Schoolhouse wastewater purification in a LWA-filled hybrid constructed wetland in Estonia

This paper analyses the purification efficiency and mass removal of organic material, suspended solids, nitrogen and phosphorus in a hybrid constructed wetland (CW) system treating wastewater from a basic school in Paistu, Estonia. The CW consists of two subsurface flow filter beds using lightweight aggregates (LWA): a two-chamber vertical subsurface flow (VSSF) filter bed followed by a horizontal subsurface flow (HSSF) filter bed, with a total area of 432 m2. This CW was constructed in summer 2002 by the Centre for Ecological Engineering in Tartu (CEET). Eighteen series of water samples (from 30.10.2003 to 15.10.2005) were undertaken. The analyses show the outstanding purification effect of the system: for BOD7 the average purification efficiency is 91%; for total suspended solids (TSS)-78%, for total P-89%, for total N-63%, and for NH4N-77%. The average outlet values for the above-listed parameters were 5.5, 7.0, 0.4, 19.2 and 9.1 mg L−1, respectively. According to our results, the purification parameters meet the standards set by the Water Act of Estonia for wastewater treatment plants of 2000-9999 PE: 15, 25, and 1.5 mg L−1 for BOD7, TSS and total P, respectively. The results show that hybrid CW systems consisting of subsurface flow filter beds can work efficiently in conditions of changing hydraulic loading and relatively cold climate. We did not find significant differences between the removal efficiency, mass removal, and values of the first-order rate-constant k for most water quality indicators during the warm (May-October) and cold (November-April) periods. Locally produced LWA as a filter material in CWs has shown good hydraulic conductivity and phosphorus sorption capacity (k = 17.1 ± 12.4 m yr−1). The Paistu CW, with its proper design and outstanding purification results, can be considered one of the best systems in Estonia.