The performance of a multi-stage system of constructed wetlands for urban wastewater treatment in a semiarid region of SE Spain

This paper describes the results obtained in an experimental multi-stage sy stem of created wetlands in Mojacar, in semiarid SE Spain, operating from J une to October 1997. We compare the removal efficiency of four different se ries of treatments each consisting of three stages, using different flow ra tes of sewage, flow regimes, types of substrate and influents. Pretreated w ater from an anaerobic stabilization pond and treated water from the last p ond of a lagoon system were used, the latter to test the system's suitabili ty as a complementary system for removing nitrogen and phosphorus. In spite of the initial high wastewater concentrations, the effluent conforms to th e strictest European norms (directive 91/271) for primary and secondary ret ention. A net treatment area of 2.3 m(2)/PE showed a high performance for S S (90-96%), COD (87%) and BOD5 removal (90%) during the early stages of ope ration; however, nutrient removal was lower than was expected as compared w ith other studies. The addition of iron to the substrate improved phosphoru s retention significantly (from 55 to 66%). The decrease of the net treatme nt area to 1.2 m(2)/PE did not significantly affect the wetland performance , with the exception of COD removal (78%). Series fed with treated water fr om the lagoon system (1.6 m(2)/PE) noticeably improved the quality of the e ffluent (average values of 7 mg/l total-N and 3 mg/l total-P). (C) 2001 Els evier Science B.V. All rights reserved.