolved oxygen (DO) content of the influent and the effluent wastewater of each system during the first two years of operation. The growth of Phragmites australis in each system was recorded by measuring the height and observing their general conditions. The treated domestic wastewater presented similar chemical-physical characteristics in the two systems which operated in analogous environmental conditions. The median influent characteristics were: TN 81.9mgL(-1), NO3-N 0.19mgL(-1), NH4-N 33.5mgL(-1), TP 11.9mgL(-1) and COD 354.5mgL(-1). During the whole monitoring period median reductions in the v-SSF were TN 71%, NH4-N 94%, TP 27% and COD 92% whereas in the h-SSF they were TN 59%, NH4-N 21%, TP 52% and COD 70%. Internal production of NO3-N was observed, mainly in the v-SSF probably due to the difference in oxygen availability in the medium and the design of the system. DO concentration increased in the effluents in both years, with higher values measured in v-SSF than in h-SSF. The reduction performance increased in the second year, particularly in v-SSF, whereas no statistical differences were observed between spring-summer and autumn-winter periods. P. australis reached maximum development at the end of summer in both systems and maintained a stable height during autumn-winter. In h-SSF the vegetation located close to the influent showed lower growth than in the rest of the bed.