Performance dynamics of a LWA-filled hybrid constructed wetland in Estonia

Purification efficiency and mass removal of organic matter, suspended solids, nitrogen and phosphorus was analysed in a hybrid constructed wetland (CW) system with two subsurface flow filter beds, using Light Weight Aggregates (LWA), a two-chamber vertical subsurface flow filter (VSSF) followed by a horizontal subsurface flow filter (HSSF) with a total area of 432 m2. The analyses show purification effect: for BOD7 the average purification efficiency is 86%, for suspended solids (SS) 84%, for total P 89%, for total N 53%, and for NH4-N 73%. The average outlet values for the above-listed parameters were 7.2, 6.0, 0.46, 20.4 and 9.0 mg dm–3 respectively. The purification parameters meet the standards set by the Water Act of Estonia for wastewater treatment plants. The results show that hybrid CW systems consisting of subsurface flow filters can work efficiently in Estonia's cold climate. Using locally produced LWA as a filter material in CWs has shown good hydraulic conductivity and phosphorus sorption capacity. The Paistu CW, with its proper design and outstanding purification results, can be considered one of the best systems in Estonia.