Removal efficiency of organic pollutants and suspended solids in constructed wetlands

Abstract

Constructed wetlands (CW) is one of wastewater treatment methods described as plant soil filters. They may be free-water surface wetlands, the so-called macrophytes filters (MF); another type of CW are subsurface flow filters including filters of vertical flow (VF) and horizontal flow (HF). To evaluate BOD5, COD and suspended solids removal efficiency, the database of the studies on different construction of CW collected within the period of 1995–2006 was used. The objective of the studies was to compare BOD5, COD and SS removal efficiency in constructed wetlands of different construction under the climatic conditions of Lithuania. As the comparative analysis has shown, the most efficient removal of mentioned pollutants (when calculating for area unit of a filter) is observed in filters of vertical flow (VF); in filters of horizontal flow (HF) the process is less efficient, while in macrophytes filters (MF) pollutants are removed least efficiently. When the load according to BOD5 is the same in all filters (4 g/m2/d), treatment efficiency in VF reaches 97.0%, in HF and MF it is 77.5% and 34.5% respectively. To reach the normative treatment level according to BOD5 (25 mgO2/l), possible load of VF would be 19.0 g/m2/d BOD5, while the load of HF and MF would be only 6.0 and 3.6 g/m2/d BOD5 respectively. Having calculated the average load of filters according to COD (8 g/m2/d), treatment efficiency in VF reaches 97.5%, in HF it is 80.8%, in MF it is only 32.3%. Under the conditions of normative treatment efficiency, according to COD (125 mgO2/l), COD concentrations contained in wastewater after the treatment process are as follows: VF – 83.7, HF – 72.8, MF – 75.6 mgO2/l. When the load of filters according to suspended solids (SS) is 4 g/m2/d, treatment efficiency in VF reaches 87.3%, in HF it is 67.5%, in MF it is 59.0%.