Lab scale constructed wetlands were used to evaluate organic load removal efficiency. Bioreactors were fed with synthetic wastewater (SW) with varying concentrations of nitrogen and potassium. Reactors were planted with species Phragmites australis. Fed theoretic COD was adjusted to 240.0mg-O 2L -1, nitrogen levels were 10 and 40mg-NL -1 (ammonium sulfate), potassium levels were 5 and 31mg-KL -1 (potassium monobasic phosphate). The higher biomass yield, for 0.5 and 0.775 N:K ratios, was related with higher organic load removal. The ratio N:K showed significant differences for organic load abatement, when 1:0.5 and 1:0.775 N:K ratios were applied, 96.8% efficiency was obtained, whereas N:K ratio of 1:0.125 had efficiency of 92.1% and N:K ratio of 1:3.1 showed an efficiency of 90.5%. For planted bioreactor E H decreased in 162.7mV from sample port to 5cm down to 35cm depth, while for the bioreactor without plant showed an E H decrement of only 17.7mV. © 2011 Elsevier Ltd.