Reed bed filters to process urban wet weather effluent

During wet weather, levels of urban effluent increase. Loaded with substances from atmospheric fallout or leaching from roofs and waterproofed surfaces, they intensify pollution levels in rivers and aquatic environments. This effluent comes from rainwater collection networks, runoff or "spillover" from storm overflows built to avoid the saturation of combined sewage systems (combined collection of wastewater and rainwater) and wastewater treatment plants during rainy weather.

Over the years, this source of pollution linked to rain has become increasingly significant, to the point where regulation now imposes limits on untreated rainwater being mixed in with wastewater. "These new regulations impose new constraints on local authorities in relation to the management of urban effluent. In addition to the need to reduce the amount of rainwater entering sewage systems at the source, solutions consist either of increasing the size of the existing network, which represents an expensive redevelopment, or to implement processing systems at various points in the network to provide in-line treatment of spillover from storm overflows, an option that presents a particularly interesting cost-benefit ratio if used on large systems1," specifies Pascal Molle, senior research engineer specializing in plant-based filters at Irstea.

To deal with these environmental and economic challenges, specialists in reed bed filters - and Irstea in particular (as the source of the network used in France to process domestic wastewater) - are looking to design a reed bed filter network adapted to processing urban rainwater runoff.

Good to know

Reed bed filters work by circulating water to be processed through a layer of filtering material (gravel, sand) in which bacteria, filtering biomass, and plants can grow.

Experiments with a life-sized system

During the Segteup2 project coordinated by Irstea, experiments in pilot systems made it possible to compare various reed bed structures and create an optimal system to process runoff water from storm overflows. A life-sized prototype was then created and installed in 2012 (in Marcy l'Etoile near Lyon) in order to evaluate the performance of the process under actual conditions (pollution loads, maintenance, acceptance by neighbors, etc.). As a continuation of this work, the Adepte project 3 was started by a group of partners invested in the process (scientists, private companies, local authorities). Its aim is to use scientific understanding of how reed bed filters for rainwater work along with design, size and management recommendations in order to adapt them to various geographic and climate conditions. "By monitoring the prototype developed as part of the Segteup project and three existing reed bed filtration sites used to treat rainwater, we were able to evaluate the on-site performance of these processes when dealing with key pollutants and their ability to meet legally required thresholds. We were also able to define clear operational rules for these structures," specifies Molle.

An optimized network, even down to the size of the structures

Now finished, the Adepte has delivered several significant results. In relation to processing performance, the project revealed that the network is capable of guaranteeing adequate concentration thresholds for key parameters (COD, BOD 5, suspended matter, NK4). It also showed its effectiveness against certain micropollutants (particular and/or biodegradable, such as polycyclic aromatic hydrocarbons, some metals or glyphosphates). In relation to environmental impact, life cycle analysis (LCA) studies were used to identify key design elements (energy consumption, consumable resources, ecotoxicology, etc.) that need to be taken into consideration to limit the environmental impact of the network.

The high point of the project was the design of a software program able to simulate both the hydraulic and the purifying processes to optimize the sizing of these structures. "Specifically, by entering overflow data gathered over several years for a specific network, the software will provide the user with optimized design parameters for their structure, including the filter surface area needed, the types of material to include in the structure and the volume of stored water to plan for," explains Molle. Designed to support sewage professionals and local authorities in implementing and managing this type of structure, the sizing support software will be rolled out in 2018. Irstea experts involved in the project have been contacted by several local authorities, including the Dordogne region, who would like to study the feasibility of using these structures across their region.

Project profile

Name: Sizing support to manage rain water through extended processing

Dates: 2013-2017

Partners: Irstea Lyon, Cerema (scientific partners); Syntea, Megao Informatique (private companies); Union des Marais de la Charente-Maritime, Syndicat de l’Orge, Essonne (local authorities).

Funding*: AFB, Water agencies: Rhône-Mediterranean-Corsica, Seine-Normandie, Rhin-Meuse, Adour-Garonne.

*Project arising from the Innovative environmental engineering projects grant offer as part of the national biodiversity strategy 2011-2020 from the Ministry for Ecology, Sustainable Development and Energy.

For more information

1 - Processing technique capable of operating without energy or reagents, approaching a natural balance.
2 - Extended systems to manage and process urban rainwater ANR Project (2009-2013).
3 - Sizing support to manage rain water through extended processing
4 - Total amount of organic and ammoniacal nitrogen contained in water.