Constructed wetlands for wastewater treatment in Estonia

In Estonia many natural/semi-natural wetlands have been used for municipal or agricultural wastewater. During the last 6 years twelve constructed wetlands for wastewater purification were established. Budgets of organic matter (BOD), total-N and total-P of three systems (a sand/plant filter with vertical flow, established in 1992, area 90 m², loading 3.8 g BOD m⁻² d⁻¹; a combined overland flow-vertical flow root-zone system on a Phalaris arundinacea-slope, 1989, 2400 m² ha, 1 g BOD m⁻² d⁻¹; an aquatic macrophyte channel (bioditch), 140 m², 40 g BOD m⁻² d⁻¹), located in southern Estonia, are analyzed. Except for nitrogen, the efficiency of the sand/plant filter was found satisfactory: 82%, 36%, and 74% for BOD₅, total-N, and total-P, respectively. The poor performance with respect to nitrogen may be caused by weak vegetation. In thePhalaris -system, 65% of organic matter, 67% N, and 80% P was removed. The average output concentrations of this system were always lower than the recommended limits (BOD₅ <10 mg l⁻¹, total-N <10 mg l⁻¹, and total-P <2 mg l⁻¹). Due to the high input load, the BOD₅, total-N, and total-P values in the outlet of the bioditch were high and extremely variable: 5–100, 6–16, and 1–4 mg l⁻¹, respectively. To guarantee the recommendable output values from the bioditch, either a significant decrease of input load or an enlargement of the area of the wetlands would be needed. All results show that compared to other seasons the winter performance was not reduced.