Multicriteria analysis for the use of CW in a multipurpose project in the Venice Lagoon watershed G. Conte*, A. Nardini**, F. Masi*, N. Martinuzzi*, C. Passoni ***, M. Bacci ****, G. Baldo***** * IRIDRA, Srl, Via Lorenzo il Magnifico 70 50129 Firenze, Italy, conte@iridra.com ** Centro Italiano di Riqualificazione Fluviale (CIRF), V.le Garibaldi 44/A, 30174, Mestre (VE), Italy (www.cirf.org ) *** Studio Paoletti-ETATEC Srl, Via Bassini, 23, 20133, Milano Italy, **** IRIS sas, Via Volterrana 18, 50020 Cerbaia Val di Pesa (FI), Italy ***** Consorzio di Bonifica Dese Sile, Via Rovereto, 12, 30100, Mestre (VE), Italy Abstract The Consorzio di Bonifica Dese Sile is a drainage authority located near Venice, Italy , created to improve the hydraulic functionality of rivers , canals and ditches of its territory, reducing hydraulic risk whereas it exists. The Regional Government asked it to define actions and projects in order to reduce the load of nutrients carried through the minor river network to the lagoon. Particular emphasis was requested to consider natural treatment technologies. The project, took place in a densely inhabitated and intensively farmed area and therefore there was the need to minimize any environmental and social impact, while addressing the two potentially conflicting objectives: minimizing hydraulic risk and removing the maximum load. So the Consorzio Dese Sile, together with a working group selected through a public bid, decided to develop the Environmental Impact Assessment (EIA) procedure – envisaged by law – as a multicriteria analysis. Such an analysis was intended to consider different project alternatives and compare their pros and cons. The EIA has been used as a Decision Support tool to facilitate public participation, and to identify a more rational project design. The project selected (between 5 alternatives) is now undergoing the final executive phase, and will realize two multipurpose constructed wetlands (for a cumulative area of 65.000 square meters), one flooding area, the enlargement and rehabilitation of part of the main river channel, the creation – in a critical urban area – of a new “naturally shaped” by-pass channel, the elimination of a few localized flow constrictions. Keywords Diffuse pollution; constructed wetland; multicriteria analysis. INTRODUCTION To reduce eutrophication of Venice Lagoon, Italian Government and Regione Veneto started a policy aimed at reduction of nitrogen load generated in the lagoon watershed: nitrogen, in fact, has been identified as the limiting factor in the lagoon ecosystem. One significant part of the policy has been devoted to increase the self-purification capacity of the drainage network, composed by a few “natural” rivers and a diffused system of small artificial channels. To implement such a policy, drainage authorities were invited to present projects able to improve self-purification capacity of the system: project should possibly be “multipurpose” (besides nitrogen load reduction, should be able to reduce hydraulic risk and to increase the natural value of the system) and with low environmental and social impact. Consorzio Dese Sile – one of the Drainage Authority of Venice Lagoon watershed – worked out a multipurpose project, able to remove nitrogen from a sub-basin of 916 hectars, while reducing flooding problems occurring in the area. Together with the working group selected through a public bid, Consorzio Dese Sile decided to develop the Environmental Impact Assessment (EIA) procedure – envisaged by law for any project occurring on watercourses – as a multicriteria analysis (MCA). Such an analysis was intended to consider different project alternatives and compare their positive and negative impacts.