Constructed wetlands for combined sewer overflow treatment

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General presentation of Orage objectives

The main goal of the ADEPTE project is to develop a software to help designing CSO-CWs. The project is composed of 5 main tasks (Fig. 15). First, a state of the art of extensive techniques was done to identify parameters influencing design choices for CSO treatment and to establish first design rules. Then, four sites are monitored during three years to evaluate technical and environmental aspects of CSO-CWs. The core model was developed based on German design-tool, state of the art and monitoring of the sites. These three tasks lead to the creation of the Orage software and the dissemination of knowledge acquired during the project. This section will present the objectives of Orage, the core model and the scientific approach regarding the calibration and the sensitivity analysis done on the model.
Designing an optimized CSO-CW is complex and requires a dynamic approach taking into account the stochastic aspect of the rain (intensity, duration and frequency), water qualities, climate, catchment and surface characteristics. This is why monitoring different sites is important to identify and mitigate uncertainties from the different aspects described above and the wetland itself.
Orage is a design and decision support-tool software which incorporates a core model, iterative algorithms for the selection of material, dimensions and environment-dependent parameters and variables (seasons, dry period…) and a graphical user interface. The goal of the software is to optimize and facilitate the design of vertical flow constructed wetlands treating both combined sewer overflow and separate sewer outlet.

The tool has two main functions:

• A sewer simulation which calculates optimized design parameters based on measured or simulated series of sewer flows, NH4-N and COD rates and catchment characteristics given by the user. From these data, the software selects a one-year period with the highest inflow around the five consecutive days in the whole dataset which contain the heaviest NH4-N load. Then, the software identifies the simples material for which a solution exist and scales the CW. The tool will interpolate the optimal dimensions (e.g. surface area and filter depth) and material based on inflow data series (Palfy et al., 2015). It will also calculate the 24h peak moving average concentration of COD and NH4-N at the selected design and the overflow volume. The output data given by Orage are the total area of the wetland, the suggested materials and the filter depth.
• An applicability test based on precipitation data series and catchment parameters which indicates if a constructed wetland is a feasible solution for the treatment of combined sewer overflow. The user will give the catchment size and the population equivalent and will select input data corresponding to his catchment using roll-down menus. For the climate region, the user choose between 5 cities the one which best describe his location and his climate. The number of cities has been set to 5 for the moment but it can be increased to give more possibilities to the user to best represent his location. Then, he has to select an imperviousness coefficient among 9 coefficients to describe the land use. For this test, 45 simulations corresponding to 9 different imperviousness coefficients multiplied by 5 climate regions were run and scaled to catchment area and population equivalent to estimate combined sewer overflow. Based on these data, the software will define if it is feasible or not to implement CSO-CW.

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Core model presentation

Orage incorporates a core model with equations similar to RSF_Sim (Meyer et al., 2015), a tool developed by Daniel Meyer to help dimensioning retention soil filters in Germany. However, the numerical model in the core was adapted by Tamás Gábor PÁLFY to the French legislation and takes into account dual filter basin concept, shortcutting effects and environmental-dependent parameters such as seasons and dry periods which have an effect on performances.
In the model, filter compartments are represented by a series of continuously stirred tank reactors in parallel plus a common basin as shown on Fig. 16. The model is described with one dimensional water flow and variable water contents (Meyer et al, 2008). According to the real structure of CSO-CWs, filters in the model are represented by a retention layer, a process layer and a drainage layer.
The model is composed of 13 user-defined parameters, 38 internal parameters, 19 calculated parameters and 47 variables. It simulates hydraulics and the removals of COD and NH4-N in the filter layer. The multiple processes involved in the modeling are governed by equations and laws which are used to model the hydraulics, the removal performances of COD, the adsorption of NH4-N and nitrification.

COD removal performances

The processes used in the model for carbon degradation are based on lab-scale column and full scale experiments from Germany and France (Meyer 2011, Fournel 2012). The removal of COD can be represented as a three-stage linear performance curve (Fig. 17).
Removal performances represent the correlation between the water in the basin (complete mixing of inflow concentration) and the outflow concentration from the filter layer. This correlation was found based on experiments from the SEGTEUP project.
The removal is happening between the process and the drainage layer. K represents the background concentration in the outflow; C1 and C2 are two concentration thresholds and ƞ1 and ƞ2 are yield rates. Then, linear equations permit to calculate the removal performances coefficients and over a certain threshold value (C2), the performances are higher. Values of K, C1, C2, ƞ1 and ƞ2 depend on the previous dry period (inter-event) and temperature. The model takes into account this dependency with 3 season multipliers which multiply the inter-event values of these parameters by a constant according to the climate region. Different day inter-event period are also implemented to take into account the previous dry period. All these parameters will be calibrated based on field measurements at Marcy-l’Etoile.

Table of contents :

I. Project presentation
1. Presentation of the company
2. Constructed wetlands for combined sewer overflow treatment
3. Presentation of the full-scale CSO-CW at Marcy- L’Etoile.
II. Materials and methods
1. General presentation of Orage objectives
2. Core model presentation
3. Scientific approach
III. Results
1. Applicability test
2. First calibration and parameters test
3. Statistical calculation and sensitivity analysis results
Conclusion

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