Constructed Wetlands for the treatment of Combined Sewer Overflow

Combined sewer collect both wastewater and stormwater intercepted by waterproof surfaces that are present in urban areas (roads, squares, roofs). During storms great quantities of rainwater flow in the sewage system: only a part of them is collected to the treatment plants, due to the fact that they are designed for average hydraulic loads; through apposite floodways placed along the drainage net, excess flows are diverted and ( most of the times) directly discharged into the environment.

To ensure the respect of the quality of the receiving body, it is necessary to avoid that these doscharges come to be unacceptable pollution sources: water collected by a combined sewer, in fact, contains pollutants of civil and industrial wastewater and, at the same time, contains pollutants coming from runoff on urban surfaces.

Along with strategies for the reduction of overflow volumes (among all the realization of separate sewage systems for wastewater and stormwater), the mitigation of environmental impacts is achieved through the treatment of overflows before they are discharged. Among different solutions, natural tecnhiques as constructed wetlands are recognized at the international level as Best Management Practices for this problem: the achievement of appropriate purification goals, in fact, comes with cheap realization and easy management.

Concerning the design of contructed wetlands for the treatment of combined sewer overflow, many experiences are known in the world (USA, Australia, Canada, Germany, England) showing positive results. Except for some differences due to particular characteristics of combined sewer overflow (variability and unpredictability of flows, lower concentrations fo pollutants respect to civil wastewater), constructed wetlands used in the treatment of overflows are similar to those used in the treatment of civil wastewater: systems are divided in Free Water Systems and Subsurface Flow Systems.

In general, a constructed wetland for the treatment of a combined sewer overflow is able to strongly reduce suspended solids and pollutants and to increase the hydraulic safety through detention and peak flow cutting.

Another important advantage of constructed wetlands for this issue is represented by the great flexibility, allowing the integration of the treatment systems in multi-function areas as natural parks and sport zones.

Commissioned by the Autorità di Bacino del Fiume Po (agency for the control of the river Po), our society is the leader of a work group for the feasibilty study of the application of natural purification systems for the treatment fo combined sewer overflow in the Lambro-Seveso-Olona basin.