Constructed Wetlands for combined sewr overflows (CSOs)

IRIDRA Fitodepurazione per sfioratori di reti fognarie miste (CSO)

Nature-based solutions such as constructed wetlands fit very well with the aim of combined sewer overflow (CSO) treatment. Indeed, constructed wetlands treat CSOs in situ, avoiding the use of first flush tanks, which request to pump the entrapped CSO event again into the sewer at the end of the rainfall. Moreover, constructed wetlands for CSO treatment can be multi objectives, exploiting different ecosystem services such as flood mitigation, biodiversity increase or fruition.

1
In situ treatment of CSO, contributing to a return to pre-development river hydrographs
2
Reduction of diluted wastewater conveyed to centrilezed WWTPs
3
Continuous treatment of CSO, intercepting higher pollutant loads in comparison to first flush tanks
4
Peak flow reduction
5
CAPEX costs comparable with first flush tanks, but with added ecosystem services
6
Low OPEX and simple management
7
Landscaping
8
Possibility of peri-urban area restoration
9
Biodiversity increase in wildlife corridors, improving river habitats

CSO - Combined sewr overflow - IRIDRA's solutions

Classical constructed wetlands

  • Constructed wetlands with vertical subsurface flow (VF)
  • Constructed wetlands with free water surface (FWS)
  • Hybrid constructed wetlands (VF + FWS)

Intensified constructed wetlands (Constructed wetlands 2.0)

  • Aerated constructed wetlands (FBA™)


Additional integrated solutions

  • Combination with additional ecosystem services
  • Combination with landscaping solutions for water, green, and cities
  • Combination with river restoration solutions

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 discharges 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 techniques 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 constructed wetlands (CWs) 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 into 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 hydraulic safety through detention and peak flow cutting.

Esperienza IRIDRA

Commissioned by the Autorità di Bacino del Fiume Po (agency for the control of the river Po), our society is the leader of a workgroup for the feasibility study of the application of natural purification systems for the treatment fo combined sewer overflow in the Lambro-Seveso-Olona basin. This study leads to the design of several nature-based solutions for combined sewer overflow in Lombardia Region, both in line with the sewer (Gorla Maggiore, Villaguardia, Capiago Intimiano) and upstream big centralized WWTPs (Merone, Carimate).

The CSO-CW of Gorla Maggiore was sited in the new Gorla Maggiore "Water Park" and was monitored as a case study of the European funded project OpenNESS. The results showed the benefits of the multiple ecosystem services of the green infrastructure in comparison to the effect of conventional grey one (first flush tank plus a dry detention basin) (Masi et al., 2016; Liquete et al., 2016):

higher water quality improvement;
same flood mitigation;
higher support to biodiversity increase;
social benefits linked to the new park.
The other CSO-CWs (Villaguardia, Capiago Intimiano, Carimate, and Merone) will be monitored, in order to confirm the capability of nature-based solutions for the mitigation of combined sewer overflow environmental impact.


cso gorla  web

Constructed wetland for the treatment of combined sewer overflow of Gorla Maggiore (VA - Italy). The wetland was designed a multipurpose, exploiting different ecosystem services in the new "Water Park" of Gorla Maggiore: water quality improvement of Olona river, flood mitigation, public recreational area, and biodiversity increase. The "Water Park" of Gorla Maggiore was one of the case studies of the EU OpenNESS project.

cso merone web 

Aerated wetland for the treatment of combined sewer overflow upstream the WWTP of Merone (CO - Italy), 120.000 PE, designed by IRIDRA

cso carimate ii  web

 Constructed wetland for the treatment of combined sewer overflow upstream the WWTP of Carimate (CO), 70.000 A.E., designed by IRIDRA

IRIDRA progetto per Sfioratore nel comune di Gorla Maggiore (VA)

Combined sewer overflow of Gorla Maggiore (VA - Italy): The "Water Park"

Yearly treated volumes of first flush : 150.000 m3/year

Chosen Nature-based solution : VF + FWS

Peculiarity : Multipurpose nature-based solution integrating different ecosystem services within the new "Water Park" of Gorla Maggiore: water quality improvement of Olona river, flood mitigation, recreational site, and biodiversity increase. Case study of EU  funded project OpenNESS.

Year of realization : 2012

Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage (+ eventual industrial wastewater) in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy rainfall, the wastewater volume in the combined can exceed the capacity of sewer system. For this reason, combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers or other waterbodies. These overflows, called combined sewer overflows (CSOs), contain not only stormwater but also untreated wastewater. The GORLA MAGGIORE CSO TREATMENT PLANT, designed by IRIDRA in collaboration with Studio Maione, is the first system realised in Italy for this kind of application. It consists of 4 sand filters vertical beds aimed to treat the first flush coming from the CSO device and from an extended retention basin for the accumulation and the slow release in the river of the second flush, with the main aim of flood protection but also with some benefits in terms of pollution control. The system has been designed, under the sponsorship or the Lombardia Regional Authority and the co-funding by Fondazione Cariplo, through a participatory process, which ha s brought to the recovery of an abandoned industrial area and the creation of a "Water Park" along the Olona River banks. A small flux is permanently diverted from the river for feeding a small pool located on the bottom of the extended retention basin, so to permit the development of a highly diversified ecosystem which will offer multiple pathways for the removal/control of several pollutants.
IRIDRA progetto per Sfioratore in testa al depuratore di Merone (CO)

Combined sewer overflow upstream the WWTP of Merone (CO - Italy)

Yearly treated volumes of CSOs : 564.000 m3/year

Chosen Nature-based solution : FBA™ + FWS

Peculiarity : First aerated constructed wetland for CSO treatment in Italy, and among the first in the World

Year of design : 2015

The centralised WWTP of Merone (ex Asil Spa, now Como Acque Spa) treats the wastewater from the combined sewer serving 38 towns in Como province (120,000 pe). The aim was to limit as much as possible the pollutant loads discharghed in the Lambro River by treating also the combined sewer overflow (CSO) upstream the WWTP. To this aim, a constructed wetland for treatment of combined sewer overflow (CSO-CW) was designed. The project was planned as an action of the LIFE11 ENV/IT/004 Lambro vivo "Interventi per il miglioramento delle acque e degli habitat nella Valle del Lambro" (Action B2). The preliminary design was done by the Parco Regionale della Valle del Lambro in 2013. The further detailed design (definitive and executive) were done by IRIDRA Srl in 2014. The original design was revised and, in order to optise the treatment efficiency of the system according to the limited available site, IRIDRA designed an intensified aerated wetland using the Forced Bed Aeration TM technology. The CSO-CW is a two-stage system.The first stage comprises two aerated FBA wetland beds, each one further divided into 2 separated hydraulic sectors for a total area of 4000 m2. The second stage is a free water surface (FWS) CW of 1500 m2. The system is fed by a pumping system and automatically regulated by a PLC to properly aerate and treat the first more polluted fraction of the CSO events. The CSO-CW is able to intercept up to 565.000 m3/year (about 40% of the total estimated average CSO volume per year) and a COD load of 120 tCOD/year (about 60% of the total estimated CSO COD load per year). In other words, the CSO-CW is planned to intercept a pollutant load of 13000 p.e. during rainy days (expressed in terms of COD), which were previously discharged untreated in the Lambro River. The maximum exploitation of CW ecosystem services is planned thanks to the FWS stage, which also aims to increase biodiversity and to recreate an area suitable for environmental educational activities. IRIDRA Srl is following the start-up phases.
IRIDRA progetto per Sfiorature in testa al depuratore di Carimate (CO)

Combined sewer overflow upstream the WWTP of Carimate (CO - Italy)

Yearly treated volumes of CSOs : 515.000 m3/year

Chosen Nature-based solution : VF + FWS

Peculiarity : Nature-based solution integrating also intervention for environmental education and river restoration. Particularly, the project implements a restored  riparian zone of Seveso river and a didactic wood area

Year of design : 2015

The centralised WWTP of Carimate treats the wastewater from the combined sewer serving 11 towns in Como province (70,040 inhabitants). The Water Utiliy (Sud Seveso Servizi spa) wants to further reduce the pollutant load discharged in the Seveso River by treating also the combined sewer overflow (CSO) upstream the WWTP. To this aim, a constructed wetland for treatment of combined sewer overflow (CSO-CW) was designed by IRIDRA.

The CSO-CW is a two-stage system. The first stage comprises two VF CW beds, each one further divided into 2 separated hydraulic sectors for a total area of 8500 m2. The second stage is a free water surface (FWS) CW of 4500 m2. The system is fed by a pumping system and automatically regulated by a PLC to properly treat the first more polluted fraction of the CSO events.

The CSO-CW is able to intercept up to 500.000 m3/year (about 58% of the total estimated average CSO volume per year) and a COD load of 60 tCOD/year (about 60% of the total estimated CSO COD load per year). In other words, the CSO-CW is planned to intercept a pollutant load of 7700 p.e. (expressed in terms of COD), which were previously discharged untreated in the Seveso River.

The maximum exploitation of CW ecosystem services is planned thanks to the FWS stage, which also aims to increase biodiversity and to recreate an area suitable for environmental educational activities. The riparian area along the left side of the Seveso river is planned to be involved in river restoration activities, with plantation of vegetation more suitable for riparian environments.
IRIDRA progetto per Sfioratore nel comune di Villaguardia (CO)

Combined sewer overflow of Villaguardia (CO - Italy)

Treated first flush flow rate : Up to 140 l/s

Chosen Nature-based solution : VF + FWS

Peculiarity : Multipurpose nature-based solution integrating different ecosystem services: water quality improvement, recreational site, and biodiversity increase.

Year of design : 2015

Il progetto rientra nella strategia adottata dalla Regione Lombardia per il contenimento degli impatti ambientali generati dagli sfioratori di fognatura mista sul territorio regionale mediante il ricorso a sistemi naturali di trattamento, progettati secondo una logica multi-obiettivo in modo da concorrere, mediante la creazione di servizi ecosistemici, sia al miglioramento della qualità delle acque che al controllo idraulico, all’arricchimento della biodiversità, all'educazione ambientale e alla fruizione. I lavori sono cominciati a Marzo 2016.

Constructed Wetland for CSO in Villa Guardia, Northern Italy

The intervention is one of the several CW system provided by Lombardia Region with the aims to reduce the environmental impact of Combined Sewer Overflows through the use of Constructed Wetland Technology designed with a multi-target approach, considering not only the water quality aspects but also the hydraulic protection, the biodiversity improvement and the educational opportunities. A real Ecosystem Service! The works started in March 2016
IRIDRA progetto per Sfioratore nel comune di Carugo (CO)

Combined sewer overflow of Carugo (CO - Italy)

Treated first flush flow rate : Up to 40 l/s

Chosen Nature-based solution : VF

Year of design : 2016

Testo
IRIDRA progetto per Sfioratore nel comune di Mozzate (CO)

Combined sewer overflow of Mozzate (CO - Italy)

Treated first flush flow rate : Volumes higher than the 2050 m3 captured by the first flush tank

Chosen Nature-based solution : FWS

Peculiarity : Multipurpose nature-based solution integrating different ecosystem services: water quality improvement of second flushes, flood mitigation, recreational site (bicycle lane, birdwatching), and biodiversity increase.

Year of  realization : 2016

The FWS CW (13,500 m3) in Mozzate (VA - Italy) has been designed to provide multiple ecosystem services: (i) treatment of combined sewer overflow (CSO) second flushes not captured by the first flush tank (volume of 2050 m3); (ii) buffer tank (accumulation capacity of 65,000 m3); (iii) biodiversity increase; (iv) recreational activities (e.g., cycle lane, birdwacthing).
IRIDRA progetto per Sfioratore nel comune di Capiago Intimiano (CO)

Combined sewer overflow of Capiago Intimiano (CO - Italy)

Treated volumes of CSOs : up to 130 m3 per CSO event

Chosen Nature-based solution : VF + FWS

Peculiarity : Multipurpose nature-based solution integrating different ecosystem services: water quality improvement, flood mitigation, recreational site, and biodiversity increase.

Year of design : 2016

Volume prima pioggia trattato: max 130 mc per evento di scolmo
Configurazione: VF + FWS

Peculiarità
Intervento multiobiettivo che integra diversi servizi ecosistemici (ecosystem services): riduzione carico inquinante, laminazione per riduzione rischio allagamenti a valle, area ricreativa, aumento della biodiversità.

IRIDRA ha svolto la progettazione (preliminare, definitiva ed esecutiva) e la direzione lavori.

First flush volume treated: max 130 mc per CSO event
Scheme: VF + FWS

Characteristics
Multiobjective wetland for the exploitation of different ecosystem services: water quality, flood mitigation, increase of biodiversity and recreational activities

IRIDRA has done the design and the work supervision.
IRIDRA progetto per Sfioratore in testa depuratore di Mariano Comense (CO)

Combined sewer overflow upstream the WWTP of Mariano Comense (CO - Italy)

Yearly treated volumes of CSOs : 10.000 m3/year Chosen Nature-based solution FBA™ + FWS

Peculiarity : Pilot plant to optimise aerated wetland for combined sewer overflow treatment.

Year of realization : 2020

The pilot plant is a two-stage system: 1st, Forced Bed Aeration (FBA - 200 m2); 2nd, Free Water Surface (FWS - 100 m2). The pilot plant is designed to test the functioning of intensified constructed wetlands (aerated wetland) for the treatment of combined sewer overflow (CSO) upstream of the centralised WWTP of Mariano Comense (CO, Lombardia Region, Italy). The aim is to investigate optimal aeration strategies to have both high CSO treatment and a low area footprint. The system is designed to receive up to 10000 m3/year of CSOs.
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