Role of Terrestrial Ecosystems Planted with Ligneous Species in M.H.E.A. System Receiving Urban Wastewaters

Tests were carried out in a small-scale experimental plant. The system consisted of aquatic ecosystems without macrophytes (stage I) followed by semi-aquatic ecosystems planted with Typha latifolia L. (stage II). The outflow of stage II was evenly distributed into 10 units of terrestrial ecosystems (stage III), planted with ligneous species or without vegetation (control). The whole plant received a hydraulic load of 384 1/day of urban wastewaters. The net treatment area per person equivalent (PE) was 8 m2. The study focused on the evaluation of the role of terrestrial ecosystems mainly in tertiary treatment (nitrogen and phosphorus removal). Stages I and II with a surface area of 4 m2/PE assure a primary and secondary purification level that conforms with European standards. As for the tertiary treatment, removal efficiency remains inadequate. The primary and secondary efficiencies were considerably enhanced by installing terrestrial ecosystems after the two previous stages. The outflow water of all tested successions respects the European norms for suspended solids (35 mg/l), COD (125 mg/l) and BOD5 (25 mg/l). Concerning tertiary efficiency, the planted ecosystems systematically enhance nitrogen and phosphorus removal compared to the control sand. The succession with Salix has shown the best results. The outflow concentrations were 0.8 and 3.7 mg/l of total nitrogen and 1.8 and 1.3 mg/l of total phosphorus respectively (average values for periods 1995 and 1996). The control successions containing non-planted unit (sand) were significantly the least efficient for both nitrogen and phosphorus removal.