Pergamon PIT: S0273-1223(99)00463-1 War. Sci. Tech Vol. 40, No.3, pp. 241-247,1999 e 19991AWQ Published by Elsevier Science Ltd Printed m Great Britain, All nghts reserved 0273-1223/99 520.00 + 0.00 ROLE OF SCIRPUS LACUSTRIS IN BACTERIAL AND NUTRIENT REMOVAL FROM WASTEWATER F. Soto, M. Garcia, E. de Luis and E. Becares Area de Ecologia, Facu/tad de Biologla, Universidad de Leon, 24071 Leon. Spain ABSTRACT Experimental microcosms using macrophytes were set up to determine the role of the plants and their rhizosphere in the removal of nutrients and fecal indicators from rural wastewater. Scirpus lacustris was grown in hydroponic culture and in siliceous gravel to compare them with the efficiency of gravel beds without macrophytes. Design parameters for the different experiments were as foIlows (surface loads in glm 2/d): 1.1-6.4 BOD, 0.8-1.6 VSS, 0.3-3.5 TN, 0.1-0.3 TP; hydraulic load 4-7 cmld; and retention time 4-8 d. Organic carbon removal was not significantly correlated to the presence of S. /acustris. The removal of phosphorus and nitrogen was enhanced in the presence of plants, even with extremely low C:N ratios. The presence of S. lacustris was responsible for 30% of TN and 20% ofTP removal in summer. Probably due to changes in plant activity, removal efficiencies for these nutrients increased 10% from spring to summer. Concerning pathogen removal, efficiencies of gravel beds with macrophytes were significantly higher than those from the other treatments, reaching up to 99.999%. iCI 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved KEYWORDS Constructed wetlands; macrophytes; nitrogen; pathogen removal; phosphorus. INTRODUCTION The removal of organic and inorganic matter from wastewater, as well as bacteria. using macrophytes has been explained by means of several mechanisms such as, sedimentation, mechanical filtration or nutrient assimilation into plants, and also due to the role of the roots as substratum for attached bacteria that may degrade or uptake nutrients and organic carbon (Brix, 1995). This latter process is favored by oxygen release into the rhizosphere (Gersberg et aJ., 1986), and by plant exudates (Stengel, 1985). Despite this evidence, there is still a controversy about the function of macrophytes for wastewater treatment in artificial wetlands. Some researchers have found an improvement in wastewater treatment in presence of macrophytes (Rogers et al.; 1991; Farahbakhshazad et al., 1995), while other studies did not detect significant differences between planted and unplanted systems (Tanner et al., 1995). Nevertheless, comparisons between studies are difficult because they utilize diverse aquatic plant species and water flows. In free-water systems with surface flow, the hydraulic conductivity of the gravel bed and that of the rhizosphere is negligible (Kadlec and Knight, 1996), and the main role of macrophytes is to provide an extra surface for development of a biofilm on the submerged parts of the plants. In those systems with sub-surface water flow the hydraulic conductivity of the substrate is an important design parameter to take into account. 241